Greenland is the 800-pound gorilla of sea-level rise. The world’s biggest island holds 680,000 cubic miles of ice, and if it all melted, the oceans would rise by more than 20 feet. That’s not going to happen any time soon, even with global warming. But some of the ice is melting and some is also sliding into the sea — and that could still add up to significantly higher seas by the end of the century, bringing all sorts of nasty problems. The big question facing scientists is: how much higher will the seas get?
A new paper published today in the journal Science doesn’t answer that question definitively, but it brings glaciologists a lot closer. By using satellite-based observations, scientists at the University of Washington and Ohio State University have clocked the flow rates of more than 200 of the so-called outlet glaciers that drain ice from Greenland’s central ice cap into the ocean. On average, they say, these rivers of ice sped up by about 30 percent between 2000 and 2010 — a good part of the reason that Greenland has been losing ice at a much faster pace over the past decade than it was before.
Earlier studies of glacier dynamics have been based on relatively few measurements taken over a much shorter period of time. In this case, the authors looked at data from Canadian, Japanese and German radar satellites over the first decade of this century.
Unlike NASA’s ICESat, which uses lasers to measure the thickness of ice, or the agency’sGRACE orbiters, which calculate ice loss by looking at changes in local gravity, radar creates a kind of topographic map of each glacier, showing patterns of roughness in the ice’s surface and mapping the locations of major crevasses. By seeing how far these features move downstream, and how fast, you can gauge the overall motion of the ice.
The good news stemming from this study is that the worst-case scenarios scientists have been entertaining for sea-level rise by the end of the century — two meters, or about six feet, by 2100 — appear less likely given the rate of observed ice motion. The middle range projections, however, are still well within reach.
“It’s not unreasonable to expect [sea level] increases of a meter or so,” said lead author lead author Twila Moon, a University of Washington grad student, in an interview. Considering that the eight inches the world has already seen during the 20th century pose major risks to life and property, however, the good news isn’t all that good.
Besides, said Zwally, “this doesn’t address the contribution to sea-level rise from melting.” Unlike Antarctica, where all of the ice loss comes from glaciers sliding into the ocean, on the Greenland ice cap during the summer, temperatures rise high enough for substantial, temporary lakes to form, which can then drain down to the sea. Moon went boating on one during her first field season, in fact.
“We didn’t realize at the time how fast they can empty,” she said. “This one drained abruptly some time later, in about an hour. The flow rate was greater than Niagara Falls. We were smarter after that.”
In Zwally’s view, melting has been downplayed as a factor in sea-level rise. Not only does meltwater lubricate glaciers, making them flow faster (this is known as the Zwally Effect) but the water itself adds to sea level directly. “The glaciers may accelerate in the future, or they may decelerate, but as temperature rises, the melting doesn’t decelerate,” Zwally said. “I think we’re seeing that melting is likely to be a more important and sustained factor in the future.”
And the glaciers could keep sliding faster, in addition to the melting. “There’s the caveat that this 10-year time series is too short to really understand long-term behavior,” co-author Ian Howatt, of Ohio State University, said in a press release. “So there still may be future events — tipping points — that could cause large increases in glacier speed to continue.”
If that happens, the worst-case scenario — a devastating six feet of sea-level rise by 2100 — might not be so farfetched after all.