In Greenland, a great deal of meltwater rushes down through drainage holes in the summer.
Reading On the Greenland ice sheet strange things are going on. Melting water, which collects in larger lakes on the surface of the ice in summer, can disappear in depth within a few hours. The water penetrates through cracks in the ice and finally makes a tubular drain hole through kilometer-thick ice to the ground. The measured flow can become as strong as the Niagara Falls, researchers report by Sarah Das of the Woods Hole Oceanographic Institution. On satellite imagery, glaciologists have long noted that the bright blue meltwater lakes that form at the surface of the ice at temperatures above zero degrees Celsius sometimes disappear quickly. So far, it was unclear where the water is going. So, in 2006, she and her colleagues monitored two lakes with seismographs, GPS receivers, and other gauges to solve the mystery. At the end of July, one of the two lakes reached an area of ​​5.6 square kilometers, at a depth of twelve meters. On July 29, the water level dropped, initially slowly at a speed of 1.5 centimeters per hour.

At four o'clock in the afternoon, the runoff increased dramatically. The entire lake was drained like a bathtub in 90 minutes, with the stopper pulled out. In this phase, 8, 700 cubic meters of water per second rushed into the depth, the researchers report that this is more than the average amount of water in the Niagara Falls. The researchers found that the water under the ice evidently moved to the north.A GPS station north of the former lake was raised by 1.20 meters, also slipped the ice along with the station 80 centimeters to the north.

As the researchers write, the higher specific gravity of water compared to ice is responsible for the fact that melt water dilates existing cracks and finally makes its way through the entire glacier. Nobody expected the speed of the events. "It's hard to imagine how a small meltwater pond can penetrate all the way from the surface through cold, solid ice to the ground, " says Sarah Das. Theoretical considerations had hinted at the possibility, but there had been heated debate among researchers about whether such a bathtub effect was possible.

As the frogs write, meltwater accelerates the movement of the glaciers. The water acts like a lubricant: Usually the ice is frozen on the rock and hardly moves. If pressurized water collects between ice and rock, a glacier can slip. However, the meltwater in the thick inland ice only causes an increase in speed of ten percent, the researchers write. The fast-flowing glaciers near the sea, some of which advance several kilometers per year and contribute significantly to sea-level rise, are hardly affected by meltwater on the ground. Their high speed is rather caused by the fact that icebergs constantly break off at the glacier tongue. As a result, ice quickly slips from behind. display

"The study shows that the Greenland ice sheet melts faster in a warmer world than previously thought, " commented the glaciologist Richard Alley of Pennsylvania State University. "But this mechanism will not lead to a catastrophic melt."

Sarah's (Woods Hole Oceanographic Institution, Woods Hole, Massachusetts) et al .: Science Express, April 18 Ian Joughin (University of Washington, Seattle) et al .: Science Express, April 18 Ute Kehse

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