Satellite observations indicate that Greenland's glaciers have been dumping ice into the Atlantic Ocean at a rate that's doubled over the past five years, researchers reported here on Thursday. The findings add yet another factor to the long-running debate over the effect of climate change on the world's ice sheets and sea levels.
"The evolution of the ice sheet, in the context of climate warming, is more rapid than has been predicted by models," one of the researchers, Eric Rignot of NASA's Jet Propulsion Laboratory, told MSNBC.com. As a result, Greenland's ice sheet — second only to Antarctica's ice sheet, with almost as much area as Mexico — could contribute more than expected to rising sea levels in a warming world, he said.
Other climate experts said the study, which was revealed in St. Louis at the annual meeting of the American Association for the Advancement of Science and published in Friday's issue of the journal Science, added an important piece to the climate puzzle.
"This is a big, major finding," said Gino Casassa, a glaciologist at Chile's Center for Scientific Studies. He noted that some glaciers in South America's Patagonia region have been shrinking faster than expected, and said "acceleration may be the missing link." Other scientists said a similar effect might be at work within glaciers in Alaska and Antarctica.
The Greenland Ice Sheet's role in climate predictions is not crystal-clear, however. Researchers have to account not only for the loss of ice around the edges of the sheet, but also for the buildup of ice in Greenland's interior. The influence of smaller-scale weather cycles on Greenland's waxing and waning ice adds to the complexity.
Climate skeptics point to the buildup of snow and ice in Greenland's interior as evidence that the ice sheet is not thawing out. But Rignot and others said that the buildup is taken into account in the computerized climate models, as a meteorological side effect of the global warming trend.
When all the effects are considered, the Greenland Ice Sheet's annual loss has risen from 21.6 cubic miles (90 cubic kilometers) in 1996 to 36 cubic miles (150 cubic kilometers) in 2005, according to Rignot and his co-author, Pannir Kanagaratnam of the University of Kansas. Their conclusions are based on nearly a decade's worth of radar data from the Radarsat-1, ERS-1, ERS-2 and Envisat satellites, as well as radio echo sounding experiments.
How much, and how fast?
Virtually everyone agrees that the complete disappearance of the 2-mile-thick (3-kilometer-thick) Greenland Ice Sheet would cause an estimated 23-foot (7-meter) rise in global sea levels. That would inundate coastal regions around the world. At the same time, virtually everyone also agrees that even under the worst-case scenario, it would take centuries of warmer weather for Greenland's ice to disappear completely.
It's the rate of change in the ice sheet, and its variability over time, that is at issue.
Rignot and Kanagaratnam say their calculations indicate that the Greenland melt currently contributes about two-hundredths of an inch (0.5 millimeters) to the annual 0.12-inch (3-millimeter) rise in global sea levels. The glacier speed-up is responsible for more than two-thirds of that contribution, they say.
Moreover, the type of speed-up seen in Greenland may be affecting glaciers elsewhere as well, Rignot said.
"We think something very similar is happening in the Antarctic Peninsula, where the ice shelves in front of these glaciers has collapsed," he told MSNBC.com, specifically pointing to 2002's demise of the Larsen B ice shelf.
Mark Chandler, a climate researcher at Columbia University, said the fate of the world's ice sheets is "probably the biggest concern that people are looking at right now" in the field of climate prediction.
"There's a lot of fear out there right now, even among scientists, that ice caps are not all that stable," he told MSNBC.com. If the pace of global ice loss accelerates, sea levels might conceivably rise 6 to 16 feet (2 to 5 meters) over the course of a century, which he said would be "devastating."
Why is it happening?
Rignot said scientists still had much to learn about the dynamics of glacial ice movement under warming conditions.
One theory is that the meltwater serves as a lubricant for the moving ice, hastening its push to the sea. "This is a complex process, because we don't exactly know how the meltwater reaches the bed," Rignot said. "Some of [the glaciers] apparently accumulate this meltwater for quite a while before they start responding."
The scientists found that the speed-up has affected Southeast Greenland's glaciers since 1996, with glaciers further north speeding up after 2000. They speculated that the pattern was due to the northward spread of warmer temperatures in Greenland.
Effect on the climate debate
The latest report provides further ammunition for those who contend industrial greenhouse-gas emissions are causing higher global mean temperatures, with potentially alarming effects over the long term. However, some researchers note that other cyclical factors could be at work.
For example, weather patterns that vary on a scale of decades, such as the North Atlantic Oscillation, are known to affect Greenland's weather. Indeed, Los Alamos National Laboratory's Petr Chylek and his colleagues noted in 2004 that Greenland went through a rapid warming trend starting in the 1920s, which was followed by cooling temperatures.
But in a 2005 study, Chylek and Swiss researcher Ulrike Lohmann found that the North Atlantic Oscillation couldn't account for Greenland's current warming trend. They estimated that the warming rate in Greenland was 2.2 times faster than the global norm — which is in line with U.N. climate models.
Rignot agreed that Greenland has gone through warming trends in the past, including the rise in the 1920s.
"It's quite probably that the glaciers reacted to that, possibly in a similar way," he said. "The difference we have here is that the warming in the 1920s lasted less than a decade, and then it stopped. We have no indication from the climate record that things are starting to cool off. We are now entering a zone of warming that has not been experienced by the ice sheet over the last century."