https://www.sciencemag.org/content/337/6102/1591.full

Science 28 September 2012:
Vol. 337 no. 6102 p. 1591
DOI: 10.1126/science.337.6102.1591

NEWS & ANALYSIS
CLIMATE CHANGE

Ice-Free Arctic Sea May Be Years, Not Decades, Away
Richard A. Kerr



Going, going, … This month north of Alaska at 80°N, scientists found sea ice's record-setting edge.
CREDIT: PHOTO COURTESY OF THE U.S. COAST GUARD AND IGNATIUS RIGOR, UNIVERSITY OF WASHINGTON

Last week's record-shattering shrinkage of summertime sea ice was yet another reminder that scientists do not understand how global warming is driving the Arctic toward inevitable ice-free summers. Even the latest climate models clearly aren't getting it right, still calling for the Arctic Ocean to retain substantial summer ice well toward the end of the century.

No one believes that anymore. The now-clearly-accelerating decline of summer ice—punctuated by exceptional losses in 2007 and now in 2012—has persuaded everyone that summer Arctic sea ice will be a goner far sooner than the end of the century. So the full knock-on effects of an ice-free Arctic Ocean—from the loss of polar bear habitat to possible increases of weather extremes at mid-latitudes—could be here in many people's lifetimes.

How far wrong the models might be, however, is still very much in dispute. Researchers who take climate model forecasts of ice area as a starting point tend to give summer ice several more decades. “I think 2030 to 2040 is pretty realistic” for an ice-free summer Arctic, says sea ice specialist Julienne Stroeve of the National Snow and Ice Data Center (NSIDC) at the University of Colorado, Boulder.

But researchers who follow the even-faster-plunging volume of sea ice are more pessimistic still. “There are a lot of deficiencies in the state-of-the-art [climate] models,” says oceanographer Wieslaw Maslowski of the Naval Postgraduate School (NPS) in Monterey, California. If the rapidly declining ice volume is taken to be the better guide, he says, Arctic sea ice could be gone by the end of the decade. So a few more years of watching and waiting could clarify the ice's fate.

Both the area and the volume of sea ice have been waning in recent decades. In area, this summer's low of 3.41 million square kilometers is 18% below the previous record of 2007—a loss the size of Texas—and 49% below the summer low of 1979, when the satellite record begins. And eyeballing the figure (below), it's easy to see that ice area has been shrinking at an accelerating rate. From 1979 through 2001, ice area was declining 6.5% per decade, according to Stroeve. Since then, it has been falling on average twice as fast.

Ice-volume trends are even more alarming. The record of ice thickness is shorter and far spottier than that for ice area, so a group including sea ice specialist Michael Steele of the University of Washington's Applied Physics Laboratory (APL) in Seattle runs a model that calculates the volume of Arctic sea ice. Inputs include measurements of ice area and thickness and observations of atmospheric and oceanic properties such as temperature, cloud cover, and circulation. According to this model, ice volume in August 2012 was 4400 cubic kilometers, 76% lower than in 1979. That's 2.1 standard deviations below the long-term trend, thanks to the more than doubled rate of volume decline during the past decade or so.



No stopping now. The area of Arctic summer sea ice has been declining rapidly since satellite observations began. CREDIT: NATIONAL SNOW AND ICE DATA CENTER

Scientists see these trends as increasingly threatening. The less area covered by highly reflective ice, the more sunlight warms Arctic waters, and the faster the ice melts. Rapidly shrinking ice volume denotes another destructive feedback: Ice volume lost through thinning leaves the thinner ice more vulnerable to destruction by rising temperatures and summer storms. “The rules are changing,” says sea ice specialist Mark Serreze of NSIDC. Much of “the ice is so thin, it just can't survive the summer,” he says.

The climate models don't handle changing rules very well, but Serreze takes a shot at a date for an ice-free Arctic Ocean by adjusting the models' projections downward by folding in current trends in ice area and volume. “I'm on record [as of 2007] saying 2030 is a reasonable time for” ice-free conditions, he says. “Ice-free” is generally considered to be less than 1 million square kilometers of ice cover, most of it blown against Canada and Greenland. In that state, “you look at the Arctic from space,” Serreze says, “and it's a blue ocean.”

Others are looking for a blue Arctic Ocean even sooner. Maslowski and colleagues at NPS run a model simulating Arctic sea ice in more detail than the global climate models do. The model's decline of sea ice volume of late looks much like that from APL's model, but in a paper in the May 2012 issue of Annual Review of Earth and Planetary Sciences, Maslowski and colleagues go further and extrapolate recent trends to near-zero volume. Using “only ice extent is not sufficient if you believe volume can change much faster,” Maslowski says. Large uncertainties remain, he notes, but their extrapolation gives a date of 2016 for a nearly ice-free Arctic Ocean, with the end likely to come by the end of the decade.

Serreze and others think Maslowski's volume extrapolation exaggerates the problem. “It could happen [by 2016,” Serreze says. “I just don't think so. I think he's being too aggressive.” There is, however, a hint that enhanced pessimism may be appropriate. Stroeve is just back from a cruise to 83°N, beyond northern Greenland. She saw only 30% to 40% ice cover there. “We never expected that,” she says, because satellite data had not suggested it.