That means snowfall on top of the ice sheet is not enough to replace what is lost through surface melting and ice chucked out in the fjords by faster-flowing glaciers. In the process, sea levels rise as towering icebergs plunge into the Atlantic Ocean and displace water — much like an ice cube dropped into a drink.
The dynamics of the ice sheet on Greenland — and the much larger ones on Antarctica — were not included in sea level rise projections by the UN expert panel on climate change in 2007 because the phenomenon was poorly mapped at the time.
The picture of what happened in Greenland is just starting to come together, and scientists are still in the dark about how the underlying causes were set in motion, how much was owed to natural variances and how much to man's tinkering with the global climate system.
"This is like medical science in the 15th century," says David Holland, director of the Center for Atmosphere Ocean Science at New York University. "It's going to take a while to find out what's going on with the patient here."
The most popular explanation is that the patient — Greenland's ice sheet — contracted its ailment not from warmer air, but a warmer ocean.
Scientists earlier believed that the biggest factor for the faster flow speeds was meltwater seeping down to the base of the glaciers, lubricating the bedrock. They're now shifting attention to ocean currents believed to have sent pulses of warmer water from southern latitudes to Greenland's glacial fjords.
Holland found that such water was reaching the edge of western Greenland's biggest glacier, the Sermeq Kujalleq. A team led by Fiamma Stranneo, of the Woods Hole Oceanographic Institution in Woods Hole, Mass., made a similar discovery last month with probes plunged into the chilly depths of Sermilik fjord, where the Helheim Glacier ends.