- Historic Sites
Footprints Of The Great Ice
The glacier that covered most of North America scarred the land, turned rivers in their courses, and deeply influenced our history
April 1960 | Volume 11, Issue 3
Since the Northern hemisphere has had four separate glaciations in recent geological times, the inevitable question is whether we can expect a fifth. The answer appears to be in the affirmative.
Until recently, attempts to explain the recurring times of ice have assumed a temporary cooling of the earth’s climate for one reason or another. None of these theories has quite explained all the facts. Within the past several years, however, a new theory has been advanced and has gained wide acceptance. Its authors are Maurice Ewing, a geophysicist at Columbia University, and William Bonn, a geologist-meteorologist; their argument is that glacial periods result from periodic warming of the Arctic Ocean.
The Arctic Ocean, they point out, is a sea almost separate from the other waters of the earth. Its connection with the Pacific at the Bering Strait is so narrow and shallow it allows no significant interchange of waters. The passage between the Arctic and the North Atlantic is broader, but across its bottom, between Norway and Greenland, extends a shallow sill, less than three hundred feet deep in most places.
The currents flowing across the sill bring warm Atlantic water into the polar sea, and although the net gain each year is tiny, over thousands of years it is enough to make the Arctic Ocean very much warmer. As a result, the ice on the water becomes thinner, patches of open water grow larger, and eventually the time comes when the ocean is completely free of ice.
A polar sea without ice opens a new stage in the glacial cycle. The warm, open water gives off a great deal of water vapor by evaporation; the moisture is swept south and overland by the winds where it cools off and falls as rain or snow. The open Arctic is such a prolific producer of precipitation that the increased winter snowfall amounts to more than the oblique rays of the sun can melt away during the short northern summer. The snow accumulates and packs into ice until, after tens of thousands of years, the ice cap has become two miles or so thick. The day comes when the tremendous pressure begins to push the ice outward and another continental glacier is on its way.
But even as they form, icecaps carry the mechanism for their own destruction—according to the Ewing-Donn theory. With so much moisture becoming locked up in ice, ocean levels slowly drop until the Bering Strait becomes dry land and the flow over the Norway-Greenland sill becomes greatly restricted, so that not enough warm water can flow into the polar ocean to keep it from freezing. The Arctic Ocean cools off and finally freezes over again. Since very little moisture evaporates from ice, the snowfalls become much lighter—and far to the south the glacier front comes to a stop.
Once again the situation is changed. With snowfall reduced, the glacier is not replenished at the old rate, and so begins to shrink. Water from the melting ice makes the oceans rise, only a fraction of an inch a year but, in the fullness of time, enough to let the currents increase their flow over the northern sill, bringing ever more warm water into the gelid Arctic.
In this year 1960 A.D., we are (still according to the Ewing-Donn theory) at the point in the cycle where the Arctic Ocean is almost ready to shed its old, old, covering of ice. The north-polar ice is going very fast; it covers twelve per cent less area and is forty per cent thinner than it was only fifteen years ago. At that rate it could disappear completely in less than a human lifetime, although we cannot say whether this is a normal climate fluctuation or a steady trend.
But even making allowances, the beginning of the next glacial era might still be breathing down our necks. Eventually the ice will pile up high enough to begin pushing outward from its own weight and will make its slow but inevitable way into the United States again. How long will it be? Much time and study and additional evidence will be necessary before even an informed guess can be made. But we do know one thing: the beginnings will be completely undramatic. Winter snowfalls will increase, so that at the end of the summer some snow will still linger in the tundra lands of northern Canada, probably nothing more than a few patches of slush in the shade of rocks and hummocks of moss. The next autumn the bits of slush will be just a little larger and deeper when the freeze-up comes.
Only a few Eskimos and scientists will notice.