Surface melt of Greenland’s Ice Sheet may not lead to sea level rise.

Mike Carlowicz of the Woods Hole Oceanographic Institution released some information on interesting research from the University of Washington. “Thousands of lakes form on the top of Greenland’s glaciers in the summer as sunlight and warm air melt ice on the surface”. In the past satellite observations had shown that these can disappear in as little as a day but scientists did not know where the water was going or how quickly nor how this impacted on ice flow he said. University of Washington’s Ian Joughin and Woods Hole Oceanographic Institution’s Sarah Das set out to examine whether the melting of the surface, which is sensitive to climate change, could influence how fast the ice can flow. The result of their research is posted in two papers this week on Science magazine’s Science Express: Fracture Propagation to the Base of the Greenland Ice Sheet During Supraglacial Lake Drainage,” and Seasonal Speedup along the Western Flank of the Greenland Ice Sheet.

The scientists have, for the first time, observed the sudden and complete drainage of a lake of meltwater from the top of the Greenland ice sheet down to its base. The report confirmed what scientists had thought was happening, that surface meltwater is reaching bedrock further inland under the Greenland Ice Sheet.

The release stated that the lake was 2 to 2.5 miles wide and 40 feet deep, holding 11.6 billion gallons of water. The scientists liken the mechanism of the emptying to a bathtub draining. The entire lake emptied from the bottom in 24 hours with the majority of the water flowing out in a 90 minute span. The report is quoted as saying that the maximum drainage rate was faster than the average flow rate over Niagara Falls. The data revealed that the pressure of the water from the lake split open the ice sheet from top to bottom through 3,200 feet of ice. A 2,400 foot block of ice in the middle of the lake was raised by 20 feet by the water that penetrated.

The report also discussed the mechanism involved in the drainage: “This shows that as cracks and crevasses form and become filled with water, the greater weight and density of the water forces the ice to crack open. As water pours down through these cracks moulins, cylindrical, vertical conduits form through the ice sheet that allows rapid drainage and remain open for the season”. Joughin, a glaciologist with the UW’s Applied Physics Laboratory is reported as saying

“Considered together, the new findings indicate that while surface melt plays a substantial role in ice sheet dynamics, it may not produce large instabilities leading to sea level rise, there are still other mechanisms that are contributing to the current ice loss and likely will increase this loss as climate warms.”

Over an entire year, surface meltwater was responsible for only a few percent of the movement of the six outlet glaciers monitored, says Joughin. Even in the summer it appears to contribute at most 15 percent, and often considerably less, to the total annual movement of these fast-moving outlet glaciers. These two pieces of research help explain the mechanisms of meltwater on glacial ice and the Greenland Ice Sheet.

The findings show that surface meltwater plays an inconsequential role in the movement of outlet glaciers, but that meltwater is responsible for 50 to 100 percent of the summer speed up for the large stretches near the edge of the ice sheet where there are no major outlet glaciers, a finding consistent with, but somewhat larger than, earlier observations.

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~ by abstraktbiblos on Tuesday, 22 April, 2008.

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