Scientists overwhelmed by evidence a large meltwater lake atop the Greenland Ice Sheet drained in two hours -- faster than the flow of Niagara Falls -- now have the answer.
Scientists have long suspected that surface meltwater could drain to the base of an ice sheet and lubricate the ice sheet's flow out to sea. But no one had ever observed the phenomenon — until now.
The lake water, they say, flowed all the way down to the bed of the ice sheet, through almost 0.6 miles (1 kilometer) of ice. The flood doubled the average speed of the ice sheet's slide across the bedrock underneath it.
The new observations, detailed in an early online April 17 issue of the journal Science, show that while the seepage can significantly accelerate the summer movements of large stretches of the ice sheet, it has little effect on the movements of outlet glaciers, the tongues of the ice sheet that go out to the ocean where their fronts break off as icebergs.
Researchers placed monitoring instruments in and around the 2.2 square-mile (5.6 square-kilometer) glacial lake in the summer of 2006. Ten days after they left the area, a large crack developed over almost the whole span of the lake and all 11.6 billion gallons of water started to drain like a big bathtub.
"We found clear evidence that supraglacial lakes — the pools of meltwater that form on the surface in summer — can actually drive a crack through the ice sheet in a process called hydrofracture," said study team member Sarah Das of the Woods Hole Oceanographic Institution. "If there is a crack or defect in the surface that is large enough, and a sufficient reservoir of water to keep that crack filled, it can create a conduit all the way down to the bed of the ice sheet."
These new findings showed scientists that the conduits that drain surface meltwater, called moulins, can funnel it all the way to the base of the ice and confirmed some long-held suspicions that this meltwater could enhance the seaward movement of the ice.
Das and colleague Ian Joughin's work shows that "we can expect the ice sheet in a warming world to shrink somewhat faster than previously expected, but [that] this mechanism will not cause greatly faster shrinkage," said Richard Alley of Penn State, who was not involved in the study.
Source: Xinhua\agencies
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