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.
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.)