If overall global warming exceeds
5.5°F (3°C), many parts of the world are likely to see substantially
increased risks of drought, floods, and wildfires, climate scientists say.
In addition, there will probably be large-scale changes in vegetation
types, a U.K.-based research team reports in tomorrow's issue of the
Proceedings of the National Academy of Science.
But if global warming is less than 3.5°F (2°C), the risks of such changes
are much lower.
"The most important message from the paper is that with stronger global
warming, the risk of severe events becomes much more pronounced," said the
study's lead author, Marko Scholze of the University of Bristol's QUEST
(Quantifying and Understanding the Earth System) project.
In the past, making climate-change projections has been complicated by
the fact that different models produce different answers.
So rather than trying to pick one "best" climate model, Scholze's team
looked at the degree to which 52 previous forecasts were in agreement.
The result, Scholze says, is like a weather report.
"[Weather forecasters] don't say, It's likely to rain tomorrow," he said.
"They say, There's a 50 percent probability. It's the same with our paper."
"This work isn't brand new science," added Sarah Cornell, science manager
of the QUEST program. Rather, she said, "it's a new way of communicating."
Worldwide Havoc
With average global temperature increases of more than 5.5°F (3°C), there
will be a high probability of droughts along Europe's Mediterranean
coastline, the scientists report.
The eastern U.S. would also become prone to severe dry spells.
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.
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."
