April 7, 2013 5:05 PM
Last September, at the end of the northern hemisphere summer, the Arctic Ocean's icy cover shrank to its lowest extent on record, continuing a long-term trend and diminishing to about half the size of the average summertime extent from 1979 to 2000.
During the cold and dark of Arctic winter, sea ice refreezes and achieves its maximum extent, usually in late February or early March. According to a NASA analysis, this year the annual maximum extent was reached on Feb. 28 and it was the fifth lowest sea ice winter extent in the past 35 years.
The new maximum —5.82 million square miles (15.09 million square kilometers)— is in line with a continuing trend in declining winter Arctic sea ice extent: nine of the ten smallest recorded maximums have occurred during the last decade. The 2013 winter extent is 144,402 square miles (374,000 square kilometers) below the average annual maximum extent for the last three decades.
"The Arctic region is in darkness during winter and the predominant type of radiation is long-wave or infrared, which is associated with greenhouse warming," said Joey Comiso, senior scientist at NASA Goddard Space Flight Center, Greenbelt, Md., and a principal investigator of NASA's Cryospheric Sciences Program. "A decline in the sea ice cover in winter is thus a manifestation of the effect of the increasing greenhouse gases on sea ice."
Satellite data retrieved since the late 1970s show that sea ice extent, which includes all areas of the Arctic Ocean where ice covers at least 15 percent of the ocean surface, is diminishing. This decline is occurring at a much faster pace in the summer than in the winter; in fact, some models predict that the Arctic Ocean could be ice-free in the summer in just a few decades.
The behavior of the winter sea ice maximum is not necessarily predictive of the following melt season. The record shows there are times when an unusually large maximum is followed by an unusually low minimum, and vice versa.
"You would think the two should be related, because if you have extensive maximum, that means you had an unusually cold winter and that the ice would have grown thicker than normal. And you would expect thicker ice to be more difficult to melt in the summer," Comiso said. "But it isn't as simple as that. You can have a lot of other forces that affect the ice cover in the summer, like the strong storm we got in August last year, which split a huge segment of ice that then got transported south to warmer waters, where it melted."
The NASA Goddard sea ice record is one of several analyses, along with those produced by the National Snow and Ice Data Center (NSIDC) in Boulder, Colo. The two institutions use slightly different methods in their sea ice tally, but overall, their trends show close agreement. NSIDC announced that Arctic sea ice reached its winter maximum on Mar. 15, at an extent of 5.84 million square miles (15.13 million square kilometers) -- a difference of less than half a percent compared to the NASA maximum extent
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