Burning the bushes. Prescribed burns can save big trees and keep more carbon sequestered.

U.S. National Park Service

Burn the Brush, Save Carbon Dioxide

Smokey the Bear would not approve, but researchers have found that setting small fires in forests can keep a large amount of carbon dioxide (CO2) from reentering the atmosphere as a greenhouse gas.

As long as trees stay moist and healthy, they can retain the carbon in their wood for many years. But drought is a recipe for disaster. Wildfires caused by lightning strikes or careless campers can race through woodlands, particularly if the forest floor is chock-full of dry branches and leaves. The worst fires destroy entire forests and expose the ground to erosion, and U.S. fires alone release 100 million metric tons of CO2 into the atmosphere per year—nearly 2% of the country's total.

The situation has become acute in the western United States, where decades of fire fighting have led to a buildup of brush. Compounding that, chronic drought has rendered millions of forested hectares vulnerable to massive blazes. One approach to reducing the risk of a major fire is to clear out the brush, either by logging or by setting small, controlled fires called prescribed burns. Until now, no one knew whether these burns, which spare big trees, cut back the amount of CO2 released by forest fires.

So, atmospheric chemist Christine Wiedinmyer of the National Center for Atmospheric Research in Boulder, Colorado, and her colleague Matthew Hurteau of Northern Arizona University in Flagstaff, decided to find out. They had been studying the effects of wildfires on air quality and had developed a model that predicts emissions, including CO2, from various types of forest fires. "We realized that we could use the model to look at prescribed burning in the western U.S," Wiedinmyer says.

Over several years, the duo ran simulations of forest wildfires and prescribed burns. "We wanted to quantify the amount of CO2 that could potentially be reduced if prescribed burning was used widely and wildfires were reduced," Wiedinmyer says. "We didn't know if this was a big number or inconsequential."

The result, she and Hurteau report this week in Environmental Science & Technology, was that prescribed burns can reduce CO2 emissions from fires by as much as 60% in certain forest systems. The strategy could produce several benefits, Wiedinmyer says. It could result in lower carbon emissions than wildfires, and it could reduce the risk of severe wildfires. "Prescribed burns also offer air-quality benefits," she says, by reducing other pollutants such as particulate matter, mercury, and the chemical precursors of ozone.

Fire climatologist Anthony Westerling of the University of California, Merced, agrees that the research shows that prescribed burns can curb forest fire severity and preserve carbon sequestration. "These forests will eventually burn," he says, so the choice is between severe and uncontrolled blazes or fires that proceed under managed conditions.