Climatic Changes: Their Nature and Causes

by Ellsworth Huntington; Stephen Sargent Visher

For all these reasons, so long as storminess remained great, the Pleistocene snowfields, according to the solar hypothesis, must have deepened and expanded. In due

time some of the snow was converted into glacial ice. When that occurred, the growth of the snowfield as well as of the ice cap must have been accelerated by glacial movement. Under such circumstances, as the ice crowded southward toward the source of the moisture by which it grew, the area of high pressure produced by its low temperature would expand. This would force the storm track southward in spite of the contrary tendency due to the sun. When the ice sheet had become very extensive, the track would be crowded relatively near to the northern margin of the trade-wind belt. Indeed, the Pleistocene ice sheets, at the time of their maximum extension, reached almost as far south as the latitude now marking the northern limit of the trade-wind belt in summer. As the storm track with its frequent low pressure and the subtropical belt with its high pressure were forced nearer and nearer together, the barometric gradient between the two presumably became greater, winds became stronger, and the storms more intense.

This zonal crowding would be of special importance in summer, at which time it would also be most pronounced. In the first place, the storms would be crowded far upon the ice cap which would then be protected from the sun by a cover of fog and cloud more fully than at any other season. Furthermore, the close approach of the trade-wind belt to the storm belt would result in a great increase in the amount of moisture drawn from the belt of evaporation which the trade winds dominate. In the trade-wind belt, clear skies and high temperature make evaporation especially rapid. Indeed, in spite of the vast deserts it is probable that more than three-fourths of the total evaporation now taking place on the earth occurs in the belt of trades, an area which includes about one-half of the earth's surface.

The agency which could produce this increased drawing northward of moisture from the trade-wind belt would be the winds blowing into the lows. According to the cyclonic hypothesis, many of these lows would be so strong that they would temporarily break down the subtropical belt of high pressure which now usually prevails between the trades and the zone of westerly winds. This belt is even now often broken by tropical cyclones. If the storms of more northerly regions temporarily destroyed the subtropical high-pressure belt, even though they still remained on its northern side, they would divert part of the trade winds. Hence the air which now is carried obliquely equatorward by those winds would be carried spirally northward into the cyclonic lows. Precipitation in the storm track on the margin of the relatively cold ice sheet would thus be much increased, for most winds from low latitudes carry abundant moisture. Such a diversion of moisture from low latitudes probably explains the deficiency of precipitation along the heat equator at times of solar activity, as shown in Fig. 3. Taken as a whole, the summer conditions, according to the cyclonic hypothesis, would be such that increased evaporation in low latitudes would coöperate with increased storminess, cloudiness, and fog in higher latitudes to preserve and increase the accumulation of ice upon the borders of the ice sheet. The greater the storminess, the more this would be true and the more the ice sheet would be able to hold its own against melting in summer. Such a combination of precipitation and of protection from the sun is especially important if an ice sheet is to grow.

The meteorologist needs no geologic evidence that the storm track was shoved equatorward by the growth of the ice sheet, for he observes a similar shifting whenever a winter's snow cap occupies part of the normal storm

tract. The geologist, however, may welcome geologic evidence that such an extreme shift of the storm track actually occurred during the Pleistocene. Harmer, in 1901, first pointed out the evidence which was repeated with approval by Wright of the Ireland Geological Survey in 1914.^[44] According to these authorities, numerous boulders of a distinctive chalk were deposited by Pleistocene icebergs along the coast of Ireland. Their distribution shows that at the time of maximum glaciation the strong winds along the south coast of Ireland were from the northeast while today they are from the southwest. Such a reversal could apparently be produced only by a southward shift of the center of the main storm track from its present position in northern Ireland, Scotland, and Norway to a position across northern France, central Germany, and middle Russia. This would mean that while now the centers of the lows commonly move northeastward a short distance north of southern Ireland, they formerly moved eastward a short distance south of Ireland. It will be recalled that in the northern hemisphere the winds spiral into a low counter-clockwise and that they are strongest near the center. When the centers pass not far north of a given point, the strong winds therefore blow from the west or southwest, while when the centers pass just south of that point, the strong winds come from the east or northeast.

In addition to the consequences of the crowding of the storm track toward the trade-wind belt, several other conditions presumably operated to favor the growth of the ice sheet. For example, the lowering of the sea level by the removal of water to form the snowfields and glaciers interfered with warm currents. It also increased the rate of erosion, for it was equivalent to an uplift of

all the land. One consequence of erosion and weathering was presumably a diminution of the carbon dioxide in the atmosphere, for although the ice covered perhaps a tenth of the lands and interfered with carbonation to that extent, the removal of large quantities of soil by accelerated erosion on the other nine-tenths perhaps more than counterbalanced the protective effect of the ice. At the same time, the general lowering of the temperature of the ocean as well as the lands increased the ocean's capacity for carbon dioxide and thus facilitated absorption. At a temperature of 50°F. water absorbs 32 per cent more carbon dioxide than at 68°. The high waves produced by the severe storms must have had a similar effect on a small scale. Thus the percentage of carbon dioxide in the atmosphere was presumably diminished. Of less significance than these changes in the lands and the air, but perhaps not negligible, was the increased salinity of the ocean which accompanied the removal of water to form snow, and the increase of the dissolved mineral load of the rejuvenated streams. Increased salinity slows up the deep-sea circulation, as we shall see in a later chapter. This increases the contrasts from zone to zone.

At times of great solar activity the agencies mentioned above would apparently coöperate to cause an advance of ice sheets into lower latitudes. The degree of solar activity would have much to do with the final extent of the ice sheets. Nevertheless, certain terrestrial conditions would tend to set limits beyond which the ice would not greatly advance unless the storminess were extraordinarily severe. The most obvious of these conditions is the location of oceans and of deserts or semi-arid regions. The southwestward advance of the European ice sheet and the southeastward advance of the Labradorean sheet in America were stopped by the Atlantic. The semi-aridity

of the Great Plains, produced by their position in the lee of the Rocky Mountains, stopped the advance of the Keewatin ice sheet toward the southwest. The advance of the European ice sheet southeast seems to have been stopped for similar reasons. The cessation of the advance would be brought about in such an area not alone by the light precipitation and abundant sunshine, but by the dryness of the air, and also by the power of dust to absorb the sun's heat. Much dust would presumably be drawn in from the dry regions by passing cyclonic storms and would be scattered over the ice.