Smoke from wildfires may be worse for climate than thought

Wildfire smoke can harm the climate and atmosphere much more than previously estimated, a new study based on data collected by a NASA aircraft warns.

Researchers from Georgia Institute of Technology in the US found that carbon particles released into the air from burning trees and other organic matter are much more likely to travel to the upper levels of the atmosphere, where they can interfere with rays from the Sun - sometimes cooling the air and at other times warming it.

The team analysed air samples collected in 2012 and 2013 by NASA aircraft from the upper troposphere - about 11 kilometres above the Earth's surface - at locations across the US. They found surprising levels of brown carbon in the samples but much less black carbon.

Black carbon can be seen in the dark smoke plumes rising above burning fossil or biomass fuels at high temperature, researchers said.

While brown carbon is produced from the incomplete combustion that occurs when grasses, wood or other biological matter smolders, as is typical for wildfires.

As particulate matter in the atmosphere, both can interfere with solar radiation by absorbing and scattering the Sun's rays. The climate is more sensitive to those particulates as their altitude increases.

The team found that brown carbon appears much more likely than black carbon to travel through the air to the higher levels of the atmosphere where it can have a greater impact on climate.

After the brown carbon is carried by smoke plumes into the lower atmosphere, it mixes with clouds. Then it hitches a ride on the deep convection forces that exist in clouds to travel to the upper atmosphere, researchers said.

"Most of the brown carbon released into the air stays in the lower atmosphere, but a fraction of it does get up into the upper atmosphere, where it has a disproportionately large effect on the planetary radiation balance - much stronger than if it was all at the surface," said Rodney Weber, professor at Georgia Tech.

"The surprise here is that the brown carbon gets promoted when you go through the cloud, compared to black carbon. This suggests that there may be in-cloud production of brown carbon that we were not aware of before," said Athanasios Nenes, professor at Georgia Tech.

The study was published in the journal Nature Geoscience.