The polar bear has become an icon of global warming. In 1973, the five "polar bear nations" (the United States, Canada, Denmark, Norway, and the former Soviet Union) agreed to limit polar bear hunting to the Inuit and other native peoples. The bear population recovered, only to face a new threat: global climate change.

Changes in sea ice, permafrost, and prey species in the Arctic and subarctic are linked to reduced body condition and smaller litter size in polar bears. In western Hudson Bay, long-term studies conducted by wildlife biologist Ian Stirling of the Canadian Wildlife Service and his colleagues document climate warming and a significant reduction in the amount, location, and persistence of sea ice adjacent to the shore. The polar bear is heavily dependent on the ringed seal as a food source.

Its usual hunting technique requires sea ice. A bear will locate the breathing hole of a seal and then ambush the animal when it comes up for air or will prey upon young seals in their dens. As sea ice diminishes, bears are less able to catch their prey. Hungry polar bears waiting on shore for sea ice to form have become a problem, invading northern villages and encountering native hunters. At one time, this increase in sightings was interpreted as evidence of increasing numbers and used to justify higher quotas for the taking of polar bears.

A series of studies requested by U.S. secretary of the interior Dirk Kempthorne, coordinated by the United States Geological Survey, and released on September 7, 2007, predicted that two-thirds of polar bears would perish by the year 2050. All polar bears in Alaska would be gone. The studies-- conducted by American and Canadian scientists-- used conservative assumptions to project loss of sea ice due to global warming. At the time of the studies, there were an estimated twenty thousand to twenty-five thousand polar bears worldwide.

Because the polar bear, Ursus maritimus, is thought to have evolved from the brown bear, Ursus arctos, 200,000 years ago, some have suggested that the polar bear might reverse course and adapt to and live in the terrestrial ecological niche now occupied by the brown bear. Brown bears are primarily vegetarian, whereas polar bears are primarily carnivorous. Brown bear claws are adapted for digging, polar bear claws for grabbing and holding prey. Polar bears are slow to reproduce, and it is unlikely that they could change both their behavior and their physical characteristics quickly enough to cope with disappearing sea ice. Sea ice is central to the life of a polar bear. It provides habitat to prey animals and hunting, mating, and denning sites for the bears themselves. In western Hudson Bay, polar bears gather on the shore each year to wait for sea ice to form (Stirling, 1999).

While polar bears and their response to global warming have been studied longer and in greater depth than has any other Arctic species, they are far from the only animal affected by a changing climate. Gray whales migrating south to their breeding grounds off the coast of Mexico have recently appeared to be emaciated, suggesting a shortage of their normal diet of tube worms and amphipods in their summer feeding grounds of the Bering and Chukchi Seas.

Other marine animals that are affected by the loss of sea ice and climate warming in the Arctic include ringed seals, bearded seals, ribbon seals, sea lions, walruses, narwhals, fish, and many sea birds (Kazlowski, 2008). Biologists have only begun to study how these creatures will respond to the profound changes in their habitat. Polar bears--known to eat dead whales washed up on the shore as well as other carrion-- might benefit temporarily from the die-off of marine animals, but will suffer eventually as their prey species are reduced.

On May 14, 2008, Secretary Kempthorne announced the listing of the polar bear as threatened under the provisions of the Endangered Species Act. This designation is one step down from that of endangered species and offers less protection from human activities. Kempthorne made clear that the Department of the Interior would not allow the polar bear's plight to be used to justify limits on greenhouse gases (GHGs) saying: "When the Endangered Species Act was adopted in 1973, I don't think terms like 'climate change' were part of our vernacular." The act, he said, "is not the instrument that's going to be effective" to deal with climate change.

Images of polar bears have been used by conservation groups to advocate reductions in GHGs in hopes of slowing and perhaps reversing global warming and also in fund-raising by these groups. Global warming skeptics complain that such images appeal to emotion and not to reason and demand further proof that a warming climate will result in great reduction and even extinction of polar bear populations by mid-century.

References

1. Kazlowski, Steve. The Last Polar Bear: Facing the Truth of a Warming World. Seattle, Wash.: Mountaineers Books, 2008.

2. Stirling, Ian. Polar Bears. Ann Arbor: University of Michigan Press, 1999.

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