Part 10 (1/2)
[Footnote 60: Chas. Schuchert: Review of Knowlton's Evolution of Geological Climates, in Am. Jour. Sci., 1921.]
[Footnote 61: G. R. Wieland: Distribution and Relations.h.i.+ps of the Cycadeoids; Am. Jour. Bot., Vol. 7, 1920, pp. 125-145.]
[Footnote 62: D. T. MacDougal: Botanical Features of North American Deserts; Carnegie Inst.i.t. of Wash., No. 99, 1908.]
[Footnote 63: _Loc. cit._]
[Footnote 64: H. H. Clayton: Variation in Solar Radiation and the Weather; Smiths. Misc. Coll., Vol. 71, No. 3, Was.h.i.+ngton, 1920.]
[Footnote 65: B. h.e.l.land Hansen and F. Nansen: Temperature Variations in the North Atlantic Ocean and in the Atmosphere; Misc. Coll., Smiths.
Inst., Vol. 70, No. 4, Was.h.i.+ngton, 1920.]
[Footnote 66: The climatic significance of ocean currents is well discussed in Croll's Climate and Time, 1875, and his Climate and Cosmogony, 1889.]
[Footnote 67: F. J. B. Cordeiro: The Gyroscope, 1913.]
[Footnote 68: W. W. Garner and H. A. Allard: Flowering and Fruition of Plants as Controlled by Length of Day; Yearbook Dept. Agri., 1920, pp.
377-400.]
[Footnote 69: Report of Committee on Sedimentation, National Research Council, April, 1922.]
CHAPTER XI
TERRESTRIAL CAUSES OF CLIMATIC CHANGES
The major portion of this book has been concerned with the explanation of the more abrupt and extreme changes of climate. This chapter and the next consider two other sorts of climatic changes, the slight secular progression during the hundreds of millions of years of recorded earth history, and especially the long slow geologic oscillations of millions or tens of millions of years. It is generally agreed among geologists that the progressive change has tended toward greater extremes of climate; that is, greater seasonal contrasts, and greater contrasts from place to place and from zone to zone.[70] The slow cyclic changes have been those that favored widespread glaciation at one extreme near the ends of geologic periods and eras, and mild temperatures even in subpolar regions at the other extreme during the medial portions of the periods.
As has been pointed out in an earlier chapter, it has often been a.s.sumed that all climatic changes are due to terrestrial causes. We have seen, however, that there is strong evidence that solar variations play a large part in modifying the earth's climate. We have also seen that no known terrestrial agency appears to be able to produce the abrupt changes noted in recent years, the longer cycles of historical times, or geological changes of the shorter type, such as glaciation.
Nevertheless, terrestrial changes doubtless have a.s.sisted in producing both the progressive change and the slow cyclic changes recorded in the rocks, and it is the purpose of this chapter and the two that follow to consider what terrestrial changes have taken place and the probable effect of such changes.
The terrestrial changes that have a climatic significance are numerous.
Some, such as variations in the amount of volcanic dust in the higher air, have been considered in an earlier chapter. Others are too imperfectly known to warrant discussion, and in addition there are presumably others which are entirely unknown. Doubtless some of these little known or unknown changes have been of importance in modifying climate. For example, the climatic influence of vegetation, animals, and man may be appreciable. Here, however, we shall confine ourselves to purely physical causes, which will be treated in the following order: First, those concerned with the solid parts of the earth, namely: (I) amount of land; (II) distribution of land; (III) height of land; (IV) lava flows; and (V) internal heat. Second, those which arise from the salinity of oceans, and third, those depending on the composition and amount of atmosphere.
The terrestrial change which appears indirectly to have caused the greatest change in climate is the contraction of the earth. The problem of contraction is highly complex and is as yet only imperfectly understood. Since only its results and not its processes influence climate, the following section as far as page 196 is not necessary to the general reader. It is inserted in order to explain why we a.s.sume that there have been oscillations between certain types of distribution of the lands.
The extent of the earth's contraction may be judged from the shrinkage indicated by the shortening of the rock formations in folded mountains such as the Alps, Juras, Appalachians, and Caucasus. Geologists are continually discovering new evidence of thrust faults of great magnitude where ma.s.ses of rock are thrust bodily over other rocks, sometimes for many miles. Therefore, the estimates of the amount of shrinkage based on the measurements of folds and faults need constant revision upward.
Nevertheless, they have already reached a considerable figure. For example, in 1919, Professor A. Heim estimated the shortening of the meridian pa.s.sing through the modern Alps and the ancient Hercynian and Caledonian mountains as fully a thousand miles in Europe, and over five hundred miles for the rest of this meridian.[71] This is a radial shortening of about 250 miles. Possibly the shrinkage has been even greater than this. Chamberlin[72] has compared the density of the earth, moon, Mars, and Venus with one another, and found it probable that the radial shrinkage of the earth may be as much as 570 miles. This result is not so different from Heim's as appears at first sight, for Heim made no allowance for unrecognized thrust faults and for the contraction incident to metamorphism. Moreover, Heim did not include shrinkage during the first half of geological time before the above-mentioned mountain systems were upheaved.
According to a well-established law of physics, contraction of a rotating body results in more rapid rotation and greater centrifugal force. These conditions must increase the earth's equatorial bulge and thereby cause changes in the distribution of land and water. Opposed to the rearrangement of the land due to increased rotation caused by contraction, there has presumably been another rearrangement due to tidal r.e.t.a.r.dation of the earth's rotation and a consequent lessening of the equatorial bulge. G. H. Darwin long ago deduced a relatively large r.e.t.a.r.dation due to lunar tides. A few years ago W. D. MacMillan, on other a.s.sumptions, deduced only a negligible r.e.t.a.r.dation. Still more recently Taylor[73] has studied the tides of the Irish Sea, and his work has led Jeffreys[74] and Brown[75] to conclude that there has been considerable r.e.t.a.r.dation, perhaps enough, according to Brown, to equal the acceleration due to the earth's contraction. From a prolonged and exhaustive study of the motions of the moon Brown concludes that tidal friction or some other cause is now lengthening the day at the rate of one second per thousand years, or an hour in almost four million years if the present rate continues. He makes it clear that the r.e.t.a.r.dation due to tides would not correspond in point of time with the acceleration due to contraction. The r.e.t.a.r.dation would occur slowly, and would take place chiefly during the long quiet periods of geologic history, while the acceleration would occur rapidly at times of diastrophic deformation. As a consequence, the equatorial bulge would alternately be reduced at a slow rate, and then somewhat suddenly augmented.
The less rigid any part of the earth is, the more quickly it responds to the forces which lead to bulging or which tend to lessen the bulge.
Since water is more fluid than land, the contraction of the earth and the tidal r.e.t.a.r.dation presumably tend alternately to increase and decrease the amount of water near the equator more than the amount of land. Thus, throughout geological history we should look for cyclic changes in the relative area of the lands within the tropics and similar changes of opposite phase in higher lat.i.tudes. The extent of the change would depend upon (a) the amount of alteration in the speed of rotation, and (b) the extent of low land in low lat.i.tudes and of shallow sea in high lat.i.tudes. According to Slichter's tables, if the earth should rotate in twenty-three hours instead of twenty-four, the great Amazon lowland would be submerged by the inflow of oceanic water, while wide areas in Hudson Bay, the North Sea, and other northern regions, would become land because the ocean water would flow away from them.[76]
Following the prompt equatorward movement of water which would occur as the speed of rotation increased, there must also be a gradual movement or creepage of the solid rocks toward the equator, that is, a bulging of the ocean floor and of the lands in low lat.i.tudes, with a consequent emergence of the lands there and a relative rise of sea level in higher lat.i.tudes. Tidal r.e.t.a.r.dation would have a similar effect. Suess[77] has described widespread elevated strand lines in the tropics which he interprets as indicating a relatively sudden change in sea level, though he does not suggest a cause of the change. However, in speaking of recent geological times, Suess reports that a movement more recent than the old strands ”was an acc.u.mulation of water toward the equator, a diminution toward the poles, and (it appears) as though this last movement were only one of the many oscillations which succeed each other with the same tendency, i.e., with a positive excess at the equator, a negative excess at the poles.” (Vol. II, p. 551.) This creepage of the rocks equatorward seemingly might favor the growth of mountains in tropical and subtropical regions, because it is highly improbable that the increase in the bulge would go on in all longitudes with perfect uniformity. Where it went on most rapidly mountains would arise. That such irregularity of movement has actually occurred is suggested not only by the fact that many Cenozoic and older mountain ranges extend east and west, but by the further fact that these include some of our greatest ranges, many of which are in fairly low lat.i.tudes. The Himalayas, the Javanese ranges, and the half-submerged Caribbean chains are examples. Such mountains suggest a thrust in a north and south direction which is just what would happen if the solid ma.s.s of the earth were creeping first equatorward and then poleward.
A fact which is in accord with the idea of a periodic increase in the oceans in low lat.i.tudes because of renewed bulging at the equator is the exposure in moderately high lat.i.tudes of the greatest extent of ancient rocks. This seems to mean that in low lat.i.tudes the frequent deepening of the oceans has caused the old rocks to be largely covered by sediments, while the old lands in higher lat.i.tudes have been left more fully exposed to erosion.
Another suggestion of such periodic equatorward movements of the ocean water is found in the reported contrast between the relative stability with which the northern part of North America has remained slightly above sea level except at times of widespread submergence, while the southern parts have suffered repeated submergence alternating with great emergence.[78] Furthermore, although the northern part of North America has been generally exposed to erosion since the Proterozoic, it has supplied much less sediment than have the more southern land areas.[79]
This apparently means that much of Canada has stood relatively low, while repeated and profound uplift alternating with depression has occurred in subtropical lat.i.tudes, apparently in adjustment to changes in the earth's speed of rotation. The uplifts generally followed the times of submergence due to equatorward movement of the water, though the buckling of the crust which accompanies shrinkage doubtless caused some of the submergence. The evidence that northern North America stood relatively low throughout much of geological time depends not only on the fact that little sediment came to the south from the north, but also on the fact that at times of especially widespread epicontinental seas, the submergence was initiated at the north.[80] This is especially true for Ordovician, Silurian, Devonian, and Jura.s.sic times in North America.