Climate models suggest that forest fires drive global warming by releasing greenhouse gases. The resulting climate change then lengthens the forest fire season and increases the number of fires each year, thereby pumping more greenhouse gas into the atmosphere and further exacerbating atmospheric warming. But a new study says that despite emitting heat-trapping methane and carbon dioxide into the atmosphere, fires in temperate zone (or boreal) forests may actually cool the climate significantly, because they leave behind a landscape that reflects sunlight.

Jim Randerson and his colleagues at the University of California, Irvine, measured the amount of radiation being absorbed and reflected by an area in central Alaska's Donnelly Flats that was ravaged by a fire in 1999. They found that the burning boreal forest immediately released large amounts of greenhouse gases. These gases absorb the sun's radiation and trap heat in the atmosphere, thereby causing warming in the first year after the fire. Black ash from the blaze fell on snow and sea ice, which soaked up additional solar radiation. During the spring following the fire, however, the researchers noted that the area had fewer trees and that the exposed snow reflected more sunlight, slightly offsetting the increased amount of absorbed energy.

As the area recovered from the fire in the following years, deciduous birch and aspen trees replaced the charred conifers. In summer, the bright green leaves reflected more light compared with the darker spruce needles of the prefire forest. In winter, because the new trees had lost their leaves, the snow-covered ground was exposed and reflected more sunlight. Using satellite images of nearby areas scarred from fires over the past eight decades, the researchers measured how the reflectivity of a fire-ravaged landscape changes over time. In the 80 years following the fire, the researchers predict that the surface reflection will cancel the impact of greenhouse gases initially emitted from the fire and, while causing local cooling, the fire would have no net effect on the global climate.

The study, which appears in today's issue of Science, focused on the boreal forests of the northern hemisphere, and Randerson predicts that fires in the tropics may have a different role in global climate because of the lack of snow. The lighter-colored tropical forest canopies also reflect radiation and exert a cooling effect, according to Ken Caldeira, a climate scientist at the Carnegie Institution in Washington, D.C. Whereas losing the dark, boreal forests to fire may have a local cooling effect and no overall influence on global climate, destruction of the tropical forests could contribute to global warming, making it "doubly important to protect tropical forests," Caldeira says.

Randerson's findings have implications for tree-planting projects designed to sequester carbon by trapping it in forests. If large areas are reforested, he says, "it might be that you accumulate carbon in the forest, but you might darken the surface, too." Darkening the surface with forests could cause more radiation to be absorbed, which in turn could lead to climate warming. "Planting forests everywhere isn't necessarily going to lead to net global cooling, and forest fires don't necessarily lead to net global warming," sums up Chris Field, also from the Carnegie Institution. "It reminds us that we need to have a sophisticated, multidimensional view of the way ecosystems affect climate."