Background Conditions Influence the Estimated Cloud Radiative Effects ofAnthropogenic Aerosol Emissions from Different Source Regions

Due to transboundary transport, anthropogenic aerosol emissions from a given region can influence aerosol abundance in other regions. With nonlinearity in the response of clouds to aerosols, this could likely lead to a result that aerosol emitted from a major source region, e.g. a given continent or country or even international shipping over a given area, could influence cloud radiative effects in other regions. Using the Community Earth System Model or CESM with an interactive aerosol module, we have investigated and demonstrated such an influence, which can be substantial in certain cases. Due to the same reason, our results actually further suggest that the estimated aerosol radiative effects, whether for a region or the globe, are sensitive to the configuration of aerosol emissions or burden in all the major sources. For example, the shortwave cloud radiative effect of global shipping when derived in our modeling with other anthropogenic emissions sources are set to the “present-day background”, is only about 65% of that when derived with 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 to 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 region.