Man and the Glacial Period

by G. Frederick Wright

Fig. 100.—Map showing course of currents in the Atlantic Ocean: b and b′ are currents set in motion by opposite trade-winds; meeting, they produce the equatorial current, which divides into c and c′, continuing on as a and a′ and e.

But why are the southeast trade-winds of the Atlantic stronger than the northeast? The ultimate reason, of course, is to be found in the fact that the northern hemisphere is warmer than the southern. The atmosphere over the northern-central portion of the Atlantic region is more thoroughly rarefied by the sun’s heat than is that over the region south of the equator. The strong southeast trades are simply the rush of atmosphere from the South Atlantic to fill the vacuum caused by the heat of the sun north of the equator.

But, again, why is this? Because, says Mr. Croll, we are now in that stage of astronomical development favourable to the increased warmth of the northern hemisphere. In the northern hemisphere the summers are longer than the winters. Perihelion occurs in winter and aphelion in summer. This is the reason why the North Atlantic is warmer than the South Atlantic, and why the trade-winds of the south are drawn to the north of the equator. Ten thousand five hundred years ago, however, the conditions were reversed, and the greater rarefaction of the atmosphere would have taken place south of the equator, thus drawing the trade-winds in that direction.

By again inspecting the map, one will see how far-reaching the effect on the climate of northern countries this change in the prevalences of the trades would have been. Then, instead of having the northwest current leading along the northeast coast of South America into the Gulf of Mexico augmented by the warm currents circulating south of the equator, the warm currents of the north would have been pushed down so far that they would augment the current running to the southwest beyond Cape St. Roque, along the southeast shore of South America; thus the northern portion of the Atlantic, instead of robbing the southern portion of heat, would itself be robbed of its warm currents to contribute to the superfluous heat of the South Atlantic.

This theory is certainly very ingenious. There is a weak point in it, however. Mr. Croll assumes that when the winters of the northern hemisphere occur in aphelion, they must necessarily be colder than now. But, evidently, this assertion implies a fuller knowledge than we possess of the laws by which the heat received from the sun is distributed over the earth.

For it appears from observation that the equator is by no means so hot now as, theoretically, it ought to be, and that the arctic regions are not so cold as, according to theory, they should be, and this in places which could not be affected by oceanic currents. For example, at Iquitos, on the Amazon, only three hundred feet above tide, three degrees and a half south of the equator, and more than a thousand miles from the Atlantic (so that ocean-currents cannot abstract the heat from its vicinity), the mean yearly temperature is but 78° Fahr.; while at Verkhojansk, in northeast Siberia, which is 67° north of the equator, and is situated where it is out of the reach of ocean-currents, and where the conditions for the radiation of heat are most favourable, and where, indeed, the winter is the coldest on the globe (January averaging—56° Fahr.), the mean yearly temperature is two degrees and a half above zero; so that the difference between the temperature upon the equator and that at the coldest point on the sixty-seventh parallel is only about 75° Fahr.; whereas, if temperature were in proportion to heat received from the sun, the difference ought to be 172°. Again, the difference between the actual January temperature on the fiftieth parallel and that upon the sixtieth is but 20° Fahr., whereas, the quantity of solar heat received on the fiftieth parallel during the month of January is three times that received upon the sixtieth, and the difference in temperature ought to be about 170° Fahr. upon any known law in the case.

Woeikoff, a Russian meteorologist, and one of the ablest critics of Mr. Croll’s theory, and to whom we are indebted for these facts, ascribes the greater present warmth of the northern Atlantic basin, not to the astronomical cause invoked by Mr. Croll, but to the relatively small extent of sea in the middle latitudes of the northern hemisphere. The extent and depth of the oceans of the southern hemisphere would of themselves give greater steadiness and force to its trade-winds, and lead to a general lowering of the temperature; so that it is doubtful if the astronomical causes introduced by Mr. Croll, even with Dr. Ball’s re-enforcement, would produce any appreciable effect while the distribution of land and water remains substantially what it is at the present time.

Still another variation in the astronomical theory has been set forth and defended by Major-General A. W. Drayson, F. R. A. S., instructor in the Royal Military School at Woolwich, England. He contends that what has been called the precession of the equinoxes, and supposed to be “a conical movement of the earth’s axis in a circle around a point as a centre, from which it continually decreases its distance,”[DW] is really a second rotation of the earth about its centre. As a consequence of this second rotation, he endeavours to show that the inclination of the earth’s axis varies as much as 12°; so that, whereas the Arctic and Antarctic Circles and the tropics extend to only about 23° from the poles and the equator, respectively, about thirteen thousand five hundred years ago they extended more than 35°; thus bringing the frigid zones in both cases 12° nearer the equator than now. This, he contends, would have produced the Glacial period at the time now more generally assigned to it by direct geological evidence.

[DW] Untrodden Ground in Astronomy and Geology, p. 26.

The difficulty with this theory, even if the mathematical calculations upon which it is based are correct, would be substantially the same as those already urged against that of Mr. Croll. It is specially difficult to see how General Drayson would account for the prolonged temperate climate in high northern latitudes during the larger part of the Tertiary epoch.