Wildfire Below: Smoldering peat disgorges huge volumes of carbon

Once set alight by wildfires, deep beds of decaying tropical plant matter pump massive amounts of carbon into the sky. According to new research, emissions of globe-warming gases from smoldering peat eclipse those from burning surface vegetation and can rival carbon gases produced globally each year by the combustion of fossil fuels.

SCORCHED EARTH. Flames and smoke rise from a peat forest in Central Kalimantan, Indonesia, spewing the soil’s abundant carbon skyward. Page and J. Rieley

Disastrous wildfires swept through Borneo and several other Indonesian islands in 1997 and 1998. The flames burned surface vegetation and the peat that’s abundant in the region’s tropical forests. The fires coincided with a worldwide spike in atmospheric carbon concentrations, prompting scientists to investigate peat’s carbon-releasing role.

Using pairs of satellite images taken before and after the 1997–1998 fires, Susan E. Page of the University of Leicester in England and her colleagues examined nearly 25,000 square kilometers of Central Kalimantan province on Borneo. The researchers determined that about 32 percent of the area burned during the fires.

Page and her team then visited 43 burned sites within a section of the study area and directly measured the depth of peat lost. On average, more than half a meter of peat had burned off, they found. From their measurements, the scientists judge that as much as 8 percent of the section’s total carbon stored in peat was lost during the fires.

In the Nov. 7 Nature, the researchers report that peat combustion in the study area during 1997 and 1998 released between 0.19 and 0.23 billion metric tons of carbon. By contrast, they estimate that burnt surface vegetation of the same area produced just 0.05 billion tons of atmospheric carbon.

The researchers extrapolate that across Indonesia, the 1997–1998 fires released a total of 0.81 to 2.57 billion tons of carbon into the air. That’s 13 to 40 percent of the average annual amount produced globally from combustion of fossil fuels.

The finding highlights the neglected role of wildfires as a source of carbon dioxide emissions, says Joel S. Levine, an atmospheric scientist at NASA’s Langley Research Center in Hampton, Va. Levine notes that the new estimates of the effects of peat burning on Borneo are consistent with his published calculations based on a different approach. He cautions, however, that it’s difficult to accurately extrapolate from findings in a limited area to an entire country, as Page’s team has attempted to do.

A separate study reports annual fluctuations in global atmospheric carbon from 1992 to 2000. Tropical fires in 1997 and 1998 account for the largest 1-year rise in carbon emissions, suggest Ray L. Langenfelds of the Commonwealth Scientific and Industrial Research Organisation in Aspendale, Australia, and his colleagues in the Fall Global Biogeochemical Cycles.

The impact of the wildfires seems “extraordinarily large” and far-reaching, given that they weren’t global phenomena, say David Schimel and David Baker of the National Center for Atmospheric Research in Boulder, Colo., in a second article in the Nov. 7 Nature. Nevertheless, they say, the data support the case that local, episodic events can significantly affect atmospheric carbon worldwide.

If so, the carbon boost from wildfires could reinforce itself. Global warming caused by carbon dioxide and other greenhouse gases may gradually reduce the amount of moisture stored in soils, leaving forests increasingly susceptible to wildfires, Levine says. This fall, severe fires again struck Indonesia, and researchers are already studying the conflagrations’ aftermath.

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