2008-09-08

Greenland, the world's largest island, holds enough ice to raise global sea levels by 23 feet (seven meters). Add the ice sheets of Antarctica and the oceans would deepen more than 200 feet (60 meters). Satellite measurements from space and speed measurements on land confirm that Greenland's glaciers are melting and on the move. And although the picture is less clear in Antarctica, the global warming seems to be having an impact there, too.

So the question is: How much—and how soon—will sea level rise?

New research from glaciologist Tad Pfeffer of the University of Colorado at Boulder and colleagues published in Science attempts to better estimate the possible sea level rise over the next century by measuring the speed at which the world's glaciers—in Greenland and Antarctica but also the many mountain ice sheets throughout the globe—are actually speeding to the sea as well as how swiftly they may melt.

"What would the flow velocities of the ocean-ending outlet glaciers have to be," if Greenland alone was to raise sea level by just six feet (two meters)? "The answer turned out to be huge: about 49 kilometers [30 miles] per year, 70 times faster than those glaciers move today," Pfeffer says, "and three times faster than we've ever observed an outlet glacier to move."

Given that Greenland's glaciers are not presently moving anywhere close to that pace—Kangerdlugssuaq Glacier, the fastest, reached speeds above nine miles (14 kilometers) per year in 2005—the researchers also looked at ice that could contribute from the rest of the world. Assuming that the largest remaining ice shelves in East Antarctica—Filchner-Ronne and Ross—will remain intact, sea level rise from all other melting ice and the expansion of seawater as the weather gets warmer over the next century would be somewhere between 2.6 feet (0.8 meter) and six feet (two meters)—or nearly twice as much as projected last year by the U.N. Intergovernmental Panel on Climate Change (IPCC).

This does not take into account how much sea level might swell from the metldown of the numerous small glaciers in Alaska, Argentina, Canada and Russia, which already contribute 60 percent of sea level rise from glacial melt. (In fact, Pfeffer notes that they are melting faster and therefore adding to sea levels more rapidly than Greenland and Antarctica combined currently do.) Nor is it clear whether something might suddenly occur to change that upper estimate. "If those two big ice shelves [in Antarctica] go out, then it's an entirely different situation," Pfeffer says. "But there's no good evidence that that's going to happen over the next century."

Ancient melting events suggest that glaciers can disappear in a hurry, however, and raise sea levels by more than half an inch a year. The Laurentide ice sheet that stretched as far south as New York State and Ohio some 20,000 years ago had retreated to eastern Canada, just across the water from Greenland, by roughly 11,000 years ago thanks to increased sunlight (due to the periodic wobble in Earth's axis known as precession). It then completely disappeared by 6,800 years ago in two geologically rapid bursts, shedding enough ice to raise sea levels by as much as four feet (1.3 meters) per century, according to research published this week in Nature Geoscience.

"The inspiration came from the IPCC report stating that we don't know how fast ice sheets will retreat and raise sea level in the future," says Anders Carlson, a paleoclimatologist at the University of Wisconsin–Madison and lead author of that study. "Well, we do have past records of ice sheet retreat under climates warmer than present so we decided to determine what those rates of retreat were."

At the same time, Greenland's glaciers were also smaller, though they persisted because of snowfall generated from the ocean. "More snow can partially offset the greater melting, helping the Greenland Ice Sheet to survive this interval," Carlson says. "But note it did retreat and this is what we are predicting for the future."

Based on this historical record and the fact that the Laurentide melted away under summertime temperatures similar to those expected in Greenland by the end of this century, Carlson and his colleagues forecast glacial melting that contributes somewhere between 2.8 inches (seven centimeters) and 5.1 inches (13 centimeters) of sea level rise per year, or as much as a 4.3-foot (1.3-meter) increase by 2100. Current rates are just 0.1 inch (3 millimeter) per year—and Greenland is contributing roughly 0.02 inches (0.4 millimeters) of that rise annually.

Pfeffer notes that the Laurentide and other ice sheets that disappeared in the past had an easier path to the sea than the glaciers in Greenland or Antarctica. "The analogies between those past climates and today aren't strong enough to say anything specific about the rate of sea level rise in the next century," he says.

The bottom line: sea levels will rise much more than predicted by the IPCC, based on both present understanding of current glacial melt as well as evidence from the geologic record. "The IPCC noted that their estimates should be seen as minimum estimates," Carlson notes, "and they are right."

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