Climatic Changes: Their Nature and Causes

by Ellsworth Huntington; Stephen Sargent Visher

Knowlton^[61] 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^[62] 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^[62] 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,^[63] 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^[64] 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.