On July 29, 2006, there was a roughly 11-billion-gallon (0.044–cubic kilometer) lake that stretched more than two square miles (5.6 square kilometers) and covered the western portion of Greenland's massive ice sheet. In the span of 16 hours, it was gone. The reason: water pressure cracked through the more than half-mile (980-meter) thick ice, draining the lake as its water rushed through the new funnel and gathered below the giant ice sheet, raising it nearly four feet (1.2 meters) and moving it nearly three feet (0.8 meter) to the north.

"My co-workers and I had proposed models [in which] meltwater gets to the bed when a lake fills a crevasse, thus driving the crack down," says glaciologist Richard Alley of Pennsylvania State University who was not involved in the study. Now glaciologist "[Sarah] Das [and her colleagues] have observed it—more than Niagara plunging into Greenland!"

In fact, Niagara Falls's flow per second can be as fast as 202,000 cubic feet (5,720 cubic meters) of water; the glacial lake drained at pace of 307,237 cubic feet (8,700 cubic meters) per second. The meltwater below the glacier then flowed away through channels in the rock below, allowing the ice to subside back to its normal position.

Das of the Woods Hole Oceanographic Institution in Massachusetts, geophysicist Ian Joughin of the University of Washington in Seattle and an international team were the first scientists to observe and report such rapid drainage of a meltwater lake. They also used satellite and ground-level observations from September 2004 to August 2007 to determine that the glaciers of western Greenland are speeding up in their flow to the sea due to a combination of meltwater and more icebergs "calving," or breaking off of the glaciers.

"Over wide areas, the ice sheet and outlet glaciers speed up in response to the surface melt reaching the bed and lubricating the flow" of the glacier, Joughin says.

Ultimately, the fate of Greenland in a changing climate will help determine the future of coastal cities around the globe. As the ice sheet on the world's largest island melts, worldwide sea level will rise as a result of extra water entering the oceans.

The only question is by how much: Scientific estimates range from less than an inch to as much as 20 feet (six meters). The answer is up in the air, says glaciologist Eric Rignot of NASA's Jet Propulsion Laboratory in Pasadena, Calif., because scientists do not know the thickness of Greenland's glaciers nor how fast or by how much the surrounding oceans are warming. "Until we have such data," he says, "it will remain difficult to make realistic prediction of the evolution of the ice sheet in a warmer climate."

Joughin notes, however, that warming climes could accelerate the melting of Greenland's ice. Global warming might trigger icebergs to break free from the leading edge of glaciers more quickly and frequently, thereby opening the way for the latter to race even faster toward the sea, according to the team's findings published this week in the online edition of Science.

"Calving of an iceberg changes the balance of forces near the front, causing the glacier to speed up," Joughin says. "Calving rates are generally greater in summer than winter and we expect them to increase in a warming climate, which our observations over the last several years as temperatures warm in Greenland seem to confirm."

The warmer climate could also have an impact on meltwater. "There should be more melt higher [on the ice sheet] and, therefore, more lakes form[ing] inland," Joughin says. "If these lakes cause fractures to the bed in regions not currently being lubricated, this could be significant." In other words, if such meltwater speeds up the slower moving glaciers, Greenland's ice sheet might melt even more quickly.

The vast ice sheet is already changing much more swiftly than anticipated just a decade ago when, Joughin says, scientists believed that ice sheets responded slowly to climate change. "The recent observations of rapid change both in Greenland and Antarctica," he notes, "defy this wisdom."