Wildfires May Briefly Slow Arctic Warming, Study Says

Sara Goudarzi
for National Geographic News

July 25, 2008

The rapidly warming Arctic may be given a brief annual reprieve by smoke from North American wildfires, which cools the surface for weeks or months at a time, a new study found.

The smoke creates a veil of aerosols—tiny liquid and solid particles suspended in air—that reduces the amount of sunlight, temporarily lowering surface air temperatures.

The effect may last weeks to months during late spring through autumn if smoke is widely dispersed, potentially offsetting some of the warming caused by greenhouse gases, researchers say.

(Learn how the greenhouse effect works.)

When wildfires raged through Alaska and Canada in the summer of 2004, study lead author Robert Stone, from the University of Colorado and the National Oceanic and Atmospheric Administration (NOAA), and colleagues analyzed the fires' impacts on the amount of the sun's energy reaching Earth's surface.

The team used data collected at a NOAA climate observatory near Barrow, Alaska.

"That particular summer, the smoke from those fires drifted directly over Barrow, creating a natural laboratory to study this event … ," Stone said.

The smoke layer was so thick that the total absorption and scattering of the sun's energy rose a hundredfold above typical summer values.

Stone's study is detailed this week in the Journal of Geophysical Research—Atmospheres.

Dark vs. Light

Like clouds, smoke cover can impact how much sunlight surfaces absorb.

Dark surfaces, such as land or open ocean, absorb most of the sunlight that reaches the surface. This process can make surface air temperatures warmer over dark regions.

But bright surfaces, such as sea ice or snow, reflect as much as 88 percent of that light back into the atmosphere, experts say.

When the surface is obscured by clouds, sunlight is blocked, cooling the region underneath.

In recent years sea ice has been retreating and snow is melting earlier each year across much of the Arctic, which means that more dark surfaces become exposed to the sun, Stone explained.

"But then you bring in the smoke and it [absorbs more] over those dark surfaces. This represents a negative feedback in a warming world," he said.

Another factor to consider is the so-called dirty snow effect, according to Alan Robock of Rutgers University.

Ice and snow will become darker and melt faster if smoke particles settle on those surfaces.

"So it's not clear that the effects of the smoke are good anyway, as people and animals breathe the smoke. And the net effects on the climate system are still not well quantified," said Robock, who was not involved in the study.

Additionally, it's important to note that smoke absorbs sunlight to warm the atmosphere.

"If the atmosphere is heated by the aerosols, some of that energy also heats the surface," Robock said.

Carbon Dioxide Impact

The study authors did not quantify carbon dioxide—a leading greenhouse gas—released as a result of the fires.

"There is probably some counterbalance," study author Stone said. "The aerosol [from smoke] has a very significant short-term effect on the region where smoke is dispersed, while emitted [carbon dioxide] is more widely dispersed over time and can persist for years."

Carbon dioxide most likely has a negligible short-term effect in the Arctic, he added.

Rutgers' Robock agrees.

"This was carbon that was taken out of the atmosphere when the trees grew, and so if forests are in balance there's no net effect on carbon dioxide."

"After the fires burn there will be more trees growing there taking the [carbon dioxide] back out of the atmosphere," he said. "So that's not really a concern."