Probably no animal has become more of an environmental symbol than the polar bear. Recently, however, beyond just being a symbol for conservation, polar bears have become closely linked with global warming. We see polar bears everywhere now, on ads, in documentaries, on environmental websites, and on protest signs. There's just one problem: Global warming, by itself, isn't a threat to polar bears.
The Polar Bear evolved roughly 200,000 to 300,000 years ago, after diverging from the common brown bear. Since that time, there have been interglacial periods, including times where much of the polar ice cap melted. Today, polar bears hunt seals from ice platforms, and presumably they did this in the past as well. When the ice melts, their habitat is reduced, and this means they have less access to seals, or they must travel further to find them, which can take more energy. Yet, they have survived such transitions in the past. As Dr. Andrew Miall has pointed out, as recently as 4000 to 8000 years ago there was a period of very limited ice cover, which they survived without difficulty.
Of course, today polar bears are indeed a threatened species. Their numbers have fallen dramatically in the past hundred years, and they are at serious risk of extinction. Global warming is not the cause of this however, humans are. We have killed many of them, and disrupted their activities. We have also eliminated much of their food supply. The number of whales, walruses and seals are not what they used to be in the past. Indeed, many species of whale that used to live in the arctic are now extinct thanks to us. If that weren't enough, we've polluted the water, the land, and the air and many of these toxins have entered the food chain, where they can bioaccumulate in top predators.
During previous interglacial periods, polar bears did have to change their behaviour to survive. This adaptation was possible because the ocean was literally teeming with life in a way that is hard to imagine today. Thousands of whales swam in the ocean close to the smaller ice cap. Many of them would die and their carcasses would wash ashore. This would provide an abundant and consistent source of food for polar bears, which they could use to supplement a smaller diet of seals and other creatures.
Of course, today, global warming is a threat to polar bears, but only because their previous methods of adaptation are no longer possible. They are very few whales in the sea today, and limited numbers of other animals. Certainly there would not be enough to provide carrion for the polar bears. Also, their already reduced numbers means they have less of a cushion to absorb any losses during a transition period. They are too close to extinction. As they move to different areas, they are also likely to encounter more humans, with the predictable outcome.
This is why I said at the beginning, global warming, by itself, is not a threat to polar bears. If we had not destroyed the environment and caused so much extinction, polar bears would likely survive global warming just fine. The rate of warming today, though, is much higher than in the past, so there would be increased pressure, but likely not enough to threaten the species as whole. When the other factors are considered, unfortunately, their extinction seems pretty much assured.
It may seem as though I am making a somewhat academic distinction here, given that the polar bears are probably not going to survive in any event. But I think it's important to remember that as serious as global warming is, it is not our only sin, and even if we found a way to stop it, without stopping the rest of our destructive ways, the planet would still be in trouble. We need to move towards full sustainability in all areas, and this goes beyond carbon emissions. Real sustainability means we make the world a little bit better each year than the year before. That is something new and something hard and something absolutely necessary.