Canada's High Arctic could become the last stable refuge for polar bears as climate change melts away their hunting grounds, a U.S. government report says.

Populations elsewhere — in Alaska, Russia, Norway and around Hudson Bay, northern Quebec and Newfoundland and Labrador — are likely to decrease or greatly decrease by the year 2050 as global temperatures rise, the report projects.

But under a moderate scenario for greenhouse gas emissions, with enough reductions worldwide to keep the average global temperature hike to no more than two degrees, the polar bear population in northern Nunavut is most likely to remain stable and even has a decent chance of increasing, researchers say.

The 124-page research report comes from the U.S. Geological Survey, an entity of the U.S. Department of the Interior.

It looks at polar bear populations in four "eco regions," including an area known as the Canadian Arctic Archipelago, home to perhaps 5,000 or more of the animals — about a quarter of the global total.

The archipelago has the best "potential to serve as a long-term refugium" for polar bears, the authors say.

But even then, if countries continue with "business as usual" and nothing is done to curb the world's greenhouse gas emissions, the long-term viability of polar bears would be in doubt.

Sea ice essential

Polar bear populations are thought to be sensitive to global warming mainly because the animals spend the winter and spring on sea ice hunting for fatty seals as well as mating and giving birth.

When the ice retreats in the summer, the bears are forced onto land. But land-based food can't satisfy their dietary needs.

"The terrestrial resources are just not sufficient. It's the difference between eating fat and eating a few berries," said Andrew Derocher, a polar bear expert and professor at the University of Alberta, who wasn't involved in the U.S. government report.

Polar bear with dead seal

A polar bear drags a seal along a floe in Baffin Bay, above the Arctic Circle in Canada's North. The bears need sea ice to hunt seals, their main source of food. (Jonathan Hayward/Canadian Press)

"The whole fate of polar bears depends on how fast the sea ice disappears."

Scientists have warned for years that climate change threatens polar bear populations. The U.S. Geological Survey study compares that risk against others like oil and gas shipping through the North, pollution and hunting of the bears, which is legal in Canada, the U.S. and Greenland.

It concludes that sea ice loss is the greatest menace to their survival, by a significant margin.

And it says about a third of the world's polar bears — those in Alaska, Russia and Norway — could be in imminent danger from greenhouse gas emissions in as soon as a decade. Those areas of the Arctic have suffered some of the most dramatic declines in sea ice.

The scientists saw no rebound in overall population numbers in the projections that stretched to the year 2100 under either of the two scenarios they looked at: one in which greenhouse gas emissions stabilized, and the other in which they continued unabated.

"Polar bears are in big trouble," said Rebecca Noblin, Alaska director for the Center for Biological Diversity. "There are other steps we can take to slow the decline of polar bears, but in the long run, the only way to save polar bears in the Arctic is to reduce greenhouse gas emissions."

Other marine animals at risk

Polar bears aren't the only marine species at risk from climate change.

In separate research released this week, an international team of scientists looked at the effects on sea creatures, concluding that under the "business as usual" scenario of unchecked greenhouse gas emissions, "most marine organisms evaluated will have very high risk of impacts."

The effects will be felt "across all latitudes," the authors write, "making this a global concern beyond the north/south divide."

As more greenhouse gases enter the atmosphere, oceans will warm and become more acidic, says the study, published in the journal Science.

Fish will have to find new habitats in cooler waters. Warm-water corals and sea grasses at mid-latitudes are already being affected.

Even if the world commits and sticks to the most stringent of the proposed emissions targets, creatures like mussels, oysters, clams and scallops "will be at high risk" by the year 2100, the scientists say.

"All the species and services we get from the ocean will be impacted and everyone, including Canadians, who benefit from these goods and services are vulnerable," said William Cheung, a co-author of the paper and an associate professor at the University of British Columbia's fisheries centre.

With files from The Associated Press