New Study Links Polar Bear Population Decline to Climate Change

Researchers at the University of Toronto Scarborough have discovered that the decline in polar bear populations in Western Hudson Bay is directly linked to diminishing sea ice caused by climate change.

In a sobering revelation, researchers at the University of Toronto Scarborough have demonstrated a direct connection between climate change-driven sea ice shrinkage and the decline in the population of polar bears in Western Hudson Bay. According to their study, the varying energy levels due to shorter hunting seasons have created an urgent survival crisis for the bears.

A novel “bio-energetic” model was developed by the research team to monitor the energy intake of the bears from hunting seals and the energy needed for their growth and reproduction.

This model uniquely maps out the lifecycle of individual polar bears, from infancy to adulthood, and was rigorously compared with four decades of monitoring data (1979-2021) concerning the Western Hudson Bay polar bear population.

The findings are stark: during these years, the polar bear population has plummeted by almost 50%. Furthermore, there has been a significant decline in the average body mass of polar bears in the region, with adult females losing 39 kg (86 lbs) and one-year-old cubs shedding 26 kg (47 lbs).

“A loss of sea ice means bears spend less time hunting seals and more time fasting on land,” lead author Louise Archer, a U of T Scarborough postdoctoral researcher, said in a news release. “This negatively affects the bears’ energy balance, leading to reduced reproduction, cub survival and, ultimately, population decline.”

This urgent issue was explained further by co-author Péter Molnár, an associate professor in the Department of Biological Sciences at U of T Scarborough.

“Our model goes one step further than saying there’s a correlation between declining sea ice and population decline,” Molnár said in the news release. “It provides a mechanism that shows what happens when there is less ice, less feeding time and less energy overall. When we run the numbers, we get a near one-to-one match to what we’re seeing in real life.”

Particularly vulnerable to these changes are the polar bear mothers and their cubs. Shorter hunting periods result in mothers producing less milk, which subsequently jeopardizes cub survival. Cubs face reduced survival rates during their first fasting period if they fail to gain sufficient weight.

“It’s pretty simple — the survival of cubs directly impacts the survival of the population,” added Archer.

Over the nearly 40-year period analyzed, there has been an 11% decrease in cub litter sizes, and mothers are keeping their cubs for longer periods due to their inability to thrive independently.

Western Hudson Bay is viewed as an environmental indicator for the global polar bear population. Given that the Arctic is warming at an alarming rate — four times faster than the global average — the model suggests similar declines might occur in other polar bear populations worldwide.

Molnár emphasized the importance of the model and its potential broader applications.

“There’s every reason to believe what is happening to polar bears in this region will also happen to polar bears in other regions, based on projected sea ice loss trajectories. This model basically describes their future,” he said.

The research, published in Science, underscores the urgent need for concerted global efforts to address climate change and protect vulnerable polar bear populations from the escalating energy deficits driven by diminishing sea ice.