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Ice is Flowing Slower on Greenland than Many Feared

News is not all that good for sea level rise

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.

Glacier near Qaanaaq, Northwest Greenland. Credit: Mike Lemonick.

“It’s a very important paper,” said Jay Zwally, project scientist for NASA’s upcoming ICESat2 mission who was not involved in this research, in an interview.

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’s GRACE 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.

Comments

By Kevin
on May 3rd, 2012

In my opinion - and this is JUST my opinion - this article would have been better if the last paragraph had been omitted.  In an article about how the worst case scenario is unlikely to close with a sentence about the worst case scenario and to include the word “devastating” just feeds into the hands of those who criticize us as alarmists.

Kevin

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By Wes
on May 3rd, 2012

Considering that many of the worst case scenarios have been steadily revised upwards over the last decade, and that no significant actions seem to be in the plans for the western nations, I would class that last sentence as a reasonable forecast , not as an extreme alarmist statement.

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By Tamsin Edwards
on May 4th, 2012

The Zwally effect has been superseded by better understanding now. Recent evidence shows that in high melt years glaciers slow down, because the drainage system changes. There is support from theoretical considerations from Christian Schoof too.

See for example:

http://www.sciencedaily.com/releases/2011/01/110126131538.htm

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By mr Keith Thurman (lakewood, wa. 98499)
on May 4th, 2012

The article was excellent. But there is a timidity to the author that is troubling. The seriousness of the consequences of sea level rise is routinely understated, and common sense extrapolations of death , property destruction,sudden but catostrophic neccesary public works repairs all created by a 20’  sea level rise are never ever discussed. This is wrong; and it will be dead wrong and unspeakably inconveneint, for every single country bordering bays inlets and the ocean coast lines. Towns, city’s,and especialy all the nearby low lying areas—- the salton sea for example.

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By Steven (Vancouver,BC,V5N2A3)
on May 10th, 2012

When you look at Antarctica there are all those lakes with Lake Vostok being perhaps one of the biggest.  And yet you try and see what’s under the ice of Greenland.  Google it.  There is zilch?  I don’t expect a Lake Vostok but I would expect something. I know scientists think there is a hot spot in northeast corner of Greenland.  Greenland isn’t a small place. Why isn’t this info readily available?

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