Reversing Greenland ice sheet's melt-off could be impossible

  • Article by: AMY HUBBARD , Los Angeles Times
  • Updated: March 17, 2012 - 5:03 PM

If that ice disappears, sea level could rise 23 feet.

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July 19, 2007 file photo, an iceberg off Ammassalik Island in Eastern Greenland.

Photo: John Mcconnico, Associated Press - Ap

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LOS ANGELES - The Greenland ice sheet has a lower melting point than previously thought, with scientists saying not only that it could melt completely at a lower temperature than once believed, but also that the melting process could soon become irreversible.

"Once the process of melting the ice begins, it is very hard for it to change course even if we can lower temperatures in the future," Alex Robertson, lead author of a new study, said by e-mail.

Next century tells the tale

"So even though melting the whole ice sheet could take a really long time, we will essentially decide the fate of Greenland within the next century."

The study was published Sunday in Nature Climate Change.

The Greenland ice sheet is about 1,490 miles at its longest and more than a mile thick, according to Science Daily. Why is it important?

"Understanding the sensitivity of the ice sheet to climate change is extremely important," Robertson said, "because it contains enough water volume to raise sea level by 7 meters," or about 23 feet.

Earth's freshwater reservoirs

This ice sheet and the one that covers most of Antarctica contain more than 99 percent of the freshwater ice on Earth, says the National Snow and Ice Data Center. The sheets influence weather and climate, the center's site says, with plateaus at higher altitudes changing storm tracks and conjuring cold winds that move downslope close to the surface of the ice.

A news release on the study, by the Potsdam Institute for Climate Impact Research, explains that as the thick sections of the Greenland ice sheet melt, the surfaces at high altitude eventually sink to lower and warmer altitudes, which in turn accelerates the melting.

"Also, the ice reflects a large part of solar radiation back into space," the release says. "When the area covered by ice decreases, more radiation is absorbed, and this adds to regional warming."

The temperature threshold for melting the ice sheet entirely is about half what was once thought, according to the study.

"We expect that (2.9 degrees Fahrenheit) global warming above preindustrial temperatures is enough to switch Greenland from being a stable ice sheet to one that is in decline and will eventually disappear," Robertson said.

He called that threshold a "tipping point" for the ice sheet. The previous best estimate, he said, was 5.6 degrees.

How long will it take to melt the sheet completely?

That "very strongly depends on how high the temperatures are above the estimated threshold," Robertson said. If temperatures remain close to the threshold, the melting would happen much more gradually, "on the order of tens of thousands of years."

Dire prediction

But if global warming continued apace, which Robertson called "a business-as-usual scenario" on greenhouse gas emissions, "the entire ice sheet would melt within 2,000 years."

"In that case, about 20 percent of the ice sheet would disappear in the first 500 years," he said, "which implies continuous sea level rise into the foreseeable future."

Once the melting has begun, Robertson said, it can prove impossible to put on the brakes: "What we show is that even just losing 10 percent of the ice sheet can be enough to make the process irreversible."

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