Variability in Biomass Burning Emissions Weakens Aerosol Forcing Due To Nonlinear Aerosol-Cloud Interactions

The magnitude of the aerosol forcing remains among the largest unknowns when assessing climate sensitivity over the historical period. Here, we quantify and explain a crucial but often overlooked source of uncertainty in aerosol forcing: the temporal variability of aerosol emissions. We show that time-variability in biomass burning (BB) emissions weakens the time-averaged total aerosol forcing, particularly in the Northern Hemisphere mid- to high-latitudes. BB emissions variability produces weaker (less negative) mean effective radiative forcing (ERF) compared to scenarios with no interannual variability in emissions. Satellite-estimated BB emissions (and associated variability) result in a June-September absolute ERF (relative to zero BB emissions) of -7.7 W m(-2) from 50 degrees to 70 degrees N, compared to -10.4 W m(-2) when no emissions variability is used in the Community Earth System Model version 2 (CESM2). This difference in forcing is attributable to nonlinear aerosol-cloud interactions. Aerosol forcing will be overestimated (i.e., more negative) if emissions are temporally-smoothed.