PNNL

Abstract

Understanding the preindustrial (PI) to present-day (PD) change in cloud droplet number concentration (?Nd) is important to constrain the anthropogenic influence on clouds and aid future model projections. Perturbed parameter ensemble simulations of PI and PD conditions in the Energy Exascale Earth System Model (E3SMv3) reveal two locations with consistent negative ?Nd signal across parameter space–the Southeastern US and the United Kingdom–despite a trend of positive ?Nd globally from increased fossil fuel consumption. These negative ?Nd signals are driven by higher anthropogenic biofuel emissions in the PI. By varying the PI aerosol mass and/or number emissions across these datasets’ uncertainty ranges, we can change the magnitude and sign of ?Nd and the effective radiative forcing from aerosol-cloud interactions (ERFaci) regionally and globally, with near-zero ?Nd leading to near-zero ERFaci. These results highlight the need to constrain PI emission uncertainty to better understand Earth system responses to aerosol-cloud interactions.