Rignot, speaking at the annual meeting of the American Association for the Advancement of Science, said rising surface air temperatures appear to be triggering the increases in glacier speed in the southern half of Greenland. As a result, he said, many estimates of Greenland's future contributions to sea-level rise could be too low.

"You should view Greenland's expected contribution to sea-level rise as conservative," he said. "It could be two or three times more than predicted."

Rignot explained that current computer models used to predict the contribution of Greenland's ice loss to sea-level rise are inadequate, because they do not account for the changes evident by new satellite surveys. The surveys have been available only within the last 10 years, and they are much more comprehensive and accurate than anything done at ground level, because of the expense - and the dangers - involved in attempting to gauge changes in glacier movement.

Taking those higher glacier speeds into account, Rignot calculates that Greenland currently contributes about 0.5 millimeters per year to overall sea-level rise, which averages 3 mm per year. He said recent increases in Greenland glacier speed are responsible for more than two-thirds of the giant island's share of sea-level rise.

"It's not going to raise sea level by one meter tomorrow," he said. "It's going to take some time, but it's faster than expected."

Since 1996, Rignot explained - and he and co-author Pannir Kanagaratnam write in the Feb. 17 issue of the journal Science - Greenland's southeastern outlet glaciers have been largely responsible for increases in overall glacier flow. After 2000, glaciers farther north also began to increase in speed, and the northward spread of warmer temperatures may be responsible.

The authors write that over the past 20 years, air temperatures in southeastern Greenland have risen by 3 degrees Celsius (5.4 degrees Fahrenheit). The warmer temperatures are increasing the amount of melt water reaching the glacier-rock interface, where it serves as a lubricant that facilitates the glaciers' march to the ocean.

"Climate warming can work in different ways, but generally speaking, if you warm up the ice sheet, the glacier will flow faster," Rignot said. At present, he added, "these are very active glaciers, once you start pushing them out of equilibrium."

He also cautioned that the processes by which the glaciers accelerate are complex and not well-understood at present, but he said if the warming continues, the recent trend of faster-moving glaciers in the southern half of Greenland also could apply to the glaciers in the northwestern part of the island, where so far they have not moved as fast.

The Greenland Ice Sheet gains mass through snowfall and loses mass when ice melts, erodes or evaporates 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.

Using satellite data, Rignot and Kanagaratnam generated a glacier-velocity map for nearly all of Greenland during 2000. By incorporating satellite measurements from 1996 and 2005, they analyzed how glacier velocity has changed over the last 10 years, and they combined that information with ice thickness estimates to calculate changes in Greenland's total annual ice loss and mass balance over the same period.

The result: Ice loss due to glacier flow has increased from 50 cubic kilometers (12 cubic miles) per year in 1996 to 150 cubic km (36 cubic miles) per year in 2005. For comparison, the City of Los Angeles uses about one cubic mile of freshwater per year, Rignot said. "It is a lot of fresh water," he added.

Glacier-draining regions of Greenland that receive lots of snow, such as 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 the Greenland Ice Sheet's overall mass loss has increased from 90 cubic kilometers (21 cubic miles) per year in 1996 to 224 cubic kilometers (54 cubic miles) per year in 2005.

The Greenland Ice Sheet's area is 1.7 million square kilometers (about 650,000 square miles), or nearly the size of Mexico. It also is up to 3 kilometers (1.8 miles) thick. If the ice sheet melted completely, it would raise global sea level by about 7 meters (23 feet).

"The southern half of Greenland is reacting to what we think is climate warming," Rignot said. "The northern half is waiting, but I don't think it's going to take long." He added he already is seeing northern glaciers hinting that they are speeding up their march to the ocean.

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