Greenland glacial lake vanishes in warming drama

By Will Dunham

WASHINGTON (Reuters) - Surface melting fueled by climate warming can trigger dramatic events on the vast Greenland ice sheet such as a lake suddenly vanishing through a crack with force of Niagara Falls, experts said on Thursday.

Rising global temperatures are expected to cause an increase in meltwater in frozen expanses like the Greenland ice sheet, and this meltwater often forms sizable lakes.

Scientists have worried that when this increase in meltwater reaches the base of the Greenland ice sheet, it could further lubricate its slide over bedrock toward the sea, causing it to shrink more quickly than expected.

But researchers at Woods Hole Oceanographic Institution in Massachusetts and the University of Washington found that while this surface melt indeed does lubricate the bottom of the ice sheet, that process by itself does not seem to be enough to cause catastrophic loss of ice sheet mass as some have feared.

Surface meltwater was responsible for only a small amount of the movement of six outlet glaciers -- those that discharge ice to the ocean -- that the scientists monitored.

In the summers of 2006 and 2007, the scientists used seismic instruments, water-level monitors and Global Positioning System sensors to study two such lakes and the motion of the surrounding ice sheet.

They also used helicopter surveys and satellite imagery to to track the progress of glaciers moving toward the coast.

In July 2006, the scientists documented the sudden, complete draining of a lake measuring 2.2 square miles (5.7 sq km). The lake split open the ice sheet from top to bottom. Like a draining bathtub, the entire lake emptied from the bottom, disappearing in 24 hours -- through 3,200 feet (980 metres) of ice -- mostly in a 90-minute span.