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Where there’s fire, there’s smoke

Quantifying the impact of fire on air chemistry and climate


by David Hosansky

christine wiedinmyer

Christine Wiedinmyer. (Photo by Carlye Calvin.)

Whether they burn across the vast evergreen forests of western Canada or the piney woods of the South, wildland fires can have a surprisingly large effect on climate, both in the short and long term. Below are two recent studies from NCAR on what fire means for our atmosphere.

High latitudes, high emissions

With climate change expected to increase the frequency and intensity of boreal wildfires, emissions from these fires are, in turn, expected to influence climate. But scientists are unsure about exactly how this will play out. Much is unknown about the amounts and characteristics of gases and particles from fires and how they influence the radiation budget and chemistry of the atmosphere.

A team of scientists from NCAR’s Earth and Sun Systems Laboratory recently approached this problem by studying conditions within the atmosphere and at the very top of it. The researchers, led by Gabriele Pfister, integrated satellite observations and model simulations to look at changes in both solar radiation and the particle loading of upper-latitude skies.

Their study, which appeared in January in the Journal of Geophysical Research–Atmospheres, focused on 2000, a quiet fire season, and 2004, when the fire season in North American boreal forests was unusually severe.

The team found that the short-term impact of wildfires may be to cool regional temperatures, because a large fraction of the particles in the smoke block incoming solar radiation. In the long term, fire emissions appear to warm the climate, because they emit large amounts of carbon dioxide and other greenhouse gases that remain in the atmosphere far longer than the smoke particles. The researchers also found that the emission inventories and climate models appear to underestimate both the amount of particles that are emitted by fires and the extent to which the particles absorb solar radiation.

The impact of U.S. fires

Large-scale fires in a single western or southeastern state can pump as much carbon dioxide into the atmosphere in a few weeks as the state’s entire motor vehicle traffic does in a year. These and other findings by NCAR’s Christine Wiedinmyer and Jason Neff (University of Colorado) were published last October in the journal Carbon Balance and Management.

firefighter

Burning more than 200 square kilometers (77 square miles), the Poomacha fire was the third-largest in San Diego County during Southern California’s disastrous round of wildland fires last October. (Photo by Andrea Booker, FEMA.)

The authors used satellite observations of fires and a new computer model developed by Wiedinmyer that estimates CO2 emissions based on the mass of vegetation burned. Overall, the study estimates that fires in the contiguous United States and Alaska release about 290 million metric tons of carbon dioxide a year, which is the equivalent of 4–6% of the nation’s CO2 emissions from fossil fuel burning. They caution that their estimates have a margin of error of about 50%, 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.

Long after a fire sweeps through an area, new vegetation over the course of several decades to a century may absorb as much carbon dioxide as was released during the blaze. But fires are likely to become more frequent and widespread as temperatures warm around much of the globe, which could complicate government efforts to rely on forests to help absorb carbon dioxide.

“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,” says Neff. “But as these forests now begin to burn, that stored twentieth-century carbon is moving back into the atmosphere, where it may compound our current problems with CO2.”

According to the study, evergreen forests in the South and West are the dominant U.S. sources for carbon dioxide emissions from U.S. fires. Fires in grasslands and agricultural areas, where vegetation is less dense, emit far less carbon dioxide.

 

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