April 4, 2013
Long-term Trend Of Diminishing Sea Ice In The Arctic
[ Watch the Video: An Interesting Year for Arctic Sea Ice ]
April Flowers for redOrbit.com - Your Universe Online
The Arctic Ocean's ice cover shrank to its lowest extent on record last September at the end of the northern hemisphere summer. This continues a long-term trend diminishing the ice to about half the size of the average summertime extent from 1979 to 2000.
The sea ice refreezes during the cold and dark Arctic winter, achieving its maximum extent usually in late February or early March. This year, according to NASA analysis, the annual maximum extent was reached on Feb. 28. It was the fifth lowest sea ice winter extent in 35 years.
The new maximum is 5.82 million square miles, which is in line with a continuing trend in declining winter Arctic sea ice extent. Nine out of the ten smallest maximums recorded have happened during the last ten years. The 2013 winter extent is 144,402 square miles below the average annual maximum extent for the last 30 years.
"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, 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."
Since the late 1970s, satellite data has shown 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. Summer decline is happening at a much faster rate than winter, with some models predicting that the Arctic Ocean could be ice-free in the summer in just a few decades.
The following melt season cannot always be predicted by the behavior of the winter sea ice maximum. There are times in the record 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 sea ice record kept by Goddard is just one of several analyses. The National Snow and Ice Data Center (NSIDC) produces another. Though the two institutions use slightly different methods to construct their sea ice tallies, their overall trends show close agreement. For example, NSIDC announced that the winter sea ice maximum had been reached on Mar. 15, at an extent of 5.84 million square miles. Compared to the NASA tally, this was a difference of less than half a percent.
Sea ice "area" is another measurement that allows scientists to analyze the evolution of the sea ice maximum. Area measurements discard regions of open water among ice floes, as opposed to extent measurements, and only tally the parts of the Arctic Ocean that are completely covered in ice. For 2013, the winter maximum area was 5.53 million square miles. This was also the fifth lowest on record since 1979.
Although the winter sea ice extent's downward trend has been at a less drastic rate than summer sea ice, the fraction of the sea ice cover that has survived at least two melt seasons remains much smaller than at the beginning of the satellite era. This "multi-year" ice, which is older, thicker and buttresses the ice cap against more severe melting in the summer grew slightly this past winter. It now covers 1.03 million square miles, which is about 39,000 square miles more than last winter. Even so, its extent is less than half what it was in the early 1980s.
"I think the multi-year ice cover will continue to decline in the upcoming years," Comiso said. "There´s a little bit of oscillation, so there still might be a small gain in some years, but it continues to go down and before you know it we´ll lose the multi-year ice altogether."
The negative phase of the Arctic Oscillation kept temperatures warmer than average this winter in the northernmost latitudes. In an area of seasonal ice, large cracks were opened in the ice cover on the Beaufort Sea along the northern coasts of Alaska and Canada by a series of storms this February and early March. Those cracks froze over again quickly, but the new layers of thin ice covering them might melt again now that the sun has re-appeared in the Arctic. This could split the ice pack into smaller ice floes.
"If you put a large chunk of ice in a glass of water, it is going to melt slowly, but if you break up the ice into small pieces, it will melt faster," said Nathan Kurtz, a sea ice scientist at NASA Goddard. "If the ice pack breaks up like that and the melt season begins with smaller-sized floes, that could impact melt."
Data collected by NASA's Operation IceBridge, an airborne mission currently surveying Arctic sea ice and the Greenland ice sheet, will be analyzed by Kurtz to see if the sea ice in the cracked areas is abnormally thin.