Assessing Regional Differences in Aerosol-Cloud Interactions and their Contribution to the Global Indirect Effect

Anthropogenic aerosol perturbations to clouds and climate (the indirect effect, or AIE) contribute significant uncertainty towards understanding contemporary climate change. However, AIE - usually defined as a change in cloud radiative forcing associated with and resulting from changes in anthropogenic aerosol emissions - is just one global metric for assessing aerosols' influence on clouds. In fact, AIE is the end result of a chain or sequence of aerosol-cloud microphysical interactions starting at droplet activation and ending in the radiative/optical properties of a cloud. In this work, we utilize a suite of global aerosol-climate models to assess differences in each chain of these microphysical interactions, including one model with different sets of aerosol-cloud parameterizations. While each model simulates different cloud droplet number burdens in the global average, we find these differences to be greatest over remote maritime regions with little anthropogenic pollution. The interactions - and subsequent indirect effect - critically depend on the baseline cloud droplet number predicted in these regions. Models with lower droplet number, on average, in these regions tend to simulate much stronger indirect effects. These results motivate an improvement in observing the microphysical properties of clouds - particularly droplet number and size distributions across their lifetime - in remote, clean, maritime regions as a means to reduce uncertainty in AIE.