LOGGERS and gold miners have done their worst, but the Amazon rainforest may be facing an even more formidable adversary: global warming. A new global model developed in Britain shows that if warming continues apace, whole swathes of the Amazon will die off by the end of the century.
Richard Betts, a biosphere modeller at the Hadley Centre for Climate Prediction and Research, and his colleagues developed a model which takes into account how different types of vegetation respond and contribute to climate change. They then used it to track the changes in global climate from 1860 to 2099 in response to so-called “business as usual” increases in greenhouse gas emissions.
Betts was not surprised to see the Amazon getting hotter and drier – that effect has shown up in earlier climate simulations. But he was shocked to see that the Amazon would dry radically and warm by more than 6 °C, changes that would decimate parts of the rainforest. “We saw quite an extreme die-back in the north-eastern Amazon. It surprised many people,” he says. While all of the Amazon will be affected to some degree, the extremely hot, dry conditions could turn a third of the rainforest to grassland or bare soil by the end of the century. “This region is tending toward desert,” Betts says.
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The researchers can’t, however, pin the changes on a single cause. All their models show the band of rainfall that normally hugs the equator moving farther north as global warming increases. But in their vegetation model, broadleaf trees become less competitive and begin to die back. As the trees pump less water from the soil into the atmosphere, rainfall in the Amazon decreases even more. “But we’re still not completely clear why the rainfall decreases so much,” Betts says.
The researchers are quick to point out the limitations of their simulation. “Nobody would treat this result as an actual prediction,” says Betts. “It’s more of a possibility, the kind of thing that could happen.” But Andrew Friend at NASA’s Goddard Institute for Space Studies in Greenbelt, Maryland, is impressed by the study. “It’s extremely important for climate models to include vegetation,” he says. If climate change speeds up later this century as predicted, feedback from soils and vegetation may dwarf the direct effect of burning fossil fuels. He says Betts’s study “should really highlight how sensitive these processes are.”
