"There's a limit to how long they can fast," Amstrup said.
In other regions polar bears live on the sea ice year-round.
These bears prefer to hunt from the ice that hangs over shallow continental shelf waters, which contain more prey than deeper waters offshore.
But as the ice retreats farther from the shoreline, the bears may have less success with their hunts (related: "Arctic Ice at All-Time Low" [August 20, 2007]).
In addition, the rapid movement of sea ice in these regions may leave some bears stranded in food-poor areas, or it might require long and exhausting swims from food-rich areas back to ice, Amstrup noted.
By mid-century polar bears will likely be eliminated from the southern end of their range and in the Polar Basin, where sea ice is moving farther from land.
These two areas are currently home to approximately two-thirds of the world's polar bear population, Amstrup said.
Polar bears are only predicted to hang on in the Canadian Arctic islands and some regions along the northern coast of Greenland where sea ice is expected to persist even in summer (see map).
The USGS findings are based on the best available models of future climate, Amstrup said.
Warming temperatures, however, are melting the sea ice quicker than even the best climate models projected, according to the National Snow and Ice Data Center in Boulder, Colorado.
As of September 3, 2007, sea-ice extent had fallen to 1.70 million square miles (4.42 million square kilometers)—beating the previous record low of 2.05 million square miles (5.32 million square kilometers) set on September 21, 2005.
The United Nations Intergovernmental Panel on Climate Change earlier this year determined that the ice-melting trend is "very likely" being caused by human emissions of greenhouse gases, Amstrup noted.
So hopes of long-term polar bear survival hinges on humans taking action now, he said.
Atmospheric scientists believe that gases already in the climate system will continue to warm the Earth and melt sea ice until about 2050, even if cuts to emissions are made today.
But if polar bears do persist through the end of the century in the Canadian Arctic islands, those bears could seed a rebounding population.
"Emissions reductions now could assure polar bears will still be around to repopulate sea ice when it returns to other areas," Amstrup said in a followup email.
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