Scientists, too, have been reporting thinning glaciers at the edge of Greenland's ice sheet. So the question is not whether things are changing, but why and how quickly.
On the Greenland ice sheet, a team of NASA glaciologists is collecting data.
On Thinning Ice
Scientists have long known that "meltwater" helps narrower mountain glaciers, for example, move faster in the summer. By contrast, the vast continental ice sheets were thought to move slowly and consistently, which implied that it would take centuries for a warming climate to affect them.
But a new study reveals that large ice sheets can respond to warm weather in a matter of weeks, with implications for sea levels, and seashores, worldwide.
In 1996, scientists were skeptical when Jay Zwally, a glaciologist based at NASA's Goddard Space Flight Center in Greenbelt, Maryland, started taking GPS measurements to gauge whether the Greenland ice sheet moved faster during the summer.
"They said, 'It's just going to show the same thing all the time,'" Zwally recalls.
Zwally's team mounted GPS equipment at the Swiss Camp90 miles (140 kilometers) east of Ilulissatthat recorded the ice's position at about ten-day intervals for five years.
Zwally and his colleagues found that when temperatures were warmer, the massive ice sheet indeed moved faster. Their study was published in the July 12 issue of Science.
"It was surprising, and we were quite excited when we saw [the results]," Zwally says.
Rising Sea Levels?
Under the summer sun, he explains, regions of surface ice turn into brilliant blue puddles and lakes of "meltwater." The meltwater pours through large channels called "moulins" until it reaches the base of the ice sheetthe interface between ice and bedrock.
As the water squeezes between the rock and ice, it behaves like a lubricant and allows the ice sheet to slide faster toward the sea.
"These [ice sheets] are tremendous reservoirs of ice that could cause the sea level to rise," says co-author of the study Waleed Abdalati, a glaciologist and manager of NASA's Cryospheric Sciences Program, also in Washington.
If the Greenland ice sheet melted, sea level could potentially rise by 23 feet (7 meters), and the Antarctic ice sheet by 200 feet (61 meters), according to the U.N.-affiliated Intergovernmental Panel on Climate Change.
But this doesn't necessarily mean we're all going to get wet. In a warmer climate, more snow may fall.
"In that case, Greenland may actually gain ice in its high center," Zwally says. "It's a race between increased melting at the edges and increased snowfall in the center."
ICESat is a significant new research tool to understand the meteorological mysteries. After December, as the ice melts and freezes, an eye in the sky will be watching.
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