Cause and Development of an Ice Age

Abstract

Late Cenozoic glaciation as a whole can be attributed to the present near-polar position of North America. The continent had undergone no extensive previous glaciation for about years, nor had it been located near a pole during that interval. The specific onset of Pleistocene glaciation can, perhaps, be assigned to the time of a hypothetical (but geologically probable) tectonic event, in the Bering Strait area, which reduced influx of warm Pacific water into the Arctic basin, thereby converting the Arctic Ocean into a large "continental lake." This history has been greatly favored by the polar position of Antarctica. Glacial oscillation on the continental scale is thought to be due to a meteorological phenomenon: an ice sheet migrates toward its source, and hence (in North America) essentially toward the south. As such an icecap rim migrates, it grows higher, until ultimately it starves its own starting or "kernel" area, while the southern ridge has extended itself into latitudes which are too warm and altitudes where moisture is insufficient to maintain an icecap. The meteorological model, taken by itself, would not permit long interglacials. Isostasy, which moves the land surface up and down (in response to loading and unloading) over a vertical range of perhaps as much as 1.0 km., provides the necessary time lag. The meteorological model can operate effectively only on "wide" continents such as North America and Europe. Greenland and Antarctic ice sheets are, therefore, safe from the dangers of migrating into warmer areas, and hence will not melt until over-all glacial conditions have been terminated. Field evidence suggests that complete melting did not take place during any of the Pleistocene interglacials. The combined model can be used to predict that sea level willl not return, in the near future, to its "normal" position, but rather that, in an interval on the order of years, it will begin to drop again as the earth enters a new cold epoch. The precise timing will depend on the rate at which isostatic rebound re-elevates pertinent parts of North America and Europe to the critical position, and perhaps also on the timing of an ''extreme value" sequence of heavy-snowfall years. In the light of the rate at which North America has approached its polar position over the last years, the over-all history of Pleistocene glaciation can be expected to last for approximately years, interrupted by various interglacials as the ice sheets starve themselves.