Large-scale fires in western and southeastern states can pump as much carbon dioxide into the atmosphere in a few weeks as some states' entire motor vehicle traffic in a year, according to newly published research by scientists at the National Center for Atmospheric Research and the University of Colorado at Boulder.
The paper, titled "Estimates of CO2 From Fires in the United States: Implications for Carbon Management," is being published online in the journal "Carbon Balance and Management." Primary funding for the study was by the National Science Foundation.
The authors, Christine Wiedinmyer of NCAR and Jason Neff of CU-Boulder, used satellite observations of fires and a new computer model developed by Wiedinmyer that estimates carbon dioxide emissions based on the mass of vegetation burned. They caution that their estimates have a margin of error of about 50 percent, both because of inexact data about the extent of fires and varying estimates of the amount of carbon dioxide emitted by different types of blazes.
Overall, the study estimates that U.S. fires release about 290 million metric tons of carbon dioxide a year, which is the equivalent of 4 percent to 6 percent of the nation's carbon dioxide emissions from fossil fuel burning. But fires contribute a higher proportion of the potent greenhouse 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 rapidly into the atmosphere, according to the authors.
"A striking implication of very large wildfires is that a severe fire season lasting only one or two months can release as much carbon as the annual emissions from the entire transportation or energy sector of an individual state," the study states.
Although last week's fires in southern California broke out after the paper was written, Wiedinmyer applied the new computer model to analyze their emissions. Her preliminary estimates indicate that the fires emitted 7.9 million metric tons of carbon dioxide in just the one-week period of Oct. 19 to Oct. 26 -- equivalent to 25 percent of the monthly emissions from all fossil fuel burning throughout California.
"Enormous fires like this pump a large amount of carbon dioxide quickly into the atmosphere," Wiedinmyer says. "This can complicate efforts to understand our carbon budget and ultimately fight global warming."
Carbon dioxide emissions from fires pose a significant challenge as policymakers focus on limiting greenhouse gases because of concerns over climate change. Some jurisdictions, such as California, have not yet decided whether to include 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 eventually may absorb as much carbon dioxide as was released during the blaze, according to the authors.
But fires are likely to become more frequent and widespread as temperatures warm around much of the globe, which means that more carbon dioxide may be released into the atmosphere, they said. The fires could complicate government efforts to rely on forests to help absorb carbon dioxide.
"The fires that are burning today in the United States are part of the legacy of the past century of fire suppression," says Neff, an assistant professor in CU-Boulder's geological sciences department and environmental studies program.
"Our attempts to control fire have had the unintended benefit of sequestering more carbon in our forests and reducing the impact of human combustion of fossil fuels," said Neff. "But as these forests now begin to burn, that stored 20th 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 the dominant U.S. sources for carbon dioxide emissions from fires. Fires in grasslands and agricultural areas, where vegetation is less dense, emit far less carbon dioxide. The extent of the fires varies widely from year to year, but typically the emissions have a small peak in the spring from fires in the southeastern and central U.S. and a larger peak in the summer during the fire season in the West.