One large, overlooked factor in global warming: tropical forest fires

Forest fires have become a wildcard in the global-warming game. New research shows that, under the right circumstances, they can emit carbon dioxide at a rate to rival fossil-fuel emissions of the heat-trapping gas. This is a factor that computer simulations of climate change cannot yet take into account.

That's what happened when Indonesian fires polluted air over Southeast Asia in 1997-98. Susan Page at Britain's University of Leicester, together with colleagues in England, Germany, and Indonesia, now have analyzed satellite photos and data gathered on the ground to estimate how much of the fire area's living vegetation and peat deposits were burned.

In a paper published today in Nature, they report that the carbon dioxide (CO2) released was "equivalent to 13 to 40 percent of the mean annual global carbon emissions from fossil fuels, and contributed greatly to the largest annual increase in atmospheric CO2 concentration detected since records began in 1957."

That's a whopping global impact for a fire that affected only a small part of Earth's surface. Many locally severe forest fires lack such global reach. They burn off the surface vegetation and then die out.

In Indonesia, nature and human activity had prepared a massive subsurface fuel reservoir. Tropical forests built up thick peat deposits as vegetation died and decayed over many centuries. Forest clearance and drainage for logging and farming have tended to dry the peat. Drought due to the 1997 El Niño was all that was needed to make the circumstances right for a sustained conflagration when forest-clearing fires were lit that year.

As Dr. Page and her colleagues explain, it's hard to pin down exactly how much CO2 the fires emitted. However, the emissions were massive, whether they were at the low or the high end of the estimated range.

Commenting on that estimate in Nature, David Schimel and David Baker at the National Center for Atmospheric Research in Boulder, Colo., note that two independent studies of atmospheric CO2 concentrations in that time period support the conclusion that the fires were a major contributor.

Drs. Schimel and Baker explain that computer climate simulations assume that processes that emit CO2 and remove it from the atmosphere operate smoothly and continuously.

Now it is obvious that catastrophic events in small areas that release carbon locked away in peat or other rich reservoirs have to be taken into account. At the moment, no climate modeler knows how to do this. Yet these events "can evidently have a huge impact on the global carbon balance," Schimel and Baker say.

It now seems clear that efforts to slow CO2 buildup in the atmosphere by curbing use of fossil fuels can be undercut if fires release the carbon locked up in large tropical peat deposits. This gives new urgency to efforts to control tropical forest development, which is a daunting political and economic challenge for countries like Indonesia.