Calorie counting for polar bears: New study shows link between sea ice loss and polar bear decline

A fat polar bear is a bear more likely to thrive in its extreme Arctic environment. So, when scientists discovered polar bear populations were dropping rapidly, they created a framework to understand what is standing in the way of the species’ survival.

Using data collected since 1979 by Environment and Climate Change Canada (ECCC) in the western Hudson’s Bay region, a group of scientists from the University of Toronto, ECCC, and Polar Bears International set out to understand the mechanism linking sea ice loss with polar bear survival.

Results of their study published in Science found the key to understanding this relationship lies in polar bears’ energy budget — the balance between energy gathered from feeding and energy spent from movement, metabolism, reproduction and growth. 

“It's like calorie counting,” said Louise Archer, lead author of the study and Polar Bear International research fellow at the University of Toronto Scarborough.

“What our model does is it tracks that balance of energy in, energy out, across the entire lifetime of a bear,” she said. “How fat a bear is determines how much they'll reproduce over their life, and ultimately their survival is affected by this.”

When the sea ice melts, polar bears lose hunting time, which means they have fewer hours to eat and take in energy, she said. Due to climate warming, which is melting the ice, the bears have been spending three to four weeks longer on land where their hunting is restricted, instead of on ice. This affects their reproduction and survival, Archer added.

They can’t catch anything nearly as nourishing as a seal when they are on land, Archer said. “They're really in an energy deficit during this time.”

The model showed mother polar bears with cubs are particularly vulnerable to sea ice loss. When the mother is short on energy because she has less time to hunt on the sea ice, she’ll produce less milk for her cubs. Cubs normally stay with their mother for the first two years. But the model showed that cubs were staying with their mother longer because their mothers were producing less milk, which impedes their growth.

Cubs are less likely to survive their first fasting period in the summer and fall if they don’t gain enough weight during the shorter hunting season in the spring, and polar bear mothers were also found to be having fewer cubs throughout their lifetime.

While this study shows historical trends in this specific region, Archer said her team would like to build a similar model for other regions of the Arctic to see if different areas show similar trends.

The fate of polar bears is largely dependent on humans and how successful we are at curbing carbon emissions which cause global warming. The impacts are particularly severe in the Arctic, she noted, which has warmed four times faster than anywhere else on earth.

“The future for polar bears lies in what we do now and what we do in the near future,” Archer said. “We can alter trajectories.”