A sharp wind knifes at the hands of the scientist as he struggles - gloves off in the bitter cold - to make adjustments to his radar. His instrument is strapped to an unwieldy wooden sled adorned with batteries and cables and two sets of flat antennas that extend like flapping wings over the snow. He hopes it will peer through the ice to the ground two miles below.

Dozens of scientific teams are scattered over the frigid Greenland snowscape, sent by the National Science Foundation, NASA and universities around the world. They are drilling the ice to collect samples, flying over it with radar and lasers, listening to its creaks and groans with seismometers, fitting it with GPS receivers to measure its pace, and photographing it as it slides to the sea and breaks into icebergs.

Their quest is crucial: If all the ice on Greenland were to melt, the seas around the world would rise by 23 feet, submerging countless coastal cities. A modest three-foot rise would endanger 70 million people. "Greenland has the potential to put a lot of water, a lot of ice, into the sea," said Tsoflias, a researcher from the University of Kansas.

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Greenland's ice cap contains 800 trillion gallons of water and several outlet glaciers, huge rivers of ice that act as faucets from the ice cap. Those faucets are running faster. The Jakobshavn Glacier where Tsoflias works has doubled its speed in five years and every day dumps enough ice into the sea to supply 20 to 30 New York Cities with water.

From the air, the Jakobshavn looks like a still-life portrait of a river in white, rippled with frozen waves, sinuous as it moves toward the ocean at a rate of 135 feet per day.

"It's the fastest-flowing glacier in the world," said Don Voigt, huddled in a tent a few yards from Tsoflias's people, on a 3-degree day, trying to warm up with hot chocolate and the tiny blue flame of a camp stove. "The question is, why is it flowing so fast?"

Voigt, 53, a white-bearded veteran of 14 field seasons in Greenland and Antarctica, leads a team from Penn State University mapping the bottom of the ice by setting off explosives and recording the seismic reverberations.

They will live out here for a month, sleeping in tents in the brutal weather, preparing explosive charges with cold-numbed hands, wiping snow off their instruments. They give nicknames to their jobs: "shooter," "recorder" and "potato planter" - the one who shoves the small charges into the snow.

Scientists have a working theory for the glacier's speed. The Jakobshavn is churning toward the sea over land that forms a trough deeper than the Grand Canyon. As higher temperatures melt ice and snow on the surface, the water is pouring down through crevasses to the rock. There, it is acting as a lubricant, lifting and carrying the glacier faster toward the sea.

As ice on glaciers moves over rock, it snags and lurches. That creates "icequakes" measurable in the same way as earthquakes. In 1993, there were seven such quakes in Greenland. In 2005, as the ice accelerated, there were 32.

In geologic terms, "quickly" still means decades or centuries. But some scientists say the Earth is approaching a point when the process cannot be stopped. Only in recent years did scientists conclude that sea levels are rising twice as fast as they had estimated, said Jay Zwally, a senior research scientist from NASA's Goddard Space Flight Center in Greenbelt, Md.

"We are seeing things taking place in the ice now that weren't expected, that five years ago we didn't even know about," said Zwally, who will spend his 14th summer on the Greenland ice cap this year. "I think eventually Greenland will reach a point that the change is irreversible in the current climate."

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By DOUG STRUCK

The Washington Post