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Canadian researchers predict 55 to 100 per cent of pregnant polar bears in western Hudson Bay will not have any cubs if the ice breaks up two months earlier than in the 1990s. ((Steve Amstrup/U.S. Fish and Wildlife Service/Associated Press))

Polar bears will have drastically fewer cubs and more miscarriages as the ice melts on Hudson Bay, Alberta researchers predict.

In fact, if the ice melts one month earlier than in the 1990s, 40 to 73 per cent of pregnant female bears living on the western side of the bay will not carry their pregnancies to term, says the study led by University of Alberta biostatistician Péter Molnár.

The study was published Tuesday in Nature Communications.

According to the paper, sea ice on Hudson Bay has been breaking up about a week earlier per decade in recent decades.

The researchers based their calculations on studies in the 1990s that showed a close link between the number of cubs each polar bear has — typically one to three per litter — and the amount of energy they have stored up as fat. The data indicated that 28 per cent of energy-deprived pregnant bears did not end up having a single cub.

Polar bears hunt on the sea ice for seals, but when it melts in summer, they are forced ashore where there is little food available. The earlier the ice melts, the longer they go without food, and the fewer energy reserves they have for pregnancy and for providing milk to their young, the paper said.

Using the information from previous studies, the researchers created a mathematical model linking the date of sea ice melt to the amount of energy the bears can store up. They then used that to predict the number of cubs the bears would likely have, based on data from previous studies.

The model indicated that 55 to 100 per cent of pregnant polar bears in western Hudson Bay will not have any cubs if the ice breaks up two months earlier than in the 1990s.

An estimated 900 polar bears now live in the western Hudson Bay area, down from 1,200 a decade ago. They are the southernmost population of polar bears in the world.

The study's predictions "serve as another indicator that the western Hudson Bay population will probably not remain viable under predicted climatic conditions," the paper said.

It added that more than a third of the estimated 20,000 to 25,000 polar bears across the Arctic similarly feed on the ice and fast on the shore after the ice melts, suggesting birth rates of other polar bear populations might also fall as the climate warms.