Nov. 1, 2007 -- The carbon dioxide released into the skies by large wildfires in California and other parts of the west can be almost as much as weeks of car exhaust in many states, say scientists. That said, the carbon emissions from burning plants are not thought to be a player in global warming since the emissions come with their own offset -- the regrowth of the burned lands.
A new satellite-based study published in the Nov. 1 issue of the journal Carbon Balance and Management estimates that burning lands in the United States release about 290 million metric tons of carbon dioxide every year. That's four to six percent of that emitted by cars.
"In the Western U.S. you have fossil fuels by far emitting the majority," said atmospheric scientist Christine Wiedinmyer of the National Center for Atmospheric Research in Boulder, Colo. Figuring out the role of wildfires, even if it's small, is critical for getting a handle on where the climate-warming gases are coming from and where they are going -- what's called the carbon budget.
When wildfires are raging they can briefly contribute much higher percentages of carbon dioxide. For instance, Wiedinmyer has done a quick analysis of the California fires and estimates they released 7.9 million metric tons of carbon dioxide in the week of October 19-26. That is about equal to the carbon emissions of all fossil fuel burning in the state over the same week.
"It's just part of a bigger puzzle," said Wiedinmyer. "This just gives us a constraint, a first approximation."
That puzzle includes sorting out the different long-term greenhouse gas contributions of wildfires versus fossil fuels, and their respective roles in global warming. For one thing, Wiedinmyer explained, it's not quite fair to compare carbon from fossil fuels to that released from wildfires.
"They are fundamentally two types of emissions," Wiedinmyer told Discovery News.
Fossil fuels release vast amounts of carbon which has been locked up underground for millions of years, she said. What's worse, fossil fuels offer no mechanism for reabsorbing carbon.
Carbon released from burning plants during wildfires, on the other hand, is carbon that was absorbed from the atmosphere by the plants in recent decades. After a fire the re-growth of burned lands re-absorbs large amounts of carbon from the atmosphere. So there is essentially no net increase in carbon to the atmosphere over the course of decades.
"The biosphere is like a sponge," said carbon emissions researcher Jason Neff of the University of Colorado, who is Wiedinmyer's coauthor. The biosphere absorbs carbon until it is filled, then fires squeeze the carbon back out, he said.
The size of the sponge could be changing in some places where earlier spring melts caused by global warming are tied to greater wildfire frequency, said climate researcher Anthony Westerling of the University of California at Merced.
That's the case in the northern Rockies, he said, as well as parts of California's Sierra Nevada. Changes in the frequency of fires could, in turn, lead to entirely different plant communities taking over -- some of which may absorb less carbon, he said.
The National Center for Atmospheric Research
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