US: April 18, 2008


WASHINGTON - Surface melting fuelled 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 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.

"It's extremely dramatic," scientist Sarah Das of Woods Hole Oceanographic Institution, who helped lead the research published in the journal Science, said in a telephone interview. "The discharge during that period exceeded the flow of Niagara Falls."

As sunlight and warm air melt surface ice, thousands of so-called supraglacial lakes appear atop the Greenland ice sheet every summer. From past satellite images, scientists have known that these supraglacial lakes can disappear quickly but did not know precisely how this was occurring.

"Greenland is losing significant (ice) mass each year and it has been adding a growing contribution of ice to the ocean -- and therefore a growing contribution to sea level rise. That has been accelerating," Das said.

"What we can show from our findings is that the mechanism responsible for most of that acceleration is not from surface meltwater enhanced flow, which had been proposed as perhaps one of the mechanisms," Das said.

The University of Washington's Ian Joughin, another of the researchers, said scientists are trying to figure out the other mechanisms contributing to the current ice loss in Greenland that likely will increase as the climate warms further.

(Editing by Eric Walsh)


Story by Will Dunham


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