Two-thirds of the world’s polar bears will be killed off by 2050 — and the entire population gone from Alaska — because of thinning sea ice from global warming in the Arctic, US government scientists forecast on Friday.

Only in the northern Canadian Arctic islands and the west coast of Greenland are any of the world's 16,000 polar bears expected to survive through the end of the century, said the US Geological Survey, which is the scientific arm of the interior department.