Sparks fly over study suggesting wildfires cut CO2

Call it a hot topic. A study suggesting that intentional forest blazes could significantly cut carbon dioxide (CO2) emissions from wildfires in the Western United States has prompted a piquant scholarly quarrel. The exchange highlights the challenge forest managers may face in balancing plans to use fire to restore forest ecosystems with efforts to curb carbon emissions.

Forests have emerged as a key player in climate change because trees can suck huge amounts of CO2 out of the atmosphere and "sequester" the carbon for decades. A raging wildfire, however, can reverse those gains in just a few days by vaporizing vast swathes of timber. In the Western United States, concerns about the climate impacts of wildfires have grown, as centuries of fire suppression has left forests packed with tinder ripe for combustion. And researchers fear fire risks could rise in the future, as the western climate become hotter and drier.

To reduce the threat of runaway infernos – and help restore fire-starved forest ecosystems — some researchers advocate "prescribed burns." These intentional fires tend to burn cooler and vaporize less wood, leading some researchers to wonder just how much they might also help reduce CO2 emissions from wildfires.

To find out, Christine Wiedinmyer of the National Center for Atmospheric Research in Boulder, Colorado and Matthew Hurteau Northern Arizona University in Flagstaff, Arizona, estimated how much CO2 had been released by wildfires in the western U.S. from 2001 to 2008. Then, they estimated what the total might have been if the wildfires had been replaced by cooler, more controlled prescribed burns. The result, they reported in the 11 February online issue of Environmental Science & Technology (ES&T), is that the planned fires might have cut CO2 emissions by 18% to 25% in the western U.S., and by as much as 60% in specific forest types.

Those numbers, however, are the product of "a fundamentally unrealistic scenario," argue Garrett W. Meigs and John L. Campbell of Oregon State University in Corvallis. In a comment published online in ES&T on July 23, the pair says the original study makes some "completely unrealistic" assumptions, such as that prescribed fires would be 100% effective in eliminating wildfires and that no prescribed fire would escape control. Still, the critics say Wiedinmyer and Hurteau did make some "important
improvements" in clarifying the climate implications of forest fires. For instance, they concluded that wildfires release about twice as much carbon per-unit-area as prescribed fires, far less than the 10-fold increase assumed by earlier studies. Overall, however, Miegs and Campbell say prescribed burns to improve ecosystem health may well reduce the carbon-storing capacity of forests over the short run. And they worry that "the authors present misleading conclusions that could result in flawed forest carbon policies."

Wideinmyer and Hurteau fire back in a response in the same issue, writing that the criticism stems from a "misunderstanding" of a key term, and a "faulty assessment" of their methods. Their goal, they note, was to set an "upper bound" on the potential benefits of prescribed burns, and not to advocate for particular policies. Not surprisingly, both sides suggest that the fire and climate issue would benefit from more research – perhaps after a cooling off period. – David Malakoff

Sources: Wiedinmyer, C., & Hurteau, M. (2010). Prescribed Fire As a Means of Reducing Forest Carbon Emissions in the Western United States. Environmental Science & Technology, DOI: 10.1021/es902455e

Meigs, G., & Campbell, J. (2010). Comment on "Prescribed Fire As a Means of Reducing Forest Carbon Emissions in the Western United States." Environmental Science & Technology DOI: 10.1021/es101595t

Hurteau, M., & Wiedinmyer, C. (2010). Response to Comment on "Prescribed Fire As a Means of Reducing Forest Carbon Emissions in the Western United States." Environmental Science & Technology DOI: 10.1021/es102186b