The effect of atmospheric sulfate reductions on diffuse radiation and photosynthesis in the United States during 1995–2013

Aerosol optical depth (AOD) has been shown to influence the global carbon sink by increasing the fraction of diffuse light, which increases photosynthesis over a greater fraction of the vegetated canopy. Between 1995 and 2013, U.S. SO 2 emissions declined by over 70%, coinciding with observed AOD reductions of 3.0 ± 0.6% yr −1 over the eastern U.S. In the Community Earth System Model (CESM), these trends cause diffuse light to decrease regionally by almost 0.6% yr −1 , leading to declines in gross primary production (GPP) of 0.07% yr −1 . Integrated over the analysis period and domain, this represents 0.5 Pg C of omitted GPP. A separate upscaling calculation that used published relationships between GPP and diffuse light agreed with the CESM model results within 20%. The agreement between simulated and data‐constrained upscaling results strongly suggests that anthropogenic sulfate trends have a small impact on carbon uptake in temperate forests due to scattered light.