Greenland's glaciers are dumping twice as much ice into the Atlantic Ocean now as five years ago because glaciers are moving and melting more quickly, researchers said this evening.
This could mean oceans will rise even faster than forecast, and rising surface air temperatures appear to be to blame, the researchers report in tomorrow's issue of the journal Science.
"This change, combined with increased melting, suggests that existing estimates of future sea level rise are too low," Julian Dowdeswell of the Scott Polar Research Institute at Britain's Cambridge University wrote in a commentary.
"At 1.7 million square km (656,000 square miles), up to 3 km (nearly two miles) thick and a little smaller than Mexico, the Greenland Ice Sheet would raise global sea level by about 7 meters (22 feet) if it melted completely."
The study did not explore what is causing the rising air temperatures in Greenland, but most scientists agree that human activity, notably the burning of fossil fuels, is playing an important role in global warming.
Eric Rignot of NASA's Jet Propulsion Laboratory at the California Institute of Technology and Pannir Kanagaratnam of the University of Kansas used satellite data to track the movement of Greenland's glaciers, which slide slowly down to the sea and deposit ice.
They calculated that Greenland contributes about 0.02 inch (half a millimetre) to the annual 0.1 inch (3 mm) rise in global sea levels.
Since 1996, southeast Greenland's outlet glaciers have been flowing more quickly and since 2000 glaciers farther north have also sped up.
Mr Rignot and Kanagaratnam found that ice loss due to glacier flow has increased from 12 cubic miles (50 cubic kilometers) of ice loss per year in 1996 to 36 cubic miles (150 cubic kilometers) of ice loss per year in 2005.
"It takes a long time to build and melt an ice sheet, but glaciers can react quickly to temperature changes," Mr Rignot said in a statement.
He said the models now used to predict how much ice Greenland will lose, and what effect that will have on sea levels, may underestimate the outcome.
Agencies