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Forest fires in the Amazon not only threaten biodiversity and destroy valuable resources, but also have a dramatic effect on climate, according to research published in this week's Science.

Two teams of scientists independently studied how small particles known as aerosols, produced as part of smoke from burning forests, reduce rainfall and certain types of cloud cover.

The findings have implications both for local climate over areas where forest is burnt, but also could affect climate on a global scale, the researchers say.

One study, carried out by an international team of researchers led by Meinrat Andreae of the Max Planck Institute for Chemistry in Mainz, Germany, measured how smoke affects the development of clouds and rainfall in the tropical rainforests of Brazil, where thousands of fires are started each year to clear the forest for farming or other developments.

Cloud droplets form when water vapour condenses around particles in the air. Thousands of these droplets have to collide in order to form a drop that is large and heavy enough to fall down as rain.

The researchers found that the increased number of aerosol particles produced by the fires meant that droplets tend to remain smaller, and often do not become heavy enough to produce rain.

"Smoke from human-induced fires reduced the cloud droplet size dramatically," says Andreae. "Due to this, precipitation was suppressed."

The effect on climate could be widespread, as aerosol particles and water vapour can be carried high up in the atmosphere, where they can spread to much larger regions, possibly all over the globe.

The other research team, led by Ilan Koren of the US-based NASA Goddard Space Flight Center and the US National Research Council in Washington, DC, also found that smoke from burning forests across the Amazon basin inhibits cloud formation.

Their findings show that smoke suppresses cloud formation through a combination of processes. The smoke particles absorb sunlight, warming the aerosol layer and cooling the surface below. This stabilises the air near the Earth's surface, reducing the movement of air and the flow of moisture needed to form clouds.

"We used to think of smoke mainly as a reflector, reflecting sunlight back to space, but here we show that, due to absorption, it chokes off cloud formation," says Koren. "This suggests that the cooling of global climate by aerosols may be smaller than previously estimated."

Koren says his study shows that, by reducing cloud cover, aerosols allow more sunlight to pass through the atmosphere and warm it. This warming could dramatically affect the water cycle and energy balance of the Amazon basin, he says.

"This effect is a very important aspect of aerosols to be studied," says Koren. "We know that the Intergovernmental Panel on Climate Change (IPCC) did not account for aerosols' inhibition of clouds and the associated warming influence on Earth's climate."

"Aerosol-clouds interactions are seen as one of the most important single forces that drive climate change," says Hans Graf from the University of Cambridge, United Kingdom and the Max-Planck Institute for Metereology, Germany. "Although these two reports concentrate on aerosols from biomass burning in Amazonia, the importance of their findings may well go beyond this specific case."

Link to paper by Andreae et al in Science

Link to paper by Koren et al in Science

References: Science 303, 1337 (2004) / Science 303, 1342 (2004)

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