In 1990, when climatologist Konrad Steffen established Swiss Camp, one of the first automatic weather stations on Greenland's ice sheet, global warming wasn't high on his agenda. Steffen wanted to study the interaction of ice and atmosphere at the "equilibrium line," the altitude where summer melt and winter snowfall are historically in perfect balance. "We probably have more information on nearby planets than we do on Greenland," he says. "Parts of Greenland have never been measured, because few satellites can see that latitude, and those that can haven't been up long enough. And it's difficult to deploy surface instruments in those conditions." Steffen's aim was to begin filling in gaps in scientists' understanding of the processes that drive-and are affected by-changes on the vast body of ice that holds roughly 8 percent of the world's freshwater supply.

But near the Earth's poles, equilibrium isn't what it used to be. A few years after Steffen built his research station, he noticed that temperatures on Greenland's ice cap were rising-and then rising faster. Over a decade, the average winter temperature shot up 7F, an increase so improbable that at first Steffen declined to publish it, fearing an error in his calculations.

Then again, he didn't need to double-check his data to see that the ice cap was changing. Swiss Camp's weather towers, which hold solar-powered monitoring equipment atop bases set 13 feet deep in the ice, began toppling over. In 1997 Steffen flew over Jakobshavn glacier in west Greenland and was shocked to see that its tongue had collapsed, "as if somebody had hit it with a massive hammer." A speed check showed that Jakobshavn, already the world's fastest-moving glacier, was accelerating; its velocity would double between 1997 and 2003...