While there's still debate on how much human activity is contributing to global warming, the effects of the warming itself are starting to show up in studies of individual species. The latest report on this concerns the polar bears of the Beaufort Sea region of Alaska's northern coast.
Two years of study by Eric Regehr of the United States Geological Survey (USGS) indicate that warming has reduced the sea ice in Canada's Hudson Bay area (which is to the east of the Beaufort Sea coast but at a similar latitude), and contributed to a 22% decline in polar bear numbers. Polar bears spend much of their lives on the sea ice along the coast, hunting seals. A decline in the ice cover shrinks the polar bears' range, increasing the competition for the small number of seals frequenting an area. If the ice melts entirely, the bears are forced onto shore, where they are sometimes driven to invade garbage dumps and come in close contact with humans. Younger bears are likely to lose out in this more competitive and dangerous environment, and if fewer young animals survive, the population inevitably drops.
While it's not clear yet whether the population in Alaska has not shown the same effects, the Center for Biological Diversity, Natural Resources Defense Council, and Greenpeace have petitioned the U.S. Fish and Wildlife Service (USFWS) to take action to protect the Alaskan population.