Greenland Meltwater Can Drain Faster Than Niagara Falls
for National Geographic News
|April 17, 2008|
Giant lakes of meltwater pooled on top of Greenland's ice sheets can suddenly drain to the bedrock, allowing the sheet to more easily slip forward, according to new research.
One lake that covered 2.2 square miles (5.6 square kilometers) and held 11.6 billion gallons (43.9 billion liters) of fresh water drained completely in about 90 minutes, scientists observed recently.
The maximum drainage rate was faster than the average flow rate over Niagara Falls.
"It's a lot of water flowing onto the ice pretty quickly," said study lead author Ian Joughin, a glaciologist at the University of Washington in Seattle.
The same phenomenon repeats itself over and over throughout the summer months in Greenland as long, warm days melt the ice on the surface. (See a Greenland map.)
The contribution of Greenland's rapidly melting ice sheet to sea-level rise is one of the biggest unknowns highlighted in the UN Intergovernmental Panel on Climate Change's most recent report.
(Related: "Not So Fast: Greenland Ice Melting, But Slower Than Thought" [October 19, 2006].)
No Catastrophic Floods
During the recent rapid release of meltwater, water funneled through a 3,200-foot-long (980-meter-long) crack in the ice.
The deluge caused the ice sheet to rise by 3.9 feet (1.2 meters) in one location like a blister, Joughin said.
"As it rose, it got to the point where it couldn't support that anymore, and then this big block fractured out of there and lifted up," he added.
The drained water creates a lubricating effect that accelerates ice flow by 50 to 100 percent in the broad, slow-moving areas of the ice sheet, Joughin and colleagues found.
"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.
(See a map of the world's disappearing ice sheets.)
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."
(Read about the effects of global warming.)
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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