Forest fires cool Amazonia

The impact of ozone needs further investigation

By BBC News Online's Jonathan Amos in Boston

Scientists have shown for the first time how fires in the Amazon are making temperatures fall across vast areas of the forest.

The fires put masses of tiny particles into the atmosphere which reflect incoming sunlight back out into space.

The loss of heating at ground level is leading to an average three Celsius drop in temperatures during the burning season that runs from August to October.

But this is only part of the story; the particles are also "drying out" the clouds, reducing precipitation in some regions by as much as 30%.

Sun shield

Fires in the Amazon are confined to a relatively small area - about 15% has been burnt or cleared for pasture - but the effects of fire pollution can be felt over a much wider area.

"The untouched, prime forest covers about 5.5 million square kilometres," Professor Paulo Artaxo, from the University of Sao Paulo, Brazil, told the annual meeting of the American Association for the Advancement of Science. "We think perhaps as much as a half of that could be being affected by secondary pollution."

The fine carbon particles thrown up by fires will rise 10 or 15 kilometres into the sky and spread out over the region. Scientists have known for some time that these particulates can act as a shield, absorbing solar radiation or bouncing it back into space and cooling the planet's surface below.

Professor Artaxo's research is the first to measure this impact in the Amazon basin.

"It is having a very strong influence on the radiation balance in Amazonia," he said. "In some areas on the ground, we are losing about 40% of the total radiation that could be available for the ecosystem and this is bound to have a profound effect on its health."

Plant efficiency

The particles are also interfering with clouds, making it more difficult for water drops to come together to form rain.

"In some areas that are being burned we have seen a 30% reduction in precipitation. Satellites will soon measure the drop in precipitation over the prime forest."

Forest fires further affect air quality by emitting carbon monoxide and other gases that lead to the production of ozone, which can at elevated levels interfere with plants.

Professor Artaxo says research is continuing into the precise effects lower levels of solar radiation and raised levels of ozone could be having on the forest ecosystem.

"It is interesting to note that there are some studies, from China for example, that show high ozone levels can reduce the efficiency of crops by 20-25%."