Polar bear numbers in parts of Alaska and Canada have declined by almost half, as thinning sea ice makes it increasingly difficult for them to hunt down seals, which are a key part of their diet.

Scientists, led by researchers at the U.S. Geological Survey, found polar bear numbers in the southern Beaufort Sea dropped about 40 percent, to 900, from 2000 to 2010. Survival rates were especially dire from 2004 to 2006 and they began to recover in 2007 and stabilized two years later. Why survival improved at the end of the study is unknown.

"Of the 80 cubs observed in Alaska from 2004 to 2007, only two are known to have survived," said Jeff Bromaghin, USGS research statistician and lead author of the study in Ecological Applications which also included researchers from Environment Canada, University of Alberta, U.S. Fish and Wildlife Service, Polar Bears International, and Western Ecosystems Technology.

The plight of polar bears has become a potent symbol of how climate change is altering the world's ecosystem. Polar bears were listed as threatened under the Endangered Species Act in 2008 due to concerns about the effects of sea ice loss on their populations.

The Polar Bear Specialists' Group of the International Union for the Conservation of Nature will use the new estimate for the southern Beaufort Sea population to track historic and current trends in the 19 populations worldwide. Currently, four populations, including the southern Beaufort Sea population, are considered to be declining, five are stable, one is increasing, with the remainder considered to be data deficient.

The impacts are clearly visible in the Arctic, where sea ice has been on the decline for decades. As a result, polar bears are finding it increasingly difficult to gain access to seals during both summer and winter months.

Scientists said most bears stay with the sea ice as it retreats north into the Arctic Basin and far from shore, where most seals are thought to occur. At the same time, the mobile winter ice is susceptible to breaking up and rafting, which can create rough and jumbled ice conditions make it harder for polar bears to capture seals.

Margaret Williams, managing director for WWF's Arctic program in Alaska, said the latest polar bear study was a reminder the world needed to act to cut greenhouse gas emissions which are to blame for rising global temperatures.

"Here are concrete numbers to show us that the impacts of climate change are happening now," Williams said. "We know human activities have caused global wildlife populations to drop by over half in the last 40 years. We need to change course if we want to stop further habitat loss and ensure resilient wildlife populations, both in the Arctic and around the world."