What does global warming have to do with the decline in the polar bear population?

Polar Bear Population on Thin Ice

Polar bears are strong swimmers but they can't outswim a seal.
Polar bears are strong swimmers but they can't outswim a seal.
Paul Nicklen/National Geographic/Getty Images

There are approximately 25,000 polar bears in the world, living in 19 separate populations throughout the Arctic. Twenty years ago, that number was closer to 33,000 [source: Daily Mail].

With sea ice melting at record levels, polar bears face two massive, connected threats to their lives: an inability to access prey and an increasing distance between sea ice and land masses. Polar bears move to coastal regions when the weather warms up and the ice shrinks for the season. Because the bears aren't built to hunt on land, they mostly lie around as their bodies rely on stored-up fat for sustenance. When winter comes and the ice returns, it's time to hunt again.

The bears are strong swimmers and are skilled at hunting on the ice. The largest bears can swim 100 miles at a time [source: Daily Mail]. They're powerful animals, and they have thick layers of blubber that keep them warm in the water. But as the ice continues to melt, the time and energy it takes to swim from land to an ice sheet is increasing beyond the bears' capacity. Polar bears are being forced either to stay on land for extended periods without food or, when they get hungry enough, to make the swim and risk drowning or freezing along the way.

The other problem with having to hunt on increasingly shrinking bodies of ice is that the polar bears' main prey, the seal, is a stronger swimmer than the bears are. The bears are faster than seals on the ice, but with less ice to move across, the seals are getting away. And with less food to eat -- especially less fatty food like seals -- the polar bears are thinner than they used to be, and have less body fat to keep them warm. This lack of insulation makes the extended swim to and from shore even more dangerous, and it makes it harder to live comfortably for an extended period of time on land.

With their means of hunting and catching prey vastly diminished, and their blubber level shrinking, polar bears are wandering more and more into campsites and other areas where humans keep food out in the open. They're even Dumpster diving. It's a state of desperation, and none of the experts expect it to ease up.

How can the species survive under these conditions? On the next page, we'll see what the future holds for the world's dwindling  polar bear population.