Diminishing Arctic sea ice endangering polar bears
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Global warming is having a significantly negative effect on polar bears, US and Canadian researchers said on Sept. 13. A new joint study by NASA and the Canadian Wildlife Service finds that Arctic sea ice in prime polar bear habitat is breaking up earlier, with direct impacts on the species' ability to survive and reproduce.
Sea ice provides polar bears with a vital platform from which to hunt seals and other marine mammals for food. The reduction in sea ice is forcing Arctic polar bears to fast for longer and longer periods, with adverse consequences for the species' survival.
The researchers say the findings, published in the September issue of the journal Artic, reject the claim that increased polar bear sightings by Arctic tourists indicate the species' population is increasing.
They studied the sea ice in regions that are home to five different polar bear populations: western Hudson Bay, eastern Hudson Bay, Foxe Basin, Baffin Bay and Davis Strait-Labrador Sea.
Increased human sightings of polar bears may not be a good sign for the species.
The findings suggests that progressively earlier breakup of the Arctic sea ice–stimulated by climate warming due largely to increased human emissions of greenhouse gases–shortens the spring hunting season for female polar bears in western Hudson Bay and is likely responsible for the continuing fall in the average weight of these bears.
As females become lighter, their ability to reproduce and the survival of their young decline.
"In 1980 the average weight of adult females in western Hudson Bay was 650 pounds," said study coauthor Ian Stirling, a senior scientist with the Canadian Wildlife Service. "Their average weight in 2004 was just 507 pounds–a 143-pound reduction."
A 1992 study in the Canadian Journal of Zoology indicated that no females weighing less than 416 pounds gave birth the following spring.
Furthermore, as the bears become thinner, they are more likely to push into human settlements for food, giving the impression that the population is increasing.
Sea ice is most scarce during the summer months, the researchers said, and this causes the bears to retreat to land and fast on their stored fat reserves until sea ice comes back in the fall.
"Our concern is that if the length of the sea ice season continues to decrease, then the polar bears will have shorter periods on the ice, when they can feed, and longer periods on the land, during the open-water season in summer and early fall," said study coauthor Claire Parkinson, a scientist at NASA's Goddard Space Flight Center.
"Their stored fat from life on the ice will likely not provide enough nourishment for the fasting period on land, posing a clear danger to their health and, in the long term, possibly to their species."
Sea-ice cover in the regions studied by Stirling and Parkinson has decreased since at least 1978, the beginning of consistent satellite monitoring. Two other studies released Wednesday by NASA scientists confirm that Arctic sea ice is melting at a dramatic rate, faster than at any time in the last three decades.
"By reviewing satellite data, we found that sea-ice cover break-up in western Hudson Bay took place about seven to eight days earlier per decade," said Stirling. "An extra month of fasting resulting from this phenomenon over four decades can significantly impact the polar bears' eating habits and survival."
In addition to monitoring sea-ice changes, the researchers incorporated data from previous polar bear studies in the same Arctic regions that also indicated the likelihood that progressively earlier break-up of sea ice was likely to cause problems for polar bears.
The research adds to growing concern about human impacts on the Arctic environment and on polar bears in particular.
Some 22,000 polar bears are believed to remain in the wild, spread across the Arctic.
Sources: ENS Photo courtesy Greenpeace