Distant Wildfires Cause Arctic Cooling

It's hard to imagine that the raging blazes of wildfires could cool things down, but that is the conclusion of a new study.

Robert Stone of the University of Colorado, Boulder, and colleagues report that wildfire smoke that reaches the Arctic has the net effect of cooling the surface by reducing the amount of sunlight that makes it through.

The effect of smoke and other aerosol particles in the atmosphere is a large source of uncertainty in climate models. The new work removes some of that uncertainty.

"They have given us a much better understanding of what the effect of smoke is on the climate system," said Alan Robock of Rutgers University in New Brunswick, N.J.

The team made measurements in 2004 when widespread wildfires in Alaska and western Canada sent large plumes of smoke into the Arctic, including over their research station in Barrow, Alaska.

They measured the density of the smoke in the atmosphere and the net amount of energy from the sun hitting the surface to find the relationship between the amount of smoke and the reduction in energy reaching the ground.

When plugged into models, this information helped predict how the smoke would behave over other surfaces, such as the ocean, where the researchers could not make measurements.

The team found that smoke particles tend to absorb energy well above ground, but because they also reflect incoming radiation, the net effect at the surface is cooling. They published their results today in the Journal of Geophysical Research-Atmospheres.

The effect is greatest over dark surfaces, including the ocean, where most of the energy that makes it through is absorbed. Smoke still reduces the amount of radiation that hits ice-covered surfaces, but since these bright areas already reflect most of the radiation that hits them, smoke cover doesn't change the net amount of energy at the surface by as much.

"If the climate warms and you have more severe and frequent wildfires, that will have this tendency to cool the surface," Stone said. It is not yet possible to say how significant of an effect this might be, he added.

California's ongoing wildfires are unlikely to have a large effect on the Arctic, he said, because circulation patterns don't favor California's air masses reaching the Arctic.

However, "every year there are tremendous natural wildfires in Siberia," Stone said. "Once they take off they can burn for weeks and months." That air is readily carried to the Arctic, he added.

The smoke may have additional effects that Stone's models don't account for. For instance, the warming effect of smoke particles at higher altitudes may evaporate clouds, or the particles may also act to seed cloud formation, increasing cloudiness.

"This is a natural phenomenon, in that lightning starts the fires," Robock said. "But it's an anthropogenic phenomenon if the trees are more susceptible to fires because of greenhouse gases."