Atmospheric Composition and Climate Responses to U.S. Air Pollution Controls

Regional-scale perturbations in anthropogenic emissions, such as those due to national air quality controls like the United States Clean Air Act (CAA) of 1970, have significantly altered the composition of short-lived climate forcers, namely ozone (O₃) and aerosols, which have counteracting radiative effects. We utilize a 13-member initial-condition ensemble generated by a fully coupled global chemistry-climate model (CESM2–WACCM6) that simulates the “world avoided” by the CAA from 1970–2014 with increasing US emissions. We compare this to a matching historical base ensemble. By differencing across ensemble means, we reduce the noise from climate variability and quantify the change in anthropogenically forced signal between scenarios, detecting regions and seasons where climate and atmospheric composition variables respond robustly to the divergent emissions trends. We find that reductions in tropospheric O₃ in the historical scenario relative to the “world avoided” are greatest in the summer and near the source region (US), with significant decreases relative to climate variability extending across the Northern Hemisphere. Reductions in aerosols are greatest in the summer, with significance relative to climate variability localized over the US. We find the forced response of annual US surface temperature to be an increase of 0.4 °C, suggesting a stronger and more localized warming effect from US aerosol reductions than the simultaneous cooling effect from O₃ reductions.