The speed at which the vast Greenland ice sheet is disappearing has more than doubled in the past 10 years, according to the latest satellite observations. Detailed maps from hundreds of satellite images reveal that scores of major glaciers are moving much faster, tripling the amount of ice dumped into the Atlantic since 1996.
The most alarming changes in glacier movement have been spotted in southern Greenland. The Kangerdlugssuaq glacier, stable since 1962, accelerated 210% between 2000 and 2005 to nine miles a year, dumping seven times as much ice into the ocean. The researchers used radar images from four satellites to map nearly the entire surface of Greenland. Crevasses and other features are locked into glaciers, so images taken at different times reveal how fast the glaciers are moving.
"What surprised us was the magnitude of the changes," said Eric Rignot, lead researcher at Nasa's Jet Propulsion Laboratory in Pasadena, California. Until 2000 only glaciers below 60 degrees north on Greenland were known to be accelerating. But the latest images showed that by last year glaciers at up to 70 degrees north had also speeded up, suggesting warmer temperatures were creeping north.
The work was presented at the American Association for the Advancement of Science meeting yesterday and appears in the journal Science this week.
Dr Rignot's team believes climate change, which has seen temperatures in south-eastern Greenland rise by 3C in 20 years, has triggered the sudden change in the glaciers' movement. Rising air temperatures produce more surface meltwater, and if enough seeps through the ice it can form a lubricating layer on the rock the glacier rests on, making it slide more easily. "It's as if they reach a threshold and then they just go," said Dr Rignot.
At 1m square miles and up to almost two miles thick, the Greenland ice sheet is the world's largest and would raise world sea levels by seven metres (23ft) if it melted completely, a process that could take thousands of years.
The researchers calculate Greenland contributes 0.5mm a year to global sea level rise, which is running at 3mm a year.
Liz Morris, Arctic science adviser for the Natural Environmental Research Council at the Scott Polar Research Institute in Cambridge, said: "Greenland is the one place where there's a sufficient quantity of water that if released could cause serious environmental problems and it appears to be just on a turning point."