Two geologists, Maurice Ewing and William L. Donn, have accounted for the beginning of the Ice Age by accepting the theory that the north and south poles had moved from the north Pacific and the south Atlantic to their present positions. They account rather ingeniously for the advance and the retreat of the four glaciations. With the north pole where it is now, the ice-free Arctic Ocean would supply moisture by evaporation. This moisture, owing to cold over the northern areas of Asia and North America, would fall as snow to nourish glaciers. But how to stop this process and reverse it? It happens that the sea floor forms a rather high sill between the Atlantic and the Arctic Oceans. When the sea level dropped, as its water piled up in the great glaciers, the sill came too close to the surface to allow much of the warmer water from the Atlantic to reach the Arctic Ocean and to keep it from freezing over. Once the ice pack formed, evaporation diminished abruptly. The glaciers lost their nourishment. The summer melt returned their waters to the ocean. Sea level rose. The currents from the Atlantic could flow over the sill again and melt the ice pack. Then conditions would be ripe for a second advance of snow and ice across the northern world. The shallow waters of Bering Strait probably had little effect upon the Arctic Ocean.
This theory has been criticized adversely by various authorities, despite the geologic, oceanographic, and meteorological evidence that Ewing and Donn have brought to bear upon each step of their reasoning. Their theory is particularly attractive to the archaeologist; it requires an ice-free Arctic coast when the land-bridge at Bering Strait would have been available to early man. Climatic conditions at that time would have been severe along the land-bridge and coastline, but not impossible for the survival of early man.[12]
Another explanation is a general decrease in solar energy; Zeuner holds this in reserve for lack of evidence. But some present-day geologists seize on the possibility that the heat of the sun may have changed from time to time, and use the theory in a curious, almost paradoxical way. The author of this hypothesis, Sir George C. Simpson, believes that the great masses of ice resulted from an initial increase instead of a decrease in temperature.[13] As the weather grew slightly warmer, cloudiness and rain and snow increased, the snow-line fell, and glaciation resulted. As the weather grew still warmer, the ice melted. (Simpson demonstrated the basic principle of this through an ingenious laboratory experiment.) His glacial theory, which is explained in more detail on the opposite page, postulates two increases in solar energy, and draws from them a meteorological pattern that provides the four glacial and three interglacial periods. The first and last interglacials would be warm and wet, the second cold and dry. Zeuner objects to this theory on the ground that the last interglacial—which, according to Simpson, should have been warm and wet—was mainly cool and dry.[14] But, while it may have been cool and dry in the German area which Zeuner has most closely studied, other areas probably had other climates. Simpson’s hypothesis would account for the heavy rains, or pluvial periods, of Nilotic Africa, which may link up with the glaciations of Europe; but he provides only two pluvials, and there are evidences of three or more in Africa.
GLACIATION THROUGH WARMTH
A somewhat modified graph of Simpson’s theory of the cause of the Great Ice Age. The following summary by Carl Sauer includes quotations from Simpson: [First] “Increased solar radiation received by the earth leads to increase in the general circulation of the atmosphere, which forms a great cloud blanket and causes increased precipitation in appropriate areas. In particular, in high latitudes and altitudes there is increased snowfall or glaciers. [Second] ‘As the radiation increases still further, the ice melts away and we have overcast skies and much precipitation but no ice accumulation.’ [Third] ‘When the solar radiation decreases, conditions are reversed and the whole sequence is gone through in reverse order.’” (After Simpson, 1938; quotation from Sauer, 1944.)
Zeuner has an explanation of why the periodic decrease in the heat from the sun produced glaciation during the last million years and not for 200,000,000 years before. He introduces the geological factor called Eustatism,[15] meaning by it simply a progressive drop in sea level. According to the hypothesis, this began before the Great Ice Age, and was caused by the sinking of very deep portions of the sea floor. As the sea level sank, the temperature of the mountains and plains dropped also, for the higher we rise above the surface of the ocean the cooler the air grows. The snowline fell, the mountain glaciers grew larger, and the snow and ice on the northern plains could not be completely melted by the reduced summer heat, and gradually grew deeper and more extensive. Thus the lowering of general temperature made it possible for the periodic decrease in solar radiation to cause the glaciations of the Great Ice Age. The theory of a general and steady lowering of the sea level is based on a series of four raised beaches occurring uniformly in many parts of the world. Other students believe these terraces were products of a regional rise of land.
Considering that the Great Ice Age ranges back at least 600,000 years—and probably 1,000,000, if we credit evidence of three earlier Danubian glaciations—it is small wonder that scientists are not entirely agreed on many factors in its story. “The difficulties are such,” says the French archaeologist A. Vayson de Pradenne, “that after fifty years of study to which the greatest geologists have devoted all their energies, there is no certainty yet as to the exact number of glaciations and the way in which the faunal changes are related to them.”[16]
Much more important, of course, than the cause of glaciation is its effect on early man. Ice covered 27 per cent of the earth’s surface during the Würm-Wisconsin period, according to Flint. This created the land-bridge over Bering Strait. It connected Santa Rosa Island with the coast of California. It broadened the Isthmus of Panama, so that man did not have to pass through a semi-mountainous jungle, which suggests that he came south during the Wisconsin glaciation. It seems to have been the ice that urged man to the south in the Americas and provided freeways.