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. 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. USGS projects that polar bears during the next half-century will disappear along the north coasts of Alaska and Russia and lose 42 percent of the Arctic range they need to live in during summer in the Polar Basin when they hunt and breed. A polar bear's life usually lasts about 30 years. "Projected changes in future sea ice conditions, if realized, will result in loss of approximately two-thirds of the world's current polar bear population by the mid 21st century," said the report released Friday.