Rignot expressed concern that the models now used to predict Greenland's ice loss and contribution to sea level rise are inadequate because they do not account for changes in the speed of outlet glaciers that flow into the sea.

Taking higher glacier speeds into account, the authors calculate that Greenland contributes about 0.5 millimeters per year to global sea level rise which currently stands at three millimeters per year.

The Greenland Ice Sheet is 1.7 million square kilometers, up to three kilometers thick and a little smaller than Mexico. If the Greenland Ice Sheet completely melted, it would raise global sea level by about seven meters (23 feet).

The mass balance of the great ice sheets of Greenland and Antarctica represents the largest unknown in predictions of global sea level rise over the coming decades, writes Julian Dowdeswell from University of Cambridge in Cambridge, UK, in a related "Perspective" article. He notes that the mass balance of large ice sheets can depend on the behavior of a small number of outlet glaciers.

"The southern half of Greenland is reacting to what we think is climate warming. The northern half is waiting, but I don't think it's going to take long," said Rignot who said he is already seeing northern glaciers beginning to accelerate their movement to the ocean.

Recent increases in glacier speed on Greenland are responsible for more than two-thirds of Greenland's contribution to sea level rise, the authors say.

Dr. Eric Rignot is a research scientist for the Jet Propulsion Laboratory's Radar Science and Engineering Section. He is a principal investigator on several NASA-funded projects to study the mass balance of the Greenland and Antarctic ice sheets, the interactions of ice shelves with the ocean; and the retreat of Patagonian glaciers. (Photo courtesy NASA)

Over the last 20 years, the air temperature in southeast Greenland has risen by three degrees Celsius.

The warmer temperatures increase the amount of melt water reaching the glacier-rock interface where it serves to lubricate glaciers' march to the ocean, the authors say.

"Climate warming can work in different ways, but generally speaking, if you warm up the ice sheet, the glacier will flow faster," said Rignot, who commented that the processes by which the glaciers accelerate are complex and not that well understood at present.

If warming continues, the recent trend toward faster moving glaciers in the southern half of Greenland may reach the glaciers in northwest Greenland, the authors say.

The Greenland Ice Sheet gains mass through snowfall and loses mass when ice melts, erodes or vaporizes off the surface, when ice breaks off and forms icebergs due to glacier flow, and when ice melts from the base of floating ice connected to glaciers.

Due to the recent acceleration, more ice is being dumped into the sea. The component of ice loss due to glacier flow has tripled during the 10 years covered by the study - from 50 cubic kilometers of ice loss per year in 1996 to 150 cubic kilometers of ice loss per year in 2005.

Understanding patterns of snow accumulation is also important for the overall picture of Greenland's role in sea level rise. Glaciers draining regions of Greenland that receive lots of snow, like the southeast and northwest, have the capacity for a greater contribution to sea level rise because more snow leads to more ice that can be dumped into the ocean.

When the researchers included findings from other groups on ice loss from glacier melting and ice accumulation from snowfall, they found that the Greenland Ice Sheet's overall mass loss has more than doubled from 90 cubic kilometers of ice loss per year in 1996 to 224 cubic kilometers of ice loss per year in 2005.

This research was performed at the Jet Propulsion Laboratory and the University of Kansas, Lawrence under a contract with NASA's Cryospheric Science Program.