A polar bear walks in the Arctic National Wildlife Refuge. More than two-thirds of the world's polar bears will be killed off by 2050 - the species completely gone from Alaska - because of thinning sea ice from global warming in the Arctic, government scientists said Friday.

Washington ? Two-thirds of the world’s population of polar bears could die out over the next 50 years, as warmer weather melts away the thick sheets of sea ice where the bears spend much of their lives, according to studies released Friday by the U.S. Geological Survey.

The studies were intended to inform the U.S. Fish and Wildlife Service, which is now considering whether polar bears should be protected as a “threatened” species. Their findings sketched out a bleak prognosis for the world’s largest land carnivore, whose dependence on fast-eroding ice shelves has already made it a symbol of climate change’s cost.

The studies used computer models to estimate how sea ice would form and melt across the Arctic in the coming decades. They concluded that, as temperatures rise, large areas once covered by ice would become open water, at least in summer.

That would be a crucial blow to the world’s roughly 24,500 polar bears, which venture out onto the ice to hunt seals, their primary food. By 2050 or so, the studies estimated, about 42 percent of “optimal polar bear habitat” could be lost – a decline that could trigger a major drop in the population.

“Polar bears are dependent on the sea ice for just about all of their life history,” said George Durner, a research zoologist at the geological survey’s Alaska Science Center, in an interview last week. “When polar bears are removed from that environment … it removes them from the food source that they’re adapted to.”

Scientists have already begun to notice problems in certain sub-populations of polar bears, including a population drop in the area of Canada’s Hudson Bay and incidents of cannibalism in the Beaufort Sea, near Alaska. In both places, sea ice has declined in recent years.

The Fish and Wildlife Service intends to make a decision about listing polar bears as “threatened” by January. Environmentalists hope that, if the bears are listed, that will push the government to impose limits on the “greenhouse gas” pollution blamed for climate change.