And as researchers come to grips with current conditions, their forecasts for the future grow gloomier.
Expectations and Timing
In 2003 Stanford's Terry Root found many examples of global-warming-spurred behavioral shifts in a review of 143 scientific studies covering 1,473 species of plants and animals.
"If you look at all the studies that have been done on species and climate change and find the same signature for species all around the globe by many, many individuals instead of just one or two, it gives circumstantial evidence" that global warming is driving the changes, she said.
And the changes are "not moving in lockstep," Root noted. Some species are moving to new habitats quickly, others slowly, some not at all. This breaks up established predator-prey interactions, for example, leaving ecosystems disrupted.
Galbraith, who co-authored a 2004 report for the Washington, D.C.-based Pew Center on Global Climate Change, said the behavioral shifts are anticipated, but they are coming about 15 years earlier than he expected.
"One important message from the Pew report is we're already seeing these effects, and they're widespread … the effect is way ahead of when many people in the field would have predicted," he said.
Looking to the Future
According to the UN's Intergovernmental Panel on Climate Change, the average global temperature could rise by an additional 2.5º to 10.5ºF (1.4º to 5.8ºC) by 2100, if the amount of carbon dioxide in the atmosphere doubles as expected.
"If we are seeing changes at one degree [Fahrenheit], what might we expect at four and five degrees?" Galbraith said.
Though the climate has shifted several times in the past, scientists say the pace of the temperature swings were much slower, giving plants and animals more time to adapt to changing conditions.
"The other factor is, the face of the planet doesn't look anything like it did when the other changes happened," Root said. "Now we have Kmart parking lots everywhere. If you are an organism that needs to disperse north in North America, and you run into the city of San Diego, say, what do you do?"
According to Root, many species will be unable to navigate through the human landscape. When the temperature gets too hot, they'll die. A temperature rise of a few degrees may cause an eighth to a quarter of all species to go extinct, she speculated.
"If we go six to ten degrees [higher], we're really right on the brink of a huge mass-extinction event. And I don't know that people understand what that means," she said.
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