Herky-jerky motion suggests worst-case sea level rise unlikely.
Time-lapse snapshots showing Greenland’s glaciers racing toward the sea in recent years have turned up some good news, and some bad news.
As the island’s glaciers disintegrate over coming decades, they won’t raise the world’s oceans as much as the most pessimistic forecasts had shown possible, researchers report in the May 3 Science. But the blocks of ice are still melting rapidly and may contribute worrisome centimeters to sea level rise by the end of the century.
“We’re certainly looking at significant rises in sea level, but some of the worst-case scenarios that people have imagined don’t seem likely,” says glaciologist Twila Moon of the University of Washington in Seattle.
Moon’s team used satellite measurements from 2000 to 2011 to clock the speeds of more than 200 outlet glaciers — flowing tongues of frozen water that carry ice away from the vast ice sheet that blankets most of the country. Where the glaciers extend offshore, they tend to fall apart and dump ice into the ocean.
Some of these icy conveyor belts have already been spotted moving — and thus melting — faster in recent years. A 2008 study in Science estimated how much such acceleration might contribute to rising sea level. If every glacier could suddenly zip along as quickly as the ultrafast glacier Jakobshavn Isbrae, about 14 kilometers per year, sea level would rise about half a meter by 2100, researchers found. A more realistic doubling of speed between 2000 and 2010, followed by leveling off, would contribute a smaller rise of about 9 centimeters.
“We were trying to set some really firm upper limits on sea level rise using values that seemed within the realm of possibility,” says glaciologist Tad Pfeffer of the University of Colorado at Boulder, a coauthor on the 2008 study.
The new data show that glaciers as a whole haven’t accelerated that much, or that uniformly, from winter to winter. On average, they moved about 30 percent faster at the end of the first decade of the 21st century than they did at the beginning. Some moved at a constant velocity the whole time. Others floored it for the first five years, then put on the brakes — or vice versa. Large changes in speed happened quickly and often.
“We don’t know what caused the speed changes,” says Leigh Stearns, a glaciologist at the University of Kansas in Lawrence who wasn’t involved in the new study. “Neighboring glaciers are behaving very differently, and that’s confusing.” Examining this complicated variability could help scientists figure out how landscapes, ocean temperatures and other factors drive the motion of glaciers.
Looking ahead, Moon cautions that 10 years is still a relatively short record, and that the future of Greenland is difficult to predict. Speeds could continue to pick up, she says, putting the glaciers back on track for 9 centimeters of sea level rise.