Polar Bears Face "Tipping Point"

Kieran Mulvaney
Analysis by Kieran Mulvaney
Tue May 25, 2010 04:48 PM ET

Polar bear populations are unlikely to decrease steadily and predictably as their sea ice habitat shrinks and thins due to climate change, says a new study by Canadian scientists. Rather, numbers are likely to decline gradually and then suddenly fall off a cliff as the bears reach a "tipping point" in relation to their deteriorating environment.

The reason polar bears have frequently been held up as the iconic species of climate change is that their survival is intimately and inextricably linked to sea ice. As Arctic sea ice declines in extent and thickness, the species' ongoing viability, at least in some areas of its range, is a cause for concern.

For example, this study documented a decline in the polar bear population in western Hudson Bay from 1984 to 2004, and found a strong correlation between the physical conditions of bears in the population and the state of the sea ice. Specifically, the longer the sea ice season, the healthier the bears; if the ice broke up earlier, or froze later, the bears tended to be skinnier. Polar_Bear_0319_-_23-11-06

In the new study, published in the journal Biological Conservation, Peter Molnar, Andrew Derocher, Gregory Thiemann and Mark Lewis developed mechanistic models to assess likely polar bear population trends under a variety of different sea ice scenarios. These scenarios considered the ability of adult polar bears to both hunt and find mates. As Matt Walker explains in this article for BBC News:

Male polar bears find females by wandering the ice, sniffing bear tracks they come across. If the tracks have been made by a female in mating condition, the male follows the tracks to her. The researchers modeled how this behavior would change as warming temperatures fragment sea ice. They also modeled the impact on the bears' survival. Southern populations of polar bears fast in summer, forced ashore as the sea ice melts. As these ice-free seasons lengthen, fewer bears are expected to have enough fat and protein stores to survive the fast. By developing a physiological model that estimates how fast a bear uses up its fat and protein stores, the researchers could estimate how long it takes a bear to die of starvation.

Using this method, Molnar and colleagues calculated that 3-6 percent of adult male polar bears in western Hudson Bay would die of starvation if sea ice conditions forced them to spend 120 days ashore. However, that starvation rate leaped to 28-48 percent if they were obliged to spend 180 days ashore.

They found similar non-linear trends when modeling mating behavior, and found that, for example, mating success in the Lancaster Sound sub-population in the Canadian Arctic could decline from 99 percent to 91 percent or even 72 percent, depending on the extent to which declining sea ice affected the efficiency of males' ability to search for females.

As Molnar told the BBC:

"As the climate warms, we may not see any substantial effect on polar bear reproduction and survival for a while, up until some threshold is passed, at which point reproduction and survival will decline dramatically and very rapidly."


Images: Angsar Walk (top) and Mila Zinkova