As Canadian wildfire smoke continues to impact large swaths of the United States, resulting in poor air quality and negative health outcomes for millions of Americans, more people than ever are feeling the effects of longer fire seasons and a changing climate. Now, researchers at Washington University in St. Louis have discovered that wildfires may have even bigger climate impacts than previously thought.

In a new study published Aug. 7 in Nature Geoscience, researchers led by Rajan Chakrabarty, the Harold D. Jolley Career Development Associate Professor in the Department of Energy, Environmental & Chemical Engineering at the McKelvey School of Engineering, found that wildfires are causing a much greater warming effect than has been accounted for by climate scientists. The work, which focuses on the role of “dark brown carbon” — an abundant but previously unknown class of particles emitted as part of wildfire smoke — highlights an urgent need to revise climate models and update approaches for the changing environment.

To conduct a comprehensive analysis of what makes up wildfire smoke plumes, Chakrabarty’s team spent 45 days traveling to different wildfire locations in the western United States, where they sampled gaseous smoke and aerosol species and analyzed their chemical and optical properties. This research was conducted as part of the Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) field campaign, a joint venture led by the National Aeronatics and Space Administration and the National Oceanic and Atmospheric Administration.

“The conventional understanding has been that dark plumes of wildfire smoke contain black carbon soot, which absorbs solar radiation, while lighter plumes contain mostly organic carbon that scatters sunlight, meaning it offsets the absorption or warming effect of soot,” Chakrabarty said. “Typically, climate models ignore or dismiss organic carbon as insignificant compared to black carbon when it comes to warming, but that is not what field observations reveal.

“This is not a binary picture. Instead, we are looking at an entire continuum where there is strong light absorption by organic carbon, or dark brown carbon, similar to black carbon,” he said.

During their sampling of ground and airborne smoke from largescale wildfires, Chakrabarty’s team encountered an abnormally strong light absorber in plumes that wasn’t black carbon, yet it accounted for more than half of observed total absorption.