A polar bear trudges across ice in northern Alaska. Because of climate change, sea ice is thinning and moving faster. Polar bears have to burn more calories to keep pace with the ice, a new study says. (Mike Lockhart/Associated Press)

Rapid global warming has sped up the movement of sea ice off Alaska’s coasts, and already- at-risk polar bears are paying a price, a new U.S. study says.

Most sea ice moves throughout the year, and the white bears must continuously walk to stay within their preferred habitat, said U.S. Geological Survey research ecologist George Durner, lead author of the study.

He compares it to living on a treadmill that has picked up speed because ice is thinner, more brittle and moving faster because of wind and ocean currents.

Polar bears were listed as threatened under the Endangered Species Act in 2008 amid the alarming loss of summer sea ice in recent decades and climate models indicating the trend would persist. However, the U.S. government said the act would not be used to regulate greenhouse gas emissions.

The U.S. polar bear recovery plan says that without action to address climate change — the primary cause of thinning sea ice — it is unlikely that polar bears could be saved. The plan, released in January, noted positive signs, such as emission goals in the Paris climate agreement.

President Donald Trump, however, announced this month that the United States will withdraw from the international deal, arguing that it had disadvantaged the nation to the “benefit of other countries,” leaving U.S. businesses and taxpayers to be responsible for the cost.

Trump also has called for more petroleum development in polar bear habitat, including offshore Alaska waters and parts of the National Petroleum Reserve-Alaska and the Arctic National Wildlife Refuge.

Margaret Williams, Arctic program managing director for the World Wildlife Fund, called withdrawal from the Paris agreement frightening but said that private, state and local efforts to reduce carbon emissions and turn to renewable energy are a positive sign.

“There’s still a lot of hope,” she said.

The political moves come as the Geological Survey and University of Wyoming study documented how polar bears use more energy to stay within their home range.

Researchers made a conservative estimate that bears will have to kill one to three more seals a year to make up for faster-moving ice. The study did not address whether they walk faster or spend more time moving to keep up with the ice.

Killing more seals is a challenge for bears already facing fewer chances to hunt.

Durner compared their situation to people living in a town hit by both higher prices and fewer jobs.

“For the polar bear, the Arctic is becoming more expensive to live in,” he said.

Polar bears can go for extended periods without eating but then gorge on ringed and bearded seals. Ringed seals give birth on sea ice in spring and early summer, and polar bears sniff out their lairs.

Thinning sea ice has reduced access to prey in the southern Beaufort Sea, off Alaska’s north coast. Researchers have documented the polar bear’s declining body condition, reproduction and survival rate.

In the new study, researchers looked at data on female polar bears in the Beaufort Sea and Chukchi Sea, off Alaska’s northwest coast, during two periods: 1987 to 1998 and 1999 to 2013.

Sea ice changed in amount, thickness and composition in the second period as melting seasons lengthened.

Bears in the Chukchi Sea had to walk farther and burn more calories than south Beaufort bears but are in better shape because more food is available, Durner said.

Amy Cutting, animal curator at the Oregon Zoo, said in an email that the paper is a key piece in the growing research examining challenges faced by polar bears.

With decades of location and movement data about bears from radio collars, the Geological Survey is able to show effects of changes in sea ice, she said.

“This kind of research is critical to understanding the specific mechanisms through which climate change is impacting polar bear biology,” Cutting said.