Rising surface air temperatures appear to be triggering the increases in glacier speed in the southern half of Greenland, according to the study's authors who say that many estimates of Greenland's future contributions to sea-level rise could be too low.
This is the first study to incorporate recent changes in glacier velocity into estimates of the overall mass of ice being lost for nearly all of Greenland.
These findings, which will be announced at the AAAS Annual Meeting in St. Louis, Mo. on Thursday, 16 February 2006, are scheduled to appear in the 17 February 2006 issue of the journal Science , published by AAAS, the nonprofit science society.
"The behavior of the glaciers that dump ice into the sea is the most important aspect of understanding how an ice sheet will evolve in a changing climate," said Science author Eric Rignot from the Jet Propulsion Laboratory, California Institute of Technology in Pasadena, Calif. "It takes a long time to build and melt an ice sheet, but glaciers can react quickly to temperature changes."
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 3 millimeters per year.
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.
Since 1996, Southeast Greenland's outlet glaciers have been largely responsible for increases in overall glacier flow. After 2000, glaciers further north have also rapidly increased in speed, and the northward spread of warmer temperatures may be responsible, according to Rignot.
Over the last 20 years, the air temperature in southeast Greenland has risen by 3 Celsius degrees.
The warmer temperatures increase the amount of melt water reaching the glacier-rock interface where it serves as a lubricant that eases 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 noted 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.
Using data from satellites capable of monitoring glacier movement from space, the researchers generated a glacier velocity map for nearly all of Greenland for 2000. By incorporating satellite measurements from 1996 and 2005, the researchers analyzed how glacier velocity has changed over the last 10 years. They combined this glacier velocity information with ice thickness estimates to calculate changes in Greenland's total annual ice loss and mass balance over the same period.
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 speed up, more ice is being dumped into the sea. The component of ice loss due to glacier flow has increased 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 increased from 90 cubic kilometers of ice loss per year in 1996 to 224 cubic kilometers of ice loss per year in 2005.
The Greenland Ice Sheet is 1.7 million square kilometers, up to 3 kilometers thick and a little smaller than Mexico. If the Greenland Ice Sheet completely melted, it would raise global sea level by about 7 meters.
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 the author of a related "Perspective" article who 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 hinting at a speed up in their march to the ocean.
Eric Rignot is at the Jet Propulsion Laboratory, California Institute of Technology in Pasadena, Calif. Pannir Kanagaratnam is at the University of Kansas in Lawrence, Kan. This work was performed at the Jet Propulsion Laboratory and the University of Kansas, Lawrence under a contract with NASA's Cryospheric Science Program.
The related "Perspective" article is by Julian Dowdeswell from University of Cambridge in Cambridge, UK.
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