for National Geographic News
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.
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