  We quantify the effective radiative forcing (ERF) of anthropogenic aerosols
modelled by the aerosol-climate model CAM5.3-MARC-ARG. CAM5.3-MARC-ARG is a new
configuration of the Community Atmosphere Model version 5.3 (CAM5.3) in which
the default aerosol module has been replaced by the two-Moment, Multi-Modal,
Mixing-state-resolving Aerosol model for Research of Climate (MARC).
CAM5.3-MARC-ARG uses the default ARG aerosol activation scheme, consistent with
the default configuration of CAM5.3. We compute differences between simulations
using year-1850 aerosol emissions and simulations using year-2000 aerosol
emissions in order to assess the radiative effects of anthropogenic aerosols.
We compare the aerosol column burdens, cloud properties, and radiative effects
produced by CAM5.3-MARC-ARG with those produced by the default configuration of
CAM5.3, which uses the modal aerosol module with three log-normal modes (MAM3).
Compared with MAM3, we find that MARC produces stronger cooling via the direct
radiative effect, stronger cooling via the surface albedo radiative effect, and
stronger warming via the cloud longwave radiative effect. The global mean cloud
shortwave radiative effect is similar between MARC and MAM3, although the
regional distributions differ. Overall, MARC produces a global mean net ERF of
-1.75$\pm$0.04 W m$^{-2}$, which is stronger than the global mean net ERF of
-1.57$\pm$0.04 W m$^{-2}$ produced by MAM3. The regional distribution of ERF
also differs between MARC and MAM3, largely due to differences in the regional
distribution of the cloud shortwave radiative effect. We conclude that the
specific representation of aerosols in global climate models, including aerosol
mixing state, has important implications for climate modelling.
