<img alt=“Bedrockb260.jpg” src=“http://blogs.nature.com/news/thegreatbeyond/Bedrockb260.jpg” width=“260” height=“198” align=right />
New results from a drilling project in Greenland suggest that the ice sheet there may be more stable—and Antarctica’s may be less stable—than previously thought. The findings, which come from the North Greenland Eemian Ice Drilling (NEEM) project, were presented on Thursday at a symposium on Antarctic science at the University of Wisconsin in Madison.
Researchers concerned about the potential for catastrophic sea level rise have long worried that global warming might melt parts of the West Antarctica Ice Sheet and the Greenland Ice Sheet. Substantial loss from either could raise sea levels by several meters, inundating parts of New York, London, and many other of the world’s major cities.
One way to assess the stability of the ice sheets is to look at how they fared during the Eemian period, the warm spell from 130,000 to 115,000 years ago that came just before the last ice age. During the Eemian, temperatures in Greenland were as much as 5 degrees C higher than recent values, a temperature hike on par with what that area might experience by 2100 if greenhouse-gas concentrations continue to rise. So researchers have been chasing Eemian ice for decades by drilling holes through the thick Greenland ice cap. But those earlier efforts have all come up empty. The Eemian ice had either been melted or mangled by the slow movement of the ice sheets.
The NEEM team—a collaboration of 14 nations—sought to get around that problem by selecting a site in northern Greenland where the ice sheet is relatively stable, so the lower layers would not be disrupted by too much movement. After three years of drilling, the researchers hit bedrock last July at a depth of 2,537 metres below the ice surface.
Analysis of the ice over the past year shows that the Eemian layers are folded to some degree, says Dorthe Dahl-Jensen of the University of Copenhagen, leader of the NEEM project. But the ice is still producing valuable data, she says.
By combining information from NEEM and other drill sites around Greenland with ice-sheet models, Dahl-Jensen and her colleagues have produced an estimate of how big the ice sheet was during the Eemian. While it did lose mass, the Greenland Ice Sheet did not melt as much as some earlier reports had suggested. The melting from Greenland caused global sea levels to rise by 1 to 2 meters, says Dahl-Jensen.
During the Eemian, sea levels rose a total of 7 to 8 meters, she says, so there must have been a substantial amount of melting somewhere else. The implication is that Antarctica, particularly the West Antarctic Ice Sheet, lost a lot of ice during the Eemian, which could raise concerns about the future stability of southern ice. Dahl-Jensen and her colleagues are readying a manuscript for publication about their findings.
Their conclusions will not get a warm response from some other ice-sheet researchers. Many have argued on the basis of other evidence that ice loss from Greenland contributed more than 2 meters to global sea levels, says Richard Alley, a glaciologist at Penn State University. An analysis in Nature in 2009 estimated that both northern and southern ice contributed at least 2.5 meters and possibly much more to the oceans during the Eemian.
Alley is eager to see the results from the NEEM core, even if the crucial layers were disturbed. “Ideally, you’d want everything perfectly in order but as long as you’ve got ice from [the Eemian], it’s a big win.”
photo of Dorthe Dahl-Jensen with ice core, courtesy of NEEM ice core drilling project, www.neem.ku.dk