"It's a significant effect that needs to be included in the models," Joughin said. "But it's not like a catastrophic one that's going to flood New York any time soon."
Joughin and Sarah Das of the Woods Hole Oceanographic Institution in Massachusetts, lead author of a separate study on the phenomenon, describe their findings in papers published online today in the journal Science.
Meltwater draining to the bedrock speeds up the flow of the entire ice sheet, including outlet glaciers—streams of ice that flow outward—by about 160 to 330 feet (50 to 100 meters) a year, Joughin said.
During the summer, this speedup accounts for half to all of the movement of the broad ice sheet.
The outlet glaciers are moving much faster, and the lubrication effect of the meltwater is contributing only fractionally to their movement.
But the added slippage has only a minor effect on the pace of Greenland's outlet glaciers, which have been galloping toward the ocean in recent years.
Jay Zwally is a glaciologist at the NASA Goddard Space Flight Center in Greenbelt, Maryland, who first described the lubricating effect of water on Greenland's ice in a 2002 Science paper.
He was not involved in the current research, which he said confirms the effect occurs along much of the ice-sheet margins but may underestimate the effect on the outlet glaciers.
The new research, he said, primarily focuses on a single large outlet glacier that has unique characteristics. Other outlet glaciers show more of a speedup.
"So the question is still open," Zwally said.
Ice Sheet Dynamics
Richard Alley is a glaciologist at Pennsylvania State University in University Park who was not involved in the new research.
He said the findings are a vital clue to understanding how much ice could melt from Greenland, pushing global sea levels higher over the course of the next century.
(Related: "Sea Level May Rise 40 Percent Higher Than Predicted, Study Says" [December 14, 2006].)
The recent IPCC report, on which Alley was an author, noted that scientists need to determine the physics that control ice flow in Greenland and then plug that data in a computer model.
"They're finding out what happens," Alley said of the new work. "And [they're doing it] really well."
Zwally of NASA said the research is breaking new ground, but that more questions remain—for instance, how far inland does the lubrication extend?
"We may be looking at a couple of feet of sea-level contribution during this century," he said.
"But as this builds up, it is headed towards a point where it will continue to speed up."
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