An icy crust covering part of Antarctica has receded steadily for the last 20,000 years and may continue to do so regardless of human impacts on climate, a new study suggests. At its current rate of retreat, the ice will disappear in 7000 years, researchers report in tomorrow's Science . However, the team cannot rule out a sudden collapse of the ice sheet in the next few centuries--an event that would raise sea levels worldwide by 5 or 6 meters.
The West Antarctic Ice Sheet is a mass of thick glaciers spanning nearly a million square kilometers. It rests on bedrock that sits below sea level. Contact with the gradually rising ocean may make the sheet more unstable than ice at higher elevations elsewhere on Antarctica. Indeed, when glaciologists drilled through the ice sheet, they found signs that it has disintegrated completely sometime within the last 1.3 million years, in a climate not much warmer than today's (Science, 3 July 1998, p. 17 ). However, researchers had few details about the timing and extent of melting during recent millennia.
The new study pins down several dates and locations that trace the ice sheet's shrinkage. The team--led by glaciologist Howard Conway of the University of Washington in Seattle and geologist Brenda Hall of the University of Maine, Orono--used radar to image old layers within the ice, which revealed its growth and decay over time. The researchers also used radiocarbon dating to analyze the ages of organic material left high and dry on beaches as the ice sheet melted away to expose land, which rises as the weight is lifted during the ice's retreat. These and other methods revealed a pattern of steady retreat at an average rate of 120 meters per year over the last 7500 years. "We suspect this is an ongoing and long-term natural cycle triggered by rising sea levels at the end of the last ice age," says Conway. The relentless melting would continue even without human-induced climate changes, he believes.
"This is top-notch work," says glaciologist Robert Bindschadler of NASA's Goddard Space Flight Center in Greenbelt, Maryland. The study bolsters a fragile consensus that gradual retreat, rather than sudden collapse, is the most likely fate of the ice sheet during the next few thousand years, he says. However, Bindschadler cautions, atmospheric warming may soften the ice and hasten its flow to the sea.