Climate change threatens bears’ long-term survival

Polar bears rely on sea ice to hunt seals, their main food source. But the Arctic is warming more quickly than any other place on Earth, and the bears’ sea-ice platform is rapidly melting away. LOUISE ARCHER explains that while models can help us understand how polar bears might fare under different climate change scenarios, their future ultimately depends on our action.

For a polar bear roaming the Arctic, the next meal is always an uncertain prospect. Sea ice is a dynamic habitat that is subject to cycles of break-up, refreeze and drift, so polar bears’ ability to access prey varies with the changing sea-ice landscape. By accumulating large amounts of body fat when conditions are favourable for hunting, polar bears have evolved to cope with bouts of food scarcity. However, rising temperatures are causing unprecedented declines in Arctic sea-ice extent, reducing the bears’ access to essential habitat and threatening the long-term persistence of all but a few subpopulations.

Regional habits and challenges

Because sea-ice dynamics vary across the Arctic, different subpopulations face distinct challenges. In the past, bears from the southern Beaufort Sea could pile on masses of body fat by autumn because even in the summer, sea ice provided access to productive hunting areas over shallow coastal waters. But because of severe declines in the extent of summer sea ice over the last four decades, the ice now recedes over deeper Arctic waters, where seals are absent and hunting opportunities are scarce. The bears here face a choice: follow the retreating ice to less productive waters, or move onshore to wait for the sea ice to return. Both options translate into food deprivation.

At the southern edge of their range, the ice has historically disappeared entirely in the summer, forcing polar bears onto land. Fuelled by hefty stores of body fat, these bears fast until winter temperatures cause the ice to refreeze and they can resume hunting. But earlier sea-ice melt and later refreeze dates in these seasonal ice regions mean the bears are enduring longer and longer periods of on-land fasting. For Canada’s western Hudson Bay subpopulation, the ice-free period has extended by an average of almost nine days per decade since 1979, accompanied by declines in the bears’ body condition, cub survival rates and population size.

Study warns of population collapse

The polar bear specialist group with the International Union for Conservation of Nature reports that three out of 19 subpopulations of polar bears are showing long-term declining trends. But we don’t have enough long-term data to determine the status of many subpopulations. Despite indicators that some are already struggling, it’s hard to know exactly how population dynamics will respond to never-before-seen sea-ice lows, or at what point different subpopulations will be affected.

A 2020 study published in Nature Climate Change aimed to tackle these uncertainties by linking climate models and sea-ice projections to estimates of how long polar bears can go without food based on their body condition and the amount of energy they burn each day. The researchers compared these “fasting thresholds” with projections of when different emissions scenarios might lead to periods of sea-ice loss that would exceed the bears’ fasting capacity. The study found that if warming continued at current rates, 12 of the 13 subpopulations in the study would reach their fasting limits before 2100, with declines in reproduction and survival likely leading to population collapse.

Things appear a little less bleak in a scenario of moderate emissions mitigation (average warming of about 2.4 °C by the end of the century), with nine out of 13 subpopulations at risk of reproductive failure by 2080 in this scenario.

Extensions to this research are now considering how additional aspects of polar bear “energy budgets” might alter future population dynamics—for example, if patterns of energy uptake and use change during periods when bears are on-ice feeding.

The study found that if warming continued at current rates, 12 of the 13 subpopulations in the study would reach their fasting limits before 2100, with declines in reproduction and survival likely leading to population collapse.

Multiple existential threats

A variety of factors are poised to make the future even more precarious for polar bears. Because the bears are apex predators, the effects of Arctic change on other species may percolate through the food web to impact them. Human activities encroaching on their territories—such as new shipping routes or oil and gas exploration in denning areas—are an added risk. In addition to being less hospitable for polar bears, a warmer Arctic will create opportunities for southerly species to move north, potentially exposing bears to more pathogens, new diseases or increased competition. The potential for human-polar bear conflict may also rise as hungrier bears spend longer stretches of time on land.

A few subpopulations of bears who range in areas of naturally high productivity or in regions with thicker, multi-year ice may be insulated from the worst risks in the short term. However, in the long term, the loss of Arctic sea-ice poses an existential threat to all polar bears. Swift, ambitious action on the climate crisis is essential to ensuring they can persist in the Arctic long into the future.