Polar Bears

Polar bears have been around in the Arctic for at least the past 200,000 years, but recent trends and events have made many wonder about the bear's future in a warming world. A study of the best observed population of polar bears, in Hudson Bay, Canada (Stirling and Parkinson, 2006), found polar bears are getting smaller, females are having fewer cubs, survival rate for cubs is declining, and adult bears are not reaching the sizes they were previously. Scientists attribute all of these changes in the polar bear population to nutritional stress brought on by the disappearance of sea ice in a warming Arctic climate.
Figure 1. Female polar bear and two cubs, Guillemot Island, Ukkusiksalik National Park, Nunavut, Canada (Photo taken by Ansgar Walk)

Polar bears are highly dependent upon sea ice primarily for hunting. Seals, walruses, and whales--the bear's main prey items--can all be found on or near sea ice. In fact, polar bears live almost entirely off of the seal and walrus calves that they find at the sea ice edge. When the sea ice melts during the summer, polar bears live off their fat reserves until the sea ice begins expanding in the fall and they can hunt again. Fall is also the time when pregnant females are preparing to retreat to dens on or near sea ice to give birth. During this time, they eat so much as to double their size. They don't emerge from their dens until the following spring after the cubs have been born. Record low sea ice extents combined with longer summers and shorter winters force the bear to fast longer and makes hunting more difficult.

The polar bear is not adapted to hunting land animals for a number of reasons, one of which is polar bears simply can't keep up with them. The bears may be good swimmers, but they are slow runners and overheat very quickly. Additionally, land animals do not provide nearly the same amount of nutrients that the bears can get from marine animals. Polar bears will actually follow the sea ice hundreds of miles as it retreats during the summer so that they can keep hunting seals.

Melting sea ice not only threatens the bear's hunting grounds, but its primary food source as well--seals. Populations of seals are also coming under pressure from the warming climate because they, like the polar bears, are dependent upon sea ice. Icebergs and ice floes provide seals with places to bear and raise their young, feed, and rest. Monnett and Gleason (2006) reported that during an aerial survey in September 2004, four polar bear carcasses were seen floating in open water and had, presumably, drowned. The authors stated: We speculate that moralities due to offshore swimming during late-ice (or mild ice) years may be an important and unaccounted source of natural mortality given energetic demands placed on individual bears engaged in long-distance swimming. We further suggest that drowning-related deaths of polar bears may increase in the future if the observed trend of regression of pack ice and/or longer open water periods continues..

Polar Bears and the last interglacial period:
Weren't polar bears able to make it the last time the Arctic climate was this warm?

During the last interglacial period, which ended 125,000 years ago, the forerunners of the modern polar bear, Ursus maritimus, were more similar to the brown bear (grizzly bear) than the polar bear of today, and likely had no trouble finding food in the birch forests that used to reach the Arctic Circle. They only started developing those traits that we associate with the modern polar bear when a warming climate stranded a small group of brown bears on glaciers in Siberia or Alaska. From that point on, they underwent rapid evolutionary changes in order to adapt to the harsh environment of the Arctic--an environment which has only gotten colder since their arrival. Their coat color changed from brown to white or yellow so it could blend in with its icy surroundings, they developed a layer of blubber in addition to thick fur help insulate them from frigid air and water temperatures, their ears and tails became shorter to guard against heat loss, their heads became smaller while their bodies grew longer to streamline them for quick swimming, and they developed sharp teeth for hunting. All of these differentiate the polar bear from its mostly vegetarian brown bear relations.

The polar bear of the last interglacial period is not the polar bear of today. It didn't "survive" the last interglacial period so much as it was created by it. Whether today's top Arctic carnivore can survive a warming period with significantly reduced or no summer sea ice must be determined by looking at the bears' current physiology, habitat needs, etc. Many polar bear scientists believe that the polar bear will be able to evolve to survive in a warmer Arctic climate. In fact, scientists and Arctic residents have noticed some changes in polar bear behavior. For example, the bears are spending more time on land both for denning and finding food. Some bears have adapted by foraging for food on beaches and in town garbage bins. However, many wonder if the polar bear will be able to keep up with the unprecedented rapid pace at which the Arctic is warming.

The polar bear already enjoys protection under international agreements and is listed as threatened or endangered internationally. Due to concerns about the impact of continued changes in climate in the region on polar bear populations, the U.S. government recently received a proposal to add the polar bear to its Endangered Species List. A decision will be announced in January of 2008.

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Monnett, C., and J.S. Gleason, "Observations of mortality associated with extended open-water swimming by polar bears in the Alaskan Beaufort Sea", Polar Biology Volume 29, Number 8, July 2006.

Stirling, I., and C.L. Parkinson, "Possible effects of climate warming on selected populations of polar bears (Ursus maritimus) in the Canadian Arctic," Arctic, v. 59, no. 3, Sept. 2006, p. 261-275.

Further Reading

Arctic Climate Impact Assessment
Wikipedia: Polar Bears
Polar Bear International
USGS news release, Future Retreat of Arctic Sea Ice Will Lower Polar Bear Populations and Limit Their Distribution
USGS (2006), Polar Bear Population Status in the Southern Beaufort Sea
Arctic Climate Impact Assessment
Wikipedia: Polar Bears
Polar Bear International

USGS publications regarding polar bears
Specialist Group of the IUCN Species Survival Commission
NSIDC All About Sea Ice: Polar bears
US House Committee on Science and Technology hearing, Disappearing Polar Bears and Permafrost: Is a Global Warming Tipping Point Embedded in the Ice?