(Related story: Global Warming Likely Causing More Heat Waves, Scientists Say [August 1, 2006].)
But with an increase of less than 3.5°F (2°C), Scholze said, "we don't see this. There might actually be floods."
In addition, the study found a high likelihood that global warming beyond 3.5 to 5.5°F (2 to 3°C) would cause a high likelihood of floods at latitudes above 50 degrees north.
The temperature rise would also cause prairies to expand northward all the way to Canada's Baffin Bay, replacing present-day forests.
Farther north, Canada's forests would push into regions now covered by brush and tundra (map of Canada).
Wildfire zones would also shift because of two factors: Forest growth would provide additional fuel, and drought would create tinderbox conditions.
The models show a high probability, for example, that the eastern U.S. would become prone to forest fires, much as the western U.S. is today (map of U.S.).
The Amazon's "Double Whammy"
Substantial warming could also have dire consequences for the Amazon rain forest.
If global warming is modest, the models are "not quite sure" what will happen in that part of South America, Scholze said. "Some say it would get drier and some say it would get wetter."
But with warming of more than 5.5°F (3°C), he says, there is a high probability that the region would get drier and face a risk of forest fires.
Steven Running, a climate researcher from the University of Montana in Missoula who was not part of Scholze's team, calls this a "double whammy." The Amazon would first endure forest-killing droughts and then see massive wildfires that turn it from rain forest to grassland.
(See a photo gallery of global warming's geological effects.)
Forecast for the Future
Scholze and his colleagues hope that using data from many climate models will make policymakers more willing to rely on the study's results.
If a prediction is based on a single model, Scholze says, politicians are likely to shrug it off as a fluke.
"But if we say we've looked at 52 [models] and half of them say this is going to happen, I think they'll take it more seriously." The new study represents an important step in climate research, the University of Montana's Running said by email.
"Policymakers are challenging scientists to define when climate change will become 'dangerous,'" he said.
Scientists know that defining "dangerous" requires some level of value judgment. But researchers can look for "threshold triggers" and flag them as potential tipping points for ecological upheaval, he says.
"This paper provides new evidence that humanity should work hard to limit climate change to less than 2°C [3.5°F] as we sail progressively into uncharted waters, as temperatures go higher."
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