The Coming Ice Age

by C. A. M. Taber

At this date the observant navigators who have visited the antarctic seas report that the surface currents above the latitude of Cape Horn, while being drifted eastward by the prevailing westerly winds, also set toward the antarctic ice cliffs, as shown on [map No. 2].

The reason why this southerly set of the surface currents becomes noticeable above the latitude of 55° south is because the tropical currents which set southward from the torrid latitudes on the western sides of the Atlantic, Indian, and Pacific Oceans, although largely turned away from the high latitudes by the westerly winds and drift currents, are also able to send sufficient water into the great belt of westerly winds to furnish water for the deep under-currents setting northward from the antarctic shores. Thus the surface waters moving from the north in order to gain the higher latitudes, after entering the westerly wind-belt, are moved in drift currents by the impelling winds easterly over many degrees of longitude, and also at the same time slowly southward among the cooling icebergs, because of the attraction caused by the difference of temperature and density between the northern drift waters and the icy seas of the antarctic ice barrier. Consequently, the gradual movement of the surface waters of the westerly wind-belt southward before entering the higher latitudes is not generally apparent; for it is after they enter latitudes where the globe becomes much reduced in circumference that their southern movement in the contracted seas becomes more noticeable. The impact of this southerly current, which finds its outlet in deep under-currents, and retards somewhat the increase of ice on the southern continent at this date, also largely prevents the small icebergs and field-ice from floating northward, away from the antarctic ice barrier; for it is such large icebergs as penetrate the deep under-currents that are the best able to move into the more temperate latitudes.

From the above explanations it will be seen that the impact of surface water against the antarctic ice barrier when the Cape Horn channel was closed would greatly assist the tropical waters attracted to the great low sea-level to the leeward of the obstructed strait to wash the antarctic shores while being drifted eastward by the westerly winds over the southern ocean against the Patagonian coast and the Pacific side of the closed channel, and there causing a high sea-level. This movement of the winds and currents encircling the antarctic continent is shown on [map No. 1].

The vast, high sea-level caused by the westerly winds drifting the surface waters against the Patagonian coast would obtain a much higher plain, were it not that so much of the water of the great drift current was required to feed the antarctic under-current which constantly sets northward from the antarctic shores; yet it would be sufficient to greatly increase the volume of the Humboldt current, which would flow in the same direction it now flows, down the South American coast to the equatorial latitudes, where it would become the main source of the great equatorial stream, and thus offset the increased southward flow of the equatorial waters through the Brazil and Mozambique streams.

The equatorial stream, with its increased volume, would also move, as it moves to-day, across the Pacific; and, on gaining the western side, after sending off large streams to the northern and southern latitudes, it would pass through the East India passages into the Indian Ocean, where it would be drifted westward by the trade winds and cause a high sea-level abreast the east coast of Africa, and so become the source of the great Mozambique current, which would flow southward along the east coast of Africa, and, with the Cape Horn channel closed, would gain a much higher latitude than it would with the channel open. At this age, when the continuation of this great equatorial stream gains the latitude of the Cape of Good Hope, its waters are largely turned eastward by the great drift current of the southern ocean.

Still, a considerable portion of its waters turns toward the west, forming the Agulhas current, which flows around the Cape of Good Hope into the Atlantic, where it mingles with the cooler currents which branch off from the great southern drift current; and so, in connection with the latter, it is attracted to the low sea-level caused by the south-east trade winds abreast the south-western coast of Africa, and from thence moved as a drift current by the trade winds to the equatorial Atlantic and coast of Brazil. Thus it will be seen that the Agulhas current, even with the Cape Horn channel in possession of its present wide capacity, serves to retard somewhat the advance of a cold period.

The Agulhas current at this date also partly serves to replenish the water which is forced from the South Atlantic by strong westerly winds into the Southern Indian and Southern Pacific Oceans. For it appears that more water is now removed by such winds from the South Atlantic than enters it from the South Pacific, even through the enlarged Cape Horn channel of this date; and this fact seems to favor an impression that a portion of this enlarged channel existed prior to the glacial periods, but with its waters so much reduced as to be unable to give the southern ocean an independent circulation sufficient to exclude the tropical currents from reaching the high southern latitudes in adequate volume to maintain a mild climate in the southern hemisphere.

For previous to the glacial age, with little or no ice gathered on the antarctic lands, it seems that a strait possessing one-half the capacity of the Cape Horn channel of the present age could not prevent the Brazil current and the Agulhas stream from flowing into the southern ocean in quantities sufficient to make it impossible for glaciers to form on southern lands.

Thus it is probable that a reduced channel separated the western continent from the antarctic lands even in the mild eras previous to the glacial epochs.

The Cape Horn channel, at the present age, with a capacity sufficient to largely maintain an independent circulation for the southern ocean, is still only one-third of the breadth of the westerly wind-belt of the southern seas. Therefore, the drift currents do not all pass through it from the Pacific into the Atlantic. Consequently, a considerable portion of the drifted water turns northward west of Cape Horn, and so forms the Humboldt current.