The uncertainty in present-day anthropogenic forcing is dominated by
uncertainty in the strength of the contribution from aerosol. Much of
the uncertainty in the direct aerosol forcing can be attributed to
uncertainty in the anthropogenic fraction of aerosol in the present-day
atmosphere, due to a lack of historical observations. Here we present a
robust relationship between total present-day aerosol optical depth and
the anthropogenic contribution across two multi-model ensembles and a
large single-model perturbed parameter ensemble. Using observations of
aerosol optical depth, we determine a reduced likely range of the
anthropogenic component and hence a reduced uncertainty in the direct
forcing of aerosol.