Tuesday, June 18, 2013

In Ancient Ice, Clues That Scientists Are Underestimating Future Sea Levels

The skies do strange things at the NEEM camp, a remote ice-drilling and research facility on the northern Greenland ice sheet. Midnight sunshine. Low clouds of sparkling ice crystals known as “diamond dust.” But when rain fell instead of snow last summer, complete with a rainbow arcing over the camp, the NEEM scientists couldn’t believe it. “I’ve been all over that ice sheet, and to have it rain that far north—that’s a shock,” says James White, a paleoclimatologist at the University of Colorado who led the American team working alongside those from 13 other countries at NEEM.
It’s fitting that part of the NEEM study’s fieldwork, which retrieved a two-and-a-half-kilometer shaft of ice, took place during one of the hottest Greenland summers on record. What that ice core has revealed about a warm period 130,000 years ago could be one of the most critical new tools for predicting how our planet will respond to a warmer future.
The NEEM ice core has provided the first picture of the Greenland ice sheet during the entire Eemian interglacial period, a 15,000-year span of natural warming that occurred between the two most recent ice ages. (NEEM is a rough acronym for North Greenland Eemian ice drilling.) During the Eemian, natural variations in Earth’s orbit brought the planet closer to the sun, making global mean temperatures up to 2°C warmer than right before the industrial revolution (the Arctic regions were made even warmer, between 3-5°C). That makes the Eemian an especially attractive period for scientists to study, because 2°C of global warming matches the temperature ceiling the UN and other international organizations have established as the limit of tolerable warming during the next century (a limit many climatologists believe we'll meet or even exceed by the end of this century). And what scientists are learning about the Eemian period could be cause for present-day concern.
Based on the study of the paleoclimate record in ice cores as well as the former locations of beaches and coral reefs, researchers believe that the Eemian temperature increase likely pushed global sea levels as high as eight meters (26 feet) above where they are today. That would put many coastal cities deep under water including Miami, the Los Angeles metropolitan area, and large parts of New York [see graphic]. During the Eemian, the polar ice sheets melted over several thousand years; an abrupt increase within the next century—seen by many scientists as inevitable, despite international goals—will not result in a 26-foot rise right away. “Even if you stabilize temperature by 2100, sea levels will keep rising for many centuries after that,” says Gavin Schmidt, a NASA climate modeler at the Goddard Institute for Space Studies who specializes in paleoclimate data. The ice sheets will take hundreds of years to fully react to warmer ocean temperatures, he says, “And there won’t be very much you can do about it.” Read More