The present-day aerosol effective radiative forcing (ERF_aer) relative to the pre-industrial era is a critical metric for climate models: climate sensitivity and ERF_aer are the most important two uncertain properties controlling a model’s ability to simulate historical temperature trends that agree with the observational record. ERF_aer is impacted by multiple factors, including the representation of both natural and anthropogenic aerosol species, and the radiation and cloud processes driving the model’s response to these aerosols.

In this presentation, we highlight the critical role of the natural background aerosol as a major factor driving an Earth System Model’s simulated ERF_aer, especially through its impacts on the forcing associated with aerosol-cloud interactions (ERF_aci). Using the Department of Energy’s Energy Exascale Earth System Model version 3 (E3SMv3), we demonstrate that the absolute magnitude of the (negative) ERF_aci can be substantially reduced by manipulations to the model that increase the concentration of natural background aerosol. Direct manipulation of the background cloud droplet number concentration through a prescribed minimum bound is an even more effective method of reducing ERF_aci. We compare the impacts of these two different mechanisms for reducing ERF_aci in E3SMv3, showing both commonalities and differences in the impacts on both aerosols and clouds.

Our results, together with similar findings from other climate modeling centers, highlight a need for increased attention to improving (1) fundamental understanding of the processes that control the natural background concentrations of cloud condensation nuclei, (2) the representation of these processes in Earth System models, and (3) the development and application of improved observational constraints that can reduce uncertainties in natural aerosol processes at Earth System scales.