Supraglacial lakes in Greenland have been draining suddenly due to climate change, and now scientists know how.

The Greenland Ice Sheet, the second largest in the world, is of great concern to scientists. A huge amount of water is contained in it, as well as in lakes which form on top of the glaciers. Until now, scientists were puzzled as to how these lakes could drain in a matter of hours.

Scientists Show Why Greenland's Glacial Lakes Are Melting

Study reveals how supraglacial lakes drain suddenly

As they drain, torrents of water rush to the base of the ice sheets and make the area where the ice sheet meets the sea more lubricated, which in turn causes the ice sheet to discharge ice into the ocean.

Now scientists have discovered what causes this sudden drainage. Researchers previously thought that the weight of the water caused the icy lake bottom to crack, but that is not the case. By deploying 16 GPS units around North Lake, in southwest Greenland, the team gathered data which showed that tension below the ice sheet, and not from the lakes, caused the cracks in the ice.

Data shows that 6 to 12 hours before cracks appeared, the ice around the lake pushed upwards and slipped horizontally due to meltwater draining to the base of the ice sheet. The water that accumulated below the ice sheet leads to a bulge which creates tension on the lake floor, which is suddenly released when the ice cracks.

Greenland – Both ice tension and lake volume contribute to draining

“In some ways, ice behaves like Silly Putty – if you push up on it slowly, it will stretch; if you do it with enough force, it will crack,” lead study author Laura Stevens from the Woods Hole Oceanographic Institution (WHOI) explained. “Ordinarily, pressure at the ice sheet surface is directed into the lake basin, compressing the ice together. But, essentially, if you push up on the ice sheet and create a dome instead of a bowl, you get tension that stretches the ice surface apart. You change the stress state of the surface ice from compressional to tensional, which promotes crack formation.”

Although that tension is responsible for forming the crack, the amount of water contained in the lake also influences the crack, widening it and forming hydrofractures. Both conditions are necessary to cause a sudden draining of a supraglacial lake.

Understanding how huge quantities of water suddenly enter the sea will help scientists to predict how supraglacial lakes will contribute to an overall rise in sea levels.