Background Conditions Influence the Estimated Cloud Radiative Effects of Anthropogenic Aerosol Emissions From Different Source Regions.

Using the Community Earth System Model, with the Community Atmosphere Model version 5.3, we investigate the cloud radiative effects of anthropogenic aerosols emitted from different source regions and global shipping. We also analyze aerosol burdens, cloud condensation nuclei concentration, liquid water path, and ice water path. Due to transboundary transport and sublinearity in the response of clouds to aerosols, the cloud radiative effects of emissions from a given source region are influenced by emissions from other source regions. For example, the shortwave cloud radiative effect of shipping is −0.39 ± 0.03 W/m2 when other anthropogenic emissions sources are present (the "present‐day background" assumption) compared with −0.60 ± 0.03 W/m2 when other anthropogenic emissions sources are absent (the "natural background" assumption). In general, the cloud radiative effects are weaker if present‐day background conditions are assumed compared with if natural background conditions are assumed. Assumptions about background conditions should be carefully considered when investigating the climate impacts of aerosol emissions from a given source region. Plain Language Summary: Particulate air pollutants interact with clouds, influencing Earth's radiation budget. We use a global climate model to investigate the cloud radiative effects of particulate emissions from different source regions and global shipping. Due to nonlinearity, the radiative effects of emissions from one region are influenced by emissions from other regions—this should be taken into account when investigating the climate impacts of particulate pollutants. Key Points: The responses of the shortwave and the longwave cloud radiative effects to aerosol emissions are sub‐linearThe cloud radiative effects of anthropogenic aerosol emissions from a given source region are sensitive to emissions from other regionsThe cloud radiative effects are weaker under present‐day background conditions compared with under natural background conditions [ABSTRACT FROM AUTHOR]