GLACIAL SPEED: Greenland may get much of the scientific attention but it is smaller glaciers such as the Columbia Glacier in Alaska pictured here that are already contributing to sea level rise--and will continue to do so in future.
Courtesy of W.T. Pfeffer, INSTAAR/University of Colorado

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.