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Forest fires could play a more significant role in the build-up of greenhouse gases in the atmosphere than was previously thought, according to new research published in the journal Science.

Levels of gases that cause climate change, such as carbon dioxide and methane, have been rising since measurements began in the 1950s, mainly as a result of human activities such as burning fossil fuels. But each year there is considerable variability in the extent of the increase, and scientists have been trying to understand the reasons for this.

Now, researchers from the United States have found that forest fires during the drought of 1997/98 contributed most of the extra methane, carbon dioxide and carbon monoxide found in the atmosphere above that which would be expected from the burning of fossil fuels and other factors during those years.

Using a combination of satellite data and computer-based climate models, Guido van der Werf and colleagues from the US Department of Agriculture and NASA, the US space agency, found that 60 per cent of this 'extra' increase in greenhouse gas emissions came from Southeast Asia, 30 per cent from Central and South America and 10 per cent from wooded areas of Europe, Asia and North America.

The higher emissions coincide with a drought that caused severe forest fires in Indonesia, Central America, parts of Amazonia, and both north and southern Africa, as well as North America, Europe and Asia. The drought was caused by the El Niño Southern Oscillation, the periodic reversal of currents in the Pacific Ocean, which disrupts the world’s weather.

The conclusions have important implications for understanding the current process of global warming, which is predicted to lead to a doubling of the frequency of the El Niño Southern Oscillation from once every seven years to every three years.

The research suggests that more frequent El Niño events could lead to forest fires becoming more frequent, and thus to larger quantities of carbon being pumped into the atmosphere as a result. Another implication, according to the researchers is “that regions that have long served as carbon sinks may suddenly become sources [of carbon]”. Both conclusions are likely to require the modification of existing computer-based models of anticipated climate change in the years ahead.

Link to research paper by Guido van der Werf et al in Science

Reference: Science 303, 73 (2003)