The rôle of climate in the life of today suggests its importance in the past and in the future. No human being can escape from the fact that his food, clothing, shelter, recreation, occupation, health, and energy are all profoundly influenced by his climatic surroundings. A change of season brings in its train some alteration in practically every phase of human activity. Animals are influenced by climate even more than man, for they have not developed artificial means of protecting themselves. Even so hardy a creature as the dog becomes notably different with a change of climate. The thick-haired "husky" of the Eskimos has outwardly little in common with the small and almost hairless canines that grovel under foot in Mexico. Plants are even more sensitive than animals and men. Scarcely a single species can flourish permanently in regions which differ more than 20°C. in average yearly temperature, and for most the limit of successful growth is 10°.[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 algæ 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 algæ there must have been many other kinds of life. The Proterozoic fossils thus far discovered include not only microscopic radiolarians such as still form the red ooze of the deepest ocean floors, but the much more significant tubes of annelids or worms. The presence of the annelids, which are relatively high in the scale of organization, is generally taken to mean that more lowly forms of animals such as coelenterates and probably even the mollusca and primitive arthropods must already have been evolved. That there were many kinds of marine invertebrates living in the later Proterozoic is indicated by the highly varied life and more especially the trilobites found in the oldest Cambrian strata of the next succeeding period. In fact the Cambrian has sponges, primitive corals, a great variety of brachiopods, the beginnings of gastropods, a wonderful array of trilobites, and other lowly forms of arthropods. Since, under the postulate of evolution, the life of that time forms an unbroken sequence with that of the present, and since many of the early forms differ only in minor details from those of today, we infer that the climate then was not very different from that of today. The same line of reasoning leads to the conclusion that even in the middle of the Proterozoic, when multicellular marine animals must already have been common, the climate of the earth had already for an enormous period been such that all the lower types of oceanic invertebrates had already evolved.
| [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.