How cleaning up America dried up the Amazon

by Mason Inman, New Scientist news service, May 7, 2008

We have all heard how a butterfly flapping its wings in the Amazon may cause storms in far off places. But it seems that environmental effects can go in the other direction too – reductions in air pollution in North America have led to severe droughts in the Amazon rainforest, according to a new study.

In 2005, the Amazon suffered one of the worst droughts of the past century. Rivers ran so low that they were unnavigable to shipping, and thousands of forest fires raged.

El Niño effects are usually suspected, but there was a problem – there were no El Niño effects that year.

Peter Cox of the University of Exeter, UK, wondered if changes in sulphate particles in the atmosphere could be responsible.

Falling pollution

Sulphates, largely produced from coal-burning power plants, are known to reflect sunlight back into space, cooling the land and ocean below, and counteracting some of heating from greenhouse gases such as carbon dioxide.

Levels of sulphates over North America rose until the 1970s, and then fell as increases in respiratory disease and acid rain led to tougher pollution controls.

What this also meant, Cox realised, is that some of the effects of climate change might have been underestimated in models, which have not traditionally included sulphates.

Cox and his colleagues looked again at the triggers of Amazonian rainfall, using a global climate model developed by the Hadley Centre in Devon, UK – one of the few that simulates the Amazon and the surrounding ocean fairly well. The researchers ran the model with and without emissions of the sulphate particles, or aerosols.

Only with realistic sulphate emissions included, both from human activity and from volcanic eruptions, did the model correctly predict the Amazon's rainfall over the 20th century.

Storm shift

"I thought [sulphate] aerosols were not a big player, but it turns out they are," Cox says.

By cutting back on sulphate aerosols, the greenhouse effect adds extra heat to certain parts of the ocean, the model showed. It also revealed that when the tropical North Atlantic ocean warms up more than in the tropical South Atlantic, then the ocean's storm tracks shift northward.

This shift, says Cox, extends the Amazon's dry season – and this effect will only grow stronger with rising greenhouse-gas levels and falling sulphate aerosols.

Most experts advise limiting CO₂ in the atmosphere to 450 parts per million in order to avoid dangerous climate change, and the study suggests this target would be sufficient to protect the Amazon.

Amazon 'dieback'

"The model shows a catastrophic dieback [of the Amazon] by about 500 or 550 parts per million," Cox says. "As we clean up air quality, as we have to do, it is even more urgent to reduce CO₂ emissions."

"This is first time the possible strong impact of sulphate aerosols [on rainfall] has been presented clearly," says Rong Fu of the Georgia Institute of Technology in Atlanta, Georgia. The results "seem plausible," she adds.

Yadvinder Malhi of the University of Oxford in the UK says that, because of the masking effect of aerosols, we may have been underestimating how sensitive the Earth is to climate change. "If the projections for the Amazon are correct, that's quite severe," he says.

Journal reference: Nature (DOI: 10.1038/nature06960)

Link to article: http://environment.newscientist.com/article/dn13851-how-cleaning-up-america-dried-up-the-amazon.html