Prolonged ice-free periods putting Hudson Bay polar bear population at risk of extinction: Study

A segment of the polar bear population is at risk of extinction should ice-free periods continue to get longer, researchers have determined using analysis of the sea ice as well as the health of the bears themselves.

Ice-free periods in Canada's Hudson Bay have been lengthening over the past few decades, scientists say. Global warming is projected to extend the ice-free periods even more, reducing the ability for the region's resident polar bear population to hunt, putting them at risk of extinction if greenhouse gas emissions continue at projected levels, a study published in Communications Earth & Environment on Thursday found.

The Hudson Bay is historically one of the few places in the Arctic where polar bears have routinely come ashore when all of the sea ice has melted, Geoff York, senior director of research and policy of Polar Bears International and co-author of the paper, told ABC News.

What used to be an ice-free period of about four months has now extended far beyond, to the point that the polar bears in the southern and western Hudson Bay have been stretched to the point of genetic adaptation and the length of time they can fast, York said. Today's Hudson Bay polar bears are spending five weeks or longer on land than their grandparents did, York said.

Collar cameras strapped to Hudson Bay polar bears showed that the longer extents of ice-free periods are putting the Hudson Bay polar bears at risk of starvation, according to a paper published in Nature earlier this year.

Polar bears that were forced to find food on land -- by foraging on berries and eating birds -- lost about the same amount of weight as the bears that simply fasted, proving that to these bears, food on land had nowhere near the same amount of nutrients as seals, which are full of fat and blubber, the researchers found.

"Their primary food sources require them to be out on the sea ice," Alex Crawford, assistant professor at the University of Manitoba's Department of Environment and Geography and co-author of the paper, told ABC News. "So that means that if they are spending [a] longer time on land in the summer, they are spending a longer amount of time effectively fasting."

The early disappearance of sea ice, now in the late spring and early months, occurs at the same time that pups are being born and when mother bears put on most of their fat that will get them through the winter, York said. Putting them on shore early puts them in a precarious body condition for the upcoming cold months.

"Females are going to be less successful taking a pregnancy to term," York said. "Even if they do carry it to term, they'll be less successful in raising those cubs in the first year."

Between 1.6 degrees Celsius and 2.6 degrees Celsius of warming since pre-Industrial times is the range in which the Hudson Bay polar bears are expected to go extinct, Crawford said.

"That low end, we're knocking on the door of that already," Crawford said.

The paper brings together the latest climate models with two components that greatly affect polar bear survivability -- snow depth and ice thickness -- for the first time, York said. Ringed seals, polar bears' primary source of prey, need adequate snow depth to successfully den and protect their young in the springtime.

Even when accounting for bias in the climate models, the paper's findings were still grim, Crawford said.

Predictions made in the early ‘90s about warming impacts on polar bears are happening sooner than expected, York said.

Researchers have observed "dramatic" drops in the Hudson Bay subpopulation of up to 26% in the last decade, York said. There are only about half the number of polar bears in the population than 40 years ago.

"That decline is already kind of in progress, and if anything, I think now we'll see it likely accelerating," York said.

There is a chance for the sea ice to recover drastically, Crawford said.

If greenhouse gas emissions were to stop altogether, the extent of the sea ice could reverse in just a matter of years, Crawford said.