Polar Bear Migration: How Climate Change has Altered it

But today the Hudson Bay population—and the rest of the world's 20,000 to 25,000 polar bears—is being affected by changes. Since 1979, sea ice cover has declined by about 30 percent in the Arctic. As greenhouse gases continue to warm the Earth, Amstrup says, polar bears are being forced ashore for longer periods of time.

The bears of Hudson Bay, for instance, now spend an average of nearly 30 days longer on land than they did 30 years ago. Stirling and his colleague Andrew Derocher found that bears lose nearly two pounds of body weight each day they're on land—meaning the bears here are, in effect, 60 pounds lighter on average than they were three decades ago.

http://news.nationalgeographic.com/news/2013/11/131122-polar-bear-migration-in-hudson-bay-science/