It's hard to predict the full impact of global warming on the Earth's biota. Some biological communities will surely be disrupted, with changes occurring in both the timing of migration and in the ranges of plants and animals. The greatest impacts will likely be felt at high latitudes, putting animals like polar bears—as well as migratory birds with regular stopovers in the Arctic or Antarctic—at special risk. A 2002 National Wildlife Federation study found that by the end of the 21st century, the climatic ranges of state birds in California, Georgia, Iowa, Maryland, Massachusetts, New Hampshire, and Washington may shift entirely outside their official states.
Current models are still sketchy in the details, yet some ecological responses to climate change have already become visible. Birds, butterflies, and other animals are moving farther north, while starting their journeys earlier. Recent observations show, for example, that 23 bird species migrating from Europe or Africa to Scandinavia—every species, in fact, for which sufficient data is available—pass by the North Sea island of Helgoland two to twelve days earlier than they did four decades ago.
While some species may adapt well to a shifting range, others may be unable to find suitable habitat. Warming temperatures have forced the Quino checkerspot butterfly to largely abandon its southern range in Mexico, but many desirable spots in southern California have fallen victim to urban sprawl. Only the most efficient colonizers can find shelter within the fragmented landscapes common today.
Altering the timing of migration also has its perils: A species' breeding cycle and food supply can easily get out of synch. It remains to be seen whether birds and other migratory creatures can adjust their travel schedules to match times of peak food availability. Although migratory species are clearly at risk from climate change, animals that don't migrate may be even more vulnerable to global and regional temperature increases.
Efforts to save whooping cranes, the most endangered of the world's 15 crane subspecies, offer a rare instance of animals learning about migration by watching what human do. Starting in 2001, humans flying ultralight aircraft have led whooping crane chicks on their first migratory flight south from a wildlife refuge in Wisconsin to a newly-created wintering habitat in a Florida preserve--a 1,200-mile journey.