National Geographic: 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.
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….
Aerial photograph of Greenland, taken during an intercontinental flight, taken by "Turelio," Wikimedia Commons, icensed under the Creative Commons Attribution ShareAlike 2.5 License.
National Geographic: Giant lakes of meltwater pooled on top of 
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