Smoke from wildfires cools planet more than climate models recognize

A new study on biomass-burning aerosols has shown that that smoke from wildfires has more of a cooling effect on the climate system than most climate models assume.

Xiaohong Liu, professor in the Department of Atmospheric Sciences , served as corresponding author of the research, which was recently published in Nature Communications.

When wildfires burn forests and grasslands, biomass-burning aerosols are produced, forming smoke clouds. The composition, size and mixing state of biomass-burning aerosols determine the optical properties of the smoke plumes in the atmosphere, and those properties determine how much the smoke clouds absorb and scatter the solar radiation. The researchers compared climate models’ predictions to large amounts of wildfire data collected in field sites around the world as well in laboratory experiments, and they found major differences.

“We collected field data from across the globe, and we found that the models make a lot of assumptions about the physical and optical properties of the biomass-burning aerosols, and those assumptions were not accurate,” Liu said.

Climate models such as those examined in this study are used to create important climate change reports, such as the Intergovernmental Panel on Climate Change (IPCC) Assessment that is published about every seven years, he explained.

“In this study we evaluate the climate models used in the IPCC assessment, and we found out that these models assume biomass-burning smoke is more light-absorbing than our data actually shows,” Liu said. “Our study shows that biomass-burning has a more net-cooling effect than previously thought.”

The study was led by Hunter Brown, who recently received his doctorate from the University of Wyoming and was advised by Liu during his time as a professor at the University of Wyoming from 2013-2019. Brown recently joined Liu’s team at Texas A&M as a postdoctoral researcher to continue his research on biomass-burning aerosols.

“We found that many of the most advanced climate models simulate biomass-burning aerosols or smoke that is darker, or more light absorbing, than what we see in observations,” Brown said. “This has implications for the climate predictions made by these models.”

Observations and models used in the study covered a wide temporal range. Africa, South America and Southeastern Asia, in addition to boreal fire regions, were chosen because these are the largest contributors to biomass-burning smoke emissions in the world, Brown said.

These biomass-burning aerosols are extremely complicated, Liu said. The complexity involves the aerosols’ physical and chemical properties, which are determined by the fire’s burning conditions and the fuel types — such as dry grasslands or forests.

“The smoke is very complicated — North American forest fire smoke is very different than Australian bush fire smoke, because of the different types of fuel and burning conditions,” he said. “They can produce darker or brighter smoke, and the chemical evolution of the smoke in the atmosphere can be very complicated.”

From the College of Geosciences