Climatic Changes.

by Ellsworth Huntington and Stephen Sargent Visher.

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 cooperation, so that by working together scientists discover much that would else have remained hid.

This book ill.u.s.trates 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 alt.i.tude 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 ill.u.s.trates the unity of modern science because it is preeminently a cooperative product. Not only have the two authors shared in its production, but several of the Yale Faculty have also cooperated. From the geological standpoint, Professor Charles Schuchert has read the entire ma.n.u.script 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 a.s.sist, and either in writing or by word of mouth has contributed many pages. In astronomy, the same cordial cooperation 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 cooperation. 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 cooperation is much deeper than would appear from this brief paragraph.

Outside the Yale Faculty we have received equally cordial a.s.sistance.

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 s.p.a.ce; Professor Andrew E. Dougla.s.s 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. c.u.mings 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 ma.n.u.script 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.

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 20C. in average yearly temperature, and for most the limit of successful growth is 10.[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 a.s.sumes that causes which can now be observed in operation, as explained in a companion volume ent.i.tled _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 a.s.sumes 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 a.n.a.lysis 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 p.r.o.nounced 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 ma.s.sive 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 quant.i.ties 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 acc.u.mulation.

_Permian Period._ Rise of reptiles. Another great ice age.

MESOZOIC ERA. Age of reptile dominance.

_Tria.s.sic Period._ Rise of dinosaurs. The period closes with a cool climate.

_Jura.s.sic 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 lat.i.tudes, 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 30C. 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 75C. (167F.). 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 45C. (113F.) 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 lat.i.tudes 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 50C.

even in the warmest portions. This is putting the limit too high rather than too low, but even so the warmest parts of the earth can scarcely have averaged much more than 20 warmer than at present.

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 0C. If those places should be colder than now by 30C., 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.