It's not just the heat: why animals suffer

Changes to habitats caused by global warming affect the world's wildlife in a wide variety of ways, including some that aren't obvious. A few examples:

POLAR BEARS

The polar bear is the world's largest, land-based carnivore. Scientists estimate that 20,000 to 25,000 roam the areas around – and atop the ice of – the Arctic Ocean. In the past 30 years, the Arctic has warmed at nearly twice the rate of the rest of the earth. The sea ice, as measured in late summer each year, has shrunk more than 1 million square miles in that time – a 15 to 20 percent decrease.

Some scientists predict that by 2040 the Arctic could be completely ice-free during summer. Although there is some disagreement about current polar bear population trends – many observers say that some populations are becoming more numerous, not less – scientists generally agree that the loss of sea ice doesn't bode well: The bears hunt marine animals from the ice. Without it, they'll quite literally lose the ground they stand on. Recently, the United States announced it was considering listing polar bears as threatened under the Endangered Species Act.

RINGED SEALS

Polar bears may not only lose their hunting ground (sea ice), a principal food source may be disappearing, too. Ringed seals, a significant portion of the bears' diet, dig dens in snowdrifts. Sheltered from the Arctic cold and protected from predators, the seals give birth to their pups in these snow caves. But with snow melting earlier each season, scientists worry that ringed seal pups will be exposed before they're weaned and ready, bad news for the seals – and for the bears that depend on them.

KRILL

If you were to put all the world's marine mammals on a scale, you'd find half that mass came from the waters surrounding Antarctica. The Southern Ocean sustains half of the world's marine mammal biomass, including one-fifth of the world's whales. At the base of this huge food chain are krill, a tiny shrimp-like species. Several whale species feed directly on them.

All the larger animals, including seals and penguins, rely on krill in some form or another. But global warming may imperil krill. The tiny crustaceans winter on the underside of the sea ice, where they feed on algae. Scientists worry that global warming will diminish this ice cover, and that less ice will lead to fewer krill. Indeed, studies show that krill populations have declined by up to 80 percent in those areas where ice has already diminished compared with the 1970s.

THE HAWAIIAN HONEYCREEPER

The Hawaiian Islands are home to many bird species found nowhere else. In 1826, humans inadvertently introduced mosquitoes there. The mosquitoes carried avian malaria and pox. In the past 25 years, six species of honeycreeper, a small colorful bird, have disappeared due to habitat loss and mosquito-borne disease. Many remaining species live only at higher elevations beyond the mosquitoes' reach.

Now scientists worry that a warming climate will allow the mosquito to move farther up the mountains. On Kauai, only 5,000 feet high, mosquitoes could push the 1,400 remaining honeycreepers off the mountain entirely. In an effort to lessen the threat, scientists are attempting to eliminate pools of water that serve as mosquito breeding grounds. Humans and feral pigs – another pair of invasive species to Hawaii – often create these stagnant bodies of water.

THE PINE BEETLE

In recent years, the tiny pine beetle has wreaked havoc on pine forests from New Mexico to Alaska. This year, there are reports that its European counterpart is eating its way through Scandinavian forests. Scientists suspect higher winter temperatures are allowing more beetles to survive the winters. Warmer conditions permit them to move up mountains and into forests where conditions were previously too cold. Drought also may be playing a role.

Pine trees repel insect attacks by secreting sap. But more frequent hot and dry spells have weakened these defenses. And because the beetle's life cycle is largely determined by temperature, scientists think warmer summers allow them to mature more quickly.

In parts of Alaska, the beetle used to complete its life cycle in two years, but now takes only one.

In the northern Rockies, the beetle has decimated whitebark pine forests that produce a pinecone essential to the grizzly bear's diet. Hoping to control further infestation, foresters are researching ways to manipulate the beetle's behavior with its own pheromones, chemicals that direct them when and where to infest new trees.