Reconstructing Earth's past climate is a scientific puzzle whose pieces are slowly dropping into place. The latest piece whose fit has been confirmed is the role of the Antarctic ice sheet in global glaciation. Two assessments published in the journal Nature earlier this year support an idea proposed in 1964 by a New Zealand scientist, Alex T. Wilson.He suggested that rapid expansion of, or a surge in, the Antarctic ice sheet could trigger an ice age.
Dr. Wilson's theory, simply stated, is that the ice sheet is basically unstable. During interglacial periods, the ice builds up on the polar continent. At a certain point, however, it begins to flow rapidly over the adjoining ocean to form "a great ice shelf" as much as 10 million square miles in extent. This new area of ice would act as a giant radiator-reflector -- reflecting more of the sunshine while efficiently radiating infrared (heat) energy into space. Thus it would act to cool the entire globe. This temperature drop would be enough to start the glaciers in North America and Europe growing, Dr. wilson suggested.
More recently, the theory has been refined. The general opinion now is that, rather than a giant ice shelf, the ice surging onto the sea would break up into a flotilla of icebergs. Also, proponents of the Wilson theory have realized that the Antarctic consists of five glacial basins. Each basin, in all probability, acts independently of the others. So the idea of the entire ice sheet thinning and spreading rapidly at the same time is oversimplified. Instead, it now is thought that each basin surges with its own rhythm. At certain times, two or more basins would surge more or less simultaneously.
Shortly after Dr. Wilson put forward his theory, John T. Hollin -- now with the Institute of Arctic and Alpine Research here -- pointed out that, if such a tremendous quantity of ice surged off the land over the ocean, this would cause a world-wide increase in sea level of as much as 45 feet -- a rise that should be revealed by a re-establishment of ancient coastlines.
Now "the required geological test for Wilson's theory appears fulfilled," comments D. Q. Bowen of university College of Wales. The support comes from the work of Dr. Hollin and a group of Australian scientists working in Papua New Guinea. The Huon Peninsula in New Guinea is an area of geologic uplift. As a result, past shorelines now are clearly visible on land.
Paul Aharon, John Chappel, and William Compston of the Australian National University studied these ancient shorelines and found evidence that a sea-level rise of 25 feet occurred 120,000 years ago, just before the onset of the last ice age. From analysis of the shells of giant clams living at the time, the scientists also conclude that the water was 3 degrees cooler. That would have been a temperature drop even greater than would be expected from the effect of melting Antarctic ice alone.
". . . we conclude that an Antarctic ice surge seems likely to have initiated the last glaciation," the scientists state.
Dr. Hollin, on the other hand, has drawn from a number of sources to support his contention of an even greater surge 95,000 years ago, at the beginning of a "stadial," or colder, period within the last ice age.
Various types of geological evidence from Europe, Africa, Australia, and North America indicate the occurrence of a sudden 52-foot rise in sea level at that time.
Periodic ice surges now are well recognized in glaciously. Ice floes that normally move at an imperceptible rate suddenly begin surgin as much as a mile a day for a brief period. The largest surge that has been seen occurred in Spitzbergen. A 20-mile front of ice pushed 20 miles out into the Barents Sea in a matter of months.
Surging appears to depend more on the depth of the ice sheet than directly on the climate. Once ice covers an area to sufficient depth, geothermal heat begins melting the bottom of the glacier. This water acts as a lubricant and, after a period of time ranging from a decade to a century, the glacier surges.
"The process is relatively independent of changes in the climate," Dr. Hollin says.
In 1976, a group of scientists demonstrated convincingly that, for the past 500,000 years, Earth's climate has varied in tune with changes in the amount of solar radiation the planet receives. These isolation changes are due to variations in Earth's orbit and the tilt of its axis. They were first suggested as a cause for climatic change by a Yugoslav geophysicist, Milutin Milankovitch.
The variations are slight, however. The question of how they could trigger the major climate change between glacial and interglacial periods remained open. Now it seems that the best candidate for such a "climatic amplier" is the Antarctic ice sheet.
As suggested by Professor Bowen, "Perhaps major periods of continental glaciation only took place when a combination of Antarctic surge and favorable Milankovitch conditions arose."
According to this theory, when a major Antarctic surge took place during a Milankovitch cold period, an ice age resulted. There is evidence for abrupt temperature drops during interglacial periods which cannot be explained by orbital variations. These could be the result of surges during Milankovitch warm periods.
"Surges initiate a period of glaciation, and Milankovitch factors may control the outcome," Dr. Hollin says.
There remain a number of loose ends; but at least the broad outline of a plausible theory for the start of an ice age is emerging.