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diff --git a/37855.txt b/37855.txt new file mode 100644 index 0000000..a13fa45 --- /dev/null +++ b/37855.txt @@ -0,0 +1,11720 @@ +The Project Gutenberg EBook of Climatic Changes, by +Ellsworth Huntington and Stephen Sargent Visher + +This eBook is for the use of anyone anywhere at no cost and with +almost no restrictions whatsoever. 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: ASCII + +*** 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) + + + + + + + + [TRANSCRIBER'S NOTE: 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.] + + + + + CLIMATIC CHANGES + + THEIR NATURE AND CAUSES + + + PUBLISHED ON THE FOUNDATION + ESTABLISHED IN MEMORY OF + THEODORE L. GLASGOW + + + + + OTHER BOOKS BY THE SAME AUTHORS + + + ELLSWORTH HUNTINGTON + + A. _Four books showing the development of knowledge as to Historical + Pulsations of Climate._ + + The Pulse of Asia. Boston, 1907. Explorations in Turkestan. + Expedition of 1903. Washington, 1905. + Palestine and Its Transformation. Boston, 1911. + The Climatic Factor, as Illustrated in Arid America. Washington, + 1914. + + B. _Two books illustrating the effect of climate on man._ + + Civilization and Climate. New Haven, 1915. + World Power and Evolution. New Haven, 1919. + + C. _Four books illustrating the general principles of Geography._ + + Asia: A Geography Reader. Chicago, 1912. + The Red Man's Continent. New Haven, 1919. + Principles of Human Geography (with S. W. Cushing). New York, + 1920. + Business Geography (with F. E. Williams). New York, 1922. + + D. _A companion to the present volume._ + + Earth and Sun: An Hypothesis of Weather and Sunspots. New Haven. + In press. + + + STEPHEN SARGENT VISHER + + Geography, Geology and Biology of Southern Dakota. Vermilion, 1912. + The Biology of Northwestern South Dakota. Vermilion, 1914. + The Geography of South Dakota. Vermilion, 1918. + Handbook of the Geology of Indiana (with others). Indianapolis, 1922. + Hurricanes of Australia and the South Pacific. Melbourne, 1922. + + + + + CLIMATIC CHANGES + + THEIR NATURE AND CAUSES + + BY + + ELLSWORTH HUNTINGTON + Research Associate in Geography in Yale University + + AND + + STEPHEN SARGENT VISHER + Associate Professor of Geology in Indiana University + + [Illustration] + + NEW HAVEN + YALE UNIVERSITY PRESS + LONDON: HUMPHREY MILFORD: OXFORD UNIVERSITY PRESS + MDCCCCXXII + + + + COPYRIGHT 1922 BY + YALE UNIVERSITY PRESS + Published 1922. + + + + THE THEODORE L. GLASGOW MEMORIAL + PUBLICATION FUND + + + 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. + + + + TO + + THOMAS CHROWDER CHAMBERLIN + + OF THE UNIVERSITY OF CHICAGO + + 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 + + + + _There 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._ + + FRANCIS BACON + + + + +PREFACE + + +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 cooeperation, so that by working together +scientists discover much that would else have remained hid. + +This book illustrates the modern trend toward unity in all of these +ways. First, it is a companion volume to _Earth and Sun_. 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. + +In the third place, this volume illustrates the unity of modern science +because it is preeminently a cooeperative product. Not only have the two +authors shared in its production, but several of the Yale Faculty have +also cooeperated. 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 cooeperation 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 cooeperation. 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 cooeperation is much deeper than would appear from this brief +paragraph. + +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 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. + +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. + + + + +CONTENTS + + + PAGE + + I. The Uniformity of Climate 1 + + II. The Variability of Climate 16 + + III. Hypotheses of Climatic Change 33 + + IV. The Solar Cyclonic Hypothesis 51 + + V. The Climate of History 64 + + VI. The Climatic Stress of the Fourteenth Century 98 + + VII. Glaciation According to the Solar Cyclonic Hypothesis 110 + + VIII. Some Problems of Glacial Periods 130 + + IX. The Origin of Loess 155 + + X. Causes of Mild Geological Climates 166 + + XI. Terrestrial Causes of Climatic Changes 188 + + XII. Post-Glacial Crustal Movements and Climatic Changes 215 + + XIII. The Changing Composition of Oceans and Atmosphere 223 + + XIV. The Effect of Other Bodies on the Sun 242 + + XV. The Sun's Journey through Space 264 + + XVI. The Earth's Crust and the Sun 285 + + + + +LIST OF ILLUSTRATIONS + + + PAGE + + Fig. 1. Climatic changes and mountain building 25 + + Fig. 2. Storminess at sunspot maxima vs. minima 54 + + Fig. 3. Relative rainfall at times of increasing and + decreasing sunspots 58, 59 + + Fig. 4. Changes of climate in California and in western + and central Asia 75 + + Fig. 5. Changes in California climate for 2000 years, as + measured by growth of Sequoia trees 77 + + Fig. 6. Distribution of Pleistocene ice sheets 123 + + Fig. 7. Permian geography and glaciation 145 + + Fig. 8. Effect of diminution of storms on movement of water 175 + + Fig. 9. Cretaceous Paleogeography 201 + + Fig. 10. Climatic changes of 140,000 years as inferred from + the stars 279 + + Fig. 11. Sunspot curve showing cycles, 1750 to 1920 283 + + Fig. 12. Seasonal distribution of earthquakes 299 + + Fig. 13. Wandering of the pole from 1890 to 1898 303 + + + + +TABLES + + + PAGE + + 1. The Geological Time Table 5 + + 2. Types of Climatic Sequence 16 + + 3. Correlation Coefficients between Rainfall and Growth of + Sequoias in California 80 + + 4. Correlation Coefficients between Rainfall Records in + California and Jerusalem 84 + + 5. Theoretical Probability of Stellar Approaches 260 + + 6. Thirty-Eight Stars Having Largest Known Parallaxes 276, 277 + + 7. Destructive Earthquakes from 1800 to 1899 Compared with + Sunspots 289 + + 8. Seasonal March of Earthquakes 295 + + 9. Deflection of Path of Pole Compared with Earthquakes 305 + + 10. Earthquakes in 1903 to 1908 Compared with Departures of + the Projected Curve of the Earth's Axis from the + Eulerian Position 306 + + + + +CHAPTER I + +THE UNIFORMITY OF CLIMATE + + +The role 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 deg.C. in average yearly temperature, and for most the +limit of successful growth is 10 deg..[1] 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 reproduction and a tendency toward changes +in specific characteristics. Any great departure means suffering or +death for the individual and destruction for the species. + +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 _Earth and Sun_, and in such books as +Humphreys' _Physics of the Air_, 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. + +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 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. + +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.[2] 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.[3] Although +the Archeozoic has yielded no generally admitted fossils, yet what seem +to be massive algae and sponges have been 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. + +In the next geologic era, the Proterozoic, the researches of Walcott +have shown that besides the marine algae 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. + +TABLE 1 + +THE GEOLOGICAL TIME TABLE[4] + + COSMIC TIME + + FORMATIVE ERA. Birth and growth of the earth. Beginnings of the + atmosphere, hydrosphere, continental platforms, oceanic basins, + and possibly of life. No known geological record. + + GEOLOGIC TIME + + ARCHEOZOIC ERA. Origin of simplest life. + + PROTEROZOIC ERA. Age of invertebrate origins. An early and a late + ice age, with one or more additional ones indicated. + + PALEOZOIC ERA. Age of primitive vertebrate dominance. + + _Cambrian Period._ First abundance of marine animals and dominance + of trilobites. + + _Ordovician Period._ First known fresh-water fishes. + + _Silurian Period._ First known land plants. + + _Devonian Period._ First known amphibians. "Table Mountain" ice + age. + + _Mississippian Period._ Rise of marine fishes (sharks). + + _Pennsylvanian Period._ Rise of insects and first period of marked + coal accumulation. + + _Permian Period._ Rise of reptiles. Another great ice age. + + MESOZOIC ERA. Age of reptile dominance. + + _Triassic Period._ Rise of dinosaurs. The period closes with a + cool climate. + + _Jurassic Period._ Rise of birds and flying reptiles. + + _Comanchean Period._ Rise of flowering plants and higher insects. + + _Cretaceous Period._ Rise of archaic or primitive mammalia. + + CENOZOIC ERA. Age of mammal dominance. + + _Early Cenozoic or Eocene and Oligocene time._ Rise of higher + mammals. Glaciers in early Eocene of the Laramide Mountains. + + _Late Cenozoic or Miocene and Pliocene time._ Transformation of + ape like animals into man. + + _Glacial or Pleistocene time._ Last great ice age. + + PRESENT TIME + + PSYCHOZOIC ERA. Age of man or age of reason. Includes the present or + "Recent time," estimated to be probably less than 30,000 years. + +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. + +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 deg.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 deg.C. (167 deg.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 deg.C. (113 deg.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 deg.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 deg. warmer than at present. + +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 found in places where the present average temperature +approaches 0 deg.C. If those places should be colder than now by 30 deg.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. + +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 deg.C. above or below the present level. +Even this range of 60 deg.C. (108 deg.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. + +A change of 60 deg.C. or even of 20 deg. in the average temperature of the +earth's surface may seem large when viewed from the limited standpoint +of terrestrial experience. 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 deg.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 deg.C., while +thousands of miles in the interior it may rise to 20,000 deg. or 100,000 deg. 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 deg.C., or +287 deg. above the absolute zero. Toward the interior, the temperature in +mines and deep wells rises about 1 deg.C. for every 100 meters. At this rate +it would be over 500 deg.C. at a depth of ten miles, and over 5000 deg. at 100 +miles. + +Let us confine ourselves to surface temperatures, which are all that +concern us in discussing climate. It has been calculated by Poynting[5] +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 deg.C.; at the distance of Venus, 85 deg.; the earth 27 deg.; Mars +-30 deg.; Neptune -219 deg.. 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 anywhere from near -273 deg.C. up to perhaps +5000 deg.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 deg., the earth claims only 60 deg. 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 deg. includes what seem to +be the two most critical of all possible temperatures, namely, the +freezing point of water, 0 deg.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. + +To appreciate the extraordinary significance of this last fact, it is +necessary to realize how extremely critical are the temperatures from +about 0 deg. to 40 deg.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 +_The Fitness of the Environment_, 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 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_{2}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. + +Again, as a solvent there is literally nothing to compare with water. As +Henderson[6] 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, caesium, 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 role in the processes of life. +Finally, no common liquid except mercury equals water in its power of +capillarity. This fact is of enormous moment in biology, most obviously +in respect to the soil. + +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 deg.C. or 59 deg.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 water. In earth, air, fire, and water alike these two +substances are always associated. + +"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."[7] + +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 formulae. 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. + +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 deg.C. and the boiling point, where all water finally turns +into vapor. In the whole range of temperature, so far as 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. + +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[8] 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. + +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 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 nebulae 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 deg. 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.[9] + +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 _uniformity_ +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 from the fact that its surface was cool enough so +that great ice sheets could accumulate on low lands within 40 deg. 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. + +FOOTNOTES: + +[Footnote 1: W. A. Setchell: The Temperature Interval in the +Geographical Distribution of Marine Algae; Science, Vol. 52, 1920, +p. 187.] + +[Footnote 2: J. Barrell: Rhythms and the Measurements of Geologic Time; +Bull. Geol. Soc. Am., Vol. 28, Dec., 1917, pp. 745-904.] + +[Footnote 3: Pirsson and Schuchert: Textbook of Geology, 1915, pp. +538-550.] + +[Footnote 4: 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.] + +[Footnote 5: J. H. Poynting: Radiation in the Solar System; Phil. Trans. +A, 1903, 202, p. 525.] + +[Footnote 6: L. J. Henderson: The Fitness of the Environment, 1913.] + +[Footnote 7: Henderson: _loc. cit._, p. 138.] + +[Footnote 8: F. R. Moulton: Introduction to Astronomy, 1916.] + +[Footnote 9: Moulton: _loc. cit._] + + + + +CHAPTER II + +THE VARIABILITY OF CLIMATE + + +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. + + +-----------------------------------------------------------+ + | | + | TABLE 2 | + | | + | TYPES OF CLIMATIC SEQUENCE | + | | + | 1. Cosmic uniformity. 7. Brueckner periods. | + | 2. Secular progression. 8. Sunspot cycles. | + | 3. Geologic oscillations. 9. Seasonal alternations. | + | 4. Glacial fluctuations. 10. Pleionian migrations. | + | 5. Orbital precessions. 11. Cyclonic vacillations. | + | 6. Historical pulsations. 12. Daily vibrations. | + | | + +-----------------------------------------------------------+ + +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 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. + +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 preeminently 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 deg.F. in the morning to 60 deg. 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 deg. by day and sink no lower than 75 deg. at night. + +The reasons for the secular progression of the earth's 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 +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. + +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. + +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, 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. + +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 deg.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 deg.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. + +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, 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. + +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 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. + +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. 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. + +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 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. + +[Illustration: _Fig. 1. Climatic changes and mountain building._ + +(_After Schuchert, in The Evolution of the Earth and Its Inhabitants, +edited by R. S. Lull._) 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.] + +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 chief hypotheses are outlined in the next +chapter, and are frequently referred to throughout the volume. + +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. + +We come now to Brueckner periods and sunspot cycles. The Brueckner 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 _Klimaschwankungen seit +1700_, Brueckner 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 growth of trees, Douglass, in his +book on _Climatic Cycles and Tree Growth_, 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 +_Earth and Sun_ 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. + +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 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 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. + +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 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. + +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 +until after the third group of sequences has been discussed. This group +includes glacial fluctuations, historic pulsations, Brueckner 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 +_Earth and Sun_ 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. + + + + +CHAPTER III + +HYPOTHESES OF CLIMATIC CHANGE + + +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. + +The conditions which any satisfactory climatic hypothesis must satisfy +are briefly as follows: + +(1) 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. + +(2) 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 _redistribution_ of the heat through changes in +atmospheric and oceanic circulation. At present the great majority of +authorities are on the side of a diminution of heat, but the other view +also deserves study. + +(3) 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. + +(4) 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. + +(5) 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. + +I. _Croll's Eccentricity Theory._ One of the most ingenious and most +carefully elaborated scientific hypotheses is Croll's[10] 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 +place in popular but unscientific books, such as Wells' _Outline of +History_, and even in scientific works like Wright's _Quaternary Ice +Age_. 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. + +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 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. + +II. _The Carbon Dioxide Theory._ 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,[11] adopting an idea suggested by 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. + +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 _Physics of the Air_, 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 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 deg.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. + +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_{2} 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_{2}, 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_{2}, would have an effect of more than +1.3 deg.C. A change of even 2 deg.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 deg. to 8 deg.C. lower than now. + +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. + +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 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. + +Suppose that while such a reversed circulation prevailed, the +atmospheric CO_{2} 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 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. + +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 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. + +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. + +(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. + +(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. + +(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 such material on the sea floor +seem to indicate that the early oceans must have been much fresher than +those of today. + +(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. + +(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. + +(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. + +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. + +III. _The Form of the Land._ Another great cause of climatic change +consists of a group of connected phenomena dependent upon movements of +the earth's crust. 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 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. + +IV. _The Volcanic Hypothesis._ 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,[12] have concluded +that volcanic eruptions are the missing agency. In _Physics of the Air_, +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 deg.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. + +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 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. + +In addition to this the _degree_ 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,[13] 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." + +Koeppen,[14] 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 Koeppen, 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. + +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 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. + +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. + +V. _The Hypothesis of Polar Wandering._ 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 and Pleistocene periods does not seem to conform to this +hypothesis. + +VI. _The Thermal Solar Hypothesis._ 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 _redistribution of the +earth's atmospheric pressure_ and hence produce changes in winds, ocean +currents, and especially storms. This second, or "cyclonic," hypothesis +is the subject of a book entitled _Earth and Sun_, 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 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." + +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. + +FOOTNOTES: + +[Footnote 10: James Croll: Climate and Time, 1876.] + +[Footnote 11: 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. + +T. C. Chamberlin and R. D. Salisbury: Geology, Vol. II, 1906, pp. +93-106, 655-677, and Vol. III, pp. 432-446. + +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. + +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. + +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.] + +[Footnote 12: C. G. Abbot and F. E. Fowle: Volcanoes and Climate; +Smiths. Misc. Coll., Vol. 60, 1913, 24 pp. + +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.] + +[Footnote 13: 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.] + +[Footnote 14: W. Koeppen: Ueber mehrjaehrige Perioden der Witterung +ins besondere uezer die II-jaehrige Periode der Temperatur. Also, +Lufttemperaturen Sonnenflecke und Vulcanausbrueche; Meteorologische +Zeitschrift, Vol. 7, 1914, pp. 305-328.] + + + + +CHAPTER IV + +THE SOLAR CYCLONIC HYPOTHESIS + + +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 _Earth and Sun_; 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. + +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, since they are given in _Earth and Sun_. 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 _Earth and Sun_, +and then to condemn only those parts that are wrong and not the whole +argument. + +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, Koeppen, have shown beyond question that +the temperature of the earth's surface varies in harmony with variations +in the number and area of sunspots.[15] 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 +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. + +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, +extending from Wyoming through Missouri to Nova Scotia, the number of +storms and the amount of rainfall decrease. + +[Illustration: _Fig. 2. Storminess at sunspot maxima vs. minima._ +(_After Kullmer._) + +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.] + +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 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. + +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 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. + +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. + +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 +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 essential point is +that when the solar atmosphere is active the storminess of the earth +increases, and that is a matter of direct observation. + +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, 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. + +[Illustration: _Fig. 3A. Relative rainfall at times of increasing and +decreasing sunspots._ + +Heavy shading, more rain with increasing spots. Light shading, more rain +with decreasing spots. No data for unshaded areas. + +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.] + +[Illustration: _Fig. 3B. Relative rainfall at times of increasing and +decreasing sunspots._ + +Heavy shading, more rain with increasing spots. Light shading, more rain +with decreasing spots. No data for unshaded areas. 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.] + +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. + +The rest of this chapter will be devoted to a _resume_ 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 +faculae, 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 _Earth and Sun_, 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 _Earth and Sun_ support this +conclusion. The chief difficulty in accepting it hitherto has been that +although Jupiter, because of its size, would be 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. + +If the preceding reasoning is correct, any disturbance of the solar +atmosphere must have an effect upon the 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. + +Here we must leave the cyclonic hypothesis of climate and must refer the +reader once more to _Earth and Sun_ 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. + +FOOTNOTES: + +[Footnote 15: 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.] + + + + +CHAPTER V + +THE CLIMATE OF HISTORY[16] + +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.[17] 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. + +In the hot debate over this problem during recent 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. + +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. + +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 deg. and 40 deg. from the equator, and preeminently 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,[18] pronounces it "ueberzeugend." + +A single quotation from scores that might be used will illustrate the +conclusions of some of the most careful archaeologists.[19] + + 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. + + 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 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. + + 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 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. + + 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 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.[20] 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. + +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. + +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. + +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.[21] 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. + +The hypotheses of climatic uniformity and of progressive 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. + +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 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. + +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.[22] The work of Douglass[23] 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 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 _The Climatic Factor_. + +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 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. + +[Illustration: _Fig. 4. Changes of climate in California (solid line) +and in western and central Asia (dotted line)._ + +Note. The curves of Figs. 4 and 5 are reproduced as published in _The +Solar Hypothesis_ 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.] + +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.[24] 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 Stein[25] 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. + +[Illustration: _Fig. 5. Changes in California climate for 2000 years, +as measured by growth of Sequoia trees._ + +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.] + +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 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. + +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. + +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. + + +-------------------------------------------------------------------+ + | TABLE 3 | + | | + | CORRELATION COEFFICIENTS BETWEEN RAINFALL AND | + | GROWTH OF SEQUOIAS IN CALIFORNIA[26] | + | | + | (_r_) = _Correlation coefficient_ | + | (_e_) = _Probable error_ | + | (_r_/_e_) = _Ratio of coefficient to probable error_ | + | | + | A. SACRAMENTO RAINFALL AND GROWTH OF 18 SEQUOIAS IN DRY | + | LOCATIONS, 1861-1910 | + | | + | (_r_) (_e_) (_r_/_e_) | + | ------ ------ ----- | + | 1 year of rainfall -0.059 +-0.096 0.6 | + | 2 years of rainfall +0.288 +-0.090 3.2 | + | 3 years of rainfall +0.570 +-0.066 8.7 | + | 4 years of rainfall +0.470 +-0.076 6.2 | + | | + | B. SACRAMENTO RAINFALL AND GROWTH OF 112 SEQUOIAS MOSTLY IN | + | MOIST LOCATIONS, 1861-1910 | + | | + | 3 years of rainfall +0.340 +-0.087 3.9 | + | 4 years of rainfall +0.371 +-0.084 4.5 | + | 5 years of rainfall +0.398 +-0.082 4.9 | + | 6 years of rainfall +0.418 +-0.079 5.3 | + | 7 years of rainfall +0.471 +-0.076 6.2 | + | 8 years of rainfall (+0.520) +-0.071 7.3 | + | 9 years of rainfall +0.575 +-0.065 8.8 | + | 10 years of rainfall +0.577 +-0.065 8.8 | + | | + | C. SACRAMENTO RAINFALL AND GROWTH OF 80 SEQUOIAS IN MOIST | + | LOCATIONS, 1861-1910 | + | | + | 10 years of rainfall +0.605 +-0.062 9.8 | + | | + | D. ANNUAL SEQUOIA GROWTH AND RAINFALL OF PRECEDING 5 YEARS | + | AT STATIONS ON SOUTHERN PACIFIC RAILROAD | + | | + | 1 = _Years_ | + | 2 = _Altitude_ (_feet_) | + | 3 = _Rainfall_ (_inches_) | + | 4 = _Approximate distance from sequoias_ (_miles_) | + | | + | 1 2 3 4 (_r_) (_e_) (_r_/_e_) | + | --------- ---- ----- --- ------ ------ --------- | + | Sacramento, 1861-1910 70 19.40 200 +0.398 +-0.081 4.9 | + | Colfax, 1871-1909 2400 48.94 200 +0.122 +-0.113 1.1 | + | Summit, 1871-1909 7000 48.07 200 +0.148 +-0.113 1.3 | + | Truckee, 1871-1909 5800 27.12 200 +0.300 +-0.105 2.9 | + | Boca, 1871-1909 5500 20.34 200 +0.604 +-0.076 8.0 | + | Winnemucca, 1871-1909 4300 8.65 300 +0.492 +-0.089 5.5 | + | | + +-------------------------------------------------------------------+ + +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. + +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 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. + +If a rainfall record were available for the place where the trees +actually grow, the relationship would probably be still closer. + +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 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. + +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 deg.N versus 37 deg.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. + + +-------------------------------------------------------------------+ + | TABLE 4 | + | | + | CORRELATION COEFFICIENTS BETWEEN | + | RAINFALL RECORDS IN CALIFORNIA | + | AND JERUSALEM | + | | + | (_r_) = _Correlation coefficient_ | + | (_e_) = _Probable error_ | + | (_r_/_e_) = _Ratio of coefficient to probable error_ | + | | + | A. JERUSALEM RAINFALL FOR 3 YEARS AND VARIOUS GROUPS OF | + | SEQUOIAS[27] | + | | + | (_r_) (_e_) (_r_/_e_)| + | ------ ----- ---------| + | 11 trees measured by Douglass +0.453 +-0.078 5.8 | + | 80 trees, moist locations, Groups IA, | + | IIA, IIIA, VA +0.500 +-0.073 6.8 | + | 101 trees, 69 in moist locations, 32 in | + | dry, I, II, III +0.616 +-0.061 10.1 | + | 112 trees, 80 in moist locations, 32 in | + | dry, I, II, III, V +0.675 +-0.053 12.7 | + | | + | B. RAINFALL AT JERUSALEM AND AT STATIONS IN CALIFORNIA AND NEVADA | + | | + | 1 = _Altitude_ (_feet_) | + | 2 = _Years_ | + | | + | -- 3 years -- -- 5 years -- | + | 1 2 (_r_) (_r_/_e_) (_r_) (_r_/_e_) | + | ---- --------- ------ ------- ------ ------- | + | Sacramento, 70 1861-1910 +0.386 4.7 +0.352 4.2 | + | Colfax, 2400 1871-1909 +0.311 3.1 +0.308 3.0 | + | Summit, 7000 1871-1909 +0.099 0.9 +0.248 2.3 | + | Truckee, 5800 1871-1909 +0.229 2.2 +0.337 3.3 | + |[A]Boca, 5500 1871-1909 +0.482 6.4 +0.617 8.6 | + | Winnemucca, 4300 1871-1909 +0.235 2.2 +0.260 2.4 | + | San Bernardino, 1050 1871-1909 +0.275 2.7 +0.177 1.8 | + | | + | C. RAINFALL FOR 3 YEARS AT CALIFORNIA AND NEVADA STATIONS, | + | 1871-1909 | + | | + | (_r_) (_r_/_e_) | + | ------ ------- | + | Sacramento and San Bernardino +0.663 10.7 | + | San Bernardino and Winnemucca +0.291 2.8 | + | | + +-------------------------------------------------------------------+ + +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. + +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 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. + +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 +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 _World Power and Evolution_. 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. + +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 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. + +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 _Overland to India_. 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 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: + + +------------------------------------------------------------+ + | | + | _Mean rainfall | + | during period | + | _1885_ _1905_ since | + | observations | + | began_ | + | Yuma, Arizona, 2.72 11.41 3.13 | + | Phoenix, Arizona, 3.77 19.73 7.27 | + | Tucson, Arizona, 5.26 24.17 11.66 | + | Lordsburg, New Mexico, 3.99 19.50 8.62 | + | El Paso, Texas (on New | + | Mexico border), 7.31 17.80 9.06 | + | ---- ----- ----- | + | Average, 4.61 18.52 7.95 | + | | + +------------------------------------------------------------+ + +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 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. + +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,[28] 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 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: + + 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, + _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_.... 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 deg.F. The isothermal of + 69 deg. 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. + + 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 deg.. Between the isotherms of 68 deg. and 64 deg., + 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 deg., 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. + + 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. + + The date palm therefore affords a test of variations in mean annual + temperature of three grades between 62 deg. and 69 deg.. + + 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. + + 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.... + + 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 _the_ city of palm trees.[29] + +In the main Gregory's conclusions seem to be well grounded, although +even according to his data a change of 2 deg. or 3 deg. 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 deg. or 6 deg.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. + +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 deg.C. On the other hand, the ten years of least +rainfall had an average of 483 mm. and a mean temperature of 9.35 deg.. In +other words, a difference of 137 mm., or 39 per cent, in rainfall was +accompanied by a difference of only 0.2 deg.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 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,[30] 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 deg.F. during the more rainy decade and 66.94 deg.F. during +the less rainy decade--a difference of only 0.42 deg.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 _Quarterly Journal of the Palestine +Exploration Fund_ 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 deg.F. The twelve with least rain +had 24.4 inches and a temperature of 62.44 deg.. A difference of 40 per cent +in rainfall was accompanied by a difference of only 0.4 deg.F. in +temperature. + +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 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. + +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 _The Climatic Factor_, 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 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. + +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 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. + +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. + +FOOTNOTES: + +[Footnote 16: Much of this chapter is taken from The Solar Hypothesis of +Climatic Changes; Bull. Geol. Soc. Am., Vol. 25, 1914.] + +[Footnote 17: 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.] + +[Footnote 18: J. Hann: Klimatologie, Vol. 1, 1908, p. 352.] + +[Footnote 19: H. C. Butler: Desert Syria, the Land of a Lost +Civilization; Geographical Review, Feb., 1920, pp. 77-108.] + +[Footnote 20: 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.] + +[Footnote 21: H. Leiter: Die Frage der Klimaanderung waherend +geschichtlicher Zeit in Nordafrika. Abhandl. K. K. Geographischen +Gesellschaft, Wien, 1909, p. 143.] + +[Footnote 22: 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.] + +[Footnote 23: 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.] + +[Footnote 24: The dotted line is taken from Palestine and Its +Transformation, pp. 327 and 403.] + +[Footnote 25: M. A. Stein: Ruins of Desert Cathay, London, 1912.] + +[Footnote 26: In the preparation and interpretation of this table the +help of Mr. G. B. Cressey is gratefully acknowledged.] + +[Footnote 27: 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.] + +[Footnote A: One year interpolated.] + +[Footnote 28: J. W. Gregory: Is the Earth Drying Up? Geog. Jour., Vol. +43, 1914, pp. 148-172 and 293-318.] + +[Footnote 29: Geog. Jour., Vol. 43, pp. 159-161.] + +[Footnote 30: See A. J. Henry: Secular Variation of Precipitation in the +United States; Bull. Am. Geog. Soc., Vol. 46, 1914, pp. 192-201.] + + + + +CHAPTER VI + +THE CLIMATIC STRESS OF THE FOURTEENTH CENTURY + + +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 _World Power and +Evolution_, but its importance and the fact that new evidence is +constantly coming to light warrant a fuller discussion. + +To begin with Europe; according to the careful account of Pettersson[31] +the fourteenth century shows + + 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 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." + +Pettersson goes on to speak of two oceanic phenomena on which the old +chronicles lay greater stress than on all others: + + 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 to note that + of the nineteen great floods on record eighteen occurred in the cold + season between the autumnal and vernal equinoxes. + + 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. + +Norlind[32] 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. + +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: + + 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 Luebeck, 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. In 1367 corn was imported from Luebeck 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 + Asbjoern Selsbane the chieftains in Trondhenaes [still farther north + in latitude 69 deg.] 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. + +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[33] 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 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. + +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. + +These facts regarding northern and central Europe during the fourteenth +century are particularly significant when compared with the conclusions +which we have drawn in _Earth and Sun_ 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 +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. + +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. + +Turning now to the east of Europe, Brueckner's[34] study 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.[35] + +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. + +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 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. + +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 case. Today no navigator would think of keeping close inland. The +old route also went _north_ 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 _Kings' Mirror_, 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: + + ... 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. + + 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. + + 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 a more suitable + neighborhood. This they found in the land colonized by the Norsemen + whom they attacked and finally annihilated. + +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.[36] Among the Mayas, as +we have seen, relatively dry periods were the times of greatest +progress. + +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 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. + +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 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,[37] 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. + +FOOTNOTES: + +[Footnote 31: O. Pettersson: The connection between hydrographical and +meteorological phenomena; Quarterly Journal of the Royal Meteorological +Society, Vol. 38, pp. 174-175.] + +[Footnote 32: A. Norlind: Einige Bemerkungen ueber das Klima der +historischen Zeit nebst einem Verzeichnis mittelaltlicher Witterungs +erscheinungen; Lunds Univ. Arsskrift, N. F., Vol. 10, 1914, 53 pp.] + +[Footnote 33: Thorwald Rogers: A History of Agriculture and Prices in +England.] + +[Footnote 34: E. Brueckner: Klimaschwankungen seit 1700, Vienna, 1891.] + +[Footnote 35: For a full discussion of the changes in the Caspian Sea, +see The Pulse of Asia, pp. 329-358.] + +[Footnote 36: S. Q. Morley: The Inscriptions at Copan; Carnegie Inst. of +Wash., No. 219, 1920. + +Ellsworth Huntington: The Red Man's Continent, 1919.] + +[Footnote 37: See summary of Wolf's work with additional information by +H. Fritz; Zuerich Vierteljahrschrift, Vol. 38, 1893, pp. 77-107.] + + + + +CHAPTER VII + +GLACIATION ACCORDING TO THE SOLAR-CYCLONIC HYPOTHESIS[38] + + +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. + +Facts not considered at the time of framing an hypothesis 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. + +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[39] estimates that eastern North America in general stood not +more than a few hundred feet higher than now, and W. B. Wright[40] +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 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. + +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. + +The conditions which prevail at times of increased storminess have been +discussed in detail in _Earth and Sun_. 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 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.[B] + +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 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. + +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. + +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 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. + +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.[41] 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 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. + +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. + +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 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. + +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. + +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 cooeperate 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. + +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 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.[42] 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. + +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 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 deg.F. water absorbs 32 +per cent more carbon dioxide than at 68 deg.. 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. + +At times of great solar activity the agencies mentioned above would +apparently cooeperate 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 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. + +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 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. + +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. + +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.[43] 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 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 debris 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. + +[Illustration: _Fig. 6. Distribution of Pleistocene ice sheets._ +(_After Schuchert._)] + +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 deg. 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. + +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.[44] 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. + +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 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 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. + +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. + +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 fierce animals and a fiercer +climate to maintain life in an area whose habitability had long been +decreasing. + +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 _Earth +and Sun_, 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 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. + +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.[45] 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. + +When the sun's activity began to diminish, all these conditions, as well +as several others, would cooeperate 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. + +FOOTNOTES: + +[Footnote 38: 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.] + +[Footnote 39: R. D. Salisbury: Physical Geography of the Pleistocene, in +Outlines of Geologic History, by Willis, Salisbury, and others, 1910, p. +265.] + +[Footnote 40: The Quaternary Ice Age, 1914, p. 364.] + +[Footnote B: For fuller discussion of climatic controls see S. S. +Visher: Seventy Laws of Climate, Annals Assoc. Am. Geographers, 1922.] + +[Footnote 41: Many of these alterations are implied or discussed in the +following papers: + +1. F. W. Harmer: Influence of Winds upon the Climate of the Pleistocene; +Quart. Jour. Geol. Soc., Vol. 57, 1901, p. 405. + +2. 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; _op. cit._, Vol. 47, 1921, pp. 173-194. + +3. W. H. Hobbs: The Role of the Glacial Anticyclone in the Air +Circulation of the Globe; Proc. Am. Phil. Soc., Vol. 54, 1915, pp. +185-225.] + +[Footnote 42: W. B. Wright: The Quaternary Ice Age, 1914, p. 100.] + +[Footnote 43: 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.] + +[Footnote 44: Chamberlin and Salisbury: Geology, 1906, Vol. 3, and W. H. +Hobbs: Characteristics of Existing Glaciers, 1911.] + +[Footnote 45: S. S. Visher: The Geography of South Dakota; S. D. Geol. +Surv., 1918.] + + + + +CHAPTER VIII + +SOME PROBLEMS OF GLACIAL PERIODS + + +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. + +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 carbon dioxide, fail to account for localization. The +cyclonic form of the solar hypothesis, however, seems to afford a +satisfactory explanation. + +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 deg. in Kansas and Kentucky, and 37 deg. 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? + +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. + +In the southern hemisphere glaciation at low latitudes 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. + +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 deg.C., while the mean for the +whole year is below -10 deg.C. If the temperature during a glacial period +averaged 6 deg.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. + +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 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. + +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. + +Although the incompetence of low temperature to cause glaciation, and +the relative unimportance of the mountains 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 _Earth and Sun_ 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. + +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, Hobbs,[46] 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. + +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 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. + +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 deg.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 deg.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 deg.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 deg. 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. + +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 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. + +II. The sudden coming of glaciation is another problem which has been a +stumbling-block in the way of every glacial hypothesis. In his _Climates +of Geologic Times_, 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. + +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. + +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 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. + +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.[47] 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 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.[48] 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. Koeppen'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. + +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 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. + +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. + +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. 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,[49] in an interesting but neglected volume, +was one of the first to marshal the evidence of this sort. More recently +Free[50] 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 _Explorations in Turkestan_, _The Pulse +of Asia_, and _Palestine and Its Transformation_. 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. + +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 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 deg.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.[51] Many small lakes in the Old World expanded still more.[52] +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.[53] 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.[54] + +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 +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.[55] Some of the stronger cyclones +locally yield more rain in a day or two than other sources yield in a +year. + +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 deg. 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 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. + +[Illustration: _Fig. 7. Permian geography and glaciation._ +(_After Schuchert._)] + +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 +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 deg. 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. + +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, 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. + +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 _Earth and Sun_, occur in the autumn +and 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 cooeperate to produce a +similar result. For example, the general low temperature would cause the +sea to be covered with 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. + +One other possibility may be mentioned, although little stress should be +laid on it. In _Earth and Sun_ 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 24-1/2 deg. W. of N. and in 1906 17 deg. 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. +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. + +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 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. + +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. + +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. + +Nevertheless it is not yet possible to determine how warm the oceans +were at the actual time of the Permian 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 those several +circumstances, changes in cyclonic storms due apparently to activity of +the sun's atmosphere must always be reckoned. + +FOOTNOTES: + +[Footnote 46: W. H. Hobbs: Characteristics of Existing Glaciers, 1911. +The Role of the Glacial Anticyclones in the Air Circulation of the +Globe; Proc. Am. Phil. Soc., Vol. 54, 1915, pp. 185-225.] + +[Footnote 47: R. D. Salisbury: Physiography, 1919.] + +[Footnote 48: Griffith Taylor: Australian Meteorology, 1920, p. 283.] + +[Footnote 49: J. D. Whitney: Climatic Changes of the Later Geological +Times, 1882.] + +[Footnote 50: 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.] + +[Footnote 51: G. K. Gilbert: Lake Bonneville; Monograph 1, U. S. Geol. +Surv.] + +[Footnote 52: C. E. P. Brooks: Quart. Jour. Royal Meteorol. Soc., 1914, +pp. 63-66.] + +[Footnote 53: H. J. L. Beadnell: A. Egyptian Oasis, London, 1909. +Ellsworth Huntington: The Libyan Oasis of Kharga; Bull. Am. Geog. Soc., +Vol. 42, Sept., 1910, pp. 641-661.] + +[Footnote 54: S. S. Visher: The Bajada of the Tucson Bolson of Southern +Arizona; Science, N. S., Mar. 23, 1913. + +Ellsworth Huntington: The Basins of Eastern Persia and Seistan, in +Explorations in Turkestan.] + +[Footnote 55: Griffith Taylor: Australian Meteorology, 1920, p. 189.] + + + + +CHAPTER IX + +THE ORIGIN OF LOESS + + +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 America 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.[56] + +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. + +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 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. + +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. + +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 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. + +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. + +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 +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. + +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 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 deg.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. + +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 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. + +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 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. + +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 Wuerm 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 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. + +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 _Earth and Sun_. Hence it would +have a maximum effect in producing aridity. This would favor the +formation of loess, but it is doubtful whether the aridity would become +extreme 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. + +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. + +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 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. + +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.[57] 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 debris. 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. + +FOOTNOTES: + +[Footnote 56: Chamberlin and Salisbury: Geology, 1906, Vol. III, pp. +405-412.] + +[Footnote 57: It may have retreated soon after reaching its maximum. If +so, the general lack of thick terminal moraines would be explained. See +page 122.] + + + + +CHAPTER X + +CAUSES OF MILD GEOLOGICAL CLIMATES + + +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. + +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[58] 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 during the course of geological +history. Our task in this chapter is to explain them. + +Knowlton[59] 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 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." + +Knowlton is so strongly impressed by the widespread fossil floras that +usually occur in the middle parts of the geological periods, that as +Schuchert[3] 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 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[60] 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." + +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,[61] +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, many +of them must have adapted themselves for living in cold climates also. +Of this I have not the least doubt." + +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. + +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 of +coal are found in red beds in many parts of the world, which implies the +presence of swamps but little above sea-level." + +Students of desert botany are likely to doubt the force of these +considerations. As MacDougal[62] 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. + +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 marked as +that of today prevailed. As Schuchert[63] puts it: "Today the variation +on land between the tropics and the poles is roughly between 110 deg. and +-60 deg.F., in the oceans between 85 deg. and 31 deg.F. In the geologic past the +temperature of the oceans for the greater parts of the periods probably +was most often between 85 deg. and 55 deg.F., while on land it may have varied +between 90 deg. and 0 deg.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." + +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. Today, as is +shown in _Earth and Sun_, 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 deg.C. in the temperature at the earth's +surface. This includes not only an actual lowering of 0.6 deg.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 deg.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 deg. or +10 deg.C. + +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 reenforced in a paper by Clayton[64] 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 +_Earth and Sun_, 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 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." + +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.[65] 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. + +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 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. + +[Illustration: _Fig. 8. Effect of diminution of storms on movement of +water._] + +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 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.[66] + +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. + +Consider now the magnitude of the probable effect of a diminution in +storms. Today off the coast of Norway in latitude 65 deg.N. and longitude +10 deg.E., the mean temperature in January is 2 deg.C. and in July 12 deg.C. This +represents a plus anomaly of about 22 deg. in January and 2 deg. 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. + +If such conditions raised the winter temperature only 8 deg.C. and the +summer temperature 4 deg.C., the climate would be as warm as that of the +northern island of New Zealand (latitude 35 deg.-43 deg.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 general temperature of the earth's surface were raised 5 deg. +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 deg.C., the January temperature would be 18 deg.C. and the July +temperature 22 deg.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. + +Another factor would cooeperate 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. + +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, +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 deg.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 deg.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. + +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 +only by driving more warm water poleward but by causing more polar water +to reach low latitudes. + +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 deg.N. The +ocean temperature would then have to average scarcely lower than 20 deg.C. +and even in the coldest month the water could scarcely fall below about +15 deg.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 deg.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 deg.C. for the coldest part of the ocean may perhaps be too low for a +time when corals thrived in latitude 70 deg.. + +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 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. + +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,[67] 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 formulae to it, on the following +assumptions: (1) That the original position of the axis was nearly +vertical to the 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. + +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. + +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 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. + +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[68] 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,[69] 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 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. + +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 caused a flowerbud in its early stages not +only to stop developing but to return once more to a vegetative shoot. + + 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. + + 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. + + 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 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. + +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 +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. + +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. + +FOOTNOTES: + +[Footnote 58: Rollin T. Chamberlin: Personal Communication.] + +[Footnote 59: F. H. Knowlton: Evolution of Geologic Climates; Bull. +Geol. Soc. Am., Vol. 30, 1919, pp. 499-566.] + +[Footnote 60: Chas. Schuchert: Review of Knowlton's Evolution of +Geological Climates, in Am. Jour. Sci., 1921.] + +[Footnote 61: G. R. Wieland: Distribution and Relationships of the +Cycadeoids; Am. Jour. Bot., Vol. 7, 1920, pp. 125-145.] + +[Footnote 62: D. T. MacDougal: Botanical Features of North American +Deserts; Carnegie Instit. of Wash., No. 99, 1908.] + +[Footnote 63: _Loc. cit._] + +[Footnote 64: H. H. Clayton: Variation in Solar Radiation and the +Weather; Smiths. Misc. Coll., Vol. 71, No. 3, Washington, 1920.] + +[Footnote 65: 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.] + +[Footnote 66: The climatic significance of ocean currents is well +discussed in Croll's Climate and Time, 1875, and his Climate and +Cosmogony, 1889.] + +[Footnote 67: F. J. B. Cordeiro: The Gyroscope, 1913.] + +[Footnote 68: 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.] + +[Footnote 69: Report of Committee on Sedimentation, National Research +Council, April, 1922.] + + + + +CHAPTER XI + +TERRESTRIAL CAUSES OF CLIMATIC CHANGES + + +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.[70] 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. + +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 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. + +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. + +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. + +The extent of the earth's contraction may be judged 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.[71] This is a radial +shortening of about 250 miles. Possibly the shrinkage has been even +greater than this. Chamberlin[72] 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. + +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 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[73] has studied the tides of the Irish Sea, and his work +has led Jeffreys[74] and Brown[75] 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. + +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 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.[76] + +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[77] 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 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. + +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. + +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.[78] Furthermore, although 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.[79] +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.[80] 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. + +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. + +Although the processes which cause contraction probably go on steadily, +their effect, as Chamberlin[81] 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, +reenforces 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. + +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,[82] 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.[83] This modification would occur chiefly +because water becomes warm far more slowly than land, and cools off far +more slowly. + +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 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. + +(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. + +(e) As the extent of the ocean diminished, there would normally be a +decrease in the amount of water vapor for 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.[84] + +Whether or not there has been a _progressive_ 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. + +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. + +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 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,[85] 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.[86] + +[Illustration: _Fig. 9. Cretaceous Paleogeography._ +(_After Schuchert._)] + +Although there is much evidence of periodic increase of the sea in +equatorial latitudes and of land in high latitudes, it has remained for +the zooelogist 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. + + 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.[87] + +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. + +(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. + +(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. + +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 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. + +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 +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.[88] + +(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. + +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 that glaciation depends on the +availability of moisture even more than upon low temperature. + +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. + +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.[89] 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 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[90] 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. + +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 +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,[91] 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. + +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 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. + +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. + +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 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. + +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. Hence even after they +are cool they increase the climatic diversity of the land. + +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. + +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,[92] 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 areas. It is +estimated to raise the average temperature of the earth's surface less +than 0.1 deg.C.[93] + +In order to contribute enough heat to raise the surface temperature +1 deg.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[94] +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. + +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 +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. + +FOOTNOTES: + +[Footnote 70: 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.] + +[Footnote 71: Quoted by J. Cornet: Cours de Geologie, 1920, p. 330.] + +[Footnote 72: T. C. Chamberlin: The Order of Magnitude of the Shrinkage +of the Earth; Jour. Geol., Vol. 28, 1920, pp. 1-17, 126-157.] + +[Footnote 73: G. I. Taylor: Philosophical Transactions, A. 220, 1919, +pp. 1-33; Monthly Notices Royal Astron. Soc., Jan., 1920, Vol. 80, +p. 308.] + +[Footnote 74: J. Jeffreys: Monthly Notices Royal Astron. Soc., Jan., +1920, Vol. 80, p. 309.] + +[Footnote 75: E. W. Brown: personal communication.] + +[Footnote 76: C. S. Slichter: The Rotational Period of a Heterogeneous +Spheroid; in Contributions to the Fundamental Problems of Geology, by +T. C. Chamberlin, _et al._, Carnegie Inst. of Wash., No. 107, 1909.] + +[Footnote 77: E. Suess: The Face of the Earth, Vol. II, p. 553, 1901.] + +[Footnote 78: Chas. Schuchert: The Earth's Changing Surface and Climate; +in Lull: The Evolution of the Earth and Its Inhabitants, 1918, p. 78.] + +[Footnote 79: J. Barren: Rhythms and the Measurement of Geologic Time; +Bull. Geol. Soc. Am., Vol. 28, 1917, p. 838.] + +[Footnote 80: Chas. Schuchert: _loc. cit._, p. 78.] + +[Footnote 81: T. C. Chamberlin: Diastrophism, the Ultimate Basis of +Correlation; Jour. Geol., Vol. 16, 1909; Chas. Schuchert: _loc. cit._] + +[Footnote 82: 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.] + +[Footnote 83: 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.] + +[Footnote 84: Chas. Schuchert: Climates of Geologic Time; in The +Climatic Factor; Carnegie Institution, 1914, p. 286.] + +[Footnote 85: A. de Lapparent: Traite de Geologie, 1906.] + +[Footnote 86: Chas. Schuchert: Historical Geology, 1915, p. 464.] + +[Footnote 87: 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.] + +[Footnote 88: Chas. Schuchert: Paleogeography of North America; Bull. +Geol. Soc. Am., Vol. 20, 1910; and Willis, Salisbury, and others: +Outlines of Geologic History, 1910.] + +[Footnote 89: Chas. Schuchert: The Earth's Changing Surface and Climate; +in Lull: The Evolution of the Earth and Its Inhabitants, 1918, p. 50.] + +[Footnote 90: A. J. Henry: The Decrease of Precipitation with Altitude; +Monthly Weather Review, Vol. 47, 1919, pp. 33-41.] + +[Footnote 91: 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.] + +[Footnote 92: F. H. Knowlton: Evolution of Geologic Climates; Bull. +Geol. Soc. Am., Vol. 30, Dec., 1919, pp. 499-566.] + +[Footnote 93: Talbert, quoted by I. Bowman: Forest Physiography, 1911, +p. 63.] + +[Footnote 94: J. Barrell: Rhythms and the Measurement of Geologic Time; +Bull. Geol. Soc. Am., Vol. 28, 1917, pp. 745-904.] + + + + +CHAPTER XII + +POST-GLACIAL CRUSTAL MOVEMENTS AND CLIMATIC CHANGES + + +An interesting practical application of some of the preceding +generalizations is found in an attempt by C. E. P. Brooks[95] 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: + + _Phase_ _Climate_ _Date_ + + 1. The Last Great Arctic climate. 30,000-18,000 B. C. + Glaciation. + 2. The Retreat of the Severe continental 18,000-6000 B. C. + Glaciers. climate. + 3. The Continental Phase. Continental climate. 6000-4000 B. C. + 4. The Maritime Phase. Warm and moist. 4000-3000 B. C. + 5. The Later Forest Phase. Warm and dry. 3000-1800 B. C. + 6. The Peat-Bog Phase. Cooler and moister. 1800 B. C.-300 A. D. + 7. The Recent Phase. Becoming drier. 300 A. D.- + +Brooks bases his chronology largely on De Geer's measurements of the +annual layers of clay in lake 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 deg.C. in the +temperature of the British Isles and of 2 deg.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. + +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.[96] 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 deg.N. Then came the Fennoscandian pause, or Gschnitz stage, when for +about 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 deg. greater than at present. This he +calculates to have caused the climate of Germany and Sweden to be 1 deg.F. +colder than at present in winter and 1 deg.F. warmer in summer. + +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 deg.. 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. + +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 deg.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. + +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 deg.F. higher than at present and in +southwestern Norway 2 deg.. 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. + +At this point Brooks turns to other parts of the world. 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: + + Maxima 3500 B. C.--------2100 B. C.--------350 B. C.-------A. D. 1434 + Minima ---------2800 B. C.--------1200 B. C.-------A. D. 530--------- + +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. + +To return to Brooks' sketch of the relation of climatic pulsations to +the altitude of the lands, by 3000 B. C., that 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. + +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 peculiarly wet and damp. At this point Brooks again departs +from northwest Europe to a wider field: + + 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. + +Brooks ends his paper with a brief resume 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: + + 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. + + 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 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 _great_ climatic changes is not misplaced. + +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. + +FOOTNOTES: + +[Footnote 95: C. E. P. Brooks: The Evolution of Climate in Northwest +Europe. Quart. Jour. Royal Meteorol. Soc., Vol. 47, 1921, pp. 173-194.] + +[Footnote 96: 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.] + + + + +CHAPTER XIII + +THE CHANGING COMPOSITION OF OCEANS AND ATMOSPHERE + + +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. + +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 of increased salinity. Yet it +seems clear that an increase in salinity must have a slight effect upon +climate. + +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. + +(1) According to the experiments of Mazelle and Okada, as reported by +Kruemmel,[97] 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. + +(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 deg.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 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. + +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.[98] 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 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. + +(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 deg.C. from the present mean +temperature of 2 deg.C. would alter the ability of the entire ocean to +absorb carbon dioxide by about 4 per cent. This, according to F. W. +Clarke,[99] 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[100] when the partially combined carbon dioxide +is also considered. Moreover, the capacity of water for carbon dioxide +varies sharply with the temperature.[101] Hence a rise in temperature of +only 1 deg.C. would theoretically cause the oceans to give up from 30 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. + +(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.[102] + +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 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 Kruemmel and +Mill,[103] 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 deg.C.) to the highest temperatures which ever +prevail in the ocean (30 deg.C. or 86 deg.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 Kruemmel, 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. + +If in earlier times a more rapid circulation occurred, low latitudes +must have been cooled more than now by 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. + +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.[104] + +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 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:[105] "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. Zooelogists who see these +wonderful fossils are at once 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." + +At present, as we have repeatedly seen, little growth takes place either +among animals or plants at temperatures below 0 deg.C. or above 40 deg.C., and +for most species the limiting temperatures are about 10 deg. and 30 deg.. 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 deg.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. + +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 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 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. + +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;[106] +(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. + +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 +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_{2}, 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. + +As to the absolute amounts of oxygen, Barrell[107] 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 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,[108] 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,[109] 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 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. + +After this chapter was in type, St. John[C] 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. + +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. + +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 scatter the short waves of light and thus +interfere with their reaching the earth. Abbot and Fowle,[110] 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. + +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 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. + +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[111] 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 deg.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. + +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 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. + +Other effects of the increase in air pressure which we are here assuming +during at least the early part of geological 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 reenforce 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. + +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. + +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 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. + +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. + +FOOTNOTES: + +[Footnote 97: Encyclopaedia Britannica, 11th edition: article "Ocean."] + +[Footnote 98: 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.] + +[Footnote 99: Data of Geochemistry, Fourth Ed., 1920; Bull. No. 695, U. +S. Geol. Survey.] + +[Footnote 100: Quoted by Schuchert in The Evolution of the Earth.] + +[Footnote 101: Smithsonian Physical Tables, Sixth Revision, 1914, p. +142.] + +[Footnote 102: 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 _en route_. 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.] + +[Footnote 103: Encyclopaedia Britannica: article "Ocean."] + +[Footnote 104: Chamberlin and Salisbury: Geology, Vol. II, pp. 1-132, +1906; and T. C. Chamberlin: The Origin of the Earth, 1916.] + +[Footnote 105: Personal communication.] + +[Footnote 106: R. T. Chamberlin: Gases in Rocks, Carnegie Inst. of +Wash., No. 106, 1908.] + +[Footnote 107: 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.] + +[Footnote 108: F. W. Clarke: Data of Geochemistry, Fourth Ed., 1920, +Bull. No. 695, U. S. Geol. Survey, p. 256.] + +[Footnote 109: F. W. Clarke: _loc. cit._, pp. 27-34 et al.] + +[Footnote C: Chas. E. St. John: Science Service Press Reports from the +Mt. Wilson Observatory, May, 1922.] + +[Footnote 110: Abbot and Fowle: Annals Astrophysical Observatory; +Smiths. Inst., Vol. II, 1908, p. 163. + +F. E. Fowle: Atmospheric Scattering of Light; Misc. Coll. Smiths. Inst., +Vol. 69, 1918.] + +[Footnote 111: Abbot and Fowle: _loc. cit._, p. 172.] + + + + +CHAPTER XIV + +THE EFFECT OF OTHER BODIES ON THE SUN + + +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. + +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 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. + +In _Earth and Sun_, 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 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. + +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 _Earth and Sun_ 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 +_Earth and Sun_ 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. + +Let us begin by scanning the available evidence as to 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. + +The second bit of evidence is found in recent exhaustive studies of +periodicities by Turner[112] 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 show similar cycles +suggests that the sun's variations are the cause. + +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. + +Allied to the preceding hypothesis is Shapley's[113] nebular hypothesis. +At frequent intervals, averaging about once a year during the last +thirty years, astronomers have discovered what are known as novae. 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 novae +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 nebulae, as is also the case with novae. 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 nebulae 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. + +In a personal communication Shapley amplifies the nebular climatic +hypothesis as follows: + + 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 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. + + 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. + + 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 novae 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. + + 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 nebulae are hardly as yet generally known. + +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 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 nebulae 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 novae, 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. + +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. + +Still another suggestion is that the tidal action of the stars and other +bodies which may chance to approach 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. + +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 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. + +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. + +In the astronomical portion of the following discussion we shall follow +Jeans[114] 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 nebulae, which are thought by some astronomers 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) Nebulae 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 nebulae, 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 nebulae, +planetary nebulae, 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. + +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 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. + +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. 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. + +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 _tidal_ 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 would be +only 40 per cent farther from the sun than Neptune, and would almost +pass within the solar system. + +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. + +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 consideration here +advanced militate so strongly against the tidal hypothesis of solar +disturbances that it seems scarcely worth while to consider it further. + +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 deg. C. or 300 deg. on the +absolute scale, and the other with 600 deg. 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.[115] Third, the +number of electrons 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. + +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[116] 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. + +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 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[117] 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 deg.C., or approximately 900 deg. 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 deg.C. on the +absolute scale. The other stars will be taken as averaging the same, +although of course they vary greatly. + +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 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 deg.C. lower than now, but the difference may have been +only 6 deg.. Again, the temperature of the outer atmosphere of Jupiter where +the electrons are shot out may be only 500 deg. or 700 deg. absolute, instead of +900 deg.. 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. + +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 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 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. + +TABLE 5 + +THEORETICAL PROBABILITY OF STELLAR APPROACHES + + --------------------------------------------------------------------- + | 1 | 2 | 3 | 4 | + | | |_Average | | + | _Dark Stars_ | _Sun_ | Star_ |_Betelgeuse_| + --------------------------------------------------------------------- + A. Approximate | | | | | + radius in miles | 430,000 | 860,000 | 2,150,000|218,000,000 | + | | | | | + B. Assumed | | | | | + temperature above| | | | | + absolute zero. | 900 deg. C. | 6300 deg. C.| 5400 deg. C.| 3150 deg. C. | + | | | | | + C. Approximate | | | | | + theoretical | | | | | + distance at which| | | | | + star would cause | | | | | + solar disturbance| | | | | + great enough to | | | | | + cause glaciation | | | | | + (billions[118] | | | | | + of miles). | 1.2 | 120 | 220 | 3200 | + | | | | | + D. Average | | | | | + interval between | | | | | + approaches | | | | | + close enough to | | | | | + cause glaciation | | | | | + if all stars |130,000,000,000| | | | + were of given |[119] | | | | + type. Years. | |130,000,000|38,000,000| 700,000 | + --------------------------------------------------------------------- + +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. + +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 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 x 10^{10}) 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. + +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. + +The two main components of Alpha Centauri are separated 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. + +FOOTNOTES: + +[Footnote 112: 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; _idem_, +pp.793-808.] + +[Footnote 113: Harlow Shapley: Note on a Possible Factor in Geologic +Climates; Jour. Geol., Vol. 29, No. 4, May, 1921; Novae and Variable +Stars, Pub. Astron. Soc. Pac., No. 194, Aug., 1921.] + +[Footnote 114: J. H. Jeans: Problems of Cosmogony and Stellar Dynamics, +Cambridge, 1919.] + +[Footnote 115: 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. + +"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, _in vacuo_, 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^{4}, 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."] + +[Footnote 116: Quoted by Moulton in his Introduction to Astronomy.] + +[Footnote 117: Introduction to Astronomy.] + +[Footnote 118: The term billions, here and elsewhere, is used in the +American sense, 10^{9}.] + +[Footnote 119: The assumed number of stars here is ten times as great as +in the other parts of this line.] + + + + +CHAPTER XV + +THE SUN'S JOURNEY THROUGH SPACE + + +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. + +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. According to Schuchert, the known +periods of glacial or semi-glacial climate have been approximately as +follows: + +LIST OF GLACIAL PERIODS + + 1. Archeozoic. + (1/4 of geological time or perhaps much more) + + No known glacial periods. + + 2. Proterozoic. + (1/4 of geological time) + + a. Oldest known glacial period near base of Proterozoic in + Canada. Evidence widely distributed. + + b. Indian glacial period; time unknown. + + c. African glacial period; time unknown. + + d. Glaciation near end of Proterozoic in Australia, Norway, + and China. + + 3. Paleozoic. + (1/4 of geological time) + + a. Late Ordovician(?). Local in Arctic Norway. + + b. Silurian. Local in Alaska. + + c. Early Devonian. Local in South Africa. + + d. Early Permian. World-wide and very severe. + + 4. Mesozoic and Cenozoic. + (1/4 of geological time) + + 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. + + c. Severe glacial period during Pleistocene. + +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 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 is like the +spiral nebulae 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. + +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 +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. + +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. + +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.[120] 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 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[121] 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. + +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.[122] R. T. Chamberlin, as already +stated, finds that 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. + +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. + +4. Post-glacial climatic stages are so well known that in Europe they +have definite names. Their sequence has 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.[123] 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.[124] + +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 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 cooeperate 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. + +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 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. + +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 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. + +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, faculae, 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. + +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. + +After the preceding parts of this book had been 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. + +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. + +TABLE 6 + +THIRTY-EIGHT STARS HAVING LARGEST KNOWN PARALLAXES + + Star + Code + 1 Groombr. 34 + 2 ++[Greek: e] Cassiop. + 3 + 4 ++[Greek: k] Tucanae + 5 [Greek: t] Ceti + 6 [Greek: d]_2 Eridani + 7 ++[Greek: e] Eridani + 8 ++40(0)^2 Eridani + 9 Cordoba Z. 243 + 10 Weisse 592 + 11 ++[Greek: a] Can. Maj. (Sirius) + 12 ++[Greek: a] Can. Min. (Procyon) + 13 ++Fedorenko 1457-8 + 14 Groombr. 1618 + 15 Weisse 234 + 16 Lalande 21185 + 17 Lalande 21258 + 18 + 19 Lalande 25372 + 20 ++[Greek: a] Centauri + 21 ++[Greek: x] Bootes + 22 ++Lalande 27173 + 23 Weisse 1259 + 24 Lacaille 7194 + 25 ++[Greek: b] 416 + 26 Argel -0.17415-6 + 27 Barnard's star + 28 ++70p Ophiuchi + 29 ++[Greek: S] 2398 + 30 [Greek: s] Draconis + 31 ++[Greek: a] Aquilae (Altair) + 32 ++61 Cygni + 33 Lacaille 8760 + 34 [Greek: e] Indi + 35 ++Krueger 60 + 36 Lacaille 9352 + 37 Lalande 46650 + 38 C. G. A. 32416 + + (++ Double star.) + + (1) (2) (3) (4) (5) (6) + Right Declination Visual Spectrum Proper Radial + Star Ascension [Greek: d] Mag. m Motion Velocity + code [Greek: a] 1900 km. per + 1900 sec. + ------------------------------------------------------------------ + 1 0^h 12^m.7 +43 deg.27' 8.1 Ma 2".89 + 3 + 2 43 .0 +57 17 3.6 F8 1 .24 + 10 + 3 43 .9 +4 55 12.3 F0 3 .01 ..... + 4 1 12 .4 -69 24 5.0 F8 .39 + 12 + 5 39 .4 -16 28 3.6 K0 1 .92 - 16 + ------------------------------------------------------------------ + 6 3 15 .9 -43 27 4.3 G5 3 .16 + 87 + 7 28 .2 - 9 48 3.8 K0 .97 + 16 + 8 4 10 .7 - 7 49 4.5 G5 4 .08 - 42 + 9 5 7 .7 -44 59 9.2 K2 8 .75 +242 + 10 26 .4 - 3 42 8.8 K2 2 .22 ..... + ------------------------------------------------------------------ + 11 6 40 .7 -16 35 -1.6 A0 1 .32 - 8 + 12 7 34 .1 + 5 29 0.5 F5 1 .24 - 4 + 13 9 7 .6 +53 7 7.9 Ma 1 .68 + 10 + 14 10 5 .3 +49 58 6.8 K5p 1 .45 - 30 + 15 14 .2 +20 22 9.0 ... .49 ..... + ------------------------------------------------------------------ + 16 57 .9 +36 38 7.6 Mb 4 .78 - 87 + 17 11 0 .5 +44 2 8.5 K5 4 .52 + 65 + 18 12 .0 -57 2 12.0 ... 2 .69 ..... + 19 13 40 .7 +15 26 8.5 K5 2 .30 ..... + 20 14 32 .8 -60 25 0.2 G 3 .68 + 22 + ------------------------------------------------------------------ + 21 14 46 .8 +19 31 4.6 K5p .17 + 4 + 22 51 .6 -20 58 5.8 Kp 1 .96 + 20 + 23 16 41 .4 +33 41 8.4 ... .37 ..... + 24 17 11 .5 -46 32 5.7 K .97 ..... + 25 12 .1 -34 53 5.9 K5 1 .19 - 4 + ------------------------------------------------------------------ + 26 37 .0 +68 26 9.1 K 1 .33 ..... + 27 52 .9 + 4 25 9.7 Mb 10 .30 - 80 + 28 18 0 .4 + 2 31 4.3 K 1 .13 ..... + 29 41 .7 +59 29 8.8 K 2 .31 ..... + 30 19 32 .5 +69 29 4.8 G5 1 .84 + 26 + ------------------------------------------------------------------ + 31 45 .9 + 8 36 1.2 A5 .66 - 33 + 32 21 2 .4 +38 15 5.6 K5 5 .20 - 64 + 33 11 .4 -39 15 6.6 G 3 .53 + 13 + 34 55 .7 -57 12 4.8 K5 4 .70 - 39 + 35 22 24 .4 +57 12 9.2 ... .87 ..... + ------------------------------------------------------------------ + 36 59 .4 -36 26 7.1 K 6 .90 + 12 + 37 23 44 .0 + 1 52 8.7 Ma 1 .39 ..... + 38 59 .5 -37 51 8.2 G 6 .05 + 26 + + + (7) (9) (11) (13) (14) + Present Minimum Magnitude Luminosity Effective + Parallax Distance at Min. Dist. | radiation + [Greek: p] Light Yrs. | | at + | | | | minimum + | (8) | (10) | (12) | distance + Star | Maximum | Time of | Absolute | from sun + Code | Parallax | Minimum | Magnitude | | + | | | Distance | | | | + ----------------------------------------------------------------------- + 1 ".28 ".28 11.6 -4000 8.1 10.3 0.0063 0.000051 + 2 .18 .19 17.1 -47000 3.5 4.9 0.91 0.003110 + 3 .24 .... .... ...... .... 14.2 0.00017 ........ + 4 .16 .23 14.2 -264000 4.2 6.0 0.33 0.001610 + 5 .32 .37 8.8 +46000 3.3 6.1 0.30 0.003840 + ----------------------------------------------------------------------- + 6 .16 .22 14.8 -33000 3.6 5.3 0.63 0.002960 + 7 .31 .46 7.1 -106000 3.0 6.3 0.25 0.004970 + 8 .21 .23 14.2 +19000 4.3 6.1 0.30 0.001470 + 9 .32 .68 4.8 -10000 7.6 11.7 0.0017 0.000074 + 10 .17 .... .... ...... .... 9.9 0.009 ........ + ----------------------------------------------------------------------- + 11 .37 .41 8.0 +65000 -1.8 1.2 27.50 0.429000 + 12 .31 .32 10.2 +34000 0.5 3.0 5.25 0.051300 + 13 .16 .16 20.4 -24000 7.9 8.9 0.023 0.000055 + 14 .18 .23 14.2 +69000 6.3 8.1 0.048 0.000238 + 15 .19 .... .... ...... .... 10.4 0.0057 ........ + ----------------------------------------------------------------------- + 16 .41 .76 4.3 +20000 6.2 10.7 0.0044 0.000238 + 17 .19 .22 14.8 -20000 8.2 9.9 0.009 0.000041 + 18 .34 .... .... ...... .... 14.7 0.00011 ........ + 19 .19 .... .... ...... .... 9.9 0.009 ........ + 20 .76 1.03 3.2 -28000 -0.5 4.6 1.20 0.117500 + ----------------------------------------------------------------------- + 21 .17 .22 14.8 -598000 4.0 5.8 0.40 0.001815 + 22 .18 .19 17.1 -36000 5.6 7.1 0.12 0.000412 + 23 .18 .... .... ...... .... 9.7 0.011 ........ + 24 .19 .... .... ...... .... 7.1 0.12 ........ + 25 .17 .17 19.2 +21000 5.7 7.1 0.12 0.000329 + ----------------------------------------------------------------------- + 26 .22 .... .... ...... .... 10.8 0.004 ........ + 27 .53 .70 4.7 +10000 9.1 13.3 0.0025 0.000114 + 28 .19 .... .... ...... .... 5.7 0.44 ........ + 29 .29 .... .... ...... .... 11.1 0.0030 ........ + 30 .20 .23 14.2 -49000 4.5 6.3 0.25 0.001238 + ----------------------------------------------------------------------- + 31 .21 .51 6.4 +117000 -0.7 2.8 6.30 0.153600 + 32 .30 .38 8.6 +19000 5.1 8.0 0.053 0.000715 + 33 .25 .26 12.6 -11000 6.6 8.6 0.030 0.000189 + 34 .28 .31 10.5 +17000 4.6 7.0 0.13 0.001230 + 35 .26 .... .... ....... .... 11.3 0.0025 ........ + ----------------------------------------------------------------------- + 36 .29 .29 11.2 -3000 7.1 9.4 0.014 0.000111 + 37 .17 .... .... ....... .... 9.9 0.009 ........ + 38 .22 .22 14.8 -7000 8.2 9.9 0.009 0.000041 + ----------------------------------------------------------------------- + +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. + +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. + +[Illustration: _Fig. 10. Climatic changes of 140,000 years as inferred +from the stars._] + +In preparing Fig. 10 it has been necessary to make 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. + +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 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. + +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 indicates that the change of +climate is still persisting, a conclusion in harmony with the evidence +as to historic changes. + +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. + +[Illustration: _Fig. 11. Sunspot curve showing cycles, 1750 to 1920._ + +_Note._ 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.] + +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. + +FOOTNOTES: + +[Footnote 120: Lewis Boss: Convergent of a Moving Cluster in Taurus; +Astronom. Jour., Vol. 26, No. 4, 1908, pp. 31-36.] + +[Footnote 121: F. R. Moulton: in Introduction to Astronomy, 1916.] + +[Footnote 122: A. Penck: Die Alpen im Eiszeitalter, Leipzig, 1909.] + +[Footnote 123: R. D. Salisbury: Physical Geography of the Pleistocene, +in Outlines of Geologic History, by Willis and Salisbury, 1910, pp. +273-274.] + +[Footnote 124: Davis, Pumpelly, and Huntington: Explorations in +Turkestan, Carnegie Inst. of Wash., No. 26, 1905. + +In North America the stages have been the subject of intensive studies +on the part of Taylor, Leverett, Goldthwait, and many others.] + + + + +CHAPTER XVI + +THE EARTH'S CRUST AND THE SUN + + +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. + +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 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. + +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 cooeperate with +other agencies in setting the time when the earth's crust shall be +deformed. + +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. + +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 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.[125] Thus +we conclude that while climatic changes and crustal movements usually +occur together, they may occur separately. + +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 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. + +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 _Milne's Catalogue of +Destructive Earthquakes_ 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. Other catalogues +such as those of Perry and Fuchs have been treated similarly."[126] + +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. 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. + +TABLE 7 + +DESTRUCTIVE EARTHQUAKES FROM 1800 TO 1899 COMPARED WITH SUNSPOTS + + A: _Sunspot numbers_ + B: _Number of months per Wolf's Table_ + C: _Number of earthquakes_ + D: _Average number of earthquakes per month_ + E: _Number of earthquakes in succeeding month_ + F: _Average number of earthquakes in succeeding month_ + + A B C D E F + + 0-15 344 522 1.52 512 1.49 + 15-30 194 306 1.58 310 1.60 + 30-50 237 433 1.83 439 1.85 + 50-70 195 402 2.06 390 2.00 + 70-100 135 286 2.12 310 2.30 + over 100 95 218 2.30 175 1.84 + +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. + +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 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: + + Jan. 2.8 May 2.4 Sept. 2.5 + Feb. 2.4 June 2.3 Oct. 2.6 + Mar. 2.5 July 2.4 Nov. 2.7 + Apr. 2.4 Aug. 2.4 Dec. 2.8 + +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: + + Sunspots and earthquakes of same month: +0.042, or 1.5 times the + probable error. + + Sunspots of a given month and earthquakes of that month and the + next: +0.084, or 3.1 times the probable error. + + 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. + +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 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. + +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. + +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 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[127] 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. + +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 +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. + +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. + +The most thorough investigation of this subject seems to have been that +of Davisson.[128] His results have been worked over and amplified by +Knott,[129] who has tested them by Schuster's exact mathematical +methods. His results are given in Table 8.[130] Here the northern +hemisphere 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 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. + +TABLE 8 + +SEASONAL MARCH OF EARTHQUAKES + +AFTER DAVISSON AND KNOTT + + A: _Region_ + B: _Limiting Dates_ + C: _Number of Shocks_ + D: _Maximum Month_ + E: _Amplitude_ + F: _Expected Amplitude_ + G: _Ratio of Actual to Expected Amplitude_ + + A B C D E F G + + Northern Hemisphere 223-1850 5879 Dec. 0.110 0.023 4.8 + Northern Hemisphere 1865-1884 8133 Dec. 0.290 0.020 14.5 + Europe 1865-1884 5499 Dec. 0.350 0.024 14.6 + Europe 306-1843 1961 Dec. 0.220 0.040 5.5 + Southeast Europe 1859-1887 3470 Dec. 0.210 0.030 7.0 + Vesuvius District 1865-1883 513 Dec. 0.250 0.078 3.2 + Italy: + Old Tromometre 1872-1887 61732 Dec. 0.490 0.007 70.0 + Old Tromometre 1876-1887 38546 Dec. 0.460 0.009 49.5 + Normal Tromometre 1876-1887 38546 Dec. 0.490 0.009 52.8 + Balkan, etc. 1865-1884 624 Dec. 0.270 0.071 3.8 + Hungary, etc. 1865-1884 384 Dec. 0.310 0.090 3.4 + Italy 1865-1883 2350 Dec.(Sept.)0.140 0.037 3.8 + Grecian Archip. 1859-1881 3578 Dec.-Jan. 0.164 0.030 5.5 + Austria 1865-1884 461 Jan. 0.370 0.083 4.4 + Switzerland, etc. 1865-1883 524 Jan. 0.560 0.077 7.3 + Asia 1865-1884 458 Feb. 0.330 0.083 4.0 + North America 1865-1884 552 Nov. 0.350 0.075 4.7 + California 1850-1886 949 Oct. 0.300 0.058 5.2 + Japan 1878-1881 246 Dec. 0.460 0.113 4.1 + Japan 1872-1880 367 Dec.-Jan. 0.256 0.093 2.8 + Japan 1876-1891 1104 Feb. 0.190 0.053 3.6 + Japan 1885-1889 2997 Oct. 0.080 0.032 2.5 + Zante 1825-1863 1326 Aug. 0.100 0.049 2.0 + Italy, North 1865-1883 1513 Sept.(Nov.) 0.210 0.046 4.6 + of Naples + East Indies 1873-1881 515 Aug., Oct., 0.071? 0.078 0.9 + or Dec.? + Malay Archip. 1865-1884 598 May 0.190 0.072 2.6 + New Zealand 1869-1879 585 Aug.-Sept. 0.203 0.073 2.8 + Chile 1873-1881 212 July 0.480 0.122 3.9 + Southern Hemisphere 1865-1884 751 July 0.370 0.065 5.7 + New Zealand 1868-1890 641 March, May 0.050 0.070 0.7 + Chile 1865-1883? 316 July, Dec. 0.270 0.100 2.7 + Peru, Bolivia 1865-1884 350 July 0.480 0.095 5.1 + +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.[131] If equatorial regions were included in +Fig. 12, they would give an almost straight line. + +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.[132] Wherever sufficient data are available, +earthquakes 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 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 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.[133] Such storms, as +we have seen, are much more frequent and severe when sunspots are +numerous than at other times. Moreover, as Veeder[134] 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. + +[Illustration: _Fig. 12. Seasonal distribution of earthquakes. (After +Davisson and Knott.)_ + + solid line ---- Northern Hemisphere. + dashed line .... Southern Hemisphere.] + +Other meteorological factors, such as rain, snow, winds, and currents, +probably have some effect on earthquakes 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. + +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 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[135] 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. + +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[136] 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 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 several feet from its normal position in +respect to the axis. + +[Illustration: _Fig. 13. Wandering of the pole from 1890 to 1898._ +(_After Moulton._)] + +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[137] and amplified by Knott.[138] 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 deg. +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 deg. and 25 deg., while in fifty-six cases it was from 25 deg. to 40 deg.. 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 deg.. + +TABLE 9 + +DEFLECTION OF PATH OF POLE COMPARED WITH EARTHQUAKES + + _No. of _No. of _Average No. + _Deflection_ Deflections_ Earthquakes_ of Earthquakes_ + 0-10 deg. 9 179 19.9 + 10-25 deg. 32 263 8.2 + 25-40 deg. 56 722 12.9 + 40-55 deg. 19 366 19.3 + over 55 deg. 7 164 23.4 + +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. + +TABLE 10 + +EARTHQUAKES IN 1903-1908 COMPARED WITH DEPARTURES OF THE PROJECTED +CURVE OF THE EARTH'S AXIS FROM THE EULERIAN POSITION + + _Average angle of deflection_ _Average daily number + (_10 periods of 1/10 year each_) of earthquakes_ + -10.5 deg. 8.31 + 11.5 deg. 8.35 + 25.8 deg. 8.23 + 40.2 deg. 8.14 + 54.7 deg. 8.86 + 90.3 deg. 11.81 + +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 deg. 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. + +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 relation between climate and +earthquakes. At its end we are inclined to believe that the relation is +close and important. + +It must not be supposed, however, that meteorological conditions are the +_cause_ 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. + +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 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. + +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 +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. + +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 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. + +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[139] 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. In the long periods between great earthquakes the +energy necessary for such shocks is being stored up in the rocks as +elastic compression." + +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. + +FOOTNOTES: + +[Footnote 125: E. Kirk: Paleozoic Glaciation in Alaska; Am. Jour. Sci., +1918, p. 511.] + +[Footnote 126: J. Milne: Catalogue of Destructive Earthquakes; Rep. +Brit. Asso. Adv. Sci., 1911.] + +[Footnote 127: Wm. Bowie: Lecture before the Geological Club of Yale +University. See Am. Jour. Sci., 1921.] + +[Footnote 128: Chas. Davisson: On the Annual and Semi-annual Seismic +Periods; Roy. Soc. of London, Philosophical Transactions, Vol. 184, +1893, 1107 _ff._] + +[Footnote 129: C. G. Knott: The Physics of Earthquake Phenomena, Oxford, +1908.] + +[Footnote 130: 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.] + +[Footnote 131: N. F. Drake: Destructive Earthquakes in China; Bull. +Seism. Soc. Am., Vol. 2, 1912, pp. 40-91, 124-133.] + +[Footnote 132: 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.] + +[Footnote 133: 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.] + +[Footnote 134: Ellsworth Huntington: The Geographic Work of Dr. M. A. +Veeder; Geog. Rev., Vol. 3, March and April, 1917, Nos. 3 and 4.] + +[Footnote 135: 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.] + +[Footnote 136: Harold Jeffreys: Causes Contributory to the Annual +Variations of Latitude; Monthly Notices, Royal Astronomical Soc., Vol. +76, 1916, pp. 499-525.] + +[Footnote 137: John Milne: British Association Reports for 1903 and +1906.] + +[Footnote 138: C. G. Knott: The Physics of Earthquake Phenomena, Oxford, +1908.] + +[Footnote 139: A. C. Lawson: The Mobility of the Coast Ranges of +California; Univ. of Calif. Pub., Geology, Vol. 12, No. 7, pp. 431-473.] + + + + +CONCLUSION + + +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, 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 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. + +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 _Earth and Sun_, 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 role 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 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. + +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 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 +role. 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. + +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 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, may +excel that of today as far as the highest known type of mentality excels +the primitive plasma from which all life appears to have arisen. + + + + +INDEX + +* Indicates illustrations. + + + Abbot, C. G., cited, 45, 52, 237, 238, 239. + + Aboskun, 104. + + Africa, earthquakes, 301; + East, _see_ East Africa; + lakes, 143; + North, _see_ North Africa. + + African glaciation, 266. + + Air, _see_ Atmosphere. + + Alaska, glacial till in, 287; + Ice Age in, 221. + + Albrecht, cited, 304. + + Alexander, march of, 88 f. + + Allard, H. A., cited, 183, 184. + + Alpha Centauri, companion of, 280; + distance from sun, 262; + luminosity, 278; + speed of, 281; + variations, 282. + + Alps, loess in, 159; + precipitation in, 141; + snow level in, 139. + + Altair, companion of, 280; + luminosity, 278; + speed of, 281. + + Amazon forest, temperature, 17. + + Ancylus lake, 217. + + Andes, snow line, 139. + + Animals, climate and, 1. + + Antarctica, mild climate, 219; + thickness of ice in, 125; + winds, 135, 161. + + Anti-cyclonic hypothesis, 135 ff. + + Appalachians, effect on ice sheet, 121. + + Arabia, civilization in, 67. + + Aral, Sea of, 108. + + Archean rocks, 211. + + Archeozoic, 3 f.; + climate of, 267. + + Arctic Ocean, submergence, 219. + + Arctowski, H., cited, 29, 46, 244. + + Argon, increase of, 236. + + Arizona, rainfall, 89, 108; + trees measured in, 73. + + Arrhenius, S., cited, 36, 254. + + Arsis, of pulsation, 24. + + Asbjoern Selsbane, corn of, 101. + + Asia, atmospheric pressure, 298; + central, changes of climate, *75; + central, post-glacial climate, 271; + climate, 66; + glaciation in, 131; + storminess in, 60; + western, climate in, 84 f. + + Atlantic Ocean, storminess, 57. + + Atmosphere, changes, 19 f., 229; + composition of, 223-241; + effect on temperature, 231. + + Atmospheric circulation, glaciation and, 42. + + Atmospheric electricity, solar relations of, 56. + + Atmospheric pressure, earthquakes and, 298; + evaporation and, 237; + increase in, 239; + redistribution of, 49; + variation, 53. + + Australia, East, mild climate, 219; + precipitation, 144. + + Axis, earth's, 48; + wabbling of, 301. + + + Bacon, Sir Francis, cited, 27. + + Bacubirito, meteor at, 246. + + Baltic Sea, as lake, 217; + freezing of, 100; + ice, 26; + storm-floods, 99; + submergence, 219. + + Bardsson, Ivar, 106. + + Barkow, cited, 135. + + Barometric pressure, solar relations of, 56. + + Barrell, J., cited, 3, 200, 213, 234. + + Bartoli, A. G., cited, 257. + + Bauer, L. A., cited, 150. + + Beaches, under water, 97. + + Beadnell, H. J. L., cited, 143. + + Beluchistan, rainfall, 89. + + Bengal, Bay of, cyclones in, 149. + + Bengal, famine in, 104 f. + + Berlin, rainfall and temperature, 93. + + Betelgeuse, 259 f.; + distance from sun, 262. + + Bible, climatic evidence in, 91 f.; + palms in, 92. + + Binary stars, 252. + + Birkeland, K., cited, 244. + + Black Earth region, loess in, 159. + + Boca, Cal., correlation coefficients, 83, 85. + + Boltzmann, L., cited, 257. + + Bonneville, Lake, 142, 143. + + Borkum, storm-flood in, 99. + + Boss, L. cited, 268, 269. + + Botanical evidence of mild climates, 167 ff. + + Boulders, on Irish coast, 119. + + Bowie, W., cited, 293. + + Bowman, I., cited, 213. + + Britain, forests, 220; + level of land, 220. + + British Isles, height of land, 111; + temperature, 216. + + Brooks, C. E. P., cited, 115, 143, 196, 215, 225. + + Brooks, C. F., cited, 209. + + Brown, E. W., cited, 191, 244. + + Brueckner, E., cited, 27. + + Brueckner periods, 27 f. + + Bufo, habitat of, 202. + + Buhl stage, 216. + + Bull, Dr., cited, 100, 101. + + Butler, H. C., cited, 66, 67 ff., 70, 76. + + + California, changes of climate, *75; + correlations of rainfall, 86; + measurements of sequoias in, 73, 74 ff.; + rainfall, 108. + + Cambrian period, 4 f. + + Canada, storminess, 53 f., 57; + storm tracks in, 113. + + Cape Farewell, shore ice at, 105. + + Carbon dioxide, erosion and, 119 f.; + from volcanoes, 23; + hypothesis, 139; + importance of, 9, 11 f.; + in Permian, 148; + in atmosphere, 20, 96, 238; + in ocean, 226; + nebular hypothesis and, 232; + theory of glaciation, 36 ff. + + Caribbean mountains, origin of, 193. + + Carnegie Institution of Washington, 74. + + Caspian Sea, climatic stress, 104; + rainfall, 107 f.; + rise and fall, 27; + ruins in, 71. + + Cenozoic, climate, 266; + fossils, 21. + + Central America, Maya ruins, 95. + + Chad, Lake, swamps of, 171. + + Chamberlin, R. T., cited, 166, 233, 269. + + Chamberlin, T. C., cited, 19, 36, 38, 39, 42 f., 48, 122, 125, + 152, 156, 190, 195, 227, 269. + + Chandler, S. C., cited, 301. + + Chinese earthquakes, periodicity of, 245. + + Chinese, sunspot observations, 108 f. + + Chinese Turkestan, desiccation in, 66. + + Chronology, glacial, 215. + + Clarke, F. W., cited, 226, 235. + + Clayton, H. H., cited, 173 f. + + Climate, effect of contraction, 189 ff.; + affect of salinity, 224; + in history, 64-97; + uniformity, 1-15; + variability, 16-32. + + Climates, mild, causes of, 166-187; + mild, periods of, 274. + + Climatic changes, and crustal movements, 285 ff.; + hypotheses of, 33-50; + mountain-building and, *25; + post-glacial crustal movements and, 215-222; + terrestrial causes of, 188-214. + + Climatic sequence, 16 f. + + Climatic stages, post-glacial, 270. + + Climatic stress, in fourteenth century, 98-109. + + Climatic uniformity, hypothesis of, 65, 71 f. + + Climatic zoning, 169. + + Cloudiness, glaciation and, 114, 147. + + Clouds, as protection, 197. + + Colfax, Cal., correlation coefficients, 83. + + Cologne, flood at, 99. + + Compass, variations, 150. + + Continental climate, variations, 103. + + Continents, effect on climate, 111 f. + + Contraction, effect on climate, 189 ff., 199, 207; + effect on lands, 207; + heat of sun and, 13 f.; + irregular, 195; + of the earth, 18; + of the sun, 249; + stresses caused by, 310. + + Convection, carbon dioxide and, 239. + + Corals, in high latitudes, 21, 39, 167, 178. + + Cordeiro, F. J. B., cited, 181, 183, 186. + + Correlation coefficients, earthquakes and sunspots, 291; + Jerusalem rainfall and sequoia growth, 83 ff.; + rainfall and tree growth, 79 ff. + + Cosmos, effect of light, 185. + + Cressey, G. B., cited, 80. + + Cretaceous, lava, 211; + mountain ranges, 44; + paleogeography, *201; + submergence of North America, 200. + + Croll, J., cited, 34 ff., 176. + + Croll's hypothesis, snow line, 139. + + Crust, climate and movements of, 63, 287, 310; + movements of, 43; + strains in, 22. + + Currents and planetary winds, 174. + + Cycads, 169. + + Cyclonic hypothesis, 97; + loess and, 163; + Permian glaciation and, 148; + snow line, 139. + + Cyclonic storms, in glacial epochs, 140 f.; + solar electricity and, 243 (_see_ Storms, Storminess). + + Cyclonic vacillations, 30 f.; + nature of, 57 ff. + + + Daily vibrations, 28 f. + + Danube, frozen, 98. + + Darwin, G. H., cited, 191. + + Daun stage, 217. + + Davis, W. M., cited, 271. + + Davisson, C., cited, 294, 295, 299. + + Day, C. P., cited, 239. + + Day, length of, 18, 191. + + Dead Sea, palms near, 92. + + Death Valley, 142. + + De Ballore, M., cited, 297, 298. + + Deep-sea circulation, rapidity, 227; + salinity and, 176; + solar activity and, 179. + + De Geer, S., cited, 215, 221. + + De Lapparent, A., cited, 200. + + Denmark, fossils, 271. + + "Desert pavements," 161. + + Deserts, abundant flora of, 171; + and pulsations theory, 88 ff.; + red beds of, 170. + + Devonian, climate, 266; + mountains, 209. + + Dog, climate and, 1. + + Donegal County, Ireland, 220. + + Double stars, 272, 280; + electrical effect of, 261. + + Douglass, A. E., cited, 28, 73, 74 f., 84, 85, 107. + + Dragon Town, destruction of, 104, 108. + + Drake, N. F., cited, 297, 298. + + Droughts, and pulsations theory, 87 f.; + in England, 102; + in India, 104 f. + + Drumkelin Bog, Ireland, log cabin in, 220. + + Dust, at high levels, 240. + + + Earth, crust of and the sun, 285-317; + internal heat, 212; + nature of mild climate, 274; + position of axis, 181; + rigidity of, 307; + temperature gradient, 213; + temperature of surface, 8. + + Earthquakes, and seasons, 294, 297; + and sunspots, 288 f.; + and tropical hurricanes, 300; + and wandering of pole, 304 f.; + cause of, 307; + compared with departures from Eulerian position, 306; + seasonal distribution of, 299; + seasonal march, 295. + + "Earthquake weather," 298. + + East Africa, mild climate, 219. + + East Indies, earthquakes of, 296. + + Eberswalde, tree growth at, 102 f. + + Ecliptic, obliquity of, 217. + + Electrical currents, in solar atmosphere, 261. + + Electrical emissions, variation of, 275. + + Electrical hypothesis, 150, 250 f., 256 ff. + + Electrical phenomena, storminess and, 56. + + Electricity, and earthquakes, 292; + solar, 243. + + Electro-magnetic hypothesis, 244. + + Electrons, solar, 56; + variation of, 256. + + Electro-stellar hypothesis, 274. + + Elevation, climatic changes and, 39. + + Engedi, palms in, 92. + + England, climatic stress, 101 f.; + storminess and rainfall, 107. + + Eocene, climate, 266. + + Equinoxes, precession of, 96. + + Erosion, storminess and, 309. + + Eskimo, in Greenland, 106. + + Eulerian movement, 301, 304. + + Euphrates, 67. + + Europe, climatic stress, 98 ff., 102 f.; + climatic table, 215; + glaciation in, 131; + ice sheet, 121; + inundations of rivers, 99; + post-glacial climate, 271; + rainfall, 107; + submergence, 196, 200. + + Evaporation, and glaciation, 112, 114; + atmospheric pressure and, 237; + from plants, 179; + importance, 129; + in trade-wind belt, 117; + rapidity of, 224. + + Evening primrose, effect of light, 184. + + Evolution, climate and, 20; + geographical complexity and, 241; + glaciation and, 33; + of the earth, 311. + + + Faculae, cause of, 61. + + False Point Lighthouse, barometric pressure at, 299. + + Famine, cause of, 103; + in England, 101 f.; + in India, 104 f.; + pulsations theory and, 87 f. + + Faunas, and mild climates, 168 f.; + in Permian, 152 f. + + Fennoscandian pause, 216. + + Flowering, light and, 184. + + Fog, and glaciation, 116; + as protection, 197; + temperature and, 178. + + Forests, climate and, 66. + + Form of the land, 43 ff. + + Fossil floras, and mild climates, 168; + in Antarctica, 273; + in Greenland, 273. + + Fossils, 169, 230; + and loess, 158; + Archeozoic, 3 f.; + Cenozoic, 21; + dating of, 153; + glaciation and, 138; + in peat bogs, 271; + mild climate, 167; + Proterozoic, 4, 6 f. + + Fourteenth century, climatic stress in, 98-109. + + Fowle, F. E., cited, 45, 237, 238, 239. + + Frech, F., cited, 36. + + Free, E. E., cited, 142. + + Freezing, salinity and, 224. + + Fresno, rainfall record, 82. + + "Friction variables," 247. + + Frisian Islands, storm-flood, 99. + + Fritz, H., cited, 109. + + Frogs, distribution of, 202. + + Fuchs, cited, 289. + + + Galaxy, 252. + + Galveston, Tex., rainfall and temperature, 94. + + Garner, W. W., cited, 183, 184. + + Gasses, in air, 233. + + Geographers, and climatic changes, 65 ff. + + Geological time table, *5. + + Geologic oscillations, 18 f., 21 ff., 188, 240. + + Geologists, changes in ideas of, 64 f. + + Germanic myths, 219. + + Germany, forests, 220; + growth of trees in, 102; + storms in, 102. + + Gilbert, G. K., cited, 143. + + Glacial epochs, causes of, 268; + dates of, 216; + intervals between, 264 f.; + length of, 166 f. + + Glacial fluctuations, 24 ff.; + nature of, 57 ff. + + Glacial period, at present, 272; + ice in, 57 f.; + length of, 269; + list, 265; + temperature, 38. + + Glaciation, and loess, 155 f.; + and movement of crust, 287; + conditions favorable for, 111; + extent of, 124; + hypotheses of, 33 ff.; + in southern Canada, 18; + localization of, 130 ff.; + Permian, *145; + solar-cyclonic hypothesis of, 110-129; + suddenness of, 138; + upper limit of, 141. + + Goldthwait, J. W., cited, 271. + + Gondwana land, 21, 204. + + Gravitation, effect on sun, 250; + pull of, 244. + + Great Basin, in glacial period, 126; + salt lakes in, 142. + + Great Ice Age, see Pleistocene. + + Great Plains, effect on ice sheet, 120. + + Greenland, climatic stress, 105 ff.; + ice, 26; + rainfall, 108; + storminess, 57; + submergence, 219; + vegetation, 21, 37, 287; + winds, 135, 161. + + Gregory, J. W., cited, 90 ff., 97. + + Gschnitz stage, 216. + + Guatemala, ruins in, 95. + + Guervain, cited, 135. + + Gyroscope, earth as, 181. + + + Hale, G. E., cited, 56, 62. + + Hamdulla, cited, 104. + + Hann, J., cited, 66. + + Hansa Union, operations of, 100. + + Harmer, F. W., cited, 115, 119. + + Heat, and earthquakes, 292; + earth's internal, 18. + + Hedin, S., cited, 88. + + Heim, A., cited, 190. + + Heligoland, flood in, 99. + + Helland-Hansen, B., cited, 174. + + Helmert, F. R., cited, 302. + + Henderson, L. J., cited, 9, 10, 11, 12. + + Henry, A. J., cited, 94, 208. + + Hercynian Mountains, 45. + + High pressure and glaciation, 115, 135. + + Himalayas, glaciation, 144; + origin of, 193; + snow line, 139. + + Himley, cited, 104. + + Historic pulsations, 24 f.; + nature of, 57 ff. + + History, climate of, 64-97; + climatic pulsations and, 26. + + Hobbs, W. H., cited, 115, 125, 135, 161. + + Hot springs, temperature of, 6. + + Humphreys, W. J., cited, 2, 37 f., 45, 46, 50, 56, 238. + + Hurricanes, in arid regions, 144; + sunspots and, 53. + + Hyades, cluster in, 268. + + + Ice, accumulations, 57 f.; + advances of, 122; + distribution of, 131; + drift, 105. + + Ice sheets, disappearance, 128; + limits, 120; + localization, 130 ff.; + rate of retreat, 165; + thickness, 125. + + Iceland, submergence, 219. + + Iowan ice sheet, rapid retreat, 165. + + Iowan loess, 158. + + India, drought, 104 f.; + famine, 104 f.; + rainfall, 108. + + Indian glaciation, 266. + + Inter-glacial epoch, Permian, 153. + + Internal heat of earth, 212. + + Ireland, Drumkelin Bog, 220; + in glacial period, 119; + level of land, 220; + storminess and rainfall, 107; + submergence, 219. + + Irish Sea, tides, 191. + + Irrigation ditches, abandoned, 97. + + Isostasy, 307 ff. + + Italy, southern, climate of, 86 f. + + + Japan, earthquakes of, 296. + + Javanese mountains, origin of, 193. + + Jaxartes, 108. + + Jeans, J. H., cited, 251, 252, 253, 266, 272. + + Jeffreys, H., cited, 302, 303, 306. + + Jeffreys, J., cited, 191. + + Jericho, palms in, 92. + + Jerusalem, rainfall, 86; + rainfall and temperature, 94; + rainfall in, and sequoia growth, 83 ff. + + Johnson, cited, 226. + + Judea, palms in, 92. + + Jupiter, and sunspots, 243; + effect of, 253; + periodicity of, 61 f.; + temperature of, 258; + tidal effect of, 250. + + Jurassic, climate, 266; + mountain ranges, 44. + + + Kansas, variations of seasons, 103. + + Kara Koshun marsh, Lop Nor, 104. + + Keewatin center, 113; + evaporation in, 129. + + Keewatin ice sheet, 121. + + Kelvin, Lord, cited, 13 f. + + Keyes, C. R., cited, 156. + + Kirk, E., cited, 287. + + Knott, C. G., cited, 294, 295, 297, 299, 304, 306. + + Knowlton, F. H., cited 167, 169, 170, 212, 232. + + Koeppen, W., 47, 52, 140. + + Krakatoa, glaciation and, 48; + volcanic hypothesis and, 45. + + Kruemmel, O., cited, 224, 228. + + Kullmer, C. J., cited, 113, 115, 128; + map of storminess, *54. + + _Kungaspegel_, sea routes described, 106. + + + Labor, price in England, 102. + + Labradorean center of glaciation, 113. + + Lahontan, Lake, 142. + + Lake strands, _see_ Strands. + + Lake Superior, lava, 211. + + Lakes, during glacial periods, 141 f.; + in semi-arid regions, 60; + of Great Basin, 126; + ruins in, 97. + + Land, and water, climatic effect of, 196 ff.; + distribution of, 200, form of, 43 ff.; + range of temperature and, 196. + + Lavas, climatic effect of, 211. + + Lawson, A. C., cited, 310. + + Lebanon, cedars of, 83. + + Leiter, H., cited, 71. + + Leverett, F., cited, 271. + + Life, atmosphere and, 229 f.; + chemical characteristic of, 12; + effect of salinity, 225; + of glacial period, 127; + persistence of forms, 230. + + Light, effect of atmosphere on, 236; + effect on plants, 184 ff.; + ultra-violet, storminess and, 56; + variation of, 275. + + Litorina sea, 218. + + Loess, date of, 156 ff.; + origin of, 155, 165. + + Lop Nor, rise of, 104; + swamps, 171. + + Lows, and glacial lobes, 122; + movements of, 126; + see Storms and Cyclones. + + Lulan, 104. + + Lull, R. S., cited, 5, 188. + + + MacDougal, D. T., cited, 171. + + McGee, W. J., cited, 156. + + Macmillan, W. D., cited, 191. + + Magdalenian period, 216. + + Magnetic fields of sunspots, 56. + + Magnetic poles, relation to storm tracks, 150. + + Makran, climate, 89; + rainfall, 89. + + Malay Archipelago, earthquakes of, 296. + + Mallet, R., cited, 288. + + Malta, rainfall, 86. + + Manson, M., cited, 147. + + Mayas, civilization, 26; + ruins, 95. + + Mayence, flood at, 99. + + Mazelle, E., cited, 224. + + Mediterranean, climate of, 72; + rainfall records, 86; + storminess in, 60. + + Mercury, and sunspots, 243. + + Mesozoic, climate, 266; + crustal changes, 286; + emergence of lands, 287. + + Messier, 8; + variables, 248. + + Metcalf, M. M., cited, 202. + + Meteorological factors and earthquakes, 300 f. + + Meteorological hypothesis of crustal movements, 294. + + Meteors, and sun's heat, 13, 246. + + Michelson, A. A., cited, 259. + + Middle Silurian, fauna in Alaska, 287. + + Mild climates, _see_ Climates, mild. + + Milky Way, 252. + + Mill, H. R., cited, 228. + + Milne, J., cited, 288, 290, 294, 304, 306. + + Miocene, crustal changes, 287. + + Mississippi Basin, loess in, 159. + + Mogul emperor, and famine, 104. + + Monsoons, character of, 146; + direction of, 208; + Indian famines and, 105. + + Moulton, F. R., cited, 13, 258, 269. + + Mountain building, climatic changes and, *25. + + Mountains, folding of, 190; + rainfall, on, 208. + + Multiple stars, 252. + + + Nansen, F., cited, 122, 174. + + Naples, rainfall, 86. + + Nathorst, cited, 169. + + Nebulae, 247. + + Nebular hypothesis, 232, 267. + + Neolithic period, 218. + + Nevada, correlations of rainfall, 86. + + New England, height of land, 111. + + New Mexico, rainfall, 89. + + New Orleans, La., rainfall and temperature, 94. + + New Zealand, climate, 177; + tree ferns, 179. + + Newcomb, S., cited, 52. + + Nile floods, periodicity in, 245. + + Nitrogen, in atmosphere, 19. + + Niya, Chinese Turkestan, desiccation at, 66. + + Nocturnal cooling, changes in, 238 f. + + Norlind, A., cited, 100. + + Norsemen, route to Greenland, 26. + + Norse sagas, 219. + + North Africa, climate of, 71; + Roman aqueducts in, 71. + + North America, at maximum glaciation, 122 ff.; + emergence of lands, 193; + glaciation in, 131; + height of land, 111; + interior sea in, 200; + inundations, 196; + loess in, 155; + submergence of lands, 19, 21. + + North Atlantic Ocean, salinity, 228. + + North Sea, climatic stress, 98 ff.; + floods around, 26, 99; + rainfall, 107; + storminess, 57. + + Northern hemisphere, earthquakes of, 294. + + Norway, decay, 100; + temperature, 177. + + Novae, 247. + + + Oceanic circulation, carbon dioxide and, 39 ff. + + Oceanic climate, characteristics, 103. + + Oceanic currents, diversion, 44; + influence of land distribution, 203. + + Oceans, age of, 223; + composition of, 223-241; + deepening of, 199; + salinity, 19, 223; + temperature, 6, 152, 180, 226. + + Okada, T., cited, 224. + + Old Testament, temperature, 92. + + Orbital precessions, 27. + + Ordovician, climate, 266. + + Organic evolution, glacial fluctuations and, 26. + + Orion, nebulosity near, 247; + stars near, 248. + + Orontes, 67. + + Osborn, H. F., cited, 216. + + Owens-Searles, lakes, 142. + + Oxus, 108. + + Oxygen, in atmosphere, 20, 234; + in Permian, 152. + + Ozone, cause of, 56. + + + Paleolithic, 216. + + Paleozoic, climate, 266; + mountains in, 209. + + Palestine, change of climate, 91 f. + + Palms, climatic change and, 91 f.; + in Ireland, 179. + + Palmyra, ruins of, 66. + + Parallaxes of stars, 276 f. + + Patrician center, 134. + + Peat-bog period, first, 218. + + Penck, A., cited, 139, 156, 157, 158, 269. + + Pennsylvanian, life of, 26. + + Periodicities, 245 f. + + Periodicity, of climatic phenomena, 60 f.; + of glaciation, 268; + of sunspots, 243. + + Permian, climate, 266; + distribution of glaciation, 152; + glaciation, 60, 144, *145, 226; + glaciation and mountains, 45; + life of, 26; + red beds, 151; + temperature, 146 f. + + Perry, cited, 289. + + Persia, lakes, 143; + rainfall, 89. + + Pettersson, O., cited, 98 ff., 100 f., 103, 106, 219. + + Pirsson, L. V., cited, 3, 196. + + Planetary hypothesis, 253, 267. + + Planetary nebulae, 252. + + Planets, and sunspots, 243; + effect of star on, 255; + sunspot cycle and, 62; + temperatures, 8 f. + + Plants, climate and, 1 f.; + effect of light, 184 ff. + + Pleion, defined, 29. + + Pleionian migrations, 29 f. + + Pleistocene, climate, 266; + duration of, 48; + glaciation, 110 ff.; + ice sheets, *123. + + Pluvial climate, causes of, 143; + during glacial periods, 141. + + Po, frozen, 98. + + Polaris, 272. + + Polar wandering, hypothesis of, 48 f. + + Pole and earthquakes, 305. + + Post-glacial crustal movements and climatic changes, 215-222. + + Poynting, J. H., cited, 8. + + Precessional hypothesis, 34 f. + + Precipitation, and glaciation, 114, 133; + during glacial period, 118; + snow line and, 139; + temperature and, 94. + + Procyon, companion of, 280; + luminosity, 278; + speed of, 281. + + Progressive change, 241. + + Progressive desiccation, hypothesis of, 65 ff. + + Proterozoic, 4 f.; + fossils, 6 f.; + glaciation, 18, 144, 226, 266; + lava, 211; + mountains in, 209; + oceanic salinity, 42 f.; + oxygen in air, 234; + red beds, 151; + temperature, 146 f. + + Pulsations, hypothesis of, 65, 72 ff. + + Pulsatory climatic changes, 72 ff. + + Pulsatory hypothesis, 272. + + Pumpelly, R., cited, 271. + + + Radiation, variation of, 275. + + Radioactivity, heat of sun and, 14 f. + + Rainfall, changes in, 93 f.; + glaciation and, 50; + sunspots and, 53, *58, 59; + tree growth and, 79. + + Red beds, 151, 170. + + Rhine, flood, 99; + frozen, 98. + + Rho Ophiuchi, variables, 248. + + "Rice grains," 61. + + Richardson, O. W., cited, 256. + + Rigidity, of earth, 307. + + Roads, climate and, 66. + + Rogers, Thorwald, cited, 101. + + Romans, aqueduct of, 71. + + Rome, history of, 87. + + Rotation, of earth, 18 f. + + Ruden, storm-flood, 99. + + Rugen, storm-flood, 99. + + Ruins, as climatic evidence, 66; + rainfall and, 60. + + + Sacramento, correlation coefficients, 82 f., 85; + rainfall, 86; + rainfall record, 79. + + Sagas, cited, 105 f. + + St. John, C. E., cited, 236. + + Salinity, deep-sea circulation and, 176; + effect on climate, 224; + in North Atlantic, 228; + ocean temperature and, 226; + of ocean, 19, 120. + + Salisbury, R. D., cited, 111, 125, 129, 139, 156, 206, 269, 271. + + Salt, in ocean, 223. + + San Bernardino, correlation of rainfall, 85. + + Saturn, and sunspots, 243; + sunspot cycle and, 62. + + Sayles, R. W., cited, 183. + + Scandinavia, climatic stress, 100 f.; + fossils, 271; + post-glacial climate, 271; + rainfall, 107; + storminess, 57, 107; + temperature, 216. + + Scandinavian center of glaciation, 113. + + Schlesinger, F., cited, 275, 278, 298, 301, 305. + + Schuchert, C., cited, 3, 5, 23, *25, *123, 138, *145, 168, 169, + 172, 188, 193, 196, 198, 200, *201, 206, 211, 230, 265. + + Schuster, A., cited, 61, 244, 294, 296. + + Sculpture, Maya, 96. + + Sea level and glaciation, 119. + + Seasonal alternations, 28 f. + + Seasonal banding, 183 f. + + Seasonal changes, geological, 183. + + Seasons, and earthquakes, 294, 295, 297, 299; + evidences of, 169. + + Secular progression, 17 ff., 188. + + Seistan, swamps, 171. + + Sequoias, measurements of, 74 ff.; + rainfall record, 79. + + Setchell, W. A., cited, 1. + + Shackleton, E., cited, 125. + + Shapley, H., cited, 246, 247, 254, 256, 275. + + Shimek, E., cited, 157, 161. + + Shreveport, La., rainfall and temperature, 93 f. + + Shrinkage of the earth, 190. + + Siberia, and glaciation, 132. + + Sierras, rainfall records, 82. + + Simpson, G. C., cited, 222. + + Sirius, companion of, 280; + distance from sun, 262; + luminosity, 278; + speed of, 281. + + Slichter, C. S., cited, 192. + + Smith, J. W., cited, 73. + + Snowfall, glaciation and, 50, 114. + + Snowfield, climatic effects of, 115. + + Snow line, height of, 138; + in Andes, 139; + in Himalayas, 139. + + Solar activity, cycles of, 245; + deep-sea circulation and, 179; + ice and, 134. + + Solar constant, 114. + + Solar-cyclonic hypothesis, 51-63, 287; + glaciation and, 110-129. + + Solar prominences, cause of, 61. + + Solar system, 252; + conservation of, 243; + proximity to stars, 63. + + Solar variations, storms and, 31. + + South America, earthquakes, 301. + + South Pole, thickness of ice at, 125. + + Southern hemisphere, earthquakes, 296; + glaciation in, 131 f. + + Southern Pacific railroad, rainfall records along, 82. + + Soy beans, effect of light, 185 f. + + Space, sun's journey through, 264-284. + + Spiral nebulae, 251 f.; + universe of, 267. + + Spitzbergen, submergence, 219. + + Springs, climate and, 66. + + Stars, approach to sun, 253; + binary, 252; + clusters, 252, 268; + effect on solar atmosphere, 63; + dark, 254; + parallaxes of, 276 f.; + tidal action of, 249. + + Stefan's Law, 257. + + Stein, M. A., cited, 78. + + Stellar approaches, probability of, 260. + + Storm belt in arid regions, 144. + + Storm-floods, in fourteenth century, 99. + + Storminess, and erosion, 309; + and ice, 134; + effect on glaciation, 112; + sunspots and, 163; + temperature and, 94, 173. + + Storms, blows of, 300, 302; + increase, 60; + movement of, 125 f.; + movement of water and, *175; + origin of, 30 f.; + sunspots and, 28, 53; + _see_ Cyclones and Lows. + + Storm tracks, during glacial period, 117; + location, 113; + relation to magnetic poles, 150; + shifting of, 119. + + Strands, climate and, 66; + in semi-arid regions, 60; + of salt lakes, 142. + + Suess, E., cited, 192. + + Sun, and the earth's crust, 285-317; + approach to star, 253; + atmosphere of, 61, 274; + atmosphere of, and weather, 52; + cooling of, 49; + contraction of, 249; + disturbances of, 172; + effect of other bodies on, 242-263; + heat, 13; + journey through space, 264-284; + Knowlton's hypothesis of, 168. + + Suncracks, 232. + + Sunspot cycles, 27 f. + + Sunspots, and earthquakes, 289; + causes of, 61; + magnetic field of, 261; + maximum of, 109; + mild climates and, 172; + number, 108 f.; + periodicity, 243; + planetary hypothesis of, 253; + records, 245; + storminess and, 163; + storms and, 300; + temperature of earth and, 52, 173. + + Sunspot variations, 282. + + Swamps, as desert phenomena, 171. + + Sylt, storm-flood, 99. + + Syria, civilization in, 67; + inscriptions in, 76; + Roman aqueducts in, 71. + + Syrian Desert, ruins in, 66. + + + Talbert, cited, 213. + + Tarim Basin, red beds, 151. + + Tarim Desert, desiccation, 66. + + Tarim River, swamps, 171. + + Taylor, G., cited, 140, 144, 191, 271. + + Temperature, change of in Atlantic, 174; + changes in, 93; + climatic change and, 49; + critical, 9; + geological time and, 3; + glacial period, 38; + glaciation and, 42, 132, 139; + gradient of earth, 213; + of ocean, 180; + in Norway, 177; + in Permian, 146 f.; + in Proterozoic, 146 f.; + limits, 6 ff.; + precipitation and, 94; + range of, 3, 8; + solar activity and, 140; + storminess and, 94, 112, 173; + sunspots and, 28, 173; + volcanic eruptions and, 46; + zones, 172. + + Terrestial causes of climatic changes, 188-214. + + Tertiary, lava, 211. + + Thames, frozen, 98. + + Thermal solar hypothesis, 49 f., 97. + + Thermo-pleion, movements of, 30. + + Thesis, of pulsations, 24. + + Thiryu, storm-flood, 99. + + Tian-Shan Mountains, irrigation in, 71. + + Tidal action of stars, 249. + + Tidal effect, of Jupiter, 253; + of planets, 244. + + Tidal hypothesis, 251. + + Tidal retardation, effect on land and sea, 191; + rotation of earth and, 18 f.; + stress caused by, 310. + + Tides, cycles of, 219. + + Time, geological, _see_ Geological time. + + Toads, distribution of, 202. + + Tobacco plant, effect of light, 184. + + Topography, and glaciation, 132. + + Transcaspian Basin, red beds, 151. + + Tree ferns, in New Zealand, 179. + + Tree growth, periodicity in, 245; + rainfall and, 79. + + Trees, in California, 219; + measurement of, 73 ff. + + Triassic, climate, 266. + + Trifid Nebula, variables, 248. + + Trondheim, wheat in, 101. + + Trondhenaes, corn in, 101. + + Tropical cyclones, in glacial epochs, 140 f.; + occurrence, 148; + solar activity and, 113. + + Tropical hurricanes, earthquakes and, 300; + sunspots and, 149. + + Turfan, temperature, 17. + + Turner, H. H., cited, 245. + + Tyler, J. M., cited, 216. + + Tyndall, J., cited, 36, 37. + + Typhoon region, "earthquake weather," 298. + + Typhoons, occurrence, 300. + + + United States, rainfall and temperature in Gulf region, 93 f.; + salt lakes in, 142; + southwestern, climate, 66; + storminess, 53 f., 60. + + + Variables, 247. + + Veeder, M. A., cited, 300. + + Vegetation, theory of pulsations and, 90. + + Venus, atmosphere of, 236. + + Vesterbygd, invasion of, 106. + + Vicksburg, Miss., rainfall and temperature, 93 f. + + Volcanic activity, climate and, 210; + movement of the earth's crust and, 285; + times of uplifting lands and, 23. + + Volcanic dust, climatic changes and, 97. + + Volcanic hypothesis, climatic change and, 45 ff.; + snow line, 139. + + Volcanoes, activity of, 96. + + Volga, 108. + + + Walcott, C. D., cited, 4, 230. + + Wandering of the pole, 302. + + Water, importance, 9. + + Water vapor, condensation of, 56; + effect on life, 231; + in atmosphere, 19. + + Wave, effect on movement of water, 176. + + Weather, changes of, 31 f.; + origin of, 174; + variations, 52. + + Wells, H. G., cited, 35. + + Wendingstadt, storm-flood, 99. + + Westerlies, 21 f. + + Wheat, price in England, 102. + + White Sea, submergence, 219. + + Whitney, J. D., cited, 142. + + Wieland, G. R., cited, 169. + + Williamson, E. D., cited, 226. + + Willis, B., cited, 206. + + Winds, at ice front, 162; + effect on currents, 174; + glaciation and, 133; + in Antarctica, 161; + in glacial period, 119; + in Greenland, 161; + planetary system of, 174; + velocity, 240. + + Witch hazel, effect of light, 184. + + Wolf, J. R., cited, 61, 109, 288. + + Wolfer, cited, 244. + + Wright, W. B., cited, 35, 111, 119. + + Writing, among Mayas, 96. + + + Yucatan, Maya civilization, 26, 107; + rainfall, 108; + ruins, 95. + + Yukon, Ice Age in, 221. + + + Zante, earthquakes of, 296. + + Zonal crowding, 117. + + +PRINTED IN THE UNITED STATES OF AMERICA + + + + + +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.txt or 37855.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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