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You may copy it, give it away or +re-use it under the terms of the Project Gutenberg License included +with this eBook or online at www.gutenberg.org + + +Title: Climatic Changes + Their Nature and Causes + +Author: Ellsworth Huntington + Stephen Sargent Visher + +Release Date: October 26, 2011 [EBook #37855] + +Language: English + +Character set encoding: ISO-8859-1 + +*** START OF THIS PROJECT GUTENBERG EBOOK CLIMATIC CHANGES *** + + + + +Produced by Robin Monks, Wayne Hammond and the Online +Distributed Proofreading Team at http://www.pgdp.net (This +file was produced from images generously made available +by The Internet Archive/Canadian Libraries) + + + + + + +</pre> + + +<div class="book"><!-- Begin Book --> +<div class="transnote"> +<p><b>TRANSCRIBER'S NOTE:</b> Spelling maintained as closely as possible to the +original document, while obvious typos have been corrected. Emdashes +in original text for negative temperatures changed to minus signs to +standardize temperatures.</p> +</div> + +<!--File: cc007.png--> +<a class="pagenum break" id="Page_vi" title="pg vi"></a> + + + +<h1>CLIMATIC CHANGES</h1> + +<h2>THEIR NATURE AND CAUSES</h2> + + +<!--File: cc008.png--> +<a class="pagenum break" id="Page_vii" title="pg vii"></a> +<!--[Blank Page]--> +<!--File: cc009.png Page --> +<a class="pagenum break" id="Page_viii" title="pg viii"></a> + + + +<h3>PUBLISHED ON THE FOUNDATION<br /> +ESTABLISHED IN MEMORY OF<br /> +THEODORE L. GLASGOW</h3> +<hr /> +<!--File: cc010.png--> +<a class="pagenum break" id="Page_ix" title="pg 1x"></a> + + + +<h3>OTHER BOOKS BY THE SAME AUTHORS</h3> + + +<h3>ELLSWORTH HUNTINGTON</h3> + +<ol style="list-style-type:upper-alpha"> + +<li><em>Four books showing the development of knowledge as to Historical Pulsations of Climate.</em> + <ul class="none"> + <li>The Pulse of Asia. Boston, 1907.</li> + <li>Explorations in Turkestan. Expedition of 1903. Washington, 1905.</li> + <li>Palestine and Its Transformation. Boston, 1911.</li> + <li>The Climatic Factor, as Illustrated in Arid America. Washington, 1914.</li> + </ul> +</li> + +<li><em>Two books illustrating the effect of climate on man.</em> +<ul class="none"> + <li>Civilization and Climate. New Haven, 1915.</li> + <li>World Power and Evolution. New Haven, 1919.</li> + </ul> +</li> + +<li><em>Four books illustrating the general principles of Geography.</em> + <ul class="none"> + <li>Asia: A Geography Reader. Chicago, 1912.</li> + <li>The Red Man's Continent. New Haven, 1919.</li> + <li>Principles of Human Geography (with S. W. Cushing). New York, 1920.</li> + <li>Business Geography (with F. E. Williams). New York, 1922.</li> + </ul> +</li> + +<li><em>A companion to the present volume.</em> + <ul class="none"> + <li>Earth and Sun: An Hypothesis of Weather and Sunspots. New Haven. In press.</li> + </ul> +</li> +</ol> + + +<h3>STEPHEN SARGENT VISHER</h3> + + +<p>Geography, Geology and Biology of Southern Dakota. Vermilion, 1912.<br /> +The Biology of Northwestern South Dakota. Vermilion, 1914.<br /> +The Geography of South Dakota. Vermilion, 1918.<br /> +Handbook of the Geology of Indiana (with others). Indianapolis, 1922.<br /> +Hurricanes of Australia and the South Pacific. Melbourne, 1922.</p> + +<!--File: cc011.png--> +<a class="pagenum break" id="Page_x" title="pg x"></a> + + +<hr /> + +<h1>CLIMATIC CHANGES</h1> + +<h3>THEIR NATURE AND CAUSES<br /><br /> + +BY<br /><br /> + +ELLSWORTH HUNTINGTON</h3> +<h5>Research Associate in Geography in Yale University</h5> + +<h3>AND<br /><br /> + +STEPHEN SARGENT VISHER</h3> +<h5>Associate Professor of Geology<br /> +in Indiana University</h5> + +<div class="figcenter"> +<img src="images/front.png" width="170" height="179" alt="" title="Shield with motto "Lux et Veritas"" /> +</div> +<h4>NEW HAVEN<br /> +YALE UNIVERSITY PRESS<br /> +LONDON: HUMPHREY MILFORD: OXFORD UNIVERSITY PRESS<br /> +MDCCCCXXII</h4> + +<!--File: cc012.png--> + + + +<hr /> + + +<h3>COPYRIGHT 1922 BY</h3> +<h3>YALE UNIVERSITY PRESS</h3> + +<h3>Published 1922.</h3> +<hr /> +<!--File: cc013.png--> + + + + +<h3>THE THEODORE L. GLASGOW MEMORIAL</h3> +<h3>PUBLICATION FUND</h3> + + +<blockquote><p>The present volume is the fifth work published by the Yale +University Press on the Theodore L. Glasgow Memorial Publication +Fund. This foundation was established September 17, 1918, +by an anonymous gift to Yale University in memory of Flight +Sub-Lieutenant Theodore L. Glasgow, R.N. He was born in +Montreal, Canada, and was educated at the University of Toronto +Schools and at the Royal Military College, Kingston. In August, +1916, he entered the Royal Naval Air Service and in July, 1917, +went to France with the Tenth Squadron attached to the Twenty-second +Wing of the Royal Flying Corps. A month later, August +19, 1917, he was killed in action on the Ypres front.</p></blockquote> +<!--File: cc014.png--> + +<!--[Blank Page]--> +<!--File: cc015.png--> + + + + +<h3>TO<br /><br /> + +THOMAS CHROWDER CHAMBERLIN<br /><br /> + +OF THE UNIVERSITY OF CHICAGO<br /><br /> + +WHOSE CLEAR AND MASTERLY DISCUSSION +OF THE GREAT PROBLEMS OF TERRESTRIAL EVOLUTION +HAS BEEN ONE OF THE MOST INSPIRING FACTORS +IN THE WRITING OF THIS BOOK</h3> + +<!--File: cc016.png--> +<blockquote> +<p><span class="smcap">There</span> <em>is a toy, which I have heard, and I would not have +it given over, but waited upon a little. They say it is observed +in the Low Countries (I know not in what part), +that every five and thirty years the same kind and suit +of years and weathers comes about again; as great frosts, +great wet, great droughts, warm winters, summers with +little heat, and the like, and they call it the prime; it is a +thing I do the rather mention, because, computing backwards, +I have found some concurrence.</em></p> + +<p style="text-align:right;">FRANCIS BACON</p> +</blockquote> + +<!--File: cc017.png--> + + + +<h2 id="preface">PREFACE</h2> + + +<p>Unity is perhaps the keynote of modern science. +This means unity in time, for the present is but +the outgrowth of the past, and the future of the +present. It means unity of process, for there seems to be +no sharp dividing line between organic and inorganic, +physical and mental, mental and spiritual. And the unity +of modern science means also a growing tendency toward +coöperation, so that by working together scientists discover +much that would else have remained hid.</p> + +<p>This book illustrates the modern trend toward unity in +all of these ways. First, it is a companion volume to +<em>Earth and Sun</em>. That volume is a discussion of the causes +of weather, but a consideration of the weather of the +present almost inevitably leads to a study of the climate +of the past. Hence the two books were written originally +as one, and were only separated from considerations of +convenience. Second, the unity of nature is so great that +when a subject such as climatic changes is considered, it +is almost impossible to avoid other subjects, such as the +movements of the earth's crust. Hence this book not only +discusses climatic changes, but considers the causes of +earthquakes and attempts to show how climatic changes +may be related to great geological revolutions in the +form, location, and altitude of the lands. Thus the book +has a direct bearing on all the main physical factors +which have molded the evolution of organic life, including +man.</p> +<!--File: cc018.png--> +<p>In the third place, this volume illustrates the unity of +modern science because it is preëminently a coöperative +product. Not only have the two authors shared in its +production, but several of the Yale Faculty have also +coöperated. From the geological standpoint, Professor +Charles Schuchert has read the entire manuscript in its +final form as well as parts at various stages. He has +helped not only by criticisms, suggestions, and facts, but +by paragraphs ready for the printer. In the same way +in the domain of physics, Professor Leigh Page has repeatedly +taken time to assist, and either in writing or by +word of mouth has contributed many pages. In astronomy, +the same cordial coöperation has come with equal +readiness from Professor Frank Schlesinger. Professors +Schuchert, Schlesinger, and Page have contributed so +materially that they are almost co-authors of the volume. +In mathematics, Professor Ernest W. Brown has been +similarly helpful, having read and criticised the entire +book. In certain chemical problems, Professor Harry W. +Foote has been our main reliance. The advice and suggestions +of these men have frequently prevented errors, and +have again and again started new and profitable lines of +thought. If we have made mistakes, it has been because +we have not profited sufficiently by their coöperation. If +the main hypothesis of this book proves sound, it is +largely because it has been built up in constant consultation +with men who look at the problem from different +points of vision. Our appreciation of their generous and +unstinted coöperation is much deeper than would appear +from this brief paragraph.</p> + +<p>Outside the Yale Faculty we have received equally +cordial assistance. Professor T. C. Chamberlin of the University +of Chicago, to whom, with his permission, we take +great pleasure in dedicating this volume, has read the +<!--File: cc019.png--> +entire proof and has made many helpful suggestions. +We cannot speak too warmly of our appreciation not +only of this, but of the way his work has served for years +as an inspiration in the preliminary work of gathering +data for this volume. Professor Harlow Shapley of Harvard +University has contributed materially to the chapter +on the sun and its journey through space; Professor +Andrew E. Douglass of the University of Arizona has +put at our disposal some of his unpublished results; +Professors S. B. Woodworth and Reginald A. Daly, and +Mr. Robert W. Sayles of Harvard, and Professor Henry +F. Reid of Johns Hopkins have suggested new facts and +sources of information; Professor E. R. Cumings of +Indiana University has critically read the entire proof; +conversations with Professor John P. Buwalda of the +University of California while he was teaching at Yale +make him another real contributor; and Mr. Wayland +Williams has contributed the interesting quotation from +Bacon on page x of this book. Miss Edith S. Russell has +taken great pains in preparing the manuscript and in +suggesting many changes that make for clearness. Many +others have also helped, but it is impossible to make due +acknowledgment because such contributions have become +so thoroughly a part of the mental background of the +book that their source is no longer distinct in the minds +of the authors.</p> + +<p>The division of labor between the two authors has not +followed any set rules. Both have had a hand in all parts +of the book. The main draft of Chapters VII, VIII, IX, +XI, and XIII was written by the junior author; his contributions +are also especially numerous in Chapters X +and XV; the rest of the book was written originally by +the senior author.</p> + +<!--File: cc020.png--> +<!--[Blank Page]--> +<!--File: cc021.png--> + + +<p> </p> +<table> +<tr><th colspan="3"><a id="content"></a>CONTENTS</th></tr> +<tr> +<td class="tdl">I</td> +<td class="tdl"><a href="#Chapt_I">The Uniformity of Climate</a></td> +<td class="tdl">1</td> +</tr> +<tr> +<td class="tdl">II.</td> +<td class="tdl"><a href="#Chapt_II">The Variability of Climate</a></td> +<td class="tdl">16</td> +</tr> +<tr> +<td class="tdl">III.</td> +<td class="tdl"><a href="#Chapt_III">Hypotheses of Climatic Change</a></td> +<td class="tdl">33</td> +</tr> +<tr> +<td class="tdl">IV.</td> +<td class="tdl"><a href="#Chapt_IV">The Solar Cyclonic Hypothesis</a></td> +<td class="tdl">51</td> +</tr> +<tr> +<td class="tdl">V.</td> +<td class="tdl"><a href="#Chapt_V">The Climate of History</a></td> +<td class="tdl">64</td> +</tr> +<tr> +<td class="tdl">VI.</td> +<td class="tdl"><a href="#Chapt_IV">The Climatic Stress of the Fourteenth Century</a></td> +<td class="tdl">98</td> +</tr> +<tr> +<td class="tdl">VII.</td> +<td class="tdl"><a href="#Chapt_VII">Glaciation According to the Solar Cyclonic Hypothesis</a></td> +<td class="tdl">110</td> +</tr> +<tr> +<td class="tdl">VIII.</td> +<td class="tdl"><a href="#Chapt_VIII">Some Problems of Glacial Periods </a></td> +<td class="tdl">130</td> +</tr> +<tr> +<td class="tdl">IX.</td> +<td class="tdl"><a href="#Chapt_IX">The Origin of Loess</a></td> +<td class="tdl">155</td> +</tr> +<tr> +<td class="tdl">X.</td> +<td class="tdl"><a href="#Chapt_X">Causes of Mild Geological Climates </a></td> +<td class="tdl">166</td> +</tr> +<tr> +<td class="tdl">XI.</td> +<td class="tdl"><a href="#Chapt_XI">Terrestrial Causes of Climatic Changes</a></td> +<td class="tdl">188</td> +</tr> +<tr> +<td class="tdl">XII.</td> +<td class="tdl"><a href="#Chapt_XII">Post-Glacial Crustal Movements and Climatic Changes</a></td> +<td class="tdl">215</td> +</tr> +<tr> +<td class="tdl">XIII.</td> +<td class="tdl"><a href="#Chapt_XIII">The Changing Composition of Oceans and Atmosphere</a></td> +<td class="tdl">223</td> +</tr> +<tr> +<td class="tdl">XIV.</td> +<td class="tdl"><a href="#Chapt_XIV">The Effect of Other Bodies on the Sun</a></td> +<td class="tdl">242</td> +</tr> +<tr> +<td class="tdl">XV.</td> +<td class="tdl"><a href="#Chapt_XV">The Sun's Journey through Space</a></td> +<td class="tdl">264</td> +</tr> +<tr> +<td class="tdl">XVI.</td> +<td class="tdl"><a href="#Chapt_XVI">The Earth's Crust and the Sun</a></td> +<td class="tdl">285</td> +</tr> +</table> + +<!--File: cc022.png--> + + +<p> </p> +<table> +<tr> +<th colspan="3">LIST OF ILLUSTRATIONS</th> +</tr> +<tr> +<td class="tdl">Fig. 1.</td> +<td class="tdl"><a href="#Fig_1">Climatic changes and mountain building</a></td> +<td class="tdl">25</td> +</tr> +<tr> +<td class="tdl">Fig. 2.</td> +<td class="tdl"><a href="#Fig_2">Storminess at sunspot maxima vs. minima</a></td> +<td class="tdl">54</td> +</tr> +<tr> +<td class="tdl">Fig. 3.</td> +<td class="tdl"><a href="#Fig_3a">Relative rainfall at times of increasing and decreasing sunspots</a></td> +<td class="tdl">58, 59</td> +</tr> +<tr> +<td class="tdl">Fig. 4.</td> +<td class="tdl"><a href="#Fig_4">Changes of climate in California and in western and central Asia</a></td> +<td class="tdl">75</td> +</tr> +<tr> +<td class="tdl">Fig. 5.</td> +<td class="tdl"><a href="#Fig_5">Changes in California climate for 2000 years, as measured by growth of Sequoia trees</a></td> +<td class="tdl">77</td> +</tr> +<tr> +<td class="tdl">Fig. 6.</td> +<td class="tdl"><a href="#Fig_6">Distribution of Pleistocene ice sheets</a></td> +<td class="tdl">123</td> +</tr> +<tr> +<td class="tdl">Fig. 7.</td> +<td class="tdl"><a href="#Fig_7">Permian geography and glaciation </a></td> +<td class="tdl">145</td> +</tr> +<tr> +<td class="tdl">Fig. 8.</td> +<td class="tdl"><a href="#Fig_8">Effect of diminution of storms on movement of water</a></td> +<td class="tdl">175</td> +</tr> +<tr> +<td class="tdl">Fig. 9.</td> +<td class="tdl"><a href="#Fig_9">Cretaceous Paleogeography</a></td> +<td class="tdl">201</td> +</tr> +<tr><td class="tdl">Fig. 10.</td> +<td class="tdl"><a href="#Fig_10">Climatic changes of 140,000 years as inferred from the stars</a></td> +<td class="tdl">279</td> +</tr> +<tr><td class="tdl">Fig. 11.</td> +<td class="tdl"><a href="#Fig_11">Sunspot curve showing cycles, 1750 to 1920</a></td> +<td class="tdl">283</td> +</tr> +<tr><td class="tdl">Fig. 12.</td> +<td class="tdl"><a href="#Fig_12">Seasonal distribution of earthquakes</a></td> +<td class="tdl">299</td> +</tr> +<tr><td class="tdl">Fig. 13.</td> +<td class="tdl"><a href="#Fig_13">Wandering of the pole from 1890 to 1898</a></td> +<td class="tdl">303</td> +</tr> +</table> + + +<!--File: cc023.png--> + + + +<p> </p> +<table> +<tr><th id="tables" colspan="3">TABLES</th></tr> +<tr> +<td class="tdl">1.</td> +<td class="tdl"><a href="#table_1">The Geological Time Table</a></td> +<td class="tdl">5</td> +</tr> +<tr> +<td class="tdl">2.</td> +<td class="tdl"><a href="#table_2">Types of Climatic Sequence</a></td> +<td class="tdl">16</td> +</tr> +<tr> +<td class="tdl">3.</td> +<td class="tdl"><a href="#table_3">Correlation Coefficients between Rainfall and Growth of Sequoias in California</a></td> +<td class="tdl">80</td> +</tr> +<tr> +<td class="tdl">4.</td> +<td class="tdl"><a href="#table_4">Correlation Coefficients between Rainfall Records in California and Jerusalem</a></td> +<td class="tdl">84</td> +</tr> +<tr> +<td class="tdl">5.</td> +<td class="tdl"><a href="#table_5">Theoretical Probability of Stellar Approaches</a></td> +<td class="tdl"> 260</td> +</tr> +<tr> +<td class="tdl">6.</td> +<td class="tdl"><a href="#table_6">Thirty-Eight Stars Having Largest Known Parallaxes</a></td> +<td class="tdl">276, 277</td> +</tr> +<tr><td class="tdl">7.</td> +<td class="tdl"><a href="#table_7">Destructive Earthquakes from 1800 to 1899 Compared with Sunspots</a></td> +<td class="tdl">289</td> +</tr> +<tr> +<td class="tdl">8.</td> +<td class="tdl"><a href="#table_8">Seasonal March of Earthquakes</a></td> +<td class="tdl">295</td> +</tr> +<tr> +<td class="tdl">9.</td> +<td class="tdl"><a href="#table_9">Deflection of Path of Pole Compared with Earthquakes</a></td> +<td class="tdl">305</td> +</tr> +<tr> +<td class="tdl">10.</td> +<td class="tdl"><a href="#table_10">Earthquakes in 1903 to 1908 Compared with Departures of the Projected Curve of the Earth's Axis from the Eulerian Position</a></td> +<td class="tdl">306</td> +</tr> +</table> + +<!--File: cc024.png--> + +<!--[Blank Page]--> + +<!--File: cc025.png Page 1--> +<a class="pagenum break" id="Page_1" title="pg 1"></a> + +<h2 id="Chapt_I"><a href="#content">CHAPTER I</a></h2> + +<h3>THE UNIFORMITY OF CLIMATE</h3> + + +<p>The rôle of climate in the life of today suggests its +importance in the past and in the future. No human +being can escape from the fact that his food, +clothing, shelter, recreation, occupation, health, and +energy are all profoundly influenced by his climatic surroundings. +A change of season brings in its train some +alteration in practically every phase of human activity. +Animals are influenced by climate even more than man, +for they have not developed artificial means of protecting +themselves. Even so hardy a creature as the dog +becomes notably different with a change of climate. The +thick-haired "husky" of the Eskimos has outwardly +little in common with the small and almost hairless +canines that grovel under foot in Mexico. Plants are even +more sensitive than animals and men. Scarcely a single +species can flourish permanently in regions which differ +more than 20°C. in average yearly temperature, and for +most the limit of successful growth is 10°.<sup><a href="#fn1" id="r1">[1]</a></sup> +So far as we +yet know every living species of plant and animal, including +man, thrives best under definite and limited conditions +of temperature, humidity, and sunshine, and of the composition +and movement of the atmosphere or water in +which it lives. Any departure beyond the limits means +lessened efficiency, and in the long run a lower rate of + +<!--File: cc026.png Page_2--> +<a class="pagenum break" id="Page_2" title="pg 2"></a> + +reproduction and a tendency toward changes in specific +characteristics. Any great departure means suffering or +death for the individual and destruction for the species.</p> + +<p>Since climate has so profound an influence on life +today, it has presumably been equally potent at other +times. Therefore few scientific questions are more important +than how and why the earth's climate has varied +in the past, and what changes it is likely to undergo in +the future. This book sets forth what appear to be the +chief reasons for climatic variations during historic and +geologic times. It assumes that causes which can now be +observed in operation, as explained in a companion +volume entitled <em>Earth and Sun</em>, and in such books as +Humphreys' <em>Physics of the Air</em>, should be carefully +studied before less obvious causes are appealed to. It +also assumes that these same causes will continue to +operate, and are the basis of all valid predictions as to +the weather or climate of the future.</p> + +<p>In our analysis of climatic variations, we may well +begin by inquiring how the earth's climate has varied +during geological history. Such an inquiry discloses three +great tendencies, which to the superficial view seem contradictory. +All, however, have a similar effect in providing +conditions under which organic evolution is able to +make progress. The first tendency is toward uniformity, +a uniformity so pronounced and of such vast duration +as to stagger the imagination. Superposed upon this +there seems to be a tendency toward complexity. During +the greater part of geological history the earth's climate +appears to have been relatively monotonous, both from +place to place and from season to season; but since the +Miocene the rule has been diversity and complexity, a +condition highly favorable to organic evolution. Finally, +the uniformity of the vast eons of the past and the + +<!--File: cc027.png Page_3--> +<a class="pagenum break" id="Page_3" title="pg 3"></a> + +tendency toward complexity are broken by pulsatory +changes, first in one direction and then in another. To +our limited human vision some of the changes, such as +glacial periods, seem to be waves of enormous proportions, +but compared with the possibilities of the universe +they are merely as the ripples made by a summer zephyr.</p> + +<p>The uniformity of the earth's climate throughout the +vast stretches of geological time can best be realized by +comparing the range of temperature on the earth during +that period with the possible range as shown in the entire +solar system. As may be seen in Table 1, the geological +record opens with the Archeozoic era, or "Age of Unicellular +Life," as it is sometimes called, for the preceding +cosmic time has left no record that can yet be read. +Practically no geologists now believe that the beginning +of the Archeozoic was less than one hundred million +years ago; and since the discovery of the peculiar properties +of radium many of the best students do not hesitate +to say a billion or a billion and a half.<sup><a href="#fn2" id="r2">[2]</a></sup> +Even in the +Archeozoic the rocks testify to a climate seemingly not +greatly different from that of the average of geologic +time. The earth's surface was then apparently cool +enough so that it was covered with oceans and warm +enough so that the water teemed with microscopic life. +The air must have been charged with water vapor and +with carbon dioxide, for otherwise there seems to be no +possible way of explaining the formation of mudstones +and sandstones, limestones of vast thickness, carbonaceous +shales, graphites, and iron ores.<sup><a href="#fn3" id="r3">[3]</a></sup> Although the +Archeozoic has yielded no generally admitted fossils, yet +what seem to be massive algæ and sponges have been + +<!--File: cc028.png Page_4--> +<a class="pagenum break" id="Page_4" title="pg 4"></a> + +found in Canada. On the other hand, abundant life is +believed to have been present in the oceans, for by no +other known means would it be possible to take from the +air the vast quantities of carbon that now form carbonaceous +shales and graphite.</p> + +<p>In the next geologic era, the Proterozoic, the researches +of Walcott have shown that besides the marine +algæ there must have been many other kinds of life. The +Proterozoic fossils thus far discovered include not only +microscopic radiolarians such as still form the red ooze +of the deepest ocean floors, but the much more significant +tubes of annelids or worms. The presence of the +annelids, which are relatively high in the scale of organization, +is generally taken to mean that more lowly forms +of animals such as coelenterates and probably even the +mollusca and primitive arthropods must already have +been evolved. That there were many kinds of marine +invertebrates living in the later Proterozoic is indicated +by the highly varied life and more especially the trilobites +found in the oldest Cambrian strata of the next +succeeding period. In fact the Cambrian has sponges, +primitive corals, a great variety of brachiopods, the +beginnings of gastropods, a wonderful array of trilobites, +and other lowly forms of arthropods. Since, under the +postulate of evolution, the life of that time forms an unbroken +sequence with that of the present, and since many +of the early forms differ only in minor details from those +of today, we infer that the climate then was not very +different from that of today. The same line of reasoning +leads to the conclusion that even in the middle of the +Proterozoic, when multicellular marine animals must +already have been common, the climate of the earth had +already for an enormous period been such that all the +lower types of oceanic invertebrates had already evolved.</p> + +<!--File: cc029.png Page_5--> +<a class="pagenum break" id="Page_5" title="pg 5"></a> + +<table id="table_1" class="borders"> + +<tr><th><a href="#tables">TABLE 1</a></th></tr> + +<tr><th>THE GEOLOGICAL TIME TABLE<sup><a href="#fn4" id="r4">[4]</a></sup></th></tr> + + +<tr><th>COSMIC TIME</th></tr> +<tr><td class="tdtxt"><span class="smcap">Formative Era.</span> Birth and growth of the earth. Beginnings of +the atmosphere, hydrosphere, continental platforms, oceanic +basins, and possibly of life. No known geological record.</td></tr> +<tr><th>GEOLOGIC TIME</th></tr> +<tr><td class="tdtxt"><span class="smcap">Archeozoic Era.</span> Origin of simplest life.</td></tr> +<tr><td class="tdtxt"><span class="smcap">Proterozoic Era.</span> Age of invertebrate origins. An early and a late +ice age, with one or more additional ones indicated.</td></tr> +<tr><td class="tdtxt"><span class="smcap">Paleozoic Era.</span> Age of primitive vertebrate dominance.</td></tr> +<tr><td class="tdi2"><em>Cambrian Period.</em> First abundance of marine animals and dominance +of trilobites.</td></tr> +<tr><td class="tdi2"><em>Ordovician Period.</em> First known fresh-water fishes.</td></tr> +<tr><td class="tdi2"><em>Silurian Period.</em> First known land plants.</td></tr> +<tr><td class="tdi2"><em>Devonian Period.</em> First known amphibians. "Table Mountain" ice age.</td></tr> +<tr><td class="tdi2"><em>Mississippian Period.</em> Rise of marine fishes (sharks).</td></tr> +<tr><td class="tdi2"><em>Pennsylvanian Period.</em> Rise of insects and first period of marked coal accumulation.</td></tr> +<tr><td class="tdi2"><em>Permian Period.</em> Rise of reptiles. Another great ice age.</td></tr> +<tr><td class="tdtxt"><span class="smcap">Mesozoic Era.</span> Age of reptile dominance.</td></tr> +<tr><td class="tdi2"><em>Triassic Period.</em> Rise of dinosaurs. The period closes with a cool +climate.</td></tr> +<tr><td class="tdi2"><em>Jurassic Period.</em> Rise of birds and flying reptiles.</td></tr> +<tr><td class="tdi2"><em>Comanchean Period.</em> Rise of flowering plants and higher insects.</td></tr> +<tr><td class="tdi2"><em>Cretaceous Period.</em> Rise of archaic or primitive mammalia.</td></tr> +<tr><td class="tdtxt"><span class="smcap">Cenozoic Era.</span> Age of mammal dominance.</td></tr> +<tr><td class="tdi2"><em>Early Cenozoic or Eocene and Oligocene time.</em> Rise of higher +mammals. Glaciers in early Eocene of the Laramide Mountains.</td></tr> +<tr><td class="tdi2"><em>Late Cenozoic or Miocene and Pliocene time.</em> Transformation of +ape like animals into man.</td></tr> +<tr><td class="tdi2"><em>Glacial or Pleistocene time.</em> Last great ice age.</td></tr> +<tr><th>PRESENT TIME</th></tr> +<tr><td class="tdtxt"><span class="smcap">Psychozoic Era</span>. Age of man or age of reason. Includes the +present or "Recent time," estimated to be probably less than +30,000 years.</td></tr> +</table> + +<!--File: cc030.png Page_6--> +<a class="pagenum break" id="Page_6" title="pg 6"></a> + +<p>Moreover, they could live in most latitudes, for the indirect +evidences of life in the Archeozoic and Proterozoic +rocks are widely distributed. Thus it appears that +at an almost incredibly early period, perhaps many hundred +million years ago, the earth's climate differed only +a little from that of the present.</p> + +<p>The extreme limits of temperature beyond which the +climate of geological times cannot have departed can be +approximately determined. Today the warmest parts of +the ocean have an average temperature of about 30°C. +on the surface. Only a few forms of life live where the +average temperature is much higher than this. In deserts, +to be sure, some highly organized plants and animals can +for a short time endure a temperature as high as 75°C. +(167°F.). In certain hot springs, some of the lowest unicellular +plant forms exist in water which is only a little +below the boiling point. More complex forms, however, +such as sponges, worms, and all the higher plants and +animals, seem to be unable to live either in water or air +where the temperature averages above 45°C. (113°F.) +for any great length of time and it is doubtful whether +they can thrive permanently even at that temperature. +The obvious unity of life for hundreds of millions of +years and its presence at all times in middle latitudes so +far as we can tell seem to indicate that since the beginning +of marine life the temperature of the oceans +cannot have averaged much above 50°C. even in the +warmest portions. This is putting the limit too high +rather than too low, but even so the warmest parts of +the earth can scarcely have averaged much more than +20° warmer than at present.</p> + +<p>Turning to the other extreme, we may inquire how +much colder than now the earth's surface may have been +since life first appeared. Proterozoic fossils have been + +<!--File: cc031.png Page _7--> +<a class="pagenum break" id="Page_7" title="pg 7"></a> + +found in places where the present average temperature +approaches 0°C. If those places should be colder than +now by 30°C., or more, the drop in temperature at the +equator would almost certainly be still greater, and the +seas everywhere would be permanently frozen. Thus +life would be impossible. Since the contrasts between +summer and winter, and between the poles and the +equator seem generally to have been less in the past than +at present, the range through which the mean temperature +of the earth as a whole could vary without utterly +destroying life was apparently less than would now be +the case.</p> + +<p>These considerations make it fairly certain that for at +least several hundred million years the average temperature +of the earth's surface has never varied more than +perhaps 30°C. above or below the present level. Even this +range of 60°C. (108°F.) may be double or triple the range +that has actually occurred. That the temperature has not +passed beyond certain narrow limits, whatever their +exact degree, is clear from the fact that if it had done so, +all the higher forms of life would have been destroyed. +Certain of the lowest unicellular forms might indeed have +persisted, for when dormant they can stand great extremes +of dry heat and of cold for a long time. Even +so, evolution would have had to begin almost anew. The +supposition that such a thing has happened is untenable, +for there is no hint of any complete break in the record +of life during geological times,—no sudden disappearance +of the higher organisms followed by a long period +with no signs of life other than indirect evidence such as +occurs in the Archeozoic.</p> + +<p>A change of 60°C. or even of 20° in the average temperature +of the earth's surface may seem large when +viewed from the limited standpoint of terrestrial experience. +<!--File: cc032.png Page_8--> +<a class="pagenum break" id="Page_8" title="pg 8"></a> +Viewed, however, from the standpoint of +cosmic evolution, or even of the solar system, it seems +a mere trifle. Consider the possibilities. The temperature +of empty space is the absolute zero, or -273°C. +To this temperature all matter must fall, provided it +exists long enough and is not appreciably heated by collisions +or by radiation. At the other extreme lies the +temperature of the stars. As stars go, our sun is only +moderately hot, but the temperature of its surface is +calculated to be nearly 7000°C., while thousands of miles +in the interior it may rise to 20,000° or 100,000° or some +other equally unknowable and incomprehensible figure. +Between the limits of the absolute zero on the one hand, +and the interior of a sun or star on the other, there is +almost every conceivable possibility of temperature. +Today the earth's surface averages not far from 14°C., +or 287° above the absolute zero. Toward the interior, +the temperature in mines and deep wells rises about 1°C. +for every 100 meters. At this rate it would be over 500°C. +at a depth of ten miles, and over 5000° at 100 miles.</p> + +<p>Let us confine ourselves to surface temperatures, +which are all that concern us in discussing climate. It +has been calculated by Poynting<sup><a href="#fn5" id="r5">[5]</a></sup> that if a small sphere +absorbed and re-radiated all the heat that fell upon it, +its temperature at the distance of Mercury from the sun +would average about 210°C.; at the distance of Venus, +85°; the earth 27°; Mars -30°; Neptune 219°. A planet +much nearer the sun than is Mercury might be heated to +a temperature of a thousand, or even several thousand, +degrees, while one beyond Neptune would remain almost +at absolute zero. It is well within the range of possibility +that the temperature of a planet's surface should be +<!--File: cc033.png Page 9--> +<a class="pagenum break" id="Page_9" title="pg 9"></a> +anywhere from near -273°C. up to perhaps 5000°C. or +more, although the probability of low temperature is +much greater than of high. Thus throughout the whole +vast range of possibilities extending to perhaps 10,000°, +the earth claims only 60° at most, or less than 1 per cent. +This may be remarkable, but what is far more remarkable +is that the earth's range of 60° includes what seem +to be the two most critical of all possible temperatures, +namely, the freezing point of water, 0°C., and the temperature +where water can dissolve an amount of carbon +dioxide equal to its own volume. The most remarkable +fact of all is that the earth has preserved its temperature +within these narrow limits for a hundred million years, +or perchance a thousand million.</p> + +<p>To appreciate the extraordinary significance of this +last fact, it is necessary to realize how extremely critical +are the temperatures from about 0° to 40°C., and how +difficult it is to find any good reason for a relatively +uniform temperature through hundreds of millions of +years. Since the dawn of geological time the earth's +temperature has apparently always included the range +from about the freezing point of water up to about the +point where protoplasm begins to disintegrate. Henderson, +in <em>The Fitness of the Environment</em>, rightly says that +water is "the most familiar and the most important of +all things." In many respects water and carbon dioxide +form the most unique pair of substances in the whole +realm of chemistry. Water has a greater tendency than +any other known substance to remain within certain +narrowly defined limits of temperature. Not only does it +have a high specific heat, so that much heat is needed to +raise its temperature, but on freezing it gives up more +heat than any substance except ammonia, while none of +the common liquids approach it in the amount of additional +<!--File: cc034.png Page_10--> +<a class="pagenum break" id="Page_10" title="pg 10"></a> +heat required for conversion into vapor after the +temperature of vaporization has been reached. Again, +water substance, as the physicists call all forms of H<sub>2</sub>O, +is unique in that it not only contracts on melting, but +continues to contract until a temperature several degrees +above its melting point is reached. That fact has a vast +importance in helping to keep the earth's surface at a +uniform temperature. If water were like most liquids, +the bottoms of all the oceans and even the entire body of +water in most cases would be permanently frozen.</p> + +<p>Again, as a solvent there is literally nothing to compare +with water. As Henderson<sup><a href="#fn6" id="r6">[6]</a></sup> puts it: "Nearly the +whole science of chemistry has been built up around +water and aqueous solution." One of the most significant +evidences of this is the variety of elements whose presence +can be detected in sea water. According to Henderson +they include hydrogen, oxygen, nitrogen, carbon, +chlorine, sodium, magnesium, sulphur, phosphorus, which +are easily detected; and also arsenic, cæsium, gold, +lithium, rubidium, barium, lead, boron, fluorine, iron, +iodine, bromine, potassium, cobalt, copper, manganese, +nickel, silver, silicon, zinc, aluminium, calcium, and +strontium. Yet in spite of its marvelous power of solution, +water is chemically rather inert and relatively +stable. It dissolves all these elements and thousands of +their compounds, but still remains water and can easily +be separated and purified. Another unique property of +water is its power of ionizing dissolved substances, a +property which makes it possible to produce electric +currents in batteries. This leads to an almost infinite +array of electro-chemical reactions which play an almost +dominant rôle in the processes of life. Finally, no +common liquid except mercury equals water in its power +<!--File: cc035.png Page_11--> +<a class="pagenum break" id="Page_11" title="pg 11"></a> +of capillarity. This fact is of enormous moment in +biology, most obviously in respect to the soil.</p> + +<p>Although carbon dioxide is far less familiar than +water, it is almost as important. "These two simple substances," +says Henderson, "are the common source of +every one of the complicated substances which are produced +by living beings, and they are the common end +products of the wearing away of all the constituents of +protoplasm, and of the destruction of those materials +which yield energy to the body." One of the remarkable +physical properties of carbon dioxide is its degree of +solubility in water. This quality varies enormously in +different substances. For example, at ordinary pressures +and temperatures, water can absorb only about 5 per +cent of its own volume of oxygen, while it can take up +about 1300 times its own volume of ammonia. Now for +carbon dioxide, unlike most gases, the volume that can +be absorbed by water is nearly the same as the volume +of the water. The volumes vary, however, according to +temperature, being absolutely the same at a temperature +of about 15°C. or 59°F., which is close to the ideal temperature +for man's physical health and practically the +same as the mean temperature of the earth's surface +when all seasons are averaged together. "Hence, when +water is in contact with air, and equilibrium has been +established, the amount of free carbonic acid in a given +volume of water is almost exactly equal to the amount +in the adjacent air. Unlike oxygen, hydrogen, and nitrogen, +carbonic acid enters water freely; unlike sulphurous +oxide and ammonia, it escapes freely from water. Thus +the waters can never wash carbonic acid completely out +of the air, nor can the air keep it from the waters. It is +the one substance which thus, in considerable quantities +relative to its total amount, everywhere accompanies +<!--File: cc036.png Page_12--> +<a class="pagenum break" id="Page_12" title="pg 12"></a> +water. In earth, air, fire, and water alike these two substances +are always associated.</p> + +<p>"Accordingly, if water be the first primary constituent +of the environment, carbonic acid is inevitably +the second,—because of its solubility possessing an +equal mobility with water, because of the reservoir of the +atmosphere never to be depleted by chemical action in +the oceans, lakes, and streams. In truth, so close is the +association between these two substances that it is +scarcely correct logically to separate them at all; together +they make up the real environment and they never +part company."<sup><a href="#fn7" id="r7">[7]</a></sup></p> + +<p>The complementary qualities of carbon dioxide and +water are of supreme importance because these two are +the only known substances which are able to form a vast +series of complex compounds with highly varying chemical +formulæ. No other known compounds can give off +or take on atoms without being resolved back into their +elements. No others can thus change their form freely +without losing their identity. This power of change without +destruction is the fundamental chemical characteristic +of life, for life demands complexity, change, and +growth.</p> + +<p>In order that water and carbon dioxide may combine +to form the compounds on which life is based, the water +must be in the liquid form, it must be able to dissolve +carbon dioxide freely, and the temperature must not be +high enough to break up the highly complex and delicate +compounds as soon as they are formed. In other words, +the temperature must be above freezing, while it must +not rise higher than some rather indefinite point between +50°C. and the boiling point, where all water finally turns +into vapor. In the whole range of temperature, so far as +<!--File: cc037.png Page_13--> +<a class="pagenum break" id="Page_13" title="pg 13"></a> +we know, there is no other interval where any such complex +reactions take place. The temperature of the earth +for hundreds of millions of years has remained firmly +fixed within these limits.</p> + +<p>The astonishing quality of the earth's uniformity of +temperature becomes still more apparent when we consider +the origin of the sun's heat. What that origin is +still remains a question of dispute. The old ideas of a +burning sun, or of one that is simply losing an original +supply of heat derived from some accident, such as collision +with another body, were long ago abandoned. The +impact of a constant supply of meteors affords an almost +equally unsatisfactory explanation. Moulton<sup><a href="#fn8" id="r8">[8]</a></sup> states that +if the sun were struck by enough meteorites to keep up +its heat, the earth would almost certainly be struck by +enough so that it would receive about half of 1 per cent +as much heat from them as from the sun. This is millions +of times more heat than is now received from meteors. +If the sun owes its heat to the impact of larger bodies at +longer intervals, the geological record should show a +series of interruptions far more drastic than is actually +the case.</p> + +<p>It has also been supposed that the sun owes its heat +to contraction. If a gaseous body contracts it becomes +warmer. Finally, however, it must become so dense that +its rate of contraction diminishes and the process ceases. +Under the sun's present condition of size and density a +radial contraction of 120 feet per year would be enough +to supply all the energy now radiated by that body. This +seems like a hopeful source of energy, but Kelvin calculated +that twenty million years ago it was ineffective +and ten million years hence it will be equally so. Moreover, +if this is the source of heat, the amount of radiation +<!--File: cc038.png Page_14--> +<a class="pagenum break" id="Page_14" title="pg 14"></a> +from the sun would have to vary enormously. +Twenty million years ago the sun would have extended +nearly to the earth's orbit and would have been so tenuous +that it would have emitted no more heat than some +of the nebulæ in space. Some millions of years later, +when the sun's radius was twice as great as at present, +that body would have emitted only one-fourth as much +heat as now, which would mean that on the earth's surface +the theoretical temperature would have been 200° +below the present level. This is utterly out of accord with +the uniformity of climate shown by the geological record. +In the future, if the sun's contraction is the only source +of heat, the sun can supply the present amount for only +ten million years, which would mean a change utterly +unlike anything of which the geological record holds +even the faintest hint.<sup><a href="#fn9" id="r9">[9]</a></sup></p> + +<p>Altogether the problem of how the sun can have remained +so uniform and how the earth's atmosphere and +other conditions can also have remained so uniform +throughout hundreds of millions of years is one of the +most puzzling in the whole realm of nature. If appeal is +taken to radioactivity and the breaking up of uranium +into radium and helium, conditions can be postulated +which will give the required amount of energy. Such is +also the case if it be supposed that there is some unknown +process which may induce an atomic change like radioactivity +in bodies which are now supposed to be stable +elements. In either case, however, there is as yet no +satisfactory explanation of the <em>uniformity</em> of the earth's +climate. A hundred million or a thousand million years +ago the temperature of the earth's surface was very +much the same as now. The earth had then presumably +ceased to emit any great amount of heat, if we may judge +<!--File: cc039.png Page_15--> +<a class="pagenum break" id="Page_15" title="pg 15"></a> +from the fact that its surface was cool enough so that +great ice sheets could accumulate on low lands within 40° +of the equator. The atmosphere was apparently almost +like that of today, and was almost certainly not different +enough to make up for any great divergence of the sun +from its present condition. We cannot escape the stupendous +fact that in those remote times the sun must +have been essentially the same as now, or else that some +utterly unknown factor is at work.</p> + +<!--File: cc040.png Page_16--> +<a class="pagenum break" id="Page_16" title="pg 16"></a> + + + +<h2 id="Chapt_II"><a href="#content">CHAPTER II</a></h2> + +<h3>THE VARIABILITY OF CLIMATE</h3> + + +<p>The variability of the earth's climate is almost as +extraordinary as its uniformity. This variability +is made up partly of a long, slow tendency in one +direction and partly of innumerable cycles of every conceivable +duration from days, or even hours, up to millions +of years. Perhaps the easiest way to grasp the full complexity +of the matter is to put the chief types of climatic +sequence in the form of a table.</p> + + +<table id="table_2" class="borders"> +<tr><th colspan="4"><a href="#tables">TABLE 2</a></th></tr> +<tr><th colspan="4">TYPES OF CLIMATIC SEQUENCE</th> </tr> +<tr><td class="tdl">1.</td><td class="tdl">Cosmic uniformity. </td><td class="tdl">7.</td><td class="tdl">Brückner periods.</td></tr> +<tr><td class="tdl">2.</td><td class="tdl">Secular progression.</td><td class="tdl">8.</td><td class="tdl">Sunspot cycles.</td></tr> +<tr><td class="tdl">3.</td><td class="tdl">Geologic oscillations.</td><td class="tdl">9.</td><td class="tdl">Seasonal alternations.</td></tr> +<tr><td class="tdl">4.</td><td class="tdl">Glacial fluctuations.</td><td class="tdl">10.</td><td class="tdl">Pleionian migrations. </td></tr> +<tr><td class="tdl">5.</td><td class="tdl">Orbital precessions.</td><td class="tdl">11.</td><td class="tdl">Cyclonic vacillations.</td></tr> +<tr><td class="tdl">6.</td><td class="tdl">Historical pulsations.</td><td class="tdl">12.</td><td class="tdl">Daily vibrations.</td></tr> +</table> + +<p>In assigning names to the various types an attempt +has been made to indicate something of the nature of the +sequence so far as duration, periodicity, and general +tendencies are concerned. Not even the rich English +language of the twentieth century, however, furnishes +words with enough shades of meaning to express all that +<!--File: cc041.png Page 17--> +<a class="pagenum break" id="Page_17" title="pg 17"></a> +is desired. Moreover, except in degree, there is no sharp +distinction between some of the related types, such as +glacial fluctuations and historic pulsations. Yet, taken as +a whole, the table brings out the great contrast between +two absolutely diverse extremes. At the one end lies well-nigh +eternal uniformity, or an extremely slow progress in +one direction throughout countless ages; at the other, +rapid and regular vibrations from day to day, or else +irregular and seemingly unsystematic vacillations due to +cyclonic storms, both of which types are repeated millions +of times during even a single glacial fluctuation.</p> + +<p>The meaning of cosmic uniformity has been explained +in the preceding chapter. Its relation to the other types +of climatic sequences seems to be that it sets sharply +defined limits beyond which no changes of any kind have +ever gone since life, as we know it, first began. Secular +progression, on the other hand, means that in spite of all +manner of variations, now this way and then the other, +the normal climate of the earth, if there is such a thing, +has on the whole probably changed a little, perhaps becoming +more complex. After each period of continental +uplift and glaciation—for such are preëminently the +times of complexity—it is doubtful whether the earth has +ever returned to quite its former degree of monotony. +Today the earth has swung away from the great diversity +of the glacial period. Yet we still have contrasts of what +seem to us great magnitude. In low depressions, such as +Turfan in the central deserts of Eurasia, the thermometer +sometimes ranges from 0°F. in the morning to 60° +in the shade at noon. On a cloudy day in the Amazon +forest close to the seashore, on the contrary, the temperature +for months may rise to 85° by day and sink no lower +than 75° at night.</p> + +<p>The reasons for the secular progression of the earth's +<!--File: cc042.png Page 18--> +<a class="pagenum break" id="Page_18" title="pg 18"></a> +climate appear to be intimately connected with those +which have caused the next, and, in many respects, more +important type of climatic sequence, which consists of +geological oscillations. Both the progression and the +oscillations seem to depend largely on three purely terrestrial +factors: first, the condition of the earth's interior, +including both internal heat and contraction; +second, the salinity and movement of the ocean; and +third, the composition and amount of the atmosphere. +To begin with the earth's interior—its loss of heat appears +to be an almost negligible factor in explaining +either secular progression or geologic oscillation. According +to both the nebular and the planetesimal hypotheses, +the earth's crust appears to be colder now than it was +hundreds or thousands of millions of years ago. The +emission of internal heat, however, had probably ceased +to be of much climatic significance near the beginning of +the geological record, for in southern Canada glaciation +occurred very early in the Proterozoic era. On the other +hand, the contraction of the earth has produced remarkable +effects throughout the whole of geological time. It +has lessened the earth's circumference by a thousand +miles or more, as appears from the way in which the +rocks have been folded and thrust bodily over one +another. According to the laws of dynamics this must +have increased the speed of the earth's rotation, thus +shortening the day, and also having the more important +effect of increasing the bulge at the equator. On the other +hand, recent investigations indicate that tidal retardation +has probably diminished the earth's rate of rotation +more than seemed probable a few years ago, thus lengthening +the day and diminishing the bulge at the equator. +Thus two opposing forces have been at work, one causing +acceleration and one retardation. Their combined +<!--File: cc043.png Page 19--> +<a class="pagenum break" id="Page_19" title="pg 19"></a> +effect may have been a factor in causing secular progression +of climate. It almost certainly was of much importance +in causing pronounced oscillations first one way +and then the other. This matter, together with most of +those touched in these first chapters, will be expanded in +later parts of the book. On the whole the tendency appears +to have been to create climatic diversity in place +of uniformity.</p> + +<p>The increasing salinity of the oceans may have been +another factor in producing secular progression, although +of slight importance in respect to oscillations. +While the oceans were still growing in volume, it is generally +assumed that they must have been almost fresh +for a vast period, although Chamberlin thinks that the +change in salinity has been much less than is usually +supposed. So far as the early oceans were fresher than +those of today, their deep-sea circulation must have been +less hampered than now by the heavy saline water which +is produced by evaporation in warm regions. Although +this saline water is warm, its weight causes it to descend, +instead of moving poleward in a surface current; this +descent slows up the rise of the cold water which has +moved along in the depths of the ocean from high latitudes, +and thus checks the general oceanic circulation. +If the ancient oceans were fresher and hence had a freer +circulation than now, a more rapid interchange of polar +and equatorial water presumably tended to equalize the +climate of all latitudes.</p> + +<p>Again, although the earth's atmosphere has probably +changed far less during geological times than was +formerly supposed, its composition has doubtless varied. +The total volume of nitrogen has probably increased, for +that gas is so inert that when it once becomes a part of +the air it is almost sure to stay there. On the other hand, +<!--File: cc044.png Page 20--> +<a class="pagenum break" id="Page_20" title="pg 20"></a> +the proportions of oxygen, carbon dioxide, and water +vapor must have fluctuated. Oxygen is taken out constantly +by animals and by all the processes of rock +weathering, but on the other hand the supply is increased +when plants break up new carbon dioxide derived from +volcanoes. As for the carbon dioxide, it appears probable +that in spite of the increased supply furnished by +volcanoes the great amounts of carbon which have gradually +been locked up in coal and limestone have appreciably +depleted the atmosphere. Water vapor also may +be less abundant now than in the past, for the presence +of carbon dioxide raises the temperature a little and +thereby enables the air to hold more moisture. When the +area of the oceans has diminished, and this has recurred +very often, this likewise would tend to reduce the water +vapor. Moreover, even a very slight diminution in the +amount of heat given off by the earth, or a decrease in +evaporation because of higher salinity in the oceans, +would tend in the same direction. Now carbon dioxide and +water vapor both have a strong blanketing effect whereby +heat is prevented from leaving the earth. Therefore, the +probable reduction in the carbon dioxide and water +vapor of the earth's atmosphere has apparently tended +to reduce the climatic monotony and create diversity and +complexity. Hence, in spite of many reversals, the general +tendency of changes, not only in the earth's interior +and in the oceans, but also in the atmosphere, appears to +be a secular progression from a relatively monotonous +climate in which the evolution of higher organic forms +would scarcely be rapid to an extremely diverse and +complex climate highly favorable to progressive evolution. +The importance of these purely terrestrial agencies +must not be lost sight of when we come to discuss other +agencies outside the earth.</p> + +<!--File: cc045.png Page 21--> +<a class="pagenum break" id="Page_21" title="pg 21"></a> + +<p>In Table 2 the next type of climatic sequence is geologic +oscillation. This means slow swings that last +millions of years. At one extreme of such an oscillation +the climate all over the world is relatively monotonous; +it returns, as it were, toward the primeval conditions at +the beginning of the secular progression. At such times +magnolias, sequoias, figs, tree ferns, and many other +types of subtropical plants grew far north in places like +Greenland, as is well known from their fossil remains of +middle Cenozoic time, for example. At these same times, +and also at many others before such high types of plants +had evolved, reef-making corals throve in great abundance +in seas which covered what is now Wisconsin, +Michigan, Ontario, and other equally cool regions. Today +these regions have an average temperature of only about +70°F. in the warmest month, and average well below +freezing in winter. No reef-making corals can now live +where the temperature averages below 68°F. The resemblance +of the ancient corals to those of today makes +it highly probable that they were equally sensitive to low +temperature. Thus, in the mild portions of a geologic +oscillation the climate seems to have been so equable and +uniform that many plants and animals could live 1500 +and at other times even 4000 miles farther from the +equator than now.</p> + +<p>At such times the lands in middle and high latitudes +were low and small, and the oceans extended widely over +the continental platforms. Thus unhampered ocean currents +had an opportunity to carry the heat of low latitudes +far toward the poles. Under such conditions, especially +if the conception of the great subequatorial +continent of Gondwana land is correct, the trade winds +and the westerlies must have been stronger and steadier +than now. This would not only enable the westerlies, +<!--File: cc046.png Page 22--> +<a class="pagenum break" id="Page_22" title="pg 22"></a> +which are really southwesterlies, to carry more heat than +now to high latitudes, but would still further strengthen +the ocean currents. At the same time, the air presumably +contained an abundance of water vapor derived from +the broad oceans, and an abundance of atmospheric +carbon dioxide inherited from a preceding time when +volcanoes contributed much carbon dioxide to the air. +These two constituents of the atmosphere may have +exercised a pronounced blanketing effect whereby the +heat of the earth with its long wave lengths was kept in, +although the energy of the sun with its shorter wave +lengths was not markedly kept out. Thus everything may +have combined to produce mild conditions in high latitudes, +and to diminish the contrast between equator and +pole, and between summer and winter.</p> + +<p>Such conditions perhaps carry in themselves the seeds +of decay. At any rate while the lands lie quiet during a +period of mild climate great strains must accumulate in +the crust because of the earth's contraction and tidal +retardation. At the same time the great abundance of +plants upon the lowlying plains with their mild climates, +and the marine creatures upon the broad continental +platforms, deplete the atmospheric carbon dioxide. Part +of this is locked up as coal and part as limestone derived +from marine plants as well as animals. Then something +happens so that the strains and stresses of the crust are +released. The sea floors sink; the continents become +relatively high and large; mountain ranges are formed; +and the former plains and emergent portions of the +continental platforms are eroded into hills and valleys. +The large size of the continents tends to create deserts +and other types of climatic diversity; the presence of +mountain ranges checks the free flow of winds and also +creates diversity; the ocean currents are likewise +<!--File: cc047.png Page 23--> +<a class="pagenum break" id="Page_23" title="pg 23"></a> +checked, altered, and diverted so that the flow of heat +from low to high latitudes is diminished. At the same +time evaporation from the ocean diminishes so that a +decrease in water vapor combines with the previous depletion +of carbon dioxide to reduce the blanketing effect +of the atmosphere. Thus upon periods of mild monotony +there supervene periods of complexity, diversity, and +severity. Turn to Table 1 and see how a glacial climate +again and again succeeds a time when relative mildness +prevailed almost everywhere. Or examine Fig. 1 and +notice how the lines representing temperatures go up and +down. In the figure Schuchert makes it clear that when +the lands have been large and mountain-making has been +important, as shown by the high parts of the lower shaded +area, the climate has been severe, as shown by the descent +of the snow line, the upper shaded area. In the diagram +the climatic oscillations appear short, but this is merely +because they have been crowded together, especially in +the left hand or early part. There an inch in length may +represent a hundred million years. Even at the right-hand +end an inch is equivalent to several million years.</p> + +<p>The severe part of a climatic oscillation, as well as the +mild part, will be shown in later chapters to bear in itself +certain probable seeds of decay. While the lands are +being uplifted, volcanic activity is likely to be vigorous +and to add carbon dioxide to the air. Later, as the mountains +are worn down by the many agencies of water, +wind, ice, and chemical decay, although much carbon +dioxide is locked up by the carbonation of the rocks, the +carbon locked up in the coal is set free and increases the +carbon dioxide of the air. At the same time the continents +settle slowly downward, for the earth's crust though +rigid as steel is nevertheless slightly viscous and will +flow if subjected to sufficiently great and enduring pressure. +<!--File: cc048.png Pae 24--> +<a class="pagenum break" id="Page_24" title="pg 24"></a> +The area from which evaporation can take place +is thereby increased because of the spread of the oceans +over the continents, and water vapor joins with the carbon +dioxide to blanket the earth and thus tends to keep it +uniformly warm. Moreover, the diminution of the lands +frees the ocean currents from restraint and permits them +to flow more freely from low latitudes to high. Thus in +the course of millions of years there is a return toward +monotony. Ultimately, however, new stresses accumulate +in the earth's crust, and the way is prepared for another +great oscillation. Perhaps the setting free of the stresses +takes place simply because the strain at last becomes +irresistible. It is also possible, as we shall see, that an +external agency sometimes adds to the strain and thereby +determines the time at which a new oscillation shall +begin.</p> + +<p>In Table 2 the types of climatic sequences which follow +"geologic oscillations" are "glacial fluctuations," +"orbital precessions" and "historical pulsations." +Glacial fluctuations and historical pulsations appear to +be of the same type, except as to severity and duration, +and hence may be considered together. They will be +treated briefly here because the theories as to their +causes are outlined in the next two chapters. Oddly +enough, although the historic pulsations lie much closer +to us than do the glacial fluctuations, they were not +discovered until two or three generations later, and are +still much less known. The most important feature of +both sequences is the swing from a glacial to an inter-glacial +epoch or from the arsis or accentuated part of an +historical pulsation to the thesis or unaccented part. In a +glacial epoch or in the arsis of an historic pulsation, +storms are usually abundant and severe, the mean temperature +is lower than usual, snow accumulates in high +<!--File: cc049.png Page 25--> +<a class="pagenum break" id="Page_25" title="pg 25"></a> + +<!--File: cc050.png Page 26--> +<a class="pagenum break" id="Page_26" title="pg 26"></a> + +latitudes or upon lofty mountains. For example, in the +last such period during the fourteenth century, great +floods and droughts occurred alternately around the +North Sea; it was several times possible to cross the +Baltic Sea from Germany to Sweden on the ice, and the +ice of Greenland advanced so much that shore ice caused +the Norsemen to change their sailing route between Iceland +and the Norse colonies in southern Greenland. At +the same time in low latitudes and in parts of the continental +interior there is a tendency toward diminished +rainfall and even toward aridity and the formation of +deserts. In Yucatan, for example, a diminution in tropical +rainfall in the fourteenth century seems to have given +the Mayas a last opportunity for a revival of their decaying +civilization.</p> +<img src="images/fig1.png" width="800" height="284" alt="Fig. 1" id="Fig_1" /> +<p class="center"><em>Fig. 1. Climatic changes and mountain building.<br /> +(After Schuchert, in The Evolution of the Earth and Its Inhabitants, edited by R. S. Lull.)</em></p> + +<p class="blockquot">Diagram showing the times and probable extent of the more or less marked climate changes in +the geologic history of North America, and of its elevation into chains of mountains.</p> + +<p>Among the climatic sequences, glacial fluctuations are +perhaps of the most vital import from the standpoint of +organic evolution; from the standpoint of human history +the same is true of climatic pulsations. Glacial epochs +have repeatedly wiped out thousands upon thousands of +species and played a part in the origin of entirely new +types of plants and animals. This is best seen when the +life of the Pennsylvanian is contrasted with that of the +Permian. An historic pulsation may wipe out an entire +civilization and permit a new one to grow up with a radically +different character. Hence it is not strange that the +causes of such climatic phenomena have been discussed +with extraordinary vigor. In few realms of science has +there been a more imposing or more interesting array of +theories. In this book we shall consider the more important +of these theories. A new solar or cyclonic hypothesis +and the hypothesis of changes in the form and altitude of +the land will receive the most attention, but the other + +<!--File: cc051.png Page 27--> +<a class="pagenum break" id="Page_27" title="pg 27"></a> + +chief hypotheses are outlined in the next chapter, and are +frequently referred to throughout the volume.</p> + +<p>Between glacial fluctuations and historical pulsations +in duration, but probably less severe than either, come +orbital precessions. These stand in a group by themselves +and are more akin to seasonal alternations than +to any other type of climatic sequence. They must have +occurred with absolute regularity ever since the earth +began to revolve around the sun in its present elliptical +orbit. Since the orbit is elliptical and since the sun is in +one of the two foci of the ellipse, the earth's distance +from the sun varies. At present the earth is nearest the +sun in the northern winter. Hence the rigor of winter in +the northern hemisphere is mitigated, while that of the +southern hemisphere is increased. In about ten thousand +years this condition will be reversed, and in another ten +thousand the present conditions will return once more. +Such climatic precessions, as we may here call them, +must have occurred unnumbered times in the past, but +they do not appear to have been large enough to leave in +the fossils of the rocks any traces that can be distinguished +from those of other climatic sequences.</p> + +<p>We come now to Brückner periods and sunspot cycles. +The Brückner periods have a length of about thirty-three +years. Their existence was suggested at least as long ago +as the days of Sir Francis Bacon, whose statement about +them is quoted on the flyleaf of this book. They have +since been detected by a careful study of the records of +the time of harvest, vintage, the opening of rivers to +navigation, and the rise or fall of lakes like the Caspian +Sea. In his book on <em>Klimaschwankungen seit 1700</em>, +Brückner has collected an uncommonly interesting assortment +of facts as to the climate of Europe for more than +two centuries. More recently, by a study of the rate of + +<!--File: cc052.png Page 28--> +<a class="pagenum break" id="Page_28" title="pg 28"></a> + +growth of trees, Douglass, in his book on <em>Climatic Cycles +and Tree Growth</em>, has carried the subject still further. +In general the nature of the 33-year periods seems to be +identical with that of the 11- or 12- year sunspot cycle, +on the one hand, and of historic pulsations on the other. +For a century observers have noted that the variations +in the weather which everyone notices from year to year +seem to have some relation to sunspots. For generations, +however, the relationship was discussed without leading +to any definite conclusion. The trouble was that the same +change was supposed to take place in all parts of the +world. Hence, when every sort of change was found +somewhere at any given sunspot stage, it seemed as +though there could not be a relationship. Of late years, +however, the matter has become fairly clear. The chief +conclusions are, first, that when sunspots are numerous +the average temperature of the earth's surface is lower +than normal. This does not mean that all parts are cooler, +for while certain large areas grow cool, others of less +extent become warm at times of many sunspots. Second, +at times of many sunspots storms are more abundant +than usual, but are also confined somewhat closely to +certain limited tracks so that elsewhere a diminution of +storminess may be noted. This whole question is discussed +so fully in <em>Earth and Sun</em> that it need not detain +us further in this preliminary view of the whole problem +of climate. Suffice it to say that a study of the sunspot +cycle leads to the conclusion that it furnishes a clue to +many of the unsolved problems of the climate of the +past, as well as a key to prediction of the future.</p> + +<p>Passing by the seasonal alternations which are fully +explained as the result of the revolution of the earth +around the sun, we may merely point out that, like the +daily vibrations which bring Table 2 to a close, they +<!--File: cc053.png Page 29--> +<a class="pagenum break" id="Page_29" title="pg 29"></a> +emphasize the outstanding fact that the main control of +terrestrial climate is the amount of energy received from +the sun. This same principle is illustrated by pleionian +migrations. The term "pleion" comes from a Greek word +meaning "more." It was taken by Arctowski to designate +areas or periods where there is an excess of some +climatic element, such as atmospheric pressure, rainfall, +or temperature. Even if the effect of the seasons is eliminated, +it appears that the course of these various elements +does not run smoothly. As everyone knows, a period +like the autumn of 1920 in the eastern United States may +be unusually warm, while a succeeding period may be +unseasonably cool. These departures from the normal +show a certain rough periodicity. For example, there is +evidence of a period of about twenty-seven days, corresponding +to the sun's rotation and formerly supposed to +be due to the moon's revolution which occupies almost +the same length of time. Still other periods appear to +have an average duration of about three months and of +between two and three years. Two remarkable discoveries +have recently been made in respect to such pleions. One +is that a given type of change usually occurs simultaneously +in a number of well-defined but widely separated +centers, while a change of an opposite character arises +in another equally well-defined, but quite different, set +of centers. In general, areas of high pressure have one +type of change and areas of low pressure the other type. +So systematic are these relationships and so completely +do they harmonize in widely separated parts of the earth, +that it seems certain that they must be due to some outside +cause, which in all probability can be only the sun. +The second discovery is that pleions, when once formed, +travel irregularly along the earth's surface. Their paths +have not yet been worked out in detail, but a general +<!--File: cc054.png Page 30--> +<a class="pagenum break" id="Page_30" title="pg 30"></a> +migration seems well established. Because of this, it is +probable that if unusually warm weather prevails in one +part of a continent at a given time, the "thermo-pleion," +or excess of heat, will not vanish but will gradually move +away in some particular direction. If we knew the path +that it would follow we might predict the general temperature +along its course for some months in advance. +The paths are often irregular, and the pleions frequently +show a tendency to break up or suddenly revive. Probably +this tendency is due to variations in the sun. When +the sun is highly variable, the pleions are numerous and +strong, and extremes of weather are frequent. Taken as +a whole the pleions offer one of the most interesting and +hopeful fields not only for the student of the causes of +climatic variations, but for the man who is interested in +the practical question of long-range weather forecasts. +Like many other climatic phenomena they seem to represent +the combined effect of conditions in the sun and +upon the earth itself.</p> + +<p>The last of the climatic sequences which require explanation +is the cyclonic vacillations. These are familiar +to everyone, for they are the changes of weather which +occur at intervals of a few days, or a week or two, at all +seasons, in large parts of the United States, Europe, +Japan, and some of the other progressive parts of the +earth. They do not, however, occur with great frequency +in equatorial regions, deserts, and many other regions. +Up to the end of the last century, it was generally supposed +that cyclonic storms were purely terrestrial in +origin. Without any adequate investigation it was assumed +that all irregularities in the planetary circulation +of the winds arise from an irregular distribution of heat +due to conditions within or upon the earth itself. These +irregularities were supposed to produce cyclonic storms +<!--File: cc055.png Page 31--> +<a class="pagenum break" id="Page_31" title="pg 31"></a> +in certain limited belts, but not in most parts of the +world. Today this view is being rapidly modified. Undoubtedly, +the irregularities due to purely terrestrial +conditions are one of the chief contributory causes of +storms, but it begins to appear that solar variations also +play a part. It has been found, for example, that not +only the mean temperature of the earth's surface varies +in harmony with the sunspot cycle, but that the frequency +and severity of storms vary in the same way. Moreover, +it has been demonstrated that the sun's radiation is not +constant, but is subject to innumerable variations. This +does not mean that the sun's general temperature varies, +but merely that at some times heated gases are ejected +rapidly to high levels so that a sudden wave of energy +strikes the earth. Thus, the present tendency is to believe +that the cyclonic variations, the changes of weather +which come and go in such a haphazard, irresponsible +way, are partly due to causes pertaining to the earth +itself and partly to the sun.</p> + +<p>From this rapid survey of the types of climatic sequences, +it is evident that they may be divided into four +great groups. First comes cosmic uniformity, one of the +most marvelous and incomprehensible of all known facts. +We simply have no explanation which is in any respect +adequate. Next come secular progression and geologic +oscillations, two types of change which seem to be due +mainly to purely terrestrial causes, that is, to changes in +the lands, the oceans, and the air. The general tendency +of these changes is toward complexity and diversity, thus +producing progression, but they are subject to frequent +reversals which give rise to oscillations lasting millions +of years. The processes by which the oscillations take +place are fully discussed in this book. Nevertheless, because +they are fairly well understood, they are deferred +<!--File: cc056.png Page 32--> +<a class="pagenum break" id="Page_32" title="pg 32"></a> +until after the third group of sequences has been discussed. +This group includes glacial fluctuations, historic +pulsations, Brückner periods, sunspot cycles, pleionian +migrations, and cyclonic vacillations. The outstanding +fact in regard to all of these is that while they are greatly +modified by purely terrestrial conditions, they seem to +owe their origin to variations in the sun. They form the +chief subject of <em>Earth and Sun</em> and in their larger phases +are the most important topic of this book also. The last +group of sequences includes orbital precessions, seasonal +alternations, and daily variations. These may be regarded +as purely solar in origin. Yet their influence, like +that of each of the other groups, is much modified by the +earth's own conditions. Our main problem is to separate +and explain the two great elements in climatic changes,—the +effects of the sun, on the one hand, and of the earth +on the other.</p> + +<!--File: cc057.png Page 33--> +<a class="pagenum break" id="Page_33" title="pg 33"></a> + + + + + +<h2 id="Chapt_III"><a href="#content">CHAPTER III</a></h2> + +<h3>HYPOTHESES OF CLIMATIC CHANGE</h3> + + +<p>The next step in our study of climate is to review +the main hypotheses as to the causes of glaciation. +These hypotheses apply also to other types +of climatic changes. We shall concentrate on glacial +periods, however, not only because they are the most +dramatic and well-known types of change, but because +they have been more discussed than any other and have +also had great influence on evolution. Moreover, they +stand near the middle of the types of climatic sequences, +and an understanding of them does much to explain the +others. In reviewing the various theories we shall not +attempt to cover all the ground, but shall merely state +the main ideas of the few theories which have had an +important influence upon scientific thought.</p> + +<p>The conditions which any satisfactory climatic hypothesis +must satisfy are briefly as follows:</p> + + +<ol> +<li>Due weight must be given to the fact that changes +of climate are almost certainly due to the combined effect +of a variety of causes, both terrestrial and solar or +cosmic.</li> + +<li>Attention must also be paid to both sides in the +long controversy as to whether glaciation is due primarily +to a diminution in the earth's supply of heat or to +a <em>redistribution</em> of the heat through changes in atmospheric +and oceanic circulation. At present the great +<!--File: cc058.png Page 34--> +<a class="pagenum break" id="Page_34" title="pg 34"></a> +majority of authorities are on the side of a diminution of +heat, but the other view also deserves study.</li> + +<li>A satisfactory hypothesis must explain the frequent +synchronism between two great types of phenomena; +first, movements of the earth's crust whereby +continents are uplifted and mountains upheaved; and, +second, great changes of climate which are usually +marked by relatively rapid oscillations from one extreme +to another.</li> + +<li>No hypothesis can find acceptance unless it satisfies +the somewhat exacting requirements of the geological +record, with its frequent but irregular repetition of long, +mild periods, relatively cool or intermediate periods like +the present, and glacial periods of more or less severity +and perhaps accompanying the more or less widespread +uplifting of continents. At least during the later glacial +periods the hypothesis must explain numerous climatic +epochs and stages superposed upon a single general +period of continental upheaval. Moreover, although historical +geology demands cycles of varied duration and +magnitude, it does not furnish evidence of any rigid +periodicity causing the cycles to be uniform in length or +intensity.</li> + +<li>Most important of all, a satisfactory explanation +of climatic changes and crustal deformation must take +account of all the agencies which are now causing similar +phenomena. Whether any other agencies should be considered +is open to question, although the relative importance +of existing agencies may have varied. +</li> +</ol> + + +<p>I. <em>Croll's Eccentricity Theory.</em> One of the most ingenious +and most carefully elaborated scientific hypotheses +is Croll's<sup><a href="#fn10" id="r10">[10]</a></sup> precessional hypothesis as to the +effect of the earth's own motions. So well was this worked +out that it was widely accepted for a time and still finds a +<!--File: cc059.png Page 35--> +<a class="pagenum break" id="Page_35" title="pg 35"></a> +place in popular but unscientific books, such as Wells' +<em>Outline of History</em>, and even in scientific works like +Wright's <em>Quaternary Ice Age</em>. The gist of the hypothesis +has already been given in connection with the type of +climatic sequence known as orbital precessions. The earth +is 93 million miles away from the sun in January and 97 +million in July. The earth's axis "precesses," however, +just as does that of a spinning top. Hence arises what is +known as the precession of the equinoxes, that is, a +steady change in the season at which the earth is in perihelion, +or nearest to the sun. In the course of 21,000 years +the time of perihelion varies from early in January +through the entire twelve months and back to January. +Moreover, the earth's orbit is slightly more elliptical at +certain periods than at others, for the planets sometimes +become bunched so that they all pull the earth in one +direction. Hence, once in about one hundred thousand +years the effect of the elliptical shape of the earth's orbit +is at a maximum.</p> + +<p>Croll argued that these astronomical changes must +alter the earth's climate, especially by their effect on +winds and ocean currents. His elaborate argument contains +a vast amount of valuable material. Later investigation, +however, seems to have proven the inadequacy of +his hypothesis. In the first place, the supposed cause does +not seem nearly sufficient to produce the observed results. +Second, Croll's hypothesis demands that glaciation in the +northern and southern hemisphere take place alternately. +A constantly growing collection of facts, however, indicates +that glaciation does not occur in the two hemispheres +alternately, but at the same time. Third, the +hypothesis calls for the constant and frequent repetition +of glaciation at absolutely regular intervals. The geological +<!--File: cc060.png Page 36--> +<a class="pagenum break" id="Page_36" title="pg 36"></a> +record shows no such regularity, for sometimes +several glacial epochs follow in relatively close succession +at irregular intervals of perhaps fifty to two hundred +thousand years, and thus form a glacial period; and +then for millions of years there are none. Fourth, the +eccentricity hypothesis provides no adequate explanation +for the glacial stages or subepochs, the historic pulsations, +and the other smaller climatic variations which are +superposed upon glacial epochs and upon one another in +bewildering confusion. In spite of these objections, there +can be little question that the eccentricity of the earth's +orbit and the precession of the equinoxes with the resulting +change in the season of perihelion must have some +climatic effect. Hence Croll's theory deserves a permanent +though minor place in any full discussion of the +causes of climatic changes.</p> + +<p>II. <em>The Carbon Dioxide Theory.</em> At about the time +that the eccentricity theory was being relegated to a +minor niche, a new theory was being developed which +soon exerted a profound influence upon geological +thought. Chamberlin,<sup><a href="#fn11" id="r11">[11]</a></sup> adopting an idea suggested by +<!--File: cc061.png Page 37--> +<a class="pagenum break" id="Page_37" title="pg 37"></a> +Tyndall, fired the imagination of geologists by his skillful +exposition of the part played by carbon dioxide in +causing climatic changes. Today this theory is probably +more widely accepted than any other. We have already +seen that the amount of carbon dioxide gas in the atmosphere +has a decided climatic importance. Moreover, +there can be little doubt that the amount of that gas in +the atmosphere varies from age to age in response to the +extent to which it is set free by volcanoes, consumed by +plants, combined with rocks in the process of weathering, +dissolved in the ocean or locked up in the form of coal +and limestone. The main question is whether such variations +can produce changes so rapid as glacial epochs and +historical pulsations.</p> + +<p>Abundant evidence seems to show that the degree to +which the air can be warmed by carbon dioxide is sharply +limited. Humphreys, in his excellent book on the <em>Physics +of the Air</em>, calculates that a layer of carbon dioxide forty +centimeters thick has practically as much blanketing +effect as a layer indefinitely thicker. In other words, forty +centimeters of carbon dioxide, while having no appreciable +<!--File: cc062.png Page 38--> +<a class="pagenum break" id="Page_38" title="pg 38"></a> +effect on sunlight coming toward the earth, would +filter out and thus retain in the atmosphere all the outgoing +terrestrial heat that carbon dioxide is capable of +absorbing. Adding more would be like adding another +filter when the one in operation has already done all that +that particular kind of filter is capable of doing. According +to Humphreys' calculations, a doubling of the carbon +dioxide in the air would in itself raise the average temperature +about 1.3°C. and further carbon dioxide would +have practically no effect. Reducing the present supply +by half would reduce the temperature by essentially the +same amount.</p> + +<p>The effect must be greater, however, than would appear +from the figures given above, for any change in +temperature has an effect on the amount of water vapor, +which in turn causes further changes of temperature. +Moreover, as Chamberlin points out, it is not clear +whether Humphreys allows for the fact that when the +40 centimeters of CO<sub>2</sub> nearest the earth has been heated +by terrestrial radiation, it in turn radiates half its heat +outward and half inward. The outward half is all absorbed +in the next layer of carbon dioxide, and so on. +The process is much more complex than this, but the end +result is that even the last increment of CO<sub>2</sub>, that is, the +outermost portions in the upper atmosphere, must apparently +absorb an infinitesimally small amount of heat. +This fact, plus the effect of water vapor, would seem to +indicate that a doubling or halving of the amount of CO<sub>2</sub>, +would have an effect of more than 1.3°C. A change of +even 2°C. above or below the present level of the earth's +mean temperature would be of very appreciable climatic +significance, for it is commonly believed that during the +height of the glacial period the mean temperature was +only 5° to 8°C. lower than now.</p> +<!--File: cc063.png Page 39--> +<a class="pagenum break" id="Page_39" title="pg 37"></a> + +<p>Nevertheless, variations in atmospheric carbon dioxide +do not necessarily seem competent to produce the relatively +rapid climatic fluctuations of glacial epochs and +historic pulsations as distinguished from the longer +swings of glacial periods and geological eras. In Chamberlin's +view, as in ours, the elevation of the land, the +modification of the currents of the air and of the ocean, +and all that goes with elevation as a topographic agency +constitute a primary cause of climatic changes. A special +effect of this is the removal of carbon dioxide from the +air by the enhanced processes of weathering. This, as he +carefully states, is a very slow process, and cannot of +itself lead to anything so sudden as the oncoming of +glaciation. But here comes Chamberlin's most distinctive +contribution to the subject, namely, the hypothesis that +changes in atmospheric temperature arising from variations +in atmospheric carbon dioxide are able to cause a +reversal of the deep-sea oceanic circulation.</p> + +<p>According to Chamberlin's view, the ordinary oceanic +circulation of the greater part of geological time was +the reverse of the present circulation. Warm water descended +to the ocean depths in low latitudes, kept its heat +while creeping slowly poleward, and rose in high latitudes +producing the warm climate which enabled corals, +for example, to grow in high latitudes. Chamberlin holds +this opinion largely because there seems to him to be no +other reasonable way to account for the enormously long +warm periods when heat-loving forms of life lived in +what are now polar regions of ice and snow. He explains +this reversed circulation by supposing that an abundance +of atmospheric carbon dioxide, together with a broad +distribution of the oceans, made the atmosphere so warm +that the evaporation in low latitudes was far more rapid +than now. Hence the surface water of the ocean became +<!--File: cc064.png Page 40--> +<a class="pagenum break" id="Page_40" title="pg 40"></a> +a relatively concentrated brine. Such a brine is heavy +and tends to sink, thereby setting up an oceanic circulation +the reverse of that which now prevails. At present +the polar waters sink because they are cold and hence +contract. Moreover, when they freeze a certain amount +of salt leaves the ice and thereby increases the salinity +of the surrounding water. Thus the polar water sinks +to the depths of the ocean, its place is taken by warmer +and lighter water from low latitudes which moves poleward +along the surface, and at the same time the cold +water of the ocean depths is forced equatorward below +the surface. But if the equatorial waters were so concentrated +that a steady supply of highly saline water kept +descending to low levels, the direction of the circulation +would have to be reversed. The time when this would +occur would depend upon the delicate balance between +the downward tendencies of the cold polar water and of +the warm saline equatorial water.</p> + +<p>Suppose that while such a reversed circulation prevailed, +the atmospheric CO<sub>2</sub> should be depleted, and the +air cooled so much that the concentration of the equatorial +waters by evaporation was no longer sufficient to +cause them to sink. A reversal would take place, the +present type of circulation would be inaugurated, and +the whole earth would suffer a chill because the surface +of the ocean would become cool. The cool surface-water +would absorb carbon dioxide faster than the previous +warm water had done, for heat drives off gases +from water. This would hasten the cooling of the atmosphere +still more, not only directly but by diminishing +the supply of atmospheric moisture. The result would be +glaciation. But ultimately the cold waters of the higher +latitudes would absorb all the carbon dioxide they could +hold, the slow equatorward creep would at length permit +<!--File: cc065.png Page 41--> +<a class="pagenum break" id="Page_41" title="pg 41"></a> +the cold water to rise to the surface in low latitudes. +There the warmth of the equatorial sun and the depleted +supply of carbon dioxide in the air would combine to +cause the water to give up its carbon dioxide once more. +If the atmosphere had been sufficiently depleted by that +time, the rising waters in low latitudes might give up +more carbon dioxide than the cold polar waters absorbed. +Thus the atmospheric supply would increase, the air +would again grow warm, and a tendency toward deglaciation, +or toward an inter-glacial condition would +arise. At such times the oceanic circulation is not supposed +to have been reversed, but merely to have been +checked and made slower by the increasing warmth. +Thus inter-glacial conditions like those of today, or even +considerably warmer, are supposed to have been produced +with the present type of circulation.</p> + +<p>The emission of carbon dioxide in low latitudes could +not permanently exceed the absorption in high latitudes. +After the present type of circulation was finally established, +which might take tens of thousands of years, the +two would gradually become equal. Then the conditions +which originally caused the oceanic circulation to be +reversed would again destroy the balance; the atmospheric +carbon dioxide would be depleted; the air would +grow cooler; and the cycle of glaciation would be repeated. +Each cycle would be shorter than the last, for not +only would the swings diminish like those of a pendulum, +but the agencies that were causing the main depletion of +the atmospheric carbon dioxide would diminish in intensity. +Finally as the lands became lower through erosion +and submergence, and as the processes of weathering +became correspondingly slow, the air would gradually be +able to accumulate carbon dioxide; the temperature would +increase; and at length the oceanic circulation would be +<!--File: cc066.png Page 42--> +<a class="pagenum break" id="Page_42" title="pg 42"></a> +reversed again. When the warm saline waters of low latitudes +finally began to sink and to set up a flow of warm +water poleward in the depths of the ocean, a glacial +period would definitely come to an end.</p> + +<p>This hypothesis has been so skillfully elaborated, and +contains so many important elements that one can +scarcely study it without profound admiration. We believe +that it is of the utmost value as a step toward the +truth, and especially because it emphasizes the great +function of oceanic circulation. Nevertheless, we are +unable to accept it in full for several reasons, which +may here be stated very briefly. Most of them will be discussed +fully in later pages.</p> + +<p>(1) While a reversal of the deep-sea circulation would +undoubtedly be of great climatic importance and would +produce a warm climate in high latitudes, we see no +direct evidence of such a reversal. It is equally true that +there is no conclusive evidence against it, and the possibility +of a reversal must not be overlooked. There seem, +however, to be other modifications of atmospheric and +oceanic circulation which are able to produce the observed +results.</p> + +<p>(2) There is much, and we believe conclusive, evidence +that a mere lowering of temperature would not produce +glaciation. What seems to be needed is changes in atmospheric +circulation and in precipitation. The carbon +dioxide hypothesis has not been nearly so fully developed +on the meteorological side as in other respects.</p> + +<p>(3) The carbon dioxide hypothesis seems to demand +that the oceans should have been almost as saline as now +in the Proterozoic era at the time of the first known +glaciation. Chamberlin holds that such was the case, but +the constant supply of saline material brought to the +ocean by rivers and the relatively small deposition of +<!--File: cc067.png Page 43--> +<a class="pagenum break" id="Page_43" title="pg 43"></a> +such material on the sea floor seem to indicate that the +early oceans must have been much fresher than those of +today.</p> + +<p>(4) The carbon dioxide hypothesis does not attempt +to explain minor climatic fluctuations such as post-glacial +stages and historic pulsations, but these appear to be of +the same nature as glacial epochs, differing only in +degree.</p> + +<p>(5) Another reason for hesitation in accepting the +carbon dioxide hypothesis as a full explanation of glacial +fluctuations is the highly complex and non-observational +character of the explanation of the alternation of glacial +and inter-glacial epochs and of their constantly decreasing +length.</p> + +<p>(6) Most important of all, a study of the variations of +weather and of climate as they are disclosed by present +records and by the historic past suggests that there are +now in action certain other causes which are competent +to explain glaciation without recourse to a process whose +action is beyond the realm of observation.</p> + +<p>These considerations lead to the conclusion that the +carbon dioxide hypothesis and the reversal of the oceanic +circulation should be regarded as a tentative rather than +a final explanation of glaciation. Nevertheless, the action +of carbon dioxide seems to be an important factor in producing +the longer oscillations of climate from one geological +era to another. It probably plays a considerable +part in preparing the way for glacial periods and in +making it possible for other factors to produce the more +rapid changes which have so deeply influenced organic +evolution.</p> + +<p>III. <em>The Form of the Land.</em> Another great cause of +climatic change consists of a group of connected phenomena +dependent upon movements of the earth's crust. +<!--File: cc068.png Page 44--> +<a class="pagenum break" id="Page_44" title="pg 44"></a> +As to the climatic potency of changes in the lands there +is practical agreement among students of climatology +and glaciation. That the height and extent of the continents, +the location, size, and orientation of mountain +ranges, and the opening and closing of oceanic gateways +at places like Panama, and the consequent diversion of +oceanic currents, exert a profound effect upon climate +can scarcely be questioned. Such changes may be introduced +rapidly, but their disappearance is usually slow +compared with the rapid pulsations to which climate has +been subject during historic times and during stages of +glacial retreat and advance, or even in comparison with +the epochs into which the Pleistocene, Permian, and +perhaps earlier glacial periods have been divided. Hence, +while crustal movements appear to be more important +than the eccentricity of the earth's orbit or the amount of +carbon dioxide in the air, they do not satisfactorily explain +glacial fluctuations, historic pulsations, and especially +the present little cycles of climatic change. All +these changes involve a relatively rapid swing from one +extreme to another, while an upheaval of a continent, +which is at best a slow geologic process, apparently +cannot be undone for a long, long time. Hence such an +upheaval, if acting alone, would lead to a relatively long-lived +climate of a somewhat extreme type. It would help +to explain the long swings, or geologic oscillations between +a mild and uniform climate at one extreme, and a +complex and varied climate at the other, but it would not +explain the rapid climatic pulsations which are closely +associated with great movements of the earth's crust. It +might prepare the way for them, but could not cause +them. That this conclusion is true is borne out by the fact +that vast mountain ranges, like those at the close of the +Jurassic and Cretaceous, are upheaved without bringing +<!--File: cc069.png Page 45--> +<a class="pagenum break" id="Page_45" title="pg 45"></a> +on glacial climates. Moreover, the marked Permian ice +age follows long after the birth of the Hercynian Mountains +and before the rise of others of later Permian +origin.</p> + +<p>IV. <em>The Volcanic Hypothesis.</em> In the search for some +cause of climatic change which is highly efficient and yet +able to vary rapidly and independently, Abbot, Fowle, +Humphreys, and others,<sup><a href="#fn12" id="r12">[12]</a></sup> have concluded that volcanic +eruptions are the missing agency. In <em>Physics of the Air</em>, +Humphreys gives a careful study of the effect of volcanic +dust upon terrestrial temperature. He begins with +a mathematical investigation of the size of dust particles, +and their quantity after certain eruptions. He demonstrates +that the power of such particles to deflect light of +short wave-lengths coming from the sun is perhaps thirty +times more than their power to retain the heat radiated +in long waves from the earth. Hence it is estimated that +if a Krakatoa were to belch forth dust every year or +two, the dust veil might cause a reduction of about 6°C. +in the earth's surface temperature. As in every such complicated +problem, some of the author's assumptions are +open to question, but this touches their quantitative and +not their qualitative value. It seems certain that if volcanic +explosions were frequent enough and violent +enough, the temperature of the earth's surface would be +considerably lowered.</p> + +<p>Actual observation supports this theoretical conclusion. +Humphreys gathers together and amplifies all that +he and Abbot and Fowle have previously said as to observations +of the sun's thermal radiation by means of the +<!--File: cc070.png Page 46--> +<a class="pagenum break" id="Page_46" title="pg 46"></a> +pyrheliometer. This summing up of the relations between +the heat received from the sun, and the occurrence of +explosive volcanic eruptions leaves little room for doubt +that at frequent intervals during the last century and a +half a slight lowering of terrestrial temperature has +actually occurred after great eruptions. Nevertheless, it +does not justify Humphreys' final conclusion that "phenomena +within the earth itself suffice to modify its own +climate,... that these and these alone have actually +caused great changes time and again in the geologic +past." Humphreys sees so clearly the importance of the +purely terrestrial point of view that he unconsciously +slights the cosmic standpoint and ignores the important +solar facts which he himself adduces elsewhere at considerable +length.</p> + +<p>In addition to this the <em>degree</em> to which the temperature +of the earth as a whole is influenced by volcanic eruptions +is by no means so clear as is the fact that there is some +influence. Arctowski,<sup><a href="#fn13" id="r13">[13]</a></sup> for example, has prepared numerous +curves showing the march of temperature month +after month for many years. During the period from +1909 to 1913, which includes the great eruption of Katmai +in Alaska, low temperature is found to have prevailed at +the time of the eruption, but, as Arctowski puts it, on the +basis of the curves for 150 stations in all parts of the +world: "The supposition that these abnormally low temperatures +were due to the veil of volcanic dust produced +by the Katmai eruption of June 6, 1912, is completely out +of the question. If that had been the case, temperature +would have decreased from that date on, whereas it was +decreasing for more than a year before that date."</p> +<!--File: cc071.png Page 47--> +<a class="pagenum break" id="Page_47" title="pg 47"></a> + +<p>Köppen,<sup><a href="#fn14" id="r14">[14]</a></sup> in his comprehensive study of temperature +for a hundred years, also presents a strong argument +against the idea that volcanic eruptions have an important +place in determining the present temperature of +the earth. A volcanic eruption is a sudden occurrence. +Whatever effect is produced by dust thrown into the air +must occur within a few months, or as soon as the dust +has had an opportunity to be wafted to the region in +question. When the dust arrives, there will be a rapid +drop through the few degrees of temperature which the +dust is supposed to be able to account for, and thereafter +a slow rise of temperature. If volcanic eruptions actually +caused a frequent lowering of terrestrial temperature in +the hundred years studied by Köppen, there should be +more cases where the annual temperature is decidedly +below the normal than where it shows a large departure +in the opposite direction. The contrary is actually the +case.</p> + +<p>A still more important argument is the fact that the +earth is now in an intermediate condition of climate. +Throughout most of geologic time, as we shall see again +and again, the climate of the earth has been milder than +now. Regions like Greenland have not been the seat of +glaciers, but have been the home of types of plants which +now thrive in relatively low latitudes. In other words, the +earth is today only part way from a glacial epoch to what +may be called the normal, mild climate of the earth—a +climate in which the contrast from zone to zone was much +less than now, and the lower air averaged warmer. Hence +it seems impossible to avoid the conclusion that the +cause of glaciation is still operating with considerable +<!--File: cc072.png Page 48--> +<a class="pagenum break" id="Page_48" title="pg 48"></a> +although diminished efficiency. But volcanic dust is +obviously not operating to any appreciable extent at +present, for the upper air is almost free from dust a large +part of the time.</p> + +<p>Again, as Chamberlin suggests, let it be supposed that +a Krakatoan eruption every two years would produce a +glacial period. Unless the most experienced field workers +on the glacial formations are quite in error, the various +glacial epochs of the Pleistocene glacial period had a +joint duration of at least 150,000 years and perhaps twice +as much. That would require 75,000 Krakatoan eruptions. +But where are the pits and cones of such eruptions? +There has not been time to erode them away since the +Pleistocene glaciation. Their beds of volcanic ash would +presumably be as voluminous as the glacial beds, but +there do not seem to be accumulations of any such size. +Even though the same volcano suffered repeated explosions, +it seems impossible to find sufficient fresh volcanic +debris. Moreover, the volcanic hypothesis has not yet +offered any mechanism for systematic glacial variations. +Hence, while the hypothesis is important, we must search +further for the full explanation of glacial fluctuations, +historic pulsations, and the earth's present quasi-glacial +climate.</p> + +<p>V. <em>The Hypothesis of Polar Wandering.</em> Another hypothesis, +which has some adherents, especially among +geologists, holds that the position of the earth's axis has +shifted repeatedly during geological times, thus causing +glaciation in regions which are not now polar. Astrophysicists, +however, are quite sure that no agency could +radically change the relation between the earth and its +axis without likewise altering the orbits of the planets to +a degree that would be easily recognized. Moreover, the +distribution of the centers of glaciation both in the Permian +<!--File: cc073.png Page 49--> +<a class="pagenum break" id="Page_49" title="pg 49"></a> +and Pleistocene periods does not seem to conform +to this hypothesis.</p> + +<p>VI. <em>The Thermal Solar Hypothesis.</em> The only other +explanations of the climatic changes of glacial and historic +times which now seem to have much standing are +two distinct and almost antagonistic solar hypotheses. +One is the idea that changes in the earth's climate are +due to variations in the heat emitted by the sun and +hence in the temperature of the earth. The other is the +entirely different idea that climatic changes arise from +solar conditions which cause a <em>redistribution of the +earth's atmospheric pressure</em> and hence produce changes +in winds, ocean currents, and especially storms. This +second, or "cyclonic," hypothesis is the subject of a book +entitled <em>Earth and Sun</em>, which is to be published as a +companion to the present volume. It will be outlined in +the next chapter. The other, or thermal, hypothesis may +be dismissed briefly. Unquestionably a permanent change +in the amount of heat emitted by the sun would permanently +alter the earth's climate. There is absolutely no +evidence, however, of any such change during geologic +time. The evidence as to the earth's cosmic uniformity +and as to secular progression is all against it. Suppose +that for thirty or forty thousand years the sun cooled off +enough so that the earth was as cool as during a glacial +epoch. As glaciation is soon succeeded by a mild climate, +some agency would then be needed to raise the sun's +temperature. The impact of a shower of meteorites might +accomplish this, but that would mean a very sudden heating, +such as there is no evidence of in geological history. +In fact, there is far more evidence of sudden cooling than +of sudden heating. Moreover, it is far beyond the bounds +of probability that such an impact should be repeated +again and again with just such force as to bring the climate +<!--File: cc074.png Page 50--> +<a class="pagenum break" id="Page_50" title="pg 50"></a> +back almost to where it started and yet to allow for +the slight changes which cause secular progression. +Another and equally cogent objection to the thermal form +of solar hypothesis is stated by Humphreys as follows: +"A change of the solar constant obviously alters all surface +temperatures by a roughly constant percentage. +Hence a decrease of the heat from the sun would in general +cause a decrease of the interzonal temperature +gradients; and this in turn a less vigorous atmospheric +circulation, and a less copious rain or snowfall—exactly +the reverse of the condition, namely, abundant precipitation, +most favorable to extensive glaciation."</p> + +<p>This brings us to the end of the main hypotheses as to +climatic changes, aside from the solar cyclonic hypothesis +which will be discussed in the next chapter. It appears +that variations in the position of the earth at perihelion +have a real though slight influence in causing cycles with +a length of about 21,000 years. Changes in the carbon +dioxide of the air probably have a more important but +extremely slow influence upon geologic oscillations. +Variations in the size, shape, and height of the continents +are constantly causing all manner of climatic complications, +but do not cause rapid fluctuations and pulsations. +The eruption of volcanic dust appears occasionally to +lower the temperature, but its potency to explain the +complex climatic changes recorded in the rocks has probably +been exaggerated. Finally, although minor changes +in the amount of heat given out by the sun occur constantly +and have been demonstrated to have a climatic +effect, there is no evidence that such changes are the main +cause of the climatic phenomena which we are trying to +explain. Nevertheless, in connection with other solar +changes they may be of high importance.</p> +<!--File: cc075.png Page 51--> +<a class="pagenum break" id="Page_51" title="pg 51"></a> + + + + +<h2 id="Chapt_IV"><a href="#content">CHAPTER IV</a></h2> + +<h3>THE SOLAR CYCLONIC HYPOTHESIS</h3> + + +<p>The progress of science is made up of a vast succession +of hypotheses. The majority die in early +infancy. A few live and are for a time widely +accepted. Then some new hypothesis either destroys them +completely or shows that, while they contain elements of +truth, they are not the whole truth. In the previous chapter +we have discussed a group of hypotheses of this kind, +and have tried to point out fairly their degree of truth so +far as it can yet be determined. In this chapter we shall +outline still another hypothesis, the relation of which to +present climatic conditions has been fully developed in +<em>Earth and Sun</em>; while its relation to the past will be explained +in the present volume. This hypothesis is not +supposed to supersede the others, for so far as they are +true they cannot be superseded. It merely seems to explain +some of the many conditions which the other +hypotheses apparently fail to explain. To suppose that +it will suffer a fate more glorious than its predecessors +would be presumptuous. The best that can be hoped is +that after it has been pruned, enriched, and modified, it +may take its place among the steps which finally lead to +the goal of truth.</p> + +<p>In this chapter the new hypothesis will be sketched in +broad outline in order that in the rest of this book the +reader may appreciate the bearing of all that is said. +Details of proof and methods of work will be omitted, +<!--File: cc076.png Page 52--> +<a class="pagenum break" id="Page_52" title="pg 52"></a> +since they are given in <em>Earth and Sun</em>. For the sake of +brevity and clearness the main conclusions will be stated +without the qualifications and exceptions which are fully +explained in that volume. Here it will be necessary to +pass quickly over points which depart radically from accepted +ideas, and which therefore must arouse serious +question in the minds of thoughtful readers. That, however, +is a necessary consequence of the attempt which +this book makes to put the problem of climate in such +form that the argument can be followed by thoughtful +students in any branch of knowledge and not merely by +specialists. Therefore, the specialist can merely be asked +to withhold judgment until he has read all the evidence +as given in <em>Earth and Sun</em>, and then to condemn only +those parts that are wrong and not the whole argument.</p> + +<p>Without further explanation let us turn to our main +problem. In the realm of climatology the most important +discovery of the last generation is that variations in the +weather depend on variations in the activity of the sun's +atmosphere. The work of the great astronomer, Newcomb, +and that of the great climatologist, Köppen, have +shown beyond question that the temperature of the +earth's surface varies in harmony with variations in the +number and area of sunspots.<sup><a href="#fn15" id="r15">[15]</a></sup> The work of Abbot has +shown that the amount of heat radiated from the sun also +varies, and that in general the variations correspond with +those of the sunspots, although there are exceptions, +especially when the spots are fewest. Here, however, +there at once arises a puzzling paradox. The earth certainly +<!--File: cc077.png Page 53--> +<a class="pagenum break" id="Page_53" title="pg 53"></a> +owes its warmth to the sun. Yet when the sun emits +the most energy, that is, when sunspots are most numerous, +the earth's surface is coolest. Doubtless the earth +receives more heat than usual at such times, and the +upper air may be warmer than usual. Here we refer only +to the air at the earth's surface.</p> + +<p>Another large group of investigators have shown that +atmospheric pressure also varies in harmony with the +number of sunspots. Some parts of the earth's surface +have one kind of variation at times of many sunspots and +other parts the reverse. These differences are systematic +and depend largely on whether the region in question +happens to have high atmospheric pressure or low. The +net result is that when sunspots are numerous the +earth's storminess increases, and the atmosphere is +thrown into commotion. This interferes with the stable +planetary winds, such as the trades of low latitudes and +the prevailing westerlies of higher latitudes. Instead of +these regular winds and the fair weather which they +bring, there is a tendency toward frequent tropical hurricanes +in the lower latitudes and toward more frequent +and severe storms of the ordinary type in the latitudes +where the world's most progressive nations now live. +With the change in storminess there naturally goes a +change in rainfall. Not all parts of the world, however, +have increased storminess and more abundant rainfall +when sunspots are numerous. Some parts change in the +opposite way. Thus when the sun's atmosphere is particularly +disturbed, the contrasts between different parts +of the earth's surface are increased. For example, the +northern United States and southern Canada become +more stormy and rainy, as appears in Fig. 2, and the +same is true of the Southwest and along the south Atlantic +coast. In a crescent-shaped central area, however, +<!--File: cc078.png Page 54--> +<a class="pagenum break" id="Page_54" title="pg 54"></a> +extending from Wyoming through Missouri to Nova +Scotia, the number of storms and the amount of rainfall +decrease.</p> + + +<img id="Fig_2" src="images/fig2.png" width="806" height="567" alt="Fig. 2" /> +<p class="center"><em>Fig. 2. Storminess of sunspot maxima vs. minima.</em><br /> +(<em>After Kullmer.</em>)</p> + +<p class="blockquot">Based on nine years' nearest sunspot minima and nine years' nearest sunspot +maxima in the three sunspot cycles from 1888 to 1918. Heavy shading +indicates excess of storminess when sunspots are numerous. Figures indicate +average yearly number of storms by which years of maximum sunspots +exceed those of minimum sunspots.</p> + +<p>The two controlling factors of any climate are the +temperature and the atmospheric pressure, for they determine +the winds, the storms, and thus the rainfall. A +study of the temperature seems to show that the peculiar +paradox of a hot sun and a cool earth is due largely to +the increased storminess during times of many sunspots. +The earth's surface is heated by the rays of the sun, but +<!--File: cc079.png Page 55--> +<a class="pagenum break" id="Page_55" title="pg 55"></a> +most of the rays do not in themselves heat the air as +they pass through it. The air gets its heat largely from +the heat absorbed by the water vapor which is intimately +mingled with its lower portions, or from the long heat +waves sent out by the earth after it has been warmed by +the sun. The faster the air moves along the earth's surface +the less it becomes heated, and the more heat it takes +away. This sounds like a contradiction, but not to anyone +who has tried to heat a stove in the open air. If the air +is still, the stove rapidly becomes warm and so does the +air around it. If the wind is blowing, the cool air delays +the heating of the stove and prevents the surface from +ever becoming as hot as it would otherwise. That seems +to be what happens on a large scale when sunspots are +numerous. The sun actually sends to the earth more +energy than usual, but the air moves with such unusual +rapidity that it actually cools the earth's surface a trifle +by carrying the extra heat to high levels where it is lost +into space.</p> + +<p>There has been much discussion as to why storms are +numerous when the sun's atmosphere is disturbed. Many +investigators have supposed it was due entirely and +directly to the heating of the earth's surface by the sun. +This, however, needs modification for several reasons. +In the first place, recent investigations show that in a +great many cases changes in barometric pressure precede +changes in temperature and apparently cause them by +altering the winds and producing storms. This is the +opposite of what would happen if the effect of solar heat +upon the earth's surface were the only agency. In the +second place, if storms were due exclusively to variations +in the ordinary solar radiation which comes to the earth +as light and is converted into heat, the solar effect ought +<!--File: cc080.png Page 56--> +<a class="pagenum break" id="Page_56" title="pg 56"></a> +to be most pronounced when the center of the sun's +visible disk is most disturbed. As a matter of fact the +storminess is notably greatest when the edges of the +solar disk are most disturbed. These facts and others lead +to the conclusion that some agency other than heat must +also play some part in producing storminess.</p> + +<p>The search for this auxiliary agency raises many difficult +questions which cannot yet be answered. On the +whole the weight of evidence suggests that electrical +phenomena of some kind are involved, although variations +in the amount of ultra-violet light may also be +important. Many investigators have shown that the sun +emits electrons. Hale has proved that the sun, like the +earth, is magnetized. Sunspots also have magnetic fields +the strength of which is often fifty times as great as that +of the sun as a whole. If electrons are sent to the earth, +they must move in curved paths, for they are deflected +by the sun's magnetic field and again by the earth's +magnetic field. The solar deflection may cause their +effects to be greatest when the spots are near the sun's +margin; the terrestrial deflection may cause concentration +in bands roughly concentric with the magnetic poles +of the earth. These conditions correspond with the known +facts.</p> + +<p>Farther than this we cannot yet go. The calculations of +Humphreys seem to indicate that the direct electrical +effect of the sun's electrons upon atmospheric pressure +is too small to be of appreciable significance in intensifying +storms. On the other hand the peculiar way in which +activity upon the margins of the sun appears to be correlated +not only with atmospheric electricity, but with +barometric pressure, seems to be equally strong evidence +in the other direction. Possibly the sun's electrons and +its electrical waves produce indirect effects by being +<!--File: cc081.png Page 57--> +<a class="pagenum break" id="Page_57" title="pg 57"></a> +converted into heat, or by causing the formation of ozone +and the condensation of water vapor in the upper air. +Any one of these processes would raise the temperature +of the upper air, for the ozone and the water vapor would +be formed there and would tend to act as a blanket to +hold in the earth's heat. But any such change in the temperature +of the upper air would influence the lower air +through changes in barometric pressure. These considerations +are given here because the thoughtful reader +is likely to inquire how solar activity can influence +storminess. Moreover, at the end of this book we shall +take up certain speculative questions in which an electrical +hypothesis will be employed. For the main portions +of this book it makes no difference how the sun's +variations influence the earth's atmosphere. The only table 6 +essential point is that when the solar atmosphere is active +the storminess of the earth increases, and that is a matter +of direct observation.</p> + +<p>Let us now inquire into the relation between the small +cyclonic vacillations of the weather and the types of +climatic changes known as historic pulsations and glacial +fluctuations. One of the most interesting results of recent +investigations is the evidence that sunspot cycles on a +small scale present almost the same phenomena as do +historic pulsations and glacial fluctuations. For instance, +when sunspots are numerous, storminess increases +markedly in a belt near the northern border of the area +of greatest storminess, that is, in southern Canada and +thence across the Atlantic to the North Sea and Scandinavia. +(See Figs. 2 and 3.) Corresponding with this is the +fact that the evidence as to climatic pulsations in historic +times indicates that regions along this path, for instance +Greenland, the North Sea region, and southern Scandinavia, +<!--File: cc082.png Page 58--> +<a class="pagenum break" id="Page_58" title="pg 58"></a> + + +<!--File: cc083.png Page 59--> +<a class="pagenum break" id="Page_59" title="pg 59"></a> + + + +<!--File: cc084.png Page 60--> +<a class="pagenum break" id="Page_60" title="pg 60"></a> + +were visited by especially frequent and severe +storms at the climax of each pulsation. Moreover, the +greatest accumulations of ice in the glacial period were +on the poleward border of the general regions where now +the storms appear to increase most at times of solar +activity.</p> +<img id="Fig_3a" src="images/fig3a.png" width="924" height="922" alt="Fig 3a" /> + +<p class="center"><em>Fig. 3.a Relative rainfall at times of increasing and decreasing sunspots</em></p> +<p class="blockquot">Heavy shading, more rain with increasing spots. Light shading, more rain with decreasing +spots. No data for unshaded areas.</p> + +<p class="blockquot">Figures indicate percentages of the average rainfall by which the rainfall during +periods of increasing spots exceeds or falls short of rainfall during periods of decreasing +spots. The excess or deficiency is stated in percentages of the average. Rainfall +data from Walker: Sunspots and Rainfall.</p> + +<img id="Fig_3b" src="images/fig3b.png" width="919" height="909" alt="Fig.3b" /> +<p class="center"> <em>Fig. 3.b Relative rainfall at times of increasing and decreasing +sunspots.</em></p> + +<p class="blockquot">Heavy shading, more rain with increasing spots. Light shading, more rain with decreasing +spots. No data for unshaded areas.</p> + +<p class="blockquot">Figures indicate percentages of the average rainfall by which the rainfall during +periods of increasing spots exceeds or falls short of rainfall during periods of decreasing +spots. The excess or deficiency is stated in percentages of the average. Rainfall +data from Walker: Sunspots and Rainfall.</p> + +<p>Even more clear is the evidence from other regions +where storms increase at times of many sunspots. One +such region includes the southwestern United States, +while another is the Mediterranean region and the semi-arid +or desert parts of Asia farther east. In these regions +innumerable ruins and other lines of evidence show that +at the climax of each climatic pulsation there was more +storminess and rainfall than at present, just as there +now is when the sun is most active. In still earlier times, +while ice was accumulating farther north, the basins of +these semi-arid regions were filled with lakes whose +strands still remain to tell the tale of much-increased +rainfall and presumable storminess. If we go back still +further in geological times to the Permian glaciation, the +areas where ice accumulated most abundantly appear to +be the regions where tropical hurricanes produce the +greatest rainfall and the greatest lowering of temperature +at times of many sunspots. From these and many +other lines of evidence it seems probable that historic +pulsations and glacial fluctuations are nothing more than +sunspot cycles on a large scale. It is one of the fundamental +rules of science to reason from the known to the +unknown, from the near to the far, from the present to +the past. Hence it seems advisable to investigate whether +any of the climatic phenomena of the past may have +arisen from an intensification of the solar conditions +which now appear to give rise to similar phenomena on +a small scale.</p> +<p><!--File: cc085.png Page 61--> +<a class="pagenum break" id="Page_61" title="pg 61"></a> +The rest of this chapter will be devoted to a <em>résumé</em> +of certain tentative conclusions which have no bearing +on the main part of this book, but which apply to the +closing chapters. There we shall inquire into the periodicity +of the climatic phenomena of geological times, and +shall ask whether there is any reason to suppose that the +sun's activity has exhibited similar periodicity. This +leads to an investigation of the possible causes of disturbances +in the sun's atmosphere. It is generally assumed +that sunspots, solar prominences, the bright clouds +known as faculæ, and other phenomena denoting a perturbed +state of the solar atmosphere, are due to some +cause within the sun. Yet the limitation of these phenomena, +especially the sunspots, to restricted latitudes, +as has been shown in <em>Earth and Sun</em>, does not seem to be +in harmony with an internal solar origin, even though +a banded arrangement may be normal for a rotating +globe. The fairly regular periodicity of the sunspots +seems equally out of harmony with an internal origin. +Again, the solar atmosphere has two kinds of circulation, +one the so-called "rice grains," and the other +the spots and their attendant phenomena. Now the rice +grains present the appearance that would be expected in +an atmospheric circulation arising from the loss of heat +by the outer part of a gaseous body like the sun. For +these reasons and others numerous good thinkers from +Wolf to Schuster have held that sunspots owe their +periodicity to causes outside the sun. The only possible +cause seems to be the planets, acting either through +gravitation, through forces of an electrical origin, or +through some other agency. Various new investigations +which are described in <em>Earth and Sun</em> support this conclusion. +The chief difficulty in accepting it hitherto has +been that although Jupiter, because of its size, would be +<!--File: cc086.png Page 62--> +<a class="pagenum break" id="Page_62" title="pg 62"></a> +expected to dominate the sunspot cycle, its period of +11.86 years has not been detected. The sunspot cycle has +appeared to average 11.2 years in length, and has been +called the 11-year cycle. Nevertheless, a new analysis of +the sunspot data shows that when attention is concentrated +upon the major maxima, which are least subject to +retardation or acceleration by other causes, a periodicity +closely approaching that of Jupiter is evident. Moreover, +when the effects of Jupiter, Saturn, and the other planets +are combined, they produce a highly variable curve which +has an extraordinary resemblance to the sunspot curve. +The method by which the planets influence the sun's +atmosphere is still open to question. It may be through +tides, through the direct effect of gravitation, through +electro-magnetic forces, or in some other way. Whichever +it may be, the result may perhaps be slight differences of +atmospheric pressure upon the sun. Such differences +may set in motion slight whirling movements analogous +to terrestrial storms, and these presumably gather momentum +from the sun's own energy. Since the planetary +influences vary in strength because of the continuous +change in the relative distances and positions of the +planets, the sun's atmosphere appears to be swayed by +cyclonic disturbances of varying degrees of severity. The +cyclonic disturbances known as sunspots have been +proved by Hale to become more highly electrified as they +increase in intensity. At the same time hot gases presumably +well up from the lower parts of the solar atmosphere +and thereby cause the sun to emit more heat. Thus +by one means or another, the earth's atmosphere appears +to be set in commotion and cycles of climate are inaugurated.</p> + +<p>If the preceding reasoning is correct, any disturbance +of the solar atmosphere must have an effect upon the +<!--File: cc087.png Page 63--> +<a class="pagenum break" id="Page_63" title="pg 63"></a> +earth's climate. If the disturbance were great enough and +of the right nature it might produce a glacial epoch. The +planets are by no means the only bodies which act upon +the sun, for that body sustains a constantly changing +relation to millions of other celestial bodies of all sizes +up to vast universes, and at all sorts of distances. If the +sun and another star should approach near enough to one +another, it is certain that the solar atmosphere would be +disturbed much more than at present.</p> + +<p>Here we must leave the cyclonic hypothesis of climate +and must refer the reader once more to <em>Earth and Sun</em> +for fuller details. In the rest of this book we shall discuss +the nature of the climatic changes of past times and shall +inquire into their relation to the various climatic hypotheses +mentioned in the last two chapters. Then we shall +inquire into the possibility that the solar system has ever +been near enough to any of the stars to cause appreciable +disturbances of the solar atmosphere. We shall complete +our study by investigating the vexed question of why +movements of the earth's crust, such as the uplifting of +continents and mountain chains, have generally occurred +at the same time as great climatic fluctuations. This +would not be so surprising were it not that the climatic +phenomena appear to have consisted of highly complex +cycles while the uplift has been a relatively steady movement +in one direction. We shall find some evidence that +the solar disturbances which seem to cause climatic +changes also have a relation to movements of the crust.</p> +<!--File: cc088.png Page 64--> +<a class="pagenum break" id="Page_64" title="pg 64"></a> + + + + +<h2 id="Chapt_V"><a href="#content">CHAPTER V</a></h2> + +<h3>THE CLIMATE OF HISTORY<sup><a href="#fn16" id="r16">[16]</a></sup></h3> + + +<p>We are now prepared to consider the climate of +the past. The first period to claim attention is +the few thousand years covered by written +history. Strangely enough, the conditions during this +time are known with less accuracy than are those of +geological periods hundreds of times more remote. Yet +if pronounced changes have occurred since the days of +the ancient Babylonians and since the last of the post-glacial +stages, they are of great importance not only +because of their possible historic effects, but because they +bridge the gap between the little variations of climate +which are observable during a single lifetime and the +great changes known as glacial epochs. Only by bridging +the gap can we determine whether there is any genetic +relation between the great changes and the small. A full +discussion of the climate of historic times is not here +advisable, for it has been considered in detail in numerous +other publications.<sup><a href="#fn17" id="r17">[17]</a></sup> Our most profitable course would +seem to be to consider first the general trend of opinion +and then to take up the chief objections to each of the +main hypotheses.</p> + +<p>In the hot debate over this problem during recent +<!--File: cc089.png Page 65--> +<a class="pagenum break" id="Page_65" title="pg 65"></a> +decades the ideas of geographers seem to have gone +through much the same metamorphosis as have those of +geologists in regard to the climate of far earlier times.</p> + +<p>As every geologist well knows, at the dawn of geology +people believed in climatic uniformity—that is, it was +supposed that since the completion of an original creative +act there had been no important changes. This view +quickly disappeared and was superseded by the hypothesis +of progressive cooling and drying, an hypothesis +which had much to do with the development of the nebular +hypothesis, and which has in turn been greatly +strengthened by that hypothesis. The discovery of evidence +of widespread continental glaciation, however, +necessitated a modification of this view, and succeeding +years have brought to light a constantly increasing number +of glacial, or at least cool, periods distributed +throughout almost the whole of geological time. Moreover, +each year, almost, brings new evidence of the great +complexity of glacial periods, epochs, and stages. Thus, +for many decades, geologists have more and more been +led to believe that in spite of surprising uniformity, when +viewed in comparison with the cosmic possibilities, the +climate of the past has been highly unstable from the +viewpoint of organic evolution, and its changes have been +of all degrees of intensity.</p> + +<p>Geographers have lately been debating the reality of +historic changes of climate in the same way in which +geologists debated the reality of glacial epochs and +stages. Several hypotheses present themselves but these +may all be grouped under three headings; namely, the +hypotheses of (1) progressive desiccation, (2) climatic +uniformity, and (3) pulsations. The hypothesis of progressive +desiccation has been widely advocated. In many +of the drier portions of the world, especially between 30° +<!--File: cc090.png Page 66--> +<a class="pagenum break" id="Page_66" title="pg 66"></a> +and 40° from the equator, and preëminently in western +and central Asia and in the southwestern United States, +almost innumerable facts seem to indicate that two or +three thousand years ago the climate was distinctly +moister than at present. The evidence includes old lake +strands, the traces of desiccated springs, roads in places +now too dry for caravans, other roads which make detours +around dry lake beds where no lakes now exist, and +fragments of dead forests extending over hundreds of +square miles where trees cannot now grow for lack of +water. Still stronger evidence is furnished by ancient +ruins, hundreds of which are located in places which are +now so dry that only the merest fraction of the former +inhabitants could find water. The ruins of Palmyra, in +the Syrian Desert, show that it must once have been a +city like modern Damascus, with one or two hundred +thousand inhabitants, but its water supply now suffices +for only one or two thousand. All attempts to increase the +water supply have had only a slight effect and the water +is notoriously sulphurous, whereas in the former days, +when it was abundant, it was renowned for its excellence. +Hundreds of pages might be devoted to describing similar +ruins. Some of them are even more remarkable for +their dryness than is Niya, a site in the Tarim Desert of +Chinese Turkestan. Yet there the evidence of desiccation +within 2000 years is so strong that even so careful and +conservative a man as Hann,<sup><a href="#fn18" id="r18">[18]</a></sup> pronounces it "überzeugend."</p> + +<p>A single quotation from scores that might be used will +illustrate the conclusions of some of the most careful +archæologists.<sup><a href="#fn19" id="r19">[19]</a></sup></p> +<!--File: cc091.png Page 67--> +<a class="pagenum break" id="Page_67" title="pg 67"></a> + +<blockquote> +<p>Among the regions which were once populous and highly +civilized, but which are now desert and deserted, there are few +which were more closely connected with the beginnings of our +own civilization than the desert parts of Syria and northern +Arabia. It is only of recent years that the vast extent and great +importance of this lost civilization has been fully recognized and +that attempts have been made to reduce the extent of the unexplored +area and to discover how much of the territory which has +long been known as desert was formerly habitable and inhabited. +The results of the explorations of the last twenty years have been +most astonishing in this regard. It has been found that practically +all of the wide area lying between the coast range of the +eastern Mediterranean and the Euphrates, appearing upon the +maps as the Syrian Desert, an area embracing somewhat more +than 20,000 square miles, was more thickly populated than any +area of similar dimensions in England or in the United States +is today if one excludes the immediate vicinity of the large +modern cities. It has also been discovered that an enormous +desert tract lying to the east of Palestine, stretching eastward +and southward into the country which we know as Arabia, was +also a densely populated country. How far these settled regions +extended in antiquity is still unknown, but the most distant +explorations in these directions have failed to reach the end of +ruins and other signs of former occupation.</p> + +<p>The traveler who has crossed the settled, and more or less +populous, coast range of northern Syria and descended into the +narrow fertile valley of the Orontes, encounters in any farther +journey toward the east an irregular range of limestone hills +lying north and south and stretching to the northeast almost +halfway to the Euphrates. These hills are about 2,500 feet high, +rising in occasional peaks from 3,000 to 3,500 feet above sea level. +They are gray and unrelieved by any visible vegetation. On +ascending into the hills the traveler is astonished to find at every +turn remnants of the work of men's hands, paved roads, walls +which divided fields, terrace walls of massive structure. Presently +he comes upon a small deserted and partly ruined town +<!--File: cc092.png Page 68--> +<a class="pagenum break" id="Page_68" title="pg 68"></a> +composed of buildings large and small constructed of beautifully +wrought blocks of limestone, all rising out of the barren +rock which forms the ribs of the hills. If he mounts an eminence +in the vicinity, he will be still further astonished to behold +similar ruins lying in all directions. He may count ten or fifteen +or twenty, according to the commanding position of his lookout. +From a distance it is often difficult to believe that these are not +inhabited places; but closer inspection reveals that the gentle +hand of time or the rude touch of earthquake has been laid upon +every building. Some of the towns are better preserved than +others; some buildings are quite perfect but for their wooden +roofs which time has removed, others stand in picturesque ruins, +while others still are level with the ground. On a far-off hilltop +stands the ruin of a pagan temple, and crowning some lofty ridge +lie the ruins of a great Christian monastery. Mile after mile of +this barren gray country may be traversed without encountering +a single human being. Day after day may be spent in traveling +from one ruined town to another without seeing any green +thing save a terebinth tree or two standing among the ruins, +which have sent their roots down into earth still preserved in +the foundations of some ancient building. No soil is visible +anywhere except in a few pockets in the rock from which it +could not be washed by the torrential rains of the wet season; +yet every ruin is surrounded with the remains of presses for the +making of oil and wine. Only one oasis has been discovered in +these high plateaus.</p> + +<p>Passing eastward from this range of hills, one descends into a +gently rolling country that stretches miles away toward the +Euphrates. At the eastern foot of the hills one finds oneself in a +totally different country, at first quite fertile and dotted with +frequent villages of flat-roofed houses. Here practically all the +remains of ancient times have been destroyed through ages of +building and rebuilding. Beyond this narrow fertile strip the +soil grows drier and more barren, until presently another kind +of desert is reached, an undulating waste of dead soil. Few walls +or towers or arches rise to break the monotony of the unbroken +<!--File: cc093.png Page 69--> +<a class="pagenum break" id="Page_69" title="pg 69"></a> +landscape; but the careful explorer will find on closer examination +that this region was more thickly populated in antiquity +even than the hill country to the west. Every unevenness of the +surface marks the site of a town, some of them cities of considerable +extent.</p> + +<p>We may draw certain very definite conclusions as to the +former conditions of the country itself. There was soil upon the +northern hills where none now exists, for the buildings now show +unfinished foundation courses which were not intended to be +seen; the soil in depressions without outlets is deeper than it +formerly was; there are hundreds of olive and wine presses in +localities where no tree or vine could now find footing; and +there are hillsides with ruined terrace walls rising one above +the other with no sign of earth near them. There was also a large +natural water supply. In the north as well as in the south we +find the dry beds of rivers, streams, and brooks with sand and +pebbles and well-worn rocks but no water in them from one +year's end to the other. We find bridges over these dry streams +and crudely made washing boards along their banks directly +below deserted towns. Many of the bridges span the beds of +streams that seldom or never have water in them and give +clear evidence of the great climatic changes that have taken +place. There are well heads and well houses, and inscriptions +referring to springs; but neither wells nor springs exist today +except in the rarest instances. Many of the houses had their +rock-hewn cisterns, never large enough to have supplied water +for more than a brief period, and corresponding to the cisterns +which most of our recent forefathers had which were for convenience +rather than for dependence. Some of the towns in +southern Syria were provided with large public reservoirs, but +these are not large enough to have supplied water to their +original populations. The high plateaus were of course without +irrigation; but there are no signs, even in the lower flatter +country, that irrigation was ever practiced; and canals for this +purpose could not have completely disappeared. There were +forests in the immediate vicinity, forests producing timbers of +great length and thickness; for in the north and northeast practically +<!--File: cc094.png--> +<a class="pagenum break" id="Page_70" title="pg 70"></a> + all the buildings had wooden roofs, wooden intermediate +floors, and other features of wood. Costly buildings, such as +temples and churches, employed large wooden beams; but wood +was used in much larger quantities in private dwellings, shops, +stables, and barns. If wood had not been plentiful and cheap—which +means grown near by—the builders would have adopted +the building methods of their neighbors in the south, who used +very little wood and developed the most perfect type of lithic +architecture the world has ever seen. And here there exists a +strange anomaly: Northern Syria, where so much wood was +employed in antiquity, is absolutely treeless now; while in the +mountains of southern Syria, where wood must have been +scarce in antiquity to have forced upon the inhabitants an almost +exclusive use of stone, there are still groves of scrub oak and +pine, and travelers of half a century ago reported large forests +of chestnut trees.<sup><a href="#fn20" id="r20">[20]</a></sup> It is perfectly apparent that large parts of +Syria once had soil and forests and springs and rivers, while it +has none of these now, and that it had a much larger and better +distributed rainfall in ancient times than it has now.</p> +</blockquote> + +<p>Professor Butler's careful work is especially interesting +because of its contrast to the loose statements of +those who believe in climatic uniformity. So far as I am +aware, no opponent of the hypothesis of climatic changes +has ever even attempted to show by careful statistical +analysis that the ancient water supply of such ruins was +no greater than that of the present. The most that has +been done is to suggest that there may have been sources +of water which are now unknown. Of course, this might +be true in a single instance, but it could scarcely be the +case in many hundreds or thousands of ruins.</p> + +<!--File: cc095.png Page 71--> +<a class="pagenum break" id="Page_71" title="pg 71"></a> + +<p>Although the arguments in favor of a change of climate +during the last two thousand years seem too strong +to be ignored, their very strength seems to have been a +source of error. A large number of people have jumped +to the conclusion that the change which appears to have +occurred in certain regions occurred everywhere, and +that it consisted of a gradual desiccation.</p> + +<p>Many observers, quite as careful as those who believe +in progressive desiccation, point to evidences of aridity +in past times in the very regions where the others find +proof of moisture. Lakes such as the Caspian Sea fell to +such a low level that parts of their present floors were +exposed and were used as sites for buildings whose ruins +are still extant. Elsewhere, for instance in the Tian-Shan +Mountains, irrigation ditches are found in places where +irrigation never seems to be necessary at present. In +Syria and North Africa during the early centuries of the +Christian era the Romans showed unparalleled activity +in building great aqueducts and in watering land which +then apparently needed water almost as much as it does +today. Evidence of this sort is abundant and is as convincing +as is the evidence of moister conditions in the +past. It is admirably set forth, for example, in the comprehensive +and ably written monograph of Leiter on the +climate of North Africa.<sup><a href="#fn21" id="r21">[21]</a></sup> The evidence cited there and +elsewhere has led many authors strongly to advocate the +hypothesis of climatic uniformity. They have done exactly +as have the advocates of progressive change, and +have extended their conclusions over the whole world and +over the whole of historic times.</p> + +<p>The hypotheses of climatic uniformity and of progressive +<!--File: cc096.png Pag2 72--> +<a class="pagenum break" id="Page_72" title="pg 72"></a> +change both seem to be based on reliable evidence. +They may seem to be diametrically opposed to one +another, but this is only when there is a failure to group +the various lines of evidence according to their dates, and +according to the types of climate in which they happen +to be located. When the facts are properly grouped in +both time and space, it appears that evidence of moist +conditions in the historic Mediterranean lands is found +during certain periods; for instance, four or five hundred +years before Christ, at the time of Christ, and 1000 A. D. +The other kind of evidence, on the contrary, culminates +at other epochs, such as about 1200 B. C. and in the +seventh and thirteenth centuries after Christ. It is also +found during the interval from the culmination of a moist +epoch to the culmination of a dry one, for at such times +the climate was growing drier and the people were under +stress. This was seemingly the case during the period +from the second to the fourth centuries of our era. North +Africa and Syria must then have been distinctly better +watered than at present, as appears from Butler's vivid +description; but they were gradually becoming drier, and +the natural effect on a vigorous, competent people like the +Romans was to cause them to construct numerous engineering +works to provide the necessary water.</p> + +<p>The considerations which have just been set forth have +led to a third hypothesis, that of pulsatory climatic +changes. According to this, the earth's climate is not +stable, nor does it change uniformly in one direction. It +appears to fluctuate back and forth not only in the little +waves which we see from year to year or decade to +decade, but in much larger waves, which take hundreds of +years or even a thousand. These in turn seem to merge +into and be imposed on the greater waves which form +glacial stages, glacial epochs, and glacial periods. At the +<!--File: cc097.png Page 73--> +<a class="pagenum break" id="Page_73" title="pg 73"></a> +present time there seems to be no way of determining +whether the general tendency is toward aridity or toward +glaciation. The seventh century of our era was apparently +the driest time during the historic period—distinctly +drier than the present—but the thirteenth century was +almost equally dry, and the twelfth or thirteenth before +Christ may have been very dry.</p> + +<p>The best test of an hypothesis is actual measurements. +In the case of the pulsatory hypothesis we are fortunately +able to apply this test by means of trees. The +growth of vegetation depends on many factors—soil, exposure, +wind, sun, temperature, rain, and so forth. In a +dry region the most critical factor in determining how a +tree's growth shall vary from year to year is the supply +of moisture during the few months of most rapid +growth.<sup><a href="#fn22" id="r22">[22]</a></sup> +The work of Douglass<sup><a href="#fn23" id="r23">[23] +</a></sup> and others has shown +that in Arizona and California the thickness of the +annual rings affords a reliable indication of the amount +of moisture available during the period of growth. This +is especially true when the growth of several years is +taken as the unit and is compared with the growth of +a similar number of years before or after. Where a long +series of years is used, it is necessary to make corrections +to eliminate the effects of age, but this can be done by +mathematical methods of considerable accuracy. It is +difficult to determine whether the climate at the beginning +<!--File: cc098.png Page 74--> +<a class="pagenum break" id="Page_74" title="pg 74"></a> +and end of a tree's life was the same, but it is easily possible +to determine whether there have been pulsations +while the tree was making its growth. If a large number +of trees from various parts of a given district all formed +thick rings at a certain period and then formed thin ones +for a hundred years, after which the rings again become +thick, we seem to be safe in concluding that the trees have +lived through a long, dry period. The full reasons for this +belief and details as to the methods of estimating climate +from tree growth are given in <em>The Climatic Factor</em>.</p> + +<p>The results set forth in that volume may be summarized +as follows: During the years 1911 and 1912, under +the auspices of the Carnegie Institution of Washington, +measurements were made of the thickness of the rings of +growth on the stumps of about 450 sequoia trees in California. +These trees varied in age from 250 to nearly +3250 years. The great majority were over 1000 years of +age, seventy-nine were over 2000 years, and three over +3000. Even where only a few trees are available the +record is surprisingly reliable, except where occasional +accidents occur. Where the number approximates 100, +accidental variations are largely eliminated and we may +accept the record with considerable confidence. Accordingly, +we may say that in California we have a fairly +accurate record of the climate for 2000 years and an +approximate record for 1000 years more. The final results +of the measurements of the California trees are +shown in Fig. 4, where the climatic variations for 3000 +years in California are indicated by the solid line. The +high parts of the line indicate rainy conditions, the low +parts, dry. An examination of this curve shows that +during 3000 years there have apparently been climatic +variations more important than any which have taken +place during the past century. In order to bring out the +<!--File: cc099.png Page 75--> +<a class="pagenum break" id="Page_75" title="pg 75"></a> + +<!--File: cc100.png Page 76--> +<a class="pagenum break" id="Page_76" title="pg 76"></a> +details more clearly, the more reliable part of the California +curve, from 100 B. C. to the present time, has been +reproduced in Fig. 5. This is identical with the corresponding +part of Fig. 4, except that the vertical scale is +three times as great.</p> +<img src="images/fig4.png" width="800" height="192" alt="Fig. 4" id="Fig_4" /> +<p class="center"><em>Fig. 4. Changes of climate in California (solid line) and in western and +central Asia (dotted line).</em></p> + +<p class="blockquot">Note. The curves of Figs. 4 and 5 are reproduced as published in <em>The Solar Hypothesis</em> in 1914. Later work, +however, has indicated that in the Asiatic curve the dash lines, which were tentatively inserted in 1914, are +probably more nearly correct than the dotted lines. Still further evidence indicates that the Asiatic curve is +nearly like that of California in its main features.</p> + +<p>The curve of tree growth in California seems to be a +true representation of the general features of climatic +pulsations in the Mediterranean region. This conclusion +was originally based on the resemblance between the +solid line of Fig. 4, representing tree growth, and the +dotted line representing changes of climate in the eastern +Mediterranean region as inferred from the study of ruins +and of history before any work on this subject had been +done in America.<sup><a href="#fn24" id="r24">[24]</a></sup> The dotted line is here reproduced for +its historical significance as a stage in the study of climatic +changes. If it were to be redrawn today on the +basis of the knowledge acquired in the last twelve years, +it would be much more like the tree curve. For example, +the period of aridity suggested by the dip of the dotted +line about 300 A. D. was based largely on Professor +Butler's data as to the paucity of inscriptions and ruins +dating from that period in Syria. In the recent article, +from which a long quotation has been given, he shows +that later work proves that there is no such paucity. On +the other hand, it has accentuated the marked and sudden +decay in civilization and population which occurred +shortly after 600 A. D. He reached the same conclusion +to which the present authors had come on wholly different +grounds, namely, that the dip in the dotted line about 300 +A. D. is not warranted, whereas the dip about 630 A. D. +is extremely important. In similar fashion the work of +<!--File: cc101.png Page 77--> +<a class="pagenum break" id="Page_77" title="pg 77"></a> + +<!--File: cc102.pngPage 78--> +<a class="pagenum break" id="Page_78" title="pg 78"></a> +Stein<sup><a href="#fn25" id="r25">[25]</a></sup> in central Asia makes it clear that the contrast +between the water supply about 200 B. C. and in the preceding +and following centuries was greater than was +supposed on the basis of the scanty evidence available +when the dotted line of Fig. 4 was drawn in 1910.</p> +<img src="images/fig5.png" width="800" height="294" alt="Fig. 5" id="Fig_5" /> +<p class="center"><em>Fig. 5. Changes in California climate for 2000 years, as measured by +growth of Sequoia trees.</em></p> + +<p class="blockquot">Fig. 5 is the same as the later portion of Fig. 4, except that the vertical scale has been magnified threefold. +It seems probable that the dotted line at the right is more nearly correct than the solid line. During the thirty +years since the end of the curve the general tendency appears in general to have been somewhat upward.</p> + +<p>Since the curve of the California trees is the only continuous +and detailed record yet available for the climate +of the last three thousand years, it deserves most careful +study. It is especially necessary to determine the degree +of accuracy with which the growth of the trees represents +(1) the local rainfall and (2) the rainfall of remote +regions such as Palestine. Perhaps the best way to determine +these matters is the standard mathematical method +of correlation coefficients. If two phenomena vary in +perfect unison, as in the case of the turning of the wheels +and the progress of an automobile when the brakes are +not applied, the correlation coefficient is 1.00, being positive +when the automobile goes forward and negative +when it goes backward. If there is no relation between +two phenomena, as in the case of the number of miles run +by a given automobile each year and the number of +chickens hatched in the same period, the coefficient is +zero. A partial relationship where other factors enter +into the matter is represented by a coefficient between +zero and one, as in the case of the movement of the automobile +and the consumption of gasoline. In this case the +relation is very obvious, but is modified by other factors, +including the roughness and grade of the road, the +amount of traffic, the number of stops, the skill of the +driver, the condition and load of the automobile, and the +state of the weather. Such partial relationships are the +kind for which correlation coefficients are most useful, +for the size of the coefficients shows the relative importance +<!--File: cc103.png Page 79--> +<a class="pagenum break" id="Page_79" title="pg 79"></a> +of the various factors. A correlation coefficient +four times the probable error, which can always be determined +by a formula well known to mathematicians, is +generally considered to afford evidence of some kind of +relation between two phenomena. When the ratio between +coefficient and error rises to six, the relationship is regarded +as strong.</p> + +<p>Few people would question that there is a connection +between tree growth and rainfall, especially in a climate +with a long summer dry season like that of California. +But the growth of the trees also depends on their position, +the amount of shading, the temperature, insect pests, +blights, the wind with its tendency to break the branches, +and a number of other factors. Moreover, while rain +commonly favors growth, great extremes are relatively +less helpful than more moderate amounts. Again, the +roots of a tree may tap such deep sources of water that +neither drought nor excessive rain produces much effect +for several years. Hence in comparing the growth of the +huge sequoias with the rainfall we should expect a correlation +coefficient high enough to be convincing, but decidedly +below 1.00. Unfortunately there is no record of +the rainfall where the sequoias grow, the nearest long +record being that of Sacramento, nearly 200 miles to the +northwest and close to sea level instead of at an altitude +of about 6000 feet.</p> + +<p>Applying the method of correlation coefficients to the +annual rainfall of Sacramento and the growth of the +sequoias from 1863 to 1910, we obtain the results shown +in Table 3. The trees of Section A of the table grew in +moderately dry locations although the soil was fairly +deep, a condition which seems to be essential to sequoias. +In this case, as in all the others, the rainfall is reckoned +from July to June, which practically means from October +to May, since there is almost no summer rain. Thus the +tree growth in 1861 is compared with the rainfall of the +preceding rainy season, 1860-1861, or of several preceding +rainy seasons as the table indicates.</p> +<!--File: cc104.png Page 80--> +<a class="pagenum break" id="Page_80" title="pg 80"></a> + +<table id="table_3" class="borders"> +<tr><th colspan="8"><a href="#tables">TABLE 3</a></th></tr> +<tr><th colspan="8">CORRELATION COEFFICIENTS BETWEEN RAINFALL<br /> + AND GROWTH OF SEQUOIAS IN CALIFORNIA<sup><a href="#fn26" id="r26">[26]</a></sup></th></tr> +<tr><td colspan="8"> </td></tr> +<tr><td colspan="8" class="tdi2"> +(<i>r</i>)=<i>Correlation coefficient</i><br /> +(<i>e</i>)=<i>Probable error</i><br /> +(<i>r/e</i>)=<i>Ratio of coefficient to probable error</i> +</td></tr> +<tr><td colspan="8"> </td></tr> +<tr><th colspan="8"> <span class="smcap">A. Sacramento Rainfall and Growth of 18 Sequoias in Dry +Locations, 1861-1910</span> </th></tr> +<tr> +<th colspan="5"></th> +<th>(<em>r</em>)</th> +<th>(<em>e</em>)</th> +<th>(<em>r/e</em>)</th> +</tr> +<tr> +<td class="tdl" colspan="5">1 year of rainfall</td> +<td>−0.059</td> +<td>±0.096 </td> +<td>0.6</td> +</tr> +<tr> +<td class="tdl" colspan="5">2 years of rainfall</td> +<td>+0.288</td> +<td>±0.090</td> +<td>3.2</td> +</tr> +<tr> +<td class="tdl" colspan="5">3 years of rainfall</td> +<td>+0.570</td> +<td>±0.066</td> +<td>8.7</td> +</tr> +<tr> +<td class="tdl" colspan="5">4 years of rainfall</td> +<td>+0.470</td> +<td>±0.076</td> +<td>6.2</td> +</tr> +<tr><td colspan="8"> </td></tr> +<tr> +<th colspan="8"><span class="smcap">B. Sacramento Rainfall and Growth of 112 Sequoias Mostly in +Moist Locations, 1861-1910</span> </th> +</tr> +<tr> +<td class="tdl" colspan="5">3 years of rainfall</td> +<td>+0.340</td> +<td>±0.087</td> +<td>3.9</td></tr> +<tr> +<td class="tdl" colspan="5">4 years of rainfall</td> +<td>+0.371</td> +<td>±0.084</td> +<td>4.5</td> +</tr> +<tr><td class="tdl" colspan="5">5 years of rainfall</td> +<td>+0.398</td> +<td>±0.082</td> +<td>4.9</td> +</tr> +<tr> +<td class="tdl" colspan="5">6 years of rainfall</td> +<td>+0.418</td> +<td>±0.079</td> +<td>5.3</td> +</tr> +<tr> +<td class="tdl" colspan="5">7 years of rainfall</td> +<td>+0.471</td> +<td>±0.076</td> +<td>6.2</td> +</tr> +<tr> +<td class="tdl" colspan="5">8 years of rainfall </td> +<td>(+0.520)</td> +<td>±0.071</td> +<td>7.3</td> +</tr> +<tr> +<td class="tdl" colspan="5">9 years of rainfall</td> +<td>+0.575</td> +<td>±0.065</td> +<td>8.8</td> +</tr> +<tr> +<td class="tdl" colspan="5">10 years of rainfall</td> +<td>+0.577</td> +<td>±0.065</td> +<td>8.8</td> +</tr> +<tr><td colspan="8"> </td></tr> +<tr> +<th colspan="8"><span class="smcap">C. Sacramento Rainfall and Growth of 80 Sequoias in Moist +Locations, 1861-1910</span> </th> +</tr> +<tr> +<td class="tdl" colspan="5">10 years of rainfall</td> +<td>+0.605</td> +<td>±0.062</td> +<td>9.8</td> +</tr> +<tr><td colspan="8"> </td></tr> +<tr> +<th colspan="8"><span class="smcap">D. Annual Sequoia Growth and Rainfall of Preceding 5 Years +At Stations on Southern Pacific Railroad</span></th> +</tr> +<tr> +<th> </th> +<th><em>Years</em></th> +<th><em>Altitude</em> <br /> (<em>feet</em>)</th> +<th><em>Rainfall</em> <br /> (<em>inches</em>)</th> +<th><em>Approximate <br /> distance from <br /> sequoias</em> (<em>miles</em>)</th> + <th>(<em>r</em>)</th> + <th>(<em>e</em>)</th> + <th>(<em>r/e</em>)</th> +</tr> +<tr> +<td class="tdl">Sacramento,</td> +<td>1861-1910</td> +<td>70</td> +<td>19.40</td> +<td>200</td> +<td>+0.398</td> +<td>±0.081</td> +<td>4.9</td> +</tr> +<tr> +<td class="tdl">Colfax,</td> +<td>1871-1909</td> +<td>2400</td> +<td>48.94</td> +<td>200</td> +<td>+0.122</td> +<td>±0.113</td> +<td>1.1</td> +</tr> +<tr><td class="tdl">Summit,</td> +<td>1871-1909</td> +<td>7000</td> +<td>48.07</td> +<td>200</td> +<td>+0.148</td> +<td>±0.113</td> +<td>1.3</td> +</tr> +<tr> +<td class="tdl">Truckee,</td> +<td>1871-1909</td> +<td>5800</td> +<td>27.12</td> +<td>200</td> +<td>+0.300</td> +<td>±0.105</td> +<td>2.9</td> +</tr> +<tr> +<td class="tdl">Boca,</td> +<td>1871-1909</td> +<td>5500</td> +<td>20.34</td> +<td>200</td> +<td>+0.604</td> +<td>±0.076</td> +<td>8.0</td> +</tr> +<tr> +<td class="tdl">Winnemucca,</td> +<td>1871-1909</td> +<td>4300</td> +<td>8.65</td> +<td>300</td> +<td>+0.492</td> +<td>±0.089</td> +<td>5.5</td> +</tr> +</table> + +<!--File: cc105.png Page 81--> +<a class="pagenum break" id="Page_81" title="pg 81"></a> + + + +<p>In the first line of Section A a correlation coefficient +of only -0.056, which is scarcely six-tenths of the probable +error, means that there is no appreciable relation +between the rainfall of a given season and the growth +during the following spring and summer. The roots of +the sequoias probably penetrate so deeply that the rain +and melted snow of the spring months do not sink down +rapidly enough to influence the trees before the growing +season comes to an end. The precipitation of two preceding +seasons, however, has some effect on the trees, as +appears in the second line of Section A, where the correlation +coefficient is +0.288, or 3.2 times the probable +error. When the rainfall of three seasons is taken into +account the coefficient rises to +0.570, or 8.7 times the +probable error, while with four years of rainfall the coefficient +begins to fall off. Thus the growth of these eighteen +sequoias on relatively dry slopes appears to have depended +chiefly on the rainfall of the second and third +preceding rainy seasons. The growth in 1900, for +example, depended largely on the rainfall in the rainy +seasons of 1897-1898 and 1898-1899.</p> + +<p>Section B of the table shows that with 112 trees, growing +chiefly in moist depressions where the water supply +is at a maximum, the correlation between growth and +rainfall, +0.577 for ten years' rainfall, is even higher +than with the dry trees. The seepage of the underground +water is so slow that not until four years' rainfall is +taken into account is the correlation coefficient more than +four times the probable error. When only the trees growing +in moist locations are employed, the coefficient between +<!--File: cc106.png Page 82--> +<a class="pagenum break" id="Page_82" title="pg 82"></a> +tree growth and the rainfall for ten years rises to +the high figure of +0.605, or 9.8 times the probable error, +as appears in Section C. These figures, as well as many +others not here published, make it clear that the curve of +sequoia growth from 1861 to 1910 affords a fairly close +indication of the rainfall at Sacramento, provided allowance +be made for a delay of three to ten years due to the +fact that the moisture in the soil gradually seeps down +the mountain-sides and only reaches the sequoias after a +considerable interval.</p> + +<p>If a rainfall record were available for the place where +the trees actually grow, the relationship would probably +be still closer.</p> + +<p>The record at Fresno, for example, bears out this conclusion +so far as it goes. But as Fresno lies at a low altitude +and its rainfall is of essentially the Sacramento +type, its short record is of less value than that of Sacramento. +The only rainfall records among the Sierras at +high levels, where the rainfall and temperature are approximately +like those of the sequoia region, are found +along the main line of the Southern Pacific railroad. This +runs from Oakland northeastward seventy miles across +the open plain to Sacramento, then another seventy miles, +as the crow flies, through Colfax and over a high pass +in the Sierras at Summit, next twenty miles or so down +through Truckee to Boca, on the edge of the inland basin +of Nevada, and on northeastward another 160 miles to +Winnemucca, where it turns east toward Ogden and Salt +Lake City. Section D of Table 3 shows the correlation +coefficients between the rainfall along the railroad and +the growth of the sequoias. At Sacramento, which lies +fairly open to winds from the Pacific and thus represents +the general climate of central California, the coefficient +is nearly five times the probable error, thus indicating a +<!--File: cc107.png--> +<a class="pagenum break" id="Page_83" title="pg 83"></a> +real relation to sequoia growth. Then among the foothills +of the Sierras at Colfax, the coefficient drops till it is +scarcely larger than the probable error. It rises rapidly, +however, as one advances among the mountains, until at +Boca it attains the high figure of +0.604 or eight times +the probable error, and continues high in the dry area +farther east. In other words the growth of the sequoias +is a good indication of the rainfall where the trees grow +and in the dry region farther east.</p> + +<p>In order to determine the degree to which the sequoia +record represents the rainfall of other regions, let us +select Jerusalem for comparison. The reasons for this +selection are that Jerusalem furnishes the only available +record that satisfies the following necessary conditions: +(1) its record is long enough to be important; (2) it is +located fairly near the latitude of the sequoias, 32°N +versus 37°N; (3) it is located in a similar type of climate +with winter rains and a long dry summer; (4) it lies well +above sea level (2500 feet) and somewhat back from the +seacoast, thus approximating although by no means +duplicating the condition of the sequoias; and (5) it lies +in a region where the evidence of climatic changes during +historic times is strongest. The ideal place for comparison +would be the valley in which grow the cedars of Lebanon. +Those trees resemble the sequoias to an extraordinary +degree, not only in their location, but in their great age. +Some day it will be most interesting to compare the +growth of these two famous groups of old trees.</p> + +<p>The correlation coefficients for the sequoia growth +and the rainfall at Jerusalem are given in Section A, +Table 4. They are so high and so consistent that they +scarcely leave room for doubt that where a hundred or +more sequoias are employed, as in Fig. 5, their curve of +growth affords a good indication of the fluctuations of +climate in western Asia. The high coefficient for the +eleven trees measured by Douglass suggests that where +the number of trees falls as low as ten, as in the part of +Fig. 4 from 710 to 840 B. C., the relation between tree +growth and rainfall is still close even when only one +year's growth is considered. Where the unit is ten years +of growth, as in Figs. 4 and 5, the accuracy of the tree +curve as a measure of rainfall is much greater than when +a single year is used as in Table 4. When the unit is +raised to thirty years, as in the smoothed part of Fig. 4 +previous to 240 B. C., even four trees, as from 960 to +1070, probably give a fair approximation to the general +changes in rainfall, while a single tree prior to 1110 B. C. +gives a rough indication.</p> +<!--File: cc108.png Page 84--> +<a class="pagenum break" id="Page_84" title="pg 84"></a> + +<table id="table_4" class="borders"> +<tr><th colspan="7"><a href="#tables">TABLE 4</a></th></tr> +<tr><th colspan="7">CORRELATION COEFFICIENTS BETWEEN +RAINFALL RECORDS<br />IN CALIFORNIA +AND JERUSALEM</th></tr> +<tr><td colspan="7"> </td></tr> +<tr><td colspan="7" class="tdi2"> +(<i>r</i>)=<i>Correlation coefficient</i><br /> +(<i>e</i>)=<i>Probable error</i><br /> +(<i>r/e</i>)=<i>Ratio of coefficient to probable error</i> +</td></tr> +<tr><td colspan="7"> </td></tr> +<tr><th colspan="7"><span class="smcap">A. Jerusalem Rainfall for 3 Years and Various Groups of +Sequoias</span><sup><a href="#fn27" id="r27">[27]</a></sup> </th></tr> +<tr><th colspan="4"></th><th>(<em>r</em>)</th><th>(<em>e</em>)</th><th>(<em>r/e</em>)</th></tr> +<tr> +<td colspan="4" class="tdl">11 trees measured by Douglass</td> +<td>+0.453</td> +<td>±0.078</td> +<td>5.8</td> +</tr> +<tr> +<td colspan="4" class="tdl">80 trees, moist locations, Groups IA, IIA, IIIA, VA</td><td>+0.500</td> +<td>±0.073</td> +<td>6.8</td> +</tr> +<tr> +<td colspan="4" class="tdl">101 trees, 69 in moist locations, 32 in dry, I, II, III</td> +<td> +0.616</td> +<td>±0.061</td> +<td>10.1</td> +</tr> +<tr> +<td colspan="4" class="tdl">112 trees, 80 in moist locations, 32 in dry, I, II, III, V</td> +<td>+0.675</td> +<td>±0.053</td> +<td>12.7</td> +</tr> +<tr><td colspan="7"> </td></tr> + <tr><th colspan="7"><span class="smcap">B. Rainfall at Jerusalem and at Stations in California and Nevada</span></th></tr> + +<tr><td colspan="3"></td> +<td colspan="2">—— <em>3 years</em> ——</td> +<td colspan="2">—— <em>5 years</em> ——</td> +</tr> +<tr> +<th class="tdl"> </th> +<th><em>Altitude</em><br /> (<em>feet</em>)</th> +<th><em>Years</em></th> +<th>(<em>r</em>)</th> +<th>(<em>r/e</em>)</th> +<th>(<em>r</em>)</th> +<th>(<em>r/e</em>)</th> +</tr> +<tr> +<td class="tdl">Sacramento,</td> +<td>70</td> +<td>1861-1910</td> +<td>+0.386</td> +<td>4.7</td> +<td>+0.352</td> +<td>4.2</td> +</tr> +<tr> +<td class="tdl">Colfax,</td> +<td>2400</td> +<td>1871-1909</td> +<td>+0.311</td> +<td>3.1</td> +<td>+0.308</td> +<td>3.0</td> +</tr> +<tr> +<td class="tdl">Summit,</td> +<td>7000</td> +<td>1871-1909</td> +<td>+0.099</td> +<td>0.9</td> +<td>+0.248</td> +<td>2.3</td> +</tr> +<tr> +<td class="tdl">Truckee,</td> +<td>5800</td> +<td>1871-1909</td> +<td>+0.229</td> +<td>2.2</td> +<td>+0.337</td> +<td>3.3</td> +</tr> +<tr> +<td class="tdl"><sup><a href="#fn28" id="r28">[28]</a></sup>Boca,</td> +<td>5500</td> +<td>1871-1909</td> +<td>+0.482</td> +<td>6.4</td> +<td>+0.617</td> +<td>8.6</td> +</tr> +<tr> +<td class="tdl">Winnemucca,</td> +<td>4300</td> +<td>1871-1909</td> +<td>+0.235</td> +<td>2.2</td> +<td>+0.260</td> +<td>2.4</td> +</tr> +<tr> +<td class="tdl">San Bernardino,</td> +<td>1050</td> +<td>1871-1909</td> +<td>+0.275</td> +<td>2.7</td> +<td>+0.177</td> +<td>1.8</td> +</tr> + +<tr><td colspan="7"> </td></tr> +<tr><th colspan="7"><span class="smcap">C. Rainfall for 3 Years at California and Nevada Stations, 1871-1909</span></th></tr> + +<tr><th colspan="5"></th><th>(<em>r</em>)</th><th>(<em>r/e</em>)</th></tr> +<tr><td colspan="5" class="tdl">Sacramento and San Bernardino</td><td>+0.663</td> <td>10.7</td></tr> +<tr><td colspan="5" class="tdl">San Bernardino and Winnemucca</td><td>+0.291</td> <td>2.8</td></tr> +</table> + +<!--File: cc109.png Page 85--> +<a class="pagenum break" id="Page_85" title="pg 85"></a> + + +<p>Table 4 shows a peculiar feature in the fact that the +correlations of Section A between tree growth and the +rainfall of Jerusalem are decidedly higher than those +between the rainfall in the two regions. Only at Sacramento +and Boca are the rainfall coefficients high enough +to be conclusive. This, however, is not surprising, for +even between Sacramento and San Bernardino, only 400 +miles apart, the correlation coefficient for the rainfall +by three-year periods is only 10.7 times the probable +error, as appears in Section C of Table 4, while between +San Bernardino and Winnemucca 500 miles away, the +corresponding figure drops to 2.8. It must be remembered +that in some respects the growth of the sequoias is +a much better record of rainfall than are the records kept +by man. The human record is based on the amount of +water caught by a little gauge a few inches in diameter. +Every gust of wind detracts from the accuracy of the +record; a mile away the rainfall may be double what it +is at the gauge. Each sequoia, on the other hand, draws +its moisture from an area thousands of times as large as +<!--File: cc110.png Page 86--> +<a class="pagenum break" id="Page_86" title="pg 86"></a> +a rain gauge. Moreover, the trees on which Figs. 4 and 5 +are based were scattered over an area fifty miles long +and several hundred square miles in extent. Hence they +represent the summation of the rainfall over an area +millions of times as large as that of a rain gauge. This +fact and the large correlation coefficients between sequoia +growth and Jerusalem rainfall should be considered in +connection with the fact that all the coefficients between +the rainfall of California and Nevada and that of Jerusalem +are positive. If full records of the complete rainfall +of California and Nevada on the one hand and of the +eastern Mediterranean region on the other were available +for a long period, they would probably agree closely.</p> + +<p>Just how widely the sequoias can be used as a measure +of the climate of the past is not yet certain. In some +regions, as will shortly be explained, the climatic changes +seem to have been of an opposite character from those +of California. In others the Californian or eastern Mediterranean +type of change seems sometimes to prevail but +is not always evident. For example, at Malta the rainfall +today shows a distinct relation to that of Jerusalem and +to the growth of the sequoias. But the correlation coefficient +between the rainfall of eight-year periods at Naples, +a little farther north, and the growth of the sequoias at +the end of the periods is -0.132, or only 1.4 times the +probable error and much too small to be significant. This +is in harmony with the fact that although Naples has +summer droughts, they are not so pronounced as in California +and Palestine, and the prevalence of storms is +much greater. Jerusalem receives only 8 per cent of its +rain in the seven months from April to October, and +Sacramento 13, while Malta receives 31 per cent and +Naples 43. Nevertheless, there is some evidence that in +the past the climatic fluctuations of southern Italy followed +<!--File: cc111.png Page 87--> +<a class="pagenum break" id="Page_87" title="pg 87"></a> +nearly the same course as those of California and +Palestine. This apparent discrepancy seems to be explained +by our previous conclusion that changes of climate +are due largely to a shifting of storm tracks. When +sunspots are numerous the storms which now prevail in +northern Italy seem to be shifted southward and traverse +the Mediterranean to Palestine just as similar storms +are shifted southward in the United States. This perhaps +accounts for the agreement between the sequoia curve +and the agricultural and social history of Rome from +about 400 B. C. to 100 A. D., as explained in <em>World Power +and Evolution</em>. For our present purposes, however, the +main point is that since rainfall records have been kept +the fluctuations of climate indicated by the growth of the +sequoias have agreed closely with fluctuations in the +rainfall of the eastern Mediterranean region. Presumably +the same was true in the past. In that case, the sequoia +curve not only is a good indication of climatic changes or +pulsations in regions of similar climate, but may serve +as a guide to coincident but different changes in regions +of other types.</p> + +<p>An enormous body of other evidence points to the same +conclusion. It indicates that while the average climate +of the present is drier than that of the past in regions +having the Mediterranean type of winter rains and +summer droughts, there have been pronounced pulsations +during historic times so that at certain times there has +actually been greater aridity than at present. This conclusion +is so important that it seems advisable to examine +the only important arguments that have been raised +against it, especially against the idea that the general +rainfall of the eastern Mediterranean was greater in the +historic past than at present. The first objection is the +unquestionable fact that droughts and famines have +<!--File: cc112.png Page 88--> +<a class="pagenum break" id="Page_88" title="pg 88"></a> +occurred at periods which seem on other evidence to have +been moister than the present. This argument has been +much used, but it seems to have little force. If the rainfall +of a given region averages thirty inches and varies +from fifteen to forty-five, a famine will ensue if the rainfall +drops for a few years to the lower limit and does not +rise much above twenty for a few years. If the climate of +the place changes during the course of centuries, so that +the rainfall averages only twenty inches, and ranges +from seven to thirty-five, famine will again ensue if the +rainfall remains near ten inches for a few years. The +ravages of the first famine might be as bad as those of +the second. They might even be worse, because when the +rainfall is larger the population is likely to be greater +and the distress due to scarcity of food would affect a +larger number of people. Hence historic records of +famines and droughts do not indicate that the climate +was either drier or moister than at present. They merely +show that at the time in question the climate was drier +than the normal for that particular period.</p> + +<p>The second objection is that deserts existed in the past +much as at present. This is not a real objection, however, +for, as we shall see more fully, some parts of the world +suffer one kind of change and others quite the opposite. +Moreover, deserts have always existed, and when we talk +of a change in their climate we merely mean that their +boundaries have shifted. A concrete example of the mistaken +use of ancient dryness as proof of climatic uniformity +is illustrated by the march of Alexander from +India to Mesopotamia. Hedin gives an excellent presentation +of the case in the second volume of his <em>Overland +to India</em>. He shows conclusively that Alexander's army +suffered terribly from lack of water and provisions. This +certainly proves that the climate was dry, but it by no +<!--File: cc113.png Page 89--> +<a class="pagenum break" id="Page_89" title="pg 89"></a> +means indicates that there has been no change from the +past to the present. We do not know whether Alexander's +march took place during an especially dry or an especially +wet year. In a desert region like Makran, in +southern Persia and Beluchistan, where the chief difficulties +occurred, the rainfall varies greatly from year to +year. We have no records from Makran, but the conditions +there are closely similar to those of southern +Arizona and New Mexico. In 1885 and 1905 the rainfall +for five stations in that region was as follows:</p> + +<table class="borders"> +<tr> +<th class="tdl"></th> +<th><em>1885</em></th> +<th><em>1905</em></th> +<th><em>Mean rainfall during<br /> period since<br /> observations began</em></th> +</tr> +<tr><td class="tdl">Yuma, Arizona,</td> +<td>2.72</td> +<td>11.41</td> +<td>3.13</td> +</tr> +<tr> +<td class="tdl">Phoenix, Arizona,</td> +<td>3.77</td> +<td>19.73</td> +<td>7.27</td> +</tr> +<tr> +<td class="tdl">Tucson, Arizona,</td> +<td>5.26</td> +<td>24.17</td> +<td>11.66</td> +</tr> +<tr> +<td class="tdl">Lordsburg, New Mexico,</td> +<td>3.99</td> +<td>19.50</td> +<td>8.62</td> +</tr> +<tr> +<td class="tdl">El Paso, Texas (on New Mexico border),</td> +<td>7.31</td> +<td>17.80</td> +<td>9.06</td> +</tr> +<tr> +<th>Average,</th> +<th class="thl">4.61</th> +<th class="thl">18.52 </th> +<th class="thl">7.95</th> +</tr> +</table> + +<p>These stations are distributed over an area nearly 500 +miles east and west. Manifestly a traveler who spent the +year 1885 in that region would have had much more difficulty +in finding water and forage than one who traveled +in the same places in 1905. During 1885 the rainfall was +42 per cent less than the average, and during 1905 it was +134 per cent more than the average. Let us suppose, for +the sake of argument, that the average rainfall of southeastern +Persia is six inches today and was ten inches in +the days of Alexander. If the rainfall from year to year +varied as much in the past in Persia as it does now in +New Mexico and Arizona, the rainfall during an ancient +<!--File: cc114.png Page 90--> +<a class="pagenum break" id="Page_90" title="pg 90"></a> +dry year, corresponding in character to 1885, would have +been about 5.75 inches. On the other hand, if we suppose +that the rainfall then averaged less than at present,—let +us say four inches,—a wet year corresponding to 1905 in +the American deserts might have had a rainfall of about +ten inches. This being the case, it is clear that our estimate +of what Alexander's march shows as to climate +must depend largely on whether 325 B. C. was a wet year +or a dry year. Inasmuch as we know nothing about this, +we must fall back on the fact that a large army accomplished +a journey in a place where today even a small +caravan usually finds great difficulty in procuring forage +and water. Moreover, elephants were taken 180 miles +across what is now an almost waterless desert, and yet +the old historians make no comment on such a feat which +today would be practically impossible. These things seem +more in harmony with a change of climate than with +uniformity. Nevertheless, it is not safe to place much +reliance on them except when they are taken in conjunction +with other evidence, such as the numerous ruins, +which show that Makran was once far more densely +populated than now seems possible. Taken by itself, such +incidents as Alexander's march cannot safely be used +either as an argument for or against changes of climate.</p> + +<p>The third and strongest objection to any hypothesis +of climatic changes during historic times is based on +vegetation. The whole question is admirably set forth by +J. W. Gregory,<sup><a href="#fn29" id="r29">[29]</a> +</sup> who gives not only his own results, but +those of the ablest scholars who have preceded him. His +conclusions are important because they represent one of +the few cases where a definite statistical attempt has been +made to prove the exact condition of the climate of the +<!--File: cc115.png Page 91--> +<a class="pagenum break" id="Page_91" title="pg 91"></a> +past. After stating various less important reasons for +believing that the climate of Palestine has not changed, +he discusses vegetation. The following quotation indicates +his line of thought. A sentence near the beginning +is italicized in order to call attention to the importance +which Gregory and others lay on this particular kind of +evidence:</p> + +<blockquote> +<p>Some more certain test is necessary than the general conclusions +which can be based upon the historical and geographical +evidence of the Bible. In the absence of rain gauge and thermometric +records, <em>the most precise test of climate is given by the +vegetation; and fortunately the palm affords a very delicate test +of the past climate of Palestine and the eastern Mediterranean</em>.... +The date palm has three limits of growth which are determined +by temperature; thus it does not reach full maturity or +produce ripe fruit of good quality below the mean annual temperature +of 69°F. The isothermal of 69° crosses southern Algeria +near Biskra; it touches the northern coasts of Cyrenaica near +Derna and passes Egypt near the mouth of the Nile, and then +bends northward along the coast lands of Palestine.</p> + +<p>To the north of this line the date palm grows and produces +fruit, which only ripens occasionally, and its quality deteriorates +as the temperature falls below 69°. Between the isotherms of +68° and 64°, limits which include northern Algeria, most of +Sicily, Malta, the southern parts of Greece and northern Syria, +the dates produced are so unripe that they are not edible. In the +next cooler zone, north of the isotherm of 62°, which enters +Europe in southwestern Portugal, passes through Sardinia, +enters Italy near Naples, crosses northern Greece and Asia +Minor to the east of Smyrna, the date palm is grown only for +its foliage, since it does not fruit.</p> + +<p>Hence at Benghazi, on the north African coast, the date palm +is fertile, but produces fruit of poor quality. In Sicily and at +Algiers the fruit ripens occasionally and at Rome and Nice the +palm is grown only as an ornamental tree.<a class="pagenum break" id="Page_92" title="pg 92"></a></p> + +<p>The date palm therefore affords a test of variations in mean +annual temperature of three grades between 62° and 69°.</p> + +<p>This test shows that the mean annual temperature of Palestine +has not altered since Old Testament times. The palm tree now +grows dates on the coast of Palestine and in the deep depression +around the Dead Sea, but it does not produce fruit on the highlands +of Judea. Its distribution in ancient times, as far as we +can judge from the Bible, was exactly the same. It grew at +"Jericho, the city of palm trees" (Deut. xxxiv: 3 and 2 Chron. +xxviii: 15), and at Engedi, on the western shore of the Dead +Sea (2 Chron. xx: 2; Sirach xxiv: 14); and though the palm +does not still live at Jericho—the last apparently died in 1838—its +disappearance must be due to neglect, for the only climatic +change that would explain it would be an increase in cold or +moisture. In olden times the date palm certainly grew on the +highlands of Palestine; but apparently it never produced fruit +there, for the Bible references to the palm are to its beauty and +erect growth: "The righteous shall flourish like the palm" (Ps. +xcii: 12); "They are upright as the palm tree" (Jer. x: 5); +"Thy stature is like to a palm tree" (Cant. vii: 7). It is used as +a symbol of victory (Rev. vii: 9), but never praised as a source +of food.</p> + +<p>Dates are not once referred to in the text of the Bible, but +according to the marginal notes the word translated "honey" in +2 Chron. xxxi: 5 may mean dates....</p> + +<p>It appears, therefore, that the date palm had essentially the +same distribution in Palestine in Old Testament times as it has +now; and hence we may infer that the mean temperature was +then the same as now. If the climate had been moister and cooler, +the date could not have flourished at Jericho. If it had been +warmer, the palms would have grown freely at higher levels and +Jericho would not have held its distinction as <em>the</em> city of palm +trees.<sup><a href="#fn30" id="r30">[30]</a></sup></p> +</blockquote> + +<p>In the main Gregory's conclusions seem to be well +grounded, although even according to his data a change +<!--File: cc117.png Page 93--> +<a class="pagenum break" id="Page_93" title="pg 93"></a> +of 2° or 3° in mean temperature would be perfectly +feasible. It will be noticed, however, that they apply to +temperature and not to rainfall. They merely prove that +two thousand years ago the mean temperature of Palestine +and the neighboring regions was not appreciably different +from what it is today. This, however, is in no sense +out of harmony with the hypothesis of climatic pulsations. +Students of glaciation believe that during the last +glacial epoch the mean temperature of the earth as a +whole was only 5° or 6°C. lower than at present. If the +difference between the climate of today and of the time of +Christ is a tenth as great as the difference between the +climate of today and that which prevailed at the culmination +of the last glacial epoch, the change in two thousand +years has been of large dimensions. Yet this would require +a rise of only half a degree Centigrade in the mean +temperature of Palestine. Manifestly, so slight a change +would scarcely be detectable in the vegetation.</p> + +<p>The slightness of changes in mean temperature as compared +with changes in rainfall may be judged from a +comparison of wet and dry years in various regions. For +example, at Berlin between 1866 and 1905 the ten most +rainy years had an average precipitation of 670 mm. and +a mean temperature of 9.15°C. On the other hand, the ten +years of least rainfall had an average of 483 mm. and a +mean temperature of 9.35°. In other words, a difference +of 137 mm., or 39 per cent, in rainfall was accompanied +by a difference of only 0.2°C. in temperature. Such contrasts +between the variability of mean rainfall and mean +temperature are observable not only when individual +years are selected, but when much longer periods are +taken. For instance, in the western Gulf region of the +United States the two inland stations of Vicksburg, Mississippi, +and Shreveport, Louisiana, and the two maritime +<!--File: cc118.png Page 94--> +<a class="pagenum break" id="Page_94" title="pg 94"></a> +stations of New Orleans, Louisiana, and Galveston, +Texas, lie at the margins of an area about 400 miles long. +During the ten years from 1875 to 1884 their rainfall +averaged 59.4 inches,<sup><a href="#fn31" id="r31">[31]</a></sup> while during the ten years from +1890 to 1899 it averaged only 42.4 inches. Even in a +region so well watered as the Gulf States, such a change—40 +per cent more in the first decade than in the second—is +important, and in drier regions it would have a great +effect on habitability. Yet in spite of the magnitude of +the change the mean temperature was not appreciably +different, the average for the four stations being 67.36°F. +during the more rainy decade and 66.94°F. during the +less rainy decade—a difference of only 0.42°F. It is worth +noticing that in this case the wetter period was also the +warmer, whereas in Berlin it was the cooler. This is +probably because a large part of the moisture of the Gulf +States is brought by winds having a southerly component. +Similar relationships are apparent in other +places. We select Jerusalem because we have been discussing +Palestine. At the time of writing, the data available +in the <em>Quarterly Journal of the Palestine Exploration +Fund</em> cover the years from 1882-1899 and 1903-1909. +Among these twenty-five years the thirteen which had +most rain had an average of 34.1 inches and a temperature +of 62.04°F. The twelve with least rain had 24.4 inches +and a temperature of 62.44°. A difference of 40 per cent +in rainfall was accompanied by a difference of only +0.4°F. in temperature.</p> + +<p>The facts set forth in the preceding paragraphs seem +to show that extensive changes in precipitation and +storminess can take place without appreciable changes of +mean temperature. If such changed conditions can persist +<!--File: cc119.png Page 95--> +<a class="pagenum break" id="Page_95" title="pg 95"></a> +for ten years, as in one of our examples, there is no +logical reason why they cannot persist for a hundred or +a thousand. The evidence of changes in climate during the +historic period seems to suggest changes in precipitation +much more than in temperature. Hence the strongest of +all the arguments against historic changes of climate +seems to be of relatively little weight, and the pulsatory +hypothesis seems to be in accord with all the known facts.</p> + +<p>Before the true nature of climatic changes, whether +historic or geologic, can be rightly understood, another +point needs emphasis. When the pulsatory hypothesis +was first framed, it fell into the same error as the hypotheses +of uniformity and of progressive change—that +is, the assumption was made that the whole world is +either growing drier or moister with each pulsation. A +study of the ruins of Yucatan, in 1912, and of Guatemala, +in 1913, as is explained in <em>The Climatic Factor</em>, has led to +the conclusion that the climate of those regions has +changed in the opposite way from the changes which +appear to have taken place in the desert regions farther +south. These Maya ruins in Central America are in many +cases located in regions of such heavy rainfall, such dense +forests, and such malignant fevers that habitation is now +practically impossible. The land cannot be cultivated +except in especially favorable places. The people are +terribly weakened by disease and are among the lowest +in Central America. Only a hundred miles from the unhealthful +forests we find healthful areas, such as the +coasts of Yucatan and the plateau of Guatemala. Here +the vast majority of the population is gathered, the large +towns are located, and the only progressive people are +found. Nevertheless, in the past the region of the forests +was the home of by far the most progressive people who +are ever known to have lived in America previous to the +<!--File: cc120.png Page 96--> +<a class="pagenum break" id="Page_96" title="pg 96"></a> +days of Columbus. They alone brought to high perfection +the art of sculpture; they were the only American people +who invented the art of writing. It seems scarcely credible +that such a people would have lived in the worst possible +habitat when far more favored regions were close +at hand. Therefore it seems as if the climate of eastern +Guatemala and Yucatan must have been relatively dry +at some past time. The Maya chronology and traditions +indicate that this was probably at the same time when +moister conditions apparently prevailed in the subarid +or desert portions of the United States and Asia. Fig. 3 +shows that today at times of many sunspots there is +a similar opposition between a tendency toward storminess +and rain in subtropical regions and toward aridity +in low latitudes near the heat equator.</p> + +<p>Thus our final conclusion is that during historic times +there have been pulsatory changes of climate. These +changes have been of the same type in regions having +similar kinds of climate, but of different and sometimes +opposite types in places having diverse climates. As to +the cause of the pulsations, they cannot have been due to +the precession of the equinoxes nor apparently to any +allied astronomical cause, for the time intervals are too +short and too irregular. They cannot have been due to +changes in the percentage of carbon dioxide in the atmosphere, +for not even the strongest believers in the climatic +efficacy of that gas hold that its amount could fluctuate in +any such violent way as would be necessary to explain +the pulsations shown in the California curve of tree +growth. Volcanic activity seems more probable as at least +a partial cause, and it would be worth while to investigate +the matter more fully. Nevertheless, it can apparently +be only a minor cause. In the first place, the main effect +of a cloud of dust is to alter the temperature, but +<!--File: cc121.png Page 97--> +<a class="pagenum break" id="Page_97" title="pg 97"></a> +Gregory's summary of the palm and the vine shows that +variations in temperature are apparently of very slight +importance during historic times. Again, ruins on the +bottoms of enclosed salt lakes, old beaches now under the +water, and signs of irrigation ditches where none are now +needed indicate a climate drier than the present. Volcanic +dust, however, cannot account for such a condition, +for at present the air seems to be practically free +from such dust for long periods. Thus we now experience +the greatest extreme which the volcanic hypothesis permits +in one direction, but there have been greater extremes +in the same direction. The thermal solar hypothesis +is likewise unable to explain the observed phenomena, +for neither it nor the volcanic hypothesis offers any explanation +of why the climate varies in one way in Mediterranean +climates and in an opposite way in regions +near the heat equator.</p> + +This leaves the cyclonic hypothesis. It seems to fit the +facts, for variations in cyclonic storms cause some +regions to be moister and others drier than usual. At the +same time the variations in temperature are slight, and +are apparently different in different regions, some places +growing warm when others grow cool. In the next chapter +we shall study this matter more fully, for it can best +be appreciated by examining the course of events in a +specific century. +<!--File: cc122.png Page 98--> +<a class="pagenum break" id="Page_98" title="pg 98"></a> + + + + +<h2 id="Chapt_VI"><a href="#content">CHAPTER VI</a></h2> + +<h3>THE CLIMATIC STRESS OF THE FOURTEENTH +CENTURY</h3> + + +<p>In order to give concreteness to our picture of the +climatic pulsations of historic times let us take a +specific period and see how its changes of climate +were distributed over the globe and how they are related +to the little changes which now take place in the sunspot +cycle. We will take the fourteenth century of the Christian +era, especially the first half. This period is chosen +because it is the last and hence the best known of the +times when the climate of the earth seems to have taken +a considerable swing toward the conditions which now +prevail when the sun is most active, and which, if intensified, +would apparently lead to glaciation. It has already +been discussed in <em>World Power and Evolution</em>, but its +importance and the fact that new evidence is constantly +coming to light warrant a fuller discussion.</p> + +<p>To begin with Europe; according to the careful account +of Pettersson<sup><a href="#fn32" id="r32">[32]</a></sup> the fourteenth century shows</p> + +<blockquote> +<p>a record of extreme climatic variations. In the cold winters the +rivers Rhine, Danube, Thames, and Po were frozen for weeks +and months. On these cold winters there followed violent floods, +so that the rivers mentioned inundated their valleys. Such floods +are recorded in 55 summers in the 14th century. There is, of +<!--File: cc123.png Page 99--> +<a class="pagenum break" id="Page_99" title="pg 99"></a> +course, nothing astonishing in the fact that the inundations of +the great rivers of Europe were more devastating 600 to 700 +years ago than in our days, when the flow of the rivers has been +regulated by canals, locks, etc.; but still the inundations in the +13th and 14th centuries must have surpassed everything of that +kind which has occurred since then. In 1342 the waters of the +Rhine rose so high that they inundated the city of Mayence and +the Cathedral "usque ad cingulum hominis." The walls of +Cologne were flooded so that they could be passed by boats in +July. This occurred also in 1374 in the midst of the month of +February, which is of course an unusual season for disasters of +the kind. Again in other years the drought was so intense that +the same rivers, the Danube, Rhine, and others, nearly dried up, +and the Rhine could be forded at Cologne. This happened at least +twice in the same century. There is one exceptional summer of +such evil record that centuries afterwards it was spoken of as +"the old hot summer of 1357."</p> +</blockquote> + +<p>Pettersson goes on to speak of two oceanic phenomena +on which the old chronicles lay greater stress than on +all others:</p> + +<blockquote> +<p>The first [is] the great storm-floods on the coast of the North +Sea and the Baltic, which occurred so frequently that not less +than nineteen floods of a destructiveness unparalleled in later +times are recorded from the 14th century. The coastline of the +North Sea was completely altered by these floods. Thus on +January 16, 1300, half of the island Heligoland and many other +islands were engulfed by the sea. The same fate overtook the +island of Borkum, torn into several islands by the storm-flood of +January 16, which remoulded the Frisian Islands into their +present shape, when also Wendingstadt, on the island of Sylt, +and Thiryu parishes were engulfed. This flood is known under +the name of "the great man-drowning." The coasts of the Baltic +also were exposed to storm-floods of unparalleled violence. On +November 1, 1304, the island of Ruden was torn asunder from +Rugen by the force of the waves. Time does not allow me to +dwell upon individual disasters of this kind, but it will be well +<!--File: cc124.png Page 100--> +<a class="pagenum break" id="Page_100" title="pg 100"></a> +to note that of the nineteen great floods on record eighteen +occurred in the cold season between the autumnal and vernal +equinoxes.</p> + +<p>The second remarkable phenomenon mentioned by the chronicles +is the freezing of the entire Baltic, which occurred many +times during the cold winters of these centuries. On such occasions +it was possible to travel with carriages over the ice from +Sweden to Bornholm and from Denmark to the German coast +(Lubeck), and in some cases even from Gotland to the coast of +Estland.</p></blockquote> + +<p>Norlind<sup><a href="#fn33" id="r33">[33]</a></sup> says that "the only authentic accounts" of +the complete freezing of the Baltic in the neighborhood +of the Kattegat are in the years 1296, 1306, 1323, and +1408. Of these 1296 is "much the most uncertain," while +1323 was the coldest year ever recorded, as appears from +the fact that horses and sleighs crossed regularly from +Sweden to Germany on the ice.</p> + +<p>Not only central Europe and the shores of the North +Sea were marked by climatic stress during the fourteenth +century, but Scandinavia also suffered. As Pettersson +puts it:</p> + +<blockquote> +<p>On examining the historic (data) from the last centuries of +the Middle Ages, Dr. Bull of Christiania has come to the conclusion +that the decay of the Norwegian kingdom was not so +much a consequence of the political conditions at that time, as +of the frequent failures of the harvest so that corn [wheat] for +bread had to be imported from Lübeck, Rostock, Wismar and so +forth. The Hansa Union undertook the importation and obtained +political power by its economic influence. The Norwegian +land-owners were forced to lower their rents. The population +decreased and became impoverished. The revenue sank 60 to 70 +per cent. Even the income from Church property decreased. +<!--File: cc125.png Page 101--> +<a class="pagenum break" id="Page_101" title="pg 101"></a> +In 1367 corn was imported from Lübeck to a value of one-half +million kroner. The trade balance inclined to the disadvantage +of Norway whose sole article of export at that time was +dried fish. (The production of fish increased enormously in the +Baltic regions off south Sweden because of the same changes +which were influencing the lands, but this did not benefit Norway.) +Dr. Bull draws a comparison with the conditions described +in the Sagas when Nordland [at the Arctic Circle] produced +enough corn to feed the inhabitants of the country. At the time +of Asbjörn Selsbane the chieftains in Trondhenäs [still farther +north in latitude 69°] grew so much corn that they did not need +to go southward to buy corn unless three successive years of +dearth had occurred. The province of Trondheim exported wheat +to Iceland and so forth. Probably the turbulent political state +of Scandinavia at the end of the Middle Ages was in a great +measure due to unfavorable climatic conditions, which lowered +the standard of life, and not entirely to misgovernment and +political strife as has hitherto been taken for granted.</p> +</blockquote> + +<p>During this same unfortunate first half of the fourteenth +century England also suffered from conditions +which, if sufficiently intensified, might be those of a glacial +period. According to Thorwald Rogers<sup><a href="#fn34" id="r34">[34]</a></sup> the severest +famine ever experienced in England was that of 1315-1316, +and the next worst was in 1321. In fact, from 1308 +to 1322 great scarcity of food prevailed most of the time. +Other famines of less severity occurred in 1351 and 1369. +"The same cause was at work in all these cases," says +Rogers, "incessant rain, and cold, stormy summers. It +is said that the inclemency of the seasons affected the +cattle, and that numbers perished from disease and +want." After the bad harvest of 1315 the price of wheat, +which was already high, rose rapidly, and in May, 1316, +was about five times the average. For a year or more +thereafter it remained at three or four times the ordinary +<!--File: cc126.png Page 102--> +<a class="pagenum break" id="Page_102" title="pg 102"></a> +level. The severity of the famine may be judged from the +fact that previous to the Great War the most notable +scarcity of wheat in modern England and the highest +relative price was in December, 1800. At that time wheat +cost nearly three times the usual amount, instead of five +as in 1316. During the famine of the early fourteenth century +"it is said that people were reduced to subsist upon +roots, upon horses and dogs, and stories are told of even +more terrible acts by reason of the extreme famine." The +number of deaths was so great that the price of labor +suffered a permanent rise of at least 10 per cent. There +simply were not people enough left among the peasants +to do the work demanded by the more prosperous class +who had not suffered so much.</p> + +<p>After the famine came drought. The year 1325 appears +to have been peculiarly dry, and 1331, 1344, 1362, 1374, +and 1377 were also dry. In general these conditions do +little harm in England. They are of interest chiefly as +showing how excessive rain and drought are apt to +succeed one another.</p> + +<p>These facts regarding northern and central Europe +during the fourteenth century are particularly significant +when compared with the conclusions which we have +drawn in <em>Earth and Sun</em> from the growth of trees in +Germany and from the distribution of storms. A careful +study of all the facts shows that we are dealing with two +distinct types of phenomena. In the first place, the climate +of central Europe seems to have been peculiarly continental +during the fourteenth century. The winters were +so cold that the rivers froze, and the summers were so +wet that there were floods every other year or oftener. +This seems to be merely an intensification of the conditions +which prevail at the present time during periods of +many sunspots, as indicated by the growth of trees at +<!--File: cc127.png Page 103--> +<a class="pagenum break" id="Page_103" title="pg 103"></a> +Eberswalde in Germany and by the number of storms in +winter as compared with summer. The prevalence of +droughts, especially in the spring, is also not inconsistent +with the existence of floods at other seasons, for one of +the chief characteristics of a continental climate is that +the variations from one season to another are more +marked than in oceanic climates. Even the summer +droughts are typically continental, for when continental +conditions prevail, the difference between the same season +in different years is extreme, as is well illustrated in +Kansas. It must always be remembered that what causes +famine is not so much absolute dryness as a temporary +diminution of the rainfall.</p> + +<p>The second type of phenomena is peculiarly oceanic in +character. It consists of two parts, both of which are +precisely what would be expected if a highly continental +climate prevailed over the land. In the first place, at certain +times the cold area of high pressure, which is the +predominating characteristic of a continent during the +winter, apparently spread out over the neighboring +oceans. Under such conditions an inland sea, such as the +Baltic, would be frozen, so that horses could cross the ice +even in the Far West. In the second place, because of the +unusually high pressure over the continent, the barometric +gradients apparently became intensified. Hence at +the margin of the continental high-pressure area the +winds were unusually strong and the storms of corresponding +severity. Some of these storms may have +passed entirely along oceanic tracks, while others invaded +the borders of the land, and gave rise to the floods +and to the wearing away of the coast described by +Pettersson.</p> + +<p>Turning now to the east of Europe, Brückner's<sup><a href="#fn35" id="r35">[35]</a></sup> study +<!--File: cc128.png--> +<a class="pagenum break" id="Page_104" title="pg 104"></a> +of the Caspian Sea shows that that region as well as +western Europe was subject to great climatic vicissitudes +in the first half of the fourteenth century. In 1306-1307 +the Caspian Sea, after rising rapidly for several years, +stood thirty-seven feet above the present level and it +probably rose still higher during the succeeding decades. +At least it remained at a high level, for Hamdulla, the +Persian, tells us that in 1325 a place called Aboskun was +under water.<sup><a href="#fn36" id="r36">[36]</a></sup></p> + +<p>Still further east the inland lake of Lop Nor also rose +at about this time. According to a Chinese account the +Dragon Town on the shore of Lop Nor was destroyed by +a flood. From Himley's translation it appears that the +level of the lake rose so as to overwhelm the city completely. +This would necessitate the expansion of the lake +to a point eighty miles east of Lulan, and fully fifty from +the present eastern end of the Kara Koshun marsh. The +water would have to rise nearly, or quite, to a strand +which is now clearly visible at a height of twelve feet +above the modern lake or marsh.</p> + +<p>In India the fourteenth century was characterized by +what appears to have been the most disastrous drought +in all history. Apparently the decrease in rainfall here +was as striking as the increase in other parts of the +world. No statistics are available but we are told that in +the great famine which began in 1344 even the Mogul +emperor was unable to obtain the necessaries of life for +his household. No rain worth mentioning fell for years. +In some places the famine lasted three or four years, and +in some twelve, and entire cities were left without an inhabitant. +In a later famine, 1769-1770, which occurred in +Bengal shortly after the foundation of British rule in +<!--File: cc129.png Page 105--> +<a class="pagenum break" id="Page_105" title="pg 105"></a> +India, but while the native officials were still in power, +a third of the population, or ten out of thirty millions, +perished. The famine in the first half of the fourteenth +century seems to have been far worse. These Indian +famines were apparently due to weak summer monsoons +caused presumably by the failure of central Asia to warm +up as much as usual. The heavier snowfall, and the +greater cloudiness of the summer there, which probably +accompanied increased storminess, may have been the +reason.</p> + +<p>The New World as well as the Old appears to have +been in a state of climatic stress during the first half of +the fourteenth century. According to Pettersson, Greenland +furnishes an example of this. At first the inhabitants +of that northland were fairly prosperous and were able +to approach from Iceland without much hindrance from +the ice. Today the North Atlantic Ocean northeast of +Iceland is full of drift ice much of the time. The border +of the ice varies from season to season, but in general it +extends westward from Iceland not far from the Arctic +circle and then follows the coast of Greenland southward +to Cape Farewell at the southern tip and around to +the western side for fifty miles or more. Except under +exceptional circumstances a ship cannot approach the +coast until well northward on the comparatively ice-free +west coast. In the old Sagas, however, nothing is said of +ice in this region. The route from Iceland to Greenland +is carefully described. In the earliest times it went from +Iceland a trifle north of west so as to approach the coast +of Greenland after as short an ocean passage as possible. +Then it went down the coast in a region where approach +is now practically impossible because of the ice. At that +time this coast was icy close to the shore, but there is no +sign that navigation was rendered difficult as is now the +<!--File: cc130.png Page 106--> +<a class="pagenum break" id="Page_106" title="pg 106"></a> +case. Today no navigator would think of keeping close +inland. The old route also went <em>north</em> of the island on +which Cape Farewell is located, although the narrow +channel between the island and the mainland is now so +blocked with ice that no modern vessel has ever penetrated +it. By the thirteenth century, however, there appears +to have been a change. In the Kungaspegel or +<em>Kings' Mirror</em>, written at that time, navigators are +warned not to make the east coast too soon on account +of ice, but no new route is recommended in the neighborhood +of Cape Farewell or elsewhere. Finally, however, +at the end of the fourteenth century, nearly 150 years +after the Kungaspegel, the old sailing route was abandoned, +and ships from Iceland sailed directly southwest +to avoid the ice. As Pettersson says:</p> + +<blockquote> +<p>... At the end of the thirteenth and the beginning of the +fourteenth century the European civilization in Greenland was +wiped out by an invasion of the aboriginal population. The colonists +in the Vesterbygd were driven from their homes and +probably migrated to America leaving behind their cattle in the +fields. So they were found by Ivar Bardsson, steward to the +Bishop of Gardar, in his official journey thither in 1342.</p> + +<p>The Eskimo invasion must not be regarded as a common raid. +It was the transmigration of a people, and like other big movements +of this kind [was] impelled by altered conditions of +nature, in this case the alterations of climate caused by [or +which caused?] the advance of the ice. For their hunting and +fishing the Eskimos require an at least partially open arctic +sea. The seal, their principal prey, cannot live where the surface +of the sea is entirely frozen over. The cause of the favorable +conditions in the Viking-age was, according to my hypothesis, +that the ice then melted at a higher latitude in the arctic seas.</p> + +<p>The Eskimos then lived further north in Greenland and +North America. When the climate deteriorated and the sea which +gave them their living was closed by ice the Eskimos had to find +<!--File: cc131.png Page 107--> +<a class="pagenum break" id="Page_107" title="pg 107"></a> +a more suitable neighborhood. This they found in the land +colonized by the Norsemen whom they attacked and finally +annihilated.</p> +</blockquote> + +<p>Finally, far to the south in Yucatan the ancient Maya +civilization made its last flickering effort at about this +time. Not much is known of this but in earlier periods +the history of the Mayas seems to have agreed quite +closely with the fluctuations in climate.<sup><a href="#fn37" id="r37">[37]</a></sup> Among the +Mayas, as we have seen, relatively dry periods were the +times of greatest progress.</p> + +<p>Let us turn now to Fig. 3 once more and compare the +climatic conditions of the fourteenth century with those +of periods of increasing rainfall. Southern England, +Ireland, and Scandinavia, where the crops were ruined +by extensive rain and storms in summer, are places +where storminess and rainfall now increase when sunspots +are numerous. Central Europe and the coasts of the +North Sea, where flood and drought alternated, are regions +which now have relatively less rain when sunspots +increase than when they diminish. However, as appears +from the trees measured by Douglass, the winters become +more continental and hence cooler, thus corresponding to +the cold winters of the fourteenth century when people +walked on the ice from Scandinavia to Denmark. When +such high pressure prevails in the winter, the total rainfall +is diminished, but nevertheless the storms are more +severe than usual, especially in the spring. In southeastern +Europe, the part of the area whence the Caspian +derives its water, appears to have less rainfall during +times of increasing sunspots than when sunspots are few, +but in an equally large area to the south, where the mountains +<!--File: cc132.png Page 108--> +<a class="pagenum break" id="Page_108" title="pg 108"></a> +are higher and the run-off of the rain is more rapid, +the reverse is the case. This seems to mean that a slight +diminution in the water poured in by the Volga would +be more than compensated by the water derived from +Persia and from the Oxus and Jaxartes rivers, which in +the fourteenth century appear to have filled the Sea of +Aral and overflowed in a large stream to the Caspian. +Still farther east in central Asia, so far as the records go, +most of the country receives more rain when sunspots +are many than when they are few, which would agree +with what happened when the Dragon Town was inundated. +In India, on the contrary, there is a large area +where the rainfall diminishes at times of many sunspots, +thus agreeing with the terrible famine from which the +Moguls suffered so severely. In the western hemisphere, +Greenland, Arizona, and California are all parts of the +area where the rain increases with many sunspots, while +Yucatan seems to lie in an area of the opposite type. Thus +all the evidence seems to show that at times of climatic +stress, such as the fourteenth century, the conditions +are essentially the same as those which now prevail at +times of increasing sunspots.</p> + +<p>As to the number of sunspots, there is little evidence +previous to about 1750. Yet that little is both interesting +and important. Although sunspots have been observed +with care in Europe only a little more than three centuries, +the Chinese have records which go back nearly to +the beginning of the Christian era. Of course the records +are far from perfect, for the work was done by individuals +and not by any great organization which continued +the same methods from generation to generation. +The mere fact that a good observer happened to use his +smoked glass to advantage may cause a particular period +to appear to have an unusual number of spots. On the +<!--File: cc133.png Page 109--> +<a class="pagenum break" id="Page_109" title="pg 109"></a> +other hand, the fact that such an observer finds spots +at some times and not at others tends to give a valuable +check on his results, as does the comparison of one +observer's work with that of another. Hence, in spite of +many and obvious defects, most students of the problem +agree that the Chinese record possesses much value, and +that for a thousand years or more it gives a fairly true +idea of the general aspect of the sun. In the Chinese +records the years with many spots fall in groups, as +would be expected, and are sometimes separated by long +intervals. Certain centuries appear to have been marked +by unusual spottedness. The most conspicuous of these +is the fourteenth, when the years 1370 to 1385 were particularly +noteworthy, for spots large enough to be visible +to the naked eye covered the sun much of the time. Hence +Wolf,<sup><a href="#fn38" id="r38">[38]</a></sup> who has made an exhaustive study of the matter, +concludes that there was an absolute maximum of spots +about 1372. While this date is avowedly open to question, +the great abundance of sunspots at that time makes it +probable that it cannot be far wrong. If this is so, it +seems that the great climatic disturbances of which we +have seen evidence in the fourteenth century occurred at +a time when sunspots were increasing, or at least when +solar activity was under some profoundly disturbing influence. +Thus the evidence seems to show not merely that +the climate of historic times has been subject to important +pulsations, but that those pulsations were magnifications +of the little climatic changes which now take +place in sunspot cycles. The past and the present are +apparently a unit except as to the intensity of the +changes.</p> + + +<!--File: cc134.png Page 110--> +<a class="pagenum break" id="Page_110" title="pg 110"></a> + + + + +<h2 id="Chapt_VII"><a href="#content">CHAPTER VII</a></h2> + +<h3>GLACIATION ACCORDING TO THE SOLAR-CYCLONIC +HYPOTHESIS<sup><a href="#fn39" id="r39">[39]</a></sup></h3> + + +<p>The remarkable phenomena of glacial periods +afford perhaps the best available test to which +any climatic hypothesis can be subjected. In this +chapter and the two that follow, we shall apply this test. +Since much more is known about the recent Great Ice +Age, or Pleistocene glaciation, than about the more +ancient glaciations, the problems of the Pleistocene will +receive especial attention. In the present chapter the +oncoming of glaciation and the subsequent disappearance +of the ice will be outlined in the light of what would +be expected according to the solar-cyclonic hypothesis. +Then in the next chapter several problems of especial +climatic significance will be considered, such as the localization +of ice sheets, the succession of severe glacial and +mild inter-glacial epochs, the sudden commencement of +glaciation and the peculiar variations in the height of the +snow line. Other topics to be considered are the occurrence +of pluvial or rainy climates in non-glaciated regions, +and glaciation near sea level in subtropical +latitudes during the Permian and Proterozoic. Then in +Chapter IX we shall consider the development and distribution +of the remarkable deposits of wind-blown material +known as loess.</p> + +<p>Facts not considered at the time of framing an hypothesis +<!--File: cc135.png Page 111--> +<a class="pagenum break" id="Page_111" title="pg 111"></a> +are especially significant in testing it. In this particular +case, the cyclonic hypothesis was framed to explain +the historic changes of climate revealed by a study of +ruins, tree rings, and the terraces of streams and lakes, +without special thought of glaciation or other geologic +changes. Indeed, the hypothesis had reached nearly its +present form before much attention was given to geological +phases of the problem. Nevertheless, it appears +to meet even this severe test.</p> + +<p>According to the solar-cyclonic hypothesis, the Pleistocene +glacial period was inaugurated at a time when certain +terrestrial conditions tended to make the earth +especially favorable for glaciation. How these conditions +arose will be considered later. Here it is enough to state +what they were. Chief among them was the fact that the +continents stood unusually high and were unusually +large. This, however, was not the primary cause of glaciation, +for many of the areas which were soon to be +glaciated were little above sea level. For example, it +seems clear that New England stood less than a thousand +feet higher than now. Indeed, Salisbury<sup><a href="#fn40" id="r40">[40]</a></sup> estimates that +eastern North America in general stood not more than +a few hundred feet higher than now, and W. B. Wright<sup><a href="#fn41" id="r41">[41]</a></sup> +reaches the same conclusion in respect to the British +Isles. Nevertheless, widespread lands, even if they are +not all high, lead to climatic conditions which favor +glaciation. For example, enlarged continents cause low +temperature in high latitudes because they interfere with +the ocean currents that carry heat polewards. Such continents +also cause relatively cold winters, for lands cool +much sooner than does the ocean. Another result is a +<!--File: cc136.png Page 112--> +<a class="pagenum break" id="Page_112" title="pg 112"></a> +diminution of water vapor, not only because cold air +cannot hold much vapor, but also because the oceanic +area from which evaporation takes place is reduced by +the emergence of the continents. Again, when the continents +are extensive the amount of carbonic acid gas in +the atmosphere probably decreases, for the augmented +erosion due to uplift exposes much igneous rock to the +air, and weathering consumes the atmospheric carbon +dioxide. When the supply of water vapor and of atmospheric +carbon dioxide is small, an extreme type of climate +usually prevails. The combined result of all these conditions +is that continental emergence causes the climate to +be somewhat cool and to be marked by relatively great +contrasts from season to season and from latitude to +latitude.</p> + +<p>When the terrestrial conditions thus permitted glaciation, +unusual solar activity is supposed to have greatly +increased the number and severity of storms and to have +altered their location, just as now happens at times of +many sunspots. If such a change in storminess had occurred +when terrestrial conditions were unfavorable for +glaciation, as, for example, when the lands were low and +there were widespread epicontinental seas in middle and +high latitudes, glaciation might not have resulted. In the +Pleistocene, however, terrestrial conditions permitted +glaciation, and therefore the supposed increase in storminess +caused great ice sheets.</p> + +<p>The conditions which prevail at times of increased +storminess have been discussed in detail in <em>Earth and +Sun</em>. Those which apparently brought on glaciation seem +to have acted as follows: In the first place the storminess +lowered the temperature of the earth's surface in several +ways. The most important of these was the rapid upward +convection in the centers of cyclonic storms whereby +<!--File: cc137.png Page 113--> +<a class="pagenum break" id="Page_113" title="pg 113"></a> +abundant heat was carried to high levels where most of it +was radiated away into space. The marked increase in +the number of tropical cyclones which accompanies increased +solar activity was probably important in this +respect. Such cyclones carry vast quantities of heat and +moisture out of the tropics. The moisture, to be sure, +liberates heat upon condensing, but as condensation +occurs above the earth's surface, much of the heat +escapes into space. Another reason for low temperature +was that under the influence of the supposedly numerous +storms of Pleistocene times evaporation over the oceans +must have increased. This is largely because the velocity +of the winds is relatively great when storms are strong +and such winds are powerful agents of evaporation. But +evaporation requires heat, and hence the strong winds +lower the temperature.<sup><a href="#fn42" id="r42">[42]</a></sup></p> + +<p>The second great condition which enabled increased +storminess to bring on glaciation was the location of the +storm tracks. Kullmer's maps, as illustrated in Fig. 2, +suggest that a great increase in solar activity, such as is +postulated in the Pleistocene, might shift the main storm +track poleward even more than it is shifted by the milder +solar changes during the twelve-year sunspot cycle. If +this is so, the main track would tend to cross North +America through the middle of Canada instead of near +the southern border. Thus there would be an increase in +precipitation in about the latitude of the Keewatin and +Labradorean centers of glaciation. From what is known +of storm tracks in Europe, the main increase in the intensity +of storms would probably center in Scandinavia. +Fig. 3 in Chapter V bears this out. That figure, it will be +recalled, shows what happens to precipitation when solar +<!--File: cc138.png Page 114--> +<a class="pagenum break" id="Page_114" title="pg 114"></a> +activity is increasing. A high rate of precipitation is +especially marked in the boreal storm track, that is, in +the northern United States, southern Canada, and northwestern +Europe.</p> + +<p>Another important condition in bringing on glaciation +would be the fact that when storms are numerous the +total precipitation appears to increase in spite of the +slightly lower temperature. This is largely because of the +greater evaporation. The excessive evaporation arises +partly from the rapidity of the winds, as already stated, +and partly from the fact that in areas where the air is +clear the sun would presumably be able to act more effectively +than now. It would do so because at times of abundant +sunspots the sun in our own day has a higher solar +constant than at times of milder activity. Our whole +hypothesis is based on the supposition that what now +happens at times of many sunspots was intensified in +glacial periods.</p> + +<p>A fourth condition which would cause glaciation to +result from great solar activity would be the fact that +the portion of the yearly precipitation falling as snow +would increase, while the proportion of rain would diminish +in the main storm track. This would arise partly because +the storms would be located farther north than +now, and partly because of the diminution in temperature +due to the increased convection. The snow in itself would +still further lower the temperature, for snow is an excellent +reflector of sunlight. The increased cloudiness which +would accompany the more abundant storms would also +cause an unusually great reflection of the sunlight and +still further lower the temperature. Thus at times of +many sunspots a strong tendency toward the accumulation +of snow would arise from the rapid convection and +consequent low temperature, from the northern location +<!--File: cc139.png Page 115--> +<a class="pagenum break" id="Page_115" title="pg 115"></a> +of storms, from the increased evaporation and precipitation, +from the larger percentage of snowy rather than +rainy precipitation, and from the great loss of heat due +to reflection from clouds and snow.</p> + +<p>If events at the beginning of the last glacial period +took place in accordance with the cyclonic hypothesis, as +outlined above, one of the inevitable results would be the +production of snowfields. The places where snow would +accumulate in special quantities would be central Canada, +the Labrador plateau, and Scandinavia, as well as certain +mountain regions. As soon as a snowfield became +somewhat extensive, it would begin to produce striking +climatic alterations in addition to those to which it owed +its origin.<sup><a href="#fn43" id="r43">[43]</a></sup> For example, within a snowfield the summers +remain relatively cold. Hence such a field is likely to be +an area of high pressure at all seasons. The fact that the +snowfield is always a place of relatively high pressure +results in outblowing surface winds except when these +are temporarily overcome by the passage of strong cyclonic +storms. The storms, however, tend to be concentrated +near the margins of the ice throughout the year +instead of following different paths in each of the four +seasons. This is partly because cyclonic lows always +avoid places of high pressure and are thus pushed out +of the areas where permanent snow has accumulated. +On the other hand, at times of many sunspots, as Kullmer +has shown, the main storm track tends to be drawn +<!--File: cc140.png Page 116--> +<a class="pagenum break" id="Page_116" title="pg 116"></a> +poleward, perhaps by electrical conditions. Hence when +a snowfield is present in the north, the lows, instead of +migrating much farther north in summer than in winter, +as they now do, would merely crowd on to the snowfield +a little farther in summer than in winter. Thus the heavy +precipitation which is usual in humid climates near the +centers of lows would take place near the advancing +margin of the snowfield and cause the field to expand +still farther southward.</p> + +<p>The tendency toward the accumulation of snow on the +margins of the snowfields would be intensified not only +by the actual storms themselves, but by other conditions. +For example, the coldness of the snow would tend to +cause prompt condensation of the moisture brought by +the winds that blow toward the storm centers from low +latitudes. Again, in spite of the general dryness of the +air over a snowfield, the lower air contains some moisture +due to evaporation from the snow by day during the +clear sunny weather of anti-cyclones or highs. Where this +is sufficient, the cold surface of the snowfields tends to +produce a frozen fog whenever the snowfield is cooled +by radiation, as happens at night and during the passage +of highs. Such a frozen fog is an effective reflector of +solar radiation. Moreover, because ice has only half the +specific heat of water, and is much more transparent to +heat, such a "radiation fog" composed of ice crystals is +a much less effective retainer of heat than clouds or fog +made of unfrozen water particles. Shallow fogs of this +type are described by several polar expeditions. They +clearly retard the melting of the snow and thus help the +icefield to grow.</p> + +<p>For all these reasons, so long as storminess remained +great, the Pleistocene snowfields, according to the solar +hypothesis, must have deepened and expanded. In due +<!--File: cc141.png Page 117--> +<a class="pagenum break" id="Page_117" title="pg 117"></a> +time some of the snow was converted into glacial ice. +When that occurred, the growth of the snowfield as well +as of the ice cap must have been accelerated by glacial +movement. Under such circumstances, as the ice crowded +southward toward the source of the moisture by which it +grew, the area of high pressure produced by its low +temperature would expand. This would force the storm +track southward in spite of the contrary tendency due to +the sun. When the ice sheet had become very extensive, +the track would be crowded relatively near to the northern +margin of the trade-wind belt. Indeed, the Pleistocene +ice sheets, at the time of their maximum extension, +reached almost as far south as the latitude now marking +the northern limit of the trade-wind belt in summer. +As the storm track with its frequent low pressure and the +subtropical belt with its high pressure were forced nearer +and nearer together, the barometric gradient between +the two presumably became greater, winds became +stronger, and the storms more intense.</p> + +<p>This zonal crowding would be of special importance in +summer, at which time it would also be most pronounced. +In the first place, the storms would be crowded far upon +the ice cap which would then be protected from the sun +by a cover of fog and cloud more fully than at any other +season. Furthermore, the close approach of the trade-wind +belt to the storm belt would result in a great increase +in the amount of moisture drawn from the belt of +evaporation which the trade winds dominate. In the +trade-wind belt, clear skies and high temperature make +evaporation especially rapid. Indeed, in spite of the vast +deserts it is probable that more than three-fourths of the +total evaporation now taking place on the earth occurs +in the belt of trades, an area which includes about one-half +of the earth's surface.</p> +<!--File: cc142.png Page 118--> +<a class="pagenum break" id="Page_118" title="pg 118"></a> + +<p>The agency which could produce this increased drawing +northward of moisture from the trade-wind belt +would be the winds blowing into the lows. According to +the cyclonic hypothesis, many of these lows would be so +strong that they would temporarily break down the subtropical +belt of high pressure which now usually prevails +between the trades and the zone of westerly winds. This +belt is even now often broken by tropical cyclones. If the +storms of more northerly regions temporarily destroyed +the subtropical high-pressure belt, even though they still +remained on its northern side, they would divert part of +the trade winds. Hence the air which now is carried +obliquely equatorward by those winds would be carried +spirally northward into the cyclonic lows. Precipitation +in the storm track on the margin of the relatively cold ice +sheet would thus be much increased, for most winds from +low latitudes carry abundant moisture. Such a diversion +of moisture from low latitudes probably explains the +deficiency of precipitation along the heat equator at +times of solar activity, as shown in Fig. 3. Taken as a +whole, the summer conditions, according to the cyclonic +hypothesis, would be such that increased evaporation in +low latitudes would coöperate with increased storminess, +cloudiness, and fog in higher latitudes to preserve and +increase the accumulation of ice upon the borders of the +ice sheet. The greater the storminess, the more this would +be true and the more the ice sheet would be able to hold +its own against melting in summer. Such a combination +of precipitation and of protection from the sun is especially +important if an ice sheet is to grow.</p> + +<p>The meteorologist needs no geologic evidence that the +storm track was shoved equatorward by the growth of +the ice sheet, for he observes a similar shifting whenever +a winter's snow cap occupies part of the normal storm +<!--File: cc143.png Page 119--> +<a class="pagenum break" id="Page_119" title="pg 119"></a> +tract. The geologist, however, may welcome geologic +evidence that such an extreme shift of the storm track +actually occurred during the Pleistocene. Harmer, in +1901, first pointed out the evidence which was repeated +with approval by Wright of the Ireland Geological Survey +in 1914.<sup><a href="#fn44" id="r44">[44]</a></sup> According to these authorities, numerous +boulders of a distinctive chalk were deposited by Pleistocene +icebergs along the coast of Ireland. Their distribution +shows that at the time of maximum glaciation the +strong winds along the south coast of Ireland were from +the northeast while today they are from the southwest. +Such a reversal could apparently be produced only by a +southward shift of the center of the main storm track +from its present position in northern Ireland, Scotland, +and Norway to a position across northern France, central +Germany, and middle Russia. This would mean that while +now the centers of the lows commonly move northeastward +a short distance north of southern Ireland, they +formerly moved eastward a short distance south of Ireland. +It will be recalled that in the northern hemisphere +the winds spiral into a low counter-clockwise and that +they are strongest near the center. When the centers pass +not far north of a given point, the strong winds therefore +blow from the west or southwest, while when the centers +pass just south of that point, the strong winds come from +the east or northeast.</p> + +<p>In addition to the consequences of the crowding of the +storm track toward the trade-wind belt, several other +conditions presumably operated to favor the growth of +the ice sheet. For example, the lowering of the sea level +by the removal of water to form the snowfields and +glaciers interfered with warm currents. It also increased +the rate of erosion, for it was equivalent to an uplift of +<!--File: cc144.png Page 120--> +<a class="pagenum break" id="Page_120" title="pg 120"></a> +all the land. One consequence of erosion and weathering +was presumably a diminution of the carbon dioxide in +the atmosphere, for although the ice covered perhaps a +tenth of the lands and interfered with carbonation to that +extent, the removal of large quantities of soil by accelerated +erosion on the other nine-tenths perhaps more than +counterbalanced the protective effect of the ice. At the +same time, the general lowering of the temperature of the +ocean as well as the lands increased the ocean's capacity +for carbon dioxide and thus facilitated absorption. At a +temperature of 50°F. water absorbs 32 per cent more +carbon dioxide than at 68°. The high waves produced by +the severe storms must have had a similar effect on a +small scale. Thus the percentage of carbon dioxide in the +atmosphere was presumably diminished. Of less significance +than these changes in the lands and the air, but +perhaps not negligible, was the increased salinity of the +ocean which accompanied the removal of water to form +snow, and the increase of the dissolved mineral load of +the rejuvenated streams. Increased salinity slows up the +deep-sea circulation, as we shall see in a later chapter. +This increases the contrasts from zone to zone.</p> + +<p>At times of great solar activity the agencies mentioned +above would apparently coöperate to cause an advance of +ice sheets into lower latitudes. The degree of solar activity +would have much to do with the final extent of the ice +sheets. Nevertheless, certain terrestrial conditions would +tend to set limits beyond which the ice would not greatly +advance unless the storminess were extraordinarily +severe. The most obvious of these conditions is the location +of oceans and of deserts or semi-arid regions. The +southwestward advance of the European ice sheet and +the southeastward advance of the Labradorean sheet in +America were stopped by the Atlantic. The semi-aridity +<!--File: cc145.png Page 121--> +<a class="pagenum break" id="Page_121" title="pg 121"></a> +of the Great Plains, produced by their position in the lee +of the Rocky Mountains, stopped the advance of the +Keewatin ice sheet toward the southwest. The advance of +the European ice sheet southeast seems to have been +stopped for similar reasons. The cessation of the advance +would be brought about in such an area not alone by the +light precipitation and abundant sunshine, but by the +dryness of the air, and also by the power of dust to absorb +the sun's heat. Much dust would presumably be +drawn in from the dry regions by passing cyclonic storms +and would be scattered over the ice.</p> + +<p>The advance of the ice is also slowed up by a rugged +topography, as among the Appalachians in northern +Pennsylvania. Such a topography besides opposing a +physical obstruction to the movement of the ice provides +bare south-facing slopes which the sun warms effectively. +Such warm slopes are unfavorable to glacial advance. +The rugged topography was perhaps quite as effective as +the altitude of the Appalachians in causing the conspicuous +northward dent in the glacial margin in Pennsylvania. +Where glaciers lie in mountain valleys the advance +beyond a certain point is often interfered with by the +deployment of the ice at the mouths of gorges. Evaporation +and melting are more rapid where a glacier is broad +and thin than where it is narrow and thick, as in a gorge. +Again, where the topography or the location of oceans or +dry areas causes the glacial lobes to be long and narrow, +the elongation of the lobe is apparently checked in several +ways. Toward the end of the lobe, melting and +evaporation increase rapidly because the planetary +westerly winds are more likely to overcome the glacial +winds and sweep across a long, narrow lobe than across +a broad one. As they cross the lobe, they accelerate +evaporation, and probably lessen cloudiness, with a consequent +<!--File: cc146.png Page 122--> +<a class="pagenum break" id="Page_122" title="pg 122"></a> +augmentation of melting. Moreover, although +lows rarely cross a broad ice sheet, they do cross a +narrow lobe. For example, Nansen records that strong +lows occasionally cross the narrow southern part of the +Greenland ice sheet. The longer the lobe, the more likely +it is that lows will cross it, instead of following its margin. +Lows which cross a lobe do not yield so much snow +to the tip as do those which follow the margin. Hence +elongation is retarded and finally stopped even without +a change in the earth's general climate.</p> + +<p>Because of these various reasons the advances of the +ice during the several epochs of a glacial period might +be approximately equal, even if the durations of the +periods of storminess and low temperature were different. +Indeed, they might be sub-equal, even if the periods +differed in intensity as well as length. Differences in the +periods would apparently be manifested less in the extent +of the ice than in the depth of glacial erosion and in +the thickness of the terminal moraines, outwash plains, +and other glacial or glacio-fluvial formations.</p> + +<p>Having completed the consideration of the conditions +leading to the advance of the ice, let us now consider the +condition of North America at the time of maximum +glaciation.<sup><a href="#fn45" id="r45">[45]</a></sup> +Over an area of nearly four million square +miles, occupying practically all the northern half of the +continent and part of the southern half, as appears in +Fig. 6, the surface was a monotonous and almost level +plain of ice covered with snow. When viewed from a +high altitude, all parts except the margins must have +presented a uniformly white and sparkling appearance. +Along the margins, however, except to the north, the +<!--File: cc145.png Page 123--> +<a class="pagenum break" id="Page_123" title="pg 123"></a> +whiteness was irregular, for the view must have included +not only fresh snow, but moving clouds and dirty snow +or ice. Along the borders where melting was in progress +there was presumably more or less spottedness due to +morainal material or glacial débris brought to the surface +by ice shearage and wastage. Along the dry southwestern +border it is also possible that there were numerous +dark spots due to dust blown onto the ice by the +wind.</p> + + +<!--File: cc148.png Page 124--> +<a class="pagenum break" id="Page_124" title="pg 124"></a> + +<img src="images/fig6.png" width="800" height="529" alt="Fig. 6" id="Fig_6" /> +<p class="center"> <em>Fig. 6. Distribution of Pleistocene ice sheets.<br /> +(After Schuchert.</em>)</p> + + +<p>The great white sheet with its ragged border was +roughly circular in form, with its center in central +Canada. Yet there were many departures from a perfectly +circular form. Some were due to the oceans, for, +except in northern Alaska, the ice extended into the +ocean all the way from New Jersey around by the north +to Washington. On the south, topographic conditions +made the margin depart from a simple arc. From New +Jersey to Ohio it swung northward. In the Mississippi +Valley it reached far south; indeed most of the broad +wedge between the Ohio and the Missouri rivers was +occupied by ice. From latitude 37° near the junction of +the Missouri and the Mississippi, however, the ice margin +extended almost due north along the Missouri to central +North Dakota. It then stretched westward to the Rockies. +Farther west lowland glaciation was abundant as far +south as western Washington. In the Rockies, the Cascades, +and the Sierra Nevadas glaciation was common as +far south as Colorado and southern California, respectively, +and snowfields were doubtless extensive enough to +make these ranges ribbons of white. Between these lofty +ranges lay a great unglaciated region, but even in the +Great Basin itself, in spite of its present aridity, certain +ranges carried glaciers, while great lakes expanded +widely.</p> +<!--File: cc149.png Page 125--> +<a class="pagenum break" id="Page_125" title="pg 125"></a> + +<p>In this vast field of snow the glacial ice slowly crept +outward, possibly at an average speed of half a foot a +day, but varying from almost nothing in winter at the +north, to several feet a day in summer at the south.<sup><a href="#fn46" id="r46">[46]</a></sup> The +force which caused the movement was the presence of +the ice piled up not far from the margins. Almost certainly, +however, there was no great dome from the +center in Canada outward, as some early writers assumed. +Such a dome would require that the ice be many +thousands of feet thick near its center. This is impossible +because of the fact that ice is more voluminous than +water (about 9 per cent near the freezing point). Hence +when subjected to sufficient pressure it changes to the +liquid form. As friction and internal heat tend to keep +the bottom of a glacier warm, even in cold regions, the +probabilities are that only under very special conditions +was a continental ice sheet much thicker than about 2500 +feet. In Antarctica, where the temperature is much lower +than was probably attained in the United States, the ice +sheet is nearly level, several expeditions having traveled +hundreds of miles with practically no change in altitude. +In Shackleton's trip almost to the South Pole, he encountered +a general rise of 3000 feet in 1200 miles. Mountains, +however, projected through the ice even near the +pole and the geologists conclude that the ice is not very +thick even at the world's coldest point, the South Pole.</p> + +<p>Along the margin of the ice there were two sorts of +movement, much more rapid than the slow creep of the +ice. One was produced by the outward drift of snow +carried by the outblowing dry winds and the other and +more important was due to the passage of cyclonic +storms. Along the border of the ice sheet, except at the +<!--File: cc150.png Page 126--> +<a class="pagenum break" id="Page_126" title="pg 126"></a> +north, storm presumably closely followed storm. Their +movement, we judge, was relatively slow until near the +southern end of the Mississippi lobe, but when this point +was passed they moved much more rapidly, for then they +could go toward instead of away from the far northern +path which the sun prescribes when solar activity is +great. The storms brought much snow to the icefield, +perhaps sometimes in favored places as much as the hundred +feet a year which is recorded for some winters in the +Sierras at present. Even the unglaciated intermontane +Great Basin presumably received considerable precipitation, +perhaps twice as much as its present scanty +supply. The rainfall was enough to support many lakes, +one of which was ten times as large as Great Salt Lake; +and grass was doubtless abundant upon many slopes +which are now dry and barren. The relatively heavy +precipitation in the Great Basin was probably due primarily +to the increased number of storms, but may also +have been much influenced by their slow eastward movement. +The lows presumably moved slowly in that general +region not only because they were retarded and turned +from their normal path by the cold ice to the east, but +because during the summer the area between the Sierra +snowfields on the west and the Rocky Mountain and Mississippi +Valley snowfields on the east was relatively +warm. Hence it was normally a place of low pressure +and therefore of inblowing winds. Slow-moving lows are +much more effective than fast-moving ones in drawing +moisture northwestward from the Gulf of Mexico, for +they give the moisture more time to move spirally first +northeast, under the influence of the normal southwesterly +winds, then northwest and finally southwest as +it approaches the storm center. In the case of the present +lows, before much moisture-laden air can describe such +<!--File: cc151.png Page 127--> +<a class="pagenum break" id="Page_127" title="pg 127"></a> +a circuit, first eastward and then westward, the storm +center has nearly always moved eastward across the +Rockies and even across the Great Plains. A result of this +is the regular decrease in precipitation northward, northwestward, +and westward from the Gulf of Mexico.</p> + +<p>Along the part of the glacial margins where for more +than 3000 miles the North American ice entered the +Atlantic and the Pacific oceans, myriads of great blocks +broke off and floated away as stately icebergs, to scatter +boulders far over the ocean floor and to melt in warmer +climes. Where the margin lay upon the lands numerous +streams issued from beneath the ice, milk-white with +rock flour, and built up great outwash plains and valley +trains of gravel and sand. Here and there, just beyond +the ice, marginal lakes of strange shapes occupied valleys +which had been dammed by the advancing ice. In many +of them the water level rose until it reached some low +point in the divide and then overflowed, forming rapids +and waterfalls. Indeed, many of the waterfalls of the +eastern United States and Canada were formed in just +this way and not a few streams now occupy courses +through ridges instead of parallel to them, as in pre-glacial +times.</p> + +<p>In the zone to the south of the continental ice sheet, +the plant and animal life of boreal, cool temperate, and +warm temperate regions commingled curiously. Heather +and Arctic willow crowded out elm and oak; musk ox, +hairy mammoth, and marmot contested with deer, chipmunk, +and skunk for a chance to live. Near the ice on +slopes exposed to the cold glacial gales, the immigrant +boreal species were dominant, but not far away in more +protected areas the species that had formerly lived there +held their own. In Europe during the last two advances +of the great ice sheet the caveman also struggled with +<!--File: cc152.png Page 128--> +<a class="pagenum break" id="Page_128" title="pg 128"></a> +fierce animals and a fiercer climate to maintain life in an +area whose habitability had long been decreasing.</p> + +<p>The next step in our history of glaciation is to outline +the disappearance of the ice sheets. When a decrease in +solar activity produced a corresponding decrease in +storminess, several influences presumably combined to +cause the disappearance of the ice. Most of their results +are the reverse of those which brought on glaciation. A +few special aspects, however, some of which have been +discussed in <em>Earth and Sun</em>, ought to be brought to mind. +A diminution in storminess lessens upward convection, +wind velocity, and evaporation, and these changes, if they +occurred, must have united to raise the temperature of +the lower air by reducing the escape of heat. Again a +decrease in the number and intensity of tropical cyclones +presumably lessened the amount of moisture carried into +mid-latitudes, and thus diminished the precipitation. The +diminution of snowfall on the ice sheets when storminess +diminished was probably highly important. The amount +of precipitation on the sheets was presumably lessened +still further by changes in the storminess of middle +latitudes. When storminess diminishes, the lows follow a +less definite path, as Kullmer's maps show, and on the +average a more southerly path. Thus, instead of all the +lows contributing snow to the ice sheet, a large fraction +of the relatively few remaining lows would bring rain to +areas south of the ice sheet. As storminess decreased, the +trades and westerlies probably became steadier, and thus +carried to high latitudes more warm water than when +often interrupted by storms. Steadier southwesterly +winds must have produced a greater movement of atmospheric +as well as oceanic heat to high latitudes. The +warming due to these two causes was probably the chief +reason for the disappearance of the European ice sheet +<!--File: cc153.png Page 129--> +<a class="pagenum break" id="Page_129" title="pg 129"></a> +and of those on the Pacific coast of North America. The +two greater American ice sheets, however, and the +glaciers elsewhere in the lee of high mountain ranges, +probably disappeared chiefly because of lessened precipitation. +If there were no cyclonic storms to draw moisture +northward from the Gulf of Mexico, most of North +America east of the Rocky Mountain barrier would be +arid. Therefore a diminution of storminess would be +particularly effective in causing the disappearance of ice +sheets in these regions.</p> + +<p>That evaporation was an especially important factor +in causing the ice from the Keewatin center to disappear, +is suggested by the relatively small amount of water-sorted +material in its drift. In South Dakota, for example, +less than 10 per cent of the drift is stratified.<sup><a href="#fn47" id="r47">[47]</a></sup> On +the other hand, Salisbury estimates that perhaps a third +of the Labradorean drift in eastern Wisconsin is crudely +stratified, about half of that in New Jersey, and more +than half of the drift in western Europe.</p> + +<p>When the sun's activity began to diminish, all these +conditions, as well as several others, would coöperate to +cause the ice sheets to disappear. Step by step with their +disappearance, the amelioration of the climate would +progress so long as the period of solar inactivity continued +and storms were rare. If the inactivity continued +long enough, it would result in a fairly mild climate in +high latitudes, though so long as the continents were +emergent this mildness would not be of the extreme type. +The inauguration of another cycle of increased disturbance +of the sun, with a marked increase in storminess, +would inaugurate another glacial epoch. Thus a succession +of glacial and inter-glacial epochs might continue so +long as the sun was repeatedly disturbed.</p> +<!--File: cc154.png Page 130--> +<a class="pagenum break" id="Page_130" title="pg 130"></a> + + + + +<h2 id="Chapt_VIII"><a href="#content">CHAPTER VIII</a></h2> + +<h3>SOME PROBLEMS OF GLACIAL PERIODS</h3> + + +<p>Having outlined in general terms the coming of +the ice sheets and their disappearance, we are +now ready to discuss certain problems of compelling +climatic interest. The discussion will be grouped +under five heads: (I) the localization of glaciation; (II) +the sudden coming of glaciation; (III) peculiar variations +in the height of the snow line and of glaciation; +(IV) lakes and other evidences of humidity in unglaciated +regions during the glacial epochs; (V) glaciation +at sea level and in low latitudes in the Permian and +Proterozoic eras. The discussion of perhaps the most +difficult of all climatic problems of glaciation, that of the +succession of cold glacial and mild inter-glacial epochs, +has been postponed to the next to the final chapter of this +book. It cannot be properly considered until we take up +the history of solar disturbances.</p> + +<p>I. The first problem, the localization of the ice sheets, +arises from the fact that in both the Pleistocene and the +Permian periods glaciation was remarkably limited. In +neither period were all parts of high latitudes glaciated; +yet in both cases glaciation occurred in large regions in +lower latitudes. Many explanations of this localization +have been offered, but most are entirely inadequate. Even +hypotheses with something of proven worth, such as +those of variations in volcanic dust and in atmospheric +<!--File: cc155.png--> +<a class="pagenum break" id="Page_131" title="pg 131"></a> +carbon dioxide, fail to account for localization. The +cyclonic form of the solar hypothesis, however, seems to +afford a satisfactory explanation.</p> + +<p>The distribution of the ice in the last glacial period is +well known, and is shown in Fig. 6. Four-fifths of the +ice-covered area, which was eight million square miles, +more or less, was near the borders of the North Atlantic +in eastern North America and northwestern Europe. +The ice spread out from two great centers in North +America, the Labradorean east of Hudson Bay, and the +Keewatin west of the bay. There were also many glaciers +in the western mountains, especially in Canada, while +subordinate centers occurred in Newfoundland, the Adirondacks, +and the White Mountains. The main ice sheet +at its maximum extension reached as far south as latitude +39° in Kansas and Kentucky, and 37° in Illinois. +Huge boulders were transferred more than one thousand +miles from their source in Canada. The northward extension +was somewhat less. Indeed, the northern margin +of the continent was apparently relatively little glaciated +and much of Alaska unglaciated. Why should northern +Kentucky be glaciated when northern Alaska was not?</p> + +<p>In Europe the chief center from which the continental +glacier moved was the Scandinavian highlands. It pushed +across the depression now occupied by the Baltic to +southern Russia and across the North Sea depression +to England and Belgium. The Alps formed a center of +considerable importance, and there were minor centers +in Scotland, Ireland, the Pyrenees, Apennines, Caucasus, +and Urals. In Asia numerous ranges also contained large +glaciers, but practically all the glaciation was of the +alpine type and very little of the vast northern lowland +was covered with ice.</p> + +<p>In the southern hemisphere glaciation at low latitudes +<!--File: cc156.png Page 132--> +<a class="pagenum break" id="Page_132" title="pg 132"></a> +was less striking than in the northern hemisphere. Most +of the increase in the areas of ice was confined to mountains +which today receive heavy precipitation and still +contain small glaciers. Indeed, except for relatively slight +glaciation in the Australian Alps and in Tasmania, most +of the Pleistocene glaciation in the southern hemisphere +was merely an extension of existing glaciers, such as +those of south Chile, New Zealand, and the Andes. Nevertheless, +fairly extensive glaciation existed much nearer +the equator than is now the case.</p> + +<p>In considering the localization of Pleistocene glaciation, +three main factors must be taken into account, +namely, temperature, topography, and precipitation. The +absence of glaciation in large parts of the Arctic regions +of North America and of Asia makes it certain that low +temperature was not the controlling factor. Aside from +Antarctica, the coldest place in the world is northeastern +Siberia. There for seven months the average temperature +is below 0°C., while the mean for the whole year is +below -10°C. If the temperature during a glacial period +averaged 6°C. lower than now, as is commonly supposed, +this part of Siberia would have had a temperature below +freezing for at least nine months out of the twelve even if +there were no snowfield to keep the summers cold. Yet +even under such conditions no glaciation occurred, although +in other places, such as parts of Canada and +northwestern Europe, intense glaciation occurred where +the mean temperature is much higher.</p> + +<p>The topography of the lands apparently had much +more influence upon the localization of glaciation than +did temperature. Its effect, however, was always to cause +glaciation exactly where it would be expected and not in +unexpected places as actually occurred. For example, in +North America the western side of the Canadian Rockies +<!--File: cc157.png Page 133--> +<a class="pagenum break" id="Page_133" title="pg 133"></a> +suffered intense glaciation, for there precipitation was +heavy because the westerly winds from the Pacific are +forced to give up their moisture as they rise. In the same +way the western side of the Sierra Nevadas was much +more heavily glaciated than the eastern side. In similar +fashion the windward slopes of the Alps, the Caucasus, +the Himalayas, and many other mountain ranges suffered +extensive glaciation. Low temperature does not +seem to have been the cause of this glaciation, for in that +case it is hard to see why both sides of the various ranges +did not show an equal percentage of increase in the size +of their icefields.</p> + +<p>From what has been said as to temperature and topography, +it is evident that variations in precipitation have +had much more to do with glaciation than have variations +in temperature. In the Arctic lowlands and on the leeward +side of mountains, the slight development of glaciation +appears to have been due to scarcity of precipitation. +On the windward side of mountains, on the other +hand, a notable increase in precipitation seems to have +led to abundant glaciation. Such an increase in precipitation +must be dependent on increased evaporation and +this could arise either from relatively high temperature +or strong winds. Since the temperature in the glacial +period was lower than now, we seem forced to attribute +the increased precipitation to a strengthening of the +winds. If the westerly winds from the Pacific should increase +in strength and waft more moisture to the western +side of the Canadian Rockies, or if similar winds increased +the snowfall on the upper slopes of the Alps or +the Tian-Shan Mountains, the glaciers would extend +lower than now without any change in temperature.</p> + +<p>Although the incompetence of low temperature to cause +glaciation, and the relative unimportance of the mountains +<!--File: cc158.png Page 134--> +<a class="pagenum break" id="Page_134" title="pg 134"></a> +in northeastern Canada and northwestern Europe +throw most glacial hypotheses out of court, they are in +harmony with the cyclonic hypothesis. The answer of +that hypothesis to the problem of the localization of ice +sheets seems to be found in certain maps of storminess +and rainfall in relation to solar activity. In Fig. 2 a +marked belt of increased storminess at times of many +sunspots is seen in southern Canada. A comparison of +this with a series of maps given in <em>Earth and Sun</em> shows +that the stormy belt tends to migrate northward in harmony +with an increase in the activity of the sun's atmosphere. +If the sun were sufficiently active the belt of +maximum storminess would apparently pass through the +Keewatin and Labradorean centers of glaciation instead +of well to the south of them, as at present. It would +presumably cross another center in Greenland, and then +would traverse the fourth of the great centers of Pleistocene +glaciation in Scandinavia. It would not succeed in +traversing northern Asia, however, any more than it +does now, because of the great high-pressure area which +develops there in winter. When the ice sheets expanded +from the main centers of glaciation, the belt of storms +would be pushed southward and outward. Thus it might +give rise to minor centers of glaciers such as the Patrician +between Hudson Bay and Lake Superior, or the +centers in Ireland, Cornwall, Wales, and the northern +Ural Mountains. As the main ice sheets advanced, however, +the minor centers would be overridden and the +entire mass of ice would be merged into one vast expanse +in the Atlantic portion of each of the two continents.</p> + +<p>In this connection it may be well to consider briefly the +most recent hypothesis as to the growth and hence the +localization of glaciation. In 1911 and more fully in 1915, +<!--File: cc159.png Page 135--> +<a class="pagenum break" id="Page_135" title="pg 135"></a> +Hobbs,<sup><a href="#fn48" id="r48">[48]</a></sup> advanced the anti-cyclonic hypothesis of the +origin of ice sheets. This hypothesis has the great merit +of focusing attention upon the fact that ice sheets are +pronounced anti-cyclonic regions of high pressure. This +is proved by the strong outblowing winds which prevail +along their margins. Such winds must, of course, be +balanced by inward-moving winds at high levels. Abundant +observations prove that such is the case. For +example, balloons sent up by Barkow near the margin of +the Antarctic ice sheet reveal the occurrence of inblowing +winds, although they rarely occur below a height of +9000 meters. The abundant data gathered by Guervain +on the coast of Greenland indicate that outblowing winds +prevail up to a height of about 4000 meters. At that +height inblowing winds commence and increase in frequency +until at an altitude of over 5000 meters they become +more common than outblowing winds. It should be +noted, however, that in both Antarctica and Greenland, +although the winds at an elevation of less than a thousand +meters generally blow outward, there are frequent and +decided departures from this rule, so that "variable +winds" are quite commonly mentioned in the reports of +expeditions and balloon soundings.</p> + +<p>The undoubted anti-cyclonic conditions which Hobbs +thus calls to the attention of scientists seem to him to +necessitate a peculiar mechanism in order to produce +the snow which feeds the glaciers. He assumes that the +winds which blow toward the centers of the ice sheets +at high levels carry the necessary moisture by which the +glaciers grow. When the air descends in the centers of +the highs, it is supposed to be chilled on reaching the surface +<!--File: cc160.png Page 136--> +<a class="pagenum break" id="Page_136" title="pg 136"></a> +of the ice, and hence to give up its moisture in the +form of minute crystals. This conclusion is doubtful for +several reasons. In the first place, Hobbs does not seem +to appreciate the importance of the variable winds which +he quotes Arctic and Antarctic explorers as describing +quite frequently on the edges of the ice sheets. They are +one of many signs that cyclonic storms are fairly frequent +on the borders of the ice though not in its interior. +Thus there is a distinct and sufficient form of precipitation +actually at work near the margin of the ice, or +exactly where the thickness of the ice sheet would lead +us to expect.</p> + +<p>Another consideration which throws grave doubt on +the anti-cyclonic hypothesis of ice sheets is the small +amount of moisture possible in the highs because of their +low temperature. Suppose, for the sake of argument, +that the temperature in the middle of an ice sheet averages +20°F. This is probably much higher than the actual +fact and therefore unduly favorable to the anti-cyclonic +hypothesis. Suppose also that the decrease in temperature +from the earth's surface upward proceeds at the +rate of 1°F. for each 300 feet, which is 50 per cent less +than the actual rate for air with only a slight amount of +moisture, such as is found in cold regions. Then at a +height of 10,000 feet, where the inblowing winds begin +to be felt, the temperature would be -20°F. At that +temperature the air is able to hold approximately 0.166 +grain of moisture per cubic foot when fully saturated. +This is an exceedingly small amount of moisture and even +if it were all precipitated could scarcely build a glacier. +However, it apparently would not be precipitated because +when such air descends in the center of the anti-cyclone +it is warmed adiabatically, that is, by compression. On +reaching the surface it would have a temperature of 20° +<!--File: cc161.png Page 137--> +<a class="pagenum break" id="Page_137" title="pg 137"></a> +and would be able to hold 0.898 grain of water vapor per +cubic foot; in other words, it would have a relative humidity +of about 18 per cent. Under no reasonable assumption +does the upper air at the center of an ice sheet +appear to reach the surface with a relative humidity of +more than 20 or 25 per cent. Such air cannot give up +moisture. On the contrary, it absorbs it and tends to +diminish rather than increase the thickness of the sheet +of ice and snow. But after the surplus heat gained by +descent has been lost by radiation, conduction, and +evaporation, the air may become super-saturated with +the moisture picked up while warm. Hobbs reports that +explorers in Antarctica and Greenland have frequently +observed condensation on their clothing. If such moisture +is not derived directly from the men's own bodies, it is +apparently picked up from the ice sheet by the descending +air, and not added to the ice sheet by air from aloft.</p> + +<p>The relation of all this to the localization of ice sheets +is this. If Hobbs' anti-cyclonic hypothesis of glacial +growth is correct, it would appear that ice sheets should +grow up where the temperature is lowest and the high-pressure +areas most persistent; for instance, in northern +Siberia. It would also appear that so far as the topography +permitted, the ice sheets ought to move out uniformly +in all directions; hence the ice sheet ought to be +as prominent to the north of the Keewatin and Labradorean +centers as to the south, which is by no means the +case. Again, in mountainous regions, such as the glacial +areas of Alaska and Chile, the glaciation ought not to +be confined to the windward slope of the mountains so +closely as is actually the fact. In each of these cases the +glaciated region was large enough so that there was +probably a true anti-cyclonic area comparable with that +now prevailing over southern Greenland. In both places +<!--File: cc162.png Page 138--> +<a class="pagenum break" id="Page_138" title="pg 138"></a> +the correlation between glaciation and mountain ranges +seems much too close to support the anti-cyclonic hypothesis, +for the inblowing winds which on that hypothesis +bring the moisture are shown by observation to occur +at heights far greater than that of all but the loftiest +ranges.</p> + +<p>II. The sudden coming of glaciation is another problem +which has been a stumbling-block in the way of every +glacial hypothesis. In his <em>Climates of Geologic Times</em>, +Schuchert states that the fossils give almost no warning +of an approaching catastrophe. If glaciation were solely +due to uplift, or other terrestrial changes aside from vulcanism, +Schuchert holds that it would have come slowly +and the stages preceding glaciation would have affected +life sufficiently to be recorded in the rocks. He considers +that the suddenness of the coming of glaciation is one +of the strongest arguments against the carbon dioxide +hypothesis of glaciation.</p> + +<p>According to the cyclonic hypothesis, however, the +suddenness of the oncoming of glaciation is merely what +would be expected on the basis of what happens today. +Changes in the sun occur suddenly. The sunspot cycle is +only eleven or twelve years long, and even this short +period of activity is inaugurated more suddenly than it +declines. Again the climatic record derived from the +growth of trees, as given in Figs. 4 and 5, also shows that +marked changes in climate are initiated more rapidly +than they disappear. In this connection, however, it must +be remembered that solar activity may arise in various +ways, as will appear more fully later. Under certain conditions +storminess may increase and decrease slowly.</p> + +<p>III. The height of the snow line and of glaciation furnishes +another means of testing glacial hypotheses. It is +well established that in times of glaciation the snow line +<!--File: cc163.png Page 139--> +<a class="pagenum break" id="Page_139" title="pg 139"></a> +was depressed everywhere, but least near the equator. +For example, according to Penck, permanent snow extended +4000 feet lower than now in the Alps, whereas +it stood only 1500 feet below the present level near the +equator in Venezuela. This unequal depression is not +readily accounted for by any hypothesis depending solely +upon the lowering of temperature. By the carbon dioxide +and the volcanic dust hypotheses, the temperature presumably +was lowered almost equally in all latitudes, but +a little more at the equator than elsewhere. If glaciation +were due to a temporary lessening of the radiation received +from the sun, such as is demanded by the thermal +solar hypothesis, and by the longer periods of Croll's +hypothesis, the lowering would be distinctly greatest at +the equator. Thus, according to all these hypotheses, the +snow line should have been depressed most at the equator, +instead of least.</p> + +<p>The cyclonic hypothesis explains the lesser depression +of the snow line at the equator as due to a diminution of +precipitation. The effectiveness of precipitation in this +respect is illustrated by the present great difference in +the height of the snow line on the humid and dry sides of +mountains. On the wet eastern side of the Andes near the +equator, the snow line lies at 16,000 feet; on the dry +western side, at 18,500 feet. Again, although the humid +side of the Himalayas lies toward the south, the snow line +has a level of 15,000 feet, while farther north, on the dry +side, it is 16,700 feet.<sup><a href="#fn49" id="r49">[49]</a></sup> The fact that the snow line is lower +near the margin of the Alps than toward the center +points in the same direction. The bearing of all this on +the glacial period may be judged by looking again at Fig. +3 in Chapter V. This shows that at times of sunspot +activity and hence of augmented storminess, the precipitation +<!--File: cc164.png Page 140--> +<a class="pagenum break" id="Page_140" title="pg 140"></a> +diminishes near the heat equator, that is, where +the average temperature for the whole year is highest. +At present the great size of the northern continents and +their consequent high temperature in summer, cause the +heat equator to lie north of the "real" equator, except +where Australia draws it to the southward.<sup><a href="#fn50" id="r50">[50]</a></sup> When large +parts of the northern continents were covered with ice, +however, the heat equator and the true equator were +probably much closer than now, for the continents could +not become so hot. If so, the diminution in equatorial +precipitation, which accompanies increased storminess +throughout the world as a whole, would take place more +nearly along the true equator than appears in Fig. 3. +Hence so far as precipitation alone is concerned, we +should actually expect that the snow line near the equator +would rise a little during glacial periods. Another factor, +however, must be considered. Köppen's data, it will be +remembered, show that at times of solar activity the +earth's temperature falls more at the equator than in +higher latitudes. If this effect were magnified it would +lower the snow line. The actual position of the snow line +at the equator during glacial periods thus appears to be +the combined effect of diminished precipitation, which +would raise the line, and of lower temperature, which +would bring it down.</p> + +<p>Before leaving this subject it may be well to recall that +the relative lessening of precipitation in equatorial latitudes +during the glacial epochs was probably caused by +the diversion of moisture from the trade-wind belt. This +diversion was presumably due to the great number of +tropical cyclones and to the fact that the cyclonic storms +of middle latitudes also drew much moisture from the +trade-wind belt in summer when the northern position of +<!--File: cc165.png Page 141--> +<a class="pagenum break" id="Page_141" title="pg 141"></a> +the sun drew that belt near the storm track which was +forced to remain south of the ice sheet. Such diversion +of moisture out of the trade-wind belt must diminish the +amount of water vapor that is carried by the trades to +equatorial regions; hence it would lessen precipitation +in the belt of so-called equatorial calms, which lies along +the heat equator rather than along the geographical +equator.</p> + +<p>Another phase of the vertical distribution of glaciation +has been the subject of considerable discussion. In the +Alps and in many other mountains the glaciation of the +Pleistocene period appears to have had its upper limit +no higher than today. This has been variously interpreted. +It seems, however, to be adequately explained +as due to decreased precipitation at high altitudes during +the cold periods. This is in spite of the fact that precipitation +in general increased with increased storminess. +The low temperature of glacial times presumably induced +condensation at lower altitudes than now, and most of +the precipitation occurred upon the lower slopes of the +mountains, contributing to the lower glaciers, while little +of it fell upon the highest glaciers. Above a moderate +altitude in all lofty mountains the decrease in the amount +of precipitation is rapid. In most cases the decrease +begins at a height of less than 3000 feet above the base +of the main slope, provided the slope is steep. The colder +the air, the lower the altitude at which this occurs. For +example, it is much lower in winter than in summer. +Indeed, the higher altitudes in the Alps are sunny in +winter even where there are abundant clouds lower down.</p> + +<p>IV. The presence of extensive lakes and other evidences +of a pluvial climate during glacial periods in non-glaciated +regions which are normally dry is another of the +facts which most glacial hypotheses fail to explain satisfactorily. +<!--File: cc166.png Page 142--> +<a class="pagenum break" id="Page_142" title="pg 142"></a> +Beyond the ice sheets many regions appear to +have enjoyed an unusually heavy precipitation during the +glacial epochs. The evidence of this is abundant, including +numerous abandoned strand lines of salt lakes and an +abundance of coarse material in deltas and flood plains.J. D. Whitney,<sup><a href="#fn51" id="r51">[51]</a></sup> in an interesting but neglected volume, +was one of the first to marshal the evidence of this sort. +More recently Free<sup><a href="#fn52" id="r52">[52]</a></sup> has amplified this. According to him +in the Great Basin region of the United States sixty-two +basins either contain unmistakable evidence of lakes, or +belong to one of the three great lake groups named below. +Two of these, the Lake Lahontan and the Lake Bonneville +groups, comprise twenty-nine present basins, while +the third, the Owens-Searles chain, contained at least +five large lakes, the lowest being in Death Valley. In +western and central Asia a far greater series of salt lakes +is found and most of these are surrounded by strands at +high levels. Many of these are described in <em>Explorations +in Turkestan</em>, <em>The Pulse of Asia</em>, and <em>Palestine and Its +Transformation</em>. There has been a good deal of debate as +to whether these lakes actually date from the glacial +period, as is claimed by C. E. P. Brooks, for example, or +from some other period. The evidence, however, seems to +be convincing that the lakes expanded when the ice also +expanded.</p> + +<p>According to the older glacial hypotheses the lower +temperature which is postulated as the cause of glaciation +would almost certainly mean less evaporation over +the oceans and hence less precipitation during glacial +periods. To counteract this the only way in which the +<!--File: cc167.png Page 143--> +<a class="pagenum break" id="Page_143" title="pg 143"></a> +level of the lakes could be raised would be because the +lower temperature would cause less evaporation from +their surfaces. It seems quite impossible, however, that +the lowering of temperature, which is commonly taken +to have been not more than 10°C., could counteract the +lessened precipitation and also cause an enormous expansion +of most of the lakes. For example, ancient Lake +Bonneville was more than ten times as large as its +modern remnant, Great Salt Lake, and its average depth +more than forty times as great.<sup><a href="#fn53" id="r53">[53]</a></sup> Many small lakes in the +Old World expanded still more.<sup><a href="#fn54" id="r54">[54]</a></sup> For example, in eastern +Persia many basins which now contain no lake whatever +are floored with vast deposits of lacustrine salt and are +surrounded by old lake bluffs and beaches. In northern +Africa similar conditions prevail.<sup><a href="#fn55" id="r55">[55]</a></sup> Other, but less obvious, +evidence of more abundant rainfall in regions that +are now dry is found in thick strata of gravel, sand, and +fine silt in the alluvial deposits of flood plains and deltas.<sup><a href="#fn56" id="r56">[56]</a></sup></p> + +<p>The cyclonic hypothesis supposes that increased +storminess accounts for pluvial climates in regions that +are now dry just as it accounts for glaciation in the +regions of the ice sheets. Figs. 2 and 3, it will be remembered, +illustrate what happens when the sun is active. +Solar activity is accompanied by an increase in storminess +in the southwestern United States in exactly the region +where elevated strands of diminished salt lakes are +most numerous. In Fig. 3, the same condition is seen in +<!--File: cc168.png Page 144--> +<a class="pagenum break" id="Page_144" title="pg 144"></a> +the region of salt lakes in the Old World. Judging by +these maps, which illustrate what has happened since +careful meteorological records were kept, an increase in +solar activity is accompanied by increased rainfall in +large parts of what are now semi-arid and desert regions. +Such precipitation would at once cause the level of the +lakes to rise. Later, when ice sheets had developed in +Europe and America, the high-pressure areas thus caused +might force the main storm belt so far south that it would +lie over these same arid regions. The increase in tropical +hurricanes at times of abundant sunspots may also have +a bearing on the climate of regions that are now arid. +During the glacial period some of the hurricanes probably +swept far over the lands. The numerous tropical +cyclones of Australia, for example, are the chief source +of precipitation for that continent.<sup><a href="#fn57" id="r57">[57]</a></sup> Some of the stronger +cyclones locally yield more rain in a day or two than +other sources yield in a year.</p> + +<p>V. The occurrence of widespread glaciation near the +tropics during the Permian, as shown in Fig. 7, has given +rise to much discussion. The recent discovery of glaciation +in latitudes as low as 30° in the Proterozoic is correspondingly +significant. In all cases the occurrence of +glaciation in low and middle latitudes is probably due to +the same general causes. Doubtless the position and altitude +of the mountains had something to do with the +matter. Yet taken by itself this seems insufficient. Today +the loftiest range in the world, the Himalayas, is almost +unglaciated, although its southern slope may seem at first +thought to be almost ideally located in this respect. Some +parts rise over 20,000 feet and certain lower slopes receive +400 inches of rain per year. The small size of the +Himalayan glaciers in spite of these favorable conditions +<!--File: cc169.png Page 145--> +<a class="pagenum break" id="Page_145" title="pg 145"></a> + +<!--File: cc170.png Page 146--> +<a class="pagenum break" id="Page_146" title="pg 146"></a> +is apparently due largely to the seasonal character of the +monsoon winds. The strong outblowing monsoons of +winter cause about half the year to be very dry with clear +skies and dry winds from the interior of Asia. In all low +latitudes the sun rides high in the heavens at midday, +even in winter, and thus melts snow fairly effectively in +clear weather. This is highly unfavorable to glaciation. +The inblowing southern monsoons bring all their moisture +in midsummer at just the time when it is least effective +in producing snow. Conditions similar to those now +prevailing in the Himalayas must accompany any great +uplift of the lands which produces high mountains and +large continents in subtropical and middle latitudes. +Hence, uplift alone cannot account for extensive glaciation +in subtropical latitudes during the Permian and +Proterozoic.</p> + +<img src="images/fig7.png" width="800" height="532" alt="Fig. 7" id="Fig_7" /> +<p class="center"><em>Fig. 7. Permian geography and glaciation.<br /> +(After Schuchert.)</em></p> + +<p>The assumption of a great general lowering of temperature +is also not adequate to explain glaciation in +subtropical latitudes. In the first place this would require +a lowering of many degrees,—far more than in the Pleistocene +glacial period. The marine fossils of the Permian, +however, do not indicate any such condition. In the +second place, if the lands were widespread as they appear +to have been in the Permian, a general lowering of +temperature would diminish rather than increase the +present slight efficiency of the monsoons in producing +glaciation. Monsoons depend upon the difference between +the temperatures of land and water. If the general temperature +were lowered, the reduction would be much less +pronounced on the oceans than on the lands, for water +tends to preserve a uniform temperature, not only because +of its mobility, but because of the large amount of +heat given out when freezing takes place, or consumed in +evaporation. Hence the general lowering of temperature +<!--File: cc171.png Page 147--> +<a class="pagenum break" id="Page_147" title="pg 147"></a> +would make the contrast between continents and oceans +less than at present in summer, for the land temperature +would be brought toward that of the ocean. This would +diminish the strength of the inblowing summer monsoons +and thus cut off part of the supply of moisture. +Evidence that this actually happened in the cold fourteenth +century has already been given in Chapter VI. +On the other hand, in winter the lands would be much +colder than now and the oceans only a little colder, so +that the dry outblowing monsoons of the cold season +would increase in strength and would also last longer +than at present. In addition to all this, the mere fact of +low temperature would mean a general reduction in the +amount of water vapor in the air. Thus, from almost +every point of view a mere lowering of temperature +seems to be ruled out as a cause of Permian glaciation. +Moreover, if the Permian or Proterozoic glacial periods +were so cold that the lands above latitude 30° were snow-covered +most of the time, the normal surface winds in +subtropical latitudes would be largely equatorward, just +as the winter monsoons now are. Hence little or no moisture +would be available to feed the snowfields which give +rise to the glaciers.</p> + +<p>It has been assumed by Marsden Manson and others +that increased general cloudiness would account for the +subtropical glaciation of the Permian and Proterozoic. +Granting for the moment that there could be universal +persistent cloudiness, this would not prevent or counteract +the outblowing anti-cyclonic winds so characteristic +of great snowfields. Therefore, under the hypothesis of +general cloudiness there would be no supply of moisture +to cause glaciation in low latitudes. Indeed, persistent +cloudiness in all higher latitudes would apparently deprive +the Himalayas of most of their present moisture, +<!--File: cc172.png Page 148--> +<a class="pagenum break" id="Page_148" title="pg 148"></a> +for the interior of Asia would not become hot in summer +and no inblowing monsoons would develop. In fact, winds +of all kinds would seemingly be scarce, for they arise +almost wholly from contrasts of temperature and hence +of atmospheric pressure. The only way to get winds and +hence precipitation would be to invoke some other agency, +such as cyclonic storms, but that would be a departure +from the supposition that glaciation arose from cloudiness.</p> + +<p>Let us now inquire how the cyclonic hypothesis +accounts for glaciation in low latitudes. We will first +consider the terrestrial conditions in the early Permian, +the last period of glaciation in such latitudes. Geologists +are almost universally agreed that the lands were exceptionally +extensive and also high, especially in low latitudes. +One evidence of this is the presence of abundant +conglomerates composed of great boulders. It is also +probable that the carbon dioxide in the air during the +early Permian had been reduced to a minimum by the +extraordinary amount of coal formed during the preceding +period. This would tend to produce low temperature +and thus make the conditions favorable for glaciation as +soon as an accentuation of solar activity caused unusual +storminess. If the storminess became extreme when terrestrial +conditions were thus universally favorable to +glaciation, it would presumably produce glaciation in low +latitudes. Numerous and intense tropical cyclones would +carry a vast amount of moisture out of the tropics, just +as now happens when the sun is active, but on a far +larger scale. The moisture would be precipitated on the +equatorward slopes of the subtropical mountain ranges. +At high elevations this precipitation would be in the form +of snow even in summer. Tropical cyclones, however, as +is shown in <em>Earth and Sun</em>, occur in the autumn and +<!--File: cc173.png Page 149--> +<a class="pagenum break" id="Page_149" title="pg 149"></a> +winter as well as in summer. For example, in the Bay of +Bengal the number recorded in October is fifty, the +largest for any month; while in November it is thirty-four, +and December fourteen as compared with an average +of forty-two for the months of July to September. +From January to March, when sunspot numbers averaged +more than forty, the number of tropical hurricanes +was 143 per cent greater than when the sunspot numbers +averaged below forty. During the months from April to +June, which also would be times of considerable snowy +precipitation, tropical hurricanes averaged 58 per cent +more numerous with sunspot numbers above forty than +with numbers below forty, while from July to September +the difference amounted to 23 per cent. Even at this +season some snow falls on the higher slopes, while the +increased cloudiness due to numerous storms also tends +to preserve the snow. Thus a great increase in the frequency +of sunspots is accompanied by increased intensity +of tropical hurricanes, especially in the cooler autumn and +spring months, and results not only in a greater accumulation +of snow but in a decrease in the melting of the +snow because of more abundant clouds. At such times as +the Permian, the general low temperature due to rapid +convection and to the scarcity of carbon dioxide presumably +joined with the extension of the lands in producing +great high-pressure areas over the lands in middle +latitudes during the winters, and thus caused the more +northern, or mid-latitude type of cyclonic storms to be +shifted to the equatorward side of the continents at that +season. This would cause an increase of precipitation in +winter as well as during the months when tropical hurricanes +abound. Many other circumstances would coöperate +to produce a similar result. For example, the general +low temperature would cause the sea to be covered with +<!--File: cc174.png Page 150--> +<a class="pagenum break" id="Page_150" title="pg 150"></a> +ice in lower latitudes than now, and would help to create +high-pressure areas in middle latitudes, thus driving the +storms far south. If the sea water were fresher than now, +as it probably was to a notable extent in the Proterozoic +and perhaps to some slight extent in the Permian, the +higher freezing point would also further the extension +of the ice and help to keep the storms away from high +latitudes. If to this there is added a distribution of land +and sea such that the volume of the warm ocean currents +flowing from low to high latitudes was diminished, as +appears to have been the case, there seems to be no difficulty +in explaining the subtropical location of the main +glaciation in both the Permian and the Proterozoic. An +increase of storminess seems to be the key to the whole +situation.</p> + +<p>One other possibility may be mentioned, although little +stress should be laid on it. In <em>Earth and Sun</em> it has been +shown that the main storm track in both the northern +and southern hemispheres is not concentric with the +geographical poles. Both tracks are roughly concentric +with the corresponding magnetic poles, a fact which may +be important in connection with the hypothesis of an electrical +effect of the sun upon terrestrial storminess. The +magnetic poles are known to wander considerably. Such +wandering gives rise to variations in the direction of +the magnetic needle from year to year. In 1815 the compass +in England pointed 24-½° W. of N. and in 1906 +17° 45' W. Such a variation seems to mean a change of +many miles in the location of the north magnetic pole. +Certain changes in the daily march of electromagnetic +phenomena over the oceans have led Bauer and his associates +to suggest that the magnetic poles may even be +subject to a slight daily movement in response to the +changes in the relative positions of the earth and sun. +<!--File: cc175.png Page 151--> +<a class="pagenum break" id="Page_151" title="pg 151"></a> +Thus there seems to be a possibility that a pronounced +change in the location of the magnetic pole in Permian +times, for example, may have had some connection with +a shifting in the location of the belt of storms. It must be +clearly understood that there is as yet no evidence of any +such change, and the matter is introduced merely to call +attention to a possible line of investigation.</p> + +<p>Any hypothesis of Permian and Proterozoic glaciation +must explain not only the glaciation of low latitudes but +the lack of glaciation and the accumulation of red desert +beds in high latitudes. The facts already presented seem +to explain this. Glaciation could not occur extensively in +high latitudes partly because during most of the year the +air was too cold to hold much moisture, but still more +because the winds for the most part must have blown +outward from the cold northern areas and the cyclonic +storm belt was pushed out of high latitudes. Because of +these conditions precipitation was apparently limited to +a relatively small number of storms during the summer. +Hence great desert areas must have prevailed at high +latitudes. Great aridity now prevails north of the Himalayas +and related ranges, and red beds are accumulating +in the centers of the great deserts, such as those of the +Tarim Basin and the Transcaspian. The redness is not +due to the original character of the rock, but to intense +oxidation, as appears from the fact that along the edges +of the desert and wherever occasional floods carry sediment +far out into the midst of the sand, the material has +the ordinary brownish shades. As soon as one goes out +into the places where the sand has been exposed to the air +for a long time, however, it becomes pink, and then red. +Such conditions may have given rise to the high degree +of oxidation in the famous Permian red beds. If the air +of the early Permian contained an unusual percentage of +<!--File: cc176.png Page 152--> +<a class="pagenum break" id="Page_152" title="pg 152"></a> +oxygen because of the release of that gas by the great +plant beds which formed coal in the preceding era, as +Chamberlin has thought probable, the tendency to produce +red beds would be still further increased.</p> + +<p>It must not be supposed, however, that these conditions +would absolutely limit glaciation to subtropical +latitudes. The presence of early Permian glaciation in +North America at Boston and in Alaska and in the Falkland +Islands of the South Atlantic Ocean proves that at +least locally there was sufficient moisture to form glaciers +near the coast in relatively high latitudes. The possibility +of this would depend entirely upon the form of the lands +and the consequent course of ocean currents. Even in +those high latitudes cyclonic storms would occur unless +they were kept out by conditions of pressure such as have +been described above.</p> + +<p>The marine faunas of Permian age in high latitudes +have been interpreted as indicating mild oceanic temperatures. +This is a point which requires further investigation. +Warm oceans during times of slight solar activity +are a necessary consequence of the cyclonic hypothesis, +as will appear later. The present cold oceans seem to be +the expectable result of the Pleistocene glaciation and of +the present relatively disturbed condition of the sun. If +a sudden disturbance threw the solar atmosphere into +violent commotion within a few thousand years during +Permian times, glaciation might occur as described above, +while the oceans were still warm. In fact their warmth +would increase evaporation while the violent cyclonic +storms and high winds would cause heavy rain and keep +the air cool by constantly raising it to high levels where +it would rapidly radiate its heat into space.</p> + +<p>Nevertheless it is not yet possible to determine how +warm the oceans were at the actual time of the Permian +<!--File: cc177.png Page 153--> +<a class="pagenum break" id="Page_153" title="pg 153"></a> +glaciation. Some faunas formerly reported as Permian +are now known to be considerably older. Moreover, others +of undoubted Permian age are probably not strictly contemporaneous +with the glaciation. So far back in the +geological record it is very doubtful whether we can date +fossils within the limits of say 100,000 years. Yet a difference +of 100,000 years would be more than enough to +allow the fossils to have lived either before or after the +glaciation, or in an inter-glacial epoch. One such epoch +is known to have occurred and nine others are suggested +by the inter-stratification of glacial till and marine sediments +in eastern Australia. The warm currents which +would flow poleward in inter-glacial epochs must have +favored a prompt reintroduction of marine faunas driven +out during times of glaciation. Taken all and all, the +Permian glaciation seems to be accounted for by the +cyclonic hypothesis quite as well as does the Pleistocene. +In both these cases, as well as in the various pulsations +of historic times, it seems to be necessary merely to magnify +what is happening today in order to reproduce the +conditions which prevailed in the past. If the conditions +which now prevail at times of sunspot minima were magnified, +they would give the mild conditions of inter-glacial +epochs and similar periods. If the conditions which now +prevail at times of sunspot maxima are magnified a little +they seem to produce periods of climatic stress such as +those of the fourteenth century. If they are magnified +still more the result is apparently glacial epochs like +those of the Pleistocene, and if they are magnified to a +still greater extent, the result is Permian or Proterozoic +glaciation. Other factors must indeed be favorable, for +climatic changes are highly complex and are unquestionably +due to a combination of circumstances. The point +which is chiefly emphasized in this book is that among +<!--File: cc178.png Page 154--> +<a class="pagenum break" id="Page_154" title="pg 154"></a> +those several circumstances, changes in cyclonic storms +due apparently to activity of the sun's atmosphere must +always be reckoned.</p> +<!--File: cc179.png Page 155--> +<a class="pagenum break" id="Page_155" title="pg 155"></a> + + + +<h2 id="Chapt_IX"><a href="#content">CHAPTER IX</a></h2> + +<h3>THE ORIGIN OF LOESS</h3> + + +<p>One of the most remarkable formations associated +with glacial deposits consists of vast sheets +of the fine-grained, yellowish, wind-blown material +called loess. Somewhat peculiar climatic conditions +evidently prevailed when it was formed. At present +similar deposits are being laid down only near the leeward +margin of great deserts. The famous loess deposits +of China in the lee of the Desert of Gobi are examples. +During the Pleistocene period, however, loess accumulated +in a broad zone along the margin of the ice sheet +at its maximum extent. In the Old World it extended +from France across Germany and through the Black +Earth region of Russia into Siberia. In the New World +a still larger area is loess-covered. In the Mississippi +Valley, tens of thousands of square miles are mantled by +a layer exceeding twenty feet in thickness and in many +places approaching a hundred feet. Neither the North +American nor the European deposits are associated with +a desert. Indeed, loess is lacking in the western and +drier parts of the great plains and is best developed in +the well-watered states of Iowa, Illinois, and Missouri. +Part of the loess overlies the non-glacial materials of the +great central plain, but the northern portions overlie the +drift deposits of the first three glaciations. A few traces +of loess are associated with the Kansan and Illinoian, +the second and third glaciations, but most of the American +<!--File: cc180.png Page 156--> +<a class="pagenum break" id="Page_156" title="pg 156"></a> +loess appears to have been formed at approximately +the time of the Iowan or fourth glaciation, while only a +little overlies the drift sheets of the Wisconsin age. The +loess is thickest near the margin of the Iowan till sheet +and thins progressively both north and south. The +thinning southward is abrupt along the stream divides, +but very gradual along the larger valleys. Indeed, loess is +abundant along the bluffs of the Mississippi, especially +the east bluff, almost to the Gulf of Mexico.<sup><a href="#fn58" id="r58">[58]</a></sup></p> + +<p>It is now generally agreed that all typical loess is wind +blown. There is still much question, however, as to its +time of origin, and thus indirectly as to its climatic implications. +Several American and European students +have thought that the loess dates from inter-glacial times. +On the other hand, Penck has concluded that the loess +was formed shortly before the commencement of the +glacial epochs; while many American geologists hold that +the loess accumulated while the ice sheets were at approximately +their maximum size. W. J. McGee, Chamberlin +and Salisbury, Keyes, and others lean toward this +view. In this chapter the hypothesis is advanced that it +was formed at the one other possible time, namely, immediately +following the retreat of the ice.</p> + +<p>These four hypotheses as to the time of origin of loess +imply the following differences in its climatic relations. +If loess was formed during typical inter-glacial epochs, +or toward the close of such epochs, profound general +aridity must seemingly have prevailed in order to kill +off the vegetation and thus enable the wind to pick up +sufficient dust. If the loess was formed during times of +extreme glaciation when the glaciers were supplying +large quantities of fine material to outflowing streams, +less aridity would be required, but there must have been +<!--File: cc181.png Page 157--> +<a class="pagenum break" id="Page_157" title="pg 157"></a> +sharp contrasts between wet seasons in summer when +the snow was melting and dry seasons in winter when +the storms were forced far south by the glacial high pressure. +Alternate floods and droughts would thus affect +broad areas along the streams. Hence arises the hypothesis +that the wind obtained the loess from the flood plains +of streams at times of maximum glaciation. If the loess +was formed during the rapid retreat of the ice, alternate +summer floods and winter droughts would still prevail, +but much material could also be obtained by the winds +not only from flood plains, but also from the deposits +exposed by the melting of the ice and not yet covered by +vegetation.</p> + +<p>The evidence for and against the several hypotheses +may be stated briefly. In support of the hypothesis of the +inter-glacial origin of loess, Shimek and others state that +the glacial drift which lies beneath the loess commonly +gives evidence that some time elapsed between the disappearance +of the ice and the deposition of the loess. For +example, abundant shells of land snails in the loess are +not of the sort now found in colder regions, but resemble +those found in the drier regions. It is probable that if +they represented a glacial epoch they would be depauperated +by the cold as are the snails of far northern regions. +The gravel pavement discussed below seems to be strong +evidence of erosion between the retreat of the ice and +the deposition of the loess.</p> + +<p>Turning to the second hypothesis, namely, that the +loess accumulated near the close of the inter-glacial epoch +rather than in the midst of it, we may follow Penck. The +mammalian fossils seem to him to prove that the loess +was formed while boreal animals occupied the region, for +they include remains of the hairy mammoth, woolly rhinoceros, +and reindeer. On the other hand, the typical +<!--File: cc182.png Page 158--> +<a class="pagenum break" id="Page_158" title="pg 158"></a> +inter-glacial beds not far away yield remains of species +characteristic of milder climates, such as the elephant, +the smaller rhinoceros, and the deer. In connection with +these facts it should be noted that occasional remains of +tundra vegetation and of trees are found beneath the +loess, while in the loess itself certain steppe animals, +such as the common gopher or spermaphyl, are found. +Penck interprets this as indicating a progressive desiccation +culminating just before the oncoming of the next ice +sheet.</p> + +<p>The evidence advanced in favor of the hypothesis that +the loess was formed when glaciation was near its maximum +includes the fact that if the loess does not represent +the outwash from the Iowan ice, there is little else that +does, and presumably there must have been outwash. +Also the distribution of loess along the margins of +streams suggests that much of the material came from +the flood plains of overloaded streams flowing from the +melting ice.</p> + +<p>Although there are some points in favor of the hypothesis +that the loess originated (1) in strictly inter-glacial +times, (2) at the end of inter-glacial epochs, and +(3) at times of full glaciation, each hypothesis is much +weakened by evidence that supports the others. The evidence +of boreal animals seems to disprove the hypothesis +that the loess was formed in the middle of a mild inter-glacial +epoch. On the other hand, Penck's hypothesis as +to loess at the end of inter-glacial times fails to account +for certain characteristics of the lowest part of the loess +deposits and of the underlying topography. Instead of +normal valleys and consequent prompt drainage such as +ought to have developed before the end of a long inter-glacial +epoch, the surface on which the loess lies shows +many undrained depressions. Some of these can be seen +<!--File: cc183.png Page 159--> +<a class="pagenum break" id="Page_159" title="pg 159"></a> +in exposed banks, while many more are inferred from the +presence of shells of pond snails here and there in the +overlying loess. The pond snails presumably lived in +shallow pools occupying depressions in the uneven surface +left by the ice. Another reason for questioning +whether the loess was formed at the end of an inter-glacial +epoch is that this hypothesis does not provide a +reasonable origin for the material which composes the +loess. Near the Alps where the loess deposits are small +and where glaciers probably persisted in the inter-glacial +epochs and thus supplied flood plain material in large +quantities, this does not appear important. In the broad +upper Mississippi Basin, however, and also in the Black +Earth region of Russia there seems to be no way to get +the large body of material composing the loess except by +assuming the existence of great deserts to windward. +But there seems to be little or no evidence of such deserts +where they could be effective. The mineralogical character +of the loess of Iowan age proves that the material +came from granitic rocks, such as formed a large part of +the drift. The nearest extensive outcrops of granite are +in the southwestern part of the United States, nearly a +thousand miles from Iowa and Illinois. But the loess is +thickest near the ice margin and thins toward the southwest +and in other directions, whereas if its source were +the southwestern desert, its maximum thickness would +probably be near the margin of the desert.</p> + +<p>The evidence cited above seems inconsistent not only +with the hypothesis that the loess was formed at the end +of an inter-glacial epoch, but also with the idea that it +originated at times of maximum glaciation either from +river-borne sediments or from any other source. A +further and more convincing reason for this last conclusion +is the probability and almost the certainty that +<!--File: cc184.png Page 160--> +<a class="pagenum break" id="Page_160" title="pg 160"></a> +when the ice advanced, its front lay close to areas where +the vegetation was not much thinner than that which +today prevails under similar climatic conditions. If the +average temperature of glacial maxima was only 6°C. +lower than that of today, the conditions just beyond the +ice front when it was in the loess region from southern +Illinois to Minnesota would have been like those now prevailing +in Canada from New Brunswick to Winnipeg. +The vegetation there is quite different from the grassy, +semi-arid vegetation of which evidence is found in the +loess. The roots and stalks of such grassy vegetation are +generally agreed to have helped produce the columnar +structure which enables the loess to stand with almost +vertical surfaces.</p> + +<p>We are now ready to consider the probability that loess +accumulated mainly during the retreat of the ice. Such a +retreat exposed a zone of drift to the outflowing glacial +winds. Most glacial hypotheses, such as that of uplift, +or depleted carbon dioxide, call for a gradual retreat +of the ice scarcely faster than the vegetation could advance +into the abandoned area. Under the solar-cyclonic +hypothesis, on the other hand, the climatic changes may +have been sudden and hence the retreat of the ice may +have been much more rapid than the advance of vegetation. +Now wind-blown materials are derived from places +where vegetation is scanty. Scanty vegetation on good +soil, it is true, is usually due to aridity, but may also +result because the time since the soil was exposed to the +air has not been long enough for the soil to be sufficiently +weathered to support vegetation. Even when weathering +has had full opportunity, as when sand bars, mud flats, +and flood plains are exposed, vegetation takes root only +slowly. Moreover, storms and violent winds may prevent +the spread of vegetation, as is seen on sandy beaches even +<!--File: cc185.png Page 161--> +<a class="pagenum break" id="Page_161" title="pg 161"></a> +in distinctly humid regions like New Jersey and Denmark. +Thus it appears that unless the retreat of the ice +were as slow as the advance of vegetation, a barren area +of more or less width must have bordered the retreating +ice and formed an ideal source of loess.</p> + +<p>Several other lines of evidence seemingly support the +conclusion that the loess was formed during the retreat +of the ice. For example, Shimek, who has made almost +a lifelong study of the Iowan loess, emphasizes the fact +that there is often an accumulation of stones and pebbles +at its base. This suggests that the underlying till was +eroded before the loess was deposited upon it. The first +reaction of most students is to assume that of course +this was due to running water. That is possible in many +cases, but by no means in all. So widespread a sheet of +gravel could not be deposited by streams without destroying +the irregular basins and hollows of which we have +seen evidence where the loess lies on glacial deposits. On +the other hand, the wind is competent to produce a similar +gravel pavement without disturbing the old topography. +"Desert pavements" are a notable feature in most +deserts. On the edges of an ice sheet, as Hobbs has made +us realize, the commonest winds are outward. They often +attain a velocity of eighty miles an hour in Antarctica +and Greenland. Such winds, however, usually decline +rapidly in velocity only a few score miles from the ice. +Thus their effect would be to produce rapid erosion +of the freshly bared surface near the retreating ice. +The pebbles would be left behind as a pavement, while +sand and then loess would be deposited farther from the +ice where the winds were weaker and where vegetation +was beginning to take root. Such a decrease in wind +velocity may explain the occasional vertical gradation +from gravel through sand to coarse loess and then to +<!--File: cc186.png Page 162--> +<a class="pagenum break" id="Page_162" title="pg 162"></a> +normal fine loess. As the ice sheet retreated the wind in +any given place would gradually become less violent. +As the ice continued to retreat the area where loess was +deposited would follow at a distance, and thus each part +of the gravel pavement would in turn be covered with the +loess.</p> + +<p>The hypothesis that loess is deposited while the ice is +retreating is in accord with many other lines of evidence. +For example, it accords with the boreal character of the +mammal remains as described above. Again, the advance +of vegetation into the barren zone along the front of the +ice would be delayed by the strong outblowing winds. +The common pioneer plants depend largely on the wind +for the distribution of their seeds, but the glacial winds +would carry them away from the ice rather than toward +it. The glacial winds discourage the advance of vegetation +in another way, for they are drying winds, as are +almost all winds blowing from a colder to a warmer +region. The fact that remains of trees sometimes occur +at the bottom of the loess probably means that the deposition +of loess extended into the forests which almost +certainly persisted not far from the ice. This seems more +likely than that a period of severe aridity before the advance +of the ice killed the trees and made a steppe or +desert. Penck's chief argument in favor of the formation +of loess before the advance of the ice rather than after, +is that since loess is lacking upon the youngest drift sheet +in Europe it must have been formed before rather than +after the last or Würm advance of the ice. This breaks +down on two counts. First, on the corresponding (Wisconsin) +drift sheet in America, loess is present,—in small +quantities to be sure, but unmistakably present. Second, +there is no reason to assume that conditions were identical +at each advance and retreat of the ice. Indeed, the +<!--File: cc187.png Page 163--> +<a class="pagenum break" id="Page_163" title="pg 163"></a> +fact that in Europe, as in the United States, nearly all +the loess was formed at one time, and only a little is associated +with the other ice advances, points clearly against +Penck's fundamental assumption that the accumulation +of loess was due to the approach of a cold climate.</p> + +<p>Having seen that the loess was probably formed during +the retreat of the ice, we are now ready to inquire what +conditions the cyclonic hypothesis would postulate in the +loess areas during the various stages of a glacial cycle. +Fig. 2, in Chapter IV, gives the best idea of what would +apparently happen in North America, and events in Europe +would presumably be similar. During the nine maximum +years on which Fig. 2 is based the sunspot numbers averaged +seventy, while during the nine minimum years they +averaged less than five. It seems fair to suppose that the +maximum years represent the average conditions which +prevailed in the past at times when the sun was in a +median stage between the full activity which led to glaciation +and the mild activity of the minimum years which +appear to represent inter-glacial conditions. This would +mean that when a glacial period was approaching, but +before an ice sheet had accumulated to any great extent, +a crescent-shaped strip from Montana through Illinois to +Maine would suffer a diminution in storminess ranging +up to 60 per cent as compared with inter-glacial conditions. +This is in strong contrast with an increase in +storminess amounting to 75 or even 100 per cent both in +the boreal storm belt in Canada and in the subtropical +belt in the Southwest. Such a decrease in storminess in +the central United States would apparently be most +noticeable in summer, as is shown in <em>Earth and Sun</em>. +Hence it would have a maximum effect in producing +aridity. This would favor the formation of loess, but it is +In discussions of climate, as of most subjects, a +peculiar psychological phenomenon is observable. +Everyone sees the necessity of explaining conditions +different from those that now exist, but few realize that +present conditions may be abnormal, and that they need +explanation just as much as do others. Because of this +tendency glaciation has been discussed with the greatest +fullness, while there has been much neglect not only of +the periods when the climate of the earth resembled that +of the present, but also of the vastly longer periods when +it was even milder than now. +doubtful whether the aridity would become extreme +<!--File: cc188.png Page 164--> +<a class="pagenum break" id="Page_164" title="pg 164"></a> +enough to explain such vast deposits as are found +throughout large parts of the Mississippi Basin. That +would demand that hundreds of thousands of square miles +should become almost absolute desert, and it is not probable +that any such thing occurred. Nevertheless, according +to the cyclonic hypothesis the period immediately +before the advent of the ice would be relatively dry in +the central United States, and to that extent favorable to +the work of the wind.</p> + +<p>As the climatic conditions became more severe and the +ice sheet expanded, the dryness and lack of storms would +apparently diminish. The reason, as has been explained, +would be the gradual pushing of the storms southward +by the high-pressure area which would develop over the +ice sheet. Thus at the height of a glacial epoch there +would apparently be great storminess in the area where +the loess is found, especially in summer. Hence the +cyclonic hypothesis does not accord with the idea of great +deposition of loess at the time of maximum glaciation.</p> + +<p>Finally we come to the time when the ice was retreating. +We have already seen that not only the river flood +plains, but also vast areas of fresh glacial deposits would +be exposed to the winds, and would remain without vegetation +for a long time. At that very time the retreat of +the ice sheet would tend to permit the storms to follow +paths determined by the degree of solar activity, in place +of the far southerly paths to which the high atmospheric +pressure over the expanded ice sheet had previously +forced them. In other words, the conditions shown in +Fig. 2 would tend to reappear when the sun's activity +was diminishing and the ice sheet was retreating, just as +they had appeared when the sun was becoming more +active and the ice sheet was advancing. This time, however, +the semi-arid conditions arising from the scarcity +<!--File: cc189.png Page 165--> +<a class="pagenum break" id="Page_165" title="pg 165"></a> +of storms would prevail in a region of glacial deposits +and widely spreading river deposits, few or none of +which would be covered with vegetation. The conditions +would be almost ideal for eolian erosion and for the +transportation of loess by the wind to areas a little more +remote from the ice where grassy vegetation had made a +start.</p> + +<p>The cyclonic hypothesis also seems to offer a satisfactory +explanation of variations in the amount of loess +associated with the several glacial epochs. It attributes +these to differences in the rate of disappearance of the +ice, which in turn varied with the rate of decline of solar +activity and storminess. This is supposed to be the reason +why the Iowan loess deposits are much more extensive +than those of the other epochs, for the Iowan ice sheet +presumably accomplished part of its retreat much more +suddenly than the other ice sheets.<sup><a href="#fn59" id="r59">[59]</a></sup> The more sudden the +retreat, the greater the barren area where the winds +could gather fine bits of dust. Temporary readvances may +also have been so distributed and of such intensity that +they frequently accentuated the condition shown in Fig. +2, thus making the central United States dry soon after +the exposure of great amounts of glacial débris. The +closeness with which the cyclonic hypothesis accords with +the facts as to the loess is one of the pleasant surprises of +the hypothesis. The first draft of Fig. 2 and the first outlines +of the hypothesis were framed without thought of +the loess. Yet so far as can now be seen, both agree +closely with the conditions of loess formation.</p> + + +<!--File: cc190.png Page 166--> +<a class="pagenum break" id="Page_166" title="pg 166"></a> + + + + +<h2 id="Chapt_X"><a href="#content">CHAPTER X</a></h2> + +<h2>CAUSES OF MILD GEOLOGICAL CLIMATES</h2> + + +<p>In discussions of climate, as of most subjects, a +peculiar psychological phenomenon is observable. +Everyone sees the necessity of explaining conditions +different from those that now exist, but few realize that +present conditions may be abnormal, and that they need +explanation just as much as do others. Because of this +tendency glaciation has been discussed with the greatest +fullness, while there has been much neglect not only of +the periods when the climate of the earth resembled that +of the present, but also of the vastly longer periods when +it was even milder than now.</p> + +<p>How important the periods of mild climate have been +in geological times may be judged from the relative +length of glacial compared with inter-glacial epochs, and +still more from the far greater relative length of the mild +parts of periods and eras when compared with the severe +parts. Recent estimates by R. T. Chamberlin<sup><a href="#fn60" id="r60">[60]</a></sup> indicate +that according to the consensus of opinion among geologists +the average inter-glacial epoch during the Pleistocene +was about five times as long as the average glacial +epoch, while the whole of a given glacial epoch averaged +five times as long as the period when the ice was at a +maximum. Climatic periods far milder, longer, and more +monotonous than any inter-glacial epoch appear repeatedly +<!--File: cc191.png Page 167--> +<a class="pagenum break" id="Page_167" title="pg 167"></a> +during the course of geological history. Our task in +this chapter is to explain them.</p> + +<p>Knowlton<sup><a href="#fn61" id="r61">[61]</a></sup> has done geology a great service by collecting +the evidence as to the mild type of climate which +has again and again prevailed in the past. He lays special +stress on botanical evidence since that pertains to the +variable atmosphere of the lands, and hence furnishes a +better guide than does the evidence of animals that lived +in the relatively unchanging water of the oceans. The +nature of the evidence has already been indicated in +various parts of this book. It includes palms, tree ferns, +and a host of other plants which once grew in regions +which are now much too cold to support them. With this +must be placed the abundant reef-building corals and +other warmth-loving marine creatures in latitudes now +much too cold for them. Of a piece with this are the conditions +of inter-glacial epochs in Europe, for example, +when elephants and hippopotamuses, as well as many +species of plants from low latitudes, were abundant. +These conditions indicate not only that the climate was +warmer than now, but that the contrast from season to +season was much less. Indeed, Knowlton goes so far as +to say that "relative uniformity, mildness, and comparative +equability of climate, accompanied by high humidity, +have prevailed over the greater part of the earth, extending +to, or into, polar circles, during the greater part of +geologic time—since, at least, the Middle Paleozoic. This +is the regular, the ordinary, the normal condition." ... "By +many it is thought that one of the strongest arguments +against a gradually cooling globe and a humid, +non-zonally disposed climate in the ages before the Pleistocene +is the discovery of evidences of glacial action +<!--File: cc192.png Page 168--> +<a class="pagenum break" id="Page_168" title="pg 168"></a> +practically throughout the entire geologic column. +Hardly less than a dozen of these are now known, ranging +in age from Huronian to Eocene. It seems to be a very +general assumption by those who hold this view that +these evidences of glacial activities are to be classed as +ice ages, largely comparable in effect and extent to the +Pleistocene refrigeration, but as a matter of fact only +three are apparently of a magnitude to warrant such +designation. These are the Huronian glaciation, that of +the 'Permo-Carboniferous,' and that of the Pleistocene. +The others, so far as available data go, appear to be +explainable as more or less local manifestations that had +no widespread effect on, for instance, ocean temperatures, +distribution of life, et cetera. They might well have +been of the type of ordinary mountain glaciers, due entirely +to local elevation and precipitation." ... "If the +sun had been the principal source of heat in pre-Pleistocene +time, terrestrial temperatures would of necessity +have been disposed in zones, whereas the whole trend of +this paper has been the presentation of proof that these +temperatures were distinctly non-zonal. Therefore it +seems to follow that the sun—at least the present small-angle +sun—could not have been the sole or even the principal +source of heat that warmed the early oceans."</p> + +<p>Knowlton is so strongly impressed by the widespread +fossil floras that usually occur in the middle parts of the +geological periods, that as Schuchert<sup><a href="#fn62">[62]</a></sup> puts it, he neglects +the evidence of other kinds. In the middle of the periods +and eras the expansion of the warm oceans over the continents +was greatest, while the lands were small and +hence had more or less insular climates of the oceanic +type. At such times, the marine fauna agrees with the +<!--File: cc193.png Page 169--> +<a class="pagenum break" id="Page_169" title="pg 169"></a> +flora in indicating a mild climate. Large colony-forming +foraminifera, stony corals, shelled cephalopods, gastropods +and thick-shelled bivalves, generally the cemented +forms, were common in the Far North and even in the +Arctic. This occurred in the Silurian, Devonian, Pennsylvanian, +and Jurassic periods, yet at other times, such +as the Cretaceous and Eocene, such forms were very +greatly reduced in variety in the northern regions or else +wholly absent. These things, as Schuchert<sup><a href="#fn62" id="r62">[62]</a></sup> says, can only +mean that Knowlton is right when he states that "climatic +zoning such as we have had since the beginning of +the Pleistocene did not obtain in the geologic ages prior +to the Pleistocene." It does not mean, however, that +there was a "non-zonal arrangement" and that the temperature +of the oceans was everywhere the same and +"without widespread effect on the distribution of life."</p> + +<p>Students of paleontology hold that as far back as we +can go in the study of plants, there are evidences of seasons +and of relatively cool climates in high latitudes. The +cycads, for instance, are one of the types most often used +as evidence of a warm climate. Yet Wieland,<sup><a href="#fn63" id="r63">[63]</a></sup> who has +made a lifelong study of these plants, says that many of +them "might well grow in temperate to cool climates. +Until far more is learned about them they should at least +be held as valueless as indices of tropic climates." The +inference is "that either they or their close relatives had +the capacity to live in every clime. There is also a suspicion +that study of the associated ferns may compel revision +of the long-accepted view of the universality of +tropic climates throughout the Mesozoic." Nathorst is +quoted by Wieland as saying, "I think ... that during +the time when the Gingkophytes and Cycadophytes dominated, +<!--File: cc194.png Page 170--> +<a class="pagenum break" id="Page_170" title="pg 170"></a> +many of them must have adapted themselves for +living in cold climates also. Of this I have not the least +doubt."</p> + +<p>Another important line of evidence which Knowlton +and others have cited as a proof of the non-zonal arrangement +of climate in the past, is the vast red beds which are +found in the Proterozoic, late Silurian, Devonian, Permian, +and Triassic, and in some Tertiary formations. +These are believed to resemble laterite, a red and highly +oxidized soil which is found in great abundance in equatorial +regions. Knowlton does not attempt to show that +the red beds present equatorial characteristics in other +respects, but bases his conclusion on the statement that +"red beds are not being formed at the present time in +any desert region." This is certainly an error. As has +already been said, in both the Transcaspian and Takla +Makan deserts, the color of the sand regularly changes +from brown on the borders to pale red far out in the +desert. Kuzzil Kum, or Red Sand, is the native name. +The sands in the center of the desert apparently were +originally washed down from the same mountains as +those on the borders, and time has turned them red. +Since the same condition is reported from the Arabian +Desert, it seems that redness is characteristic of some of +the world's greatest deserts. Moreover, beds of salt and +gypsum are regularly found in red beds, and they can +scarcely originate except in deserts, or in shallow almost +landlocked bays on the coasts of deserts, as appears to +have happened in the Silurian where marine fossils are +found interbedded with gypsum.</p> + +<p>Again, Knowlton says that red beds cannot indicate +deserts because the plants found in them are not +"pinched or depauperate, nor do they indicate xerophytic +adaptations. Moreover, very considerable deposits +<!--File: cc195.png Page 171--> +<a class="pagenum break" id="Page_171" title="pg 171"></a> +of coal are found in red beds in many parts of the world, +which implies the presence of swamps but little above +sea-level."</p> + +<p>Students of desert botany are likely to doubt the force +of these considerations. As MacDougal<sup><a href="#fn64" id="r64">[64]</a></sup> has shown, the +variety of plants in deserts is greater than in moist +regions. Not only do xerophytic desert species prevail, +but halophytes are present in the salty areas, and hygrophytes +in the wet swampy areas, while ordinary mesophytes +prevail along the water courses and are washed +down from the mountains. The ordinary plants, not the +xerophytes, are the ones that are chiefly preserved since +they occur in most abundance near streams where deposition +is taking place. So far as swamps are concerned, few +are of larger size than those of Seistan in Persia, Lop +Nor in Chinese Turkestan, and certain others in the midst +of the Asiatic deserts. Streams flowing from the mountains +into deserts are almost sure to form large swamps, +such as those along the Tarim River in central Asia. +Lake Chad in Africa is another example. In it, too, reeds +are very numerous.</p> + +<p>Putting together the evidence on both sides in this disputed +question, it appears that throughout most of geological +time there is some evidence of a zonal arrangement +of climate. The evidence takes the form of traces of +cool climates, of seasons, and of deserts. Nevertheless, +there is also strong evidence that these conditions were +in general less intense than at present and that times of +relatively warm, moist climate without great seasonal +extremes have prevailed very widely during periods +much longer than those when a zonal arrangement as +<!--File: cc196.png Page 172--> +<a class="pagenum break" id="Page_172" title="pg 172"></a> +marked as that of today prevailed. As Schuchert<sup><a href="#fn65" id="r65">[65]</a></sup> puts it: +"Today the variation on land between the tropics and the +poles is roughly between 110° and -60°F., in the oceans +between 85° and 31°F. In the geologic past the temperature +of the oceans for the greater parts of the periods +probably was most often between 85° and 55°F., while on +land it may have varied between 90° and 0°F. At rare +intervals the extremes were undoubtedly as great as they +are today. The conclusion is therefore that at all times +the earth had temperature zones, varying between the +present-day intensity and times which were almost without +such belts, and at these latter times the greater part +of the earth had an almost uniformly mild climate, without +winters."</p> + +<p>It is these mild climates which we must now attempt +to explain. This leads us to inquire what would happen to +the climate of the earth as a whole if the conditions which +now prevail at times of few sunspots were to become +intensified. That they could become greatly intensified +seems highly probable, for there is good reason to think +that aside from the sunspot cycle the sun's atmosphere +is in a disturbed condition. The prominences which +sometimes shoot out hundreds of thousands of miles +seem to be good evidence of this. Suppose that the sun's +atmosphere should become very quiet. This would apparently +mean that cyclonic storms would be much less +numerous and less severe than during the present times +of sunspot minima. The storms would also apparently +follow paths in middle latitudes somewhat as they do +now when sunspots are fewest. The first effect of such a +condition, if we can judge from what happens at present, +would be a rise in the general temperature of the earth, +because less heat would be carried aloft by storms.</p> +<!--File: cc197.png Page 173--> +<a class="pagenum break" id="Page_173" title="pg 173"></a> +<p>Today, as is shown in <em>Earth and Sun</em>, a difference of +perhaps 10 per cent in the average storminess during +periods of sunspot maxima and minima is correlated with +a difference of 3°C. in the temperature at the earth's +surface. This includes not only an actual lowering of +0.6°C. at times of sunspot maxima, but the overcoming +of the effect of increased insolation at such times, an +effect which Abbot calculates as about 2.5°C. If the +storminess were to be reduced to one-half or one-quarter +its present amount at sunspot minima, not only would the +loss of heat by upward convection in storms be diminished, +but the area covered by clouds would diminish so +that the sun would have more chance to warm the lower +air. Hence the average rise of temperature might amount +to as much at 5° or 10°C.</p> + +<p>Another effect of the decrease in storminess would be +to make the so-called westerly winds, which are chiefly +southwesterly in the northern hemisphere and northwesterly +in the southern hemisphere, more strong and +steady than at present. They would not continually suffer +interruption by cyclonic winds from other directions, as +is now the case, and would have a regularity like that +of the trades. This conclusion is strongly reënforced in +a paper by Clayton<sup><a href="#fn66" id="r66">[66]</a></sup> which came to hand after this chapter +had been completed. From his studies of the solar +constant and the temperature of the earth which are +described in <em>Earth and Sun</em>, he reaches the following +conclusion: "The results of these researches have led +me to believe: 1. That if there were no variation in solar +radiation the atmospheric motions would establish a +stable system with exchanges of air between equator and +pole and between ocean and land, in which the only variations +<!--File: cc198.png Page 174--> +<a class="pagenum break" id="Page_174" title="pg 174"></a> +would be daily and annual changes set in operation +by the relative motions of the earth and sun. 2. The existing +abnormal changes, which we call weather, have their +origins chiefly, if not entirely, in the variations of solar +radiation."</p> + +<p>If cyclonic storms and "weather" were largely eliminated +and if the planetary system of winds with its +steady trades and southwesterlies became everywhere +dominant, the regularity and volume of the poleward-flowing +currents, such as the Gulf Stream and the +Atlantic Drift in one ocean, and the Japanese Current in +another, would be greatly increased. How important this +is may be judged from the work of Helland-Hansen and +Nansen.<sup><a href="#fn67" id="r67">[67]</a></sup> These authors find that with the passage of each +cyclonic storm there is a change in the temperature of +the surface water of the Atlantic Ocean. Winds at right +angles to the course of the Drift drive the water first in +one direction and then in the other but do not advance it +in its course. Winds with an easterly component, on the +other hand, not only check the Drift but reverse it, driving +the warm water back toward the southwest and +allowing cold water to well up in its stead. The driving +force in the Atlantic Drift is merely the excess of the +winds with a westerly component over those with an +easterly component.</p> + +<p>Suppose that the numbers in Fig. 8 represent the +strength of the winds in a certain part of the North +Atlantic or North Pacific, that is, the total number of +miles moved by the air per year. In quadrant A of the +left-hand part all the winds move from a more or less +southwesterly direction and produce a total movement +<!--File: cc199.png Page 175--> +<a class="pagenum break" id="Page_175" title="pg 175"></a> +of the air amounting to thirty units per year. Those +coming from points between north and west move twenty-five +units; those between north and east, twenty units; +and those between east and south, twenty-five units. +Since the movement of the winds in quadrants B and +D is the same, these winds have no effect in producing +currents. They merely move the water back and forth, +and thus give it time to lose whatever heat it has brought +from more southerly latitudes. On the other hand, since +the easterly winds in quadrant C do not wholly check the +currents caused by the westerly winds of quadrant A, +the effective force of the westerly winds amounts to ten, +or the difference between a force of thirty in quadrant A +and of twenty in quadrant C. Hence the water is moved +forward toward the northeast, as shown by the thick +part of arrow A.</p> +<img src="images/fig8.png" width="798" height="433" alt="Fig. 8" id="Fig_8" /> +<p class="center"><em>Fig. 8. Effect of diminution of storms on +movement of water.</em></p> + +<p>Now suppose that cyclonic storms should be greatly +reduced in number so that in the zone of prevailing +westerlies they were scarcely more numerous than tropical +<!--File: cc200.png Page 176--> +<a class="pagenum break" id="Page_176" title="pg 176"></a> +hurricanes now are in the trade-wind belt. Then the +more or less southwesterly winds in quadrant A´ in the +right-hand part of Fig. 8 would not only become more +frequent but would be stronger than at present. The +total movement from that quarter might rise to sixty +units, as indicated in the figure. In quadrants B´ and D´ +the movement would fall to fifteen and in quadrant C´ to +ten. B´ and D´ would balance one another as before. The +movement in A´, however, would exceed that in C´ by fifty +instead of ten. In other words, the current-making force +would become five times as great as now. The actual +effect would be increased still more, for the winds from +the southwest would be stronger as well as steadier if +there were no storms. A strong wind which causes whitecaps +has much more power to drive the water forward +than a weaker wind which does not cause whitecaps. In a +wave without a whitecap the water returns to practically +the original point after completing a circle beneath the +surface. In a wave with a whitecap, however, the cap +moves forward. Any increase in velocity beyond the rate +at which whitecaps are formed has a great influence upon +the amount of water which is blown forward. Several +times as much water is drifted forward by a persistent +wind of twenty miles an hour as by a ten-mile wind.<sup><a href="#fn68" id="r68">[68]</a></sup></p> + +<p>In this connection a suggestion which is elaborated in +Chapter XIII may be mentioned. At present the salinity +of the oceans checks the general deep-sea circulation and +thereby increases the contrasts from zone to zone. In the +past, however, the ocean must have been fresher than +now. Hence the circulation was presumably less impeded, +and the transfer of heat from low latitudes to high was +facilitated.</p> + +<!--File: cc201.png Page 177--> +<a class="pagenum break" id="Page_177" title="pg 177"></a> + + +<p>Consider now the magnitude of the probable effect of +a diminution in storms. Today off the coast of Norway +in latitude 65°N. and longitude 10°E., the mean temperature +in January is 2°C. and in July 12°C. This represents +a plus anomaly of about 22° in January and 2° in July; +that is, the Norwegian coast is warmer than the normal +for its latitude by these amounts. Suppose that in some +past time the present distribution of lands and seas prevailed, +but Norway was a lowland where extensive deposits +could accumulate in great flood plains. Suppose, +also, that the sun's atmosphere was so inactive that few +cyclonic storms occurred, steady winds from the west-southwest +prevailed, and strong, uninterrupted ocean +currents brought from the Caribbean Sea and Gulf of +Mexico much greater supplies of warm water than at +present. The Norwegian winters would then be warmer +than now not only because of the general increase in temperature +which the earth regularly experiences at sunspot +minima, but because the currents would accentuate +this condition. In summer similar conditions would prevail +except that the warming effect of the winds and +currents would presumably be less than in winter, but +this might be more than balanced by the increased heat +of the sun during the long summer days, for storms and +clouds would be rare.</p> + +<p>If such conditions raised the winter temperature only +8°C. and the summer temperature 4°C., the climate would +be as warm as that of the northern island of New Zealand +(latitude 35°-43°S.). The flora of that part of New Zealand +is subtropical and includes not only pines and +beeches, but palms and tree ferns. A climate scarcely +warmer than that of New Zealand would foster a flora +like that which existed in far northern latitudes during +some of the milder geological periods. If, however, the +<!--File: cc202.png Page 178--> +<a class="pagenum break" id="Page_178" title="pg 178"></a> +general temperature of the earth's surface were raised +5° because of the scarcity of storms, if the currents were +strong enough so that they increased the present anomaly +by 50 per cent, and if more persistent sunshine in summer +raised the temperature at that season about 4°C., the +January temperature would be 18°C. and the July temperature +22°C. These figures perhaps make summer and +winter more nearly alike than was ever really the case in +such latitudes. Nevertheless, they show that a diminution +of storms and a consequent strengthening and steadying +of the southwesterlies might easily raise the temperature +of the Norwegian coast so high that corals could flourish +within the Arctic Circle.</p> + +<p>Another factor would coöperate in producing mild +temperatures in high latitudes during the winter, namely, +the fogs which would presumably accumulate. It is well +known that when saturated air from a warm ocean is +blown over the lands in winter, as happens so often in the +British Islands and around the North Sea, fog is formed. +The effect of such a fog is indeed to shut out the sun's +radiation, but in high latitudes during the winter when +the sun is low, this is of little importance. Another effect +is to retain the heat of the earth itself. When a constant +supply of warm water is being brought from low latitudes +this blanketing of the heat by the fog becomes of +great importance. In the past, whenever cyclonic storms +were weak and westerly winds were correspondingly +strong, winter fogs in high latitudes must have been much +more widespread and persistent than now.</p> + +<p>The bearing of fogs on vegetation is another interesting +point. If a region in high latitudes is constantly protected +by fog in winter, it can support types of vegetation +characteristic of fairly low latitudes, for plants are +oftener killed by dry cold than by moist cold. Indeed, +<!--File: cc203.png Page 179--> +<a class="pagenum break" id="Page_179" title="pg 179"></a> +excessive evaporation from the plant induced by dry +cold when the evaporated water cannot be rapidly replaced +by the movement of sap is a chief reason why +large plants are winterkilled. The growing of transplanted +palms on the coast of southwestern Ireland, in +spite of its location in latitude 50°N., is possible only because +of the great fogginess in winter due to the marine +climate. The fogs prevent the escape of heat and ward off +killing frosts. The tree ferns in latitude 46°S. in New +Zealand, already referred to, are often similarly protected +in winter. Therefore, the relative frequency of fogs +in high latitudes when storms were at a minimum would +apparently tend not merely to produce mild winters but +to promote tropical vegetation.</p> + +<p>The strong steady trades and southwesterlies which +would prevail at times of slight solar activity, according +to our hypothesis, would have a pronounced effect on the +water of the deep seas as well as upon that of the surface. +In the first place, the deep-sea circulation would be hastened. +For convenience let us speak of the northern hemisphere. +In the past, whenever the southwesterly winds +were steadier than now, as was probably the case when +cyclonic storms were relatively rare, more surface water +than at present was presumably driven from low latitudes +and carried to high latitudes. This, of course, means that +a greater volume of water had to flow back toward the +equator in the lower parts of the ocean, or else as a cool +surface current. The steady southwesterly winds, however, +would interfere with south-flowing surface currents, +thus compelling the polar waters to find their way +equatorward beneath the surface. In low latitudes the +polar waters would rise and their tendency would be to +lower the temperature. Hence steadier westerlies would +make for lessened latitudinal contrasts in climate not +<!--File: cc204.png Page 180--> +<a class="pagenum break" id="Page_180" title="pg 180"></a> +only by driving more warm water poleward but by causing +more polar water to reach low latitudes.</p> + +<p>At this point a second important consideration must be +faced. Not only would the deep-sea circulation be hastened, +but the ocean depths might be warmed. The deep +parts of the ocean are today cold because they receive +their water from high latitudes where it sinks because of +low temperature. Suppose, however, that a diminution in +storminess combined with other conditions should permit +corals to grow in latitude 70°N. The ocean temperature +would then have to average scarcely lower than 20°C. +and even in the coldest month the water could scarcely +fall below about 15°C. Under such conditions, if the polar +ocean were freely connected with the rest of the oceans, +no part of it would probably have a temperature much +below 10°C., for there would be no such thing as ice caps +and snowfields to reflect the scanty sunlight and radiate +into space what little heat there was. On the contrary, +during the winter an almost constant state of dense fogginess +would prevail. So great would be the blanketing +effect of this that a minimum monthly temperature of +10°C. for the coldest part of the ocean may perhaps be +too low for a time when corals thrived in latitude 70°.</p> + +<p>The temperature of the ocean depths cannot permanently +remain lower than that of the coldest parts of the +surface. Temporarily this might indeed happen when a +solar change first reduced the storminess and strengthened +the westerlies and the surface currents. Gradually, +however, the persistent deep-sea circulation would bring +up the colder water in low latitudes and carry downward +the water of medium temperature at the coldest part of +the surface. Thus in time the whole body of the ocean +would become warm. The heat which at present is carried +away from the earth's surface in storms would slowly +<!--File: cc205.png Page 181--> +<a class="pagenum break" id="Page_181" title="pg 181"></a> +accumulate in the oceans. As the process went on, all +parts of the ocean's surface would become warmer, for +equatorial latitudes would be less and less cooled by cold +water from below, while the water blown from low latitudes +to high would be correspondingly warmer. The +warming of the ocean would come to an end only with +the attainment of a state of equilibrium in which the loss +of heat by radiation and evaporation from the ocean's +surface equaled the loss which under other circumstances +would arise from the rise of warm air in cyclonic storms. +When once the oceans were warmed, they would form an +extremely strong conservative force tending to preserve +an equable climate in all latitudes and at all seasons. +According to the solar cyclonic hypothesis such conditions +ought to have prevailed throughout most of geological +time. Only after a strong and prolonged solar +disturbance with its consequent storminess would conditions +like those of today be expected.</p> + +<p>In this connection another possibility may be mentioned. +It is commonly assumed that the earth's axis is +held steadily in one direction by the fact that the rotating +earth is a great gyroscope. Having been tilted to a certain +position, perhaps by some extraneous force, the axis +is supposed to maintain that position until some other +force intervenes. Cordeiro,<sup><a href="#fn69" id="r69">[69]</a></sup> however, maintains that this +is true only of an absolutely rigid gyroscope. He believes +that it is mathematically demonstrable that if an elastic +gyroscope be gradually tilted by some extraneous force, +and if that force then ceases to act, the gyroscope as a +whole will oscillate back and forth. The earth appears to +be slightly elastic. Cordeiro therefore applies his formulæ +to it, on the following assumptions: (1) That the +original position of the axis was nearly vertical to the +<!--File: cc206.png Page 182--> +<a class="pagenum break" id="Page_182" title="pg 182"></a> +plane of the ecliptic in which the earth revolves around +the sun; (2) that at certain times the inclination has been +even greater than now; and (3) that the position of the +axis with reference to the earth has not changed to any +great extent, that is, the earth's poles have remained +essentially stationary with reference to the earth, although +the whole earth has been gyroscopically tilted +back and forth repeatedly.</p> + +<p>With a vertical axis the daylight and darkness in all +parts of the earth would be of equal duration, being +always twelve hours. There would be no seasons, and the +climate would approach the average condition now experienced +at the two equinoxes. On the whole the climate +of high latitudes would give the impression of being +milder than now, for there would be less opportunity for +the accumulation of snow and ice with their strong cooling +effect. On the other hand, if the axis were tilted more +than now, the winter nights would be longer and the +winters more severe than at present, and there would be +a tendency toward glaciation. Thus Cordeiro accounts +for alternating mild and glacial epochs. The entire swing +from the vertical position to the maximum inclination +and back to the vertical may last millions of years depending +on the earth's degree of elasticity. The swing +beyond the vertical position in the other direction would +be equally prolonged. Since the axis is now supposed to +be much nearer its maximum than its minimum degree of +tilting, the duration of epochs having a climate more +severe than that of the present would be relatively short, +while the mild epochs would be long.</p> + +<p>Cordeiro's hypothesis has been almost completely +ignored. One reason is that his treatment of geological +facts, and especially his method of riding rough-shod +over widely accepted conclusions, has not commended his +<!--File: cc207.png Page 183--> +<a class="pagenum break" id="Page_183" title="pg 183"></a> +work to geologists. Therefore they have not deemed it +worth while to urge mathematicians to test the assumptions +and methods by which he reached his results. It is +perhaps unfair to test Cordeiro by geology, for he lays +no claim to being a geologist. In mathematics he labors +under the disadvantage of having worked outside the +usual professional channels, so that his work does not +seem to have been subjected to sufficiently critical +analysis.</p> + +<p>Without expressing any opinion as to the value of +Cordeiro's results we feel that the subject of the earth's +gyroscopic motion and of a possible secular change in +the direction of the axis deserves investigation for two +chief reasons. In the first place, evidences of seasonal +changes and of seasonal uniformity seem to occur more +or less alternately in the geological record. Second, the +remarkable discoveries of Garner and Allard<sup><a href="#fn70" id="r70">[70]</a></sup> show that +the duration of daylight has a pronounced effect upon +the reproduction of plants. We have referred repeatedly +to the tree ferns, corals, and other forms of life which +now live in relatively low latitudes and which cannot +endure strong seasonal contrasts, but which once lived +far to the north. On the other hand, Sayles,<sup><a href="#fn71" id="r71">[71]</a></sup> for example, +finds that microscopical examination of the banding of +ancient shales and slates indicates distinct seasonal banding +like that of recent Pleistocene clays or of the Squantum +slate formed during or near the Permian glacial +period. Such seasonal banding is found in rocks of various +ages: (a) Huronian, in cobalt shales previously +reported by Coleman; (b) late Proterozoic or early Cambrian, +<!--File: cc208.png Page 184--> +<a class="pagenum break" id="Page_184" title="pg 184"></a> +in Hiwassee slate; (c) lower Cambrian, in Georgian +slates of Vermont; (d) lower Ordovician, in Georgia +(Rockmart slate), Tennessee (Athens shale), Vermont +(slates), and Quebec (Beekmantown formation); and (e) +Permian in Massachusetts (Squantum slate). How far +the periods during which such evidence of seasons was +recorded really alternated with mild periods, when tropical +species lived in high latitudes and the contrast of +seasons was almost or wholly lacking, we have as yet no +means of knowing. If periods characterized by marked +seasonal changes should be found to have alternated with +those when the seasons were of little importance, the fact +would be of great geological significance.</p> + +<p>The discoveries of Garner and Allard as to the effect +of light on reproduction began with a peculiar tobacco +plant which appeared in some experiments at Washington. +The plant grew to unusual size, and seemed to +promise a valuable new variety. It formed no seeds, however, +before the approach of cold weather. It was therefore +removed to a greenhouse where it flowered and +produced seed. In succeeding years the flowering was +likewise delayed till early winter, but finally it was discovered +that if small plants were started in the greenhouse +in the early fall they flowered at the same time as +the large ones. Experiments soon demonstrated that the +time of flowering depends largely upon the length of the +daily period when the plants are exposed to light. The +same is true of many other plants, and there is great +variety in the conditions which lead to flowering. Some +plants, such as witch hazel, appear to be stimulated to +bloom by very short days, while others, such as evening +primrose, appear to require relatively long days. So +sensitive are plants in this respect that Garner and +Allard, by changing the length of the period of light, have +<!--File: cc209.png Page 185--> +<a class="pagenum break" id="Page_185" title="pg 185"></a> +caused a flowerbud in its early stages not only to stop +developing but to return once more to a vegetative shoot.</p> + +<blockquote> +<p>Common iris, which flowers in May and June, will not blossom +under ordinary conditions when grown in the greenhouse in +winter, even under the same temperature conditions that prevail +in early summer. Again, one variety of soy beans will regularly +begin to flower in June of each year, a second variety in July, +and a third in August, when all are planted on the same date. +There are no temperature differences during the summer months +which could explain these differences in time of flowering; and, +since "internal causes" alone cannot be accepted as furnishing +a satisfactory explanation, some external factor other than temperature +must be responsible.</p> + +<p>The ordinary varieties of cosmos regularly flower in the fall +in northern latitudes if they are planted in the spring or summer. +If grown in a warm greenhouse during the winter months the +plants also flower readily, so that the cooler weather of fall is +not a necessary condition. If successive plantings of cosmos are +made in the greenhouse during the late winter and early spring +months, maintaining a uniform temperature throughout, the +plantings made after a certain date will fail to blossom promptly, +but, on the contrary, will continue to grow till the following fall, +thus flowering at the usual season for this species. This curious +reversal of behavior with advance of the season cannot be attributed +to change in temperature. Some other factor is responsible +for the failure of cosmos to blossom during the summer months. +In this respect the behavior of cosmos is just the opposite of that +observed in iris.</p> + +<p>Certain varieties of soy beans change their behavior in a +peculiar manner with advance of the summer season. The variety +known as Biloxi, for example, when planted early in the spring +in the latitude of Washington, D. C., continues to grow throughout +the summer, flowering in September. The plants maintain +growth without flowering for fifteen to eighteen weeks, attaining +a height of five feet or more. As the dates of successive plantings +are moved forward through the months of June and July, however, +<!--File: cc210.png Page 186--> +<a class="pagenum break" id="Page_186" title="pg 186"></a> +there is a marked tendency for the plants to cut short the +period of growth which precedes flowering. This means, of course, +that there is a tendency to flower at approximately the same time +of year regardless of the date of planting. As a necessary consequence, +the size of the plants at the time of flowering is reduced +in proportion to the delay in planting.</p> +</blockquote> + +<p>The bearing of this on geological problems lies in a +query which it raises as to the ability of a genus or family +of plants to adapt itself to days of very different length +from those to which it is wonted. Could tree ferns, ginkgos, +cycads, and other plants whose usual range of location +never subjects them to daylight for more than +perhaps fourteen hours or less than ten, thrive and reproduce +themselves if subjected to periods of daylight +ranging all the way from nothing up to about twenty-four +hours? No answer to this is yet possible, but the +question raises most interesting opportunities of investigation. +If Cordeiro is right as to the earth's elastic +gyroscopic motion, there may have been certain periods +when a vertical or almost vertical axis permitted the +days to be of almost equal length at all seasons in all +latitudes. If such an absence of seasons occurred when +the lands were low, when the oceans were extensive and +widely open toward the poles, and when storms were +relatively inactive, the result might be great mildness of +climate such as appears sometimes to have prevailed in +the middle of geological eras. Suppose on the other hand +that the axis should be tilted more than now, and that +the lands should be widely emergent and the storm belt +highly active in low latitudes, perhaps because of the +activity of the sun. The conditions might be favorable for +glaciation at latitudes as low as those where the Permo-Carboniferous +ice sheets appear to have centered. The +possibilities thus suggested by Cordeiro's hypothesis are +<!--File: cc211.png Page 187--> +<a class="pagenum break" id="Page_187" title="pg 187"></a> +so interesting that the gyroscopic motion of the earth +ought to be investigated more thoroughly. Even if no +such gyroscopic motion takes place, however, the other +causes of mild climate discussed in this chapter may +be enough to explain all the observed phenomena.</p> + +<p>Many important biological consequences might be +drawn from this study of mild geological climates, but +this book is not the place for them. In the first chapter +we saw that one of the most remarkable features of the +climate of the earth is its wonderful uniformity through +hundreds of millions of years. As we come down through +the vista of years the mild geological periods appear to +represent a return as nearly as possible to this standard +condition of uniformity. Certain changes of the earth +itself, as we shall see in the next chapter, may in the long +run tend slightly to change the exact conditions of this +climatic standard, as we might perhaps call it. Yet they +act so slowly that their effect during hundreds of millions +of years is still open to question. At most they seem +merely to have produced a slight increase in diversity +from season to season and from zone to zone. The normal +climate appears still to be of a milder type than that +which happens to prevail at present. Some solar condition, +whose possible nature will be discussed later, seems +even now to cause the number of cyclonic storms to be +greater than normal. Hence the earth's climate still +shows something of the great diversity of seasons and +of zones which is so marked a characteristic of glacial +epochs.</p> +<!--File: cc212.png Page 188--> +<a class="pagenum break" id="Page_188" title="pg 188"></a> + + + + +<h2 id="Chapt_XI"><a href="#content">CHAPTER XI</a></h2> + +<h3>TERRESTRIAL CAUSES OF CLIMATIC CHANGES</h3> + + +<p>The major portion of this book has been concerned +with the explanation of the more abrupt and extreme +changes of climate. This chapter and the +next consider two other sorts of climatic changes, the +slight secular progression during the hundreds of millions +of years of recorded earth history, and especially +the long slow geologic oscillations of millions or tens of +millions of years. It is generally agreed among geologists +that the progressive change has tended toward greater +extremes of climate; that is, greater seasonal contrasts, +and greater contrasts from place to place and from zone +to zone.<sup><a href="#fn72" id="r72">[72]</a></sup> The slow cyclic changes have been those that +favored widespread glaciation at one extreme near the +ends of geologic periods and eras, and mild temperatures +even in subpolar regions at the other extreme during the +medial portions of the periods.</p> + +<p>As has been pointed out in an earlier chapter, it has +often been assumed that all climatic changes are due to +terrestrial causes. We have seen, however, that there is +strong evidence that solar variations play a large part in +modifying the earth's climate. We have also seen that no +known terrestrial agency appears to be able to produce +the abrupt changes noted in recent years, the longer +<!--File: cc213.png Page 189--> +<a class="pagenum break" id="Page_189" title="pg 189"></a> +cycles of historical times, or geological changes of the +shorter type, such as glaciation. Nevertheless, terrestrial +changes doubtless have assisted in producing both the +progressive change and the slow cyclic changes recorded +in the rocks, and it is the purpose of this chapter and the +two that follow to consider what terrestrial changes have +taken place and the probable effect of such changes.</p> + +<p>The terrestrial changes that have a climatic significance +are numerous. Some, such as variations in the +amount of volcanic dust in the higher air, have been considered +in an earlier chapter. Others are too imperfectly +known to warrant discussion, and in addition there are +presumably others which are entirely unknown. Doubtless +some of these little known or unknown changes have +been of importance in modifying climate. For example, +the climatic influence of vegetation, animals, and man +may be appreciable. Here, however, we shall confine ourselves +to purely physical causes, which will be treated in +the following order: First, those concerned with the solid +parts of the earth, namely: (I) amount of land; (II) distribution +of land; (III) height of land; (IV) lava flows; +and (V) internal heat. Second, those which arise from +the salinity of oceans, and third, those depending on the +composition and amount of atmosphere.</p> + +<p>The terrestrial change which appears indirectly to +have caused the greatest change in climate is the contraction +of the earth. The problem of contraction is +highly complex and is as yet only imperfectly understood. +Since only its results and not its processes influence climate, +the following section as far as page 196 is not +necessary to the general reader. It is inserted in order to +explain why we assume that there have been oscillations +between certain types of distribution of the lands.</p> + +<p>The extent of the earth's contraction may be judged +<!--File: cc214.png Page 190--> +<a class="pagenum break" id="Page_190" title="pg 190"></a> +from the shrinkage indicated by the shortening of the +rock formations in folded mountains such as the Alps, +Juras, Appalachians, and Caucasus. Geologists are continually +discovering new evidence of thrust faults of +great magnitude where masses of rock are thrust bodily +over other rocks, sometimes for many miles. Therefore, +the estimates of the amount of shrinkage based on the +measurements of folds and faults need constant revision +upward. Nevertheless, they have already reached a considerable +figure. For example, in 1919, Professor A. Heim +estimated the shortening of the meridian passing through +the modern Alps and the ancient Hercynian and Caledonian +mountains as fully a thousand miles in Europe, +and over five hundred miles for the rest of this meridian.<sup><a href="#fn73" id="r73">[73]</a></sup> +This is a radial shortening of about 250 miles. Possibly +the shrinkage has been even greater than this. Chamberlin<sup><a href="#fn74" id="r74">[74]</a></sup> +has compared the density of the earth, moon, Mars, +and Venus with one another, and found it probable that +the radial shrinkage of the earth may be as much as +570 miles. This result is not so different from Heim's as +appears at first sight, for Heim made no allowance for +unrecognized thrust faults and for the contraction incident +to metamorphism. Moreover, Heim did not include +shrinkage during the first half of geological time before +the above-mentioned mountain systems were upheaved.</p> + +<p>According to a well-established law of physics, contraction +of a rotating body results in more rapid rotation +and greater centrifugal force. These conditions must increase +the earth's equatorial bulge and thereby cause +changes in the distribution of land and water. Opposed +to the rearrangement of the land due to increased rotation +<!--File: cc215.png Page 191--> +<a class="pagenum break" id="Page_191" title="pg 191"></a> +caused by contraction, there has presumably been +another rearrangement due to tidal retardation of the +earth's rotation and a consequent lessening of the equatorial +bulge. G. H. Darwin long ago deduced a relatively +large retardation due to lunar tides. A few years ago +W. D. MacMillan, on other assumptions, deduced only a +negligible retardation. Still more recently Taylor<sup><a href="#fn75" id="r75">[75]</a></sup> has +studied the tides of the Irish Sea, and his work has led +Jeffreys<sup><a href="#fn76" id="r76">[76]</a></sup> and +Brown<sup><a href="#fn77" id="r77">[77]</a></sup> to conclude that there has been considerable +retardation, perhaps enough, according to +Brown, to equal the acceleration due to the earth's contraction. +From a prolonged and exhaustive study of the +motions of the moon Brown concludes that tidal friction +or some other cause is now lengthening the day at the rate +of one second per thousand years, or an hour in almost +four million years if the present rate continues. He makes +it clear that the retardation due to tides would not correspond +in point of time with the acceleration due to contraction. +The retardation would occur slowly, and would +take place chiefly during the long quiet periods of geologic +history, while the acceleration would occur rapidly +at times of diastrophic deformation. As a consequence, +the equatorial bulge would alternately be reduced at a +slow rate, and then somewhat suddenly augmented.</p> + +<p>The less rigid any part of the earth is, the more quickly +it responds to the forces which lead to bulging or which +tend to lessen the bulge. Since water is more fluid than +land, the contraction of the earth and the tidal retardation +presumably tend alternately to increase and decrease +the amount of water near the equator more than the +<!--File: cc216.png Pag2 192--> +<a class="pagenum break" id="Page_192" title="pg 192"></a> +amount of land. Thus, throughout geological history we +should look for cyclic changes in the relative area of the +lands within the tropics and similar changes of opposite +phase in higher latitudes. The extent of the change would +depend upon (a) the amount of alteration in the speed +of rotation, and (b) the extent of low land in low latitudes +and of shallow sea in high latitudes. According to +Slichter's tables, if the earth should rotate in twenty-three +hours instead of twenty-four, the great Amazon +lowland would be submerged by the inflow of oceanic +water, while wide areas in Hudson Bay, the North Sea, +and other northern regions, would become land because +the ocean water would flow away from them.<sup><a href="#fn78" id="r78">[78]</a></sup></p> + +<p>Following the prompt equatorward movement of water +which would occur as the speed of rotation increased, +there must also be a gradual movement or creepage of the +solid rocks toward the equator, that is, a bulging of the +ocean floor and of the lands in low latitudes, with a consequent +emergence of the lands there and a relative +rise of sea level in higher latitudes. Tidal retardation +would have a similar effect. Suess<sup><a href="#fn79" id="r79">[79]</a></sup> has described widespread +elevated strand lines in the tropics which he interprets +as indicating a relatively sudden change in sea +level, though he does not suggest a cause of the change. +However, in speaking of recent geological times, Suess +reports that a movement more recent than the old +strands "was an accumulation of water toward the +equator, a diminution toward the poles, and (it appears) +as though this last movement were only one of the many +oscillations which succeed each other with the same tendency, +i.e., with a positive excess at the equator, a negative +<!--File: cc217.png Page 193--> +<a class="pagenum break" id="Page_193" title="pg 193"></a> +excess at the poles." (Vol. II, p. 551.) This creepage +of the rocks equatorward seemingly might favor the +growth of mountains in tropical and subtropical regions, +because it is highly improbable that the increase in the +bulge would go on in all longitudes with perfect uniformity. +Where it went on most rapidly mountains would +arise. That such irregularity of movement has actually +occurred is suggested not only by the fact that many +Cenozoic and older mountain ranges extend east and +west, but by the further fact that these include some of +our greatest ranges, many of which are in fairly low latitudes. +The Himalayas, the Javanese ranges, and the half-submerged +Caribbean chains are examples. Such mountains +suggest a thrust in a north and south direction +which is just what would happen if the solid mass of the +earth were creeping first equatorward and then poleward.</p> + +<p>A fact which is in accord with the idea of a periodic +increase in the oceans in low latitudes because of renewed +bulging at the equator is the exposure in moderately +high latitudes of the greatest extent of ancient rocks. +This seems to mean that in low latitudes the frequent +deepening of the oceans has caused the old rocks to be +largely covered by sediments, while the old lands in +higher latitudes have been left more fully exposed to +erosion.</p> + +<p>Another suggestion of such periodic equatorward movements +of the ocean water is found in the reported contrast +between the relative stability with which the northern part +of North America has remained slightly above sea level +except at times of widespread submergence, while the +southern parts have suffered repeated submergence alternating +with great emergence.<sup><a href="#fn80" id="r80">[80]</a></sup> Furthermore, although +<!--File: cc218.png Page 194--> +<a class="pagenum break" id="Page_194" title="pg 194"></a> +the northern part of North America has been generally +exposed to erosion since the Proterozoic, it has supplied +much less sediment than have the more southern land +areas.<sup><a href="#fn81" id="r81">[81]</a></sup> This apparently means that much of Canada has +stood relatively low, while repeated and profound uplift +alternating with depression has occurred in subtropical +latitudes, apparently in adjustment to changes in the +earth's speed of rotation. The uplifts generally followed +the times of submergence due to equatorward movement +of the water, though the buckling of the crust which accompanies +shrinkage doubtless caused some of the submergence. +The evidence that northern North America +stood relatively low throughout much of geological time +depends not only on the fact that little sediment came to +the south from the north, but also on the fact that at +times of especially widespread epicontinental seas, the +submergence was initiated at the north.<sup><a href="#fn82" id="r82">[82]</a></sup> This is especially +true for Ordovician, Silurian, Devonian, and Jurassic +times in North America. General submergence of this +kind is supposed to be due chiefly to the overflowing of +the ocean when its level is slowly raised by the deposition +of sediment derived from the erosion of what once were +continental highlands but later are peneplains. The fact +that such submergence began in high latitudes, however, +seems to need a further explanation. The bulging of the +rock sphere at the equator and the consequent displacement +of some of the water in low latitudes would furnish +such an explanation, as would also a decrease in the speed +of rotation induced by tidal retardation, if that retardation +were great enough and rapid enough to be geologically +effective.</p> +<!--File: cc219.png Page 195--> +<a class="pagenum break" id="Page_195" title="pg 95"></a> + +<p>The climatic effects of the earth's contraction, which +we shall shortly discuss, are greatly complicated by the +fact that contraction has taken place irregularly. Such +irregularity has occurred in spite of the fact that the +processes which cause contraction have probably gone +on quite steadily throughout geological history. These +processes include the chemical reorganization of the minerals +of the crust, a process which is illustrated by the +metamorphism of sedimentary rocks into crystalline +forms. The escape of gases through volcanic action or +otherwise has been another important process.</p> + +<p>Although the processes which cause contraction probably +go on steadily, their effect, as Chamberlin<sup><a href="#fn83" id="r83">[83]</a></sup> and +others have pointed out, is probably delayed by inertia. +Thus the settling of the crust or its movement on a large +scale is delayed. Perhaps the delay continues until the +stresses become so great that of themselves they overcome +the inertia, or possibly some outside agency, whose +nature we shall consider later, reënforces the stresses and +gives the slight impulse which is enough to release them +and allow the earth's crust to settle into a new state of +equilibrium. When contraction proceeds actively, the +ocean segments, being largest and heaviest, are likely to +settle most, resulting in a deepening of the oceans and an +emergence of the lands. Following each considerable contraction +there would be an increase in the speed of rotation. +The repeated contractions with consequent growth +of the equatorial bulge would alternate with long quiet +periods during which tidal retardation would again decrease +the speed of rotation and hence lessen the bulge. +The result would be repeated changes of distribution of +land and water, with consequent changes in climate.</p> +<!--File: cc220.png0 Page 196--> +<a class="pagenum break" id="Page_196" title="pg 196"></a> + +<p>I. We shall now consider the climatic effect of the +repeated changes in the relative amounts of land and +water which appear to have resulted from the earth's +contraction and from changes in its speed of rotation. +During many geologic epochs a larger portion of the +earth was covered with water than at present. For example, +during at least twelve out of about twenty epochs, +North America has suffered extensive inundations,<sup><a href="#fn84" id="r84">[84]</a></sup> and +in general the extensive submergence of Europe, the +other area well known geologically, has coincided with +that of North America. At other times, the ocean has +been less extensive than now, as for example during the +recent glacial period, and probably during several of +the glacial periods of earlier date. Each of the numerous +changes in the relative extent of the lands must have +resulted in a modification of climate.<sup><a href="#fn85" id="r85">[85]</a></sup> This modification +would occur chiefly because water becomes warm far +more slowly than land, and cools off far more slowly.</p> + +<p>An increase in the lands would cause changes in several +climatic conditions. (a) The range of temperature between +day and night and between summer and winter +would increase, for lands become warmer by day and in +summer than do oceans, and cooler at night and in +winter. The higher summer temperature when the lands +are widespread is due chiefly to the fact that the land, if +not snow-covered, absorbs more of the sun's radiant +energy than does the ocean, for its reflecting power is +low. The lower winter temperature when lands are widespread +occurs not only because they cool off rapidly but +<!--File: cc221.png Page 197--> +<a class="pagenum break" id="Page_197" title="pg 197"></a> +because the reduced oceans cannot give them so much +heat. Moreover, the larger the land, the more generally +do the winds blow outward from it in winter and thus +prevent the ocean heat from being carried inland. So +long as the ocean is not frozen in high latitudes, it is +generally the chief source of heat in winter, for the nights +are several months long near the poles, and even when +the sun does shine its angle is so low that reflection from +the snow is very great. Furthermore, although on the +average there is more reflection from water than from +land, the opposite is true in high latitudes in winter +when the land is snow-covered while the ocean is relatively +dark and is roughened by the waves. Another +factor in causing large lands to have extremely low temperature +in winter is the fact that in proportion to their +size they are less protected by fog and cloud than are +smaller areas. The belt of cloud and fog which is usually +formed when the wind blows from the ocean to the relatively +cold land is restricted to the coastal zone. Thus the +larger the land, the smaller the fraction in which loss of +heat by radiation is reduced by clouds and fogs. Hence +an increase in the land area is accompanied by an increase +in the contrasts in temperature between land and +water.</p> + +<p>(b) The contrasts in temperature thus produced must +cause similar contrasts in atmospheric pressure, and +hence stronger barometric gradients. (c) The strong +gradients would mean strong winds, flowing from land +to sea or from sea to land. (d) Local convection would +also be strengthened in harmony with the expansion of +the lands, for the more rapid heating of land than of +water favors active convection.</p> + +<p>(e) As the extent of the ocean diminished, there would +normally be a decrease in the amount of water vapor for +<!--File: cc222.png Page 198--> +<a class="pagenum break" id="Page_198" title="pg 198"></a> +three reasons: (1) Evaporation from the ocean is the +great source of water vapor. Other conditions being +equal, the smaller the ocean becomes, the less the evaporation. +(2) The amount of water vapor in the air diminishes +as convection increases, since upward convection +is a chief method by which condensation and precipitation +are produced, and water vapor removed from the +atmosphere. (3) Nocturnal cooling sufficient to produce +dew and frost is very much more common upon land than +upon the ocean. The formation of dew and frost diminishes +the amount of water vapor at least temporarily. +(f) Any diminution in water vapor produced in these +ways, or otherwise, is significant because water vapor is +the most essential part of the atmosphere so far as regulation +of temperature is concerned. It tends to keep the +days from becoming hot or the nights cold. Therefore +any decrease in water vapor would increase the diurnal +and seasonal range of temperature, making the climate +more extreme and severe. Thus a periodic increase in the +area of the continents would clearly make for periodic +increased climatic contrasts, with great extremes, a type +of climatic change which has recurred again and again. +Indeed, each great glaciation accompanied or followed +extensive emergence of the lands.<sup><a href="#fn86" id="r86">[86]</a></sup></p> + +<p>Whether or not there has been a <em>progressive</em> increase +from era to era in the area of the lands is uncertain. +Good authorities disagree widely. There is no doubt, +however, that at present the lands are more extensive +than at most times in the past, though smaller, perhaps, +than at certain periods. The wide expanse of lands helps +explain the prominence of seasons at present as compared +with the past.</p> + +<!--File: cc223.png Page 199--> +<a class="pagenum break" id="Page_199" title="pg 199"></a> + +<p>II. The contraction of the earth, as we have seen, has +produced great changes in the distribution as well as in +the extent of land and water. Large parts of the present +continents have been covered repeatedly by the sea, and +extensive areas now covered with water have been land. +In recent geological times, that is, during the Pliocene and +Pleistocene, much of the present continental shelf, the +zone less than 600 feet below sea level, was land. If the +whole shelf had been exposed, the lands would have been +greater than at present by an area larger than North +America. When the lands were most elevated, or a little +earlier, North America was probably connected with +Asia and almost with Europe. Asia in turn was apparently +connected with the larger East Indian islands. In +much earlier times land occupied regions where now the +ocean is fairly deep. Groups of islands, such as the East +Indies and Malaysia and perhaps the West Indies, were +united into widespreading land masses. Figs. 7 and 9, +illustrating the paleography of the Permian and the +Cretaceous periods, respectively, indicate a land distribution +radically different from that of today.</p> + +<p>So far as appears from the scattered facts of geological +history, the changes in the distribution of land seem +to have been marked by the following characteristics: (1) +Accompanying the differentiation of continental and +oceanic segments of the earth's crust, the oceans have +become somewhat deeper, and their basins perhaps +larger, while the continents, on the average, have been +more elevated and less subject to submergence. Hence +there have been less radical departures from the present +distribution during the relatively recent Cenozoic era +than in the ancient Paleozoic because the submergence of +continental areas has become less general and less frequent. +For example, the last extensive epeiric or interior +<!--File: cc224.png Page 200--> +<a class="pagenum break" id="Page_200" title="pg 200"></a> +sea in North America was in the Cretaceous, at least ten +million years ago, and according to Barrell perhaps fifty +million, while in Europe, according to de Lapparent,<sup><a href="#fn87" id="r87">[87]</a></sup> a +smaller share of the present continent has been submerged +since the Cretaceous than before. Indeed, as in +North America, the submergence has decreased on the +average since the Paleozoic era. (2) The changes in distribution +of land which have taken place during earth +history have been cyclic. Repeatedly, at the close of each +of the score or so of geologic periods, the continents +emerged more or less, while at the close of the groups of +periods known as eras, the lands were especially large +and emergent. After each emergence, a gradual encroachment +of the sea took place, and toward the close of several +of the earlier periods, the sea appears to have +covered a large fraction of the present land areas. (3) On +the whole, the amount of land in the middle and high latitudes +of the northern hemisphere appears to have increased +during geologic time. Such an increase does not +require a growth of the continents, however, in the +broader sense of the term, but merely that a smaller +fraction of the continent and its shelf should be submerged. +(4) In tropical latitudes, on the other hand, the +extent of the lands seems to have decreased, apparently +by the growth of the ocean basins. South America and +Africa are thought by many students to have been connected, +and Africa was united with India via Madagascar, +as is suggested in Fig. 9. The most radical cyclic +as well as the most radical progressive changes in land +distribution also seem to have taken place in tropical +regions.<sup><a href="#fn88" id="r88">[88]</a></sup> +<!--File: cc225.png Page 201--> +<a class="pagenum break" id="Page_201" title="pg 201"></a></p> + + +<img src="images/fig9.png" width="800" height="531" alt="Fig. 9" id="Fig_9" /> +<p class="center"><em>Fig. 9. Cretaceous Paleogeography</em>.<br /> +(<em>After Schuchert.</em>)</p> + + +<p>Although there is much evidence of periodic increase +<!--File: cc226.png Page 202--> +<a class="pagenum break" id="Page_202" title="pg 202"></a> +of the sea in equatorial latitudes and of land in high latitudes, +it has remained for the zoölogist Metcalf to present +a very pretty bit of evidence that at certain times +submergence along the equator coincided with emergence +in high latitudes, and vice versa. Certain fresh water +frogs which carry the same internal parasite are confined +to two widely separated areas in tropical and south temperate +America and in Australia. The extreme improbability +that both the frogs and the parasites could have +originated independently in two unconnected areas and +could have developed by convergent evolution so that +they are almost identical in the two continents makes it +almost certain that there must have been a land connection +between South America and Australia, presumably +by way of Antarctica. The facts as to the parasites +seem also to prove that while the land connection existed +there was a sea across South America in equatorial latitudes. +The parasite infests not only the frogs but the +American toads known as Bufo. Now Bufo originated +north of the equator in America and differs from the +frogs which originated in southern South America in +not being found in Australia. This raises the question of +how the frogs could go to Australia via Antarctica carrying +the parasite with them, while the toads could not go. +Metcalf's answer is that the toads were cut off from the +southern part of South America by an equatorial sea +until after the Antarctic connection between the Old +World and the New was severed.</p> + +<blockquote><p> +As Patagonia let go of Antarctica by subsidence of the intervening +land area, there was a probable concomitant rise of land +through what is now middle South America and the northern +and southern portions of this continent came together.<sup><a href="#fn89" id="r89">[89]</a></sup></p> +</blockquote> + + +<!--File: cc227.png Page 203--> +<a class="pagenum break" id="Page_203" title="pg 203"></a> + +<p>These various changes in the earth's crust have given +rise to certain specific types of distribution of the lands, +which will now be considered. We shall inquire what climatic +conditions would arise from changes in (a) the +continuity of the lands from north to south, (b) the +amount of land in tropical latitudes, and (c) the amount +of land in middle and high latitudes.</p> + +<p>(a) At present the westward drift of warm waters, set +in motion by the trade winds, is interrupted by land +masses and turned poleward, producing the important +Gulf Stream Drift and Japan Current in the northern +hemisphere, and corresponding, though less important, +currents in the southern hemisphere. During the past, +quite different sets of ocean currents doubtless have +existed in response to a different distribution of land. +Repeatedly, in the mid-Cretaceous (Fig. 9) and several +other periods, the present American barrier to the westward +moving tropical current was broken in Central +America. Even if the supposed continent of "Gondwana +Land" extended from Africa to South America in equatorial +latitudes, strong currents must still have flowed +westward along its northern shore under the impulse of +the peculiarly strong trade winds which the equatorial +land would create. Nevertheless at such times relatively +little warm tropical water presumably entered the North +Atlantic, for it escaped into the Pacific. At several other +times, such as the late Ordovician and mid-Devonian, +when the isthmian barrier existed, it probably turned an +important current northward into what is now the Mississippi +Basin instead of into the Atlantic. There it +traversed an epeiric, or mid-continental sea open to both +north and south. Hence its effectiveness in warming +Arctic regions must have been quite different from that +of the present Gulf Stream.</p> +<!--File: cc228.png Page 204--> +<a class="pagenum break" id="Page_204" title="pg 204"></a> + +<p>(b) We will next consider the influences of changes in +the amount of equatorial and tropical land. As such lands +are much hotter than the corresponding seas, the intensity +and width of the equatorial belt of low pressure must +be great when they are extensive. Hence the trade winds +must have been stronger than now whenever tropical +lands were more extensive than at present. This is because +the trades are produced by the convection due to +excessive heat along the heat equator. There the air +expands upward and flows poleward at high altitudes. +The trade wind consists of air moving toward the heat +equator to take the place of the air which there rises. +When the lands in low latitudes were wide the trade +winds must also have dominated a wide belt. The greater +width of the trade-wind belt today over Africa than over +the Atlantic illustrates the matter. The belt must have +been still wider when Gondwana Land was large, as it is +believed to have been during the Paleozoic era and the +early Mesozoic.</p> + +<p>An increase in the width of the equatorial belt of +low pressure under the influence of broad tropical lands +would be accompanied not only by stronger and more +widespread trade winds, but by a corresponding strengthening +of the subtropical belts of high pressure. The chief +reason would be the greater expansion of the air in the +equatorial low pressure belt and the consequent more +abundant outflow of air at high altitudes in the form of +anti-trades or winds returning poleward above the trades. +Such winds would pile up the air in the region of the high-pressure +belt. Moreover, since the meridians converge as +one proceeds away from the equator, the air of the poleward-moving +anti-trades tends to be crowded as it +reaches higher latitudes, thus increasing the pressure. +Unless there were a corresponding increase in tropical +<!--File: cc229.png Page 205--> +<a class="pagenum break" id="Page_205" title="pg 205"></a> +cyclones, one of the most prominent results of the +strengthened trades and the intensified subtropical high-pressure +belt at times of broad lands in low latitudes +would be great deserts. It will be recalled that the trade-wind +lowlands and the extra-tropical belt of highs are the +great desert belts at present. The trade-wind lowlands +are desert because air moving into warmer latitudes +takes up water except where it is cooled by rising on +mountain-sides. The belt of highs is arid because there, +too, air is being warmed, but in this case by descending +from aloft.</p> + +<p>Again, if the atmospheric pressure in the subtropical +belt should be intensified, the winds flowing poleward +from this belt would necessarily become stronger. These +would begin as southwesterlies in the northern hemisphere +and northwesterlies in the southern. In the preceding +chapter we have seen that such winds, especially +when cyclonic storms are few and mild, are a powerful +agent in transferring subtropical heat poleward. If the +strength of the westerlies were increased because of +broad lands in low latitudes, their efficacy in transferring +heat would be correspondingly augmented. It is thus +evident that any change in the extent of tropical lands +during the geologic past must have had important climatic +consequences in changing the velocity of the +atmospheric circulation and in altering the transfer of +heat from low latitudes to high. When the equatorial and +tropical lands were broad the winds and currents must +have been strong, much heat must have been carried +away from low latitudes, and the contrast between low +and high latitudes must have been relatively slight. As +we have already remarked, leading paleogeographers +believe that changes in the extent of the lands have been +especially marked in low latitudes, and that on the average +<!--File: cc230.png Page 206--> +<a class="pagenum break" id="Page_206" title="pg 206"></a> +there has been a decrease in the extent of land within +the tropics. Gondwana Land is the greatest illustration +of this. In the same way, on the numerous paleogeographic +maps of North America, most paleogeographers +have shown fairly extensive lands south of the latitude of +the United States during most of the geologic epochs.<sup><a href="#fn90" id="r90">[90]</a></sup></p> + +<p>(c) There is evidence that during geologic history the +area of the lands in middle and high latitudes, as well as +in low latitudes, has changed radically. An increase in +such lands would cause the winters to grow colder. This +would be partly because of the loss of heat by radiation +into the cold dry air over the continents in winter, and +partly because of increased reflection from snow and +frost, which gather much more widely upon the land than +upon the ocean. Furthermore, in winter when the continents +are relatively cold, there is a strong tendency for +winds to blow out from the continent toward the ocean. +The larger the land the stronger this tendency. In Asia +it gives rise to strong winter monsoons. The effect of +such winds is illustrated by the way in which the westerlies +prevent the Gulf Stream from warming the eastern +United States in winter. The Gulf Stream warms northwestern +Europe much more than the United States because, +in Europe, the prevailing winds are onshore.</p> + +<p>Another effect of an increase in the area of the lands in +middle and high latitudes would be to interpose barriers +to oceanic circulation and thus lower the temperature +of polar regions. This would not mean glaciation in +high latitudes, however, even when the lands were widespread +as in the Mesozoic and early Tertiary. Students +of glaciology are more and more thoroughly convinced + + +<!--File: cc231.png Page 207--> +<a class="pagenum break" id="Page_207" title="pg 207"></a> +that glaciation depends on the availability of moisture +even more than upon low temperature.</p> + +<p>In conclusion it may be noted that each of the several +climatic influences of increased land area in the high +latitudes would tend to increase the contrasts between +land and sea, between winter and summer, and between +low latitudes and high. In other words, so far as the +effect upon high latitudes themselves is concerned, an +expansion of the lands there would tend in the same +direction as a diminution in low latitudes. In so far as +the general trend of geological evolution has been toward +more land in high latitudes and less in low, it would help +to produce a progressive increase in climatic diversity +such as is faintly indicated in the rock strata. On the +other hand, the oscillations in the distribution of the +lands, of which geology affords so much evidence, must +certainly have played an important part in producing the +periodic changes of climate which the earth has undergone.</p> + +<p>III. Throughout geological history there is abundant +evidence that the process of contraction has led to +marked differences not only in the distribution and area +of the lands, but in their height. On the whole the lands +have presumably increased in height since the Proterozoic, +somewhat in proportion to the increased differentiation +of continents and oceans.<sup><a href="#fn91" id="r91">[91]</a></sup> If there has been such an +increase, the contrast between the climate of ocean and +land must have been accentuated, for highlands have a +greater diurnal and seasonal range of temperature than +do lowlands. The ocean has very little range of either +sort. The large range at high altitudes is due chiefly to +the small quantity of water vapor, for this declines +<!--File: cc232.png Page 208--> +<a class="pagenum break" id="Page_208" title="pg 208"></a> +steadily with increased altitude. A diminution in the +density of the other constituents of the air also decreases +the blanketing effect of the atmosphere. In conformity +with the great seasonal range in temperature at times +when the lands stand high, the direction of the wind +would be altered. When the lands are notably warmer +than the oceans, the winds commonly flow from land to +sea, and when the continents are much colder than the +oceans, the direction is reversed. The monsoons of Asia +are examples. Strong seasonal winds disturb the normal +planetary circulation of the trade winds in low latitudes +and of the westerlies in middle latitudes. They also interfere +with the ocean currents set in motion by the planetary +winds. The net result is to hinder the transfer of +heat from low latitudes to high, and thus to increase the +contrasts between the zones. Local as well as zonal contrasts +are also intensified. The higher the land, the +greater, relatively speaking, are the cloudiness and precipitation +on seaward slopes, and the drier the interior. +Indeed, most highlands are arid. Henry's<sup><a href="#fn92" id="r92">[92]</a></sup> recent study +of the vertical distribution of rainfall on mountain-sides +indicates that a decrease sets in at about 3500 feet in the +tropics and only a little higher in mid-latitudes.</p> + +<p>In addition to the main effects upon atmospheric circulation +and precipitation, each of the many upheavals +of the lands must have been accompanied by many minor +conditions which tended toward diversity. For example, +the streams were rejuvenated, and instead of meandering +perhaps over vast flood plains they intrenched their +channels and in many cases dug deep gorges. The water +table was lowered, soil was removed from considerable +areas, the bare rock was exposed, and the type of dominant +<!--File: cc233.png Page 209--> +<a class="pagenum break" id="Page_209" title="pg 209"></a> +vegetation altered in many places. An almost barren +ridge may represent all that remains of what was once a +vast forested flood plain. Thus, increased elevation of the +land produces contrasted conditions of slope, vegetation, +availability of ground water, exposure to wind and so +forth, and these unite in diversifying climate. Where +mountains are formed, strong contrasts are sure to +occur. The windward slopes may be very rainy, while +neighboring leeward slopes are parched by a dry foehn +wind. At the same time the tops may be snow-covered. +Increased local contrasts in climatic conditions are +known to influence the intensity of cyclonic storms,<sup><a href="#fn93" id="r93">[93]</a></sup> and +these affect the climatic conditions of all middle and high +latitudes, if not of the entire earth. The paths followed +by cyclonic storms are also altered by increased contrast +between land and water. When the continents are notably +colder than the neighboring oceans, high atmospheric +pressure develops on the lands and interferes with the +passage of lows, which are therefore either deflected +around the continent or forced to move slowly.</p> + +<p>The distribution of lofty mountains has an even more +striking climatic effect than the general uplift of a region. +In Proterozoic times there was a great range in the Lake +Superior region; in the late Devonian the Acadian mountains +of New England and the Maritime Provinces of +Canada possibly attained a height equal to the present +Rockies. Subsequently, in the late Paleozoic a significant +range stood where the Ouachitas now are. Accompanying +the uplift of each of these ranges, and all others, the +climate of the surrounding area, especially to leeward, +must have been altered greatly. Many extensive salt deposits +<!--File: cc234.png Page 210--> +<a class="pagenum break" id="Page_210" title="pg 210"></a> +found now in fairly humid regions, for example, +the Pennsylvanian and Permian deposits of Kansas and +Oklahoma, were probably laid down in times of local +aridity due to the cutting off of moisture-bearing winds +by the mountains of Llanoria in Louisiana and Texas. +Hence such deposits do not necessarily indicate periods +of widespread and profound aridity.</p> + +<p>When the causes of ancient glaciation were first considered +by geologists, about the middle of the nineteenth +century, it was usually assumed that the glaciated areas +had been elevated to great heights, and thus rendered +cold enough to permit the accumulation of glaciers. The +many glaciers occurring in the Alps of central Europe +where glaciology arose doubtless suggested this explanation. +However, it is now known that most of the ancient +glaciation was not of the alpine type, and there is adequate +proof that the glacial periods cannot be explained +as due directly and solely to uplift. Nevertheless, upheavals +of the lands are among the most important factors +in controlling climate, and variations in the height +of the lands have doubtless assisted in producing climate +oscillations, especially those of long duration. Moreover, +the progressive increase in the height of the lands has +presumably played a part in fostering local and zonal +diversity in contrast with the relative uniformity of +earlier geological times.</p> + +<p>IV. The contraction of the earth has been accompanied +by volcanic activity as well as by changes in the extent, +distribution, and altitude of the lands. The probable part +played by volcanic dust as a contributory factor in producing +short sudden climatic variations has already been +discussed. There is, however, another though probably +less important respect in which volcanic activity may +have had at least a slight climatic significance. The oldest +<!--File: cc235.png Page 211--> +<a class="pagenum break" id="Page_211" title="pg 211"></a> +known rocks, those of the Archean era, contain so much +igneous matter that many students have assumed that +they show that the entire earth was once liquid. It is now +considered that they merely indicate igneous activity of +great magnitude. In the later part of Proterozoic time, +during the second quarter of the earth's history according +to Schuchert's estimate, there were again vast outflowings +of lava. In the Lake Superior district, for example, +a thickness of more than a mile accumulated over +a large area, and lavas are common in many areas where +rocks of this age are known. The next quarter of the +earth's history elapsed without any correspondingly +great outflows so far as is known, though several lesser +ones occurred. Toward the end of the last quarter, and +hence quite recently from the geological standpoint, +another period of outflows, perhaps as noteworthy as +that of the Proterozoic, occurred in the Cretaceous and +Tertiary.</p> + +<p>The climatic effects of such extensive lava flows would +be essentially as follows: In the first place so long as the +lavas were hot they would set up a local system of convection +with inflowing winds. This would interfere at +least a little with the general winds of the area. Again, +where the lava flowed out into water, or where rain fell +upon hot lava, there would be rapid evaporation which +would increase the rainfall. Then after the lava had +cooled, it would still influence climate a trifle in so far as +its color was notably darker or lighter than that of the +average surface. Dark surfaces absorb solar heat and +become relatively warm when the sun shines upon them. +Dark objects likewise radiate heat more rapidly than +light-colored objects. Hence they cool more rapidly at +night, and in the winter. As most lavas are relatively +dark they increase the average diurnal range of temperature. +<!--File: cc236.png Page 212--> +<a class="pagenum break" id="Page_212" title="pg 212"></a> +Hence even after they are cool they increase the +climatic diversity of the land.</p> + +<p>The amount of heat given to the atmosphere by an +extensive lava flow, though large according to human +standards, is small compared with the amount received +from the sun by a like area, except during the first few +weeks or months before the lava has formed a thick +crust. Furthermore, probably only a small fraction of +any large series of flows occurred in a given century or +millennium. Moreover, even the largest lava flows covered +an area of only a few hundredths of one per cent of the +earth's surface. Nevertheless, the conditions which modify +climate are so complicated that it would be rash to +state that this amount of additional heat has been of +no climatic significance. Like the proverbial "straw that +broke the camel's back," the changes it would surely +produce in local convection, atmospheric pressure, and +the direction of the wind may have helped to shift the +paths of storms and to produce other complications which +were of appreciable climatic significance.</p> + +<p>V. The last point which we shall consider in connection +with the effect of the earth's interior upon climate is +internal heat. The heat given off by lavas is merely a +small part of that which is emitted by the earth as a +whole. In the earliest part of geological history enough +heat may have escaped from the interior of the earth to +exert a profound influence on the climate. Knowlton,<sup><a href="#fn94" id="r94">[94]</a></sup> +as we have seen, has recently built up an elaborate theory +on this assumption. At present, however, accurate measurements +show that the escape of heat is so slight that +it has no appreciable influence except in a few volcanic +<!--File: cc237.png Page 213--> +<a class="pagenum break" id="Page_213" title="pg 213"></a> +areas. It is estimated to raise the average temperature +of the earth's surface less than 0.1°C.<sup><a href="#fn95" id="r95">[95]</a></sup></p> + +<p>In order to contribute enough heat to raise the surface +temperature 1°C., the temperature gradient from the +interior of the earth to the surface would need to be ten +times as great as now, for the rate of conduction varies +directly with the gradient. If the gradient were ten times +as great as now, the rocks at a depth of two and one-half +miles would be so hot as to be almost liquid according to +Barrell's<sup><a href="#fn96" id="r96">[96]</a></sup> estimates. The thick strata of unmetamorphosed +Paleozoic rocks indicate that such high temperatures +have not prevailed at such slight depths since the +Proterozoic. Furthermore, the fact that the climate was +cold enough to permit glaciation early in the Proterozoic +era and at from one to three other times before the opening +of the Paleozoic suggests that the rate of escape of +heat was not rapid even in the first half of the earth's +recorded history. Yet even if the general escape of heat +has never been large since the beginning of the better-known +part of geological history, it was presumably +greater in early times than at present.</p> + +<p>If there actually has been an appreciable decrease in +the amount of heat given out by the earth's interior, its +effects would agree with the observed conditions of the +geological record. It would help to explain the relative +mildness of zonal, seasonal, and local contrasts of climate +in early geological times, but it would not help to explain +the long oscillations from era to era which appear to have +been of much greater importance. Those oscillations, so +far as we can yet judge, may have been due in part to +solar changes, but in large measure they seem to be +<!--File: cc238.png Page 214--> +<a class="pagenum break" id="Page_214" title="pg 214"></a> +explained by variations in the extent, distribution, and +altitude of the lands. Such variations appear to be the +inevitable result of the earth's contraction.</p> +<!--File: cc239.png Page 215--> +<a class="pagenum break" id="Page_215" title="pg 215"></a> + + + + +<h2 id="Chapt_XII"><a href="#content">CHAPTER XII</a></h2> + +<h3>POST-GLACIAL CRUSTAL MOVEMENTS AND +CLIMATIC CHANGES</h3> + + +<p>An interesting practical application of some of the +preceding generalizations is found in an attempt +by C. E. P. Brooks<sup><a href="#fn97" id="r97">[97]</a></sup> to interpret post-glacial +climatic changes almost entirely in terms of crustal movement. +We believe that he carries the matter much too far, +but his discussion is worthy of rather full recapitulation, +not only for its theoretical value but because it gives a +good summary of post-glacial changes. His climatic table +for northwest Europe as reprinted from the annual report +of the Smithsonian Institution for 1917, p. 366, is +as follows:</p> + + +<table class="borders"> +<tr> +<th><em>Phase</em></th> +<th><em>Climate</em> </th> +<th> <em>Date</em></th> +</tr> +<tr> +<td class="tdl">1. The Last Great Glaciation.</td> +<td class="tdl">Arctic climate.</td> +<td>30,000-18,000 B. C.</td> +</tr> +<tr><td class="tdl">2. The Retreat of the Glaciers.</td> +<td class="tdl">Severe continental climate.</td> +<td>18,000-6000 B. C.</td> +</tr> +<tr> +<td class="tdl">3. The Continental Phase.</td> +<td class="tdl">Continental climate.</td> +<td>6000-4000 B. C.</td></tr> +<tr><td class="tdl">4. The Maritime Phase.</td> +<td class="tdl">Warm and moist.</td> +<td>4000-3000 B. C.</td> +</tr> +<tr> +<td class="tdl">5. The Later Forest Phase.</td> +<td class="tdl">Warm and dry.</td> +<td>3000-1800 B. C.</td></tr> +<tr><td class="tdl">6. The Peat-Bog Phase.</td> +<td class="tdl">Cooler and moister.</td> +<td>1800 B. C.-300 A. D.</td></tr> +<tr> +<td class="tdl">7. The Recent Phase.</td> +<td class="tdl">Becoming drier.</td> +<td>300 A. D.-</td></tr> +</table> + + +<p>Brooks bases his chronology largely on De Geer's +measurements of the annual layers of clay in lake +<!--File: cc240.png Page 216--> +<a class="pagenum break" id="Page_216" title="pg 216"></a> +bottoms but makes much use of other evidence. According +to Brooks the last glacial epoch lasted roughly from +30,000 to 18,000 B. C., but this includes a slight amelioration +of climate followed by a readvance of the ice, known +as the Buhl stage. During the time of maximum glaciation +the British Isles stood twenty or thirty feet higher +than now and Scandinavia was "considerably" more +elevated. The author believes that this caused a fall of +1°C. in the temperature of the British Isles and of 2°C. +in Scandinavia. By an ingenious though not wholly convincing +method of calculation he concludes that this +lowering of temperature, aided by an increase in the area +of the lands, sufficed to start an ice sheet in Scandinavia. +The relatively small area of ice cooled the air and gave +rise to an area of high barometric pressure. This in turn +is supposed to have caused further expansion of the ice +and to have led to full-fledged glaciation.</p> + +<p>About 18,000 B. C. the retreat of the ice began in good +earnest. Even though no evidence has yet been found, +Brooks believes there must have been a change in the distribution +of land and sea to account for the diminution of +the ice. The ensuing millenniums formed the Magdalenian +period in human history, the last stage of the Paleolithic, +when man lived in caves and reindeer were abundant +in central Europe.<sup><a href="#fn98" id="r98">[98]</a></sup> At first the ice retreated very +slowly and there were periods when for scores of years +the ice edge remained stationary or even readvanced. +About 10,000 B. C. the edge of the ice lay along the +southern coast of Sweden. During the next 2000 years it +withdrew more rapidly to about 59°N. Then came the +Fennoscandian pause, or Gschnitz stage, when for about +<!--File: cc241.png Page 217--> +<a class="pagenum break" id="Page_217" title="pg 217"></a> +200 years the ice edge remained in one position, forming +a great moraine. Brooks suggests that this pause about +8000 B. C. was due to the closing of the connection between +the Atlantic Ocean and the Baltic Sea and the +synchronous opening of a connection between the Baltic +and the White Seas, whereby cold Arctic waters replaced +the warmer Atlantic waters. He notes, however, that +about 7500 B. C. the obliquity of the ecliptic was probably +nearly 1° greater than at present. This he calculates to +have caused the climate of Germany and Sweden to be +1°F. colder than at present in winter and 1°F. warmer in +summer.</p> + +<p>The next climatic stage was marked by a rise of temperature +till about 6000 B. C. During this period the ice +at first retreated, presumably because the climate was +ameliorating, although no cause of such amelioration is +assigned. At length the ice lay far enough north to allow +a connection between the Baltic and the Atlantic by way +of Lakes Wener and Wetter in southern Sweden. This is +supposed to have warmed the Baltic Sea and to have +caused the climate to become distinctly milder. Next the +land rose once more so that the Baltic was separated +from the Atlantic and was converted into the Ancylus +lake of fresh water. The southwest Baltic region then +stood 400 feet higher than now. The result was the Daun +stage, about 5000 B. C., when the ice halted or perhaps +readvanced a little, its front being then near Ragunda +in about latitude 63°. Why such an elevation did not +cause renewed glaciation instead of merely the slight +Daun pause, Brooks does not explain, although his calculations +as to the effect of a slight elevation of the land +during the main period of glaciation from 30,000 to +18,000 B. C. would seem to demand a marked readvance.</p> +<!--File: cc242.png Page 218--> +<a class="pagenum break" id="Page_218" title="pg 218"></a> + +<p>After 5000 B. C. there ensued a period when the climate, +although still distinctly continental, was relatively +mild. The winters, to be sure, were still cold but the +summers were increasingly warm. In Sweden, for example, +the types of vegetation indicate that the summer +temperature was 7°F. higher than now. Storms, Brooks +assumes, were comparatively rare except on the outer +fringe of Great Britain. There they were sufficiently +abundant so that in the Northwest they gave rise to the +first Peat-Bog period, during which swamps replaced +forests of birch and pine. Southern and eastern England, +however, probably had a dry continental climate. Even +in northwest Norway storms were rare as is indicated by +remains of forests on islands now barren because of the +strong winds and fierce storms. Farther east most parts +of central and northern Europe were relatively dry. This +was the early Neolithic period when man advanced from +the use of unpolished to polished stone implements.</p> + +<p>Not far from 4000 B. C. the period of continental climate +was replaced by a comparatively moist maritime +climate. Brooks believes that this was because submergence +opened the mouth of the Baltic and caused the +fresh Ancylus lake to give place to the so-called Litorina +sea. The temperature in Sweden averaged about 3°F. +higher than at present and in southwestern Norway 2°. +More important than this was the small annual range of +temperature due to the fact that the summers were cool +while the winters were mild. Because of the presence of +a large expanse of water in the Baltic region, storms, as +our author states, then crossed Great Britain and followed +the Baltic depression, carrying the moisture far +inland. In spite of the additional moisture thus available +the snow line in southern Norway was higher than now.</p> + +<p>At this point Brooks turns to other parts of the world. +<!--File: cc243.png Page 219--> +<a class="pagenum break" id="Page_219" title="pg 219"></a> +He states that not far from 4000 B. C., a submergence +of the lands, rarely amounting to more than twenty-five +feet, took place not only in the Baltic region but in Ireland, +Iceland, Spitzbergen, and other parts of the Arctic +Ocean, as well as in the White Sea, Greenland, and the +eastern part of North America. Evidences of a mild climate +are found in all those places. Similar evidence of a +mild warm climate is found in East Africa, East Australia, +Tierra del Fuego, and Antarctica. The dates are +not established with certainty but they at least fall in the +period immediately preceding the present epoch. In explanation +of these conditions Brooks assumes a universal +change of sea level. He suggests with some hesitation +that this may have been due to one of Pettersson's +periods of maximum "tide-generating force." According +to Pettersson the varying positions of the moon, earth, +and sun cause the tides to vary in cycles of about 9, 90, +and 1800 years, though the length of the periods is not +constant. When tides are high there is great movement +of ocean waters and hence a great mixture of the water +at different latitudes. This is supposed to cause an +amelioration of climate. The periods of maximum and +minimum tide-generating force are as follows:</p> + +<blockquote> +<p>Maxima 3500 B. C. ———— 2100 B. C. ———— 350 B. C. ———— A. D. 1434<br /> +Minima ————— 2800 B. C. ———— 1200 B. C. ———— A. D. 530 ————</p> +</blockquote> + +<p>Brooks thinks that the big trees in California and the +Norse sagas and Germanic myths indicate a rough agreement +of climatic phenomena with Pettersson's last three +dates, while the mild climate of 4000 B. C. may really +belong to 3500 B. C. He gives no evidence confirming +Pettersson's view at the other three dates.</p> + +<p>To return to Brooks' sketch of the relation of climatic +pulsations to the altitude of the lands, by 3000 B. C., that +<!--File: cc244.png Page 220--> +<a class="pagenum break" id="Page_220" title="pg 220"></a> +is, toward the close of the Neolithic period, further elevation +is supposed to have taken place over the central +latitudes of western Europe. Southern Britain, which had +remained constantly above its present level ever since +30,000 B. C., was perhaps ninety feet higher than now. +Ireland was somewhat enlarged by elevation, the Straits +of Dover were almost closed, and parts of the present +North Sea were land. To these conditions Brooks ascribes +the prevalence of a dry continental climate. The storms +shifted northward once more, the winds were mild, as +seems to be proved by remains of trees in exposed places; +and forests replaced fields of peat and heath in Britain +and Germany. The summers were perhaps warmer than +now but the winters were severe. The relatively dry climate +prevailed as far west as Ireland. For example, in +Drumkelin Bog in Donegal County a corded oak road and +a two-story log cabin appear to belong to this time. Fourteen +feet of bog lie below the floor and twenty-six above. +This period, perhaps 3000-2000 B. C., was the legendary +heroic age of Ireland when "the vigour of the Irish +reached a level not since attained." This, as Brooks +points out, may have been a result of the relatively dry +climate, for today the extreme moisture of Ireland seems +to be a distinct handicap. In Scandinavia, civilization, or +at least the stage of relative progress, was also high at +this time.</p> + +<p>By 1600 B. C. the land had assumed nearly its present +level in the British Isles and the southern Baltic +region, while northern Scandinavia still stood lower than +now. The climate of Britain and Germany was so humid +that there was an extensive formation of peat even on +high ground not before covered. This moist stage seems +to have lasted almost to the time of Christ, and may have +been the reason why the Romans described Britain as +<!--File: cc245.png Page 221--> +<a class="pagenum break" id="Page_221" title="pg 221"></a> +peculiarly wet and damp. At this point Brooks again departs +from northwest Europe to a wider field:</p> + +<blockquote> +<p>It is possible that we have to attribute this damp period in +Northwest Europe to some more general cause, for Ellsworth +Huntington's curves of tree-growth in California and climate +in Western Asia both show moister conditions from about +1000 B. C. to A. D. 200, and the same author believes that the +Mediterranean lands had a heavier rainfall about 500 B. C. to +A. D. 200. It seems that the phase was marked by a general increase +of the storminess of the temperate regions of the northern +hemisphere at least, with a maximum between Ireland and North +Germany, indicating probably that the Baltic again became the +favourite track of depressions from the Atlantic.</p> +</blockquote> + +<p>Brooks ends his paper with a brief résumé of glacial +changes in North America, but as the means of dating +events are unreliable the degree of synchronism with +Europe is not clear. He sums up his conclusions as +follows:</p> + +<blockquote> +<p>On the whole it appears that though there is a general similarity +in the climatic history of the two sides of the North +Atlantic, the changes are not really contemporaneous, and such +relationship as appears is due mainly to the natural similarity +in the geographical history of two regions both recovering from +an Ice Age, and only very partially to world-wide pulsations +of climate. Additional evidence on this head will be available +when Baron de Geer publishes the results of his recent investigations +of the seasonal glacial clays of North America, especially +if, as he hopes, he is able to correlate the banding of these clays +with the growth-rings of the big trees.</p> + +<p>When we turn to the northwest of North America, this is +brought out very markedly. For in Yukon and Alaska the Ice +Age was a very mild affair compared with its severity in eastern +America and Scandinavia. As the land had not a heavy ice-load +to recover from, there were no complicated geographical +<!--File: cc246.png Page 222--> +<a class="pagenum break" id="Page_222" title="pg 222"></a> +changes. Also, there were no fluctuations of climate, but simply +a gradual passage to present conditions. The latter circumstance +especially seems to show that the emphasis laid on geographical +rather than astronomical factors of <em>great</em> climatic changes is +not misplaced.</p> +</blockquote> + +<p>Brooks' painstaking discussion of post-glacial climatic +changes is of great value because of the large body of +material which he has so carefully wrought together. His +strong belief in the importance of changes in the level of +the lands deserves serious consideration. It is difficult, +however, to accept his final conclusion that such changes +are the main factors in recent climatic changes. It is almost +impossible, for example, to believe that movements +of the land could produce almost the same series of +climatic changes in Europe, Central Asia, the western +and eastern parts of North America, and the southern +hemisphere. Yet such changes appear to have occurred +during and since the glacial period. Again there is no +evidence whatever that movements of the land have anything +to do with the historic cycles of climate or with the +cycles of weather in our own day, which seem to be the +same as glacial cycles on a small scale. Also, as Dr. +Simpson points out in discussing Brooks' paper, there +appears "no solution along these lines of the problem +connected with rich vegetation in both polar circles and +the ice-age which produced the ice-sheet at sea-level in +Northern India." Nevertheless, we may well believe that +Brooks is right in holding that changes in the relative +level and relative area of land and sea have had important +local effects. While they are only one of the +factors involved in climatic changes, they are certainly +one that must constantly be kept in mind.</p> +<!--File: cc247.png Page 223--> +<a class="pagenum break" id="Page_223" title="pg 223"></a> + + + + +<h2 id="Chapt_XIII"><a href="#content">CHAPTER XIII</a></h2> + +<h3>THE CHANGING COMPOSITION OF OCEANS AND +ATMOSPHERE</h3> + + +<p>Having discussed the climatic effect of movements +of the earth's crust during the course of +geological time, we are now ready to consider +the corresponding effects due to changes in the movable +envelopes—the oceans and the atmosphere. Variations in +the composition of sea water and of air and in the +amount of air must almost certainly have occurred, and +must have produced at least slight climatic consequences. +It should be pointed out at once that such variations +appear to be far less important climatically than do +movements of the earth's crust and changes in the activity +of the sun. Moreover, in most cases, they are not +reversible as are the crustal and solar phenomena. Hence, +while most of them appear to have been unimportant so +far as climatic oscillations and fluctuations are concerned, +they seemingly have aided in producing the slight secular +progression to which we have so often referred.</p> + +<p>There is general agreement among geologists that the +ocean has become increasingly saline throughout the +ages. Indeed, calculations of the rate of accumulation of +salt have been a favorite method of arriving at estimates +of the age of the ocean, and hence of the earliest marine +sediments. So far as known, however, no geologist or +climatologist has discussed the probable climatic effects +<!--File: cc248.png Page 224--> +<a class="pagenum break" id="Page_224" title="pg 224"></a> +of increased salinity. Yet it seems clear that an increase +in salinity must have a slight effect upon climate.</p> + +<p>Salinity affects climate in four ways: (1) It appreciably +influences the rate of evaporation; (2) it alters +the freezing point; (3) it produces certain indirect +effects through changes in the absorption of carbon +dioxide; and (4) it has an effect on oceanic circulation.</p> + +<p>(1) According to the experiments of Mazelle and +Okada, as reported by Krümmel,<sup><a href="#fn99" id="r99">[99]</a></sup> evaporation from ordinary +sea water is from 9 to 30 per cent less rapid than +from fresh water under similar conditions. The variation +from 9 to 30 per cent found in the experiments depends, +perhaps, upon the wind velocity. When salt water +is stagnant, rapid evaporation tends to result in the +development of a film of salt on the top of the water, +especially where it is sheltered from the wind. Such a film +necessarily reduces evaporation. Hence the relatively +low salinity of the oceans in the past probably had a +tendency to increase the amount of water vapor in the +air. Even a little water vapor augments slightly the +blanketing effect of the air and to that extent diminishes +the diurnal and seasonal range of temperature and the +contrast from zone to zone.</p> + +<p>(2) Increased salinity means a lower freezing temperature +of the oceans and hence would have an effect +during cold periods such as the present and the Pleistocene +ice age. It would not, however, be of importance +during the long warm periods which form most of +geologic time. A salinity of about 3.5 per cent at present +lowers the freezing point of the ocean roughly 2°C. below +that of fresh water. If the ocean were fresh and our +winters as cold as now, all the harbors of New England +and the Middle Atlantic States would be icebound. The +<!--File: cc249.png Page 225--> +<a class="pagenum break" id="Page_225" title="pg 225"></a> +Baltic Sea would also be frozen each winter, and even +the eastern harbors of the British Isles would be frequently +locked in ice. At high latitudes the area of permanently +frozen oceans would be much enlarged. The +effect of such a condition upon marine life in high latitudes +would be like that of a change to a warmer climate. +It would protect the life on the continental shelf from the +severe battering of winter storms. It would also lessen +the severity of the winter temperature in the water for +when water freezes it gives up much latent heat,—eighty +calories per cubic centimeter. Part of this raises the +temperature of the underlying water.</p> + +<p>The expansion of the ice near northern shores would +influence the life of the lands quite differently from that +of the oceans. It would act like an addition of land to the +continents and would, therefore, increase the atmospheric +contrasts from zone to zone and from continental interior +to ocean. In summer the ice upon the sea would tend to +keep the coastal lands cool, very much as happens now +near the Arctic Ocean, where the ice floes have a great +effect through their reflection of light and their absorption +of heat in melting. In winter the virtual enlargement +of the continents by the addition of an ice fringe would +decrease the snowfall upon the lands. Still more important +would be the effect in intensifying the anti-cyclonic +conditions which normally prevail in winter not +only over continents but over ice-covered oceans. Hence +the outblowing cold winds would he strengthened.<sup><a href="#fn100" id="r100">[100]</a></sup> The +net effect of all these conditions would apparently be a +diminution of snowfall in high latitudes upon the lands +even though the summer snowfall upon the ocean and the +<!--File: cc250.png Page 226--> +<a class="pagenum break" id="Page_226" title="pg 226"></a> +coasts may have increased. This condition may have been +one reason why widespread glaciation does not appear to +have prevailed in high latitudes during the Proterozoic +and Permian glaciations, even though it occurred farther +south. If the ocean during those early glacial epochs +were ice-covered down to middle latitudes, a lack of extensive +glaciation in high latitudes would be no more +surprising than is the lack of Pleistocene glaciation in +the northern parts of Alaska and Asia. Great ice sheets +are impossible without a large supply of moisture.</p> + +<p>(3) Among the indirect effects of salinity one of the +chief appears to be that the low salinity of the water in +the past and the greater ease with which it froze presumably +allowed the temperature of the entire ocean to be +slightly higher than now. This is because ice serves as a +blanket and hinders the radiation of heat from the underlying +water. The temperature of the ocean has a climatic +significance not only directly, but indirectly through its +influence on the amount of carbon dioxide held by the +oceans. A change of even 1°C. from the present mean +temperature of 2°C. would alter the ability of the entire +ocean to absorb carbon dioxide by about 4 per cent. This, +according to F. W. Clarke,<sup><a href="#fn101" id="r101">[101]</a></sup> is because the oceans contain +from eighteen to twenty-seven times as much carbon +dioxide as the air when only the free carbon dioxide is +considered, and about seventy times as much according +to Johnson and Williamson<sup><a href="#fn102" id="r102">[102]</a></sup> when the partially combined +carbon dioxide is also considered. Moreover, the capacity +of water for carbon dioxide varies sharply with the temperature.<sup><a href="#fn103" id="r103">[103]</a></sup> +Hence a rise in temperature of only 1°C. +would theoretically cause the oceans to give up from 30 +<!--File: cc251.png Page 227--> +<a class="pagenum break" id="Page_227" title="pg 227"></a> +to 280 times as much carbon dioxide as the air now holds. +This, however, is on the unfounded assumption that the +oceans are completely saturated. The important point is +merely that a slight change in ocean temperature would +cause a disproportionately large change in the amount +of carbon dioxide in the air with all that this implies in +respect to blanketing the earth, and thus altering temperature.</p> + +<p>(4) Another and perhaps the most important effect of +salinity upon climate depends upon the rapidity of the +deep-sea circulation. The circulation is induced by differences +of temperature, but its speed is affected at least +slightly by salinity. The vertical circulation is now dominated +by cold water from subpolar latitudes. Except in +closed seas like the Mediterranean the lower portions +of the ocean are near the freezing point. This is because +cold water sinks in high latitudes by reason of its superior +density, and then "creeps" to low latitudes. There +it finally rises and replaces either the water driven poleward +by the winds, or that which has evaporated from +the Surface.<sup><a href="#fn104" id="r104">[104]</a></sup></p> + +<p>During past ages, when the sea water was less salty, +the circulation was presumably more rapid than now. +This was because, in tropical regions, the rise of cold +<!--File: cc252.png Page 228--> +<a class="pagenum break" id="Page_228" title="pg 228"></a> +water is hindered by the sinking of warm surface water +which is relatively dense because evaporation has removed +part of the water and caused an accumulation of +salt. According to Krümmel and Mill,<sup><a href="#fn105" id="r105">[105]</a></sup> the surface salinity +of the subtropical belt of the North Atlantic commonly +exceeds 3.7 per cent and sometimes reaches 3.77 per cent, +whereas the underlying waters have a salinity of less +than 3.5 per cent and locally as little as 3.44 per cent. +The other oceans are slightly less saline than the North +Atlantic at all depths, but the vertical salinity gradients +along the tropics are similar. According to the Smithsonian +Physical Tables, the difference in salinity between +the surface water and that lying below is equivalent to +a difference of .003 in density, where the density of fresh +water is taken as 1.000. Since the decrease in density produced +by warming water from the temperature of its +greatest density (4°C.) to the highest temperatures +which ever prevail in the ocean (30°C. or 86°F.) is only +.004, the more saline surface waters of the dry tropics +are at most times almost as dense as the less saline but +colder waters beneath the surface, which have come from +higher latitudes. During days of especially great evaporation, +however, the most saline portions of the surface +waters in the dry tropics are denser than the underlying +waters and therefore sink, and produce a temporary local +stagnation in the general circulation. Such a sinking of +the warm surface waters is reported by Krümmel, who +detected it by means of the rise in temperature which it +produces at considerable depths. If such a hindrance to +the circulation did not exist, the velocity of the deep-sea +movements would be greater.</p> + +<p>If in earlier times a more rapid circulation occurred, +low latitudes must have been cooled more than now by +<!--File: cc253.png Page 229--> +<a class="pagenum break" id="Page_229" title="pg 229"></a> +the rise of cold waters. At the same time higher latitudes +were presumably warmed by a greater flow of warm +water from tropical regions because less of the surface +heat sank in low latitudes. Such conditions would tend to +lessen the climatic contrast between the different latitudes. +Hence, in so far as the rate of deep-sea circulation +depends upon salinity, the slowly increasing amount of +salt in the oceans must have tended to increase the contrasts +between low and high latitudes. Thus for several +reasons, the increase of salinity during geologic history +seems to deserve a place among the minor agencies which +help to explain the apparent tendency toward a secular +progression of climate in the direction of greater contrasts +between tropical and subpolar latitudes.</p> + +<p>Changes in the composition and amount of the atmosphere +have presumably had a climatic importance greater +than that of changes in the salinity of the oceans. The +atmospheric changes may have been either progressive +or cyclic, or both. In early times, according to the nebular +hypothesis, the atmosphere was much more dense than +now and contained a larger percentage of certain constituents, +notably carbon dioxide and water. The planetesimal +hypothesis, on the other hand, postulates an increase +in the density of the atmosphere, for according to +this hypothesis the density of the atmosphere depends +upon the power of the earth to hold gases, and this power +increases as the earth grows bigger with the infall of +material from without.<sup><a href="#fn106" id="r106">[106]</a></sup></p> + +<p>Whichever hypothesis may be correct, it seems probable +that when life first appeared on the land the atmosphere +resembled that of today in certain fundamental +respects. It contained the elements essential to life, and +<!--File: cc254.png Page 230--> +<a class="pagenum break" id="Page_230" title="pg 230"></a> +its blanketing effect was such as to maintain temperatures +not greatly different from those of the present. The +evidence of this depends largely upon the narrow limits +of temperature within which the activities of modern +life are possible, and upon the cumulative evidence that +ancient life was essentially similar to the types now +living. The resemblance between some of the oldest +forms and those of today is striking. For example, +according to Professor Schuchert:<sup><a href="#fn107" id="r107">[107]</a></sup> "Many of the living +genera of forest trees had their origin in the Cretaceous, +and the giant sequoias of California go back to the Triassic, +while Ginkgo is known in the Permian. Some of the +fresh-water molluscs certainly were living in the early +periods of the Mesozoic, and the lung-fish of today +(Ceratodus) is known as far back as the Triassic and is +not very unlike other lung-fishes of the Devonian. The +higher vertebrates and insects, on the other hand, are +very sensitive to their environment, and therefore do not +extend back generically beyond the Cenozoic, and only in +a few instances even as far as the Oligocene. Of marine +invertebrates the story is very different, for it is well +known that the horseshoe crab (Limulus) lived in the +Upper Jurassic, and Nautilus in the Triassic, with forms +in the Devonian not far removed from this genus. Still +longer-ranging genera occur among the brachiopods, for +living Lingula and Crania have specific representatives +as far back as the early Ordovician. Among living foraminifers, +Lagena, Globigerina, and Nodosaria are known +in the later Cambrian or early Ordovician. In the Middle +Cambrian near Field, British Columbia, Walcott has +found a most varied array of invertebrates among which +are crustaceans not far removed from living forms. +Zoölogists who see these wonderful fossils are at once +<!--File: cc255.png Page 231--> +<a class="pagenum break" id="Page_231" title="pg 231"></a> +struck with their modernity and the little change that has +taken place in certain stocks since that far remote time. +Back of the Paleozoic, little can be said of life from the +generic standpoint, since so few fossils have been recovered, +but what is at hand suggests that the marine +environment was similar to that of today."</p> + +<p>At present, as we have repeatedly seen, little growth +takes place either among animals or plants at temperatures +below 0°C. or above 40°C., and for most species +the limiting temperatures are about 10° and 30°. The +maintenance of so narrow a scale of temperature is a +function of the atmosphere, as well as of the sun. Without +an atmosphere, the temperature by day would mount +fatally wherever the sun rides high in the sky. By night +it would fall everywhere to a temperature approaching +absolute zero, that is -273°C. Some such temperature +prevails a few miles above the earth's surface, +beyond the effective atmosphere. Indeed, even if the +atmosphere were almost as it is now, but only lacked one +of the minor constituents, a constituent which is often +actually ignored in statements of the composition of the +air, life would be impossible. Tyndall concludes that if +water vapor were entirely removed from the atmosphere +for a single day and night, all life—except that which is +dormant in the form of seeds, eggs, or spores—would be +exterminated. Part would be killed by the high temperature +developed by day when the sun was high, and part, +by the cold night.</p> + +<p>The testimony of ancient glaciation as to the slight +difference in the climate and therefore in the atmosphere +of early and late geological times is almost as clear +as that of life. Just as life proves that the earth can never +have been extremely cold during hundreds of millions of +years, so glaciation in moderately low latitudes near +<!--File: cc256.png Page 232--> +<a class="pagenum break" id="Page_232" title="pg 232"></a> +the dawn of earth history and at several later times, +proves that the earth was not particularly hot even in +those early days. The gentle progressive change of climate +which is recorded in the rocks appears to have been only +in slight measure a change in the mean temperature of +the earth as a whole, and almost entirely a change in the +distribution of temperature from place to place and +season to season. Hence it seems probable that neither +the earth's own emission of heat, nor the supply of solar +heat, nor the power of the atmosphere to retain heat can +have been much greater a few hundred million years ago +than now. It is indeed possible that these three factors +may have varied in such a way that any variation in one +has been offset by variations of the others in the opposite +direction. This, however, is so highly improbable that it +seems advisable to assume that all three have remained +relatively constant. This conclusion together with a +realization of the climatic significance of carbon dioxide +has forced most of the adherents of the nebular hypothesis +to abandon their assumption that carbon dioxide, the +heaviest gas in the air, was very abundant until taken +out by coal-forming plants or combined with the calcium +oxide of igneous rocks to form the limestone secreted +by animals. In the same way the presence of sun cracks +in sedimentary rocks of all ages suggests that the air +cannot have contained vast quantities of water vapor +such as have been assumed by Knowlton and others in +order to account for the former lack of sharp climatic +contrast between the zones. Such a large amount of water +vapor would almost certainly be accompanied by well-nigh +universal and continual cloudiness so that there +would be little chance for the pools on the earth's water-soaked +surface to dry up. Furthermore, there is only one +way in which such cloudiness could be maintained and +<!--File: cc257.png Page 233--> +<a class="pagenum break" id="Page_233" title="pg 233"></a> +that is by keeping the air at an almost constant temperature +night and day. This would require that the chief +source of warmth be the interior of the earth, a condition +which the Proterozoic, Permian, and other widespread +glaciations seem to disprove.</p> + +<p>Thus there appears to be strong evidence against the +radical changes in the atmosphere which are sometimes +postulated. Yet some changes must have taken place, and +even minor changes would be accompanied by some sort +of climatic effect. The changes would take the form of +either an increase or a decrease in the atmosphere as a +whole, or in its constituent elements. The chief means by +which the atmosphere has increased appear to be as +follows: (a) By contributions from the interior of the +earth via volcanoes and springs and by the weathering of +igneous rocks with the consequent release of their enclosed +gases;<sup><a href="#fn108" id="r108">[108]</a></sup> (b) by the escape of some of the abundant +gases which the ocean holds in solution; (c) by the arrival +on the earth of gases from space, either enclosed in +meteors or as free-flying molecules; (d) by the release of +gases from organic compounds by oxidation, or by exhalation +from animals and plants. On the other hand, one +or another of the constituents of the atmosphere has presumably +decreased (a) by being locked up in newly +formed rocks or organic compounds; (b) by being dissolved +in the ocean; (c) by the escape of molecules into +space; and (d) by the condensation of water vapor.</p> + +<p>The combined effect of the various means of increase +and decrease depends partly on the amount of each constituent +received from the earth's interior or from space, +and partly on the fact that the agencies which tend to +deplete the atmosphere are highly selective in their +<!--File: cc258.png Page 234--> +<a class="pagenum break" id="Page_234" title="pg 234"></a> +action. Our knowledge of how large a quantity of new +gases the air has received is very scanty, but judging by +present conditions the general tendency is toward a slow +increase chiefly because of meteorites, volcanic action, +and the work of deep-seated springs. As to decrease, the +case is clearer. This is because the chemically active +gases, oxygen, CO<sub>2</sub>, and water vapor, tend to be locked +up in the rocks, while the chemically inert gases, nitrogen +and argon, show almost no such tendency. Though oxygen +is by far the most abundant element in the earth's +crust, making up more than 50 per cent of the total, it +forms only about one-fifth of the air. Nitrogen, on the +other hand, is very rare in the rocks, but makes up nearly +four-fifths of the air. It would, therefore, seem probable +that throughout the earth's history, there has been a +progressive increase in the amount of atmospheric nitrogen, +and presumably a somewhat corresponding increase +in the mass of the air. On the other hand, it is not clear +what changes have occurred in the amount of atmospheric +oxygen. It may have increased somewhat or +perhaps even notably. Nevertheless, because of the +greater increase in nitrogen, it may form no greater percentage +of the air now than in the distant past.</p> + +<p>As to the absolute amounts of oxygen, Barrell<sup><a href="#fn109" id="r109">[109]</a></sup> +thought that atmospheric oxygen began to be present +only after plants had appeared. It will be recalled that +plants absorb carbon dioxide and separate the carbon +from the oxygen, using the carbon in their tissues and +setting free the oxygen. As evidence of a paucity of +oxygen in the air in early Proterozoic times, Barrell +cites the fact that the sedimentary rocks of that remote +<!--File: cc259.png Page 235--> +<a class="pagenum break" id="Page_235" title="pg 235"></a> +time commonly are somewhat greyish or greenish-grey +wackes, or other types, indicating incomplete oxidation. +He admits, however, that the stupendous thicknesses of +red sandstones, quartzite, and hematitic iron ores of the +later Proterozoic prove that by that date there was an +abundance of atmospheric oxygen. If so, the change from +paucity to abundance must have occurred before fossils +were numerous enough to give much clue to climate. +However, Barrell's evidence as to a former paucity of +atmospheric oxygen is not altogether convincing. In the +first place, it does not seem justifiable to assume that +there could be no oxygen until plants appeared to break +down the carbon dioxide, for some oxygen is contributed +by volcanoes,<sup><a href="#fn110" id="r110">[110]</a></sup> and lightning decomposes water into its +elements. Part of the hydrogen thus set free escapes into +space, for the earth's gravitative force does not appear +great enough to hold this lightest of gases, but the oxygen +remains. Thus electrolysis of water results in the +accumulation of oxygen. In the second place, there is no +proof that the ancient greywackes are not deoxidized +sediments. Light colored rock formations do not necessarily +indicate a paucity of atmospheric oxygen, for such +rocks are abundant even in recent times. For example, +the Tertiary formations are characteristically light +colored, a result, however, of deoxidation. Finally, the +fact that sedimentary rocks, irrespective of their age, +contain an average of about 1.5 per cent more oxygen +than do igneous rocks,<sup><a href="#fn111" id="r111">[111]</a></sup> suggests that oxygen was present +in the air in quantity even when the earliest shales +and sandstones were formed, for atmospheric oxygen +seems to be the probable source of the extra oxygen they +<!--File: cc260.png Page 236--> +<a class="pagenum break" id="Page_236" title="pg 236"></a> +contain. The formation of these particular sedimentary +rocks by weathering of igneous rocks involves only a +little carbon dioxide and water. Although it seems probable +that oxygen was present in the atmosphere even at +the beginning of the geological record, it may have been +far less abundant then than now. It may have been removed +from the atmosphere by animals or by the oxidation +of the rocks almost as rapidly as it was added by +volcanoes, plants, and other agencies.</p> + +<p>After this chapter was in type, St. John<sup><a href="#fn112" id="r112">[112]</a></sup> announced +his interesting discovery that oxygen is apparently lacking +in the atmosphere of Venus. He considers that this +proves that Venus has no life. Furthermore he concludes +that so active an element as oxygen cannot be abundant +in the atmosphere of a planet unless plants continually +supply large quantities by breaking down carbon dioxide.</p> + +<p>But even if the earth has experienced a notable increase +in atmospheric oxygen since the appearance of +life, this does not necessarily involve important climatic +changes except those due to increased atmospheric density. +This is because oxygen has very little effect upon +the passage of light or heat, being transparent to all but +a few wave lengths. Those absorbed are chiefly in the +ultra violet.</p> + +<p>The distinct possibility that oxygen has increased in +amount, makes it the more likely that there has been an +increase in the total atmosphere, for the oxygen would +supplement the increase in the relatively inert nitrogen +and argon, which has presumably taken place. The climatic +effects of an increase in the atmosphere include, in +the first place, an increased scattering of light as it +approaches the earth. Nitrogen, argon, and oxygen all +<!--File: cc261.png Page 237--> +<a class="pagenum break" id="Page_237" title="pg 237"></a> +scatter the short waves of light and thus interfere with +their reaching the earth. Abbot and Fowle,<sup><a href="#fn113" id="r113">[113]</a></sup> who have +carefully studied the matter, believe that at present the +scattering is quantitatively important in lessening insolation. +Hence our supposed general increase in the volume +of the air during part of geological times would tend to +reduce the amount of solar energy reaching the earth's +surface. On the other hand, nitrogen and argon do not +appear to absorb the long wave lengths known as heat, +and oxygen absorbs so little as to be almost a non-absorber. +Therefore the reduced penetration of the air +by solar radiation due to the scattering of light would +apparently not be neutralized by any direct increase in +the blanketing effect of the atmosphere, and the temperature +near the earth's surface would be slightly lowered +by a thicker atmosphere. This would diminish the amount +of water vapor which would be held in the air, and +thereby lower the temperature a trifle more.</p> + +<p>In the second place, the higher atmospheric pressure +which would result from the addition of gases to the +air would cause a lessening of the rate of evaporation, +for that rate declines as pressure increases. Decreased +evaporation would presumably still further diminish the +vapor content of the atmosphere. This would mean a +greater daily and seasonal range of temperature, as is +very obvious when we compare clear weather with cloudy. +Cloudy nights are relatively warm while clear nights are +cool, because water vapor is an almost perfect absorber +of radiant heat, and there is enough of it in the air on +moist nights to interfere greatly with the escape of the +heat accumulated during the day. Therefore, if atmospheric +<!--File: cc262.png Page 238--> +<a class="pagenum break" id="Page_238" title="pg 238"></a> +moisture were formerly much more abundant +than now, the temperature must have been much more +uniform. The tendency toward climatic severity as time +went on would be still further increased by the cooling +which would result from the increased wind velocity discussed +below; for cooling by convection increases with +the velocity of the wind, as does cooling by conduction.</p> + +<p>Any persistent lowering of the general temperature of +the air would affect not only its ability to hold water +vapor, but would produce a lessening in the amount of +atmospheric carbon dioxide, for the colder the ocean +becomes the more carbon dioxide it can hold in solution. +When the oceanic temperature falls, part of the atmospheric +carbon dioxide is dissolved in the ocean. This +minor constituent of the air is important because +although it forms only 0.003 per cent of the earth's atmosphere, +Abbot and Fowle's<sup><a href="#fn114" id="r114">[114]</a></sup> calculations indicate that +it absorbs over 10 per cent of the heat radiated outward +from the earth. Hence variations in the amount of carbon +dioxide may have caused an appreciable variation in +temperature and thus in other climatic conditions. +Humphreys, as we have seen, has calculated that a +doubling of the carbon dioxide in the air would directly +raise the earth's temperature to the extent of 1.3°C., and +a halving would lower it a like amount. The indirect +results of such an increase or decrease might be greater +than the direct results, for the change in temperature +due to variations in carbon dioxide would alter the +capacity of the air to hold moisture.</p> + +<p>Two conditions would especially help in this respect; +first, changes in nocturnal cooling, and second, changes +in local convection. The presence of carbon dioxide diminishes +nocturnal cooling because it absorbs the heat radiated +<!--File: cc263.png Page 239--> +<a class="pagenum break" id="Page_239" title="pg 239"></a> +by the earth, and re-radiates part of it back again. +Hence with increased carbon dioxide and with the +consequent warmer nights there would be less nocturnal +condensation of water vapor to form dew and frost. +Local convection is influenced by carbon dioxide because +this gas lessens the temperature gradient. In general, the +less the gradient, that is, the less the contrast between +the temperature at the surface and higher up, the less +convection takes place. This is illustrated by the seasonal +variation in convection. In summer, when the gradient is +steepest, convection reaches its maximum. It will be recalled +that when air rises it is cooled by expansion, and +if it ascends far the moisture is soon condensed and +precipitated. Indeed, local convection is considered by +C. P. Day to be the chief agency which keeps the lower +air from being continually saturated with moisture. The +presence of carbon dioxide lessens convection because it +increases the absorption of heat in the zone above the +level in which water vapor is abundant, thus warming +these higher layers. The lower air may not be warmed +correspondingly by an increase in carbon dioxide if +Abbot and Fowle are right in stating that near the earth's +surface there is enough water vapor to absorb practically +all the wave lengths which carbon dioxide is capable of +absorbing. Hence carbon dioxide is chiefly effective at +heights to which the low temperature prevents water +vapor from ascending. Carbon dioxide is also effective +in cold winters and in high latitudes when even the lower +air is too cold to contain much water vapor. Moreover, +carbon dioxide, by altering the amount of atmospheric +water vapor, exerts an indirect as well as a direct effect +upon temperature.</p> + +<p>Other effects of the increase in air pressure which we +are here assuming during at least the early part of geological +<!--File: cc264.png Page 240--> +<a class="pagenum break" id="Page_240" title="pg 240"></a> +times are corresponding changes in barometric +contrasts, in the strength of winds, and in the mass of air +carried by the winds along the earth's surface. The increase +in the mass of the air would reënforce the greater +velocity of the winds in their action as eroding and transporting +agencies. Because of the greater weight of the +air, the winds would be capable of picking up more dust +and of carrying it farther and higher; while the increased +atmospheric friction would keep it aloft a longer time. +The significance of dust at high levels and its relation to +solar radiation have already been discussed in connection +with volcanoes. It will be recalled that on the average it +lowers the surface temperature. At lower levels, since +dust absorbs heat quickly and gives it out quickly, its +presence raises the temperature of the air by day and +lowers it by night. Hence an increase in dustiness tends +toward greater extremes.</p> + +<p>From all these considerations it appears that if the +atmosphere has actually evolved according to the supposition +which is here tentatively entertained, the general +tendency of the resultant climatic changes must have +been partly toward long geological oscillations and partly +toward a general though very slight increase in climatic +severity and in the contrasts between the zones. This +seems to agree with the geological record, although the +fact that we are living in an age of relative climatic +severity may lead us astray.</p> + +<p>The significant fact about the whole matter is that the +three great types of terrestrial agencies, namely, those +of the earth's interior, those of the oceans, and those of +the air, all seem to have suffered changes which lead to +slow variations of climate. Many reversals have doubtless +taken place, and the geologic oscillations thus induced +are presumably of much greater importance than +<!--File: cc265.png Page 241--> +<a class="pagenum break" id="Page_241" title="pg 241"></a> +the progressive change, yet so far as we can tell the +purely terrestrial changes throughout the hundreds of +millions of years of geological time have tended toward +complexity and toward increased contrasts from continent +to ocean, from latitude to latitude, from season to +season, and from day to night.</p> + +<p>Throughout geological history the slow and almost +imperceptible differentiation of the earth's surface has +been one of the most noteworthy of all changes. It has +been opposed by the extraordinary conservatism of the +universe which causes the average temperature today to +be so like that of hundreds of millions of years ago that +many types of life are almost identical. Nevertheless, the +differentiation has gone on. Often, to be sure, it has presumably +been completely masked by the disturbances of +the solar atmosphere which appear to have been the +cause of the sharper, shorter climatic pulsations. But +regardless of cosmic conservatism and of solar impulses +toward change, the slow differentiation of the earth's +surface has apparently given to the world of today much +of the geographical complexity which is so stimulating +a factor in organic evolution. Such complexity—such +diversity from place to place—appears to be largely +accounted for by purely terrestrial causes. It may be +regarded as the great terrestrial contribution to the +climatic environment which guides the development of +life.</p> +<!--File: cc266.png Page 242--> +<a class="pagenum break" id="Page_242" title="pg 242"></a> + + + + +<h2 id="Chapt_XIV"><a href="#content">CHAPTER XIV</a></h2> + +<h3>THE EFFECT OF OTHER BODIES ON THE SUN</h3> + + +<p>If solar activity is really an important factor in +causing climatic changes, it behooves us to subject +the sun to the same kind of inquiry to which we +have subjected the earth. We have inquired into the nature +of the changes through which the earth's crust, the +oceans, and the atmosphere have influenced the climate +of geological times. It has not been necessary, however, +to study the origin of the earth, nor to trace its earlier +stages. Our study of the geological record begins only +when the earth had attained practically its present mass, +essentially its present shape, and a climate so similar to +that of today that life as we know it was possible. In +other words, the earth had passed the stages of infancy, +childhood, youth, and early maturity, and had reached +full maturity. As it still seems to be indefinitely far from +old age, we infer that during geological times its relative +changes have been no greater than those which a man +experiences between the ages of perhaps twenty-five and +forty.</p> + +<p>Similar reasoning applies with equal or greater force +to the sun. Because of its vast size it presumably passes +through its stages of development much more slowly than +the earth. In the first chapter of this book we saw that +the earth's relative uniformity of climate for hundreds of +millions of years seems to imply a similar uniformity in +solar activity. This accords with a recent tendency among +<!--File: cc267.png--> +<a class="pagenum break" id="Page_243" title="pg 243"></a> +astronomers who are more and more recognizing that the +stars and the solar system possess an extraordinary degree +of conservatism. Changes that once were supposed +to take place in thousands of years are now thought to +have required millions. Hence in this chapter we shall +assume that throughout geological times the condition +of the sun has been almost as at present. It may have +been somewhat larger, or different in other ways, but it +was essentially a hot, gaseous body such as we see today +and it gave out essentially the same amount of energy. +This assumption will affect the general validity of what +follows only if it departs widely from the truth. With this +assumption, then, let us inquire into the degree to which +the sun's atmosphere has probably been disturbed +throughout geological times.</p> + +<p>In <em>Earth and Sun</em>, as already explained, a detailed +study has led to the conclusion that cyclonic storms are +influenced by the electrical action of the sun. Such action +appears to be most intense in sunspots, but apparently +pertains also to other disturbed areas in the sun's +atmosphere. A study of sunspots suggests that their +true periodicity is almost if not exactly identical with +that of the orbital revolution of Jupiter, 11.8 years. Other +investigations show numerous remarkable coincidences +between sunspots and the orbital revolution of the other +planets, including especially Saturn and Mercury. This +seems to indicate that there is some truth in the hypothesis +that sunspots and other related disturbances of the +solar atmosphere owe their periodicity to the varying +effects of the planets as they approach and recede from +the sun in their eccentric orbits and as they combine or +oppose their effects according to their relative positions. +This does not mean that the energy of the solar disturbances +is supposed to come from the planets, but merely +<!--File: cc268.png--> +<a class="pagenum break" id="Page_244" title="pg 244"></a> +that their variations act like the turning of a switch to +determine when and how violently the internal forces of +the sun shall throw the solar atmosphere into commotion. +This hypothesis is by no means new, for in one form or +another it has been advocated by Wolfer, Birkeland, +E. W. Brown, Schuster, Arctowski, and others.</p> + +<p>The agency through which the planets influence the +solar atmosphere is not yet clear. The suggested agencies +are the direct pull of gravitation, the tidal effect of the +planets, and an electro-magnetic effect. In <em>Earth and +Sun</em> the conclusion is reached that the first two are out +of the question, a conclusion in which E. W. Brown +acquiesces. Unless some unknown cause is appealed to, +this leaves an electro-magnetic hypothesis as the only one +which has a reasonable foundation. Schuster inclines to +this view. The conclusions set forth in <em>Earth and Sun</em> as +to the electrical nature of the sun's influence on the earth +point somewhat in the same direction. Hence in this +chapter we shall inquire what would happen to the sun, +and hence to the earth, on their journey through space, +if the solar atmosphere is actually subject to disturbance +by the electrical or other effects of other heavenly bodies. +It need hardly be pointed out that we are here venturing +into highly speculative ground, and that the verity or +falsity of the conclusions reached in this chapter has +nothing to do with the validity of the reasoning in previous +chapters. Those chapters are based on the assumption +that terrestrial causes of climatic changes are supplemented +by solar disturbances which produce their +effect partly through variations in temperature but also +through variations in the intensity and paths of cyclonic +storms. The present chapter seeks to shed some light on +the possible causes and sequence of solar disturbances.</p> + +<p>Let us begin by scanning the available evidence as to +<!--File: cc269.png--> +<a class="pagenum break" id="Page_245" title="pg 245"></a> +solar disturbances previous to the time when accurate +sunspot records are available. Two rather slender bits of +evidence point to cycles of solar activity lasting hundreds +of years. One of these has already been discussed in +Chapter VI, where the climatic stress of the fourteenth +century was described. At that time sunspots are known +to have been unusually numerous, and there were great +climatic extremes. Lakes overflowed in Central Asia; +storms, droughts, floods, and cold winters were unusually +severe in Europe; the Caspian Sea rose with great +rapidity; the trees of California grew with a vigor unknown +for centuries; the most terrible of recorded famines +occurred in England and India; the Eskimos were +probably driven south by increasing snowiness in Greenland; +and the Mayas of Yucatan appear to have made +their last weak attempt at a revival of civilization under +the stimulus of greater storminess and less constant +rainfall.</p> + +<p>The second bit of evidence is found in recent exhaustive +studies of periodicities by Turner<sup><a href="#fn115" id="r115">[115]</a></sup> and other +astronomers. They have sought every possible natural +occurrence for which a numerical record is available for +a long period. The most valuable records appear to be +those of tree growth, Nile floods, Chinese earthquakes, +and sunspots. Turner reaches the conclusion that all four +types of phenomena show the same periodicity, namely, +cycles with an average length of about 260 to 280 years. +He suggests that if this is true, the cycles in tree growth +and in floods, both of which are climatic, are probably +due to a non-terrestrial cause. The fact that the sunspots +<!--File: cc270.png--> +<a class="pagenum break" id="Page_246" title="pg 246"></a> +show similar cycles suggests that the sun's variations +are the cause.</p> + +<p>These two bits of evidence are far too slight to form +the foundation of any theory as to changes in solar +activity in the geological past. Nevertheless it may be +helpful to set forth certain possibilities as a stimulus to +further research. For example, it has been suggested that +meteoric bodies may have fallen into the sun and caused +it suddenly to flare up, as it were. This is not impossible, +although it does not appear to have taken place since +men became advanced enough to make careful observations. +Moreover, the meteorites which now fall on the +earth are extremely small, the average size being computed +as no larger than a grain of wheat. The largest +ever found on the earth's surface, at Bacubirito in +Mexico, weighs only about fifty tons, while within the +rocks the evidences of meteorites are extremely scanty +and insignificant. If meteorites had fallen into the sun +often enough and of sufficient size to cause glacial fluctuations +and historic pulsations of climate, it seems highly +probable that the earth would show much more evidence +of having been similarly disturbed. And even if the sun +should be bombarded by large meteors the result would +probably not be sudden cold periods, which are the most +notable phenomena of the earth's climatic history, but +sudden warm periods followed by slow cooling. Nevertheless, +the disturbance of the sun by collision with meteoric +matter can by no means be excluded as a possible cause +of climatic variations.</p> + +<p>Allied to the preceding hypothesis is Shapley's<sup><a href="#fn116" id="r116">[116]</a></sup> nebular +hypothesis. At frequent intervals, averaging about +<!--File: cc271.png--> +<a class="pagenum break" id="Page_247" title="pg 247"></a> +once a year during the last thirty years, astronomers have +discovered what are known as novæ. These are stars +which were previously faint or even invisible, but which +flash suddenly into brilliancy. Often their light-giving +power rises seven or eight magnitudes—a thousand-fold. +In addition to the spectacular novæ there are numerous +irregular variables whose brilliancy changes in every +ratio from a few per cent up to several magnitudes. Most +of them are located in the vicinity of nebulæ, as is also +the case with novæ. This, as well as other facts, makes it +probable that all these stars are "friction variables," as +Shapley calls them. Apparently as they pass through the +nebulæ they come in contact with its highly diffuse +matter and thereby become bright much as the earth +would become bright if its atmosphere were filled with +millions of almost infinitesimally small meteorites. A star +may also lose brilliancy if nebulous matter intervenes +between it and the observer. If our sun has been subjected +to any of these changes some sort of climatic effect +must have been produced.</p> + +<p>In a personal communication Shapley amplifies the +nebular climatic hypothesis as follows:</p> + +<blockquote> +<p>Within 700 light years of the sun in many directions (Taurus, +Cygnus, Ophiuchus, Scorpio) are great diffuse clouds of nebulosity, +some bright, most of them dark. The probability that stars +moving in the general region of such clouds will encounter this +material is very high, for the clouds fill enormous volumes of +space,—e.g., probably more than a hundred thousand cubic light +years in the Orion region, and are presumably composed of rarefied +gases or of dust particles. Probably throughout all our +part of space such nebulosity exists (it is all around us, we are +sure), but only in certain regions is it dense enough to affect +conspicuously the stars involved in it. If a star moving at high +velocity should collide with a dense part of such a nebulous +<!--File: cc272.png--> +<a class="pagenum break" id="Page_248" title="pg 248"></a> +cloud, we should probably have a typical nova. If the relative +velocity of nebulous material and star were low or moderate, or +if the material were rare, we should not expect a conspicuous +effect on the star's light.</p> + +<p>In the nebulous region of Orion, which is probably of unusually +high density, there are about 100 known stars, varying +between 20% and 80% of their total light—all of them irregularly—some +slowly, some suddenly. Apparently they are +"friction variables." Some of the variables suddenly lose 40% +of their light as if blanketed by nebulous matter. In the Trifid +Nebula there are variables like those of Orion, in Messier 8 also, +and probably many of the 100 or so around the Rho Ophiuchi +region belong to this kind.</p> + +<p>I believe that our sun could not have been a typical nova, at +least not since the Archeozoic, that is for perhaps a billion years. +I believe we have in geological climates final proof of this, because +an increase in the amount of solar radiation by 1000 times +as in the typical nova, would certainly punctuate emphatically +the life cycle on the earth, even if the cause of the nova would +not at the same time eliminate the smaller planets. But the sun +may have been one of these miniature novæ or friction variables; +and I believe it very probable that its wanderings through +this part of space could not long leave its mean temperature +unaffected to the amount of a few per cent.</p> + +<p>One reason we have not had this proposal insisted upon +before is that the data back of it are mostly new—the Orion +variables have been only recently discovered and studied, the +distribution and content of the dark nebulæ are hardly as yet +generally known.</p> +</blockquote> + +<p>This interesting hypothesis cannot be hastily dismissed. +If the sun should pass through a nebula it seems +inevitable that there would be at least slight climatic +effects and perhaps catastrophic effects through the +action of the gaseous matter not only on the sun but on +the earth's own atmosphere. As an explanation of the +<!--File: cc273.png--> +<a class="pagenum break" id="Page_249" title="pg 249"></a> +general climatic conditions of the past, however, Shapley +points out that the hypothesis has the objection of being +vague, and that nebulosity should not be regarded as +more than "a possible factor." One of the chief difficulties +seems to be the enormously wide distribution of as +yet undiscovered nebulous matter which must be assumed +if any large share of the earth's repeated climatic +changes is to be ascribed to such matter. If such matter +is actually abundant in space, it is hard to see how any +but the nearest stars would be visible. Another objection +is that there is no known nebulosity near at hand with +which to connect the climatic vicissitudes of the last +glacial period. Moreover, the known nebulæ are so much +less numerous than stars that the chances that the sun +will encounter one of them are extremely slight. This, +however, is not an objection, for Shapley points out that +during geological times the sun can never have varied +as much as do the novæ, or even as most of the friction +variables. Thus the hypothesis stands as one that is worth +investigating, but that cannot be finally rejected or accepted +until it is made more definite and until more information +is available.</p> + +<p>Another suggested cause of solar variations is the relatively +sudden contraction of the sun such as that which +sometimes occurs on the earth when continents are uplifted +and mountains upheaved. It seems improbable that +this could have occurred in a gaseous body like the sun. +Lacking, as it does, any solid crust which resists a change +of form, the sun probably shrinks steadily. Hence any +climatic effects thus produced must be extremely gradual +and must tend steadily in one direction for millions of +years.</p> + +<p>Still another suggestion is that the tidal action of the +stars and other bodies which may chance to approach +<!--File: cc274.png--> +<a class="pagenum break" id="Page_250" title="pg 250"></a> +the sun's path may cause disturbances of the solar +atmosphere. The vast kaleidoscope of space is never +quiet. The sun, the stars, and all the other heavenly +bodies are moving, often with enormous speed. Hence the +effect of gravitation upon the sun must vary constantly +and irregularly, as befits the geological requirements. In +the case of the planets, however, the tidal effect does not +seem competent to produce the movements of the solar +atmosphere which appear to be concerned in the inception +of sunspots. Moreover, there is only the most remote +probability that a star and the sun will approach near +enough to one another to produce a pronounced gravitational +disturbance in the solar atmosphere. For instance, +if it be assumed that changes in Jupiter's tidal effect on +the sun are the main factor in regulating the present difference +between sunspot maxima and sunspot minima, +the chances that a star or some non-luminous body of +similar mass will approach near enough to stimulate +solar activity and thereby bring on glaciation are only +one in twelve billion years, as will be explained below. +This seems to make a gravitational hypothesis impossible.</p> + +<p>Another possible cause of solar disturbances is that +the stars in their flight through space may exert an +electrical influence which upsets the equilibrium of the +solar atmosphere. At first thought this seems even more +impossible than a gravitational effect. Electrostatic +effects, however, differ greatly from those of tides. They +vary as the diameter of a body instead of as its mass; +their differentials also vary inversely as the square of +the distance instead of as the cube. Electrostatic effects +also increase as the fourth power of the temperature or +at least would do so if they followed the law of black +bodies; they are stimulated by the approach of one body +<!--File: cc275.png--> +<a class="pagenum break" id="Page_251" title="pg 251"></a> +to another; and they are cumulative, for if ions arrive +from space they must accumulate until the body to which +they have come begins to discharge them. Hence, on the +basis of assumptions such as those used in the preceding +paragraph, the chances of an electrical disturbance of +the solar atmosphere sufficient to cause glaciation on the +earth may be as high as one in twenty or thirty million +years. This seems to put an electrical hypothesis within +the bounds of possibility. Further than that we cannot +now go. There may be other hypotheses which fit the facts +much better, but none seems yet to have been suggested.</p> + +<p>In the rest of this chapter the tidal and electrical +hypotheses of stellar action on the sun will be taken up +in detail. The tidal hypothesis is considered because in +discussions of the effect of the planets it has hitherto +held almost the entire field. The electrical hypothesis will +be considered because it appears to be the best yet suggested, +although it still seems doubtful whether electrical +effects can be of appreciable importance over such vast +distances as are inevitably involved. The discussion of +both hypotheses will necessarily be somewhat technical, +and will appeal to the astronomer more than to the layman. +It does not form a necessary part of this book, for +it has no bearing on our main thesis of the effect of the +sun on the earth. It is given here because ultimately the +question of changes in solar activity during geological +times must be faced.</p> + +<p>In the astronomical portion of the following discussion +we shall follow Jeans<sup><a href="#fn117" id="r117">[117]</a></sup> in his admirable attempt at a +mathematical analysis of the motions of the universe. +Jeans divides the heavenly bodies into five main types. +(1) Spiral nebulæ, which are thought by some astronomers +<!--File: cc276.png--> +<a class="pagenum break" id="Page_252" title="pg 252"></a> +to be systems like our own in the making, and by +others to be independent universes lying at vast distances +beyond the limits of our Galactic universe, as it is called +from the Galaxy or Milky Way. (2) Nebulæ of a smaller +type, called planetary. These lie within the Galactic portion +of the universe and seem to be early stages of what +may some day be stars or solar systems. (3) Binary or +multiple stars, which are extraordinarily numerous. In +some parts of the heavens they form 50 or even 60 per +cent of the stars and in the galaxy as a whole they seem to +form "fully one third." (4) Star clusters. These consist +of about a hundred groups of stars in each of which the +stars move together in the same direction with approximately +the same velocity. These, like the spiral nebulæ, +are thought by some astronomers to lie outside the limits +of the galaxy, but this is far from certain. (5) The solar +system. According to Jeans this seems to be unique. It +does not fit into the general mathematical theory by +which he explains spiral nebulæ, planetary nebulæ, binary +stars, and star clusters. It seems to demand a special +explanation, such as is furnished by tidal disruption due +to the passage of the sun close to another star.</p> + +<p>The part of Jeans' work which specially concerns us is +his study of the probability that some other star will +approach the sun closely enough to have an appreciable +gravitative or electrical effect, and thus cause disturbances +in the solar atmosphere. Of course both the star +and the sun are moving, but to avoid circumlocution we +shall speak of such mutual approaches simply as approaches +of the sun. For our present purpose the most +fundamental fact may be summed up in a quotation from +Jeans in which he says that most stars "show evidence +of having experienced considerable disturbance by other +systems; there is no reason why our solar system should +<!--File: cc277.png--> +<a class="pagenum break" id="Page_253" title="pg 253"></a> +be expected to have escaped the common fate." Jeans +gives a careful calculation from which it is possible to +derive some idea of the probability of any given degree of +approach of the sun and some other star. Of course all +such calculations must be based on certain assumptions. +The assumptions made by Jeans are such as to make the +probability of close approaches as great as possible. For +example, he allows only 560 million years for the entire +evolution of the sun, whereas some astronomers and +geologists would put the figure ten or more times as +high. Nevertheless, Jeans' assumptions at least show +the order of magnitude which we may expect on the basis +of reasonable astronomical conclusions.</p> + +<p>According to the planetary hypothesis of sunspots, the +difference in the effect of Jupiter when it is nearest and +farthest from the sun is the main factor in starting the +sunspot cycle and hence the corresponding terrestrial +cycle. The climatic difference between sunspot maxima +and minima, as measured by temperature, apparently +amounts to at least a twentieth and perhaps a tenth of +the difference between the climate of the last glacial +epoch and the present. We may suppose, then, that a body +which introduced a gravitative or electrical factor twenty +times as great as the difference in Jupiter's effect at its +maximum and minimum distances from the sun would +cause a glacial epoch if the effect lasted long enough. Of +course the other planets combine their effects with that +of Jupiter, but for the sake of simplicity we will leave +the others out of account. The difference between Jupiter's +maximum and minimum tidal effect on the sun +amounts to 29 per cent of the planet's average effect. +The corresponding difference, according to the electrical +hypothesis, is about 19 per cent, for electrostatic action +varies as the square of the distance instead of as the cube. +<!--File: cc278.png--> +<a class="pagenum break" id="Page_254" title="pg 254"></a> +Let us assume that a body exerting four times Jupiter's +present tidal effect and placed at the average distance of +Jupiter from the sun would disturb the sun's atmosphere +twenty times as much as the present difference between +sunspot maxima and minima, and thus, perhaps, cause a +glacial period on the earth.</p> + +<p>On the basis of this assumption our first problem is to +estimate the frequency with which a star, visible or +dark, is likely to approach near enough to the sun to +produce a <em>tidal</em> effect four times that of Jupiter. The +number of visible stars is known or at least well estimated. +As to dark stars, which have grown cool, Arrhenius +believed that they are a hundred times as numerous +as bright stars; few astronomers believe that there are +less than three or four times as many. Dr. Shapley of +the Harvard Observatory states that a new investigation +of the matter suggests that eight or ten is probably a +maximum figure. Let us assume that nine is correct. +The average visible star, so far as measured, has a mass +about twice that of the sun, or about 2100 times that of +Jupiter. The distances of the stars have been measured +in hundreds of cases and thus we can estimate how many +stars, both visible and invisible, are on an average contained +in a given volume of space. On this basis Jeans +estimates that there is only one chance in thirty billion +years that a visible star will approach within 2.8 times +the distance of Neptune from the sun, that is, within about +eight billion miles. If we include the invisible stars the +chances become one in three billion years. In order to +produce four times the tidal effect of Jupiter, however, +the average star would have to approach within about +four billion miles of the sun, and the chances of that +are only one in twelve billion years. The disturbing star +<!--File: cc279.png--> +<a class="pagenum break" id="Page_255" title="pg 255"></a> +would be only 40 per cent farther from the sun than +Neptune, and would almost pass within the solar system.</p> + +<p>Even though Jeans holds that the frequency of the +mutual approach of the sun and a star was probably +much greater in the distant past than at present, the +figures just given lend little support to the tidal hypothesis. +In fact, they apparently throw it out of court. It will +be remembered that Jeans has made assumptions which +give as high a frequency of stellar encounters as is consistent +with the astronomical facts. We have assumed +nine dark stars for every bright one, which may be a +liberal estimate. Also, although we have assumed that a +disturbance of the sun's atmosphere sufficient to cause +a glacial period would arise from a tidal effect only +twenty times as great as the difference in Jupiter's effect +when nearest the sun and farthest away, in our computations +this has actually been reduced to thirteen. With all +these favorable assumptions the chances of a stellar approach +of the sort here described are now only one in +twelve billion years. Yet within a hundred million years, +according to many estimates of geological time, and +almost certainly within a billion, there have been at least +half a dozen glaciations.</p> + +<p>Our use of Jeans' data interposes another and equally +insuperable difficulty to any tidal hypothesis. Four billion +miles is a very short distance in the eyes of an +astronomer. At that distance a star twice the size of the +sun would attract the outer planets more strongly than +the sun itself, and might capture them. If a star should +come within four billion miles of the sun, its effect in +distorting the orbits of all the planets would be great. +If this had happened often enough to cause all the glaciations +known to geologists, the planetary orbits would +be strongly elliptical instead of almost circular. The considerations +<!--File: cc280.png--> +<a class="pagenum break" id="Page_256" title="pg 256"></a> +here advanced militate so strongly against +the tidal hypothesis of solar disturbances that it seems +scarcely worth while to consider it further.</p> + +<p>Let us turn now to the electrical hypothesis. Here the +conditions are fundamentally different from those of the +tidal hypothesis. In the first place the electrostatic effect +of a body has nothing to do with its mass, but depends on +the area of its surface; that is, it varies as the square of +the radius. Second, the emission of electrons varies exponentially. +If hot glowing stars follow the same law as +black bodies at lower temperatures, the emission of +electrons, like the emission of other kinds of energy, +varies as the fourth power of the absolute temperature. +In other words, suppose there are two black bodies, otherwise +alike, but one with a temperature of 27° C. or 300° +on the absolute scale, and the other with 600° on the +absolute scale. The temperature of one is twice as high +as that of the other, but the electrostatic effect will be +sixteen times as great.<sup><a href="#fn118" id="r118">[118]</a></sup> Third, the number of electrons +<!--File: cc281.png--> +<a class="pagenum break" id="Page_257" title="pg 257"></a> +that reach a given body varies inversely as the square of +the distance, instead of as the cube which is the case +with tide-making forces.</p> + +<p>In order to use these three principles in calculating +the effect of the stars we must know the diameters, distances, +temperature, and number of the stars. The distances +and number may safely be taken as given by Jeans +in the calculations already cited. As to the diameters, the +measurements of the stars thus far made indicate that +the average mass is about twice that of the sun. The +average density, as deduced by Shapley<sup><a href="#fn119" id="r119">[119]</a></sup> from the movements +of double stars, is about one-eighth the solar +density. This would give an average diameter about two +and a half times that of the sun. For the dark stars, we +shall assume for convenience that they are ten times as +numerous as the bright ones. We shall also assume that +their diameter is half that of the sun, for being cool they +must be relatively dense, and that their temperature is +the same as that which we shall assume for Jupiter.</p> + +<p>As to Jupiter we shall continue our former assumption +that a body with four times the effectiveness of that +planet, which here means with twice as great a radius, +would disturb the sun enough to cause glaciation. It +would produce about twenty times the electrostatic effect +<!--File: cc282.png--> +<a class="pagenum break" id="Page_258" title="pg 258"></a> +which now appears to be associated with the difference in +Jupiter's effect at maximum and minimum. The temperature +of Jupiter must also be taken into account. The +planet is supposed to be hot because its density is low, +being only about 1.25 that of water. Nevertheless, it is +probably not luminous, for as Moulton<sup><a href="#fn120" id="r120">[120]</a></sup> puts it, shadows +upon it are black and its moons show no sign of illumination +except from the sun. Hence a temperature of about +600°C., or approximately 900° on the absolute scale, +seems to be the highest that can reasonably be assigned +to the cold outer layer whence electrons are emitted. As +to the temperature of the sun, we shall adopt the common +estimate of about 6300°C. on the absolute scale. The +other stars will be taken as averaging the same, although +of course they vary greatly.</p> + +<p>When Jeans' method of calculating the probability of +a mutual approach of the sun and a star is applied to the +assumptions given above, the results are as shown in +Table 5. On that basis the dark stars seem to be of +negligible importance so far as the electrical hypothesis +is concerned. Even though they may be ten times as +numerous as the bright ones there appears to be only +one chance in 130 billion years that one of them will approach +the sun closely enough to cause the assumed disturbance +of the solar atmosphere. On the other hand, if +all the visible stars were the size of the sun, and as hot +as that body, their electrical effect would be fourfold +that of our assumed dark star because of their size, and +2401 times as great because of their temperature, or approximately +10,000 times as great. Under such conditions +the theoretical chance of an approach that would cause +glaciation is one in 130 million years. If the average +visible star is somewhat cooler than the sun and has a +<!--File: cc283.png--> +<a class="pagenum break" id="Page_259" title="pg 259"></a> +radius about two and one-half times as great, as appears +to be the fact, the chances rise to one in thirty-eight million +years. A slight and wholly reasonable change in our +assumptions would reduce this last figure to only five or +ten million. For instance, the earth's mean temperature +during the glacial period has been assumed as 10°C. +lower than now, but the difference may have been only 6°. +Again, the temperature of the outer atmosphere of Jupiter +where the electrons are shot out may be only 500° or +700° absolute, instead of 900°. Or the diameter of the +average star may be five or ten times that of the sun, +instead of only two and one-half times as great. All this, +however, may for the present be disregarded. The essential +point is that even when the assumptions err on the +side of conservatism, the results are of an order of magnitude +which puts the electrical hypothesis within the +bounds of possibility, whereas similar assumptions put +the tidal hypothesis, with its single approach in twelve +billion years, far beyond those limits.</p> + +<p>The figures for Betelgeuse in Table 5 are interesting. +At a meeting of the American Association for the Advancement +of Science in December, 1920, Michelson +reported that by measurements of the interference of +light coming from the two sides of that bright star in +Orion, the observers at Mount Wilson had confirmed the +recent estimates of three other authorities that the star's +diameter is about 218 million miles, or 250 times that of +the sun. If other stars so much surpass the estimates of +only a decade or two ago, the average diameter of all the +visible stars must be many times that of the sun. The low +figure for Betelgeuse in section D of the table means that +if all the stars were as large as Betelgeuse, several might +often be near enough to cause profound disturbances of +the solar atmosphere. Nevertheless, because of the low +<!--File: cc284.png--> +<a class="pagenum break" id="Page_260" title="pg 260"></a> +temperature of the giant red stars of the Betelgeuse type, +the distance at which one of them would produce a given +electrical effect is only about five times the distance at +which our assumed average star would produce the same +effect. This, to be sure, is on the assumption that the +<!--File: cc285.png--> +<a class="pagenum break" id="Page_261" title="pg 261"></a> +radiation of energy from incandescent bodies varies +according to temperature in the same ratio as the radiation +from black bodies. Even if this assumption departs +somewhat from the truth, it still seems almost certain +that the lower temperature of the red compared with the +high temperature of the white stars must to a considerable +degree reduce the difference in electrical effect which +would otherwise arise from their size.</p> + +<table id="table_5" class="borders"> +<tr><th colspan="5"><a href="#tables">TABLE 5</a></th></tr> +<tr><th colspan="5">THEORETICAL PROBABILITY OF STELLAR APPROACHES</th> +</tr> +<tr> +<td></td> +<th><em>1<br />Dark Stars</em></th> +<th><em>2<br />Sun</em></th> +<th><em>3<br />Average Star</em></th> +<th><em>4<br />Betelgeuse</em></th> +</tr> +<tr> +<td>A. Approximate radius in miles.</td> +<td>430,000</td> +<td>860,000</td> +<td>2,150,000</td> +<td>218,000,000</td> +</tr> +<tr> +<td>B. Assumed temperature above absolute zero.</td> +<td>900° C.</td> +<td>6300° C.</td> +<td>5400° C.</td> +<td>3150° C.</td> +</tr> +<tr> +<td>C. Approximate theoretical distance +at which star would cause solar disturbance great enough +to cause glaciation (billions<sup><a href="#fn121" id="r121">[121]</a></sup> of miles).</td> +<td>1.2</td> +<td>120</td> +<td>220</td> +<td>3200 </td> +</tr> +<tr> +<td>D. Average interval between approaches close enough to cause glaciation if all stars were of given type. Years.</td> +<td>130,000,000,000<sup><a href="#fn122" id="r122">[122]</a></sup></td> +<td>130,000,000</td> +<td>38,000,000</td> +<td>700,000</td> +</tr> +</table> + + +<p>Thus far in our attempt to estimate the distance at +which a star might disturb the sun enough to cause glaciation +on the earth, we have considered only the star's +size and temperature. No account has been taken of the +degree to which its atmosphere is disturbed. Yet in the +case of the sun this seems to be one of the most important +factors. The magnetic field of sunspots is sometimes 50 +or 100 times as strong as that of the sun in general. The +strength of the magnetic field appears to depend on the +strength of the electrical currents in the solar atmosphere. +But the intensity of the sunspots and, by inference, +of the electrical currents, may depend on the electrical +action of Jupiter and the other planets. If we apply a +similar line of reasoning to the stars, we are at once led +to question whether the electrical activity of double stars +may not be enormously greater than that of isolated +stars like the sun.</p> + +<p>If this line of reasoning is correct, the atmosphere of +every double star must be in a state of commotion vastly +greater than that of the sun's atmosphere even when it +is most disturbed. For example, suppose the sun were +accompanied by a companion of equal size at a distance +of one million miles, which would make it much like many +known double stars. Suppose also that in accordance with +the general laws of physics the electrical effect of the +two suns upon one another is proportional to the fourth +<!--File: cc286.png--> +<a class="pagenum break" id="Page_262" title="pg 262"></a> +power of the temperature, the square of the radius, and +the inverse square of the distance. Then the effect of each +sun upon the other would be sixty billion (6 × 10<sup>10</sup>) times +as great as the present electrical effect of Jupiter upon +the sun. Just what this would mean as to the net effect +of a pair of such suns upon the electrical potential of +other bodies at a distance we can only conjecture. The +outstanding fact is that the electrical conditions of a +double star must be radically different and vastly more +intense than those of a single star like the sun.</p> + +<p>This conclusion carries weighty consequences. At present +twenty or more stars are known to be located within +about 100 trillion miles of the sun (five parsecs, as the +astronomers say), or 16.5 light years. According to the +assumptions employed in Table 5 an average single star +would influence the sun enough to cause glaciation if it +came within approximately 200 billion miles. If the star +were double, however, it might have an electrical capacity +enormously greater than that of the sun. Then it would +be able to cause glaciation at a correspondingly great +distance. Today Alpha Centauri, the nearest known star +about twenty-five trillion miles, or 4.3 light years from +the sun, and Sirius, the brightest star in the heavens, is +about fifty trillion miles away, or 8.5 light years. If these +stars were single and had a diameter three times that of +the sun, and if they were of the same temperature as has +been assumed for Betelgeuse, which is about fifty times as +far away as Alpha Centauri, the relative effects of the +three stars upon the sun would be, approximately, Betelgeuse +700, Alpha Centauri 250, Sirius 1. But Alpha Centauri +is triple and Sirius double, and both are much hotter +than Betelgeuse. Hence Alpha Centauri and even Sirius +may be far more effective than Betelgeuse.</p> + +<p>The two main components of Alpha Centauri are separated +<!--File: cc287.png--> +<a class="pagenum break" id="Page_263" title="pg 263"></a> +by an average distance of about 2,200,000,000 miles, +or somewhat less than that of Neptune from the sun. A +third and far fainter star, one of the faintest yet measured, +revolves around them at a great distance. In mass +and brightness the two main components are about like +the sun, and we will assume that the same is true of their +radius. Then, according to the assumptions made above, +their effect in disturbing one another electrically would +be about 10,000 times the total effect of Jupiter upon the +sun, or 2500 times the effect that we have assumed to be +necessary to produce a glacial period. We have already +seen in Table 5 that, according to our assumptions, a +single star like the sun would have to approach within +120 billion miles of the solar system, or within 2 per cent +of a light year, in order to cause glaciation. By a similar +process of reasoning it appears that if the mutual electrical +excitation of the two main parts of Alpha Centauri, +regardless of the third part, is proportional to the apparent +excitation of the sun by Jupiter, Alpha Centauri +would be 5000 times as effective as the sun. In other +words, if it came within 8,500,000,000,000 miles of the sun, +or 1.4 light years, it would so change the electrical conditions +as to produce a glacial epoch. In that case Alpha +Centauri is now so near that it introduces a disturbing +effect equal to about one-sixth of the effect needed to +cause glaciation on the earth. Sirius and perhaps others +of the nearer and brighter or larger stars may also create +appreciable disturbances in the electrical condition of the +sun's atmosphere, and may have done so to a much +greater degree in the past, or be destined to do so in the +future. Thus an electrical hypothesis of solar disturbances +seems to indicate that the position of the sun in +respect to other stars may be a factor of great importance +in determining the earth's climate.</p> +<!--File: cc288.png--> +<a class="pagenum break" id="Page_264" title="pg 264"></a> + + + + +<h2 id="Chapt_XV"><a href="#content">CHAPTER XV</a></h2> + +<h3>THE SUN'S JOURNEY THROUGH SPACE</h3> + + +<p>Having gained some idea of the nature of the +electrical hypothesis of solar disturbances and +of the possible effect of other bodies upon the +sun's atmosphere, let us now compare the astronomical +data with those of geology. Let us take up five chief +points for which the geologist demands an explanation, +and which any hypothesis must meet if it is to be permanently +accepted. These are (1) the irregular intervals +at which glacial periods occur; (2) the division of glacial +periods into epochs separated sometimes by hundreds +of thousands of years; (3) the length of glacial periods +and epochs; (4) the occurrence of glacial stages and historic +pulsations in the form of small climatic waves +superposed upon the larger waves of glacial epochs; (5) +the occurrence of climatic conditions much milder than +those of today, not only in the middle portion of the great +geological eras, but even in some of the recent inter-glacial +epochs.</p> + +<p>1. The irregular duration of the interval from one +glacial epoch to another corresponds with the irregular +distribution of the stars. If glaciation is indirectly due +to stellar influences, the epochs might fall close together, +or might be far apart. If the average interval were ten +million years, one interval might be thirty million or +more and the next only one or two hundred thousand. +<!--File: cc289.png--> +<a class="pagenum break" id="Page_265" title="pg 265"></a> +According to Schuchert, the known periods of glacial or +semi-glacial climate have been approximately as follows:</p> + +<h4>LIST OF GLACIAL PERIODS</h4> +<ol> +<li>Archeozoic.<br /> + (¼ of geological time or perhaps much more) + <ul class="none"> + <li>No known glacial periods.</li> + </ul> +</li> + +<li>Proterozoic.<br /> + (¼ of geological time) + <ul class="none"> + <li>a. Oldest known glacial period near base of Proterozoic in + Canada. Evidence widely distributed.</li> + <li>b. Indian glacial period; time unknown.</li> + <li>c. African glacial period; time unknown.</li> + <li>d. Glaciation near end of Proterozoic in Australia, Norway, + and China.</li> + </ul> +</li> + +<li>Paleozoic.<br /> + (¼ of geological time) + <ul class="none"> + <li>a. Late Ordovician(?). Local in Arctic Norway.</li> + <li>b. Silurian. Local in Alaska.</li> + <li>c. Early Devonian. Local in South Africa.</li> + <li>d. Early Permian. World-wide and very severe.</li> + </ul> +</li> + +<li>Mesozoic and Cenozoic.<br /> + (¼ of geological time)<br /> + <ul class="none"> + <li>a-b. None definitely determined during Mesozoic, although there appears to have been periods of cooling + (a) in the late Triassic, and (b) in the late Cretacic, with at least local glaciation in early Eocene.</li> + <li>c. Severe glacial period during Pleistocene.</li> + </ul> +</li> +</ol> + +<p>This table suggests an interesting inquiry. During the +last few decades there has been great interest in ancient +glaciation and geologists have carefully examined rocks +of all ages for signs of glacial deposits. In spite of the +large parts of the earth which are covered with deposits +belonging to the Mesozoic and Cenozoic, which form the +<!--File: cc290.png--> +<a class="pagenum break" id="Page_266" title="pg 266"></a> +last quarter of geological time, the only signs of actual +glaciation are those of the great Pleistocene period and a +few local occurrences at the end of the Mesozoic or beginning +of the Cenozoic. Late in the Triassic and early +in the Jurassic, the climate appears to have been rigorous, +although no tillites have been found to demonstrate +glaciation. In the preceding quarter, that is, the Paleozoic, +the Permian glaciation was more severe than that +of the Pleistocene, and the Devonian than that of the +Eocene, while the Ordovician evidences of low temperature +are stronger than those at the end of the Triassic. +In view of the fact that rocks of Paleozoic age cover +much smaller areas than do those of later age, the three +Paleozoic glaciations seem to indicate a relative frequency +of glaciation. Going back to the Proterozoic, it +is astonishing to find that evidence of two highly developed +glacial periods, and possibly four, has been discovered. +Since the Indian and the African glaciations of +Proterozoic times are as yet undated, we cannot be sure +that they are not of the same date as the others. Nevertheless, +even two is a surprising number, for not only +are most Proterozoic rocks so metamorphosed that possible +evidences of glacial origin are destroyed, but rocks +of that age occupy far smaller areas than either those +of Paleozoic or, still more, Mesozoic and Cenozoic age. +Thus the record of the last three-quarters of geological +time suggests that if rocks of all ages were as abundant +and as easily studied as those of the later periods, the +frequency of glacial periods would be found to increase +as one goes backward toward the beginnings of the +earth's history. This is interesting, for Jeans holds that +the chances that the stars would approach one another +were probably greater in the past than at present. This +conclusion is based on the assumption that our universe +<!--File: cc291.png--> +<a class="pagenum break" id="Page_267" title="pg 267"></a> +is like the spiral nebulæ in which the orbits of the various +members are nearly circular during the younger stages. +Jeans considers it certain that in such cases the orbits +will gradually become larger and more elliptical because +of the attraction of one body for another. Thus as time +goes on the stars will be more widely distributed and +the chances of approach will diminish. If this is correct, +the agreement between astronomical theory and geological +conclusions suggests that the two are at least not in +opposition.</p> + +<p>The first quarter of geological time as well as the last +three must be considered in this connection. During the +Archeozoic, no evidence of glaciation has yet been discovered. +This suggests that the geological facts disprove +the astronomical theory. But our knowledge of early +geological times is extremely limited, so limited that +lack of evidence of glaciation in the Archeozoic may have +no significance. Archeozoic rocks have been studied +minutely over a very small percentage of the earth's land +surface. Moreover, they are highly metamorphosed so +that, even if glacial tills existed, it would be hard to +recognize them. Third, according to both the nebular and +the planetesimal hypotheses, it seems possible that +during the earliest stages of geological history the +earth's interior was somewhat warmer than now, and the +surface may have been warmed more than at present by +conduction, by lava flows, and by the fall of meteorites. +If the earth during the Archeozoic period emitted enough +heat to raise its surface temperature a few degrees, the +heat would not prevent the development of low forms of +life but might effectively prevent all glaciation. This +does not mean that it would prevent changes of climate, +but merely changes so extreme that their record would +be preserved by means of ice. It will be most interesting +<!--File: cc292.png--> +<a class="pagenum break" id="Page_268" title="pg 268"></a> +to see whether future investigations in geology and +astronomy indicate either a semi-uniform distribution of +glacial periods throughout the past, or a more or less +regular decrease in frequency from early times down to +the present.</p> + +<p>2. The Pleistocene glacial period was divided into at +least four epochs, while in the Permian at least one +inter-glacial epoch seems certain, and in some places the +alternation between glacial and non-glacial beds suggests +no less than nine. In the other glaciations the evidence is +not yet clear. The question of periodicity is so important +that it overthrows most glacial hypotheses. Indeed, had +their authors known the facts as established in recent +years, most of the hypotheses would never have been +advanced. The carbon dioxide hypothesis is the only one +which was framed with geologically rapid climatic alternations +in mind. It certainly explains the facts of periodicity +better than does any of its predecessors, but even +so it does not account for the intimate way in which +variations of all degrees from those of the weather up to +glacial epochs seem to grade into one another.</p> + +<p>According to our stellar hypothesis, occasional groups +of glacial epochs would be expected to occur close together +and to form long glacial periods. This is because +many of the stars belong to groups or clusters in which +the stars move in parallel paths. A good example is the +cluster in the Hyades, where Boss has studied thirty-nine +stars with special care.<sup><a href="#fn123" id="r123">[123]</a></sup> The stars are grouped about a +center about 130 light years from the sun. The stars +themselves are scattered over an area about thirty +light years in diameter. They average about the same +distance apart as do those near the sun, but toward the +<!--File: cc293.png--> +<a class="pagenum break" id="Page_269" title="pg 269"></a> +center of the group they are somewhat closer together. +The whole thirty-nine sweep forward in essentially +parallel paths. Boss estimates that 800,000 years ago +the cluster was only half as far from the sun as at present, +but probably that was as near as it has been during +recent geological times. All of the thirty-nine stars of this +cluster, as Moulton<sup><a href="#fn124" id="r124">[124]</a></sup> puts it, "are much greater in light-giving +power than the sun. The luminosities of even the +five smallest are from five to ten times that of the sun, +while the largest are one hundred times greater in light-giving +power than our own luminary. Their masses are +probably much greater than that of the sun." If the sun +were to pass through such a cluster, first one star and +then another might come so near as to cause a profound +disturbance in the sun's atmosphere.</p> + +<p>3. Another important point upon which a glacial hypothesis +may come to grief is the length of the periods +or rather of the epochs which compose the periods. +During the last or Pleistocene glacial period the evidence +in America and Europe indicates that the inter-glacial +epochs varied in length and that the later ones were +shorter than the earlier. Chamberlin and Salisbury, from +a comparison of various authorities, estimate that the +intervals from one glacial epoch to another form a declining +series, which may be roughly expressed as follows: +16-8-4-2-1, where unity is the interval from the +climax of the late Wisconsin, or last glacial epoch, to the +present. Most authorities estimate the culmination of the +late Wisconsin glaciation as twenty or thirty thousand +years ago. Penck estimates the length of the last inter-glacial +period as 60,000 years and the preceding one as +240,000.<sup><a href="#fn125" id="r125">[125]</a></sup> R. T. Chamberlin, as already stated, finds that +<!--File: cc294.png--> +<a class="pagenum break" id="Page_270" title="pg 270"></a> +the consensus of opinion is that inter-glacial epochs have +averaged five times as long as glacial epochs. The actual +duration of the various glaciations probably did not vary +in so great a ratio as did the intervals from one glaciation +to another. The main point, however, is the irregularity +of the various periods.</p> + +<p>The relation of the stellar electrical hypothesis to the +length of glacial epochs may be estimated from column +C, in Table 5. There we see that the distances at which +a star might possibly disturb the sun enough to cause +glaciation range all the way from 120 billion miles in +the case of a small star like the sun, to 3200 billion in +the case of Betelgeuse, while for double stars the figure +may rise a hundred times higher. From this we can calculate +how long it would take a star to pass from a point +where its influence would first amount to a quarter of the +assumed maximum to a similar point on the other side of +the sun. In making these calculations we will assume that +the relative rate at which the star and the sun approach +each other is about twenty-two miles per second, or 700 +million miles per year, which is the average rate of +motion of all the known stars. According to the distances +in Table 5 this gives a range from about 500 years up to +about 10,000, which might rise to a million in the case of +double stars. Of course the time might be relatively short +if the sun and a rapidly moving star were approaching +one another almost directly, or extremely long if the sun +and the star were moving in almost the same direction +and at somewhat similar rates,—a condition more +common than the other. Here, as in so many other cases, +the essential point is that the figures which we thus obtain +seem to be of the right order of magnitude.</p> + +<p>4. Post-glacial climatic stages are so well known that +in Europe they have definite names. Their sequence has +<!--File: cc295.png--> +<a class="pagenum break" id="Page_271" title="pg 271"></a> +already been discussed in Chapter XII. Fossils found in +the peat bogs of Denmark and Scandinavia, for example, +prove that since the final disappearance of the continental +ice cap at the close of the Wisconsin there has +been at least one period when the climate of Europe was +distinctly milder than now. Directly overlying the sheets +of glacial drift laid down by the ice there is a flora corresponding +to that of the present tundras. Next come remains +of a forest vegetation dominated by birches and +poplars, showing that the climate was growing a little +warmer. Third, there follow evidences of a still more +favorable climate in the form of a forest dominated by +pines; fourth, one where oak predominates; and fifth, a +flora similar to that of the Black Forest of Germany, +indicating that in Scandinavia the temperature was then +decidedly higher than today. This fifth flora has retreated +southward once more, having been driven back to its +present latitude by a slight recurrence of a cool stormy +climate.<sup><a href="#fn126" id="r126">[126]</a></sup> In central Asia evidence of post-glacial stages +is found not only in five distinct moraines but in a corresponding +series of elevated strands surrounding salt +lakes and of river terraces in non-glaciated arid regions.<sup><a href="#fn127" id="r127">[127]</a></sup></p> + +<p>In historic as well as prehistoric times, as we have +already seen, there have been climatic fluctuations. For +instance, the twelfth or thirteenth century B. C. appears +to have been almost as mild as now, as does the seventh +century B. C. On the other hand about 1000 B. C., at the +time of Christ, and in the fourteenth century there were +times of relative severity. Thus it appears that both on +<!--File: cc296.png--> +<a class="pagenum break" id="Page_272" title="pg 272"></a> +a large and on a small scale pulsations of climate are the +rule. Any hypothesis of climatic changes must satisfy +the periods of these pulsations. These conditions furnish +a problem which makes difficulty for almost all hypotheses +of climatic change. According to the present hypothesis, +earth movements such as are discussed in Chapter +XII may coöperate with two astronomical factors. One is +the constant change in the positions of the stars, a change +which we have already called kaleidoscopic, and the other +is the fact that a large proportion of the stars are double +or multiple. When one star in a group approaches the +sun closely enough to cause a great solar disturbance, +numerous others may approach or recede and have a +minor effect. Thus, whenever the sun is near groups of +stars we should expect that the earth would show many +minor climatic pulsations and stages which might or +might not be connected with glaciation. The historic +pulsations shown in the curve of tree growth in California, +Fig. 4, are the sort of changes that would be +expected if movements of the stars have an effect on the +solar atmosphere.</p> + +<p>Not only are fully a third of all the visible stars double, +as we have already seen, but at least a tenth of these are +known to be triple or multiple. In many of the double +stars the two bodies are close together and revolve so +rapidly that whatever periodicity they might create in +the sun's atmosphere would be very short. In the triplets, +however, the third star is ordinarily at least ten times +as far from the other two as they are from each other, +and its period of rotation sometimes runs into hundreds +or thousands of years. An actual multiple star in the +constellation Polaris will serve as an example. The main +star is believed by Jeans to consist of two parts which +are almost in contact and whirl around each other with +<!--File: cc297.png--> +<a class="pagenum break" id="Page_273" title="pg 273"></a> +extraordinary speed in four days. If this is true they +must keep each other's atmospheres in a state of intense +commotion. Much farther away a third star revolves +around this pair in twelve years. At a much greater distance +a fourth star revolves around the common center +of gravity of itself and the other three in a period which +may be 20,000 years. Still more complicated cases probably +exist. Suppose such a system were to traverse a +path where it would exert a perceptible influence on the +sun for thirty or forty thousand years. The varying +movements of its members would produce an intricate +series of cycles which might show all sorts of major and +minor variations in length and intensity. Thus the varied +and irregular stages of glaciation and the pulsations of +historic times might be accounted for on the hypothesis +of the proximity of the sun to a multiple star, as well as +on that of the less pronounced approach and recession +of a number of stars. In addition to all this, an almost +infinitely complex series of climatic changes of long and +short duration might arise if the sun passed through a +nebula.</p> + +<p>5. We have seen in Chapter VIII that the contrast +between the somewhat severe climate of the present and +the generally mild climate of the past is one of the great +geological problems. The glacial period is not a thing +of the distant past. Geologists generally recognize that +it is still with us. Greenland and Antarctica are both +shrouded in ice sheets in latitudes where fossil floras +prove that at other periods the climate was as mild as +in England or even New Zealand. The present glaciated +regions, be it noted, are on the polar borders of the +world's two most stormy oceanic areas, just where ice +would be expected to last longest according to the solar +cyclonic hypothesis. In contrast with the semi-glacial +<!--File: cc298.png--> +<a class="pagenum break" id="Page_274" title="pg 274"></a> +conditions of the present, the last inter-glacial epoch was +so mild that not only men but elephants and hippopotamuses +flourished in central Europe, while at earlier times +in the middle of long eras, such as the Paleozoic and +Mesozoic, corals, cycads, and tree ferns flourished within +the Arctic circle.</p> + +<p>If the electro-stellar hypothesis of solar disturbances +proves well founded, it may explain these peculiarities. +Periods of mild climate would represent a return of the +sun and the earth to their normal conditions of quiet. At +such times the atmosphere of the sun is assumed to be +little disturbed by sunspots, faculæ, prominences, and +other allied evidences of movements; and the rice-grain +structure is perhaps the most prominent of the solar +markings. The earth at such times is supposed to be +correspondingly free from cyclonic storms. Its winds are +then largely of the purely planetary type, such as trade +winds and westerlies. Its rainfall also is largely planetary +rather than cyclonic. It falls in places such as the +heat equator where the air rises under the influence of +heat, or on the windward slopes of mountains, or in regions +where warm winds blow from the ocean over cold +lands.</p> + +<p>According to the electro-stellar hypothesis, the conditions +which prevailed during hundreds of millions of +years of mild climate mean merely that the solar system +was then in parts of the heavens where stars—especially +double stars—were rare or small, and electrical disturbances +correspondingly weak. Today, on the other hand, +the sun is fairly near a number of stars, many of which +are large doubles. Hence it is supposed to be disturbed, +although not so much as at the height of the last glacial +epoch.</p> + +<p>After the preceding parts of this book had been +<!--File: cc299.png--> +<a class="pagenum break" id="Page_275" title="pg 275"></a> +written, the assistance of Dr. Schlesinger made it possible +to test the electro-stellar hypothesis by comparing +actual astronomical dates with the dates of climatic or +solar phenomena. In order to make this possible, Dr. +Schlesinger and his assistants have prepared Table 6, +giving the position, magnitude, and motions of the thirty-eight +nearest stars, and especially the date at which each +was nearest the sun. In column 10 where the dates are +given, a minus sign indicates the past and a plus sign the +future. Dr. Shapley has kindly added column 12, giving +the absolute magnitudes of the stars, that of the sun +being 4.8, and column 13, showing their luminosity or +absolute radiation, that of the sun being unity. Finally, +column 14 shows the effective radiation received by the +sun from each star when the star is at a minimum distance. +Unity in this case is the effect of a star like the +sun at a distance of one light year.</p> + +<p>It is well known that radiation of all kinds, including +light, heat, and electrical emissions, varies in direct proportion +to the exposed surface, that is, as the square +of the radius of a sphere, and inversely as the square of +the distance. From black bodies, as we have seen, the +total radiation varies as the fourth power of the absolute +temperature. It is not certain that either light or +electrical emissions from incandescent bodies vary in +quite this same proportion, nor is it yet certain whether +luminous and electrical emissions vary exactly together. +Nevertheless they are closely related. Since the light +coming from each star is accurately measured, while no +information is available as to electrical emissions, we +have followed Dr. Shapley's suggestion and used the +luminosity of the stars as the best available measure of +total radiation. This is presumably an approximate +measure of electrical activity, provided some allowance +be made for disturbances by outside bodies such as companion +stars. Hence the inclusion of column 14.</p> +<!--File: cc300.png--> +<a class="pagenum break" id="Page_276" title="pg 276"></a> + + +<table id="table_6" class="borders1"> +<tr><th colspan="15"><a href="#tables">TABLE 6</a></th></tr> + +<tr><th colspan="15">THIRTY-EIGHT STARS HAVING LARGEST KNOWN PARALLAXES</th></tr> +<tr> +<th></th> +<th>(<em>1</em>) <br /><em>Right Ascension α 1900</em></th> +<th>(<em>2</em>) <br /><em>Declination δ 1900</em></th> +<th>(<em>3</em>) <br /><em>Visual Mag. m</em></th> +<th>(<em>4</em>) <br /><em>Spectrum</em></th> +<th>(<em>5</em>) <br /><em>Proper Motion</em></th> +<th>(<em>6</em>) <br /><em>Radial Velocity<br />km. per sec.</em></th> +<th>(<em>7</em>) <br /><em>Present Parallax π</em></th> +<th>(<em>8</em>) <br /><em>Maximum Parallax</em></th> +<th>(<em>9</em>) <br /><em>Minimum Distance Light Yrs.</em></th> +<th>(<em>10</em>) <br /><em>Time of<br />Minimum<br />Distance</em></th> +<th>(<em>11</em>) <br /><em>Magnitude<br />at Min. Dist.</em></th> +<th>(<em>12</em>) <br /><em>Absolute<br /> Magnitude</em></th> +<th>(<em>13</em>) <br /><em>Luminosity</em></th> +<th>(<em>14</em>) <br /><em>Effective<br /> radiation <br />at minimum<br />distance<br />from sun</em></th> +</tr> +<tr> +<td>Groombr. 34</td> +<td>0<sup>h</sup>12<sup>m</sup>.7</td> +<td>+43°27'</td> +<td>8.1</td> +<td>Ma</td> +<td>2".89</td> +<td>+ 3</td> +<td>".28</td> +<td>".28</td> +<td>11.6</td> +<td>- 4000</td> +<td>8.1</td> +<td>10.3</td> +<td>0.0063</td> +<td>0.000051</td> +</tr> +<tr> +<td><sup><a href="#fn128" id="r128">[128]</a></sup>η Cassiop.</td> +<td>43 .0</td> +<td>+57 17</td> +<td>3.6</td> +<td>F8</td> +<td>1 .24</td> +<td>+ 10</td> +<td>.18</td> +<td>.19</td> +<td>17.1</td> +<td>- 47000</td> +<td>3.5 </td> +<td>4.9</td> +<td>0.91</td> +<td>0.003110</td> +</tr> +<tr> +<td></td> +<td>43 .9</td> +<td>+4 55</td> +<td>12.3</td> +<td>F0</td> +<td>3 .01</td> +<td>.....</td> +<td>.24</td> +<td>....</td> +<td>....</td> +<td>.......</td> +<td>....</td> +<td>14.2</td> +<td>0.00017</td> +<td>........</td> +</tr> +<tr> +<td><sup><a href="#fn128">[128]</a></sup>κ Tucanæ</td> +<td>1 12 .4</td> +<td>-69 24</td> +<td>5.0</td> +<td>F8</td> +<td>.39</td> +<td>+ 12</td> +<td>.16</td> +<td>.23</td> +<td>14.2</td> +<td>-264000</td> +<td>4.2</td> +<td>6.0</td> +<td>0.33</td> +<td>0.001610</td> +</tr> +<tr> +<td>τ Ceti</td> +<td>39 .4</td> +<td>-16 28</td> +<td>3.6</td> +<td>K0</td> +<td>1 .92</td> +<td>- 16</td> +<td>.32</td> +<td>.37</td> +<td>8.8</td> +<td>+ 46000</td> +<td>3.3</td> +<td>6.1</td> +<td>0.30</td> +<td>0.003840</td> +</tr> +<tr> +<td>δ<sub>2</sub> Eridani</td> +<td>3 15 .9</td> +<td>-43 27</td> +<td>4.3</td> +<td>G5</td> +<td>3 .16</td> +<td>+ 87</td> +<td>.16</td> +<td>.22</td> +<td>14.8</td> +<td>- 33000</td> +<td>3.6</td> +<td>5.3</td> +<td>0.63</td> +<td>0.002960</td> +</tr> +<tr> +<td><sup><a href="#fn128">[128]</a></sup>ε Eridani</td> +<td>28 .2</td> +<td>- 9 48</td> +<td>3.8</td> +<td>K0</td> +<td>.97</td> +<td>+ 16</td> +<td>.31</td> +<td>.46</td> +<td>7.1</td> +<td>-106000</td> +<td>3.0</td> +<td>6.3</td> +<td>0.25</td> +<td>0.004970</td> +</tr> +<tr> +<td><sup><a href="#fn128">[128]</a></sup>40(0)<sup>2</sup> Eridani</td> +<td>4 10 .7</td> +<td>- 7 49</td> +<td>4.5</td> +<td>G5</td> +<td>4 .08</td> +<td>- 42</td> +<td>.21</td> +<td>.23</td> +<td>14.2</td> +<td>+ 19000</td> +<td>4.3</td> +<td>6.1</td> +<td>0.30</td> +<td>0.001470</td> +</tr> +<tr> +<td>Cordoba Z. 243</td> +<td>5 7 .7</td> +<td>-44 59</td> +<td>9.2</td> +<td>K2</td> +<td>8 .75</td> +<td>+242</td> +<td>.32</td> +<td>.68</td> +<td>4.8</td> +<td>- 10000</td> +<td>7.6</td> +<td>11.7</td> +<td>0.0017</td> +<td>0.000074</td> +</tr> +<tr> +<td>Weisse 592</td> +<td>26 .4</td> +<td>- 3 42</td> +<td>8.8</td> +<td>K2</td> +<td>2 .22</td> +<td>.....</td> +<td>.17</td> +<td>....</td> +<td>....</td> +<td>.......</td> +<td>....</td> +<td>9.9</td> +<td>0.009</td> +<td>........</td> +</tr> +<tr> +<td><sup><a href="#fn128">[128]</a></sup>α Can. Maj. (Sirius)</td> +<td>6 40 .7</td> +<td>-16 35</td> +<td>-1.6</td> +<td>A0</td> +<td>1 .32</td> +<td>- 8</td> +<td>.37</td> +<td>.41</td> +<td>8.0</td> +<td>+ 65000</td> +<td>-1.8</td> +<td>1.2</td> +<td>27.50</td> +<td>0.429000</td> +</tr> +<tr> +<td><sup><a href="#fn128">[128]</a></sup>α Can. Min. (Procyon)</td> +<td>7 34 .1</td> +<td>+ 5 29</td> +<td>0.5</td> +<td>F5</td> +<td>1 .24</td> +<td>- 4</td> +<td>.31</td> +<td>.32</td> +<td>10.2</td> +<td>+ 34000</td> +<td>0.5</td> +<td>3.0</td> +<td>5.25</td> +<td>0.051300</td> +</tr> +<tr> +<td><sup><a href="#fn128">[128]</a></sup>Fedorenko 1457-8</td> +<td>9 7 .6</td> +<td>+53 7</td> +<td>7.9</td> +<td>Ma</td> +<td>1 .68</td> +<td>+ 10</td> +<td>.16</td> +<td>.16</td> +<td>20.4</td> +<td>- 24000</td> +<td>7.9</td> +<td>8.9</td> +<td>0.023</td> +<td>0.000055</td> +</tr> +<tr> +<td>Groombr. 1618</td> +<td>10 5 .3 </td> +<td>+49 58</td> +<td>6.8</td> +<td>K5p</td> +<td>1 .45</td> +<td>- 30</td> +<td>.18</td> +<td>.23</td> +<td>14.2</td> +<td>+ 69000</td> +<td>6.3</td> +<td>8.1</td> +<td>0.048</td> +<td>0.000238</td> +</tr> +<tr> +<td>Weisse 234</td> +<td>14 .2</td> +<td>+20 22</td> +<td>9.0</td> +<td>...</td> +<td>.49</td> +<td>.....</td> +<td>.19</td> +<td>....</td> +<td>....</td> +<td>.......</td> +<td>....</td> +<td>10.4</td> +<td>0.0057</td> +<td>........</td> +</tr> +<tr> +<td>Lalande 21185</td> +<td>57 .9</td> +<td>+36 38</td> +<td>7.6</td> +<td>Mb</td> +<td>4 .78</td> +<td>- 87</td> +<td>.41</td> +<td>.76</td> +<td>4.3</td> +<td>+ 20000</td> +<td>6.2</td> +<td>10.7</td> +<td>0.0044</td> +<td>0.000238</td> +</tr> +<tr> +<td>Lalande 21258</td> +<td>11 0 .5</td> +<td>+44 2</td> +<td>8.5</td> +<td>K5</td> +<td>4 .52</td> +<td>+ 65</td> +<td>.19</td> +<td>.22</td> +<td>14.8</td> +<td>- 20000</td> +<td>8.2</td> +<td>9.9</td> +<td>0.009</td> +<td>0.000041</td> +</tr> +<tr> +<td></td> +<td>12 .0</td> +<td>-57 2</td> +<td>12.0</td> +<td>...</td> +<td>2 .69</td> +<td>.....</td> +<td>.34</td> +<td>....</td> +<td>....</td> +<td>........</td> +<td>...</td> +<td>14.7</td> +<td>0.00011</td> +<td>........</td> +</tr> +<tr> +<td>Lalande 25372</td> +<td>13 40 .7</td> +<td>+15 26</td> +<td>8.5</td> +<td>K5</td> +<td>2 .30</td> +<td>.....</td> +<td>.19</td> +<td>....</td> +<td>....</td> +<td>.......</td> +<td>....</td> +<td>9.9</td> +<td>0.009</td> +<td>........</td> +</tr> +<tr> +<td><sup><a href="#fn128">[128]</a></sup>α Centauri</td> +<td>14 32 .8</td> +<td>-60 25</td> +<td>0.2</td> +<td>G</td> +<td>3 .68</td> +<td>+ 22</td> +<td>.76</td> +<td>1.03</td> +<td>3.2</td> +<td>- 28000</td> +<td>-0.5</td> +<td>4.6</td> +<td>1.20</td> +<td>0.117500</td> +</tr> +<tr> +<td><sup><a href="#fn128">[128]</a></sup>ξ Bootes</td> +<td>14 46 .8</td> +<td>+19 31</td> +<td>4.6</td> +<td>K5p</td> +<td>.17</td> +<td>+ 4</td> +<td>.17 </td> +<td>.22</td> +<td>14.8</td> +<td>-598000</td> +<td>4.0</td> +<td>5.8</td> +<td>0.40</td> +<td>0.001815</td> +</tr> +<tr> +<td><sup><a href="#fn128">[128]</a></sup>Lalande 27173</td> +<td>51 .6</td> +<td>-20 58</td> +<td>5.8</td> +<td>Kp</td> +<td>1 .96</td> +<td>+ 20</td> +<td>.18</td> +<td>.19</td> +<td>17.1</td> +<td>- 36000</td> +<td>5.6</td> +<td>7.1</td> +<td>0.12</td> +<td>0.000412</td> +</tr> +<tr> +<td>Weisse 1259</td> +<td>16 41 .4</td> +<td>+33 41</td> +<td>8.4</td> +<td>...</td> +<td>.37</td> +<td>.....</td> +<td>.18</td> +<td>....</td> +<td>....</td> +<td>.......</td> +<td>....</td> +<td>9.7</td> +<td>0.011</td> +<td>........</td> +</tr> +<tr> +<td>Lacaille 7194</td> +<td>17 11 .5</td> +<td>-46 32</td> +<td>5.7</td> +<td>K</td> +<td>.97</td> +<td>.....</td> +<td>.19</td> +<td>....</td> +<td>....</td> +<td>.......</td> +<td>....</td> +<td>7.1</td> +<td>0.12</td> +<td>........</td> +</tr> +<tr> +<td><sup><a href="#fn128">[128]</a></sup>β 416</td> +<td>12 .1</td> +<td>-34 53</td> +<td>5.9</td> +<td>K5</td> +<td>1 .19</td> +<td>- 4</td> +<td>.17</td> +<td>.17</td> +<td>19.2</td> +<td>+ 21000</td> +<td>5.7</td> +<td>7.1</td> +<td>0.12</td> +<td>0.000329</td> +</tr> +<tr> +<td>Argel <br />-0.17415-6</td> +<td>37 .0</td> +<td>+68 26</td> +<td>9.1</td> +<td> K </td> +<td>1 .33</td> +<td>.....</td> +<td>.22</td> +<td>....</td> +<td>....</td> +<td>.......</td> +<td>....</td> +<td>10.8</td> +<td>0.004</td> +<td>........</td> +</tr> +<tr> +<td>Barnard's star</td> +<td>52 .9</td> +<td>+ 4 25</td> +<td>9.7</td> +<td>Mb</td> +<td>10 .30</td> +<td>- 80</td> +<td>.53</td> +<td>.70</td> +<td>4.7</td> +<td>+ 10000</td> +<td>9.1</td> +<td>13.3</td> +<td>0.0025</td> +<td>0.000114</td> +</tr> +<tr> +<td><sup><a href="#fn128">[128]</a></sup>70p Ophiuchi</td> +<td>18 0 .4</td> +<td>+ 2 31</td> +<td>4.3</td> +<td>K</td> +<td>1 .13</td> +<td>.....</td> +<td>.19</td> +<td>....</td> +<td>....</td> +<td>.......</td> +<td>....</td> +<td>5.7</td> +<td>0.44</td> +<td>........</td> +</tr> +<tr> +<td><sup><a href="#fn128">[128]</a></sup>Σ 2398</td> +<td>41 .7</td> +<td>+59 29</td> +<td>8.8</td> +<td>K</td> +<td>2 .31</td> +<td>.....</td> +<td>.29</td> +<td>....</td> +<td>....</td> +<td>.......</td> +<td>....</td> +<td>11.1</td> +<td>0.0030</td> +<td>........</td> +</tr> +<tr> +<td>σ Draconis</td> +<td>19 32 .5</td> +<td>+69 29</td> +<td>4.8</td> +<td>G5</td> +<td>1 .84</td> +<td>+ 26</td> +<td>.20</td> +<td>.23</td> +<td>14.2</td> +<td>- 49000</td> +<td>4.5</td> +<td>6.3</td> +<td>0.25</td> +<td>0.001238</td> +</tr> +<tr> +<td><sup><a href="#fn128">[128]</a></sup>α Aquilæ (Altair)</td> +<td>45 .9</td> +<td>+ 8 36</td> +<td>1.2</td> +<td>A5</td> +<td>.66</td> +<td>- 33</td> +<td>.21</td> +<td>.51</td> +<td>6.4</td> +<td>+117000</td> +<td>-0.7</td> +<td>2.8</td> +<td>6.30</td> +<td>0.153600</td> +</tr> +<tr> +<td><sup><a href="#fn128">[128]</a></sup>61 Cygni</td> +<td>21 2 .4</td> +<td>+38 15</td> +<td>5.6</td> +<td>K5</td> +<td>5 .20</td> +<td>- 64</td> +<td>.30</td> +<td>.38</td> +<td>8.6</td> +<td>+ 19000</td> +<td>5.1</td> +<td>8.0</td> +<td>0.053</td> +<td>0.000715</td> +</tr> +<tr> +<td>Lacaille 8760</td> +<td>11 .4</td> +<td>-39 15</td> +<td>6.6</td> +<td>G</td> +<td>3 .53</td> +<td>+ 13</td> +<td>.25</td> +<td>.26</td> +<td>12.6</td> +<td>- 11000</td> +<td>6.6</td> +<td>8.6</td> +<td>0.030</td> +<td>0.000189</td> +</tr> +<tr> +<td>ε Indi</td> +<td>55 .7</td> +<td>-57 12</td> +<td>4.8</td> +<td>K5</td> +<td>4 .70</td> +<td>- 39</td> +<td>.28</td> +<td>.31</td> +<td>10.5</td> +<td>+ 17000</td> +<td>4.6</td> +<td>7.0</td> +<td>0.13</td> +<td>0.001230</td> +</tr> +<tr> +<td><sup><a href="#fn128">[128]</a></sup>Krüger 60</td> +<td>22 24 .4</td> +<td>+57 12</td> +<td>9.2</td> +<td>...</td> +<td>.87</td> +<td>.....</td> +<td>.26</td> +<td>....</td> +<td>....</td> +<td>.......</td> +<td>....</td> +<td>11.3</td> +<td>0.0025</td> +<td>.......</td> +</tr> +<tr> +<td>Lacaille 9352</td> +<td>59 .4</td> +<td>-36 26</td> +<td>7.1</td> +<td>K</td> +<td>6 .90</td> +<td>+ 12</td> +<td>.29</td> +<td>.29</td> +<td>11.2</td> +<td>- 3000</td> +<td>7.1</td> +<td>9.4</td> +<td>0.014</td> +<td>0.000111</td> +</tr> +<tr> +<td>Lalande 46650</td> +<td>23 44 .0</td> +<td>+ 1 52</td> +<td>8.7</td> +<td>Ma</td> +<td>1 .39</td> +<td>.....</td> +<td>.17</td> +<td>....</td> +<td>....</td> +<td>.......</td> +<td>....</td> +<td>9.9</td> +<td>0.009</td> +<td>.......</td> +</tr> +<tr> +<td>C. G. A. 32416</td> +<td>59 .5</td> +<td>-37 51</td> +<td>8.2</td> +<td>G</td> +<td>6 .05</td> +<td>+ 26</td> +<td>.22</td> +<td>.22</td> +<td>14.8</td> +<td>- 7000</td> +<td>8.2</td> +<td>9.9</td> +<td>0.009</td> +<td>0.000041</td> +</tr> +</table> + +<!--File: cc301.png--> +<a class="pagenum break" id="Page_277" title="pg 277"></a> + +<!--File: cc302.png--> +<a class="pagenum break" id="Page_278" title="pg 278"></a> + + +<p>On the basis of column 14 and of the movements and +distances of the stars as given in the other columns Fig. +10 has been prepared. This gives an estimate of the +approximate electrical energy received by the sun from +the nearest stars for 70,000 years before and after the +present. It is based on the twenty-six stars for which +complete data are available in Table 6. The inclusion of +the other twelve would not alter the form of the curve, +for even the largest of them would not change any part +by more than about half of 1 per cent, if as much. +Nor would the curve be visibly altered by the omission +of all except four of the twenty-six stars actually used. +The four that are important, and their relative luminosity +when nearest the sun, are Sirius 429,000, Altair +153,000, Alpha Centauri 117,500, and Procyon 51,300. +The figure for the next star is only 4970, while for this +star combined with the other twenty-one that are unimportant +it is only 24,850.</p> + +<p>Figure 10 is not carried more than 70,000 years into +the past or into the future because the stars near the +sun at more remote times are not included among the +thirty-eight having the largest known parallaxes. That +is, they have either moved away or are not yet near +enough to be included. Indeed, as Dr. Schlesinger +strongly emphasizes, there may be swiftly moving, bright +or gigantic stars which are now quite far away, but whose +inclusion would alter Fig. 10 even within the limits of +the 140,000 years there shown. It is almost certain, however, +that the most that these would do would be to raise, +but not obliterate, the minima on either side of the main +maximum.</p> + +<p>In preparing Fig. 10 it has been necessary to make +<!--File: cc303.png--> +<a class="pagenum break" id="Page_279" title="pg 279"></a> + + +<!--File: cc304.png--> +<a class="pagenum break" id="Page_280" title="pg 280"></a> +allowance for double stars. Passing by the twenty-two +unimportant stars, it appears that the companion of +Sirius is eight or ten magnitudes smaller than that star, +while the companions of Procyon and Altair are five or +more magnitudes smaller than their bright comrades. +This means that the luminosity of the faint components +is at most only 1 per cent of that of their bright companions +and in the case of Sirius not a hundredth of 1 +per cent. Hence their inclusion would have no visible +effect on Fig. 10. In Alpha Centauri, on the other hand, +the two components are of almost the same magnitude. +For this reason the effective radiation of that star as +given in column 14 is doubled in Fig. 10, while for +another reason it is raised still more. The other reason +is that if our inferences as to the electrical effect of the +sun on the earth and of the planets on the sun are correct, +double stars, as we have seen, must be much more +effective electrically than single stars. By the same +reasoning two bright stars close together must excite +one another much more than a bright star and a very +faint one, even if the distances in both cases are the same. +So, too, other things being equal, a triple star must be +more excited electrically than a double star. Hence in +preparing Fig. 10 all double stars receive double weight +and each part of Alpha Centauri receives an additional +50 per cent because both parts are bright and because +they have a third companion to help in exciting them.</p> + +<img src="images/fig10.png" alt="Fig. 10" id="Fig_10" /> +<p class="center"><em>Fig. 10. Climatic changes of 140,000 years as inferred from the stars.</em></p> + +<p>According to the electro-stellar hypothesis, Alpha Centauri +is more important climatically than any other star +in the heavens not only because it is triple and bright, but +because it is the nearest of all stars, and moves fairly +rapidly. Sirius and Procyon move slowly in respect to +the sun, only about eleven and eight kilometers per +second respectively, and their distances at minimum are +<!--File: cc305.png--> +<a class="pagenum break" id="Page_281" title="pg 281"></a> +fairly large, that is, 8 and 10.2 light years. Hence their +effect on the sun changes slowly. Altair moves faster, +about twenty-six kilometers per second, and its minimum +distance is 6.4 light years, so that its effect changes fairly +rapidly. Alpha Centauri moves about twenty-four kilometers +per second, and its minimum distance is only 3.2 +light years. Hence its effect changes very rapidly, the +change in its apparent luminosity as seen from the sun +amounting at maximum to about 30 per cent in 10,000 +years against 14 per cent for Altair, 4 for Sirius, and 2 +for Procyon. The vast majority of the stars change so +much more slowly than even Procyon that their effect is +almost uniform. All the stars at a distance of more than +perhaps twenty or thirty light years may be regarded as +sending to the sun a practically unchanging amount of +radiation. It is the bright stars within this limit which +are important, and their importance increases with their +proximity, their speed of motion, and the brightness and +number of their companions. Hence Alpha Centauri +causes the main maximum in Fig. 10, while Sirius, Altair, +and Procyon combine to cause a general rise of the curve +from the past to the future.</p> + +<p>Let us now interpret Fig. 10 geologically. The low position +of the curve fifty to seventy thousand years ago +suggests a mild inter-glacial climate distinctly less severe +than that of the present. Geologists say that such was the +case. The curve suggests a glacial epoch culminating +about 28,000 years ago. The best authorities put the climax +of the last glacial epoch between twenty-five and +thirty thousand years ago. The curve shows an amelioration +of climate since that time, although it suggests that +there is still considerable severity. The retreat of the ice +from North America and Europe, and its persistence in +Greenland and Antarctica agree with this. And the curve +<!--File: cc306.png--> +<a class="pagenum break" id="Page_282" title="pg 282"></a> +indicates that the change of climate is still persisting, a +conclusion in harmony with the evidence as to historic +changes.</p> + +<p>If Alpha Centauri is really so important, the effect of +its variations, provided it has any, ought perhaps to be +evident in the sun. The activity of the star's atmosphere +presumably varies, for the orbits of the two components +have an eccentricity of 0.51. Hence during their period +of revolution, 81.2 years, the distance between them +ranges from 1,100,000,000 to 3,300,000,000 miles. They +were at a minimum distance in 1388, 1459, 1550, 1631, +1713, 1794, 1875, and will be again in 1956. In Fig. +11, showing sunspot variations, it is noticeable that the +years 1794 and 1875 come just at the ends of periods of +unusual solar activity, as indicated by the heavy horizontal +line. A similar period of great activity seems to +have begun about 1914. If its duration equals the average +of its two predecessors, it will end about 1950. Back in +the fourteenth century a period of excessive solar activity, +which has already been described, culminated from +1370 to 1385, or just before the two parts of Alpha Centauri +were at a minimum distance. Thus in three and +perhaps four cases the sun has been unusually active +during a time when the two parts of the star were most +rapidly approaching each other and when their atmospheres +were presumably most disturbed and their electrical +emanations strongest.</p> + +<p>The fact that Alpha Centauri, the star which would be +expected most strongly to influence the sun, and hence +the earth, was nearest the sun at the climax of the last +glacial epoch, and that today the solar atmosphere is +most active when the star is presumably most disturbed +may be of no significance. It is given for what it is worth. +Its importance lies not in the fact that it proves anything, + +but that no contradiction is found when we test +the electro-stellar hypothesis by facts which were not +thought of when the hypothesis was framed. A vast +amount of astronomical work is still needed before the +matter can be brought to any definite conclusion. In case +the hypothesis stands firm, it may be possible to use the +stars as a help in determining the exact chronology of the +later part of geological times. If the hypothesis is disproved, +it will merely leave the question of solar variations +where it is today. It will not influence the main +conclusions of this book as to the causes and nature of +climatic changes. Its value lies in the fact that it calls +attention to new lines of research.</p> +<!--File: cc307.png--> +<a class="pagenum break" id="Page_283" title="pg 283"></a> + +<img src="images/fig11.png" width="800" height="193" alt="Fig. 11" id="Fig_11" /> +<p class="center"><em>Fig. 11. Sunspot curve showing cycles, 1750 to 1920.</em></p> + +<p class="blockquot"><em>Note.</em> The asterisks indicate two absolute minima of sunspots in 1810 and 1913, and the middle years (1780 and +1854) of two periods when the sunspot maxima never fell below 95. If Alpha Centauri has an effect on the sun's +atmosphere, the end of another such period would be expected not far from 1957.</p> +<!--File: cc308.png--> +<a class="pagenum break" id="Page_284" title="pg 284"></a> +<!--File: cc309.png--> +<a class="pagenum break" id="Page_285" title="pg 285"></a> + + + + +<h2 id="Chapt_XVI"><a href="#content">CHAPTER XVI</a></h2> + +<h3>THE EARTH'S CRUST AND THE SUN</h3> + + +<p>Although the problems of this book may lead far +afield, they ultimately bring us back to the earth +and to the present. Several times in the preceding +pages there has been mention of the fact that periods of +extreme climatic fluctuations are closely associated with +great movements of the earth's crust whereby mountains +are uplifted and continents upheaved. In attempting to +explain this association the general tendency has been +to look largely at the past instead of the present. Hence +it has been almost impossible to choose among three +possibilities, all beset with difficulties. First, the movements +of the crust may have caused the climatic fluctuations; +second, climatic changes may cause crustal movements; +and third, variations in solar activity or in some +other outside agency may give rise to both types of terrestrial +phenomena.</p> + +<p>The idea that movements of the earth's crust are the +main cause of geological changes of climate is becoming +increasingly untenable as the complexity and rapidity of +climatic changes become more clear, especially during +post-glacial times. It implies that the earth's surface +moves up and down with a speed and facility which +appear to be out of the question. If volcanic activity be +invoked the problem becomes no clearer. Even if volcanic +dust should fill the air frequently and completely, neither +its presence nor absence would produce such peculiar features +<!--File: cc310.png--> +<a class="pagenum break" id="Page_286" title="pg 286"></a> +as the localization of glaciers, the distribution of +loess, and the mild climate of most parts of geological +time. Nevertheless, because of the great difficulties presented +by the other two possibilities many geologists +still hold that directly or indirectly the greater climatic +changes have been mainly due to movements of the +earth's crust and to the reaction of the crustal movements +on the atmosphere.</p> + +<p>The possibility that climatic changes are in themselves +a cause of movements of the earth's crust seems so improbable +that no one appears to have investigated it with +any seriousness. Nevertheless, it is worth while to raise +the question whether climatic extremes may coöperate +with other agencies in setting the time when the earth's +crust shall be deformed.</p> + +<p>As to the third possibility, it is perfectly logical to +ascribe both climatic changes and crustal deformation to +some outside agency, solar or otherwise, but hitherto +there has been so little evidence on this point that such +an ascription has merely begged the question. If heavenly +bodies should approach the earth closely enough so that +their gravitational stresses caused crustal deformation, +all life would presumably be destroyed. As to the sun, +there has hitherto been no conclusive evidence that it is +related to crustal movements, although various writers +have made suggestions along this line. In this chapter +we shall carry these suggestions further and shall see +that they are at least worthy of study.</p> + +<p>As a preliminary to this study it may be well to note +that the coincidence between movements of the earth's +crust and climatic changes is not so absolute as is sometimes +supposed. For example, the profound crustal +changes at the end of the Mesozoic were not accompanied +by widespread glaciation so far as is yet known, although +<!--File: cc311.png--> +<a class="pagenum break" id="Page_287" title="pg 287"></a> +the temperature appears to have been lowered. Nor was +the violent volcanic and diastrophic activity in the Miocene +associated with extreme climates. Indeed, there +appears to have been little contrast from zone to zone, +for figs, bread fruit trees, tree ferns, and other plants of +low latitudes grew in Greenland. Nevertheless, both at +the end of the Mesozoic and in the Miocene the climate +may possibly have been severe for a time, although the +record is lost. On the other hand, Kirk's recent discovery +of glacial till in Alaska between beds carrying an undoubted +Middle Silurian fauna indicates glaciation at a +time when there was little movement of the crust so far +as yet appears.<sup><a href="#fn129" id="r129">[129]</a></sup> Thus we conclude that while climatic +changes and crustal movements usually occur together, +they may occur separately.</p> + +<p>According to the solar-cyclonic hypothesis such a condition +is to be expected. If the sun were especially active +when the terrestrial conditions prohibited glaciation, +changes of climate would still occur, but they would be +milder than under other circumstances, and would leave +little record in the rocks. Or there might be glaciation in +high latitudes, such as that of southern Alaska in the +Middle Silurian, and none elsewhere. On the other hand, +when the sun was so inactive that no great storminess +occurred, the upheaval of continents and the building of +mountains might go on without the formation of ice +sheets, as apparently happened at the end of the Mesozoic. +The lack of absolute coincidence between glaciation +and periods of widespread emergence of the lands is +evident even today, for there is no reason to suppose +that the lands are notably lower or less extensive now +than they were during the Pleistocene glaciation. In +fact, there is much evidence that many areas have risen +<!--File: cc312.png--> +<a class="pagenum break" id="Page_288" title="pg 288"></a> +since that time. Yet glaciation is now far less extensive +than in the Pleistocene. Any attempt to explain this difference +on the basis of terrestrial changes is extremely +difficult, for the shape and altitude of continents and +mountains have not changed much in twenty or thirty +thousand years. Yet the present moderately mild epoch, +like the puzzling inter-glacial epochs of earlier times, is +easily explicable on the assumption that the sun's atmosphere +may sometimes vary in harmony with crustal +activity, but does not necessarily do so at all times.</p> + +<p>Turning now to the main problem of how climatic +changes may be connected with movements of the earth's +crust, let us follow our usual method and examine what +is happening today. Let us first inquire whether earthquakes, +which are one of the chief evidences that crustal +movements are actually taking place in our own times, +show any connection with sunspots. In order to test this, +we have compared <em>Milne's Catalogue of Destructive +Earthquakes</em> from 1800 to 1899, with Wolf's sunspot +numbers for the same period month by month. The earthquake +catalogue, as its compiler describes it, "is an +attempt to give a list of earthquakes which have announced +changes of geological importance in the earth's +crust; movements which have probably resulted in the +creation or the extension of a line of fault, the vibrations +accompanying which could, with proper instruments, +have been recorded over a continent or the whole surface +of our world. Small earthquakes have been excluded, +while the number of large earthquakes both for ancient +and modern times has been extended. As an illustration +of exclusion, I may mention that between 1800 and 1808, +which are years taken at random, I find in Mallet's catalogue +407 entries. Only thirty-seven of these, which were +accompanied by structural damage, have been retained. +<!--File: cc313.png--> +<a class="pagenum break" id="Page_289" title="pg 289"></a> +Other catalogues such as those of Perry and Fuchs have +been treated similarly."<sup><a href="#fn130" id="r130">[130]</a></sup></p> + +<p>If the earthquakes in such a carefully selected list bear +a distinct relation to sunspots, it is at least possible and +perhaps probable that a similar relation may exist between +solar activity and geological changes in the earth's +crust. The result of the comparison of earthquakes and +sunspots is shown in Table 7. The first column gives the +sunspot numbers; the second, the number of months that +had the respective spot numbers during the century from +1800 to 1899. Column C shows the total number of earthquakes +during the months having any particular degree +of spottedness; while D, which is the significant column, +gives the average number of destructive earthquakes per +month under each of the six conditions of solar spottedness.</p> + +<table id="table_7" class="borders2"> +<tr><th colspan="6"><a href="#tables">TABLE 7</a></th></tr> +<tr><td colspan='6'> </td></tr> +<tr><th colspan="6">DESTRUCTIVE EARTHQUAKES FROM 1800 TO +1899 COMPARED WITH SUNSPOTS</th></tr> +<tr><td colspan='6'> </td></tr> +<tr> +<th><em>A</em></th> +<th><em>B</em></th> +<th><em>C</em></th> +<th><em>D</em></th> +<th><em>E</em></th> +<th><em>F</em></th> +</tr> +<tr> +<th><em>Sunspot numbers</em></th> +<th><em>Number of months per Wolf's Table</em></th> +<th><em>Number of earthquakes</em></th> +<th><em>Average number of earthquakes per month</em></th> +<th><em>Number of earthquakes in succeeding month</em></th> +<th><em>Average number of earthquakes in succeeding month</em></th> +</tr> +<tr> +<td> 0-15</td> +<td>344</td> +<td>522</td> +<td>1.52</td> +<td>512</td> +<td>1.49</td> +</tr> +<tr> +<td>15-30</td> +<td>194</td> +<td>306</td> +<td>1.58</td> +<td>310</td> +<td>1.60</td> +</tr> +<tr> +<td>30-50</td> +<td>237</td> +<td>433</td> +<td>1.83</td> +<td>439</td> +<td>1.85</td> +</tr> +<tr> +<td>50-70</td> +<td>195</td> +<td>402</td> +<td>2.06</td> +<td>390</td> +<td>2.00</td> +</tr> +<tr> +<td>70-100</td> +<td>135</td> +<td>286</td> +<td>2.12</td> +<td>310</td> +<td>2.30</td> +</tr> +<tr> +<td>over 100</td> +<td>95</td> +<td>218</td> +<td>2.30</td> +<td>175</td> +<td>1.84</td> +</tr> +</table> + +<!--File: cc314.png Page 290--> +<a class="pagenum break" id="Page_290" title="pg 290"></a> +<p>The regularity of column D is so great as to make +it almost certain that we are here dealing with a real +relationship. Column F, which shows the average number +of earthquakes in the month succeeding any given condition +of the sun, is still more regular except for the last +entry.</p> + +<p>The chance that six numbers taken at random will +arrange themselves in any given order is one in 720. In +other words, there is one chance in 720 that the regularity +of column D is accidental. But column F is as regular as +column D except for the last entry. If columns D and E +were independent there would be one chance in about +500,000 that the six numbers in both columns would +fall in the same order, and one chance in 14,400 that +five numbers in each would fall in the same order. +But the two columns are somewhat related, for although +the after-shocks of a great earthquake are never included +in Milne's table, a world-shaking earthquake in one +region during a given month probably creates conditions +that favor similar earthquakes elsewhere during the next +month. Hence the probability that we are dealing with a +purely accidental arrangement in Table 7 is less than one +in 14,400 and greater than one in 500,000. It may be one +in 20,000 or 100,000. In any event it is so slight that there +is high probability that directly or indirectly sunspots +and earthquakes are somehow connected.</p> + +<p>In ascertaining the relation between sunspots and +earthquakes it would be well if we could employ the strict +method of correlation coefficients. This, however, is impossible +for the entire century, for the record is by no +means homogeneous. The earlier decades are represented +by only about one-fourth as many earthquakes as the +later ones, a condition which is presumably due to lack of +information. This makes no difference with the method +<!--File: cc315.png--> +<a class="pagenum break" id="Page_291" title="pg 291"></a> +employed in Table 7, since years with many and few sunspots +are distributed almost equally throughout the +entire nineteenth century, but it renders the method of +correlation coefficients inapplicable. During the period +from 1850 onward the record is much more nearly homogeneous, +though not completely so. Even in these later +decades, however, allowance must be made for the fact +that there are more earthquakes in winter than in +summer, the average number per month for the fifty +years being as follows:</p> + +<table> +<tr><td class="tdl">Jan. 2.8</td> <td class="tdl">May 2.4 </td> <td class="tdl">Sept. 2.5</td></tr> +<tr><td class="tdl">Feb. 2.4</td> <td class="tdl">June 2.3</td> <td class="tdl">Oct. 2.6</td></tr> +<tr><td class="tdl">Mar. 2.5</td> <td class="tdl">July 2.4</td> <td class="tdl">Nov. 2.7</td></tr> +<tr><td class="tdl">Apr. 2.4 </td> <td class="tdl">Aug. 2.4</td> <td class="tdl">Dec. 2.8</td></tr> +</table> + +<p>The correlation coefficient between the departures from +these monthly averages and the corresponding departures +from the monthly averages of the sunspots for the +same period, 1850-1899, are as follows:</p> + +<blockquote> +<p>Sunspots and earthquakes of same month: +0.042, or 1.5 +times the probable error.</p> + +<p>Sunspots of a given month and earthquakes of that month +and the next: +0.084, or 3.1 times the probable error.</p> + +<p>Sunspots of three consecutive months and earthquakes of +three consecutive months allowing a lag of one month, i.e., sunspots +of January, February, and March compared with earthquakes +of February, March, and April; sunspots of February, +March, and April with earthquakes of March, April, and May, +etc.; +0.112, or 4.1 times the probable error.</p> +</blockquote> + +<p>These coefficients are all small, but the number of individual +cases, 600 months, is so large that the probable +error is greatly reduced, being only ±0.027 or ±0.028. +Moreover, the nature of our data is such that even if +<!--File: cc316.png--> +<a class="pagenum break" id="Page_292" title="pg 292"></a> +there is a strong connection between solar changes and +earth movements, we should not expect a large correlation +coefficient. In the first place, as already mentioned, +the earthquake data are not strictly homogeneous. +Second, an average of about two and one-half strong +earthquakes per month is at best only a most imperfect +indication of the actual movement of the earth's crust. +Third, the sunspots are only a partial and imperfect +measure of the activity of the sun's atmosphere. Fourth, +the relation between solar activity and earthquakes is +almost certainly indirect. In view of all these conditions, +the regularity of Table 7 and the fact that the most important +correlation coefficient rises to more than four +times the probable error makes it almost certain that the +solar and terrestrial phenomena are really connected.</p> + +<p>We are now confronted by the perplexing question of +how this connection can take place. Thus far only three +possibilities present themselves, and each is open to +objections. The chief agencies concerned in these three +possibilities are heat, electricity, and atmospheric pressure. +Heat may be dismissed very briefly. We have seen +that the earth's surface becomes relatively cool when +the sun is active. Theoretically even the slightest change +in the temperature of the earth's surface must influence +the thermal gradient far into the interior and hence cause +a change of volume which might cause movements of the +crust. Practically the heat of the surface ceases to be of +appreciable importance at a depth of perhaps twenty +feet, and even at that depth it does not act quickly enough +to cause the relatively prompt response which seems to be +characteristic of earthquakes in respect to the sun.</p> + +<p>The second possibility is based on the relationship +between solar and terrestrial electricity. When the sun +is active the earth's atmospheric electrical potential is +<!--File: cc317.png--> +<a class="pagenum break" id="Page_293" title="pg 293"></a> +subject to slight variations. It is well known that when +two opposing points of an ionized solution are oppositely +charged electrically, a current passes through the liquid +and sets up electrolysis whereby there is a segregation +of materials, and a consequent change in the volume of +the parts near the respective electrical poles. The same +process takes place, although less freely, in a hot mass +such as forms the interior of the earth. The question +arises whether internal electrical currents may not pass +between the two oppositely charged poles of the earth, +or even between the great continental masses and the +regions of heavier rock which underlie the oceans. Could +this lead to electrolysis, hence to differentiation in volume, +and thus to movements of the earth's crust? Could +the results vary in harmony with the sun? Bowie<sup><a href="#fn131" id="r131">[131]</a></sup> has +shown that numerous measurements of the strength and +direction of the earth's gravitative pull are explicable +only on the assumption that the upheaval of a continent +or a mountain range is due in part not merely to pressure, +or even to flowage of the rocks beneath the crust, +but also to an actual change in volume whereby the rocks +beneath the continent attain relatively great volume and +those under the oceans a small volume in proportion to +their weight. The query arises whether this change of +volume may be related to electrical currents at some +depth below the earth's surface.</p> + +<p>The objections to this hypothesis are numerous. First, +there is little evidence of electrolytic differentiation in +the rocks. Second, the outer part of the earth's crust is a +very poor conductor so that it is doubtful whether even +a high degree of electrification of the surface would have +much effect on the interior. Third, electrolysis due to any +<!--File: cc318.png--> +<a class="pagenum break" id="Page_294" title="pg 294"></a> +such mild causes as we have here postulated must be an +extremely slow process, too slow, presumably, to have +any appreciable result within a month or two. Other +objections join with these three in making it seem improbable +that the sun's electrical activity has any direct +effect upon movements of the earth's crust.</p> + +<p>The third, or meteorological hypothesis, which makes +barometric pressure the main intermediary between solar +activity and earthquakes, seems at first sight almost as +improbable as the thermal and electrical hypotheses. +Nevertheless, it has a certain degree of observational +support of a kind which is wholly lacking in the other two +cases. Among the extensive writings on the periodicity of +earthquakes one main fact stands out with great distinctness: +earthquakes vary in number according to the +season. This fact has already been shown incidentally in +the table of earthquake frequency by months. If allowance +is made for the fact that February is a short month, +there is a regular decrease in the frequency of severe +earthquakes from December and January to June. Since +most of Milne's earthquakes occurred in the northern +hemisphere, this means that severe earthquakes occur in +winter about 20 per cent oftener than in summer.</p> + +<!--File: cc319.png--> +<a class="pagenum break" id="Page_295" title="pg 295"></a> +<!--File: cc320.png--> +<table id="table_8" class="borders"> +<tr><th colspan="7"><a href="#tables">TABLE 8</a></th></tr> +<tr><td colspan='7'> </td></tr> +<tr><th colspan="7">SEASONAL MARCH OF EARTHQUAKES</th></tr> + +<tr><th colspan="7"><small>AFTER DAVISSON AND KNOTT</small></th></tr> +<tr><td colspan='7'> </td></tr> +<tr> +<th><em>A</em></th> +<th><em>B</em></th> +<th><em>C</em></th> +<th><em>D</em></th> +<th><em>E</em></th> +<th><em>F</em></th> +<th><em>G</em></th> +</tr> +<tr> +<th><em>Region</em></th> +<th><em>Limiting Dates</em></th> +<th><em>Number of Shocks</em></th> +<th><em>Maximum Month</em></th> +<th><em>Amplitude</em></th> +<th><em>Expected Amplitude</em></th> +<th><em>Ratio of<br /> Actual to<br /> Expected Amplitude</em></th> +</tr> + +<tr> +<td class="tdl">Northern Hemisphere</td> +<td>223-1850</td> +<td>5879</td> +<td>Dec.</td> +<td>0.110</td> +<td>0.023</td> +<td>4.8</td> +</tr> + +<tr> +<td class="tdl">Northern Hemisphere</td> +<td>1865-1884</td> +<td>8133</td> +<td>Dec.</td> +<td>0.290</td> +<td>0.020</td> +<td>14.5</td> +</tr> +<tr> +<td class="tdl">Europe </td> +<td>1865-1884</td> +<td>5499</td> +<td>Dec.</td> +<td>0.350</td> +<td>0.024</td> +<td>14.6</td> +</tr> +<tr> +<td class="tdl">Europe</td> +<td>306-1843</td> +<td>1961</td> +<td>Dec.</td> +<td>0.220</td> +<td>0.040</td> +<td>5.5</td> +</tr> +<tr> +<td class="tdl">Southeast Europe</td> +<td>1859-1887</td> +<td>3470</td> +<td>Dec.</td> +<td>0.210</td> +<td>0.030</td> +<td>7.0</td> +</tr> +<tr> +<td class="tdl">Vesuvius District</td> +<td>1865-1883</td> +<td>513</td> +<td>Dec.</td> +<td>0.250</td> +<td>0.078 </td> +<td>3.2</td> +</tr> +<tr> +<td class="tdl" colspan="7">Italy:</td> +</tr> +<tr> + <td class="tdi2">Old Tromometre</td> + <td>1872-1887</td> + <td>61732</td> + <td>Dec.</td> + <td>0.490</td> + <td>0.007</td> + <td>70.0</td> + </tr> +<tr> + <td class="tdi2">Old Tromometre</td> + <td>1876-1887</td> + <td>38546</td> + <td>Dec.</td> + <td>0.460</td> + <td>0.009</td> + <td>49.5</td> +</tr> +<tr> + <td class="tdi2">Normal Tromometre</td> + <td>1876-1887</td> + <td>38546</td> + <td>Dec.</td> + <td>0.490</td> + <td>0.009</td> + <td>52.8</td> +</tr> +<tr> +<td class="tdl">Balkan, etc.</td> +<td>1865-1884</td> +<td>624</td> +<td>Dec.</td> +<td>0.270</td> +<td>0.071</td> +<td>3.8</td> +</tr> +<tr> +<td class="tdl">Hungary, etc.</td> +<td>1865-1884</td> +<td>384</td> +<td>Dec.</td> +<td>0.310</td> +<td>0.090</td> +<td>3.4</td> +</tr> +<tr> +<td class="tdl">Italy</td> +<td>1865-1883</td> +<td>2350</td> +<td>Dec.(Sept.)</td> +<td>0.140</td> +<td>0.037</td> +<td>3.8</td> +</tr> +<tr> +<td class="tdl">Grecian Archip.</td> +<td>1859-1881</td> +<td>3578</td> +<td>Dec.-Jan.</td> +<td>0.164</td> +<td>0.030</td> +<td>5.5</td> +</tr> +<tr> +<td class="tdl">Austria</td> +<td>1865-1884</td> +<td>461</td> +<td>Jan.</td> +<td>0.370</td> +<td>0.083</td> +<td>4.4</td> +</tr> +<tr> +<td class="tdl">Switzerland, etc.</td> +<td>1865-1883</td> +<td>524</td> +<td>Jan.</td> +<td>0.560</td> +<td>0.077</td> +<td>7.3</td> +</tr> +<tr> +<td class="tdl">Asia</td> +<td>1865-1884</td> +<td>458</td> +<td>Feb.</td> +<td>0.330</td> +<td>0.083</td> +<td>4.0</td> +</tr> +<tr> +<td class="tdl">North America</td> +<td>1865-1884</td> +<td>552</td> +<td>Nov.</td> +<td>0.350</td> +<td>0.075</td> +<td>4.7</td> +</tr> +<tr> +<td class="tdl">California</td> +<td>1850-1886</td> +<td>949</td> +<td>Oct.</td> +<td>0.300</td> +<td>0.058</td> +<td>5.2</td> +</tr> +<tr> +<td class="tdl">Japan</td> +<td>1878-1881</td> +<td>246</td> +<td>Dec.</td> +<td>0.460</td> +<td>0.113</td> +<td>4.1</td> +</tr> +<tr> +<td class="tdl">Japan</td> +<td>1872-1880</td> +<td>367</td> +<td>Dec.-Jan.</td> +<td>0.256</td> +<td>0.093</td> +<td>2.8</td> +</tr> +<tr> +<td class="tdl">Japan</td> +<td>1876-1891</td> +<td>1104</td> +<td>Feb.</td> +<td>0.190</td> +<td>0.053</td> +<td>3.6</td> +</tr> +<tr> +<td class="tdl">Japan</td> +<td>1885-1889</td> +<td>2997</td> +<td>Oct.</td> +<td>0.080</td> +<td>0.032</td> +<td>2.5</td> +</tr> +<tr> +<td class="tdl">Zante</td> +<td>1825-1863</td> +<td>1326</td> +<td>Aug.</td> +<td>0.100</td> +<td>0.049</td> +<td>2.0</td> +</tr> +<tr> +<td class="tdl">Italy, North of Naples</td> +<td>1865-1883</td> +<td>1513</td> +<td>Sept.(Nov.)</td> +<td>0.210</td> +<td>0.046</td> +<td>4.6</td> +</tr> +<tr> +<td class="tdl">East Indies </td> +<td>1873-1881</td> +<td>515</td> +<td>Aug., Oct., or Dec.?</td> +<td>0.071?</td> +<td>0.078 </td> +<td>0.9</td> +</tr> + <tr> +<td class="tdl">Malay Archip.</td> +<td>1865-1884</td> +<td>598</td> +<td>May</td> +<td>0.190</td> +<td>0.072</td> +<td>2.6</td> +</tr> +<tr> +<td class="tdl">New Zealand</td> +<td>1869-1879</td> +<td>585</td> +<td>Aug.-Sept.</td> +<td>0.203</td> +<td>0.073</td> +<td>2.8</td> +</tr> +<tr> +<td class="tdl">Chile</td> +<td>1873-1881</td> +<td>212</td> +<td>July</td> +<td>0.480</td> +<td>0.122</td> +<td>3.9</td> +</tr> +<tr> +<td class="tdl">Southern Hemisphere</td> +<td>1865-1884</td> +<td>751</td> +<td>July</td> +<td>0.370</td> +<td>0.065</td> +<td>5.7</td> +</tr> +<tr> +<td class="tdl">New Zealand</td> +<td>1868-1890</td> +<td>641</td> +<td>March, May</td> +<td>0.050</td> +<td>0.070</td> +<td>0.7</td> +</tr> +<tr> +<td class="tdl">Chile</td> +<td>1865-1883?</td> +<td>316</td> +<td>July, Dec.</td> +<td>0.270</td> +<td>0.100</td> +<td>2.7</td> +</tr> +<tr> +<td class="tdl">Peru, Bolivia</td> +<td>1865-1884</td> +<td>350</td> +<td>July</td> +<td>0.480</td> +<td>0.095</td> +<td>5.1</td> +</tr> +</table> +<p>The most thorough investigation of this subject seems +to have been that of Davisson.<sup><a href="#fn132" id="r132">[132]</a></sup> His results have been +worked over and amplified by Knott,<sup><a href="#fn133" id="r133">[133]</a></sup> who has tested +them by Schuster's exact mathematical methods. His results +are given in Table 8.<sup><a href="#fn134" id="r134">[134]</a></sup> Here the northern hemisphere +<a class="pagenum break" id="Page_296" title="pg 296"></a> +is placed first; then come the East Indies and the Malay +Archipelago lying close to the equator; and finally the +southern hemisphere. In the northern hemisphere practically +all the maxima come in the winter, for the month +of December appears in fifteen cases out of the twenty-five +in column D, while January, February, or November +appears in six others. It is also noticeable that in sixteen +cases out of twenty-five the ratio of the actual to the expected +amplitude in column G is four or more, so that a +real relationship is indicated, while the ratio falls below +three only in Japan and Zante. The equatorial data, +unlike those of the northern hemisphere, are indefinite, +for in the East Indies no month shows a marked maximum +and the expected amplitude exceeds the actual amplitude. +Even in the Malay Archipelago, which shows a +maximum in May, the ratio of actual to expected amplitude +is only 2.6. Turning to the southern hemisphere, the +winter months of that hemisphere are as strongly marked +by a maximum as are the winter months of the northern +<!--File: cc321.png--> +<a class="pagenum break" id="Page_297" title="pg 297"></a> +hemisphere. July or August appears in five out of six +cases. Here the ratio between the actual and expected +amplitudes is not so great as in the northern hemisphere. +Nevertheless, it is practically four in Chile, and exceeds +five in Peru and Bolivia, and in the data for the entire +southern hemisphere.</p> + + + +<p>The whole relationship between earthquakes and the +seasons in the northern and southern hemispheres is +summed up in Fig. 12 taken from Knott. The northern +hemisphere shows a regular diminution in earthquake +frequency from December until June, and an increase +the rest of the year. In the southern hemisphere the +course of events is the same so far as summer and winter +are concerned, for August with its maximum comes in +winter, while February with its minimum comes in +summer. In the southern hemisphere the winter month +of greatest seismic activity has over 100 per cent more +earthquakes than the summer month of least activity. In +the northern hemisphere this difference is about 80 per +cent, but this smaller figure occurs partly because the +northern data include certain interesting and significant +regions like Japan and China where the usual conditions +are reversed.<sup><a href="#fn135" id="r135">[135]</a></sup> If equatorial regions were included +in Fig. 12, they would give an almost straight line.</p> + +<p>The connection between earthquakes and the seasons is +so strong that almost no students of seismology question +it, although they do not agree as to its cause. A meteorological +hypothesis seems to be the only logical explanation.<sup><a href="#fn136" id="r136">[136]</a></sup> +Wherever sufficient data are available, earthquakes +<!--File: cc322.png--> +<a class="pagenum break" id="Page_298" title="pg 298"></a> +appear to be most numerous when climatic conditions +cause the earth's surface to be most heavily loaded or to +change its load most rapidly. The main factor in the +loading is apparently atmospheric pressure. This acts in +two ways. First, when the continents become cold in +winter the pressure increases. On an average the air +at sea level presses upon the earth's surface at the rate +of 14.7 pounds per square inch, or over a ton per square +foot, and only a little short of thirty million tons per +square mile. An average difference of one inch between +the atmospheric pressure of summer and winter over ten +million square miles of the continent of Asia, for example, +means that the continent's load in winter is about +ten million million tons heavier than in summer. Second, +the changes in atmospheric pressure due to the passage +of storms are relatively sharp and sudden. Hence they +are probably more effective than the variations in the +load from season to season. This is suggested by the +rapidity with which the terrestrial response seems to +follow the supposed solar cause of earthquakes. It is also +suggested by the fact that violent storms are frequently +followed by violent earthquakes. "Earthquake weather," +as Dr. Schlesinger suggests, is a common phrase in the +typhoon region of Japan, China, and the East Indies. +During tropical hurricanes a change of pressure amounting +to half an inch in two hours is common. On September +<!--File: cc323.png--> +<a class="pagenum break" id="Page_299" title="pg 299"></a> +22, 1885, at False Point Lighthouse on the Bay of +Bengal, the barometer fell about an inch in six hours, +then nearly an inch and a half in not much over two +hours, and finally rose fully two inches inside of two +hours. A drop of two inches in barometric pressure +means that a load of about two million tons is removed + +<!--File: cc324.png--> +<a class="pagenum break" id="Page_300" title="pg 300"></a> +from each square mile of land; the corresponding rise of +pressure means the addition of a similar load. Such a +storm, and to a less degree every other storm, strikes a +blow upon the earth's surface, first by removing millions +of tons of pressure and then by putting them on again.<sup><a href="#fn137" id="r137">[137]</a></sup> +Such storms, as we have seen, are much more frequent +and severe when sunspots are numerous than at other +times. Moreover, as Veeder<sup><a href="#fn138" id="r138">[138]</a></sup> long ago showed, one of the +most noteworthy evidences of a connection between sunspots +and the weather is a sudden increase of pressure in +certain widely separated high pressure areas. In most +parts of the world winter is not only the season of +highest pressure and of most frequent changes of +Veeder's type, but also of severest storms. Hence a +meteorological hypothesis would lead to the expectation +that earthquakes would occur more frequently in winter +than in summer. On the Chinese coast, however, and also +on the oceanic side of Japan, as well as in some more +tropical regions, the chief storms come in summer in the +form of typhoons. These are the places where earthquakes +also are most abundant in summer. Thus, wherever +we turn, storms and the related barometric changes +seem to be most frequent and severe at the very times +when earthquakes are also most frequent.</p> + +<img src="images/fig12.png" width="715" height="867" alt="Fig. 12" id="Fig_12" /> +<p class="center"><em>Fig. 12. Seasonal distribution of earthquakes. +(After Davisson and Knott.)</em><br /> + +—— Northern Hemisphere.<br /> +- - - - Southern Hemisphere.</p> + +<p>Other meteorological factors, such as rain, snow, +winds, and currents, probably have some effect on earthquakes +<!--File: cc325.png--> +<a class="pagenum break" id="Page_301" title="pg 301"></a> +through their ability to load the earth's crust. +The coming of vegetation may also help. These agencies, +however, appear to be of small importance compared +with the storms. In high latitudes and in regions of +abundant storminess most of these factors generally +combine with barometric pressure to produce frequent +changes in the load of the earth's crust, especially in +winter. In low latitudes, on the other hand, there are few +severe storms, and relatively little contrast in pressure +and vegetation from season to season; there is no snow; +and the amount of ground water changes little. With this +goes the twofold fact that there is no marked seasonal +distribution of earthquakes, and that except in certain +local volcanic areas, earthquakes appear to be rare. In +proportion to the areas concerned, for example, there is +little evidence of earthquakes in equatorial Africa and +South America.</p> + +<p>The question of the reality of the connection between +meteorological conditions and crustal movements is so +important that every possible test should be applied. At +the suggestion of Professor Schlesinger we have looked +up a very ingenious line of inquiry. During the last +decades of the nineteenth century, a long series of extremely +accurate observations of latitude disclosed a fact +which had previously been suspected but not demonstrated, +namely, that the earth wabbles a little about its +axis. The axis itself always points in the same direction, +and since the earth slides irregularly around it the latitude +of all parts of the earth keeps changing. Chandler +has shown that the wabbling thus induced consists of +two parts. The first is a movement in a circle with a +radius of about fifteen feet which is described in approximately +430 days. This so-called Eulerian movement is a +normal gyroscopic motion like the slow gyration of a +<!--File: cc326.png--> +<a class="pagenum break" id="Page_302" title="pg 302"></a> +spinning top. This depends on purely astronomical +causes, and no terrestrial cause can stop it or eliminate +it. The period appears to be constant, but there are certain +puzzling irregularities. The usual amplitude of this +movement, as Schlesinger<sup><a href="#fn139" id="r139">[139]</a></sup> puts it, "is about 0".27, but +twice in recent years it has jumped to 0".40. Such a +change could be accounted for by supposing that the +earth had received a severe blow or a series of milder +blows tending in the same direction." These blows, which +were originally suggested by Helmert are most interesting +in view of our suggestion as to the blows struck by +storms.</p> + +<p>The second movement of the pole has a period of a +year, and is roughly an ellipse whose longest radius is +fourteen feet and the shortest, four feet; or, to put it +technically, there is an annual term with a maximum +amplitude of about 0".20. This, however, varies irregularly. +The result is that the pole seems to wander over +the earth's surface in the spiral fashion illustrated in +Fig. 13. It was early suggested that this peculiar wandering +of the pole in an annual period must be due to +meteorological causes. Jeffreys<sup><a href="#fn140" id="r140">[140]</a></sup> has investigated the +matter exhaustively. He assumes certain reasonable +values for the weight of air added or subtracted from +different parts of the earth's surface according to the +seasons. He also considers the effect of precipitation, +vegetation, and polar ice, and of variations of temperature +and atmospheric pressure in their relation to movements +of the ocean. Then he proceeds to compare all +<!--File: cc327.png--> +<a class="pagenum break" id="Page_303" title="pg 303"></a> + +these with the actual wandering of the pole from 1907 +to 1913. While it is as yet too early to say that any +special movement of the pole was due to the specific +meteorological conditions of any particular year, +Jeffreys' work makes it clear that meteorological causes, +especially atmospheric pressure, are sufficient to cause +the observed irregular wanderings. Slight wanderings +may arise from various other sources such as movements +of the rocks when geological faults occur or the rush of +a great wave due to a submarine earthquake. So far as +known, however, all these other agencies cause insignificant +displacements compared with those arising from +movements of the air. This fact coupled with the mathematical +certainty that meteorological phenomena must +produce some wandering of the pole, has caused most +astronomers to accept Jeffreys' conclusion. If we follow +their example we are led to conclude that changes in +atmospheric pressure and in the other meteorological +conditions strike blows which sometimes shift the earth +<!--File: cc328.png--> +<a class="pagenum break" id="Page_304" title="pg 304"></a> +several feet from its normal position in respect to the +axis.</p> + +<img src="images/fig13.png" width="443" height="402" alt="Fig. 13" id="Fig_13" /> +<p class="center"><em>Fig. 13. Wandering of the +pole from 1890 to 1898.</em> +(<em>After Moulton.</em>)</p> + +<p>If the foregoing reasoning is correct, the great and +especially the sudden departures from the smooth +gyroscopic circle described by the pole in the Eulerian +motion would be expected to occur at about the same time +as unusual earthquake activity. This brings us to an +interesting inquiry carried out by Milne<sup><a href="#fn141" id="r141">[141]</a></sup> and amplified +by Knott.<sup><a href="#fn142" id="r142">[142]</a></sup> Taking Albrecht's representation of the +irregular spiral-like motion of the pole, as given in Fig. +13, they show that there is a preponderance of severe +earthquakes at times when the direction of motion of the +earth in reference to its axis departs from the smooth +Eulerian curve. A summary of their results is given in +Table 9. The table indicates that during the period from +1892 to 1905 there were nine different times when the +curve of Fig. 13 changed its direction or was deflected by +less than 10° during a tenth of a year. In other words, +during those periods it did not curve as much as it ought +according to the Eulerian movement. At such times there +were 179 world-shaking earthquakes, or an average of +about 19.9 per tenth of a year. According to the other +lines of Table 9, in thirty-two cases the deflection during +a tenth of a year was between 10° and 25°, while in fifty-six +cases it was from 25° to 40°. During these periods +the curve remained close to the Eulerian path and the +world-shaking earthquakes averaged only 8.2 and 12.9. +Then, when the deflection was high, that is, when meteorological +conditions threw the earth far out of its Eulerian +course, the earthquakes were again numerous, the +number rising to 23.4 when the deflection amounted to +more than 55°.</p> + +<!--File: cc329.png--> +<a class="pagenum break" id="Page_305" title="pg 305"></a> + +<table id="table_9" class="borders2"> +<tr><th colspan="4"><a href="#tables">TABLE 9</a></th></tr> +<tr><td colspan='4'> </td></tr> +<tr><th colspan="4">DEFLECTION OF PATH OF POLE COMPARED WITH EARTHQUAKES</th></tr> +<tr><td colspan='4'> </td></tr> +<tr> +<th><em>Deflection</em></th> +<th><em>No. of Deflections</em></th> +<th><em>No. of Earthquakes</em></th> +<th><em>Average No. of Earthquakes</em></th> +</tr> +<tr> +<td>0-10°</td> +<td>9</td> +<td>179</td> +<td>19.9</td> +</tr> +<tr> +<td>10-25° </td> +<td>32</td> +<td>263 </td> +<td>8.2</td> +</tr> +<tr> +<td>25-40°</td> +<td>56</td> +<td>722</td> +<td>12.9</td> +</tr> +<tr> +<td>40-55°</td> +<td>19</td> +<td>366</td> +<td>19.3</td> +</tr> +<tr> +<td>over 55°</td> +<td>7</td> +<td>164</td> +<td>23.4</td> +</tr> +</table> + +<p>In order to test this conclusion in another way we have +followed a suggestion of Professor Schlesinger. Under +his advice the Eulerian motion has been eliminated and +a new series of earthquake records has been compared +with the remaining motions of the poles which presumably +arise largely from meteorological causes. For this +purpose use has been made of the very full records of +earthquakes published under the auspices of the International +Seismological Commission for the years 1903 +to 1908, the only years for which they are available. +These include every known shock of every description +which was either recorded by seismographs or by direct +observation in any part of the world. Each shock is given +the same weight, no matter what its violence or how +closely it follows another. The angle of deflection has +been measured as Milne measured it, but since the Eulerian +motion is eliminated, our zero is approximately +the normal condition which would prevail if there were +no meteorological complications. Dividing the deflections +into six equal groups according to the size of the angle, +we get the result shown in Table 10.</p> +<!--File: cc330.png Page 306--> +<a class="pagenum break" id="Page_306" title="pg 306"></a> + +<table id="table_10" class="borders2"> +<tr><th colspan="2"><a href="#tables">TABLE 10</a></th></tr> +<tr><td colspan='2'> </td></tr> +<tr><th colspan="2">EARTHQUAKES IN 1903-1908 COMPARED +WITH DEPARTURES OF THE PROJECTED +CURVE OF THE EARTH'S +AXIS FROM THE EULERIAN +POSITION</th> +</tr> +<tr><td colspan='2'> </td></tr> +<tr> +<th><em>Average angle of deflection<br />(10 periods of 1/10 year each)</em> </th> +<th><em>Average daily number<br />of earthquakes</em></th> +</tr> +<tr> +<td>-10.5°</td> +<td>8.31</td> +</tr> +<tr> +<td> 11.5°</td> +<td> 8.35</td> +</tr> +<tr> +<td>25.8°</td> +<td>8.23</td> +</tr> +<tr> +<td>40.2°</td> +<td> 8.14</td> +</tr> +<tr> +<td> 54.7° </td> +<td>8.86</td> +</tr> +<tr> +<td>90.3°</td> +<td>11.81</td> +</tr> +</table> + +<p>Here where some twenty thousand earthquakes are +employed the result agrees closely with that of Milne for +a different series of years and for a much smaller number +of earthquakes. So long as the path of the pole departs +less than about 45° from the smooth gyroscopic Eulerian +path, the number of earthquakes is almost constant, about +eight and a quarter per day. When the angle becomes +large, however, the number increases by nearly 50 per +cent. Thus the work of Milne, Knott, and Jeffreys is confirmed +by a new investigation. Apparently earthquakes +and crustal movements are somehow related to sudden +changes in the load imposed on the earth's crust by +meteorological conditions.</p> + +<p>This conclusion is quite as surprising to the authors +as to the reader—perhaps more so. At the beginning of +this investigation we had no faith whatever in any important +<!--File: cc331.png Page 307--> +<a class="pagenum break" id="Page_307" title="pg 307"></a> +relation between climate and earthquakes. At its +end we are inclined to believe that the relation is close +and important.</p> + +<p>It must not be supposed, however, that meteorological +conditions are the <em>cause</em> of earthquakes and of movements +of the earth's crust. Even though the load that the +climatic agencies can impose upon the earth's crust runs +into millions of tons per square mile, it is a trifle compared +with what the crust is able to support. There is, +however, a great difference between the cause and the +occasion of a phenomenon. Suppose that a thick sheet of +glass is placed under an increasing strain. If the strain +is applied slowly enough, even so rigid a material as glass +will ultimately bend rather than break. But suppose that +while the tension is high the glass is tapped. A gentle +tap may be followed by a tiny crack. A series of little +taps may be the signal for small cracks to spread in +every direction. A few slightly harder taps may cause +the whole sheet to break suddenly into many pieces. Yet +even the hardest tap may be the merest trifle compared +with the strong force which is keeping the glass in a state +of strain and which would ultimately bend it if given +time.</p> + +<p>The earth as a whole appears to stand between steel +and glass in rigidity. It is a matter of common observation +that rocks stand high in this respect and in the +consequent difficulty with which they can be bent without +breaking. Because of the earth's contraction the crust +endures a constant strain, which must gradually become +enormous. This strain is increased by the fact that sediment +is transferred from the lands to the borders of the +sea and there forms areas of thick accumulation. From +this has arisen the doctrine of isostasy, or of the equalization +of crustal pressure. An important illustration of +<!--File: cc332.png--> +<a class="pagenum break" id="Page_308" title="pg 308"></a> +this is the oceanward and equatorial creep which has +been described in Chapter XI. There we saw that when +the lands have once been raised to high levels or when a +shortening of the earth's axis by contraction has increased +the oceanic bulge at the equator, or when the +reverse has happened because of tidal retardation, the +outer part of the earth appears to creep slowly back +toward a position of perfect isostatic adjustment. If the +sun had no influence upon the earth, either direct or +indirect, isostasy and other terrestrial processes might +flex the earth's crust so gradually that changes in the +form and height of the lands would always take place +slowly, even from the geological point of view. Thus +erosion would usually be able to remove the rocks as +rapidly as they were domed above the general level. If +this happened, mountains would be rare or unknown, and +hence climatic contrasts would be far less marked than is +actually the case on our earth where crustal movements +have repeatedly been rapid enough to produce mountains.</p> + +<p>Nature's methods rarely allow so gradual an adjustment +to the forces of isostasy. While the crust is under a +strain, not only because of contraction, but because of +changes in its load through the transference of sediments +and the slow increase or decrease in the bulge at the +equator, the atmosphere more or less persistently carries +on the tapping process. The violence of that process +varies greatly, and the variations depend largely on the +severity of the climatic contrasts. If the main outlines of +the cyclonic hypothesis are reliable, one of the first effects +of a disturbance of the sun's atmosphere is increased +storminess upon the earth. This is accompanied by increased +intensity in almost every meteorological process. +The most important effect, however, so far as the earth's +crust is concerned would apparently be the rapid and +<!--File: cc333.png Page 309--> +<a class="pagenum break" id="Page_309" title="pg 309"></a> +intense changes of atmospheric pressure which would +arise from the swift passage of one severe storm after +another. Each storm would be a little tap on the tensely +strained crust. Any single tap might be of little consequence, +even though it involved a change of a billion +tons in the pressure on an area no larger than the state +of Rhode Island. Yet a rapid and irregular succession of +such taps might possibly cause the crust to crack, and +finally to collapse in response to stresses arising from +the shrinkage of the earth.</p> + +<p>Another and perhaps more important effect of variations +in storminess and especially in the location of the +stormy areas would be an acceleration of erosion in some +places and a retardation elsewhere. A great increase in +rainfall may almost denude the slopes of soil, while a +diminution to the point where much of the vegetation +dies off has a similar effect. If such changes should take +place rapidly, great thicknesses of sediment might be +concentrated in certain areas in a short time, thus disturbing +the isostatic adjustment of the earth's crust. This +might set up a state of strain which would ultimately +have to be relieved, thus perhaps initiating profound +crustal movements. Changes in the load of the earth's +crust due to erosion and the deposition of sediment, no +matter how rapid they may be from the geological standpoint, +are slow compared with those due to changes in +barometric pressure. A drop of an inch in barometric +pressure is equivalent to the removal of about five inches +of solid rock. Even under the most favorable circumstances, +the removal of an average depth of five inches +of rock or its equivalent in soil over millions of square +miles would probably take several hundred years, while +the removal of a similar load of air might occur in half +a day or even a few hours. Thus the erosion and deposition +<!--File: cc334.png Page 310--> +<a class="pagenum break" id="Page_310" title="pg 310"></a> +due to climatic variations presumably play their +part in crustal deformation chiefly by producing crustal +stresses, while the storms, as it were, strike sharp, sudden +blows.</p> + +<p>Suppose now that a prolonged period of world-wide +mild climate, such as is described in Chapter X, should +permit an enormous accumulation of stresses due to contraction +and tidal retardation. Suppose that then a +sudden change of climate should produce a rapid shifting +of the deep soil that had accumulated on the lands, with a +corresponding localization and increase in strains. Suppose +also that frequent and severe storms play their part, +whether great or small, by producing an intensive tapping +of the crust. In such a case the ultimate collapse would +be correspondingly great, as would be evident in the +succeeding geological epoch. The sea floor might sink +lower, the continents might be elevated, and mountain +ranges might be shoved up along lines of special weakness. +This is the story of the geological period as known +to historical geology. The force that causes such movements +would be the pull of gravity upon the crust surrounding +the earth's shrinking interior. Nevertheless +climatic changes might occasionally set the date when the +gravitative pull would finally overcome inertia, and thus +usher in the crustal movements that close old geologic +periods and inaugurate new ones. This, however, could +occur only if the crust were under sufficient strain. As +Lawson<sup><a href="#fn143" id="r143">[143]</a></sup> says in his discussion of the "elastic rebound +theory," the sudden shifts of the crust which seem to be +the underlying cause of earthquakes "can occur only +after the accumulation of strain to a limit and ... this +accumulation involves a slow creep of the region affected. +<!--File: cc335.png Page 311--> +<a class="pagenum break" id="Page_311" title="pg 311"></a> +In the long periods between great earthquakes the energy +necessary for such shocks is being stored up in the rocks +as elastic compression."</p> + +<p>If a period of intense storminess should occur when +the earth as a whole was in such a state of strain, the +sudden release of the strains might lead to terrestrial +changes which would alter the climate still further, making +it more extreme, and perhaps permitting the storminess +due to the solar disturbances to bring about glaciation. +At the same time if volcanic activity should +increase it would add its quota to the tendency toward +glaciation. Nevertheless, it might easily happen that a +very considerable amount of crustal movement would +take place without causing a continental ice sheet or even +a marked alpine ice sheet. Or again, if the strains in the +earth's crust had already been largely released through +other agencies before the stormy period began, the climate +might become severe enough to cause glaciation +in high latitudes without leading to any very marked +movements of the earth's crust, as apparently happened +in the Mid-Silurian period.</p> + + +<h2 id="conclusion"><a href="#content">CONCLUSION</a></h2> + +<p>Here we must bring this study of the earth's evolution +to a close. Its fundamental principle has been that the +present, if rightly understood, affords a full key to the +past. With this as a guide we have touched on many +hypotheses, some essential and some unessential to the +general line of thought. The first main hypothesis is that +the earth's present climatic variations are correlated +with changes in the solar atmosphere. This is the keynote +of the whole book. It is so well established, however, +<!--File: cc336.pngPage 312--> +<a class="pagenum break" id="Page_312" title="pg 312"></a> +that it ranks as a theory rather than as an hypothesis. +Next comes the hypothesis that variations in the solar +atmosphere influence the earth's climate chiefly by causing +variations not only in temperature but also in +atmospheric pressure and thus in storminess, wind, and +rainfall. This, too, is one of the essential foundations on +which the rest of the book is built, but though this +cyclonic hypothesis is still a matter of discussion, it +seems to be based on strong evidence. These two hypotheses +might lead us astray were they not balanced by +another. This other is that many climatic conditions are +due to purely terrestrial causes, such as the form and +altitude of the lands, the degree to which the continents +are united, the movement of ocean currents, the activity +of volcanoes, and the composition of the atmosphere and +the ocean. Only by combining the solar and the terrestrial +can the truth be perceived. Finally, the last main hypothesis +of this book holds that if the climatic conditions which +now prevail at times of solar activity were magnified +sufficiently and if they occurred in conjunction with certain +important terrestrial conditions of which there is +good evidence, they would produce most of the notable +phenomena of glacial periods. For example, they would +explain such puzzling conditions as the localization and +periodicity of glaciation, the formation of loess, and the +occurrence of glaciation in low latitudes during Permian +and Proterozoic times. The converse of this is that if +the conditions which now prevail at times when the sun +is relatively inactive should be intensified, that is, if +the sun's atmosphere should become calmer than now, +and if the proper terrestrial conditions of topographic +form and atmospheric composition should prevail, there +would arise the mild climatic conditions which appear +to have prevailed during the greater part of geological +<!--File: cc337.png Page 313--> +<a class="pagenum break" id="Page_313" title="pg 313"></a> +time. In short, there seems thus far to be no phase of +the climate of the past which is not in harmony with an +hypothesis which combines into a single unit the three +main hypotheses of this book, solar, cyclonic, and terrestrial.</p> + +<p>Outside the main line of thought lie several other +hypotheses. Several of these, as well as some of the main +hypotheses, are discussed chiefly in <em>Earth and Sun</em>, but +as they are given a practical application in this book +they deserve a place in this final summary. Each of these +secondary hypotheses is in its way important. Yet any or +all may prove untrue without altering our main conclusions. +This point cannot be too strongly emphasized, for +there is always danger that differences of opinion as to +minor hypotheses and even as to details may divert attention +from the main point. Among the non-essential +hypotheses is the idea that the sun's atmosphere influences +that of the earth electrically as well as thermally. +This idea is still so new that it has only just entered the +stage of active discussion, and naturally the weight of +opinion is against it. Although not necessary to the main +purpose of this book, it plays a minor rôle in the chapter +dealing with the relation of the sun to other astronomical +bodies. It also has a vital bearing on the further advance +of the science of meteorology and the art of weather +forecasting. Another secondary hypothesis holds that +sunspots are set in motion by the planets. Whether the +effect is gravitational or more probably electrical, or +perhaps of some other sort, does not concern us at present, +although the weight of evidence seems to point +toward electronic emissions. This question, like that of +the relative parts played by heat and electricity in terrestrial +climatic changes, can be set aside for the moment. +What does concern us is a third hypothesis, namely, that +<!--File: cc338.png--> +<a class="pagenum break" id="Page_314" title="pg 314"></a> +if the planets really determine the periodicity of sunspots, +even though not supplying the energy, the sun in +its flight through space must have been repeatedly and +more strongly influenced in the same way by many other +heavenly bodies. In that case, climatic changes like those +of the present, but sometimes greatly magnified, have presumably +arisen because of the constantly changing position +of the solar system in respect to other parts of the +universe. Finally, the fourth of our secondary hypotheses +postulates that at present the date of movements of the +earth's crust is often determined by the fact that storms +and other meteorological conditions keep changing the +load upon first one part of the earth's surface and then +upon another. Thus stresses that have accumulated in the +earth's isostatic shell during the preceding months are +released. In somewhat the same way epochs of extreme +storminess and rapid erosion in the past may possibly +have set the date for great movements of the earth's +crust. This hypothesis, like the other three in our secondary +or non-essential group, is still so new that only the +first steps have been taken in testing it. Yet it seems to +deserve careful study.</p> + +<p>In testing all the hypotheses here discussed, primary +and secondary alike, the first necessity is a far greater +amount of quantitative work. In this book there has been +a constant attempt to subject every hypothesis to the test +of statistical facts of observation. Nevertheless, we have +been breaking so much new ground that in many cases +exact facts are not yet available, while in others they +can be properly investigated only by specialists in +physics, astronomy, or mathematics. In most cases the +next great step is to ascertain whether the forces here +called upon are actually great enough to produce the +observed results. Even though they act only as a means +<!--File: cc339.png Page 315--> +<a class="pagenum break" id="Page_315" title="pg 315"></a> +of releasing the far greater forces due to the contraction +of the earth and the sun, they need to be rigidly tested +as to their ability to play even this minor rôle. Still +another line of study that cries aloud for research is a +fuller comparison between earthquakes on the one hand +and meteorological conditions and the wandering of the +poles on the other. Finally, an extremely interesting and +hopeful quest is the determination of the positions and +movements of additional stars and other celestial bodies, +the faint and invisible as well as the bright, in order to +ascertain the probable magnitude of their influence upon +the sun and thus upon the earth at various times in the +past and in the future. Perhaps we are even now approaching +some star that will some day give rise to a +period of climatic stress like that of the fourteenth century, +or possibly to a glacial epoch. Or perhaps the variations +in others of the nearer stars as well as Alpha +Centauri may show a close relation to changes in the sun.</p> + +<p>Throughout this volume we have endeavored to discover +new truth concerning the physical environment +that has molded the evolution of all life. We have seen +how delicate is the balance among the forces of nature, +even though they be of the most stupendous magnitude. +We have seen that a disturbance of this balance in one +of the heavenly bodies may lead to profound changes in +another far away. Yet during the billion years, more or +less, of which we have knowledge, there appears never +to have been a complete cataclysm involving the destruction +of all life. One star after another, if our hypothesis +is correct, has approached the solar system closely +enough to set the atmosphere of the sun in such commotion +that great changes of climate have occurred upon +the earth. Yet never has the solar system passed so close +to any other body or changed in any other way sufficiently +<!--File: cc340.png Page 316--> +<a class="pagenum break" id="Page_316" title="pg 316"></a> +to blot out all living things. The effect of climatic +changes has always been to alter the environment and +therefore to destroy part of the life of a given time, but +with this there has invariably gone a stimulus to other +organic types. New adaptations have occurred, new lines +of evolutionary progress have been initiated, and the net +result has been greater organic diversity and richness. +Temporarily a great change of climate may seem to +retard evolution, but only for a moment as the geologist +counts time. Then it becomes evident that the march of +progress has actually been more rapid than usual. Thus +the main periods of climatic stress are the most conspicuous +milestones upon the upward path toward more varied +adaptation. The end of each such period of stress has +found the life of the world nearer to the high mentality +which reaches out to the utmost limits of space, of time, +and of thought in the search for some explanation of the +meaning of the universe. Each approach of the sun to +other bodies, if such be the cause of the major climatic +changes, has brought the organic world one step nearer +to the solution of the greatest of all problems,—the problem +of whether there is a psychic goal beyond the mental +goal toward which we are moving with ever accelerating +speed. Throughout the vast eons of geological time the +adjustment of force to force, of one body of matter to +another, and of the physical environment to the organic +response has been so delicate, and has tended so steadily +toward the one main line of mental progress that there +seems to be a purpose in it all. If the cosmic uniformity +of climate continues to prevail and if the uniformity is +varied by changes as stimulating as those of the past, the +imagination can scarcely picture the wonders of the +future. In the course of millions or even billions of years +the development of mind, and perhaps of soul, many excel +<!--File: cc341.png Page 317--> +<a class="pagenum break" id="Page_317" title="pg 317"></a> +that of today as far as the highest known type of mentality +excels the primitive plasma from which all life +appears to have arisen.</p> +<!--File: cc342.png Page 318--> +<a class="pagenum break" id="Page_318" title="pg 318"></a> + +<!--[Blank Page]--> +<!--File: cc343.png Page 319--> +<a class="pagenum break" id="Page_319" title="pg 319"></a> + + + +<h2 id="index"><a href="#content">INDEX</a></h2> + +<h3>* Indicates illustrations.</h3> + +<ul class="none"> +<li>Abbot, C. G., cited, <a href="#Page_45">45</a>, <a href="#Page_52">52</a>, <a href="#Page_237">237</a>, <a href="#Page_238">238</a>, <a href="#Page_239">239.</a></li> + +<li>Aboskun, <a href="#Page_104">104.</a></li> + +<li>Africa, earthquakes, <a href="#Page_301">301</a>; + <ul class="none"><li>East, <em>see</em> East Africa;</li> + <li>lakes, <a href="#Page_143">143;</a></li> + <li> North, <em>see</em> North Africa.</li> + </ul> + </li> + +<li>African glaciation, <a href="#Page_266">266</a>.</li> + +<li>Air, <em>see</em> Atmosphere.</li> + +<li>Alaska, glacial till in, <a href="#Page_287">287</a>; + <ul class="none"> + <li>Ice Age in, <a href="#Page_221">221.</a></li> + </ul> + </li> + +<li>Albrecht, cited, <a href="#Page_304">304</a>.</li> + +<li>Alexander, march of, <a href="#Page_88">88</a> f.</li> + +<li>Allard, H. A., cited, <a href="#Page_183">183</a>, <a href="#Page_184">184</a>.</li> + +<li>Alpha Centauri, companion of, <a href="#Page_280">280</a>; +<ul class="none"> + <li>distance from sun, <a href="#Page_262">262</a>;</li> + <li>luminosity, <a href="#Page_278">278</a>;</li> + <li>speed of, <a href="#Page_281">281</a>;</li> + <li>variations, <a href="#Page_282">282</a>.</li> +</ul> + </li> + +<li>Alps, loess in, <a href="#Page_159">159</a>; +<ul class="none"> + <li>precipitation in, <a href="#Page_141">141</a>;</li> + <li>snow level in, <a href="#Page_139">139</a>.</li> + </ul> +</li> +<li>Altair, companion of, <a href="#Page_280">280</a>; +<ul class="none"> + <li>luminosity, <a href="#Page_278">278</a>;</li> + <li>speed of, <a href="#Page_281">281</a>.</li> + </ul> +</li> + +<li>Amazon forest, temperature, <a href="#Page_17">17</a>.</li> + +<li>Ancylus lake, <a href="#Page_217">217</a>.</li> + +<li>Andes, snow line, <a href="#Page_139">139</a>.</li> + +<li>Animals, climate and, <a href="#Page_1">1</a>.</li> + +<li>Antarctica, mild climate, <a href="#Page_219">219</a>; +<ul class="none"> + <li>thickness of ice in, <a href="#Page_125">125</a>;</li> + <li>winds, <a href="#Page_135">135</a>, <a href="#Page_161">161</a>.</li> + </ul> + </li> + +<li>Anti-cyclonic hypothesis, <a href="#Page_135">135</a> ff.</li> + +<li>Appalachians, effect on ice sheet, <a href="#Page_121">121</a>.</li> + +<li>Arabia, civilization in, <a href="#Page_67">67</a>.</li> + +<li>Aral, Sea of, <a href="#Page_108">108</a>.</li> + +<li>Archean rocks, <a href="#Page_211">211</a>.</li> + +<li>Archeozoic, <a href="#Page_3">3</a> f.; +<ul class="none"> + <li>climate of, <a href="#Page_267">267</a>.</li> + </ul> + </li> + +<li>Arctic Ocean, submergence, <a href="#Page_219">219</a>.</li> + +<li>Arctowski, H., cited, <a href="#Page_29">29</a>, <a href="#Page_46">46</a>, <a href="#Page_244">244</a>.</li> + +<li>Argon, increase of, <a href="#Page_236">236</a>.</li> + +<li>Arizona, rainfall, <a href="#Page_89">89</a>, <a href="#Page_108">108</a>; +<ul class="none"> + <li>trees measured in, <a href="#Page_73">73</a>.</li> +</ul> +</li> +<li>Arrhenius, S., cited, <a href="#Page_36">36</a>, <a href="#Page_254">254</a>.</li> + +<li>Arsis, of pulsation, <a href="#Page_24">24</a>.</li> + +<li>Asbjörn Selsbane, corn of, <a href="#Page_101">101</a>.</li> + +<li>Asia, atmospheric pressure, <a href="#Page_298">298</a>; +<ul class="none"> + <li>central,changes of climate, * <a href="#Page_75">75</a>;</li> + <li>central, post-glacial climate, <a href="#Page_271">271</a>;</li> + <li>climate, <a href="#Page_66">66</a>;</li> + <li>glaciation in, <a href="#Page_131">131</a>;</li> + <li>storminess in, <a href="#Page_60">60</a>;</li> + <li>western, climate in, <a href="#Page_84">84</a> f.</li> + </ul> +</li> +<li>Atlantic Ocean, storminess, <a href="#Page_57">57</a>.</li> + +<li>Atmosphere, changes, <a href="#Page_19">19</a> f., <a href="#Page_229">229</a>; +<ul class="none"> + <li>composition of, <a href="#Page_223">223-241</a>;</li> + <li>effect on temperature, <a href="#Page_231">231</a>.</li> + </ul> +</li> +<li>Atmospheric circulation, glaciation and, <a href="#Page_42">42</a>.</li> + +<li>Atmospheric electricity, solar relations of, <a href="#Page_56">56</a>.</li> + +<li>Atmospheric pressure, earthquakes and, <a href="#Page_298">298</a>; +<ul class="none"> + <li>evaporation and, <a href="#Page_237">237</a>;</li> + <li>increase in, <a href="#Page_239">239</a>;</li> + <li>redistribution of, <a href="#Page_49">49</a>;</li> + <li>variation, <a href="#Page_53">53</a>.</li> + </ul> +</li> +<li>Australia, East, mild climate, <a href="#Page_219">219</a>; +<ul class="none"> + <li>precipitation, <a href="#Page_144">144</a>.</li> + </ul> +</li> +<li>Axis, earth's, <a href="#Page_48">48</a>; +<ul class="none"> + <li>wabbling of, <a href="#Page_301">301</a>.</li> +</ul> +</li> + +<li> </li> + +<li>Bacon, Sir Francis, cited, <a href="#Page_27">27</a>.</li> + +<li>Bacubirito, meteor at, <a href="#Page_246">246</a>.</li> + +<li>Baltic Sea, as lake, <a href="#Page_217">217</a>; +<ul class="none"> + <li>freezing of, <a href="#Page_100">100</a>;</li> + <li>ice, <a href="#Page_26">26</a>;</li> + <li>storm-floods, <a href="#Page_99">99</a>;</li> + <li>submergence, <a href="#Page_219">219</a>.</li> +</ul> +</li> + +<li>Bardsson, Ivar, <a href="#Page_106">106</a>.</li> + +<li>Barkow, cited, <a href="#Page_135">135</a>.</li> + +<li>Barometric pressure, solar relations of, <a href="#Page_56">56</a>.</li> + +<li>Barrell, J., cited, <a href="#Page_3">3</a>, <a href="#Page_200">200</a>, <a href="#Page_213">213</a>, <a href="#Page_234">234</a>.</li> + +<li>Bartoli, A. G., cited, <a href="#Page_257">257</a>.</li> + +<!--File: cc344.png--> +<!--<a class="pagenum break" id="Page_320" title="pg 320"></a>--> + +<li>Bauer, L. A., cited, <a href="#Page_150">150</a>.</li> + +<li>Beaches, under water, <a href="#Page_97">97</a>.</li> + +<li>Beadnell, H. J. L., cited, <a href="#Page_143">143</a>.</li> + +<li>Beluchistan, rainfall, <a href="#Page_89">89</a>.</li> + +<li>Bengal, Bay of, cyclones in, <a href="#Page_149">149</a>.</li> + +<li>Bengal, famine in, <a href="#Page_104">104</a> f.</li> + +<li>Berlin, rainfall and temperature, <a href="#Page_93">93</a>.</li> + +<li>Betelgeuse, <a href="#Page_259">259</a> f.; +<ul class="none"> + <li>distance from sun, <a href="#Page_262">262</a>.</li> + </ul> + + </li> + +<li>Bible, climatic evidence in, <a href="#Page_91">91</a> f.; +<ul class="none"> + <li>palms in, <a href="#Page_92">92</a>.</li> + </ul> + </li> + +<li>Binary stars, <a href="#Page_252">252</a>.</li> + +<li>Birkeland, K., cited, <a href="#Page_244">244</a>.</li> + +<li>Black Earth region, loess in, <a href="#Page_159">159</a>.</li> + +<li>Boca, Cal., correlation coefficients, <a href="#Page_83">83</a>, <a href="#Page_85">85</a>.</li> + +<li>Boltzmann, L., cited, <a href="#Page_257">257</a>.</li> + +<li>Bonneville, Lake, <a href="#Page_142">142</a>, <a href="#Page_143">143</a>.</li> + +<li>Borkum, storm-flood in, <a href="#Page_99">99</a>.</li> + +<li>Boss, L. cited, <a href="#Page_268">268</a>, <a href="#Page_269">269</a>.</li> + +<li>Botanical evidence of mild climates, <a href="#Page_167">167</a> ff.</li> + +<li>Boulders, on Irish coast, <a href="#Page_119">119</a>.</li> + +<li>Bowie, W., cited, <a href="#Page_293">293</a>.</li> + +<li>Bowman, I., cited, <a href="#Page_213">213</a>.</li> + +<li>Britain, forests, <a href="#Page_220">220</a>; +<ul class="none"> + <li>level of land, <a href="#Page_220">220</a>.</li> + </ul> + </li> + +<li>British Isles, height of land, <a href="#Page_111">111</a>; +<ul class="none"> + <li>temperature, <a href="#Page_216">216</a>.</li> + </ul> + </li> + +<li>Brooks, C. E. P., cited, <a href="#Page_115">115</a>, <a href="#Page_143">143</a>, <a href="#Page_196">196</a>, <a href="#Page_215">215</a>, <a href="#Page_225">225</a>.</li> + +<li>Brooks, C. F., cited, <a href="#Page_209">209</a>.</li> + +<li>Brown, E. W., cited, <a href="#Page_191">191</a>, <a href="#Page_244">244</a>.</li> + +<li>Brückner, E., cited, <a href="#Page_27">27</a>.</li> + +<li>Brückner periods, <a href="#Page_27">27</a> f.</li> + +<li>Bufo, habitat of, <a href="#Page_202">202</a>.</li> + +<li>Buhl stage, <a href="#Page_216">216</a>.</li> + +<li>Bull, Dr., cited, <a href="#Page_100">100</a>, <a href="#Page_101">101</a>.</li> + +<li>Butler, H. C., cited, <a href="#Page_66">66</a>, <a href="#Page_67">67</a> ff., <a href="#Page_70">70</a>, <a href="#Page_76">76</a>.</li> + +<li> </li> + +<li>California, changes of climate, * <a href="#Page_75">75</a>; +<ul class="none"> + <li>correlations of rainfall, <a href="#Page_86">86</a>;</li> + <li>measurements of sequoias in, <a href="#Page_73">73</a>, <a href="#Page_74">74</a> ff.;</li> + <li>rainfall, <a href="#Page_108">108</a>.</li> + </ul> + </li> + +<li>Cambrian period, <a href="#Page_4">4</a> f.</li> + +<li>Canada, storminess, <a href="#Page_53">53</a> f., <a href="#Page_57">57</a>; +<ul class="none"> + <li>storm tracks in, <a href="#Page_113">113</a>.</li> + </ul> + </li> + +<li>Cape Farewell, shore ice at, <a href="#Page_105">105</a>.</li> + +<li>Carbon dioxide, erosion and, <a href="#Page_119">119</a> f.; +<ul class="none"> + <li>from volcanoes, <a href="#Page_23">23</a>;</li> + <li>hypothesis, <a href="#Page_139">139</a>;</li> + <li>importance of, <a href="#Page_9">9</a>, <a href="#Page_11">11</a> f.;</li> + <li>in Permian, <a href="#Page_148">148</a>;.</li> + <li>in atmosphere, <a href="#Page_20">20</a>, <a href="#Page_96">96</a>, <a href="#Page_238">238</a>;</li> + <li>in ocean, <a href="#Page_226">226</a>;</li> + <li>nebular hypothesis and, <a href="#Page_232">232</a>;</li> + <li>theory of glaciation, <a href="#Page_36">36</a> ff.</li> + </ul> +</li> +<li>Caribbean mountains, origin of, <a href="#Page_193">193</a>.</li> + +<li>Carnegie Institution of Washington, <a href="#Page_74">74</a>.</li> + +<li>Caspian Sea, climatic stress, <a href="#Page_104">104</a>; +<ul class="none"> + <li>rainfall, <a href="#Page_107">107</a> f.;</li> + <li>rise and fall, <a href="#Page_27">27</a>;</li> + <li>ruins in, <a href="#Page_71">71</a>.</li> + </ul> + </li> + +<li>Cenozoic, climate, <a href="#Page_266">266</a>; +<ul class="none"> + <li>fossils, <a href="#Page_21">21</a>.</li> + </ul> + </li> + +<li>Central America, Maya ruins, <a href="#Page_95">95</a>.</li> + +<li>Chad, Lake, swamps of, <a href="#Page_171">171</a>.</li> + +<li>Chamberlin, R. T., cited, <a href="#Page_166">166</a>, <a href="#Page_233">233</a>, <a href="#Page_269">269</a>.</li> + +<li>Chamberlin, T. C., cited, <a href="#Page_19">19</a>, <a href="#Page_36">36</a>, <a href="#Page_38">38</a>, <a href="#Page_39">39</a>, <a href="#Page_42">42</a> f., <a href="#Page_42">48</a>, <a href="#Page_122">122</a>, <a href="#Page_125">125</a>, <a href="#Page_152">152</a>, <a href="#Page_156">156</a>, <a href="#Page_190">190</a>, <a href="#Page_195">195</a>, <a href="#Page_227">227</a>, <a href="#Page_269">269</a>.</li> + +<li>Chandler, S. C., cited, <a href="#Page_301">301</a>.</li> + +<li>Chinese earthquakes, periodicity of, <a href="#Page_245">245</a>.</li> + +<li>Chinese, sunspot observations, <a href="#Page_108">108</a> f.</li> + +<li>Chinese Turkestan, desiccation in, <a href="#Page_66">66</a>.</li> + +<li>Chronology, glacial, <a href="#Page_215">215</a>.</li> + +<li>Clarke, F. W., cited, <a href="#Page_226">226</a>, <a href="#Page_235">235</a>.</li> + +<li>Clayton, H. H., cited, <a href="#Page_173">173</a> f.</li> + +<li>Climate, effect of contraction, <a href="#Page_189">189</a> ff.; +<ul class="none"> + <li>affect of salinity, <a href="#Page_224">224</a>;</li> + <li>in history, <a href="#Page_64">64-97</a>;</li> + <li>uniformity, <a href="#Page_1">1-15</a>;</li> + <li>variability, <a href="#Page_16">16-32</a>.</li> + </ul> + </li> + +<li>Climates, mild, causes of, <a href="#Page_166">166-187</a>; +<ul class="none"> + <li>mild, periods of, <a href="#Page_274">274</a>.</li> + </ul> + </li> + +<li>Climatic changes, and crustal movements, <a href="#Page_285">285</a> ff.; +<ul class="none"> + <li>hypotheses of, <a href="#Page_33">33-50</a>;</li> + <li>mountain-building and, * <a href="#Page_25">25</a>;</li> + <li>post-glacial crustal movements and, <a href="#Page_215">215-222</a>;</li> + <li>terrestrial causes of, <a href="#Page_188">188-214</a>.</li> + </ul> + </li> + +<li>Climatic sequence, <a href="#Page_16">16</a> f.</li> + +<li>Climatic stages, post-glacial, <a href="#Page_270">270</a>.</li> + +<li>Climatic stress, in fourteenth century, <a href="#Page_98">98-109</a>.</li> + +<li>Climatic uniformity, hypothesis of, <a href="#Page_65">65</a>, <a href="#Page_71">71</a> f.</li> + +<li>Climatic zoning, <a href="#Page_169">169</a>.</li> + +<!--File: cc345.png--> +<!--<a class="pagenum break" id="Page_321" title="pg 321"></a>--> + +<li>Cloudiness, glaciation and, <a href="#Page_114">114</a>, <a href="#Page_147">147</a>.</li> + +<li>Clouds, as protection, <a href="#Page_197">197</a>.</li> + +<li>Colfax, Cal., correlation coefficients, <a href="#Page_83">83</a>.</li> + +<li>Cologne, flood at, <a href="#Page_99">99</a>.</li> + +<li>Compass, variations, <a href="#Page_150">150</a>.</li> + +<li>Continental climate, variations, <a href="#Page_103">103</a>.</li> + +<li>Continents, effect on climate, <a href="#Page_111">111</a> f.</li> + +<li>Contraction, effect on climate, <a href="#Page_189">189</a> ff., <a href="#Page_199">199</a>, <a href="#Page_207">207</a>; +<ul class="none"> + <li>effect on lands, <a href="#Page_207">207</a>;</li> + <li>heat of sun and, <a href="#Page_13">13</a> f.;</li> + <li>irregular, <a href="#Page_195">195</a>;</li> + <li>of the earth, <a href="#Page_18">18</a>;</li> + <li>of the sun, <a href="#Page_249">249</a>;</li> + <li>stresses caused by, <a href="#Page_310">310</a>.</li> + </ul> + </li> + +<li>Convection, carbon dioxide and, <a href="#Page_239">239</a>.</li> + +<li>Corals, in high latitudes, <a href="#Page_21">21</a>, <a href="#Page_39">39</a>, <a href="#Page_167">167</a>, <a href="#Page_178">178</a>.</li> + +<li>Cordeiro, F. J. B., cited, <a href="#Page_181">181</a>, <a href="#Page_183">183</a>, <a href="#Page_186">186</a>.</li> + +<li>Correlation coefficients, earthquakes and sunspots, <a href="#Page_291">291</a>; +<ul class="none"> + <li>Jerusalem rainfall and sequoia growth, <a href="#Page_83">83</a> ff.;</li> + <li>rainfall and tree growth, <a href="#Page_79">79</a> ff.</li> + </ul> + </li> + +<li>Cosmos, effect of light, <a href="#Page_185">185</a>.</li> + +<li>Cressey, G. B., cited, <a href="#Page_80">80</a>.</li> + +<li>Cretaceous, lava, <a href="#Page_211">211</a>; +<ul class="none"> + <li>mountain ranges, <a href="#Page_44">44</a>;</li> + <li>paleogeography, * <a href="#Page_201">201</a>;</li> + <li>submergence of North America, <a href="#Page_200">200</a>.</li> + </ul> + </li> + +<li>Croll, J., cited, <a href="#Page_34">34</a> ff., <a href="#Page_176">176</a>.</li> + +<li>Croll's hypothesis, snow line, <a href="#Page_139">139</a>.</li> + +<li>Crust, climate and movements of, <a href="#Page_63">63</a>, <a href="#Page_287">287</a>, <a href="#Page_310">310</a>; +<ul class="none"> + <li>movements of, <a href="#Page_43">43</a>;</li> + <li>strains in, <a href="#Page_22">22</a>.</li> + </ul> + </li> + +<li>Currents and planetary winds, <a href="#Page_174">174</a>.</li> + +<li>Cycads, <a href="#Page_169">169</a>.</li> + +<li>Cyclonic hypothesis, <a href="#Page_97">97</a>; +<ul class="none"> + <li>loess and, <a href="#Page_163">163</a>;</li> + <li>Permian glaciation and, <a href="#Page_148">148</a>;</li> + <li>snow line, <a href="#Page_139">139</a>.</li> + </ul> + </li> + +<li>Cyclonic storms, in glacial epochs, <a href="#Page_140">140</a> f.; +<ul class="none"> + <li>solar electricity and, <a href="#Page_243">243</a> (<em>see</em> Storms, Storminess).</li> + </ul> + </li> + +<li>Cyclonic vacillations, <a href="#Page_30">30</a> f.; +<ul class="none"> + <li>nature of, <a href="#Page_57">57</a> ff.</li> + </ul> + </li> + +<li> </li> + +<li>Daily vibrations, <a href="#Page_28">28</a> f.</li> + +<li>Danube, frozen, <a href="#Page_98">98</a>.</li> + +<li>Darwin, G. H., cited, <a href="#Page_191">191</a>.</li> + +<li>Daun stage, <a href="#Page_217">217</a>.</li> + +<li>Davis, W. M., cited, <a href="#Page_271">271</a>.</li> + +<li>Davisson, C., cited, <a href="#Page_294">294</a>, <a href="#Page_295">295</a>, <a href="#Page_299">299</a>.</li> + +<li>Day, C. P., cited, <a href="#Page_239">239</a>.</li> + +<li>Day, length of, <a href="#Page_18">18</a>, <a href="#Page_191">191</a>.</li> + +<li>Dead Sea, palms near, <a href="#Page_92">92</a>.</li> + +<li>Death Valley, <a href="#Page_142">142</a>.</li> + +<li>De Ballore, M., cited, <a href="#Page_297">297</a>, <a href="#Page_298">298</a>.</li> + +<li>Deep-sea circulation, rapidity, <a href="#Page_227">227</a>; +<ul class="none"> + <li>salinity and, <a href="#Page_176">176</a>;</li> + <li>solar activity and, <a href="#Page_179">179</a>.</li> + </ul> + </li> + +<li>De Geer, S., cited, <a href="#Page_215">215</a>, <a href="#Page_221">221</a>.</li> + +<li>De Lapparent, A., cited, <a href="#Page_200">200</a>.</li> + +<li>Denmark, fossils, <a href="#Page_271">271</a>.</li> + +<li>"Desert pavements," <a href="#Page_161">161</a>.</li> + +<li>Deserts, abundant flora of, <a href="#Page_171">171</a>; +<ul class="none"> + <li>and pulsations theory, <a href="#Page_88">88</a> ff.;</li> + <li>red beds of, <a href="#Page_170">170</a>.</li> + </ul> + </li> + +<li>Devonian, climate, <a href="#Page_266">266</a>; +<ul class="none"> + <li>mountains, <a href="#Page_209">209</a>.</li> + </ul> + </li> + +<li>Dog, climate and, <a href="#Page_1">1</a>.</li> + +<li>Donegal County, Ireland, <a href="#Page_220">220</a>.</li> + +<li>Double stars, <a href="#Page_272">272</a>, <a href="#Page_280">280</a>; +<ul class="none"> + <li>electrical effect of, <a href="#Page_261">261</a>.</li> + </ul> + </li> + +<li>Douglass, A. E., cited, <a href="#Page_28">28</a>, <a href="#Page_73">73</a>, <a href="#Page_74">74</a> f., <a href="#Page_84">84</a>, <a href="#Page_85">85</a>, <a href="#Page_107">107</a>.</li> + +<li>Dragon Town, destruction of, <a href="#Page_104">104</a>, <a href="#Page_108">7108</a>.</li> + +<li>Drake, N. F., cited, <a href="#Page_297">297</a>, <a href="#Page_298">298</a>.</li> + +<li>Droughts, and pulsations theory, <a href="#Page_87">87</a> f.; +<ul class="none"> + <li>in England, <a href="#Page_102">102</a>;</li> + <li>in India, <a href="#Page_104">104</a> f.</li> + </ul> + </li> + +<li>Drumkelin Bog, Ireland, log cabin in, <a href="#Page_220">220</a>.</li> + +<li>Dust, at high levels, <a href="#Page_240">240</a>.</li> + +<li> </li> + +<li>Earth, crust of and the sun, <a href="#Page_285">285-317</a>; +<ul class="none"> + <li>internal heat, <a href="#Page_212">212</a>;</li> + <li>nature of mild climate, <a href="#Page_274">274</a>;</li> + <li>position of axis, <a href="#Page_181">181</a>;</li> + <li>rigidity of, <a href="#Page_307">307</a>;</li> + <li>temperature gradient, <a href="#Page_213">213</a>;</li> + <li>temperature of surface, <a href="#Page_8">8</a>.</li> + </ul> + </li> + +<li>Earthquakes, and seasons, <a href="#Page_294">294</a>, <a href="#Page_297">297</a>; +<ul class="none"> + <li>and sunspots, <a href="#Page_288">288</a> f.;</li> + <li>and tropical hurricanes, <a href="#Page_300">300</a>;</li> + <li>and wandering of pole, <a href="#Page_304">304</a> f.;</li> + <li>cause of, <a href="#Page_307">307</a>;</li> + <li>compared with departures from Eulerian position, <a href="#Page_306">306</a>;</li> + <li>seasonal distribution of, <a href="#Page_299">299</a>;</li> + <li>seasonal march, <a href="#Page_295">295</a>.</li> + </ul> + </li> + +<li>"Earthquake weather," <a href="#Page_298">298</a>.</li> + +<!--File: cc346.png--> +<!--<a class="pagenum break" id="Page_322" title="pg 322"></a>--> + +<li>East Africa, mild climate, <a href="#Page_219">219</a>.</li> + +<li>East Indies, earthquakes of, <a href="#Page_296">296</a>.</li> + +<li>Eberswalde, tree growth at, <a href="#Page_102">102</a> f.</li> + +<li>Ecliptic, obliquity of, <a href="#Page_217">217</a>.</li> + +<li>Electrical currents, in solar atmosphere, <a href="#Page_261">261</a>.</li> + +<li>Electrical emissions, variation of, <a href="#Page_275">275</a>.</li> + +<li>Electrical hypothesis, <a href="#Page_150">150</a>, <a href="#Page_250">250</a> f., <a href="#Page_256">256</a> ff.</li> + +<li>Electrical phenomena, storminess and, <a href="#Page_56">56</a>.</li> + +<li>Electricity, and earthquakes, <a href="#Page_292">292</a>; +<ul class="none"> + <li>solar, <a href="#Page_243">243</a>.</li> + </ul> + </li> + +<li>Electro-magnetic hypothesis, <a href="#Page_244">244</a>.</li> + +<li>Electrons, solar, <a href="#Page_56">56</a>; +<ul class="none"> + <li>variation of, <a href="#Page_256">256</a>.</li> + </ul> + </li> + +<li>Electro-stellar hypothesis, <a href="#Page_274">274</a>.</li> + +<li>Elevation, climatic changes and, <a href="#Page_39">39</a>.</li> + +<li>Engedi, palms in, <a href="#Page_92">92</a>.</li> + +<li>England, climatic stress, <a href="#Page_101">101</a> f.; +<ul class="none"> + <li>storminess and rainfall, <a href="#Page_107">107</a>.</li> + </ul> + </li> + +<li>Eocene, climate, <a href="#Page_266">266</a>.</li> + +<li>Equinoxes, precession of, <a href="#Page_96">96</a>.</li> + +<li>Erosion, storminess and, <a href="#Page_309">309</a>.</li> + +<li>Eskimo, in Greenland, <a href="#Page_106">106</a>.</li> + +<li>Eulerian movement, <a href="#Page_301">301</a>, <a href="#Page_304">304</a>.</li> + +<li>Euphrates, <a href="#Page_67">67</a>.</li> + +<li>Europe, climatic stress, <a href="#Page_98">98</a> ff., <a href="#Page_102">102</a> f.; +<ul class="none"> + <li>climatic table, <a href="#Page_215">215</a>;</li> + <li>glaciation in, <a href="#Page_131">131</a>,</li> + <li>ice sheet, <a href="#Page_121">121</a>;</li> + <li>inundations of rivers, <a href="#Page_99">99</a>;</li> + <li>post-glacial climate, <a href="#Page_271">271</a>;</li> + <li>rainfall, <a href="#Page_107">107</a>;</li> + <li>submergence, <a href="#Page_196">196</a>, <a href="#Page_200">200</a>.</li> + </ul> + </li> + +<li>Evaporation, and glaciation, <a href="#Page_112">112</a>, <a href="#Page_114">114</a>; +<ul class="none"> + <li>atmospheric pressure and, <a href="#Page_237">237</a>;</li> + <li>from plants, <a href="#Page_179">179</a>;</li> + <li>importance, <a href="#Page_129">129</a>;</li> + <li>in trade-wind belt, <a href="#Page_117">117</a>;</li> + <li>rapidity of, <a href="#Page_224">224</a>.</li> + </ul> + </li> + +<li>Evening primrose, effect of light, <a href="#Page_184">184</a>.</li> + +<li>Evolution, climate and, <a href="#Page_20">20</a>; +<ul class="none"> + <li>geographical complexity and, <a href="#Page_241">241</a>;</li> + <li>glaciation and, <a href="#Page_33">33</a>;</li> + <li>of the earth, <a href="#Page_311">311</a>.</li> + </ul> + </li> + +<li> </li> + +<li>Faculæ, cause of, <a href="#Page_61">61</a>.</li> + +<li>False Point Lighthouse, barometric pressure at, <a href="#Page_299">299</a>.</li> + +<li>Famine, cause of, <a href="#Page_103">103</a>; +<ul class="none"> + <li>in England, <a href="#Page_101">101</a> f.;</li> + <li>in India, <a href="#Page_104">104</a> f.;</li> + <li>pulsations theory and, <a href="#Page_87">87</a> f.</li> + </ul> + </li> + +<li>Faunas, and mild climates, <a href="#Page_168">168</a> f.; +<ul class="none"> + <li>in Permian, <a href="#Page_152">152</a> f.</li> + </ul> + </li> + +<li>Fennoscandian pause, <a href="#Page_216">216</a>.</li> + +<li>Flowering, light and, <a href="#Page_184">184</a>.</li> + +<li>Fog, and glaciation, <a href="#Page_116">116</a>; +<ul class="none"> + <li>as protection, <a href="#Page_197">197</a>;</li> + <li>temperature and, <a href="#Page_178">178</a>.</li> + </ul> + </li> + +<li>Forests, climate and, <a href="#Page_66">66</a>.</li> + +<li>Form of the land, <a href="#Page_43">43</a> ff.</li> + +<li>Fossil floras, and mild climates, <a href="#Page_168">168</a>; +<ul class="none"> + <li>in Antarctica, <a href="#Page_273">273</a>;</li> + <li>in Greenland, <a href="#Page_273">273</a>.</li> + </ul> + </li> + +<li>Fossils, <a href="#Page_169">169</a>, <a href="#Page_230">230</a>; +<ul class="none"> + <li>and loess, <a href="#Page_158">158</a>;</li> + <li>Archeozoic, <a href="#Page_3">3</a> f.;</li> + <li>Cenozoic, <a href="#Page_21">21</a>;</li> + <li>dating of, <a href="#Page_153">153</a>;</li> + <li>glaciation and, <a href="#Page_138">138</a>;</li> + <li>in peat bogs, <a href="#Page_271">271</a>;</li> + <li>mild climate, <a href="#Page_167">167</a>;</li> + <li>Proterozoic, <a href="#Page_4">4</a>, <a href="#Page_6">6</a> f.</li> + </ul> + </li> + +<li>Fourteenth century, climatic stress in, <a href="#Page_98">98</a>-109.</li> + +<li>Fowle, F. E., cited, <a href="#Page_45">45</a>, <a href="#Page_237">237</a>, <a href="#Page_238">238</a>, <a href="#Page_239">239</a>.</li> + +<li>Frech, F., cited, <a href="#Page_36">36</a>.</li> + +<li>Free, E. E., cited, <a href="#Page_142">142</a>.</li> + +<li>Freezing, salinity and, <a href="#Page_224">224</a>.</li> + +<li>Fresno, rainfall record, <a href="#Page_82">82</a>.</li> + +<li>"Friction variables," <a href="#Page_247">247</a>.</li> + +<li>Frisian Islands, storm-flood, <a href="#Page_99">99</a>.</li> + +<li>Fritz, H., cited, <a href="#Page_109">109</a>.</li> + +<li>Frogs, distribution of, <a href="#Page_202">202</a>.</li> + +<li>Fuchs, cited, <a href="#Page_289">289</a>.</li> + +<li> </li> + +<li>Galaxy, <a href="#Page_252">252</a>.</li> + +<li>Galveston, Tex., rainfall and temperature, <a href="#Page_94">94</a>.</li> + +<li>Garner, W. W., cited, <a href="#Page_183">183</a>, <a href="#Page_184">184</a>.</li> + +<li>Gasses, in air, <a href="#Page_233">233</a>.</li> + +<li>Geographers, and climatic changes, <a href="#Page_65">65</a> ff.</li> + +<li>Geological time table, * <a href="#Page_5">5</a>.</li> + +<li>Geologic oscillations, <a href="#Page_18">18</a> f., <a href="#Page_21">21</a> ff., <a href="#Page_188">188</a>, <a href="#Page_240">240</a>.</li> + +<li>Geologists, changes in ideas of, <a href="#Page_64">64</a> f.</li> + +<li>Germanic myths, <a href="#Page_219">219</a>.</li> + +<li>Germany, forests, <a href="#Page_220">220</a>; +<ul class="none"> + <li>growth of trees in, <a href="#Page_102">102</a>;</li> + <li>storms in, <a href="#Page_102">102</a>.</li> + </ul> + </li> + +<li>Gilbert, G. K., cited, <a href="#Page_143">143</a>.</li> + +<!--File: cc347.png--> +<!--<a class="pagenum break" id="Page_323" title="pg 323"></a>--> + +<li>Glacial epochs, causes of, <a href="#Page_268">268</a>; +<ul class="none"> + <li>dates of, <a href="#Page_216">216</a>;</li> + <li>intervals between, <a href="#Page_264">264</a> f.;</li> + <li>length of, <a href="#Page_166">166</a> f.</li> + </ul> + </li> + +<li>Glacial fluctuations, <a href="#Page_24">24</a> ff.; +<ul class="none"> + <li>nature of, <a href="#Page_57">57</a> ff.</li> + </ul> + </li> + +<li>Glacial period, at present, <a href="#Page_272">272</a>; +<ul class="none"> + <li>ice in, <a href="#Page_57">57</a> f.;</li> + <li>length of, <a href="#Page_269">269</a>;</li> + <li>list, <a href="#Page_265">265</a>;</li> + <li>temperature, <a href="#Page_38">38</a>.</li> + </ul> + </li> + +<li>Glaciation, and loess, <a href="#Page_155">155</a> f.; +<ul class="none"> + <li>and movement of crust, <a href="#Page_287">287</a>;</li> + <li>conditions favorable for, <a href="#Page_111">111</a>,</li> + <li>extent of, <a href="#Page_124">124</a>;</li> + <li>hypotheses of, <a href="#Page_33">33</a> ff.;</li> + <li>in southern Canada, <a href="#Page_18">18</a>;</li> + <li>localization of, <a href="#Page_130">130</a> ff.;</li> + <li>Permian, * <a href="#Page_145">145</a>;</li> + <li>solar-cyclonic hypothesis of, <a href="#Page_110">110-129</a>;</li> + <li>suddenness of, <a href="#Page_138">138</a>;</li> + <li>upper limit of, <a href="#Page_141">141</a>.</li> + </ul> + </li> + +<li>Goldthwait, J. W., cited, <a href="#Page_271">271</a>.</li> + +<li>Gondwana land, <a href="#Page_21">21</a>, <a href="#Page_204">204</a>.</li> + +<li>Gravitation, effect on sun, <a href="#Page_250">250</a>; +<ul class="none"> + <li>pull of, <a href="#Page_244">244</a>.</li> + </ul> + </li> + +<li>Great Basin, in glacial period, <a href="#Page_126">126</a>; +<ul class="none"> + <li>salt lakes in, <a href="#Page_142">142</a>.</li> + </ul> + </li> + +<li>Great Ice Age, see Pleistocene.</li> + +<li>Great Plains, effect on ice sheet, <a href="#Page_120">120</a>.</li> + +<li>Greenland, climatic stress, <a href="#Page_105">105</a> ff.; +<ul class="none"> + <li>ice, <a href="#Page_26">26</a>;</li> + <li>rainfall, <a href="#Page_108">7108</a>;</li> + <li>storminess, <a href="#Page_57">57</a>;</li> + <li>submergence, <a href="#Page_219">219</a>;</li> + <li>vegetation, <a href="#Page_21">21</a>, <a href="#Page_37">37</a>, <a href="#Page_287">287</a>;</li> + <li>winds, <a href="#Page_135">135</a>, <a href="#Page_161">161</a>.</li> + </ul> + </li> + +<li>Gregory, J. W., cited, <a href="#Page_90">90</a> ff., <a href="#Page_97">97</a>.</li> + +<li>Gschnitz stage, <a href="#Page_216">216</a>.</li> + +<li>Guatemala, ruins in, <a href="#Page_95">95</a>.</li> + +<li>Guervain, cited, <a href="#Page_135">135</a>.</li> + +<li>Gyroscope, earth as, <a href="#Page_181">181</a>.</li> + +<li> </li> + +<li>Hale, G. E., cited, <a href="#Page_56">56</a>, <a href="#Page_62">62</a>.</li> + +<li>Hamdulla, cited, <a href="#Page_104">104</a>.</li> + +<li>Hann, J., cited, <a href="#Page_66">66</a>.</li> + +<li>Hansa Union, operations of, <a href="#Page_100">100</a>.</li> + +<li>Harmer, F. W., cited, <a href="#Page_115">115</a>, <a href="#Page_119">119</a>.</li> + +<li>Heat, and earthquakes, <a href="#Page_292">292</a>; +<ul class="none"> + <li>earth's internal, <a href="#Page_18">18</a>.</li> + </ul> + </li> + +<li>Hedin, S., cited, <a href="#Page_88">88</a>.</li> + +<li>Heim, A., cited, <a href="#Page_190">190</a>.</li> + +<li>Heligoland, flood in, <a href="#Page_99">99</a>.</li> + +<li>Helland-Hansen, B., cited, <a href="#Page_174">174</a>.</li> + +<li>Helmert, F. R., cited, <a href="#Page_302">302</a>.</li> + +<li>Henderson, L. J., cited, <a href="#Page_9">9</a>, <a href="#Page_10">10</a>, <a href="#Page_11">11</a>, <a href="#Page_12">12</a>.</li> + +<li>Henry, A. J., cited, <a href="#Page_94">94</a>, <a href="#Page_208">208</a>.</li> + +<li>Hercynian Mountains, <a href="#Page_45">45</a>.</li> + +<li>High pressure and glaciation, <a href="#Page_115">115</a>, <a href="#Page_135">135</a>.</li> + +<li>Himalayas, glaciation, <a href="#Page_144">144</a>; +<ul class="none"> + <li>origin of, <a href="#Page_193">193</a>,</li> + <li>snow line, <a href="#Page_139">139</a>.</li> + </ul> + </li> + +<li>Himley, cited, <a href="#Page_104">104</a>.</li> + +<li>Historic pulsations, <a href="#Page_24">24</a> f.; +<ul class="none"> + <li>nature of, <a href="#Page_57">57</a> ff.</li> + </ul> + </li> + +<li>History, climate of, <a href="#Page_64">64</a>-97; +<ul class="none"> + <li>climatic pulsations and, <a href="#Page_26">26</a>.</li> + </ul> + </li> + +<li>Hobbs, W. H., cited, <a href="#Page_115">115</a>, <a href="#Page_125">125</a>, <a href="#Page_135">135</a>, <a href="#Page_161">161</a>.</li> + +<li>Hot springs, temperature of, <a href="#Page_6">6</a>.</li> + +<li>Humphreys, W. J., cited, <a href="#Page_2">2</a>, <a href="#Page_37">37</a> f., <a href="#Page_45">45</a>, <a href="#Page_46">46</a>, <a href="#Page_50">50</a>, <a href="#Page_56">56</a>, <a href="#Page_238">238</a>.</li> + +<li>Hurricanes, in arid regions, <a href="#Page_144">144</a>; +<ul class="none"> + <li>sunspots and, <a href="#Page_53">53</a>.</li> + </ul> + </li> + +<li>Hyades, cluster in, <a href="#Page_268">268</a>.</li> + +<li> </li> + +<li>Ice, accumulations, <a href="#Page_57">57</a> f.; +<ul class="none"> + <li>advances of, <a href="#Page_122">122</a>;</li> + <li>distribution of, <a href="#Page_131">131</a>,</li> + <li>drift, <a href="#Page_105">105</a>.</li> + </ul> + </li> + +<li>Ice sheets, disappearance, <a href="#Page_128">128</a>; +<ul class="none"> + <li>limits, <a href="#Page_120">120</a>;</li> + <li>localization, <a href="#Page_130">130</a> ff.;</li> + <li>rate of retreat, <a href="#Page_165">165</a>;</li> + <li>thickness, <a href="#Page_125">125</a>.</li> + </ul> + </li> + +<li>Iceland, submergence, <a href="#Page_219">219</a>.</li> + +<li>Iowan ice sheet, rapid retreat, <a href="#Page_165">165</a>.</li> + +<li>Iowan loess, <a href="#Page_158">158</a>.</li> + +<li>India, drought, <a href="#Page_104">104</a> f.; +<ul class="none"> + <li>famine, <a href="#Page_104">104</a> f.;</li> + <li>rainfall, <a href="#Page_108">7108</a>.</li> + </ul> + </li> + +<li>Indian glaciation, <a href="#Page_266">266</a>.</li> + +<li>Inter-glacial epoch, Permian, <a href="#Page_153">153</a>.</li> + +<li>Internal heat of earth, <a href="#Page_212">212</a>.</li> + +<li>Ireland, Drumkelin Bog, <a href="#Page_220">220</a>; +<ul class="none"> + <li>in glacial period, <a href="#Page_119">119</a>;</li> + <li>level of land, <a href="#Page_220">220</a>;</li> + <li>storminess and rainfall, <a href="#Page_107">107</a>;</li> + <li>submergence, <a href="#Page_219">219</a>.</li> + </ul> + </li> + +<li>Irish Sea, tides, <a href="#Page_191">191</a>.</li> + +<li>Irrigation ditches, abandoned, <a href="#Page_97">97</a>.</li> + +<li>Isostasy, <a href="#Page_307">307</a> ff.</li> + +<li>Italy, southern, climate of, <a href="#Page_86">86</a> f.</li> + +<li> </li> + +<li>Japan, earthquakes of, <a href="#Page_296">296</a>.</li> + +<li>Javanese mountains, origin of, <a href="#Page_193">193</a>.</li> + +<li>Jaxartes, <a href="#Page_108">7108</a>.</li> + +<li>Jeans, J. H., cited, <a href="#Page_251">251</a>, <a href="#Page_252">252</a>, <a href="#Page_253">253</a>, <a href="#Page_266">266</a>, <a href="#Page_272">272</a>.</li> + +<!--File: cc348.png--> +<!--<a class="pagenum break" id="Page_324" title="pg 324"></a>--> + +<li>Jeffreys, H., cited, <a href="#Page_302">302</a>, <a href="#Page_303">303</a>, <a href="#Page_306">306</a>.</li> + +<li>Jeffreys, J., cited, <a href="#Page_191">191</a>.</li> + +<li>Jericho, palms in, <a href="#Page_92">92</a>.</li> + +<li>Jerusalem, rainfall, <a href="#Page_86">86</a>; +<ul class="none"> + <li>rainfall and temperature, <a href="#Page_94">94</a>;</li> + <li>rainfall in, and sequoia growth, <a href="#Page_83">83</a> ff.</li> + </ul> + </li> + +<li>Johnson, cited, <a href="#Page_226">226</a>.</li> + +<li>Judea, palms in, <a href="#Page_92">92</a>.</li> + +<li>Jupiter, and sunspots, <a href="#Page_243">243</a>; +<ul class="none"> + <li>effect of, <a href="#Page_253">253</a>;</li> + <li>periodicity of, <a href="#Page_61">61</a> f.;</li> + <li>temperature of, <a href="#Page_258">258</a>;</li> + <li>tidal effect of, <a href="#Page_250">250</a>.</li> + </ul> + </li> + +<li>Jurassic, climate, <a href="#Page_266">266</a>; +<ul class="none"> + <li>mountain ranges, <a href="#Page_44">44</a>.</li> + </ul> + </li> + +<li> </li> + +<li>Kansas, variations of seasons, <a href="#Page_103">103</a>.</li> + +<li>Kara Koshun marsh, Lop Nor, <a href="#Page_104">104</a>.</li> + +<li>Keewatin center, <a href="#Page_113">113</a>; +<ul class="none"> + <li>evaporation in, <a href="#Page_129">129</a>.</li> + </ul> + </li> + +<li>Keewatin ice sheet, <a href="#Page_121">121</a>.</li> + +<li>Kelvin, Lord, cited, <a href="#Page_13">13</a> f.</li> + +<li>Keyes, C. R., cited, <a href="#Page_156">156</a>.</li> + +<li>Kirk, E., cited, <a href="#Page_287">287</a>.</li> + +<li>Knott, C. G., cited, <a href="#Page_294">294</a>, <a href="#Page_295">295</a>, <a href="#Page_297">297</a>, <a href="#Page_299">299</a>, <a href="#Page_304">304</a>, <a href="#Page_306">306</a>.</li> + +<li>Knowlton, F. H., cited <a href="#Page_167">167</a>, <a href="#Page_169">169</a>, <a href="#Page_170">170</a>, <a href="#Page_212">212</a>, <a href="#Page_232">232</a>.</li> + +<li>Köppen, W.,, <a href="#Page_52">52</a>, <a href="#Page_140">140</a>.</li> + +<li>Krakatoa, glaciation and, <a href="#Page_48">48</a>; +<ul class="none"> + <li>volcanic hypothesis and, <a href="#Page_45">45</a>.</li> + </ul> + </li> + +<li>Krümmel, O., cited, <a href="#Page_224">224</a>, <a href="#Page_228">228</a>.</li> + +<li>Kullmer, C. J., cited, <a href="#Page_113">113</a>, <a href="#Page_115">115</a>, <a href="#Page_128">128</a>; +<ul class="none"> + <li>map of storminess, * <a href="#Page_54">54</a>.</li> + </ul> + </li> + +<li><em>Kungaspegel</em>, sea routes described, <a href="#Page_106">106</a>.</li> + +<li> </li> + +<li>Labor, price in England, <a href="#Page_102">102</a>.</li> + +<li>Labradorean center of glaciation, <a href="#Page_113">113</a>.</li> + +<li>Lahontan, Lake, <a href="#Page_142">142</a>.</li> + +<li>Lake strands, <em>see</em> Strands.</li> + +<li>Lake Superior, lava, <a href="#Page_211">211</a>.</li> + +<li>Lakes, during glacial periods, <a href="#Page_141">141</a> f.; +<ul class="none"> + <li>in semi-arid regions, <a href="#Page_60">60</a>;</li> + <li>of Great Basin, <a href="#Page_126">126</a>;</li> + <li>ruins in, <a href="#Page_97">97</a>.</li> + </ul> + </li> + +<li>Land, and water, climatic effect of, <a href="#Page_196">196</a> ff.; +<ul class="none"> + <li>distribution of, <a href="#Page_200">200</a>, form of, <a href="#Page_43">43</a> ff.;</li> + <li>range of temperature and, <a href="#Page_196">196</a>.</li> + </ul> + </li> + +<li>Lavas, climatic effect of, <a href="#Page_211">211</a>.</li> + +<li>Lawson, A. C., cited, <a href="#Page_310">310</a>.</li> + +<li>Lebanon, cedars of, <a href="#Page_83">83</a>.</li> + +<li>Leiter, H., cited, <a href="#Page_71">71</a>.</li> + +<li>Leverett, F., cited, <a href="#Page_271">271</a>.</li> + +<li>Life, atmosphere and, <a href="#Page_229">229</a> f.; +<ul class="none"> + <li>chemical characteristic of, <a href="#Page_12">12</a>;</li> + <li>effect of salinity, <a href="#Page_225">225</a>;</li> + <li>of glacial period, <a href="#Page_127">127</a>;</li> + <li>persistence of forms, <a href="#Page_230">230</a>.</li> + </ul> + </li> + +<li>Light, effect of atmosphere on, <a href="#Page_236">236</a>; +<ul class="none"> + <li>effect on plants, <a href="#Page_184">184</a> ff.;</li> + <li>ultra-violet, storminess and, <a href="#Page_56">56</a>;</li> + <li>variation of, <a href="#Page_275">275</a>.</li> + </ul> + </li> + +<li>Litorina sea, <a href="#Page_218">218</a>.</li> + +<li>Loess, date of, <a href="#Page_156">156</a> ff.; +<ul class="none"> + <li>origin of, <a href="#Page_155">155</a>, <a href="#Page_165">165</a>.</li> + </ul> + </li> + +<li>Lop Nor, rise of, <a href="#Page_104">104</a>; +<ul class="none"> + <li>swamps, <a href="#Page_171">171</a>.</li> + </ul> + </li> + +<li>Lows, and glacial lobes, <a href="#Page_122">122</a>; +<ul class="none"> + <li>movements of, <a href="#Page_126">126</a>;</li> + <li>see Storms and Cyclones.</li> + </ul> + </li> + +<li>Lulan, <a href="#Page_104">104</a>.</li> + +<li>Lull, R. S., cited, <a href="#Page_5">5</a>, <a href="#Page_188">188</a>.</li> + +<li> </li> + +<li>MacDougal, D. T., cited, <a href="#Page_171">171</a>.</li> + +<li>McGee, W. J., cited, <a href="#Page_156">156</a>.</li> + +<li>Macmillan, W. D., cited, <a href="#Page_191">191</a>.</li> + +<li>Magdalenian period, <a href="#Page_216">216</a>.</li> + +<li>Magnetic fields of sunspots, <a href="#Page_56">56</a>.</li> + +<li>Magnetic poles, relation to storm tracks, <a href="#Page_150">150</a>.</li> + +<li>Makran, climate, <a href="#Page_89">89</a>; +<ul class="none"> + <li>rainfall, <a href="#Page_89">89</a>.</li> + </ul> + </li> + +<li>Malay Archipelago, earthquakes of, <a href="#Page_296">296</a>.</li> + +<li>Mallet, R., cited, <a href="#Page_288">288</a>.</li> + +<li>Malta, rainfall, <a href="#Page_86">86</a>.</li> + +<li>Manson, M., cited, <a href="#Page_147">147</a>.</li> + +<li>Mayas, civilization, <a href="#Page_26">26</a>; +<ul class="none"> + <li>ruins, <a href="#Page_95">95</a>.</li> + </ul> + </li> + +<li>Mayence, flood at, <a href="#Page_99">99</a>.</li> + +<li>Mazelle, E., cited, <a href="#Page_224">224</a>.</li> + +<li>Mediterranean, climate of, <a href="#Page_72">72</a>; +<ul class="none"> + <li>rainfall records, <a href="#Page_86">86</a>;</li> + <li>storminess in, <a href="#Page_60">60</a>.</li> + </ul> + </li> + +<li>Mercury, and sunspots, <a href="#Page_243">243</a>.</li> + +<li>Mesozoic, climate, <a href="#Page_266">266</a>; +<ul class="none"> + <li>crustal changes, <a href="#Page_286">286</a>;</li> + <li>emergence of lands, <a href="#Page_287">287</a>.</li> + </ul> + </li> + +<li>Messier, <a href="#Page_8">8</a>; +<ul class="none"> + <li>variables, <a href="#Page_248">248</a>.</li> + </ul> + </li> + +<li>Metcalf, M. M., cited, <a href="#Page_202">202</a>.</li> + +<li>Meteorological factors and earthquakes, <a href="#Page_300">300</a> f.</li> + +<!--File: cc349.png--> +<!--<a class="pagenum break" id="Page_325" title="pg 325"></a>--> + +<li>Meteorological hypothesis of crustal movements, <a href="#Page_294">294</a>.</li> + +<li>Meteors, and sun's heat, <a href="#Page_13">13</a>, <a href="#Page_246">246</a>.</li> + +<li>Michelson, A. A., cited, <a href="#Page_259">259</a>.</li> + +<li>Middle Silurian, fauna in Alaska, <a href="#Page_287">287</a>.</li> + +<li>Mild climates, <em>see</em> Climates, mild.</li> + +<li>Milky Way, <a href="#Page_252">252</a>.</li> + +<li>Mill, H. R., cited, <a href="#Page_228">228</a>.</li> + +<li>Milne, J., cited, <a href="#Page_288">288</a>, <a href="#Page_290">290</a>, <a href="#Page_294">294</a>, <a href="#Page_304">304</a>, <a href="#Page_306">306</a>.</li> + +<li>Miocene, crustal changes, <a href="#Page_287">287</a>.</li> + +<li>Mississippi Basin, loess in, <a href="#Page_159">159</a>.</li> + +<li>Mogul emperor, and famine, <a href="#Page_104">104</a>.</li> + +<li>Monsoons, character of, <a href="#Page_146">146</a>; +<ul class="none"> + <li>direction of, <a href="#Page_208">208</a>;</li> + <li>Indian famines and, <a href="#Page_105">105</a>.</li> + </ul> + </li> + +<li>Moulton, F. R., cited, <a href="#Page_13">13</a>, <a href="#Page_258">258</a>, <a href="#Page_269">269</a>.</li> + +<li>Mountain building, climatic changes and, * <a href="#Page_25">25</a>.</li> + +<li>Mountains, folding of, <a href="#Page_190">190</a>; +<ul class="none"> + <li>rainfall, on, <a href="#Page_208">208</a>.</li> + </ul> + </li> + +<li>Multiple stars, <a href="#Page_252">252</a>.</li> + +<li> </li> + +<li>Nansen, F., cited, <a href="#Page_122">122</a>, <a href="#Page_174">174</a>.</li> + +<li>Naples, rainfall, <a href="#Page_86">86</a>.</li> + +<li>Nathorst, cited, <a href="#Page_169">169</a>.</li> + +<li>Nebulæ, <a href="#Page_247">247</a>.</li> + +<li>Nebular hypothesis, <a href="#Page_232">232</a>, <a href="#Page_267">267</a>.</li> + +<li>Neolithic period, <a href="#Page_218">218</a>.</li> + +<li>Nevada, correlations of rainfall, <a href="#Page_86">86</a>.</li> + +<li>New England, height of land, <a href="#Page_111">111</a>.</li> + +<li>New Mexico, rainfall, <a href="#Page_89">89</a>.</li> + +<li>New Orleans, La., rainfall and temperature, <a href="#Page_94">94</a>.</li> + +<li>New Zealand, climate, <a href="#Page_177">177</a>; +<ul class="none"> + <li>tree ferns, <a href="#Page_179">179</a>.</li> + </ul> + </li> + +<li>Newcomb, S., cited, <a href="#Page_52">52</a>.</li> + +<li>Nile floods, periodicity in, <a href="#Page_245">245</a>.</li> + +<li>Nitrogen, in atmosphere, <a href="#Page_19">19</a>.</li> + +<li>Niya, Chinese Turkestan, desiccation at, <a href="#Page_66">66</a>.</li> + +<li>Nocturnal cooling, changes in, <a href="#Page_238">238</a> f.</li> + +<li>Norlind, A., cited, <a href="#Page_100">100</a>.</li> + +<li>Norsemen, route to Greenland, <a href="#Page_26">26</a>.</li> + +<li>Norse sagas, <a href="#Page_219">219</a>.</li> + +<li>North Africa, climate of, <a href="#Page_71">71</a>; +<ul class="none"> + <li>Roman aqueducts in, <a href="#Page_71">71</a>.</li> + </ul> + </li> + +<li>North America, at maximum glaciation, <a href="#Page_122">122</a> ff.; +<ul class="none"> + <li>emergence of lands, <a href="#Page_193">193</a>,</li> + <li>glaciation in, <a href="#Page_131">131</a>,</li> + <li>height of land, <a href="#Page_111">111</a>,</li> + <li>interior sea in, <a href="#Page_200">200</a>,</li> + <li>inundations, <a href="#Page_196">196</a>,</li> + <li>loess in, <a href="#Page_155">155</a>,</li> + <li>submergence of lands, <a href="#Page_19">19</a>, <a href="#Page_21">21</a>.</li> + </ul> + </li> + +<li>North Atlantic Ocean, salinity, <a href="#Page_228">228</a>.</li> + +<li>North Sea, climatic stress, <a href="#Page_98">98</a> ff.; +<ul class="none"> + <li>floods around, <a href="#Page_26">26</a>, <a href="#Page_99">99</a>;</li> + <li>rainfall, <a href="#Page_107">107</a>;</li> + <li>storminess, <a href="#Page_57">57</a>.</li> + </ul> + </li> + +<li>Northern hemisphere, earthquakes of, <a href="#Page_294">294</a>.</li> + +<li>Norway, decay, <a href="#Page_100">100</a>; +<ul class="none"> + <li>temperature, <a href="#Page_177">177</a>.</li> + </ul> + </li> + +<li>Novæ, <a href="#Page_247">247</a>.</li> + +<li> </li> + +<li>Oceanic circulation, carbon dioxide and, <a href="#Page_39">39</a> ff.</li> + +<li>Oceanic climate, characteristics, <a href="#Page_103">103</a>.</li> + +<li>Oceanic currents, diversion, <a href="#Page_44">44</a>; +<ul class="none"> + <li>influence of land distribution, <a href="#Page_203">203</a>.</li> + </ul> + </li> + +<li>Oceans, age of, <a href="#Page_223">223</a>; +<ul class="none"> + <li>composition of, <a href="#Page_223">223</a>-241;</li> + <li>deepening of, <a href="#Page_199">199</a>;</li> + <li>salinity, <a href="#Page_19">19</a>, <a href="#Page_223">223</a>;</li> + <li>temperature, <a href="#Page_6">6</a>, <a href="#Page_152">152</a>, <a href="#Page_180">180</a>, <a href="#Page_226">226</a>.</li> + </ul> + </li> + +<li>Okada, T., cited, <a href="#Page_224">224</a>.</li> + +<li>Old Testament, temperature, <a href="#Page_92">92</a>.</li> + +<li>Orbital precessions, <a href="#Page_27">27</a>.</li> + +<li>Ordovician, climate, <a href="#Page_266">266</a>.</li> + +<li>Organic evolution, glacial fluctuations and, <a href="#Page_26">26</a>.</li> + +<li>Orion, nebulosity near, <a href="#Page_247">247</a>; +<ul class="none"> + <li>stars near, <a href="#Page_248">248</a>.</li> + </ul> + </li> + +<li>Orontes, <a href="#Page_67">67</a>.</li> + +<li>Osborn, H. F., cited, <a href="#Page_216">216</a>.</li> + +<li>Owens-Searles, lakes, <a href="#Page_142">142</a>.</li> + +<li>Oxus, <a href="#Page_108">7108</a>.</li> + +<li>Oxygen, in atmosphere, <a href="#Page_20">20</a>, <a href="#Page_234">234</a>; +<ul class="none"> + <li>in Permian, <a href="#Page_152">152</a>.</li> + </ul> + </li> + +<li>Ozone, cause of, <a href="#Page_56">56</a>.</li> + +<li> </li> + +<li>Paleolithic, <a href="#Page_216">216</a>.</li> + +<li>Paleozoic, climate, <a href="#Page_266">266</a>; +<ul class="none"> + <li>mountains in, <a href="#Page_209">209</a>.</li> + </ul> + </li> + +<li>Palestine, change of climate, <a href="#Page_91">91</a> f.</li> + +<li>Palms, climatic change and, <a href="#Page_91">91</a> f.; +<ul class="none"> + <li>in Ireland, <a href="#Page_179">179</a>.</li> + </ul> + </li> + +<li>Palmyra, ruins of, <a href="#Page_66">66</a>.</li> + +<li>Parallaxes of stars, <a href="#Page_276">276</a> f.</li> + +<li>Patrician center, <a href="#Page_134">134</a>.</li> + +<!--File: cc350.png--> +<!--<a class="pagenum break" id="Page_326" title="pg 326"></a>--> + +<li>Peat-bog period, first, <a href="#Page_218">218</a>.</li> + +<li>Penck, A., cited, <a href="#Page_139">139</a>, <a href="#Page_156">156</a>, <a href="#Page_157">157</a>, <a href="#Page_158">158</a>, <a href="#Page_269">269</a>.</li> + +<li>Pennsylvanian, life of, <a href="#Page_26">26</a>.</li> + +<li>Periodicities, <a href="#Page_245">245</a> f.</li> + +<li>Periodicity, of climatic phenomena, <a href="#Page_60">60</a> f.; +<ul class="none"> + <li>of glaciation, <a href="#Page_268">268</a>;</li> + <li>of sunspots, <a href="#Page_243">243</a>.</li> + </ul> + </li> + +<li>Permian, climate, <a href="#Page_266">266</a>; +<ul class="none"> + <li>distribution of glaciation, <a href="#Page_152">152</a>;</li> + <li>glaciation, <a href="#Page_60">60</a>, <a href="#Page_144">144</a>, * <a href="#Page_145">145</a>, <a href="#Page_226">226</a>;</li> + <li>glaciation and mountains, <a href="#Page_45">45</a>;</li> + <li>life of, <a href="#Page_26">26</a>;</li> + <li>red beds, <a href="#Page_151">151</a>;</li> + <li>temperature, <a href="#Page_146">146</a> f.</li> + </ul> + </li> + +<li>Perry, cited, <a href="#Page_289">289</a>.</li> + +<li>Persia, lakes, <a href="#Page_143">143</a>; +<ul class="none"> + <li>rainfall, <a href="#Page_89">89</a>.</li> + </ul> + </li> + +<li>Pettersson, O., cited, <a href="#Page_98">98</a> ff., <a href="#Page_100">100</a> f., <a href="#Page_103">103</a>, <a href="#Page_106">106</a>, <a href="#Page_219">219</a>.</li> + +<li>Pirsson, L. V., cited, <a href="#Page_3">3</a>, <a href="#Page_196">196</a>.</li> + +<li>Planetary hypothesis, <a href="#Page_253">253</a>, <a href="#Page_267">267</a>.</li> + +<li>Planetary nebulæ, <a href="#Page_252">252</a>.</li> + +<li>Planets, and sunspots, <a href="#Page_243">243</a>; +<ul class="none"> + <li>effect of star on, <a href="#Page_255">255</a>;</li> + <li>sunspot cycle and, <a href="#Page_62">62</a>;</li> + <li>temperatures, <a href="#Page_8">8</a> f.</li> + </ul> + </li> + +<li>Plants, climate and, <a href="#Page_1">1</a> f.; +<ul class="none"> + <li>effect of light, <a href="#Page_184">184</a> ff.</li> + </ul> + </li> + +<li>Pleion, defined, <a href="#Page_29">29</a>.</li> + +<li>Pleionian migrations, <a href="#Page_29">29</a> f.</li> + +<li>Pleistocene, climate, <a href="#Page_266">266</a>; +<ul class="none"> + <li>duration of, <a href="#Page_48">48</a>;</li> + <li>glaciation, <a href="#Page_110">110</a> ff.;</li> + <li>ice sheets, * <a href="#Page_123">123</a>.</li> + </ul> + </li> + +<li>Pluvial climate, causes of, <a href="#Page_143">143</a>; +<ul class="none"> + <li>during glacial periods, <a href="#Page_141">141</a>.</li> + </ul> + </li> + +<li>Po, frozen, <a href="#Page_98">98</a>.</li> + +<li>Polaris, <a href="#Page_272">272</a>.</li> + +<li>Polar wandering, hypothesis of, <a href="#Page_48">48</a> f.</li> + +<li>Pole and earthquakes, <a href="#Page_305">305</a>.</li> + +<li>Post-glacial crustal movements and climatic changes, <a href="#Page_215">215-222</a>.</li> + +<li>Poynting, J. H., cited, <a href="#Page_8">8</a>.</li> + +<li>Precessional hypothesis, <a href="#Page_34">34</a> f.</li> + +<li>Precipitation, and glaciation, <a href="#Page_114">114</a>, <a href="#Page_133">133</a>; +<ul class="none"> + <li>during glacial period, <a href="#Page_118">118</a>;</li> + <li>snow line and, <a href="#Page_139">139</a>;</li> + <li>temperature and, <a href="#Page_94">94</a>.</li> + </ul> + </li> + +<li>Procyon, companion of, <a href="#Page_280">280</a>; +<ul class="none"> + <li>luminosity, <a href="#Page_278">278</a>;</li> + <li>speed of, <a href="#Page_281">281</a>.</li> + </ul> + </li> + +<li>Progressive change, <a href="#Page_241">241</a>.</li> + +<li>Progressive desiccation, hypothesis of, <a href="#Page_65">65</a> ff.</li> + +<li>Proterozoic, <a href="#Page_4">4</a> f.; +<ul class="none"> + <li>fossils, <a href="#Page_6">6</a> f.;</li> + <li>glaciation, <a href="#Page_18">18</a>, <a href="#Page_144">144</a>, <a href="#Page_226">226</a>, <a href="#Page_266">266</a>;</li> + <li>lava, <a href="#Page_211">211</a>;</li> + <li>mountains in, <a href="#Page_209">209</a>;</li> + <li>oceanic salinity, <a href="#Page_42">42</a> f.;</li> + <li>oxygen in air, <a href="#Page_234">234</a>;</li> + <li>red beds, <a href="#Page_151">151</a>;</li> + <li>temperature, <a href="#Page_146">146</a> f.</li> + </ul> + </li> + +<li>Pulsations, hypothesis of, <a href="#Page_65">65</a>, <a href="#Page_72">72</a> ff.</li> + +<li>Pulsatory climatic changes, <a href="#Page_72">72</a> ff.</li> + +<li>Pulsatory hypothesis, <a href="#Page_272">272</a>.</li> + +<li>Pumpelly, R., cited, <a href="#Page_271">271</a>.</li> + +<li> </li> + +<li>Radiation, variation of, <a href="#Page_275">275</a>.</li> + +<li>Radioactivity, heat of sun and, <a href="#Page_14">14</a> f.</li> + +<li>Rainfall, changes in, <a href="#Page_93">93</a> f.; +<ul class="none"> + <li>glaciation and, <a href="#Page_50">50</a>;</li> + <li>sunspots and, <a href="#Page_53">53</a>, * <a href="#Page_58">58</a>, <a href="#Page_59">59</a>;</li> + <li>tree growth and, <a href="#Page_79">79</a>.</li> + </ul> + </li> + +<li>Red beds, <a href="#Page_151">151</a>, <a href="#Page_170">170</a>.</li> + +<li>Rhine, flood, <a href="#Page_99">99</a>; +<ul class="none"> + <li>frozen, <a href="#Page_98">98</a>.</li> + </ul> + </li> + +<li>Rho Ophiuchi, variables, <a href="#Page_248">248</a>.</li> + +<li>"Rice grains," <a href="#Page_61">61</a>.</li> + +<li>Richardson, O. W., cited, <a href="#Page_256">256</a>.</li> + +<li>Rigidity, of earth, <a href="#Page_307">307</a>.</li> + +<li>Roads, climate and, <a href="#Page_66">66</a>.</li> + +<li>Rogers, Thorwald, cited, <a href="#Page_101">101</a>.</li> + +<li>Romans, aqueduct of, <a href="#Page_71">71</a>.</li> + +<li>Rome, history of, <a href="#Page_87">87</a>.</li> + +<li>Rotation, of earth, <a href="#Page_18">18</a> f.</li> + +<li>Ruden, storm-flood, <a href="#Page_99">99</a>.</li> + +<li>Rugen, storm-flood, <a href="#Page_99">99</a>.</li> + +<li>Ruins, as climatic evidence, <a href="#Page_66">66</a>; +<ul class="none"> + <li>rainfall and, <a href="#Page_60">60</a>.</li> + </ul> + </li> + +<li> </li> + +<li>Sacramento, correlation coefficients, <a href="#Page_82">82</a> f., <a href="#Page_85">85</a>; +<ul class="none"> + <li>rainfall, <a href="#Page_86">86</a>;</li> + <li>rainfall record, <a href="#Page_79">79</a>.</li> + </ul> + </li> + +<li>Sagas, cited, <a href="#Page_105">105</a> f.</li> + +<li>St. John, C. E., cited, <a href="#Page_236">236</a>.</li> + +<li>Salinity, deep-sea circulation and, <a href="#Page_176">176</a>; +<ul class="none"> + <li>effect on climate, <a href="#Page_224">224</a>;</li> + <li>in North Atlantic, <a href="#Page_228">228</a>;</li> + <li>ocean temperature and, <a href="#Page_226">226</a>;</li> + <li>of ocean, <a href="#Page_19">19</a>, <a href="#Page_120">120</a>.</li> + </ul> + </li> + +<li>Salisbury, R. D., cited, <a href="#Page_111">111</a>, <a href="#Page_125">125</a>, <a href="#Page_129">129</a>, <a href="#Page_139">139</a>, <a href="#Page_156">156</a>, <a href="#Page_206">206</a>, <a href="#Page_269">269</a>, <a href="#Page_271">271</a>.</li> + +<li>Salt, in ocean, <a href="#Page_223">223</a>.</li> + +<li>San Bernardino, correlation of rainfall, <a href="#Page_85">85</a>.</li> + +<li>Saturn, and sunspots, <a href="#Page_243">243</a>; +<ul class="none"> + <li>sunspot cycle and, <a href="#Page_62">62</a>.</li> + </ul> + </li> + +<!--File: cc351.png--> +<!--<a class="pagenum break" id="Page_327" title="pg 327"></a>--> + +<li>Sayles, R. W., cited, <a href="#Page_183">183</a>.</li> + +<li>Scandinavia, climatic stress, <a href="#Page_100">100</a> f.; +<ul class="none"> + <li>fossils, <a href="#Page_271">271</a>;</li> + <li>post-glacial climate, <a href="#Page_271">271</a>;</li> + <li>rainfall, <a href="#Page_107">107</a>;</li> + <li>storminess, <a href="#Page_57">57</a>, <a href="#Page_107">107</a>;</li> + <li>temperature, <a href="#Page_216">216</a>.</li> + </ul> + </li> + +<li>Scandinavian center of glaciation, <a href="#Page_113">113</a>.</li> + +<li>Schlesinger, F., cited, <a href="#Page_275">275</a>, <a href="#Page_278">278</a>, <a href="#Page_298">298</a>, <a href="#Page_301">301</a>, <a href="#Page_305">305</a>.</li> + +<li>Schuchert, C., cited, <a href="#Page_3">3</a>, <a href="#Page_5">5</a>, <a href="#Page_23">23</a>, * <a href="#Page_25">25</a>, * <a href="#Page_123">123</a>, <a href="#Page_138">138</a>, * <a href="#Page_145">145</a>, <a href="#Page_168">168</a>, <a href="#Page_169">169</a>, <a href="#Page_172">172</a>, <a href="#Page_188">188</a>, <a href="#Page_193">193</a>, <a href="#Page_196">196</a>, <a href="#Page_198">198</a>, <a href="#Page_200">200</a>, * <a href="#Page_201">201</a>, <a href="#Page_206">206</a>, <a href="#Page_211">211</a>, <a href="#Page_230">230</a>, <a href="#Page_265">265</a>.</li> + +<li>Schuster, A., cited, <a href="#Page_61">61</a>, <a href="#Page_244">244</a>, <a href="#Page_294">294</a>, <a href="#Page_296">296</a>.</li> + +<li>Sculpture, Maya, <a href="#Page_96">96</a>.</li> + +<li>Sea level and glaciation, <a href="#Page_119">119</a>.</li> + +<li>Seasonal alternations, <a href="#Page_28">28</a> f.</li> + +<li>Seasonal banding, <a href="#Page_183">183</a> f.</li> + +<li>Seasonal changes, geological, <a href="#Page_183">183</a>.</li> + +<li>Seasons, and earthquakes, <a href="#Page_294">294</a>, <a href="#Page_295">295</a>, <a href="#Page_297">297</a>, <a href="#Page_299">299</a>; +<ul class="none"> + <li>evidences of, <a href="#Page_169">169</a>.</li> + </ul> + </li> + +<li>Secular progression, <a href="#Page_17">17</a> ff., <a href="#Page_188">188</a>.</li> + +<li>Seistan, swamps, <a href="#Page_171">171</a>.</li> + +<li>Sequoias, measurements of, <a href="#Page_74">74</a> ff.; +<ul class="none"> + <li>rainfall record, <a href="#Page_79">79</a>.</li> + </ul> + </li> + +<li>Setchell, W. A., cited, <a href="#Page_1">1</a>.</li> + +<li>Shackleton, E., cited, <a href="#Page_125">125</a>.</li> + +<li>Shapley, H., cited, <a href="#Page_246">246</a>, <a href="#Page_247">247</a>, <a href="#Page_254">254</a>, <a href="#Page_256">256</a>, <a href="#Page_275">275</a>.</li> + +<li>Shimek, E., cited, <a href="#Page_157">157</a>, <a href="#Page_161">161</a>.</li> + +<li>Shreveport, La., rainfall and temperature, <a href="#Page_93">93</a> f.</li> + +<li>Shrinkage of the earth, <a href="#Page_190">190</a>.</li> + +<li>Siberia, and glaciation, <a href="#Page_132">132</a>.</li> + +<li>Sierras, rainfall records, <a href="#Page_82">82</a>.</li> + +<li>Simpson, G. C., cited, <a href="#Page_222">222</a>.</li> + +<li>Sirius, companion of, <a href="#Page_280">280</a>; +<ul class="none"> + <li>distance from sun, <a href="#Page_262">262</a>;</li> + <li>luminosity, <a href="#Page_278">278</a>;</li> + <li>speed of, <a href="#Page_281">281</a>.</li> + </ul> + </li> + +<li>Slichter, C. S., cited, <a href="#Page_192">192</a>.</li> + +<li>Smith, J. W., cited, <a href="#Page_73">73</a>.</li> + +<li>Snowfall, glaciation and, <a href="#Page_50">50</a>, <a href="#Page_114">114</a>.</li> + +<li>Snowfield, climatic effects of, <a href="#Page_115">115</a>.</li> + +<li>Snow line, height of, <a href="#Page_138">138</a>; +<ul class="none"> + <li>in Andes, <a href="#Page_139">139</a>;</li> + <li>in Himalayas, <a href="#Page_139">139</a>.</li> + </ul> + </li> + +<li>Solar activity, cycles of, <a href="#Page_245">245</a>; +<ul class="none"> + <li>deep-sea circulation and, <a href="#Page_179">179</a>;</li> + <li>ice and, <a href="#Page_134">134</a>.</li> + </ul> + </li> + +<li>Solar constant, <a href="#Page_114">114</a>.</li> + +<li>Solar-cyclonic hypothesis, <a href="#Page_51">51-63</a>, <a href="#Page_287">287</a>; +<ul class="none"> + <li>glaciation and, <a href="#Page_110">110-129</a>.</li> + </ul> + </li> + +<li>Solar prominences, cause of, <a href="#Page_61">61</a>.</li> + +<li>Solar system, <a href="#Page_252">252</a>; +<ul class="none"> + <li>conservation of, <a href="#Page_243">243</a>;</li> + <li>proximity to stars, <a href="#Page_63">63</a>.</li> + </ul> + </li> + +<li>Solar variations, storms and, <a href="#Page_31">31</a>.</li> + +<li>South America, earthquakes, <a href="#Page_301">301</a>.</li> + +<li>South Pole, thickness of ice at, <a href="#Page_125">125</a>.</li> + +<li>Southern hemisphere, earthquakes, <a href="#Page_296">296</a>; +<ul class="none"> + <li>glaciation in, <a href="#Page_131">131</a> f.</li> + </ul> + </li> + +<li>Southern Pacific railroad, rainfall records along, <a href="#Page_82">82</a>.</li> + +<li>Soy beans, effect of light, <a href="#Page_185">185</a> f.</li> + +<li>Space, sun's journey through, <a href="#Page_264">264-284</a>.</li> + +<li>Spiral nebulæ, <a href="#Page_251">251</a> f.; +<ul class="none"> + <li>universe of, <a href="#Page_267">267</a>.</li> + </ul> + </li> + +<li>Spitzbergen, submergence, <a href="#Page_219">219</a>.</li> + +<li>Springs, climate and, <a href="#Page_66">66</a>.</li> + +<li>Stars, approach to sun, <a href="#Page_253">253</a>; +<ul class="none"> + <li>binary, <a href="#Page_252">252</a>;</li> + <li>clusters, <a href="#Page_252">252</a>, <a href="#Page_268">268</a>;</li> + <li>effect on solar atmosphere, <a href="#Page_63">63</a>;</li> + <li>dark, <a href="#Page_254">254</a>;</li> + <li>parallaxes of, <a href="#Page_276">276</a> f.;</li> + <li>tidal action of, <a href="#Page_249">249</a>.</li> + </ul> + </li> + +<li>Stefan's Law, <a href="#Page_257">257</a>.</li> + +<li>Stein, M. A., cited, <a href="#Page_78">78</a>.</li> + +<li>Stellar approaches, probability of, <a href="#table_5">260</a>.</li> + +<li>Storm belt in arid regions, <a href="#Page_144">144</a>.</li> + +<li>Storm-floods, in fourteenth century, <a href="#Page_99">99</a>.</li> + +<li>Storminess, and erosion, <a href="#Page_309">309</a>; +<ul class="none"> + <li>and ice, <a href="#Page_134">134</a>;</li> + <li>effect on glaciation, <a href="#Page_112">112</a>;</li> + <li>sunspots and, <a href="#Page_163">163</a>;</li> + <li>temperature and, <a href="#Page_94">94</a>, <a href="#Page_173">173</a>.</li> + </ul> + </li> + +<li>Storms, blows of, <a href="#Page_300">300</a>, <a href="#Page_302">302</a>; +<ul class="none"> + <li>increase, <a href="#Page_60">60</a>;</li> + <li>movement of, <a href="#Page_125">125</a> f.;</li> + <li>movement of water and, * <a href="#Page_175">175</a>;</li> + <li>origin of, <a href="#Page_30">30</a> f.;</li> + <li>sunspots and, <a href="#Page_28">28</a>, <a href="#Page_53">53</a>;</li> + <li><em>see</em> Cyclones and Lows.</li> + </ul> + </li> + +<li>Storm tracks, during glacial period, <a href="#Page_117">117</a>; +<ul class="none"> + <li>location, <a href="#Page_113">113</a>;</li> + <li>relation to magnetic poles, <a href="#Page_150">150</a>;</li> + <li>shifting of, <a href="#Page_119">119</a>.</li> + </ul> + </li> + +<li>Strands, climate and, <a href="#Page_66">66</a>; +<ul class="none"> + <li>in semi-arid regions, <a href="#Page_60">60</a>;</li> + <li>of salt lakes, <a href="#Page_142">142</a>.</li> + </ul> + </li> + +<li>Suess, E., cited, <a href="#Page_192">192</a>.</li> + +<li>Sun, and the earth's crust, <a href="#Page_285">285-317</a>; +<ul class="none"> + <li>approach to star, <a href="#Page_253">253</a>;</li> + <li>atmosphere of, <a href="#Page_61">61</a>, <a href="#Page_274">274</a>;</li> + +<!--File: cc352.png--> +<!--<a class="pagenum break" id="Page_328" title="pg 328"></a>--> + + <li>atmosphere of, and weather, <a href="#Page_52">52;</a></li> + <li>cooling of, <a href="#Page_49">49;</a></li> + <li>contraction of, <a href="#Page_249">249;</a></li> + <li>disturbances of, <a href="#Page_172">172</a>;</li> + <li>effect of other bodies on, <a href="#Page_242">242-263</a>;</li> + <li>heat, <a href="#Page_13">13</a>;</li> + <li>journey through space, <a href="#Page_264">264-284</a>;</li> + <li>Knowlton's hypothesis of, <a href="#Page_168">168</a>.</li> + </ul> + </li> + +<li>Suncracks, <a href="#Page_232">232.</a></li> + +<li>Sunspot cycles, <a href="#Page_27">27</a> f.</li> + +<li>Sunspots, and earthquakes, <a href="#Page_289">289</a>; +<ul class="none"> + <li>causes of, <a href="#Page_61">61</a>;</li> + <li>magnetic field of, <a href="#Page_261">261</a>;</li> + <li>maximum of, <a href="#Page_109">109</a>;</li> + <li>mild climates and, <a href="#Page_172">172</a>;</li> + <li>number, <a href="#Page_108">7108</a> f.;</li> + <li>periodicity, <a href="#Page_243">243</a>;</li> + <li>planetary hypothesis of, <a href="#Page_253">253</a>;</li> + <li>records, <a href="#Page_245">245</a>;</li> + <li>storminess and, <a href="#Page_163">163</a>;</li> + <li>storms and, <a href="#Page_300">300</a>;</li> + <li>temperature of earth and, <a href="#Page_52">52</a>, <a href="#Page_173">173</a>.</li> + </ul> + </li> + +<li>Sunspot variations, <a href="#Page_282">282</a>.</li> + +<li>Swamps, as desert phenomena, <a href="#Page_171">171</a>.</li> + +<li>Sylt, storm-flood, <a href="#Page_99">99</a>.</li> + +<li>Syria, civilization in, <a href="#Page_67">67</a>; +<ul class="none"> + <li>inscriptions in, <a href="#Page_76">76</a>;</li> + <li>Roman aqueducts in, <a href="#Page_71">71</a>.</li> + </ul> + </li> + +<li>Syrian Desert, ruins in, <a href="#Page_66">66</a>.</li> + +<li> </li> + +<li>Talbert, cited, <a href="#Page_213">213</a>.</li> + +<li>Tarim Basin, red beds, <a href="#Page_151">151</a>.</li> + +<li>Tarim Desert, desiccation, <a href="#Page_66">66</a>.</li> + +<li>Tarim River, swamps, <a href="#Page_171">171</a>.</li> + +<li>Taylor, G., cited, <a href="#Page_140">140</a>, <a href="#Page_144">144</a>, <a href="#Page_191">191</a>, <a href="#Page_271">271</a>.</li> + +<li>Temperature, change of in Atlantic, <a href="#Page_174">174</a>; +<ul class="none"> + <li>changes in, <a href="#Page_93">93</a>;</li> + <li>climatic change and, <a href="#Page_49">49</a>;</li> + <li>critical, <a href="#Page_9">9</a>;</li> + <li>geological time and, <a href="#Page_3">3</a>;</li> + <li>glacial period, <a href="#Page_38">38</a>;</li> + <li>glaciation and, <a href="#Page_42">42</a>, <a href="#Page_132">132</a>, <a href="#Page_139">139</a>;</li> + <li>gradient of earth, <a href="#Page_213">213</a>;</li> + <li>of ocean, <a href="#Page_180">180</a>;</li> + <li>in Norway, <a href="#Page_177">177</a>;</li> + <li>in Permian, <a href="#Page_146">146</a> f.;</li> + <li>in Proterozoic, <a href="#Page_146">146</a> f.;</li> + <li>limits, <a href="#Page_6">6</a> ff.;</li> + <li>precipitation and, <a href="#Page_94">94</a>;</li> + <li>range of, <a href="#Page_3">3</a>, <a href="#Page_8">8</a>;</li> + <li>solar activity and, <a href="#Page_140">140</a>;</li> + <li>storminess and, <a href="#Page_94">94</a>, <a href="#Page_112">112</a>, <a href="#Page_173">173</a>;</li> + <li>sunspots and, <a href="#Page_28">28</a>, <a href="#Page_173">173</a>;</li> + <li>volcanic eruptions and, <a href="#Page_46">46</a>;</li> + <li>zones, <a href="#Page_172">172</a>.</li> + </ul> + </li> + +<li>Terrestial causes of climatic changes, <a href="#Page_188">188-214</a>.</li> + +<li>Tertiary, lava, <a href="#Page_211">211</a>.</li> + +<li>Thames, frozen, <a href="#Page_98">98</a>.</li> + +<li>Thermal solar hypothesis, <a href="#Page_49">49</a> f., <a href="#Page_97">97</a>.</li> + +<li>Thermo-pleion, movements of, <a href="#Page_30">30</a>.</li> + +<li>Thesis, of pulsations, <a href="#Page_24">24</a>.</li> + +<li>Thiryu, storm-flood, <a href="#Page_99">99</a>.</li> + +<li>Tian-Shan Mountains, irrigation in, <a href="#Page_71">71</a>.</li> + +<li>Tidal action of stars, <a href="#Page_249">249</a>.</li> + +<li>Tidal effect, of Jupiter, <a href="#Page_253">253</a>; +<ul class="none"> + <li>of planets, <a href="#Page_244">244</a>.</li> + </ul> + </li> + +<li>Tidal hypothesis, <a href="#Page_251">251</a>.</li> + +<li>Tidal retardation, effect on land and sea, <a href="#Page_191">191</a>; +<ul class="none"> + <li>rotation of earth and, <a href="#Page_18">18</a> f.;</li> + <li>stress caused by, <a href="#Page_310">310</a>.</li> + </ul> + </li> + +<li>Tides, cycles of, <a href="#Page_219">219</a>.</li> + +<li>Time, geological, <em>see</em> Geological time.</li> + +<li>Toads, distribution of, <a href="#Page_202">202</a>.</li> + +<li>Tobacco plant, effect of light, <a href="#Page_184">184</a>.</li> + +<li>Topography, and glaciation, <a href="#Page_132">132</a>.</li> + +<li>Transcaspian Basin, red beds, <a href="#Page_151">151</a>.</li> + +<li>Tree ferns, in New Zealand, <a href="#Page_179">179</a>.</li> + +<li>Tree growth, periodicity in, <a href="#Page_245">245</a>; +<ul class="none"> + <li>rainfall and, <a href="#Page_79">79</a>.</li> + </ul> + </li> + +<li>Tress, in California, <a href="#Page_219">219</a>; +<ul class="none"> + <li>measurement of, <a href="#Page_73">73</a> ff.</li> + </ul> + </li> + +<li>Triassic, climate, <a href="#Page_266">266</a>.</li> + +<li>Trifid Nebula, variables, <a href="#Page_248">248</a>.</li> + +<li>Trondheim, wheat in, <a href="#Page_101">101</a>.</li> + +<li>Trondhenäs, corn in, <a href="#Page_101">101</a>.</li> + +<li>Tropical cyclones, in glacial epochs, <a href="#Page_140">140</a> f.; +<ul class="none"> + <li>occurrence, <a href="#Page_148">148</a>;</li> + <li>solar activity and, <a href="#Page_113">113</a>.</li> + </ul> + </li> + +<li>Tropical hurricanes, earthquakes and, <a href="#Page_300">300</a>; +<ul class="none"> + <li>sunspots and, <a href="#Page_149">149</a>.</li> + </ul> + </li> + +<li>Turfan, temperature, <a href="#Page_17">17</a>.</li> + +<li>Turner, H. H., cited, <a href="#Page_245">245</a>.</li> + +<li>Tyler, J. M., cited, <a href="#Page_216">216</a>.</li> + +<li>Tyndall, J., cited, <a href="#Page_36">36</a>, <a href="#Page_37">37</a>.</li> + +<li>Typhoon region, "earthquake weather," <a href="#Page_298">298</a>.</li> + +<li>Typhoons, occurrence, <a href="#Page_300">300</a>.</li> + +<li> </li> + +<li>United States, rainfall and temperature in Gulf region, <a href="#Page_93">93</a> f.; +<ul class="none"> + <li>salt lakes in, <a href="#Page_142">142</a>;</li> + <li>southwestern, climate, <a href="#Page_66">66</a>;</li> + <li>storminess, <a href="#Page_53">53</a> f., <a href="#Page_60">60</a>.</li> + </ul> + </li> + +<li> </li> + +<li>Variables, <a href="#Page_247">247</a>.</li> + +<li>Veeder, M. A., cited, <a href="#Page_300">300</a>.</li> + +<li>Vegetation, theory of pulsations and, <a href="#Page_90">90</a>.</li> + +<!--File: cc353.png--> +<!--<a class="pagenum break" id="Page_329" title="pg 329"></a>--> + +<li>Venus, atmosphere of, <a href="#Page_236">236</a>.</li> + +<li>Vesterbygd, invasion of, <a href="#Page_106">106</a>.</li> + +<li>Vicksburg, Miss., rainfall and temperature, <a href="#Page_93">93</a> f.</li> + +<li>Volcanic activity, climate and, <a href="#Page_210">210</a>; +<ul class="none"> + <li>movement of the earth's crust and, <a href="#Page_285">285</a>;</li> + <li>times of uplifting lands and, <a href="#Page_23">23</a>.</li> + </ul> + </li> + +<li>Volcanic dust, climatic changes and, <a href="#Page_97">97</a>.</li> + +<li>Volcanic hypothesis, climatic change and, <a href="#Page_45">45</a> ff.; +<ul class="none"> + <li>snow line, <a href="#Page_139">139</a>.</li> + </ul> + </li> + +<li>Volcanoes, activity of, <a href="#Page_96">96</a>.</li> + +<li>Volga, <a href="#Page_108">108</a>.</li> + +<li> </li> + +<li>Walcott, C. D., cited, <a href="#Page_4">4</a>, <a href="#Page_230">230</a>.</li> + +<li>Wandering of the pole, <a href="#Page_302">302</a>.</li> + +<li>Water, importance, <a href="#Page_9">9</a>.</li> + +<li>Water vapor, condensation of, <a href="#Page_56">56</a>; +<ul class="none"> + <li>effect on life, <a href="#Page_231">231</a>;</li> + <li>in atmosphere, <a href="#Page_19">19</a>.</li> + </ul> + </li> + +<li>Wave, effect on movement of water, <a href="#Page_176">176</a>.</li> + +<li>Weather, changes of, <a href="#Page_31">31</a> f.; +<ul class="none"> + <li>origin of, <a href="#Page_174">174</a>;</li> + <li>variations, <a href="#Page_52">52</a>.</li> + </ul> + </li> + +<li>Wells, H. G., cited, <a href="#Page_35">35</a>.</li> + +<li>Wendingstadt, storm-flood, <a href="#Page_99">99</a>.</li> + +<li>Westerlies, <a href="#Page_21">21</a> f.</li> + +<li>Wheat, price in England, <a href="#Page_102">102</a>.</li> + +<li>White Sea, submergence, <a href="#Page_219">219</a>.</li> + +<li>Whitney, J. D., cited, <a href="#Page_142">142</a>.</li> + +<li>Wieland, G. R., cited, <a href="#Page_169">169</a>.</li> + +<li>Williamson, E. D., cited, <a href="#Page_226">226</a>.</li> + +<li>Willis, B., cited, <a href="#Page_206">206</a>.</li> + +<li>Winds, at ice front, <a href="#Page_162">162</a>; +<ul class="none"> + <li>effect on currents, <a href="#Page_174">174</a>;</li> + <li>glaciation and, <a href="#Page_133">133</a>;</li> + <li>in Antarctica, <a href="#Page_161">161</a>;</li> + <li>in glacial period, <a href="#Page_119">119</a>;</li> + <li>in Greenland, <a href="#Page_161">161</a>;</li> + <li>planetary system of, <a href="#Page_174">174</a>;</li> + <li>velocity, <a href="#Page_240">240</a>.</li> + </ul> + </li> + +<li>Witch hazel, effect of light, <a href="#Page_184">184</a>.</li> + +<li>Wolf, J. R., cited, <a href="#Page_61">61</a>, <a href="#Page_109">109</a>, <a href="#Page_288">288</a>.</li> + +<li>Wolfer, cited, <a href="#Page_244">244</a>.</li> + +<li>Wright, W. B., cited, <a href="#Page_35">35</a>, <a href="#Page_111">111</a>, <a href="#Page_119">119</a>.</li> + +<li>Writing, among Mayas, <a href="#Page_96">96</a>.</li> + +<li> </li> + +<li>Yucatan, Maya civilization, <a href="#Page_26">26</a>, <a href="#Page_107">107</a>; +<ul class="none"> + <li>rainfall, <a href="#Page_108">108</a>;</li> + <li>ruins, <a href="#Page_95">95</a>.</li> + </ul> + </li> + +<li>Yukon, Ice Age in, <a href="#Page_221">221</a>.</li> + +<li> </li> + +<li>Zante, earthquakes of, <a href="#Page_296">296</a>.</li> + +<li>Zonal crowding, <a href="#Page_117">117</a>.</li> + +<!--File: cc354.png--> +<!--<a class="pagenum break" id="Page_330" title="pg 330"></a>--> + +</ul> + + +<h4>PRINTED IN THE UNITED STATES OF AMERICA</h4> +<p> </p> +<p> </p> + +<!--Footnotes:--> +<div class="footnotes"> +<h3>FOOTNOTES:</h3> +<div id="fn1"><p><a href="#r1">[1]</a> W. A. Setchell: The Temperature Interval in the Geographical Distribution +of Marine Algæ; Science, Vol. 52, 1920, p. 187.</p></div> + +<div id="fn2"><p><a href="#r2">[2]</a> J. Barrell: Rhythms and the Measurements of Geologic Time; Bull. +Geol. Soc. Am., Vol. 28, Dec., 1917, pp. 745-904.</p></div> + +<div id="fn3"><p><a href="#r3">[3]</a> Pirsson and Schuchert: Textbook of Geology, 1915, pp. 538-550.</p></div> + +<div id="fn4"><p><a href="#r4">[4]</a> From Charles Schuchert in The Evolution of the Earth and Its Inhabitants: +Edited by R. S. Lull, New Haven, 1918, but with revisions by Professor Schuchert.</p></div> + +<div id="fn5"><p><a href="#r5">[5]</a> J. H. Poynting: Radiation in the Solar System; Phil. Trans. A, 1903, +202, p. 525.</p></div> + +<div id="fn6"><p><a href="#r6">[6]</a> L. J. Henderson: The Fitness of the Environment, 1913.</p></div> + +<div id="fn7"><p><a href="#r7">[7]</a> Henderson: <em>loc. cit.</em>, p. 138.</p></div> + +<div id="fn8"><p><a href="#r8">[8]</a> F. R. Moulton: Introduction to Astronomy, 1916.</p></div> + +<div id="fn9"><p><a href="#r9">[9]</a> Moulton: <em>loc. cit.</em></p></div> + +<div id="fn10"><p><a href="#r10">[10]</a> James Croll: Climate and Time, 1876.</p></div> + +<div id="fn11"><p><a href="#r11">[11]</a> T. C. Chamberlin: An attempt to frame a working hypothesis of the +cause of glacial periods on an atmospheric basis; Jour. Geol., Vol. VII, +1899, pp. 545-584, 667-685, 757-787.</p> + +<p>T. C. Chamberlin and R. D. Salisbury: Geology, Vol. II, 1906, pp. 93-106, +655-677, and Vol. III, pp. 432-446.</p> + +<p>S. Arrhenius (Kosmische Physik, Vol. II, 1903, p. 503) carried out some +investigations on carbon dioxide which have had a pronounced effect on +later conclusions.</p> + +<p>F. Frech adopted Arrhenius' idea and developed it in a paper entitled +Ueber die Klima-Aenderungen der Geologischen Vergangenheit. Compte +Rendu, Tenth (Mexico) Congr. Geol. Intern., 1907 (=1908), pp. 299-325.</p> + +<p>The exact origin of the carbon dioxide theory has been stated so variously +that it seems worth while to give the exact facts. Prompted by the suggestion, +of Tyndall that glaciation might be due to depletion of atmospheric +carbon dioxide, Chamberlin worked up the essentials of his early views +before he saw any publication from Arrhenius, to whom the idea has often +been attributed. In 1895 or earlier Chamberlin began to give the carbon +dioxide hypothesis to his students and to discuss it before local scientific +bodies. In 1897 he prepared a paper on "A Group of Hypotheses Bearing +on Climatic Changes," Jour. Geol., Vol. V (1897), to be read at the meeting +of the British Association at Toronto, basing his conclusions on Tyndall's +determination of the competency of carbon dioxide as an absorber of heat +radiated from the earth. He had essentially completed this when a paper by +Arrhenius, "On the influence of carbonic acid in the air upon the temperature +of the ground," Phil. Mag., 1896, pp. 237-276, first came to his +attention. Chamberlin then changed his conservative, tentative statement of +the functions of carbon dioxide to a more sweeping one based on Arrhenius' +very definite quantitative deductions from Langley's experiments. Both +Langley and Arrhenius were then in the ascendancy of their reputations +and seemingly higher authorities could scarcely have been chosen, nor a +finer combination than experiment and physico-mathematical development. +Arrhenius' deductions were later proved to have been overstrained, while +Langley's interpretation and even his observations were challenged. Chamberlin's +latest views are more like his earlier and more conservative statement.</p></div> + +<div id="fn12"><p><a href="#r12">[12]</a> C. G. Abbot and F. E. Fowle: Volcanoes and Climate; Smiths. Misc. +Coll., Vol. 60, 1913, 24 pp.</p> + +<p>W. J. Humphreys: Volcanic dust and other factors in the production of +climatic and their possible relation to ice ages; Bull. Mount Weather +Observatory, Vol. 6, Part 1, 1913, 26 pp. Also, Physics of the Air, 1920.</p> +</div> + +<div id="fn13"><p><a href="#r13">[13]</a> H. Arctowski: The Pleonian Cycle of Climatic Fluctuations; Am. +Jour. Sci., Vol. 42, 1916, pp. 27-33. See also Annals of the New York +Academy of Sciences, Vol. 24, 1914.</p></div> + +<div id="fn14"><p><a href="#r14">[14]</a> W. Köppen: Über mehrjährige Perioden der Witterung ins besondere +üzer die II-jährige Periode der Temperatur. Also, Lufttemperaturen +Sonnenflecke und Vulcanausbrüche; Meteorologische Zeitschrift, Vol. 7, +1914, pp. 305-328.</p></div> + +<div id="fn15"><p><a href="#r15">[15]</a> The so-called sunspot numbers to which reference is made again and +again in this book are based on a system devised by Wolf and revised by +A. Wolfer. The number and size of the spots are both taken into account. +The numbers from 1749 to 1900 may be found in the Monthly Weather +Review for April, 1902, and from 1901 to 1918 in the same journal for +1920.</p></div> + +<div id="fn16"><p><a href="#r16">[16]</a> Much of this chapter is taken from The Solar Hypothesis of Climatic +Changes; Bull. Geol. Soc. Am., Vol. 25, 1914.</p></div> + +<div id="fn17"><p><a href="#r17">[17]</a> Ellsworth Huntington: Explorations in Turkestan, 1905; The Pulse of +Asia, 1907; Palestine and Its Transformation, 1911; The Climatic Factor, +1915; World Power and Evolution, 1919.</p></div> + +<div id="fn18"><p><a href="#r18">[18]</a> J. Hann: Klimatologie, Vol. 1, 1908, p. 352.</p></div> + +<div id="fn19"><p><a href="#r19">[19]</a> H. C. Butler: Desert Syria, the Land of a Lost Civilization; Geographical +Review, Feb., 1920, pp. 77-108. +</p></div> + +<div id="fn20"><p><a href="#r20">[20]</a> This is due to the fact that where these forests occur, in Gilead for +example, the mountains to the west break down, so that the west winds with +water from the Mediterranean are able to reach the inner range without +having lost all their water. It is one of the misfortunes of Syria that its +mountains generally rise so close to the sea that they shut off rainfall from +the interior and cause the rain to fall on slopes too steep for easy cultivation.</p></div> + +<div id="fn21"><p><a href="#r21">[21]</a> H. Leiter: Die Frage der Klimaanderung waherend geschichtlicher Zeit +in Nordafrika. Abhandl. K. K. Geographischen Gesellschaft, Wien, 1909, p. 143.</p></div> + +<div id="fn22"><p><a href="#r22">[22]</a> A most careful and convincing study of this problem is embodied in +an article by J. W. Smith: The Effects of Weather upon the Yield of Corn; +Monthly Weather Review, Vol. 42, 1914, pp. 78-92. On the basis of the +yield of corn in Ohio for 60 years and in other states for shorter periods, +he shows that the rainfall of July has almost as much influence on the crop +as has the rainfall of all other months combined. See his Agricultural +Meteorology, New York, 1920.</p></div> + +<div id="fn23"><p><a href="#r23">[23]</a> See chapter by A. E. Douglass in The Climatic Factor; and his book on +Climatic Cycles and Tree-Growth; Carnegie Inst., 1919. Also article by +M. N. Stewart: The Relation of Precipitation to Tree Growth, in the +Monthly Weather Review, Vol. 41, 1913.</p></div> + +<div id="fn24"><p><a href="#r24">[24]</a> The dotted line is taken from Palestine and Its Transformation, pp. +327 and 403.</p></div> + +<div id="fn25"><p><a href="#r25">[25]</a> M. A. Stein: Ruins of Desert Cathay, London, 1912.</p></div> + +<div id="fn26"><p><a href="#r26">[26]</a> In the preparation and interpretation of this table the help of Mr. +G. B. Cressey is gratefully acknowledged.</p></div> + + +<div id="fn27"><p><a href="#r27">[27]</a> For the tree data used in these comparisons, see The Climatic Factor +P. 328, and A. E. Douglass: Climatic Cycles and Tree Growth, p. 123.</p></div> + +<div id="fn28"><p><a href="#r28">[28]</a> One year interpolated.</p></div> + +<div id="fn29"><p><a href="#r29">[29]</a> J. W. Gregory: Is the Earth Drying Up? Geog. Jour., Vol. 43, 1914, +pp. 148-172 and 293-318.</p></div> + +<div id="fn30"><p><a href="#r30">[30]</a> Geog. Jour., Vol. 43, pp. 159-161.</p></div> + +<div id="fn31"><p><a href="#r31">[31]</a> See A. J. Henry: Secular Variation of Precipitation in the United +States; Bull. Am. Geog. Soc., Vol. 46, 1914, pp. 192-201.</p></div> + +<div id="fn32"><p><a href="#r32">[32]</a> O. Pettersson: The connection between hydrographical and meteorological +phenomena; Quarterly Journal of the Royal Meteorological Society, Vol. +38, pp. 174-175.</p></div> + +<div id="fn33"><p><a href="#r33">[33]</a> A. Norlind: Einige Bemerkungen über das Klima der historischen Zeit +nebst einem Verzeichnis mittelaltlicher Witterungs erscheinungen; Lunds +Univ. Arsskrift, N. F., Vol. 10, 1914, 53 pp.</p></div> + +<div id="fn34"><p><a href="#r34">[34]</a> Thorwald Rogers: A History of Agriculture and Prices in England.</p></div> + +<div id="fn35"><p><a href="#r35">[35]</a> E. Brückner: Klimaschwankungen seit 1700, Vienna, 1891.</p></div> + +<div id="fn36"><p><a href="#r36">[36]</a> For a full discussion of the changes in the Caspian Sea see The Pulse +of Asia, pp. 329-358.</p></div> + +<div id="fn37"><p><a href="#r37">[37]</a> S. Q. Morley: The Inscriptions at Copán; Carnegie Inst. of Wash., No. +219, 1920.</p> + +<p>Ellsworth Huntington: The Red Man's Continent, 1919.</p></div> + +<div id="fn38"><p><a href="#r38">[38]</a> See summary of Wolf's work with additional information by H. Fritz; +Zürich Vierteljahrschrift, Vol. 38, 1893, pp. 77-107.</p></div> + +<div id="fn39"><p><a href="#r39">[39]</a> This chapter is an amplification and revision of the sketch of the glacial +period contained in The Solar Hypothesis of Climatic Changes; Bull. Geol. +Soc. Am., Vol. 25, 1914.</p></div> + +<div id="fn40"><p><a href="#r40">[40]</a> R. D. Salisbury: Physical Geography of the Pleistocene, in Outlines of +Geologic History, by Willis, Salisbury, and others, 1910, p. 265.</p></div> + +<div id="fn41"><p><a href="#r41">[41]</a> The Quaternary Ice Age, 1914, p. 364.</p></div> + +<div id="fn42"><p><a href="#r42">[42]</a> For fuller discussion of climatic controls see S. S. Visher: Seventy +Laws of Climate, Annals Assoc. Am. Geographers, 1922.</p></div> + +<div id="fn43"><p><a href="#r43">[43]</a> Many of these alterations are implied or discussed in the following +papers:</p> +<ol> +<li>F. W. Harmer: Influence of Winds upon the Climate of the Pleistocene; +Quart. Jour. Geol. Soc., Vol. 57, 1901, p. 405.</li> + +<li>C. E. P. Brooks: Meteorological Conditions of an Ice Sheet; Quart. +Jour. Royal Meteorol. Soc., Vol. 40, 1914, pp. 53-70, and The Evolution of +Climate in Northwest Europe; <em>op. cit.</em>, Vol. 47, 1921, pp. 173-194.</li> + +<li>W. H. Hobbs: The Rôle of the Glacial Anticyclone in the Air Circulation +of the Globe; Proc. Am. Phil. Soc., Vol. 54, 1915, pp. 185-225.</li> +</ol></div> + +<div id="fn44"><p><a href="#r44">[44]</a> W. B. Wright: The Quaternary Ice Age, 1914, p. 100.</p></div> + + +<div id="fn45"><p><a href="#r45">[45]</a> The description of the distribution of the ice sheet is based on T. C. +Chamberlin's wall map of North America at the maximum of glaciation, 1913.</p></div> + +<div id="fn46"><p><a href="#r46">[46]</a> Chamberlin and Salisbury: Geology, 1906, Vol. 3, and W. H. Hobbs: +Characteristics of Existing Glaciers, 1911.</p></div> + +<div id="fn47"><p><a href="#r47">[47]</a> S. S. Visher: The Geography of South Dakota; S. D. Geol. Surv., 1918.</p></div> + +<div id="fn48"><p><a href="#r48">[48]</a> W. H. Hobbs: Characteristics of Existing Glaciers, 1911. The Rôle of +the Glacial Anticyclones in the Air Circulation of the Globe; Proc. Am. +Phil. Soc., Vol. 54, 1915, pp. 185-225.</p></div> + +<div id="fn49"><p><a href="#r49">[49]</a> R. D. Salisbury: Physiography, 1919.</p></div> + +<div id="fn50"><p><a href="#r50">[50]</a> Griffith Taylor: Australian Meteorology, 1920, p. 283.</p></div> + +<div id="fn51"><p><a href="#r51">[51]</a> J. D. Whitney: Climatic Changes of the Later Geological Times, 1882.</p></div> + +<div id="fn52"><p><a href="#r52">[52]</a> E. E. Free: U. S. Dept. of Agriculture, Bull. 54, 1914. Mr. Free has +prepared a summary of this Bulletin which appears in The Solar Hypothesis, +Bull. Geol. Sec. of Am., Vol. 25, pp. 559-562.</p></div> + +<div id="fn53"><p><a href="#r53">[53]</a> G. K. Gilbert: Lake Bonneville; Monograph 1, U. S. Geol. Surv.</p></div> + +<div id="fn54"><p><a href="#r54">[54]</a> C. E. P. Brooks: Quart. Jour. Royal Meteorol. Soc., 1914, pp. 63-66.</p></div> + +<div id="fn55"><p><a href="#r55">[55]</a> H. J. L. Beadnell: A. Egyptian Oasis, London, 1909.</p> + +<p>Ellsworth Huntington: The Libyan Oasis of Kharga; Bull. Am. Geog. +Soc., Vol. 42, Sept., 1910, pp. 641-661.</p></div> + +<div id="fn56"><p><a href="#r56">[56]</a> S. S. Visher: The Bajada of the Tucson Bolson of Southern Arizona; +Science, N. S., Mar. 23, 1913.</p> + +<p>Ellsworth Huntington: The Basins of Eastern Persia and Seistan, in +Explorations in Turkestan.</p></div> + +<div id="fn57"><p><a href="#r57">[57]</a> Griffith Taylor: Australian Meteorology, 1920, p. 189.</p></div> + +<div id="fn58"><p><a href="#r58">[58]</a> Chamberlin and Salisbury: Geology, 1906, Vol. III, pp. 405-412.</p></div> + +<div id="fn59"><p><a href="#r59">[59]</a> It may have retreated soon after reaching its maximum. If so, the +general lack of thick terminal moraines would be explained. See page 122.</p></div> + +<div id="fn60"><p><a href="#r60">[60]</a> Rollin T. Chamberlin: Personal Communication.</p></div> + +<div id="fn61"><p><a href="#r61">[61]</a> F. H. Knowlton: Evolution of Geologic Climates; Bull. Geol. Soc. Am., +Vol. 30, 1919, pp. 499-566.</p></div> + +<div id="fn62"><p><a href="#r62">[62]</a> Chas. Schuchert: Review of Knowlton's Evolution of Geological Climates, +in Am. Jour. Sci., 1921.</p></div> + +<div id="fn63"><p><a href="#r63">[63]</a> G. R. Wieland: Distribution and Relationships of the Cycadeoids; Am. +Jour. Bot., Vol. 7, 1920, pp. 125-145.</p></div> + +<div id="fn64"><p><a href="#r64">[64]</a> D. T. MacDougal: Botanical Features of North American Deserts; +Carnegie Instit. of Wash., No. 99, 1908.</p></div> + +<div id="fn65"><p><a href="#r65">[65]</a> <em>Loc. cit.</em></p></div> + +<div id="fn66"><p><a href="#r66">[66]</a> H. H. Clayton: Variation in Solar Radiation and the Weather; Smiths. +Misc. Coll., Vol. 71, No. 3, Washington, 1920.</p></div> + +<div id="fn67"><p><a href="#r67">[67]</a> B. Helland Hansen and F. Nansen: Temperature Variations in the +North Atlantic Ocean and in the Atmosphere; Misc. Coll., Smiths. Inst., Vol. 70, No. 4, Washington, 1920.</p></div> + +<div id="fn68"><p><a href="#r68">[68]</a> The climatic significance of ocean currents is well discussed in Croll's +Climate and Time, 1875, and his Climate and Cosmogony, 1889.</p></div> + +<div id="fn69"><p><a href="#r69">[69]</a> F. J. B. Cordeiro: The Gyroscope, 1913.</p></div> + +<div id="fn70"><p><a href="#r70">[70]</a> W. W. Garner and H. A. Allard: Flowering and Fruition of Plants +as Controlled by Length of Day; Yearbook Dept. Agri., 1920, pp. 377-400.</p></div> + +<div id="fn71"><p><a href="#r71">[71]</a> Report of Committee on Sedimentation, National Research Council, +April, 1922.</p></div> + +<div id="fn72"><p><a href="#r72">[72]</a> Chas. Schuchert: The Earth's Changing Surface and Climate during +Geologic Time; in Lull: The Evolution of the Earth and Its Inhabitants, +1918, p. 55.</p></div> + +<div id="fn73"><p><a href="#r73">[73]</a> Quoted by J. Cornet: Cours de Géologie, 1920, p. 330.</p></div> + +<div id="fn74"><p><a href="#r74">[74]</a> T. C. Chamberlin: The Order of Magnitude of the Shrinkage of the +Earth; Jour. Geol., Vol. 28, 1920, pp. 1-17, 126-157.</p></div> + +<div id="fn75"><p><a href="#r75">[75]</a> G. I. Taylor: Philosophical Transactions, A. 220, 1919, pp. 1-33; +Monthly Notices Royal Astron. Soc., Jan., 1920, Vol. 80, p. 308.</p></div> + +<div id="fn76"><p><a href="#r76">[76]</a> J. Jeffreys: Monthly Notices Royal Astron. Soc., Jan., 1920, Vol. 80, +p. 309.</p></div> + +<div id="fn77"><p><a href="#r77">[77]</a> E. W. Brown: personal communication.</p></div> + +<div id="fn78"><p><a href="#r78">[78]</a> C. S. Slichter: The Rotational Period of a Heterogeneous Spheroid; in +Contributions to the Fundamental Problems of Geology, by T. C. Chamberlin, +<em>et al.</em>, Carnegie Inst. of Wash., No. 107, 1909.</p></div> + +<div id="fn79"><p><a href="#r79">[79]</a> E. Suess: The Face of the Earth, Vol. II, p. 553, 1901.</p></div> + +<div id="fn80"><p><a href="#r80">[80]</a> Chas. Schuchert: The Earth's Changing Surface and Climate; in Lull: +The Evolution of the Earth and Its Inhabitants, 1918, p. 78.</p></div> + +<div id="fn81"><p><a href="#r81">[81]</a> J. Barren: Rhythms and the Measurement of Geologic Time; Bull. +Geol. Soc. Am., Vol. 28, 1917, p. 838.</p></div> + +<div id="fn82"><p><a href="#r82">[82]</a> Chas. Schuchert: <i>loc. cit.</i>, p. 78.</p></div> + +<div id="fn83"><p><a href="#r83">[83]</a> T. C. Chamberlin: Diastrophism, the Ultimate Basis of Correlation; +Jour. Geol., Vol. 16, 1909; Chas. Schuchert: <em>loc. cit.</em></p></div> + +<div id="fn84"><p><a href="#r84">[84]</a> Pirsson-Schuchert: Textbook of Geology, 1915, Vol. II, p. 982; Chas. +Schuchert: Paleogeography of North America; Bull. Geol. Soc. Am., Vol. +20, pp. 427-606; reference on p. 499.</p></div> + +<div id="fn85"><p><a href="#r85">[85]</a> The general subject of the climatic significance of continentality is +discussed by C. E. P. Brooks: continentality and Temperature; Quart. +Jour. Royal Meteorol. Soc., April, 1917, and Oct., 1918.</p></div> + +<div id="fn86"><p><a href="#r86">[86]</a> Chas. Schuchert: Climates of Geologic Time; in The Climatic Factor; +Carnegie Institution, 1914, p. 286.</p></div> + +<div id="fn87"><p><a href="#r87">[87]</a> A. de Lapparent: Traité de Géologie, 1906.</p></div> + +<div id="fn88"><p><a href="#r88">[88]</a> Chas. Schuchert: Historical Geology, 1915, p. 464.</p></div> + +<div id="fn89"><p><a href="#r89">[89]</a> M. M. Metcalf: Upon an important method of studying problems of +relationship and of geographical distribution; Proceedings National Academy +of Sciences, Vol. 6, July, 1920, pp. 432-433.</p></div> + +<div id="fn90"><p><a href="#r90">[90]</a> Chas. Schuchert: Paleogeography of North America; Bull. Geol. Soc. +Am., Vol. 20, 1910; and Willis, Salisbury, and others: Outlines of Geologic +History, 1910.</p></div> + +<div id="fn91"><p><a href="#r91">[91]</a> Chas. Schuchert: The Earth's Changing Surface and Climate; in Lull: +The Evolution of the Earth and Its Inhabitants, 1918, p. 50.</p></div> + +<div id="fn92"><p><a href="#r92">[92]</a> A. J. Henry: The Decrease of Precipitation with Altitude; Monthly +Weather Review, Vol. 47, 1919, pp. 33-41.</p></div> + +<div id="fn93"><p><a href="#r93">[93]</a> Chas. F. Brooks: Monthly Weather Review, Vol. 46, 1918, p. 511; and +also A. J. Henry and others: Weather Forecasting in the United States, +1913.</p></div> + +<div id="fn94"><p><a href="#r94">[94]</a> F. H. Knowlton: Evolution of Geologic Climates; Bull. Geol. Soc. Am., +Vol. 30, Dec., 1919, pp. 499-566.</p></div> + +<div id="fn95"><p><a href="#r95">[95]</a> Talbert, quoted by I. Bowman: Forest Physiography, 1911, p. 63.</p></div> + +<div id="fn96"><p><a href="#r96">[96]</a> J. Barrell: Rhythms and the Measurement of Geologic Time; Bull. +Geol. Soc. Am., Vol. 28, 1917, pp. 745-904.</p></div> + +<div id="fn97"><p><a href="#r97">[97]</a> C. E. P. Brooks: The Evolution of Climate in Northwest Europe. Quart. +Jour. Royal Meteorol. Soc., Vol. 47, 1921, pp. 173-194.</p></div> + +<div id="fn98"><p><a href="#r98">[98]</a> H. F. Osborn: Men of the Old Stone Age, N. Y., 1915; J. M. Tyler: +The New Stone Age in Northwestern Europe, N. Y., 1920.</p></div> + +<div id="fn99"><p><a href="#r99">[99]</a> Encyclopædia Britannica, 11th edition: article "Ocean."</p></div> + +<div id="fn100"><p><a href="#r100">[100]</a> C. E. P. Brooks: The Meteorological Conditions of an Ice sheet and +Their Bearing on the Desiccation of the Globe; Quart. Jour. Royal Meteorol. +Soc., Vol. 40, 1914, pp. 53-70.</p></div> + +<div id="fn101"><p><a href="#r101">[101]</a> Data of Geochemistry, Fourth Ed., 1920; Bull. No. 695, U. S. Geol. +Survey.</p></div> + +<div id="fn102"><p><a href="#r102">[102]</a> Quoted by Schuchert in The Evolution of the Earth.</p></div> + +<div id="fn103"><p><a href="#r103">[103]</a> Smithsonian Physical Tables, Sixth Revision, 1914, p. 142.</p></div> + +<div id="fn104"><p><a href="#r104">[104]</a> Chamberlin, in a very suggestive article "On a possible reversal of +oceanic circulation" (Jour. of Geol., Vol. 14, pp. 363-373, 1906), discusses +the probable climatic consequences of a reversal in the direction of deep-sea +circulation. It is not wholly beyond the bounds of possibility that, +in the course of ages the increasing drainage of salt from the lands not +only by nature but by man's activities in agriculture and drainage, may +ultimately cause such a reversal by increasing the ocean's salinity until the +more saline tropical portion is heavier than the cooler but fresher subpolar +waters. If that should happen, Greenland, Antarctica, and the northern +shores of America and Asia would be warmed by the tropical heat which +had been transferred poleward beneath the surface of the ocean, without +loss <em>en route</em>. Subpolar regions, under such a condition of reversed deep-sea +circulation, might have a mild climate. Indeed, they might be among the +world's most favorable regions climatically.</p></div> + +<div id="fn105"><p><a href="#r105">[105]</a> Encyclopædia Britannica: article "Ocean."</p></div> + +<div id="fn106"><p><a href="#r106">[106]</a> Chamberlin and Salisbury: Geology, Vol. II, pp. 1-132, 1906; and T. C. +Chamberlin: The Origin of the Earth, 1916.</p></div> + +<div id="fn107"><p><a href="#r107">[107]</a> Personal communication.</p></div> + +<div id="fn108"><p><a href="#r108">[108]</a> R. T. Chamberlin: Gases in Rocks, Carnegie Inst. of Wash., No. 106, +1908.</p></div> + +<div id="fn109"><p><a href="#r109">[109]</a> J. Barrell: The Origin of the Earth, in Evolution of the Earth and +Its Inhabitants, 1918, p. 44, and more fully in an unpublished manuscript.</p></div> + +<div id="fn110"><p><a href="#r110">[110]</a> F. W. Clarke: Data of Geochemistry, Fourth Ed., 1920, Bull. No. 695, +U. S. Geol. Survey, p. 256.</p></div> + +<div id="fn111"><p><a href="#r111">[111]</a> F. W. Clarke: <em>loc. cit.</em>, pp. 27-34 et al.</p></div> + +<div id="fn112"><p><a href="#r112">[112]</a> Chas. E. St. John: Science Service Press Reports from the Mt. Wilson +Observatory, May, 1922.</p></div> + +<div id="fn113"><p><a href="#r113">[113]</a> Abbot and Fowle: Annals Astrophysical Observatory; Smiths. Inst., +Vol. II, 1908, p. 163.</p> + +<p>F. E. Fowle: Atmospheric Scattering of Light; Misc. Coll. Smiths. Inst., +Vol. 69, 1918.</p></div> + +<div id="fn114"><p><a href="#r114">[114]</a> Abbot and Fowle: <em>loc. cit.</em>, p. 172.</p></div> + +<div id="fn115"><p><a href="#r115">[115]</a> H. H. Turner: On a Long Period in Chinese Earthquake Records; Mon. +Not. Royal Astron. Soc., Vol. 79, 1919, pp. 531-539; Vol. 80, 1920, pp. 617-619; +Long Period Terms in the Growth of Trees; <em>idem</em>, pp. 793-808.</p></div> + +<div id="fn116"><p><a href="#r116">[116]</a> Harlow Shapley: Note on a Possible Factor in Geologic Climates; +Jour. Geol., Vol. 29, No. 4, May, 1921; Novæ and Variable Stars, Pub. +Astron. Soc. Pac., No. 194, Aug., 1921.</p></div> + +<div id="fn117"><p><a href="#r117">[117]</a> J. H. Jeans: Problems of Cosmogony and Stellar Dynamics, Cambridge, +1919.</p></div> + +<div id="fn118"><p><a href="#r118">[118]</a> This fact is so important and at the same time so surprising to the +layman, that a quotation from The Electron Theory of Matter by O. W. +Richardson, 1914, pp. 326 and 334 is here added.</p> +<blockquote> +<p>"It is a very familiar fact that when material bodies are heated they +emit electromagnetic radiations, in the form of thermal, luminous, and +actinic rays, in appreciable quantities. Such an effect is a natural consequence +of the electron and kinetic theories of matter. On the kinetic theory, temperature +is a measure of the violence of the motion of the ultimate particles; +and we have seen that on the electron theory, electromagnetic +radiation is a consequence of their acceleration. The calculation of this +emission from the standpoint of the electron theory alone is a very complex +problem which takes us deeply into the structure of matter and which has +probably not yet been satisfactorily resolved. Fortunately, we can find out +a great deal about these phenomena by the application of general principles +like the conservation of energy and the second law of thermodynamics +without considering special assumptions about the ultimate constitution +of matter. It is to be borne in mind that the emission under +consideration occurs at all temperatures although it is more marked the +higher the temperature.... The energy per unit volume, <em>in vacuo</em>, of the +radiation in equilibrium in an enclosure at the absolute temperature, T, is +equal to a universal constant, A, multiplied by the fourth power of the +absolute temperature. Since the intensity of the radiation is equal to the +energy per unit volume multiplied by the velocity of light, it follows that +the former must also be proportional to the fourth power of the absolute +temperature. Moreover, if E is the total emission from unit area of a +perfectly black body, we see from p. 330 that E=A´T<sup>4</sup>, where A´ is a new +universal constant. This result is usually known as Stefan's Law. It was +suggested by Stefan in the inaccurate form that the total radiant energy +of emission from bodies varies as the fourth power of the absolute temperature, +as a generalization from the results of experiments. The credit for +showing that it is a consequence of the existence of radiation pressure +combined with the principles of thermodynamics is due to Bartoli and +Boltzmann."</p></blockquote></div> + +<div id="fn119"><p><a href="#r119">[119]</a> Quoted by Moulton in his Introduction to Astronomy.</p></div> + +<div id="fn120"><p><a href="#r120">[120]</a> Introduction to Astronomy.</p></div> + +<div id="fn121"><p><a href="#r121">[121]</a> The term billions, here and elsewhere, is used in the American sense, 10<sup>9</sup>.</p></div> + +<div id="fn122"><p><a href="#r122">[122]</a> The assumed number of stars here is ten times as great as in the other +parts of this line.</p></div> + +<div id="fn123"><p><a href="#r123">[123]</a> Lewis Boss: Convergent of a Moving Cluster in Taurus; Astronom. +Jour., Vol. 26, No. 4, 1908, pp. 31-36.</p></div> + +<div id="fn124"><p><a href="#r124">[124]</a> F. R. Moulton: in Introduction to Astronomy, 1916.</p></div> + +<div id="fn125"><p><a href="#r125">[125]</a> A. Penck: Die Alpen im Eiszeitalter, Leipzig, 1909.</p></div> + +<div id="fn126"><p><a href="#r126">[126]</a> R. D. Salisbury: Physical Geography of the Pleistocene, in Outlines of +Geologic History, by Willis and Salisbury, 1910, pp. 273-274.</p></div> + +<div id="fn127"><p><a href="#r127">[127]</a> Davis, Pumpelly, and Huntington: Explorations in Turkestan, Carnegie +Inst. of Wash., No. 26, 1905.</p> + +<p>In North America the stages have been the subject of intensive studies +on the part of Taylor, Leverett, Goldthwait, and many others.</p></div> + +<div id="fn128"><p><a href="#r128">[128]</a> Double star.</p></div> + +<div id="fn129"><p><a href="#r129">[129]</a> E. Kirk: Paleozoic Glaciation in Alaska; Am. Jour. Sci., 1918, p. 511.</p></div> + +<div id="fn130"><p><a href="#r130">[130]</a> J. Milne: Catalogue of Destructive Earthquakes; Rep. Brit. Asso. Adv. +Sci., 1911.</p></div> + +<div id="fn131"><p><a href="#r131">[131]</a> Wm. Bowie: Lecture before the Geological Club of Yale University. +See Am. Jour. Sci., 1921.</p></div> + +<div id="fn132"><p><a href="#r132">[132]</a> Chas. Davisson: On the Annual and Semi-annual Seismic Periods; +Roy. Soc. of London, Philosophical Transactions, Vol. 184, 1893, 1107 <em>ff.</em></p></div> + +<div id="fn133"><p><a href="#r133">[133]</a> C. G. Knott: The Physics of Earthquake Phenomena, Oxford, 1908.</p></div> + +<div id="fn134"><p><a href="#r134">[134]</a> In Table 8 the first column indicates the region; the second, the dates; +and the third, the number of shocks. The fourth column gives the month in +which the annual maximum occurs when the crude figures are smoothed by +the use of overlapping six-monthly means. In other words, the average for +each successive six months has been placed in the middle of the period. +Thus the average of January to June, inclusive, is placed between March +and April, that for February to July between April and May, and so on. +This method eliminates the minor fluctuations and also all periodicities +having a duration of less than a year. If there were no annual periodicity +the smoothing would result in practically the same figure for each month. +The column marked "Amplitude" gives the range from the highest month +to the lowest divided by the number of earthquakes and then corrected +according to Schuster's method which is well known to mathematicians, +but which is so confusing to the layman that it will not be described. Next, +in the column marked "Expected Amplitude," we have the amplitude that +would be expected if a series of numbers corresponding to the earthquake +numbers and having a similar range were arranged in accidental order +throughout the year. This also is calculated by Schuster's method in which +the expected amplitude is equal to the square root of "pi" divided by the +number of shocks. When the actual amplitude is four or more times the +expected amplitude, the probability that there is a real periodicity in the +observed phenomena becomes so great that we may regard it as practically +certain. If there is no periodicity the two are equal. The last column gives +the number of times by which the actual exceeds the expected amplitude, +and thus is a measure of the probability that earthquakes vary systematically +in a period of a year.</p></div> + +<div id="fn135"><p><a href="#r135">[135]</a> N. F. Drake: Destructive Earthquakes in China; Bull. Seism. Soc. Am., +Vol. 2, 1912, pp. 40-91, 124-133.</p></div> + +<div id="fn136"><p><a href="#r136">[136]</a> The only other explanation that seems to have any standing is the +psychological hypothesis of Montessus de Ballore as given in Les Tremblements +de Terre. He attributes the apparent seasonal variation in earthquakes +to the fact that in winter people are within doors, and hence notice movements of the earth much more than in summer when they are out of +doors. There is a similar difference between people's habits in high latitudes +and low. Undoubtedly this does have a marked effect upon the degree +to which minor earthquake shocks are noticed. Nevertheless, de Ballore's +contention, as well as any other psychological explanation, is completely +upset by two facts: First, instrumental records show the same seasonal distribution +as do records based on direct observation, and instruments certainly +are not influenced by the seasons. Second, in some places, notably +China, as Drake has shown, the summer rather than the winter is very +decidedly the time when earthquakes are most frequent.</p></div> + +<div id="fn137"><p><a href="#r137">[137]</a> A comparison of tropical hurricanes with earthquakes is interesting. +Taking all the hurricanes recorded in August, September, and October, from +1880 to 1899, and the corresponding earthquakes in Milne's catalogue, the +correlation coefficient between hurricanes and earthquakes is +0.236, with a +probable error of ±0.082, the month being used as the unit. This is not a +large correlation, yet when it is remembered that the hurricanes represent +only a small part of the atmospheric disturbances in any given month, it +suggests that with fuller data the correlation might be large.</p></div> + +<div id="fn138"><p><a href="#r138">[138]</a> Ellsworth Huntington: The Geographic Work of Dr. M. A. Veeder; +Geog. Rev., Vol. 3, March and April, 1917, Nos. 3 and 4.</p></div> + +<div id="fn139"><p><a href="#r139">[139]</a> Frank Schlesinger: Variations of Latitude; Their Bearing upon Our +Knowledge of the Interior of the Earth; Proc. Am. Phil. Soc., Vol. 54, +1915, pp. 351-358. Also Smithsonian Report for 1916, pp. 248-254.</p></div> + +<div id="fn140"><p><a href="#r140">[140]</a> Harold Jeffreys: Causes Contributory to the Annual Variations of +Latitude; Monthly Notices, Royal Astronomical Soc., Vol. 76, 1916, pp. +499-525.</p></div> + +<div id="fn141"><p><a href="#r141">[141]</a> John Milne: British Association Reports for 1903 and 1906.</p></div> + +<div id="fn142"><p><a href="#r142">[142]</a> C. G. Knott: The Physics of Earthquake Phenomena, Oxford, 1908.</p></div> + +<div id="fn143"><p><a href="#r143">[143]</a> A. C. Lawson: The Mobility of the Coast Ranges of California; Univ. +of Calif. Pub., Geology, Vol. 12, No. 7, pp. 431-473.</p></div> +</div> +<!--End Footnotes--> +</div><!-- End of Book --> + + + + + + + +<pre> + + + + + +End of the Project Gutenberg EBook of Climatic Changes, by +Ellsworth Huntington and Stephen Sargent Visher + +*** END OF THIS PROJECT GUTENBERG EBOOK CLIMATIC CHANGES *** + +***** This file should be named 37855-h.htm or 37855-h.zip ***** +This and all associated files of various formats will be found in: + http://www.gutenberg.org/3/7/8/5/37855/ + +Produced by Robin Monks, Wayne Hammond and the Online +Distributed Proofreading Team at http://www.pgdp.net (This +file was produced from images generously made available +by The Internet Archive/Canadian Libraries) + + +Updated editions will replace the previous one--the old editions +will be renamed. + +Creating the works from public domain print editions means that no +one owns a United States copyright in these works, so the Foundation +(and you!) can copy and distribute it in the United States without +permission and without paying copyright royalties. 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