Climate is a crucial factor not only in determining where plants, animals, and microorganisms live and reproduce, but also in understanding the types of ecosystems they form. The scientific community agrees that human-induced warming, which is occurring, has the potential to trigger rapid changes, and that those changes will alter every ecosystem on Earth in some way. For many species, such as the corals that inhabit tropical reef systems, climate change is proving deadly. For others, including destructive insects like spruce budworms, climate change is leading to favorable conditions that will allow their numbers to multiply.

The impact of climate change on the two polar regions is expected to be among the greatest of any on Earth. One Arctic inhabitant, the polar bear, has become a symbol of the effects that global warming is increasingly having around the world. This animal—the largest terrestrial carnivore—has become vulnerable because its primary habitat, the Arctic ice, is shrinking.

Although they are excellent swimmers, capable of spending several hours at a time in icy waters, polar bears also hunt, breed, den, and rest on top of the ice that covers the Arctic seas most of the year. The distribution of polar bears is influenced by the type and distribution of sea ice, as well as the density and distribution of their food sources.

Polar bears prey on seals and belugas, a species of whales. In doing so they also indirectly benefit other organisms that these animals prey on—such as krill, the small crustaceans that are the foundation of marine food chains, and smaller fish. Because polar bears have a disproportionate effect on their environment, they are considered to be a "keystone" species in the Arctic ecosystem. Therefore, continued shrinkage in sea ice due to climate change will have severe repercussions on life in the Arctic that will ripple through the entire food web.

The Arctic warming trend has already resulted in a three percent decrease per decade in the extent of sea ice since the 1970s. A polar bear's access to food is critical for maintaining its body condition and ensuring its reproductive success. During the ice-free season, polar bears must fast for as long as four months. If climate change shortens the period of ice cover, bears may be forced on shore to rely on stored fat for even longer. With less available food, reproduction rates will fall. Bear cubs will be smaller, so mortality rates will rise.

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