James Randerson, assistant professor of Earth system science at UC Irvine, and colleagues report that by combining satellite data and measurements of atmospheric gases, they have quantified for the first time the amount of greenhouse gases, like carbon dioxide and methane, emitted by these fires.
The researchers found that fire emissions of greenhouse gases increased across multiple continents in 1997-98, including an increase of 60 percent in Southeast Asia, 30 percent in Central and South America, and 10 percent in the boreal forests of North America and Eurasia. Study results appear in the Jan. 2, 2004, issue of Science.
"Vast areas of the tropics dry out and become vulnerable to fire during El Niño events," Randerson said. "It appears that El Niño events accelerate carbon loss from terrestrial ecosystems because they enable humans to use fire more effectively as a tool for clearing land in the tropics."
The study is another step toward more fully understanding the carbon cycle - the movement of carbon in its many forms in the biosphere - and its effect on climate. Previous studies have suggested that global warming increases the microbial activity that decomposes plants, releasing more carbon dioxide from soils. Carbon dioxide, one of the forms of carbon, is a greenhouse gas which, when released into the atmosphere, helps raise the Earth's temperature. The new study suggests that future carbon dioxide levels in the atmosphere may actually be more sensitive to the intensity and variability of El Niños than to increased microbial activity caused by warming.
The researchers also found that large-scale fires contributed to most of the year-to-year changes in atmospheric carbon dioxide totals during the period from 1997 to 2001. "Many scientists have attributed this variability to changes in the balance between plant growth and microbial activity," Randerson said. "Our work indicates, however, this has a smaller impact on atmospheric carbon dioxide levels than previously believed."
In addition, the scientists determined that almost all of the increased levels of methane measured during 1997 and 1998 can be attributed to the worldwide fires at the time, underscoring the impact El Niño has on methane emissions.
The study, funded by the NASA's Office of Earth Science, also provides evidence that areas in southern Mexico and Guatemala experienced large scale increases in fire emissions during the 1997-98 El Niño.
"An important next step is to identify the processes that contributed to the high fire emissions, including deforestation, pasture maintenance, agricultural waste burning and savanna fires," Randerson said. "This will help us understand how quickly greenhouse gases like carbon dioxide and methane will accumulate in the atmosphere."
With other researchers in the UCI Department of Earth System Science, Randerson has embarked on a project to study the recent fires in California. Currently, he is conducting field work to study the impact of fires on forests in Alaska and Siberia.
Randerson's colleagues in the El Niño study include Guido R. van der Werf, G. James Collatz and Louis Giglio of the NASA-Goddard Space Flight Center; Prasad S. Kasibhatla and Avelino F. Arellano Jr. of Duke University; Seth Olsen of California Institute of Technology; and Eric S. Kasischke of the University of Maryland.
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