  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
analyse aerosol burdens, cloud condensation nuclei concentration, liquid water
path, and ice water path. Due to transboundary transport and sub-linearity 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
$-0.39 \pm 0.03$ W m$^{-2}$ when other anthropogenic emissions sources are
present (the `present-day background' assumption) compared with $-0.60 \pm
0.03$ W m$^{-2}$ 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.
