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Geoscientific Model Development An interactive open-access journal of the European Geosciences Union
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Discussion papers | Copyright
https://doi.org/10.5194/gmd-2018-244
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Development and technical paper 16 Oct 2018

Development and technical paper | 16 Oct 2018

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This discussion paper is a preprint. It is a manuscript under review for the journal Geoscientific Model Development (GMD).

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

Stephanie Fiedler1, Bjorn Stevens1, Matthew Gidden2, Steven J. Smith3, Keywan Riahi2, and Detlef van Vuuren4,5 Stephanie Fiedler et al.
  • 1Max Planck Institute for Meteorology, Bundesstrasse 53, 20146, Hamburg, Germany
  • 2International Institute for Applied Systems Analysis, Schlossplatz 1, 2361 Laxenburg, Austria
  • 3Joint Global Change Research Institute, Pacific Northwest National Laboratory, College Park, MD, USA
  • 4Utrecht University, Vening Meineszgebouw A, Princetonlaan 8a, 3584 CB Utrecht, the Netherlands
  • 5PBL Netherlands Environmental Assessment Agency, The Hague, the Netherlands

Abstract. 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% to 36% 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 −0.20 to −0.57Wm−2 (−0.15 to −0.54Wm−2) for RF (ERF) of anthropogenic aerosol, associated with uncertainty in the emission pathway alone, i.e., the mid-2090s forcing ranges from 33–95% (30–108%) of the mid-2000s RF (ERF). We find a larger ERF spread of −0.15 to −0.92Wm−2, when we additionally consider uncertainty in the magnitude of the Twomey effect. The year-to-year standard deviations around 0.3Wm−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.

Stephanie Fiedler et al.
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