Christine Wiedinmyer of the National Center for Atmospheric Research and Jason Neff of the University of Colorado used satellite imaging data to determine the extent of fires over the period 2002-2006. They estimated the output of carbon dioxide, CO2, based on the degree of forest cover in a particular area.
Typical annual emissions from fires are around five percent of the human-caused total for the United States. But during major fires in the western and southeastern United States, the proportion of fire contributions to CO2 emissions can increase.
'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 authors write.
Although the release of carbon dioxide in fires should be balanced over the long-term by the uptake of CO2 as new vegetation grows, the immediate impact of the fires on atmospheric CO2 is significant, the study shows.
The authors say this may create difficulties in accounting for carbon sources and sinks, and in assessing trends with current remote sensing technologies.
Wiedinmyer warns, 'There is a significant potential for additional net release of carbon from the forests of the United States due to changing fire dynamics in the coming decades.'
Although fire emissions are not currently included in most carbon dioxide emission restriction agreements, increasing fire frequency and severity can lead to greater emissions of CO2 to the atmosphere and compound the problems that are related to human-caused emissions.
The fires include wild fires, prescribed burning and burning for agricultural purposes, but the majority of these emissions come from needle leaf forest fires in the western and southeastern states.