GEOLOGICAL AND SOLAR CLIMATES,
THEIR CAUSES AND VARIATIONS.
“The most important problem in terrestrial physics * * and the one which will ultimately prove the most far reaching in its consequences, is: What are the physical causes which led to the Glacial Epoch and to all those great secular changes of climate which are known to have taken place during Geological Ages?” (Dr. Croll, Climate and Cosmology.)
“An attentive study of the physical Geography of the earth and its influences on Climate, together with a judicious application of the simplest physical theories, will enable us to gain by and by a better knowledge of Geological climates.” (Prof. A. Woeikof, Nature, March 2, 1882, p. 426.)
Since Agassiz announced[1] the past existence of an age during which ice covered temperate and tropical land areas, the cause of this wonderful phenomenon has been a problem of profound interest. Upon the correct solution of it hinges also the cause of Geological climates.
So great has been the interest attaching to this subject, that more study has been devoted to it during the past fifty years than perhaps to any other in Geology; hardly a leading scientific magazine runs through a year’s numbers without one or more articles upon it; and no Geological Society is without zealous students of glacial phenomena. Some have become so absorbed in the subject that, led by the recurrence of certain slight astronomical influences, they recognize a glacial period for slight and widely scattered evidences of possible early local glaciation, forgetful of the fact that an era of frigid climate could not intervene between two eras of tropical climates without the intervention of eras of temperate climates.
The evidences establishing the reality of the Ice Age[2] during the Quaternary period are now beyond dispute. It is difficult, however, to establish by geological evidence the synchronal glaciation of all the continental areas known to have been heavily glaciated. This difficulty arises from the fact that the identity of various strata has to be established by fossils of varying conditions and characters; it is also rare that the same geologist has visited and compared the evidence upon more than two continents, thus eliminating probable errors from unequal sub-aerial denudation and exposure in the different zones of present climates and upon different continents. Again, the proof of the contemporaneous existence of corresponding strata upon different continents in the same latitude is sometimes attempted by a comparison of land fauna and flora, with marine fauna and flora, or even by more complex comparisons. Fossil plant life is by far more reliable than animal life for comparative purposes.
Another misleading factor is found in the interpretation of the great trans-continental lines of terminal moraines into the absolute limits of glaciation. Considering the great lapse of time since the removal of glacial conditions in temperate and tropical latitudes, it is more than probable that the existing unobliterated evidences by no means mark the extreme limits of a lighter and more extended glaciation whose traces have been destroyed, but which can justly be interpolated between the existing very marked traces of enormous glacial extension during Quaternary times. It is not impossible, nor entirely improbable, that early local glaciation did not occur during the early part of the Cenozoic Era, or even earlier, but the data upon which to establish the occurrence of such early local glaciation are both meagre and obscure. Should the evidences of such early local glaciation be developed beyond dispute, they will in no way interfere with the interpretation to be given, but they will strongly corroborate certain portions of this interpretation. So far as the author has been able to examine such evidence, it has been found to be between strata containing fossil life of a torrid character, with no evidences of a gradual merging into a temperate climate above and below it, as in Quaternary glaciation.[3]
Before entering further into this discussion, it may not be out of place to briefly review the principal theories advanced to account for the Ice Age. It will be seen that physicists and astronomers have vied with geologists in the diligence of the search for the cause of this age, and their minds have been as fertile in the number of causes assigned as the true one. Not one of all the causes suggested has been sustained by argument without a flaw in the reasoning, and no demonstration has been made which has carried conviction to the scientific world.
It would not be instructive to attempt to review all of the theories which have been urged. The tendency to ascribe remote inadequate or obscure causes, rather than to interpret facts and phenomena in accordance with known laws, is apparent in many. Some writers have ascribed causes resting only upon hypotheses beyond the range of either analysis or investigation; such hypotheses can only stand in the absence or failure of all other assignable causes. Therefore the leading causes only will be briefly mentioned.
In a recent monograph on the subject, the following are given:[4]
The first of these theories is universally admitted, and taught in even elementary works on Physical Geography, but it fails to account for all the phenomena accompanying the Ice Age, or to account for the disappearance of that age, and, so far as the author is aware, has not been presented in such form as to satisfactorily account for geological and present climates in rigid conformity with the facts and known laws. Nor has it been presented in such form as to account for that era of geological climates known as the Ice Age; moreover, it fails to account for the disappearance of that Age.
The second has been proved to be a local and correlated phenomenon, but cannot be accepted as a cause, since glaciation did not solely depend in the same latitudes upon elevation above sea level.
As to the third, whilst slight changes in obliquity have occurred, and must continue to occur, the results are too slight and the distribution of glacial phenomena is too general to warrant the acceptance of such change as a prime cause.
The fourth is a necessary consequence of the first, but, like the first, fails when the crucial test of accounting for the disappearance of the continental Ice Sheets is applied.
The fifth, sixth and seventh theories are mere hypotheses, unsupported by either demonstration or observed facts.
The eighth has been presented to the scientific world through the labors and researches of that eminent geologist and physicist, Dr. James Croll, in his various articles in leading scientific magazines, and lastly, in his grand contributions to the subject under discussion, “Climate and Time” and “Climate and Cosmology.”
The ninth has been maintained by one of the greatest English naturalists, Mr. Alfred Russell Wallace. He combines the theory of Dr. Croll with that of Sir Charles Lyell, and very ably presents his views in “Island Life.”
The tenth is a presentation by Dr. Ball, F. R. S., etc., of an interesting demonstration, to the effect that 63 per cent. of solar heat reaches either hemisphere during its summer exposure, and the remaining 37 per cent. during winter exposure. Nothing is added to the Physical Theory of Dr. Croll, nor does the demonstration in any way remove the serious objections which have been urged against Dr. Croll’s views.
The strongest support that has been given to any of the above theories is made by the arguments and deductions of Dr. Croll and Mr. Wallace; yet they have failed to produce conviction, for, in a recent work on Geology, the author, after reviewing the various theories as to the cause of the Glacial Period, uses this expression: “This seems to be by far the most probable yet presented.”[6]
This opinion is directly given upon only one—the ninth; but its terms are such that it embraces all. If the ninth is “by far the most probable,” it would be difficult to fix the degree of probability or improbability of the others.
The only explanation which can be accepted is one which will admit of definite proof, and will satisfy all the conditions, and not require the distortion of known facts, by forcibly fitting them into arbitrary molds. It must start from universally admitted premises, and in rigid consonance with known laws, correctly interpret the grand eras of climate which have marked the geological history of our globe, and further, it must point out and fully elucidate wherein and why the present climates of our globe differ so radically from those vast secular variations recorded by fossil life—aye, more, it must be so general as to be of universal force and applicable to other members of the solar system constituted as our globe.
In the brief review just made of the principal theories urged by various scientists as causes producing the Ice Age, it was remarked of the first that it was universally admitted as true, and even taught in elementary works on Physical Geography, but that it failed to account for all the facts developed by the Ice Age. This first theory was a decrease in the original heat of the globe, the truth of which is established by a mass of indisputable geological evidence.
The present conditions are so radically different from any of the eras of climate known to have existed, that the explanation of this range of secular changes becomes the grandest problem in terrestrial physics, and has an important bearing in the solution of existing conditions upon the other planets.
It is universally admitted that this original heat has been so lost that it is no longer a factor in the surface temperature of the earth, and that solar energy is now the controlling source of heat.
There can then be no mistaking the first nor the present condition of the earth as regards its exposure to the only two sources of heat—(1) solar and stellar[7] heat, and (2) resident, internal, or earth heat. There can therefore be no error as to the main features of the problem.
There must have been two marked eras of climatic control—(A) a past era, during which both sources were active; (B) and the present era, in which the greater exterior source only remains, the local and lesser source having been practically exhausted.
Or, in other words, we have, first, a heated globe having resident in its mass a finite quantity of heat, undergoing loss and exposed to an exterior source of heat and light, which source may be either constant or decreasing in its energy, but so slowly that it may be considered sensibly constant during the eras under consideration; second, the same globe deprived of its heat to such an extent that a crust of non-conducting material has formed, the outer surface of which is exposed only to solar heat, and whose climates are entirely controlled thereby. The objects in view being to explain (1) the peculiar uniformity of climates prior to the exhaustion of the first source, and (2) the occurrence of an age of general glaciation in all latitudes prior to the establishment of the sole control of the exterior source; (3) the reasons of the differences between heat distribution during geological and present climates. Such explanations to be in strict conformity with admitted facts and known laws, and without omitting the one nor distorting the other.
To be explicit we will state that the prime objects are to demonstrate—
1. That in the passage of the earth from an era during which its climates have been controlled by internal heat into an era during which its climates are controlled by solar heat, eras of uniform climates must have been passed through during which isotherms were independent of latitude.
2. That before climates could have passed under solar control that an age must occur during which continental areas must be glaciated; and that this stupendous phenomenon, occurring before solar climatic control, was also independent of latitude.
3. That the direct cause of the Ice Age was a combination of the remarkable properties, in relation to heat and cold, possessed by the various forms of water. As vapor, in the form of fogs and clouds it prevented the loss or receipt of heat by radiation; as water, by reason of its high specific heat, it retained to the last moment the effective remnant of earth heat; as ice, it assumed a solid form, storing the maximum amount of cold.
4. To point out in a general way the fallacies of previous attempts to explain geological and present climates.
The problem will be given in a general proposition, which is capable of demonstration in perfect accord with known laws.
[This demonstration was first given by the Author in September, 1891, and is reproduced here slightly modified and extended from Vol. VIII of the Transactions of the Technical Society of the Pacific Coast.]