Do forest fires have a significant impact on global warming?

Illuminating answers to environmental questions.
Oct. 16 2007 7:29 AM

Dirty Burns

How much carbon does a forest fire spew into the atmosphere?

Illustration by Robert Neubecker. Click image to expand.

Whenever I see footage of a raging wildfire, I can't help but think of all the carbon that's being spewed into the atmosphere. Do forest fires have a significant impact on global warming, or is my anxiety misplaced?

A lot depends on what the fire destroys, as there is tremendous variation among tree species in terms of carbon storage. As a general rule, the most carbon-laden trees are those with high-density wood and large trunk diameters. So, if you see a fire sweeping through an expanse of mighty evergreens, the carbon emissions will be much higher than if the conflagration was consuming wispier trees. In the United States, the most consistently carbon-rich forests run from Northern California up through Washington.

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You've also got to factor in the composition of the ravaged soil. The fires that swept across Indonesia in 1997, for example, burned relatively thin-trunked tropical trees. But the devastated forests were also covered in carbon-rich peat, with deposits measuring up to 20 meters thick. As a result, the Indonesian fires were estimated to have released between 0.81 and 2.57 gigatons of carbon—between 13 percent and 40 percent of the world's annual emissions at the time.

Thankfully, the typical North American wildfire isn't nearly that calamitous, at least in terms of carbon emissions. Environment Canada estimates that for every acre of primarily coniferous forest burned, approximately 4.81 metric tons of carbon is released into the atmosphere—between 80 percent and 90 percent in the form of carbon dioxide (CO2), with the rest as carbon monoxide (CO) and methane (CH4). In 2006, a record-setting 96,385 wildfires destroyed about 9.87 million acres of forest in the United States. According to the Canadian figure, then, forest fires accounted for 47.47 million metric tons of carbon emissions in the United States last year. For comparison, the nation's annual carbon dioxide emissions are said to be around 6.049 billion metric tons.

This estimate, however, doesn't take into account the carbon released by vegetation that decays once the fires have been extinguished. Nor does it include the long-term effects of losing forests, which absorb carbon dioxide from the atmosphere and thus can help slow global warming.

That warming seems to increase the frequency of wildfires. According to a recent University of Arizona study, the length of the wildfire season has increased by roughly two and a half months since the mid-1980s, mostly due to earlier snow melts in the West. (An earlier melt means that dried, easily ignited brush spends more time lying on the ground.) And a 2004 Kansas State University study included a prediction that carbon dioxide  emitted by American wildfires will double by 2100.

The big caveat in all this is that wildfires can also play a vital role in replenishing a forest's soil and clearing away rot. And even if mankind were to suddenly vanish from the planet, wildfires would still occur, typically sparked by lightning. In 2006, in fact, naturally occurring blazes were responsible for more than half of the 9.87 million acres of forest burned in the United States. It's also worth noting that, prior to the 1800s, Native Americans are estimated to have burned roughly 4.5 million acres of land per year in order to manage game, spur the growth of edible plants, and deprive enemies of cover.

So, while you should be concerned about the growing frequency of wildfires and their attendant effects on global warming, don't lose too much sleep over the issue. There is a limit, the Lantern presumes, to how much anxiety you can sustain. It might be best if you conserved your angst for more readily tackled issues, such as our appalling fossil-fuel dependence.

Is there an environmental quandary that's been keeping you up at night? Send it to ask.the.lantern@gmail.com, and check this space every Tuesday.

Brendan I. Koerner is a contributing editor at Wired and a columnist for Gizmodo. His first book, Now the Hell Will Start, is out now.