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.