First forcing estimates from the future CMIP6 scenarios of anthropogenic aerosol optical properties and an associated Twomey effect

Abstract

<p><strong>Abstract.</strong> We present the first forcing interpretation of the future anthropogenic aerosol scenarios of CMIP6 with the simple plumes parameterisation MACv2-SP. The nine scenarios for 2015 to 2100 are based on SO₂ and NH₃ emissions for use in CMIP6 (Riahi et al., 2017; Gidden et al., in prep.). We use the emissions to scale the observationally informed anthropogenic aerosol optical properties and the associated effect on the cloud albedo of present-day (Fiedler et al., 2017; Stevens et al., 2017) into the future. The resulting scenarios in MACv2-SP are then ranked according to their strength in forcing magnitude and spatial asymmetries. Almost all scenarios show a decrease in anthropogenic aerosol by 2100 with a range of 108<span class="thinspace"></span>% to 36<span class="thinspace"></span>% of the anthropogenic aerosol optical depth in 2015. We estimate the spread in the radiative forcing associated with the scenarios in the mid-2090s by performing ensembles of simulations with the atmosphere-only configuration of MPI-ESM1.2. MACv2-SP herein translates the CMIP6 emission scenarios for inducing aerosol forcing. With the implementation in our model, we obtain forcing estimates for both the shortwave instantaneous (RF) and effective radiative forcing (ERF) relative to 1850. Here, ERF accounts for rapid atmospheric adjustments and natural variability internal to the model. The spread for the mid-2090s is &amp;minus;0.20 to &amp;minus;0.57<span class="thinspace"></span>Wm^&amp;minus;2 (&amp;minus;0.15 to &amp;minus;0.54<span class="thinspace"></span>Wm^&amp;minus;2) for RF (ERF) of anthropogenic aerosol, associated with uncertainty in the emission pathway alone, i.e., the mid-2090s forcing ranges from 33&amp;ndash;95% (30&amp;ndash;108%) of the mid-2000s RF (ERF). We find a larger ERF spread of &amp;minus;0.15 to &amp;minus;0.92<span class="thinspace"></span>Wm^&amp;minus;2, when we additionally consider uncertainty in the magnitude of the Twomey effect. The year-to-year standard deviations around 0.3<span class="thinspace"></span>Wm^&amp;minus;2 associated with natural variability highlights the necessity for averaging over sufficiently long time periods for estimating ERF, in contrast to RF that is typically well constrained after simulating just one year. The scenario interpretation of MACv2-SP will be used within the framework of CMIP6 and other cutting-edge scientific endeavours.</p>