By Deborah Zabarenko, Environment Correspondent Fri Sep 7, 6:27 PM ET WASHINGTON (Reuters) - Two-thirds of the world's polar bear population could be gone by midcentury if predictions of melting sea ice hold true, the U.S. Geological Survey reported on Friday. The fate of polar bears could be even bleaker than that estimate, because sea ice in the Arctic might be vanishing faster than the available computer models predict, the geological survey said in a report aimed at determining whether the big white bear should be listed as a threatened species. "There is a definite link between changes in the sea ice and the welfare of polar bears," said Steve Amstrup, who led the research team. Arctic sea ice is already at an all-time low this year and is expected to retreat farther this month, according to the U.S. National Snow and Ice Data Center. That means that polar bears -- some 16,000 of them -- will disappear by 2050 from parts of the Arctic where sea ice is melting most rapidly, along the north coasts of Alaska and Russia, researchers said in a telephone briefing. Other polar bear populations could survive beyond that date but many of those could be gone by 2100, Amstrup said. By century's end, the only polar bears left might live in the Canadian Arctic islands and along the west coast of Greenland. "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," the report's executive summary said. "Because the observed trajectory of Arctic sea ice decline appears to be underestimated by currently available models, this assessment of future polar bear status may be conservative." ARE POLAR BEARS 'THREATENED'? In January, the U.S. Fish and Wildlife Service proposed listing the polar bear as a threatened species under the Endangered Species Act, noting polar bears depended on sea ice as a platform to hunt seals, their main prey. The research released on Friday was sent to the Fish and Wildlife Service. A decision on the bears' status is expected in January. Without enough sea ice, polar bears would be forced onto land, but they are inefficient hunters once they get out of the water and ice, the researchers said. The bears' disappearance would probably take place as young cubs failed to survive to adulthood and females were unable to reproduce successfully. The first polar bears probably first appeared about 40,000 to 50,000 years ago, and the species has not lived through a period as warm as the one predicted by the U.N. Intergovernmental Panel on Climate Change, the scientists said. In a series of reports this year, the U.N. climate panel said with 90 percent probability that global climate change was occurring and that human activities contributed to it. The emission of greenhouse gases -- including carbon dioxide from petroleum-fueled vehicles and coal-fired power plants -- is the prime human cause of this warming trend, the panel said. Global warming was an important topic of discussions of the Asia Pacific Economic Cooperation forum this week in Australia and will be the subject of a special U.N. meeting later this month.