Protective Ice Flow in Antarctica Linked to Warmth of Volcanoes
By WILLIAM K. STEVENS
Published: February 16, 1993
NOT least among the possible catastrophes that could result from a warming of the earth's climate, some scientists say, is a collapse of the vast West Antarctic Ice Sheet, an event that would cause the global sea level to rise by some 20 feet. Now investigators have found evidence of a wholly different factor in the ice sheet's stability that could be as important as climate: volcanism.
Huge rivers of ice flowing toward the sea from a main ice reservoir in the continent's interior keep the ocean from coming into direct contact with the domed reservoir, scientists from several institutions say in the current issue of the British journal Nature. If the relatively warm ocean waters were to lap at the interior reservoir, the ice sheet, which rests on bedrock that is below sea level, would begin to disintegrate and would eventually collapse, releasing great amounts of pent-up water to the ocean.
The scientists report that they have identified what they believe is the mechanism that produces the ice streams, and they suggest some ways in which the mechanism could be undermined with disastrous results for coastal areas worldwide.
Using a variety of instruments, including a laser altimeter and a radar altimeter, the scientists surveyed the ice sheet from the air to determine the contours of both the ice sheet and the rocky base underneath, along with the magnetic and gravity fields associated with the rock base. From the resulting data, collected in the Southern Hemisphere summer of 1991-92, they concluded that an active volcano underlies one part of the area where the ice streams form. Combining this with satellite data that suggest similar and more widespread geologic forms, they suggest that other volcanoes underlie the ice as well.
The volcanoes, the investigators believe, indicate that the ground in that region produces warmth that melts just enough ice to produce a slippery layer of mud over which ice migrating toward the sea can move at a rapid pace of about half a mile a year. Each of the rivers formed this way is about 30 to 60 miles wide and flows for hundreds of miles before reaching the seashore. There it continues flowing out into the ocean to produce the Ross Ice Shelf, a huge offshore expanse of ice that forms the rampart of the barrier separating the ocean from the central ice reservoir.
The ice streams could vanish, said one of the scientists, Dr. Donald D. Blankenship of the University of Texas at Austin, if climatic changes should cause the sea level to rise enough for the ocean shore to move inland. This could happen if sea water expanded and its level rose as a result of global warming, for example, or if any change in oceanic circulation patterns resulting from climatic change produced warmer ocean waters around Antarctica.
If the shoreline moved inland beyond the area of volcanism where the ice streams form, said Dr. Blankenship, the ice streams would disappear and would no longer feed the Ross Ice Shelf. It would begin to shrink, setting off a retreat of the ice barrier that in time would allow the ocean to come into direct contact with the interior ice reservoir. The reservoir would begin to disintegrate on contact with the warmer water and ultimately the entire ice sheet would collapse. How long this would take is unknown, he said, since "that situation doesn't exist in nature right now.
In addition to Dr. Blankenship, the authors of the paper are Dr. Robin E. Bell of the Lamont-Doherty Earth Observatory of Columbia University; Dr. Steven M. Hodge, Dr. John C. Behrendt and Dr. Carol A. Finn of the United States Geological Survey, and John M. Brozena of the Naval Research Laboratory in Washington.
Diagram: "Natural Flow of Icy Rivers" shows how volcanoes may disrupt the ice flow and lead to a disastrous rise in sea levels. (Source: Nature)
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