Fires release large amounts of carbondioxide

31 October 2007

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USA — Large-scale fires in a western or southeastern state can pump as muchcarbon dioxide into the atmosphere in a few weeks as the state’s entire motorvehicle traffic does in a year, according to newly published research byscientists at the National Center for Atmospheric Research (NCAR) and theUniversity of Colorado at Boulder.

The paper, “Estimates of CO2 from fires in the United States:implications for carbon management,” is being published online today in thejournal “Carbon Balance and Management.” NCAR’s portion of theresearch was supported by the National Science Foundation, NCAR’s principalsponsor.

The authors, Christine Wiedinmyer of NCAR and Jason Neff of the University ofColorado, used satellite observations of fires and a new computer model,developed by Wiedinmyer, that estimates carbon dioxide emissions based on themass of vegetation burned. They caution that their estimates have a margin oferror of about 50 percent, both because of inexact data about the extent offires and varying estimates of the amount of carbon dioxide emitted by differenttypes of blazes.

Overall, the study estimates that fires in the contiguous United States andAlaska release about 290 million metric tons of carbon dioxide a year, which isthe equivalent of 4 to 6 percent of the nation’s carbon dioxide emissions fromfossil fuel burning. But fires contribute a higher proportion of the potentgreenhouse gas in several western and southeastern states, especially Alaska,Idaho, Oregon, Montana, Washington, Arkansas, Mississippi, and Arizona.Particularly large fires can release enormous pulses of carbon dioxide rapidlyinto the atmosphere.

“A striking implication of very large wildfires is that a severe fireseason lasting only one or two months can release as much carbon as the annualemissions from the entire transportation or energy sector of an individual state,”the authors write.

California fires

Although last week’s fires in southern California broke out after the paperwas written, Wiedinmyer applied the new computer model to analyze theiremissions. Her preliminary estimates indicate that the fires emitted 7.9 millionmetric tons of carbon dioxide in just the one-week period of October 19-26, theequivalent of about 25 percent of the average monthly emissions from all fossilfuel burning throughout California.

“Enormous fires like this pump a large amount of carbon dioxide quicklyinto the atmosphere,” Wiedinmyer says. “This can complicate efforts tounderstand our carbon budget and ultimately fight global warming.”

Challenge for policymakers

Carbon dioxide emissions from fires pose a significant challenge aspolicymakers focus on limiting greenhouse gases because of concerns over climatechange. Some jurisdictions, such as California, have not yet decided whether toinclude wildfire emissions when setting targets to reduce greenhouse gases.

The impacts of fires on climate change are complex and difficult to predict.Long after a fire sweeps through an area, new vegetation over the course ofseveral decades to a century may absorb as much carbon dioxide as was releasedduring the blaze. But fires are likely to become more frequent and widespread astemperatures warm around much of the globe, which means that more carbon dioxidemay be released into the atmosphere. The fires could complicate governmentefforts to rely on forests to help absorb carbon dioxide.

“The fires that are burning today in the United States are part of thelegacy of the past century of fire suppression,” says Neff, an assistantprofessor of environmental studies. “Our attempts to control fire have hadthe unintended benefit of sequestering more carbon in our forests and reducingthe impact of human combustion of fossil fuels. But as these forests now beginto burn, that stored twentieth century carbon is moving back into the atmosphere,where it may compound our current problems with CO2.”

The new study found that evergreen forests in the South and West are thedominant U.S. sources for carbon dioxide emissions from fires. Fires ingrasslands and agricultural areas, where vegetation is less dense, emit far lesscarbon dioxide. The extent of U.S. fires varies widely from year to year, buttypically the emissions have a small peak in the spring from fires in thesoutheastern and central United States, and a larger peak in the summer duringthe fire season in the West.

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