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Lakes on the surface of Greenland鈥檚 ice sheet are draining through the kilometre-thick ice and roaring to the bedrock with a flow rate exceeding that of Niagara Falls.

The worry has been that with further global warming such meltwater would increase and have a catastrophic effect on the ice sheet, lubricating its base and making it slide quickly into the ocean. But a new study suggests that the meltwater鈥檚 effect is not as strong as feared.

In the summer months, the surface of Greenland鈥檚 ice sheet melts and the water pools on top, forming many lakes that are kilometres across. Researchers knew these lakes could drain quickly, but it wasn鈥檛 clear exactly how they drained nor how fast.

Ice fracture

of the , US, and colleagues camped out next to a 3-kilometre-wide lake in western Greenland. In July 2006, their seismometers picked up rumbles from the ice, then 30 minutes later the water started draining.

The entire lake was swallowed up in about an hour and a half. 鈥淭he most surprising thing was the speed,鈥� Das says. 鈥淚 didn鈥檛 think it would happen that fast.鈥� The scientists think the weight of the water created a fracture in the ice that then opened the entire one-kilometre thickness.

Summer rush

Once at the base of the ice sheet, the water seems to have drained away within a day. 鈥淚t was either tapping into an existing drainage system or forming a new system鈥� at the base of the ice sheet, Das says.

To get a broader view of how such water is affecting Greenland鈥檚 ice, of the University of Washington, Seattle, US, led a separate study using satellite images and GPS measurements. His team assembled the first map of the movement of the ice sheet and glaciers in Greenland, and found that each summer, the ice sheets slid toward the ocean 50 to 100% faster than they did during the rest of the year.

However, glaciers, which already flow much faster than the surrounding ice sheet, speed up by less than 15% during the summer, according to the study.

Nonetheless, the lubrication from the meltwater could make Greenland lose from 10 to 25% more ice over the 21st century than if this effect was not at work, Joughin and colleagues estimate.

鈥淭he good news is that increased melting [with continued global warming] doesn鈥檛 seem like it鈥檚 going to cause a runaway effect like some people had predicted,鈥� Joughin says.

Added lubrication

However, global warming does seem to be substantially speeding up glacier flow through other means, he argues, such as warmer oceans that are eating away at glaciers鈥� feet.

鈥淚 agree that at present, the lubrication of the ice sheet margins by meltwater will have a substantial but not catastrophic effect,鈥� says of the University of Colorado in Boulder, US.

But Steffen thinks that the meltwater could still speed up some glaciers more than it does others. He points to Greenland鈥檚 so-called Dead Glacier, stagnant for decades until it recently became active, which speeds up by 80%t during the summer.

Journal references: (DOI: 10.1126/science.1153360, 10.1126/science.1153288)

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