Cloud-aerosol interactions in subtropical marine stratocumulus weaken in a warmer climate

TL;DR

This study uses large-eddy simulations to show that aerosol-cloud interactions weaken in a warmer climate, reducing their cooling effect by over 30%, with implications for climate feedbacks and geoengineering.

Contribution

It demonstrates that doubling CO2 attenuates aerosol-induced cloud radiative effects and highlights the sensitivity of low cloud feedbacks to aerosol concentrations.

Findings

Aerosol-induced cloud changes are inhibited in doubled-CO2 conditions.

Doubling CO2 reduces aerosol-induced radiative cooling by over 30%.

Low cloud feedbacks depend on background aerosol levels.

Abstract

Radiative effects of aerosol-cloud interactions constitute the most uncertain climate forcing of the Earth system, making it important to understand how they may change with climate. We conduct 3-day-long large-eddy simulations of a stratocumulus-to-cumulus transition along an airmass-following trajectory over the Northeast Pacific Ocean. By perturbing boundary layer aerosol concentrations, we simulate aerosol-cloud interactions in both present-day and doubled-CO2 conditions. Aerosol-induced cloud changes, including the Twomey effect and adjustments of cloud fraction and liquid water path, are inhibited in a doubled-CO2 climate. Decomposing the aerosol-induced cloud radiative effect change ($\Delta$CRE) reveals that aerosol-induced cloud fraction changes dominate $\Delta$CRE. Overall, doubling CO2 attenuates aerosol-induced $\Delta$CRE (i.e., cooling) by >30% in our simulations. Our results also show that low cloud feedbacks are sensitive to the background aerosol concentration, highlighting the interplay between climate forcings and feedbacks. These results may aid in predicting the cooling potential of marine cloud brightening in a changing climate.