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.

Automated summary

The magnitude of aerosol forcing remains uncertain in assessing historical climate sensitivity, with temporal variability of aerosol emissions being a crucial and often overlooked source of uncertainty. This study reveals that biomass burning (BB) emissions variability weakens the time-averaged total aerosol forcing, particularly in the Northern Hemisphere mid- to high-latitudes, due to nonlinear aerosol-cloud interactions. Comparisons show that satellite-estimated BB emissions result in weaker mean effective radiative forcing (ERF) compared to scenarios with no interannual variability in emissions, emphasizing the importance of considering emissions variability in aerosol forcing calculations.