To determine how much of Greenland's ice is disappearing due to warming, researchers have used a variety of high-tech tools, including global positioning systems (GPS), altitude-measuring devices on aircraft, and satellites.
"The changing mass of the great ice sheets of Greenland and Antarctica represents the largest unknown in predictions of global sea level rise over the coming decades," writes researcher Julian Dowdeswell, of the University of Cambridge in England, in an accompanying commentary in Science.
The techniques all come up with different raw numbers for the amount of ice lossor gain. But Penn State's Alley said they all paint a similar picture.
More snow is falling in the middle of Greenland, adding to the ice sheet's thickness. But the ice is rapidly thinning at the edges as the glaciers melt and slip ever more quickly into the Atlantic Ocean.
Alley said the snow piling up in the middle of Greenland is not enough to make up for the increased glacier speed and melting on the edges.
"It is a really hot topic, and the number is going to change but that is the picture that is starting to emerge," he said.
Along with colleague Pannir Kanagaratnam at the University of Kansas in Lawrence, Rignot used a satellite-based radar that monitors glacier movement from space.
In this way, he was able to generate a glacier-speed map for nearly all of Greenland for 2000.
The team then compared this data to satellite measurements from 1996 and 2005 to analyze how glacier velocity has changed over the last ten years.
Between 1996 and 2000 southeast Greenland's outlet glacierswhich are relatively small but fast flowingaccelerated 28.5 percent, on average, the researchers found.
Between 1996 and 2005 the speed of those same glaciers increased 57 percent.
The team found that, after 2000, glaciers farther north also rapidly increased in speed. Rignot said the northward advance of warmer temperatures may be responsible for the acceleration. The same factors could eventually unleash large glaciers in northwestern Greenland.
"The climate signal couldn't be more clear, in my opinion," Box, the Ohio State climate researcher, said. "And so seeing this report is just alarming. I just wouldn't have expected there to be such a clear signal."
By combining this glacier-speed data with ice-thickness measurements, Rignot and Kanagaratnam calculated Greenland's total annual ice loss.
How much of ice is Greenland losing due to glaciers sliding seaward?
In 1996 the number was 12 cubic miles (50 cubic kilometers). In 2005 it was 36 cubic miles (150 cubic kilometers), thanks to the glaciers' acceleration, the study says.
Overall, the Greenland ice sheet lost 22 cubic miles (90 cubic kilometers) of ice in 1996 but 54 cubic miles (224 cubic kilometers) in 2005, according to data from the new study and others.
To put this in perspective, Rignot said, the city of Los Angeles, California, uses about 1 cubic kilometer (35.3 billion cubic feet) of water per year.
"The exact details of climate-warming interaction with ice sheets are rather complicated. But broadly speaking, this is a direct result of climate warming in Greenland," Rignot said.
NASA's Zwally said his research, to be published later this month in the Journal of Glaciology, puts the 1996 value "very close to zero." He suggests that Rignot and Kanagaratnam "exaggerate" the loss.
The difference, Zwally added, comes from his figures that suggest that increased precipitation in the middle of Greenland is compensating for the ice loss and glacial thinning on the edges.
"How long will that offset continue? And which of the two competing factors will win over the next decades?" Zwally said.
"If we keep warming over the next decades as we have been, by the end of the century we'll be in serious trouble."
According to Ohio State's Box, the figures reported today will force an upward revision of predicted sea level rise.
The current middle-of-the-road estimate is a rise of 14 inches (35 centimeters) by the year 2100. The new findings, Box says, could double the predicted sea rise, to 28 inches (70 centimeters).
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