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Forest fires may lead to cooling of northern climate

November 17, 2006

Brighter, snowy surface offsets impact of greenhouse gases, UCI study finds

Irvine, Calif. - Countering hypotheses that forest fires in Alaska, Canada and Siberia warm the climate, scientists at UC Irvine have discovered that cooling may occur in areas where charred trees expose more snow, which reflects sunlight into space.

This finding suggests that taking steps to prevent boreal forest fires to limit the release of carbon dioxide and methane - the most influential greenhouse gases - may unintentionally warm the climate in northern regions. Unusually large fires devoured forests in these areas over the past decade, and scientists predict that with climate warming, fires may occur more frequently over the next several centuries as a result of a longer fire season. Sunlight absorbed by the Earth tends to cause warming, while heat reflected back into space tends to cause cooling.

"Boreal forest fires release greenhouse gases that contribute to climate warming, but inseparable changes in the forest canopy cause more sunlight to be reflected back into space during spring and summer for many decades after fire," said James Randerson, associate professor of Earth system science at UCI and lead author of the study. "This cooling effect cancels the impact of the greenhouse gases, so the net effect of fire is close to neutral when averaged globally, and in northern regions may lead to slightly colder temperatures."

Randerson and UCI scientists Kathleen Treseder, Michael Goulden and Charles Zender published their research in the current online edition of Science.

This is the first study to simultaneously analyze all aspects of how boreal fires influence climate. Previous studies by other scientists have suggested that fires in boreal regions accelerate climate warming because greenhouse gases from burning trees and vegetation are released into the atmosphere. Greenhouse gases absorb infrared radiation and trap heat.

The scientists focused on the Donnelly Flats fire in central Alaska, which burned about 16,549 acres in mid-June 1999. After the fire, scientists took field measurements of incoming and outgoing radiation, carbon dioxide being absorbed or emitted by plants, wind speed and other conditions in Donnelly Flats. They took similar measurements on nearby land that burned in 1987 and on land that burned in approximately 1920.

Scientists found that, right after the fire, large amounts of greenhouse gases entered the atmosphere and caused warming. Ozone levels increased, and ash from the fire fell on remote sea ice and the Greenland ice sheet, darkening the surface and causing more radiation from the sun to be absorbed. The following spring, however, the landscape within the perimeter of the fire was brighter than before the fire because fewer trees shaded the ground. Snow on the ground - more exposed after the fire - reflected more sunlight back into space, leading to cooling.

As years passed, lighter-colored deciduous trees such as aspen and birch grew to replace the dark conifer forest. When they lost their leaves in the winter, the snow-covered ground was more exposed. Younger trees also take in carbon dioxide at a faster rate than older trees. After 80 years, enough conifer trees grow back to darken the landscape and push the ecosystem toward a more climate-neutral state.

This study has implications for reforestation projects in which a primary goal is keeping carbon dioxide out of the atmosphere to slow climate warming. "We need to explore all possible ways to reduce the accumulation of greenhouse gases in the atmosphere. Improving the efficiency of our use of fossil fuels has to be our highest priority," Randerson said. "Storing carbon in terrestrial ecosystems also can help, but we have to consider all of the different ways that ecosystems can influence climate."

Scientists tracked the change in the amount of radiation entering and leaving the climate system as a result of the fire - a measurement closely related to global air temperature. Typically, fires in boreal regions occur in the same area every 80 to 150 years. Scientists, however, found that when fires occur more frequently, more radiation is lost from the Earth and net cooling results. Specifically, they determined when fire returns 20 years earlier than anticipated, 0.5 watts per square meter of area burned are absorbed by the Earth from greenhouse gases, but more snow exposure and brighter surfaces causes 0.9 watts per square meter to be reflected back into space. The net effect is cooling. Watts are used to measure the rate at which energy is gained or lost from the Earth.

University of California - Irvine