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But these bears are in trouble. Hudson Bay polar bears, some of the southernmost in the world, are feeling the heat of global warming. The region is about two degrees (F) warmer in winter than 50 years ago. The bay's ice is breaking up in early July rather than late July, says Nick Lunn of the Canadian Wildlife Service. The earlier the ice disappears, the less time bears have to feed on seals. Pregnant females need to gain at least 200 pounds to sustain them through the long fast in their dens, where they may spend eight months. In the past they've been able to kill many seal pups being weaned by their mothers in early July. Now, with the ice melting sooner, the bears can't hunt and must forsake that nutrition. Such deprivation leads to fewer cubs surviving to adulthood.

"For every week that the ice is breaking up earlier, the bears are coming ashore more than 20 pounds lighter," says Lunn. He and his colleagues—including Derocher and Ian Stirling of the Canadian Wildlife Service—estimate that the bears are 15 percent smaller than they were 20 years ago. And Lunn says the latest census shows that in just ten years the bears' population has declined from 1,200 to less than 1,000. It doesn't seem like a temporary drop. "Until recently the numbers have been stable. The last two censuses, in the mid-1980s and mid-1990s, both showed 1,200 bears," says Lunn.

By 2050, southern Hudson Bay may be ice free year-round. Will Wapusk's bears move north? "There are already 2,000 polar bears in northern Hudson Bay," Lunn says. "The area probably can't take any more. The environmental changes in Hudson Bay are happening so fast that one day the bay may no longer be able to support polar bears."

Wapusk's bears will have to stay ashore longer and longer, marooned in the park created for them, watching and waiting for the ice. To the north the rest of the world's polar bears, about 25,000 of them, roam the Arctic. As the climate warms, what about them?

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