The sudden and dramatic disappearance of a large lake which had formed on the surface of a melting ice sheet in Greenland has been documented for the first time by scientists who estimated that its outflow was greater than that of Niagara Falls.
Rising temperatures in the Arctic are causing more meltwater lakes to form on top of the Greenland ice sheet and satellite pictures have shown that they can disappear almost overnight for no apparent reason.
Thousands of the "supra-glacial" lakes form each summer – and last year was one of the warmest on record in the region – but scientists have little understanding of how the lakes can suddenly disappear or how far down the water flows into the 3,200ft (975m) deep ice sheet.
Now scientists have seen how they fall through the ice as a result of giant cracks that suddenly open up under the lake bottom, causing millions of tons of water to flow down to the base of the thick ice sheet where it grinds against the bedrock.
Instruments placed around the lake, which covered an area of 2.2sq miles and was up to 40ft deep, found that a huge crack or "moulin" (from the French for mill) opened up in the ice which allowed about 11.6 billion gallons of water to flow down to the bedrock in under 24 hours.
The pressure of the liquid water flowing between the ice sheet and the bedrock lifted the surface of the ice sheet by up to 20 feet at the point where the lake had formed in the summer of 2006, according to the findings of the study published in the journal Science.
The research is important because one of the questions scientists want answered is whether these huge volumes of water falling to the base of the ice sheet can lubricate its movement, so sending it faster towards the coast, where icebergs break off and contribute to a rise in global sea levels.
"We found clear evidence that supraglacial lakes – the pools of meltwater that form on the surface in summer – can drive a crack through the ice sheet in a process called hydrofracture," said Sarah Das of the Woods Hole Oceanographic Institution in Massachusetts.
"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 the bed of the ice sheet," Ms Das said.
"It's hard to envision how a trickle or a pool of meltwater from the surface could cut through thick, cold ice all the way to the bed. For that reason, there has been a debate as to whether such processes could exist, even though some theoretical work has hypothesised this for decades," she said.
The scientists believe that the lake emptied like a bathtub once the crack had formed, with much of the water disappearing in a cataclysmic outflow lasting about 90 minutes – a flow rate greater than the average flow rate of Niagara Falls.
Global positioning instruments recording the horizontal movements over the ice sheet found that the ice in the region of the lake began to move faster towards the ocean at about double the normal speed.
However, the scientists also found that the speed of movement of the ice sheet nearer the coastline was not affected significantly, indicating that the disappearance of the summer meltwater lakes may not influence the more complex dynamics that determine the rate at which icebergs "calve" from the ice sheet.
"We set out to examine whether the melting at the surface – which is sensitive to climate change – could influence how fast the ice can flow... if the ice sheet is frozen to the bedrock or has very little water available, then it will flow much more slowly than if it has a lubricating and pressurised layer of water underneath to reduce friction," Ms Das said.
Ian Jouglin, of Washington University, said: "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."
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