Much of the West Antarctic Ice Sheet, unlike all the other big ice sheets in the world, lies on top of land that is below sea level, filling a large valley in Antarctica. And that makes it extraordinarily vulnerable to global warming, says Reed Scherer, a geologist at Uppsala University in Sweden. A warmer ocean would melt the vast floating islands of ice at the margins of the ice sheet, which might set in motion an inexorable rise in global sea levels.
If floating ice along the continental ice sheet's edges were to melt, says Scherer, rivers of ice flowing from Antarctica's interior would have unchecked access to the sea, increasing the amount of water entering the ocean. If the ice sheet were to melt completely--a process that could take as little as 500 years according to some models--global sea levels could rise by as much as 20 feet, inundating islands and coastal areas worldwide.
The debate over whether the ice sheet is at risk hinges partly on its past history. Scherer has found the first direct evidence that the ice sheet has collapsed before. In soil samples recovered from nearly two-thirds of a mile below the ice, he found fossils of tiny marine plants called diatoms. Some of the fossils were less than 650,000 years old, and Scherer says they were deposited the last time an open ocean, not ice, covered this part of Antarctica.
"Until now, there has been no direct evidence that it's happened before," says Scherer. Although he couldn't precisely determine the age of the diatoms, he suggests that they formed 400,000 years ago, a time when geologists believe that sea-surface temperatures were about 9 to 12 degrees warmer than today and sea levels possibly 65 feet higher. If he's right, the diatoms mark the ice sheet's last collapse.
"You are now in a much better position to develop global climate models and test them," says Scherer. "Anybody can make a model that predicts the future, but if you can't accurately reconstruct changes that we know have occurred in the past, then you're not going to have very much confidence in its predictions of the future. "I have no doubt that the West Antarctic ice sheet, at some time in the future, will disappear again. In terms of the broad geologic time, I don't think there's any question."