Again, if the atmospheric pressure in the subtropical belt should be intensified, the winds flowing poleward from this belt would necessarily become stronger. These would begin as southwesterlies in the northern hemisphere and northwesterlies in the southern. In the preceding chapter we have seen that such winds, especially when cyclonic storms are few and mild, are a powerful agent in transferring subtropical heat poleward. If the strength of the westerlies were increased because of broad lands in low lat.i.tudes, their efficacy in transferring heat would be correspondingly augmented. It is thus evident that any change in the extent of tropical lands during the geologic past must have had important climatic consequences in changing the velocity of the atmospheric circulation and in altering the transfer of heat from low lat.i.tudes to high. When the equatorial and tropical lands were broad the winds and currents must have been strong, much heat must have been carried away from low lat.i.tudes, and the contrast between low and high lat.i.tudes must have been relatively slight. As we have already remarked, leading paleogeographers believe that changes in the extent of the lands have been especially marked in low lat.i.tudes, and that on the average there has been a decrease in the extent of land within the tropics.
Gondwana Land is the greatest ill.u.s.tration of this. In the same way, on the numerous paleogeographic maps of North America, most paleogeographers have shown fairly extensive lands south of the lat.i.tude of the United States during most of the geologic epochs.[88]
(c) There is evidence that during geologic history the area of the lands in middle and high lat.i.tudes, as well as in low lat.i.tudes, has changed radically. An increase in such lands would cause the winters to grow colder. This would be partly because of the loss of heat by radiation into the cold dry air over the continents in winter, and partly because of increased reflection from snow and frost, which gather much more widely upon the land than upon the ocean. Furthermore, in winter when the continents are relatively cold, there is a strong tendency for winds to blow out from the continent toward the ocean. The larger the land the stronger this tendency. In Asia it gives rise to strong winter monsoons.
The effect of such winds is ill.u.s.trated by the way in which the westerlies prevent the Gulf Stream from warming the eastern United States in winter. The Gulf Stream warms northwestern Europe much more than the United States because, in Europe, the prevailing winds are onsh.o.r.e.
Another effect of an increase in the area of the lands in middle and high lat.i.tudes would be to interpose barriers to oceanic circulation and thus lower the temperature of polar regions. This would not mean glaciation in high lat.i.tudes, however, even when the lands were widespread as in the Mesozoic and early Tertiary. Students of glaciology are more and more thoroughly convinced that glaciation depends on the availability of moisture even more than upon low temperature.
In conclusion it may be noted that each of the several climatic influences of increased land area in the high lat.i.tudes would tend to increase the contrasts between land and sea, between winter and summer, and between low lat.i.tudes and high. In other words, so far as the effect upon high lat.i.tudes themselves is concerned, an expansion of the lands there would tend in the same direction as a diminution in low lat.i.tudes.
In so far as the general trend of geological evolution has been toward more land in high lat.i.tudes and less in low, it would help to produce a progressive increase in climatic diversity such as is faintly indicated in the rock strata. On the other hand, the oscillations in the distribution of the lands, of which geology affords so much evidence, must certainly have played an important part in producing the periodic changes of climate which the earth has undergone.
III. Throughout geological history there is abundant evidence that the process of contraction has led to marked differences not only in the distribution and area of the lands, but in their height. On the whole the lands have presumably increased in height since the Proterozoic, somewhat in proportion to the increased differentiation of continents and oceans.[89] If there has been such an increase, the contrast between the climate of ocean and land must have been accentuated, for highlands have a greater diurnal and seasonal range of temperature than do lowlands. The ocean has very little range of either sort. The large range at high alt.i.tudes is due chiefly to the small quant.i.ty of water vapor, for this declines steadily with increased alt.i.tude. A diminution in the density of the other const.i.tuents of the air also decreases the blanketing effect of the atmosphere. In conformity with the great seasonal range in temperature at times when the lands stand high, the direction of the wind would be altered. When the lands are notably warmer than the oceans, the winds commonly flow from land to sea, and when the continents are much colder than the oceans, the direction is reversed. The monsoons of Asia are examples. Strong seasonal winds disturb the normal planetary circulation of the trade winds in low lat.i.tudes and of the westerlies in middle lat.i.tudes. They also interfere with the ocean currents set in motion by the planetary winds. The net result is to hinder the transfer of heat from low lat.i.tudes to high, and thus to increase the contrasts between the zones. Local as well as zonal contrasts are also intensified. The higher the land, the greater, relatively speaking, are the cloudiness and precipitation on seaward slopes, and the drier the interior. Indeed, most highlands are arid.
Henry"s[90] recent study of the vertical distribution of rainfall on mountain-sides indicates that a decrease sets in at about 3500 feet in the tropics and only a little higher in mid-lat.i.tudes.
In addition to the main effects upon atmospheric circulation and precipitation, each of the many upheavals of the lands must have been accompanied by many minor conditions which tended toward diversity. For example, the streams were rejuvenated, and instead of meandering perhaps over vast flood plains they intrenched their channels and in many cases dug deep gorges. The water table was lowered, soil was removed from considerable areas, the bare rock was exposed, and the type of dominant vegetation altered in many places. An almost barren ridge may represent all that remains of what was once a vast forested flood plain. Thus, increased elevation of the land produces contrasted conditions of slope, vegetation, availability of ground water, exposure to wind and so forth, and these unite in diversifying climate. Where mountains are formed, strong contrasts are sure to occur. The windward slopes may be very rainy, while neighboring leeward slopes are parched by a dry foehn wind.
At the same time the tops may be snow-covered. Increased local contrasts in climatic conditions are known to influence the intensity of cyclonic storms,[91] and these affect the climatic conditions of all middle and high lat.i.tudes, if not of the entire earth. The paths followed by cyclonic storms are also altered by increased contrast between land and water. When the continents are notably colder than the neighboring oceans, high atmospheric pressure develops on the lands and interferes with the pa.s.sage of lows, which are therefore either deflected around the continent or forced to move slowly.
The distribution of lofty mountains has an even more striking climatic effect than the general uplift of a region. In Proterozoic times there was a great range in the Lake Superior region; in the late Devonian the Acadian mountains of New England and the Maritime Provinces of Canada possibly attained a height equal to the present Rockies. Subsequently, in the late Paleozoic a significant range stood where the Ouachitas now are. Accompanying the uplift of each of these ranges, and all others, the climate of the surrounding area, especially to leeward, must have been altered greatly. Many extensive salt deposits found now in fairly humid regions, for example, the Pennsylvanian and Permian deposits of Kansas and Oklahoma, were probably laid down in times of local aridity due to the cutting off of moisture-bearing winds by the mountains of Llanoria in Louisiana and Texas. Hence such deposits do not necessarily indicate periods of widespread and profound aridity.
When the causes of ancient glaciation were first considered by geologists, about the middle of the nineteenth century, it was usually a.s.sumed that the glaciated areas had been elevated to great heights, and thus rendered cold enough to permit the acc.u.mulation of glaciers. The many glaciers occurring in the Alps of central Europe where glaciology arose doubtless suggested this explanation. However, it is now known that most of the ancient glaciation was not of the alpine type, and there is adequate proof that the glacial periods cannot be explained as due directly and solely to uplift. Nevertheless, upheavals of the lands are among the most important factors in controlling climate, and variations in the height of the lands have doubtless a.s.sisted in producing climate oscillations, especially those of long duration.
Moreover, the progressive increase in the height of the lands has presumably played a part in fostering local and zonal diversity in contrast with the relative uniformity of earlier geological times.
IV. The contraction of the earth has been accompanied by volcanic activity as well as by changes in the extent, distribution, and alt.i.tude of the lands. The probable part played by volcanic dust as a contributory factor in producing short sudden climatic variations has already been discussed. There is, however, another though probably less important respect in which volcanic activity may have had at least a slight climatic significance. The oldest known rocks, those of the Archean era, contain so much igneous matter that many students have a.s.sumed that they show that the entire earth was once liquid. It is now considered that they merely indicate igneous activity of great magnitude. In the later part of Proterozoic time, during the second quarter of the earth"s history according to Schuchert"s estimate, there were again vast outflowings of lava. In the Lake Superior district, for example, a thickness of more than a mile acc.u.mulated over a large area, and lavas are common in many areas where rocks of this age are known.
The next quarter of the earth"s history elapsed without any correspondingly great outflows so far as is known, though several lesser ones occurred. Toward the end of the last quarter, and hence quite recently from the geological standpoint, another period of outflows, perhaps as noteworthy as that of the Proterozoic, occurred in the Cretaceous and Tertiary.
The climatic effects of such extensive lava flows would be essentially as follows: In the first place so long as the lavas were hot they would set up a local system of convection with inflowing winds. This would interfere at least a little with the general winds of the area. Again, where the lava flowed out into water, or where rain fell upon hot lava, there would be rapid evaporation which would increase the rainfall. Then after the lava had cooled, it would still influence climate a trifle in so far as its color was notably darker or lighter than that of the average surface. Dark surfaces absorb solar heat and become relatively warm when the sun shines upon them. Dark objects likewise radiate heat more rapidly than light-colored objects. Hence they cool more rapidly at night, and in the winter. As most lavas are relatively dark they increase the average diurnal range of temperature. Hence even after they are cool they increase the climatic diversity of the land.
The amount of heat given to the atmosphere by an extensive lava flow, though large according to human standards, is small compared with the amount received from the sun by a like area, except during the first few weeks or months before the lava has formed a thick crust. Furthermore, probably only a small fraction of any large series of flows occurred in a given century or millennium. Moreover, even the largest lava flows covered an area of only a few hundredths of one per cent of the earth"s surface. Nevertheless, the conditions which modify climate are so complicated that it would be rash to state that this amount of additional heat has been of no climatic significance. Like the proverbial "straw that broke the camel"s back," the changes it would surely produce in local convection, atmospheric pressure, and the direction of the wind may have helped to shift the paths of storms and to produce other complications which were of appreciable climatic significance.
V. The last point which we shall consider in connection with the effect of the earth"s interior upon climate is internal heat. The heat given off by lavas is merely a small part of that which is emitted by the earth as a whole. In the earliest part of geological history enough heat may have escaped from the interior of the earth to exert a profound influence on the climate. Knowlton,[92] as we have seen, has recently built up an elaborate theory on this a.s.sumption. At present, however, accurate measurements show that the escape of heat is so slight that it has no appreciable influence except in a few volcanic areas. It is estimated to raise the average temperature of the earth"s surface less than 0.1C.[93]
In order to contribute enough heat to raise the surface temperature 1C., the temperature gradient from the interior of the earth to the surface would need to be ten times as great as now, for the rate of conduction varies directly with the gradient. If the gradient were ten times as great as now, the rocks at a depth of two and one-half miles would be so hot as to be almost liquid according to Barrell"s[94]
estimates. The thick strata of unmetamorphosed Paleozoic rocks indicate that such high temperatures have not prevailed at such slight depths since the Proterozoic. Furthermore, the fact that the climate was cold enough to permit glaciation early in the Proterozoic era and at from one to three other times before the opening of the Paleozoic suggests that the rate of escape of heat was not rapid even in the first half of the earth"s recorded history. Yet even if the general escape of heat has never been large since the beginning of the better-known part of geological history, it was presumably greater in early times than at present.
If there actually has been an appreciable decrease in the amount of heat given out by the earth"s interior, its effects would agree with the observed conditions of the geological record. It would help to explain the relative mildness of zonal, seasonal, and local contrasts of climate in early geological times, but it would not help to explain the long oscillations from era to era which appear to have been of much greater importance. Those oscillations, so far as we can yet judge, may have been due in part to solar changes, but in large measure they seem to be explained by variations in the extent, distribution, and alt.i.tude of the lands. Such variations appear to be the inevitable result of the earth"s contraction.
FOOTNOTES:
[Footnote 70: Chas. Schuchert: The Earth"s Changing Surface and Climate during Geologic Time; in Lull: The Evolution of the Earth and Its Inhabitants, 1918, p. 55.]
[Footnote 71: Quoted by J. Cornet: Cours de Geologie, 1920, p. 330.]
[Footnote 72: T. C. Chamberlin: The Order of Magnitude of the Shrinkage of the Earth; Jour. Geol., Vol. 28, 1920, pp. 1-17, 126-157.]
[Footnote 73: G. I. Taylor: Philosophical Transactions, A. 220, 1919, pp. 1-33; Monthly Notices Royal Astron. Soc., Jan., 1920, Vol. 80, p. 308.]
[Footnote 74: J. Jeffreys: Monthly Notices Royal Astron. Soc., Jan., 1920, Vol. 80, p. 309.]
[Footnote 75: E. W. Brown: personal communication.]
[Footnote 76: C. S. Slichter: The Rotational Period of a Heterogeneous Spheroid; in Contributions to the Fundamental Problems of Geology, by T. C. Chamberlin, _et al._, Carnegie Inst. of Wash., No. 107, 1909.]
[Footnote 77: E. Suess: The Face of the Earth, Vol. II, p. 553, 1901.]
[Footnote 78: Chas. Schuchert: The Earth"s Changing Surface and Climate; in Lull: The Evolution of the Earth and Its Inhabitants, 1918, p. 78.]
[Footnote 79: J. Barren: Rhythms and the Measurement of Geologic Time; Bull. Geol. Soc. Am., Vol. 28, 1917, p. 838.]
[Footnote 80: Chas. Schuchert: _loc. cit._, p. 78.]
[Footnote 81: T. C. Chamberlin: Diastrophism, the Ultimate Basis of Correlation; Jour. Geol., Vol. 16, 1909; Chas. Schuchert: _loc. cit._]
[Footnote 82: Pirsson-Schuchert: Textbook of Geology, 1915, Vol. II, p.
982; Chas. Schuchert: Paleogeography of North America; Bull. Geol. Soc.
Am., Vol. 20, pp. 427-606; reference on p. 499.]
[Footnote 83: The general subject of the climatic significance of continentality is discussed by C. E. P. Brooks: continentality and Temperature; Quart. Jour. Royal Meteorol. Soc., April, 1917, and Oct., 1918.]
[Footnote 84: Chas. Schuchert: Climates of Geologic Time; in The Climatic Factor; Carnegie Inst.i.tution, 1914, p. 286.]
[Footnote 85: A. de Lapparent: Traite de Geologie, 1906.]
[Footnote 86: Chas. Schuchert: Historical Geology, 1915, p. 464.]
[Footnote 87: M. M. Metcalf: Upon an important method of studying problems of relationship and of geographical distribution; Proceedings National Academy of Sciences, Vol. 6, July, 1920, pp. 432-433.]
[Footnote 88: Chas. Schuchert: Paleogeography of North America; Bull.
Geol. Soc. Am., Vol. 20, 1910; and Willis, Salisbury, and others: Outlines of Geologic History, 1910.]
[Footnote 89: Chas. Schuchert: The Earth"s Changing Surface and Climate; in Lull: The Evolution of the Earth and Its Inhabitants, 1918, p. 50.]
[Footnote 90: A. J. Henry: The Decrease of Precipitation with Alt.i.tude; Monthly Weather Review, Vol. 47, 1919, pp. 33-41.]
[Footnote 91: Chas. F. Brooks: Monthly Weather Review, Vol. 46, 1918, p.
511; and also A. J. Henry and others: Weather Forecasting in the United States, 1913.]
[Footnote 92: F. H. Knowlton: Evolution of Geologic Climates; Bull.
Geol. Soc. Am., Vol. 30, Dec., 1919, pp. 499-566.]
[Footnote 93: Talbert, quoted by I. Bowman: Forest Physiography, 1911, p. 63.]
[Footnote 94: J. Barrell: Rhythms and the Measurement of Geologic Time; Bull. Geol. Soc. Am., Vol. 28, 1917, pp. 745-904.]
CHAPTER XII
POST-GLACIAL CRUSTAL MOVEMENTS AND CLIMATIC CHANGES