For decades, glaciologists have warned that the thinning West Antarctic ice sheet is on the verge of collapse. If that happened, it would add massive amounts of water to the world's seas and flash flood the continents' coasts. The threat of global warming, most have assumed, only ups that ante. But new research presented at the American Geophysical Union's annual meeting in San Francisco on Friday and Saturday suggests otherwise. Not only does this ice sheet appear to contribute less to sea-level rise than was previously thought, it may be unlikely to fall apart anytime soon. Instead the ice streams running through it might be slowing to a stop, which some models show could possibly lead to another ice age.
The last major ice age on Earth occurred some 20,000 years ago, and earlier calculations indicated that the West Antarctic ice sheet had retreated since then. But scientists from the University of Washington led by Charles Raymond recently completed a radar analysis of a 30-by-50-mile rise, called Siple Dome, which showed that this region was not covered by a massive ice sheet in the past. And a new reconstruction of historic sea levels by W. R. Peltier of the University of Toronto reveals that a major jump in sea level happened before the West Antarctic ice sheet began its retreat, but not after. Current evidence suggests that the sheet was still growing as recently as 8,000 years ago.
"Our previous best estimates that the ice sheet is adding one millimeter per year to global sea level are almost certainly too high," says Robert Bindschadler of the NASA Goddard Space Flight Center, who gave a talk summarizing the new evidence at the AGU meeting. "The portion of the West Antarctic ice sheet we have focused on for the past 10 years appears to be in a stage of near-zero retreat now, but what it will do in the future is still uncertain."
One prediction, delivered a day earlier at the same meeting, comes from ">Slawek Tulaczyk and his colleagues at the University of California at Santa Cruz, who created a new mathematical model of the ice streams racing through the sheet. Whereas researchers had guessed that global warming might accelerate those streamsthereby advancing the sheet's disintegrationTulaczyk found that the streams may simply stop. In fact, one stream did stop moving about 150 to 200 years ago, and another has slowed by about 50 percent in the past 40 years. This stagnation is not the result of global warming, Tulaczyk says, but rather climate shifts and alterations in the ice sheet's geometry over the past 10,000 years.
If the ice streams do stop, the ice sheet could thin and shrink, triggering global ocean circulation and climate changes. "In the most extreme case, some models suggest that these changes could result in a shift from the current interglacial climate into another glacial period," Tulaczyk says. "I don't want to go that far, because we're still learning how the ice sheet would respond to changes in the ice streams, but it's an interesting possibility, especially since people have focused for so long on the possible collapse of the West Antarctic ice sheet due to global warming."
