The population of polar bears in parts of Alaska and northwestern Canada has declined by about 40 percent in the first decade of the 2000s, a new study says.

The number of polar bears found near the southern Beaufort Sea dropped to 900 from 2000 to 2010, most drastically between 2004 and 2006. The population recovered in 2007 and began stabilizing in 2009, the study, conducted by researchers with the U.S. Geological Survey, found.

"Of the 80 cubs observed in Alaska from 2004 to 2007, only two are known to have survived," said lead author Jeff Bromaghin. "We basically had very poor cub survival and we saw very few subadult bears in the years since then."

It wasn't clear why the population improved toward the end of the decade.

Beaufort Sea is home to one of four polar bear populations considered to be on the decline among 19 worldwide. The species has been threatened by declining sea ice for decades.

"The ice is much different than it used to be," Bromaghin said. "We saw several cases of bears clawing through really thick chunks of ice to get to seals."

"Here are concrete numbers to show us that the impacts of climate change are happening now," Margaret Williams, managing director for World Wildlife Fund's Arctic program in Alaska, said in a statement in response to the study's findings.