Study results appear in an article in the Jan. 2, 2004, issue of Science. "Many scientists have attributed this atmospheric variability to changes in the balance between plant growth and microbial respiration," said James Randerson of the University of California-Irvine, one of the study's authors. "Our work indicates, however, that the sum of these two processes has a smaller impact on atmospheric carbon dioxide levels than previously believed."
The scientists also determined almost all the increased levels of carbon dioxide and methane measured during 1997 and 1998 could be attributed to the worldwide fires at the time, underscoring the impact El Nino has on greenhouse gas emissions. Carbon is stored in vegetation, and when the vegetation burns, the carbon returns to the atmosphere.
By combining satellite data and measurements of atmospheric gases, researchers from NASA, the University of California- Irvine and other institutions have for the first time quantified the amount of greenhouse gases, like carbon dioxide and methane, emitted by these fires on global and interannual scales.
The researchers found that fire emissions of greenhouse gases increased across multiple continental regions in 1997-98, including Southeast Asia (60 percent of the global increase), Central and South America (30 percent), and boreal forests of North America and Eurasia (10 percent).
"The rate of accumulation of these gases in the atmosphere is highly variable," Randerson said. "Vast areas of the tropics dry out and become vulnerable to fire during El Nino events. It appears that El Nino 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."
Scientists today are trying to understand the relationship between the carbon cycle and the climate system. The carbon cycle is the movement of carbon, in its many forms, among the biosphere, atmosphere, oceans and the geosphere. The cycling of carbon affects the amount of carbon-based greenhouse gases in the atmosphere and thus the Earth's climate. This study shows carbon loss in the biosphere over the next several centuries may be sensitive to the intensity and variability of El Nino- induced droughts.
"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 due to burning."
Satellite measurements of many Earth system properties were input into fire models. "The work involves data products from instruments aboard NASA's Terra, Aqua, Tropical Rainfall Measuring Mission and Active Cavity Radiometer Irradiance Monitor satellites," said Guido R. van der Werf of NASA's Goddard Space Flight Center, Greenbelt, Md., one of the study's authors. NASA also processed data on vegetation, precipitation, surface air temperatures and changes in Earth's radiation, all used in the fire modeling. Data from the European Space Agency's Along Track Scanning Radiometer was used as well.
The study authors, in addition to Randerson and van der Werf, include G. James Collatz and Louis Giglio of NASA's Goddard Space Flight Center; Prasad S. Kasibhatla and Avelino F. Arellano Jr. of Duke University, Durham, N.C.; Seth Olsen of the California Institute of Technology, Pasadena; and Eric S. Kasischke of the University of Maryland, College Park.
The research was funded by NASA's Earth Science Enterprise, which is dedicated to understanding the Earth as an integrated system. The Enterprise applies Earth System Science to improve prediction of climate, weather and natural hazards using the unique vantage point of space.
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