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Slow Retreat of Sea Ice Lengthens Arctic Polar Bear's Fast
Many of us have watched documentary film clips or seen touching photos of mother polar bears walking on the icy surface of Arctic waters, at times with cubs in tow, hunting for food. For some time, scientists have suggested that the same icy Arctic surface -- sea ice -- is slowly retreating because of an ever-warming climate.

Image showing changes in sea ice that might be affecting polar bear populations in five regions. Image right: NASA has teamed with the Canadian Wildlife Service to examine how changes in sea ice might be affecting polar bear populations in five regions: Baffin Bay, Davis Strait, Foxe Basin, Western and Eastern Hudson Bay. Sea ice data for the following map come from the Defense Meteorological Satellite Program (DMSP) Special Sensor Microwave Imager (SSMI) for June 2000. Credit: NASA/Canadian Wildlife Service

Scientists from NASA and the Canadian Wildlife Service are now reporting that the slow reduction in sea ice is forcing Arctic polar bears to fast for longer and longer periods, posing danger to their survival.

Claire Parkinson, a senior scientist at NASA's Goddard Space Flight Center, and Ian Stirling, a research scientist specializing in polar marine mammals with the Canadian Wildlife Service in Edmonton, Alberta, also suggest that the progressively earlier breakup of sea ice, caused by climate warming, shortens the spring hunting season for female polar bears and is probably responsible for the observed weight loss in these bears. The lighter the females, the more their ability to reproduce, and ability for their young to survive declines.

As the bears become thinner, they will also have a greater tendency to seek alternative food villages and hunting camps, giving the impression to some that the population is increasing. The study will be published this week in the September issue of the Journal Arctic.

"A key result of our research is its strong suggestion that climate warming is having a significant and negative effect on a primary species reliant on the sea ice cover for survival," said Parkinson.

Photo of a polar bear Image left: Polar bear photograph courtesy of Robert Taylor

The researchers used NASA satellite observations captured over 26 years from 1979 to 2004 to show the reduction in sea ice cover in several specific areas. In most of the areas studied, they found that ice break-up in these areas has been occurring progressively earlier over time.

"Polar bears live much of their lives on the sea ice, which is fundamental for their survival, at least in terms of their traditional lifestyles. For instance, it’s the sea ice surface that provides them a platform from which to hunt seals and other marine mammals for food," said Parkinson. Sea ice is most scarce during the summer months, causing the bears to retreat to land and fast on their stored fat reserves until sea ice comes back in the fall.

"By reviewing satellite data, we found that sea ice cover break-up in Western Hudson Bay took place at 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."

The researchers' data also indicate the likelihood that progressively earlier breakup of the sea ice was likely to also cause reproductive problems for polar bears.

"In 1980 the average weight of adult females in western Hudson Bay was 650 pounds. Their average weight in 2004 was just 507 pounds – a 143-pound reduction," said Stirling. A 1992 study in the Canadian Journal of Zoology indicated that no females weighing less than 416 pounds gave birth the following spring.

Image that shows the August 5, 2005 - August 5, 2006 annual cycle of sea ice coverage

Image above: Polar bears depend on sea ice for survival. Climate warming in the Arctic has caused significant declines in overall Arctic sea ice cover and progressively earlier breakup in some areas, including western Hudson Bay. This visualization shows, twice, the August 5, 2005 - August 5, 2006 annual cycle of sea ice coverage as determined from data from the Aqua satellite's Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR-E). Click on image to view animation. Credit: NASA/JAXA

According to Stirling, if the climate continues to warm as projected by the Intergovernmental Panel on Climate Change and the ice continues to break up progressively earlier, it is likely that in another 20-30 years polar bear reproduction in western Hudson Bay will be significantly limited and may be repeated elsewhere.

Gretchen Cook-Anderson
NASA Goddard Space Flight Center