until after the third group of sequences has been discussed. This group includes glacial fluctuations, historic pulsations, Brückner 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
  3. majority of authorities are on the side of a diminution of heat, but the other view also deserves study.
  4. 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.
  5. 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.
  6. 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.