Greenland's ice sheet covers 660,235 square miles. It is the second-largest ice body in the world, after the Antarctic Ice Sheet.
Image: Flickr/Nick Russill
Portions of the Greenland ice sheet melted a "moderate" amount thousands of years ago during an extremely warm period, raising new questions about its likely behavior in the future amid rising temperatures, according to a new study from a team of international scientists.
The conclusions about the Eemian interglacial period, 130,000 to 115,000 years ago, enlighten an ongoing debate over a deceptively simple question: To what degree will Greenland add to rising seas in a warming world, and to what degree will Antarctica?
The new study, published yesterday in Nature, suggests that Antarctica may have played a larger role in the past in adding to rising sea levels than Greenland, and therefore may follow a similar pattern in the future.
"The clues for sea level rise are pointing to the south, to Antarctica," said James White, director of the Institute of Arctic and Alpine Research at the University of Colorado, Boulder, and one of hundreds of scientists from 14 countries contributing to the new research as part of the North Greenland Eemian Ice Drilling (NEEM) project. The National Science Foundation funded a portion of the research.
The findings are significant because Antarctica "is a more dynamic ice sheet than Greenland," with a base well below sea level, said White. It has the potential to become more unstable over the long term as it loses ice, he said. Greenland, by contrast, is land based, providing it with a long-term "stabilizing factor" in regards to ice loss, he said.
The team of scientists found that the thickness of the Northwest Greenland Ice Sheet in the Eemian declined about 25 percent, or roughly 400 meters, over a 6,000-year period. The change in ice volume left the sheet near the NEEM research site about 130 meters below its current surface elevation.
While the decline was not insignificant, the warming at the time did not lead to the complete disappearance of the ice, as some models would suggest could happen with Greenland, said Dorthe Dahl-Jensen, a professor at the University of Copenhagen, and leader of the NEEM project. "There was a limited response in Greenland," she explained. "The reduction in elevation was pretty moderate."
An earlier melting had limits
This was so, she said, even though the team found that the Eemian period was warmer than previously thought, peaking at the NEEM site at roughly 8 degrees Celsius above the mean of the past millennium. At the time, the Earth was closer to the sun in the summer, allowing more solar energy to reach the surface.
The "moderate" reaction of Greenland meant that its melting ice sheet contributed less than half -- or 2 meters or less -- to rising sea levels in the Eemian period, she said. At the time, seas were likely 4 to 8 meters (13 to 26 feet) higher than now, according to Dahl-Jensen.
The world's glaciers and thermal expansion of the ocean would have contributed a fraction of that rise in sea level, she said, meaning that Antarctica would have made up the rest.
Part of the reason for the "limited" Greenland response is that the warming in the Eemian lasted a short period of time in geologic terms, even though it was thousands of years, she said. "We think that saved Greenland," she added.
Additionally, west Antarctica differs from Greenland in that it is essentially "sitting in a bowl," meaning that its connection to warm water could speed up its melting dramatically over a long time frame, explained White.
By drilling an 8,333-foot-deep ice core, the researchers were able to obtain a snapshot of Greenland's past by extracting very old ice and testing its chemistry and air composition. "It's just like a tree ring in many ways," said Dahl-Jensen.
A debate continues
Ian Joughin, a glaciologist at the University of Washington who did not participate in the research, said the study is a "nice detailed piece of work" but said it would not end the Antarctica-Greenland debate about rising sea levels in the future. He noted that the NEEM project reported a very broad range of numbers with regard to ice loss.
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6 Comments
Add CommentKey point here, even though the Eemian warming happened "quickly" in geologic terms, it still took THOUSANDS of years to do what human CO2 emissions are doing in DECADES. On top of that, there wasn't a complex industrial society spread out over the globe during the Eemian either. Things are different now. We need to stop being lazy and DEAL WITH OUR MESS!
Reply | Report Abuse | Link to thisThe difference between the earlier period and now is the presence of significant amounts of black carbon in the atmosphere of the northern polar region. I assume it is much less in the Antarctic. Of course, over a thousand years black carbon might be controlled.
Reply | Report Abuse | Link to thisThe problem is that ignorant politicians are basing policy decisions on an immature science that has yet to have
Reply | Report Abuse | Link to thisANY of it's detailed predictions match experience.
The claim of omniscience by the Global Warming crowd is the reason for the backlash.
What appears to have happened is a warming that leveled off, in a typical hysteresis curve fashion. I have seen nothing to explain this.
Black carbon in the atmosphere exerts a cooling effect. That was established in the 1970s. It is the main reason to require precipitators on coal burning power plants. It seems to have worked.
Reply | Report Abuse | Link to thisTo now claim that this soot in the atmosphere is warming, goes against experience and solid science.
Soot on the surface DOES induce localized warming, however.
The 6/2010 article Melt Zone" in National Geographic talks about cryoconite --atmospheric dust which has historically been tan -- now being black. It says that altho only 5% of the cryoconite is soot, it is having a major effect on melting. Nat Geo has photos of moulins, vertical tunnels melted as lakes of meltwater plunge to the bottom of the Greenland ice cap. They talk of an 11 billion gallon lake taking 84 minutes to drain .. a flow greater than Niagara falls (average Niagara flow is about 2.5 billion gallons in 84 minutes).
Reply | Report Abuse | Link to thisGreenland, with between 836,000 and 860,000 square miles is a little larger than the Louisiana Purchase. So this 11 billion ice-melt lake is likely not unique.
A major assumption about the Greenland ice cap seems to be that it is this massive block of ice, subject to surface melt and run-off. This seems to be what we based our sea level rise predictions. The reality is that the ice cap is rotting away on the inside as all this liquid water flows --presumably to the base -- and acts as a lubricant to the glaciers flowing seaward.
this coupled with warmer seawater is causing ice shelves to break off at land's edge. Sea ice acts as a plug to slow glacial flow. Without those ice shelves, glaciers flow faster.
I also note that scientists seem to go with the more conservative predictions but.... i ran into an article a few months ago saying that a number of the predictions were off by as much as a factor of 10, and i think it likely it is true again. A foot of sea level rise in 90 years is way beyond conservative, especially when NASA photos showed 97% or so of the 800,000 square mile ice sheet turning to slush last summer. Another NASA related article said 150 gigatons of ice disappeared last summer.
True? i don't know but it's time to recalculate glacial melt and sea level rise using more realistic paramaters and data.
There are tons more glaciers down in Antarctica. Warmer ocean waters lapping at glaciers is not good. These glaciers touching the oceans are eroding from their underneath. We still don't know or can't see the bottoms of the glaciers and there are tons of them -
Reply | Report Abuse | Link to thishttp://www.cccarto.com/antarctica/