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Polar bears tread lightly on ever-thinning Arctic ice


September 15, 2006

The year-round ice cover of the Arctic Ocean has declined by nearly one-third during the summer months over the past 30 years, and a new study shows that even the maximum cover of winter ice has shrunk an area larger than Texas in just the past two years.

A second study, also using satellite data from NASA, shows that since the 1970s, ice cover in prime hunting areas for some Canadian polar bears has been breaking up earlier and earlier each summer, forcing the animals onto land and closer to native villages an average of three weeks sooner.



NASA scientists discussed both studies in a press briefing Wednesday.

"While a cold snap in August seems to have kept us from seeing quite the record level of melting we did last year, the amount of ice cover we're seeing in the Arctic just before the onset of winter is well below normal," said Mark Serreze, a senior research scientist at the National Snow and Ice Data Center in Boulder, Colo.

Thanks to recently declassified data from the Soviet and U.S. navies, researchers also know that the thickness of the perennial ice over the Arctic has declined from about 11 feet in the early 1960s to 7 feet or less today.

While ice does form over the remaining open water during the winter, that seasonal ice is only about 1 to 5 or 6 feet thick, and breaks up quickly.

That's bad news for marine life at all levels, said NASA researcher Joey Comiso, because the cycle of pack ice thawing and freezing is vital to the health of plankton, the bottom of the food chain that makes the Arctic and surrounding waters such rich fisheries.

At the top of the chain, polar bears use the year-round ice as a platform for hunting seals and other marine mammals during the winter.

Claire Parkinson, of NASA's Goddard Space Flight Center in Greenbelt, Md., and Ian Stirling of the Canadian Wildlife Service studied ice patterns between 1979 and 2004 to show the reductions in habitat for polar bears in key regions. Their report is being published this week in the journal Arctic.

"We found that sea-ice-cover breakup in western Hudson Bay took place about seven to eight days earlier per decade," Stirling said. "An extra month of fasting resulting from this phenomenon over four decades can significantly impact the polar bears' eating habits and survival."

The researchers also found that in the area with the earliest breakup of sea ice, in 1980 the average weight of adult females was 650 pounds; that average in 2004 was 507 pounds. Other studies indicate that females that weigh less than 416 pounds don't bear any surviving cubs.

Comiso said the amount of winter sea-ice decline he and colleagues at NASA's Jet Propulsion Laboratory in Pasadena, Calif., documented over the past two seasons "has not taken place before during the 27 years that satellite data has been available. This reduction of over 6 percent per year is most likely a result of warming due to greenhouse gases." That study was published in the Sept. 7 issue of the journal Geophysical Research Letters.

Computer simulations of the climate-warming effects of carbon dioxide and other emissions had predicted that winter sea ice would decline faster than ice during the summer. But the satellite data had shown otherwise until just the past two seasons.

"These findings serve to validate our understanding of the climate feedback that we've incorporated into the models," Comiso said.


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Ketchikan, Alaska