The arctic ice pack is not only shrinking in area but rapidly thinning as well, according to reports in tomorrow's Science and the 15 December issue of Geophysical Research Letters. The big question now is what's causing the shrinkage. If it's natural fluctuations in polar climate, then the loss of arctic ice should eventually reverse. But if global warming from greenhouse gases is at fault, the entire ice pack will eventually disappear--with drastic climate implications for the Northern Hemisphere.
The idea of a shrinking arctic ice pack is not new. By combining satellite observations and historical records of ice extent, various groups had found that the area covered by the ice in the summer has been decreasing by about 3% per decade during recent decades--not an alarming degree of shrinkage notes polar climate researcher Douglas Martinson of the Lamont-Doherty Earth Observatory in Palisades, New York, since at that rate it would take another 350 years for the Arctic Ocean to be ice-free in summer.
But polar researcher Ola Johannessen of the Nansen Environmental and Remote Sensing Center in Bergen, Norway, and his colleagues report in Science that the arctic ice is undergoing a much more rapid change. By compiling and analyzing satellite observations of microwave emissions from ice, which vary depending on the age of the ice, Johannessen and his colleagues found that the area of older ice had declined by 7% per decade during the last 20-years--twice the rate at which the total ice area has been shrinking.
Ice thickness seems to be dropping even faster. Oceanographers from the University of Washington, Seattle, compared measurements of polar ice thickness made by U.S. navy submarines from 1958 to 1976 with those made in 1993, 1996, and 1997. The ice over the deep-water Arctic thinned from an average of about 3.1 meters to about 1.8 meters, or about 15% per decade, the researchers report in Geophysical Research Letters. Overall, says team-member Andrew Rothrock, the arctic ice has lost 40% of its volume in less than 3 decades.
Why is the ice is thinning? Most fingers are pointing at the Arctic Oscillation (AO), an erratic atmospheric seesaw that can change wind patterns and thus affect ice thickness, notes Rothrock. "I would lean toward the view that this is a fairly extreme state [of the AO], and it will likely come back toward more normal conditions," he says.
However, climate researcher Konstantin Vinnikov of the University of Maryland, College Park, and his colleagues suggest in another report in tomorrow's Science that increasing levels of greenhouse gases may be the prime mover behind the shrinking arctic sea ice. The team compared the observed ice loss and the ice loss in two climate models simulating the strengthening greenhouse. The chances that the losses seen since 1953 are just an extreme of a natural cycle and will swing back toward normal are less than 0.1%, they say.
If the thinning continues at the current rate, notes polar researcher John Walsh of the University of Illinois, Urbana-Champaign, "there really are only a few decades before ice thickness reaches zero." That would convert the Arctic Ocean from a brilliantly white reflector sending 80% of solar energy back into space into a heat collector absorbing 80% of incident sunlight, with effects on ocean and atmospheric circulation extending into mid-latitudes. These could include shifts in storm tracks, says Walsh, with changes in precipitation.