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Discussion papers | Copyright
https://doi.org/10.5194/acp-2018-639
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 07 Sep 2018

Research article | 07 Sep 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Anthropogenic aerosol forcing – insights from multi-estimates from aerosol-climate models with reduced complexity

Stephanie Fiedler1, Stefan Kinne1, W. T. Katty Huang2, Petri Räisänen3, Declan O'Donnell3, Nicolas Bellouin4, Philip Stier5, Joonas Merikanto3, Twan van Noije6, Ken Carslaw7, Risto Makkonen3,8, and Ulrike Lohmann2 Stephanie Fiedler et al.
  • 1Max Planck Institute for Meteorology, Hamburg, Germany
  • 2Institute for Atmospheric and Climate Science, ETH Zürich, Zürich, Switzerland
  • 3Finnish Meteorological Institute, Helsinki, Finland
  • 4Department of Meteorology, University of Reading, Reading, UK
  • 5Department of Physics, University of Oxford, Oxford, UK
  • 6Royal Netherlands Meteorological Institute, De Bilt, Netherlands
  • 7School of Earth and Environment, University of Leeds, Leeds, UK
  • 8Institute for Atmospheric and Earth System Research/Physics, Faculty of Science, University of Helsinki, Finland

Abstract. The radiative forcing of anthropogenic aerosol remains a key uncertainty in the understanding of climate change. This study quantifies the model spread in aerosol forcing associated with (i) variability internal to the atmosphere and (ii) differences in the model representation of weather. We do so by performing ensembles of atmosphere-only simulations with four state-of-the-art Earth system models, three of which will be used in the sixth coupled model inter-comparison project (CMIP6, Eyring et al., 2016). In those models we reduce the complexity of the anthropogenic aerosol by prescribing the same annually-repeating patterns of the anthropogenic aerosol optical properties and associated effects on the cloud reflectivity. We quantify a comparably small model spread in the long-term averaged ERF compared to the overall possible range in annual ERF estimates associated with model-internal variability. This implies that identifying the true model spread in ERF associated with differences in the representation of meteorological processes and natural aerosol requires averaging over a sufficiently large number of annual estimates. We characterize the model diversity in clouds and use satellite products as benchmarks. Despite major inter-model differences in natural aerosol and clouds, all models show only a small change in the global-mean ERF due to the substantial change in the global anthropogenic aerosol distribution between the mid-1970s and mid-2000s, the ensemble mean ERF being −0.47Wm−2 for the mid-1970s and −0.51Wm−2 for the mid-2000s. This result suggests that inter-comparing ERF changes between two periods rather than absolute magnitudes relative to pre-industrial might provide a more stringent test for a model's ability for representing climate evolutions.

Stephanie Fiedler et al.
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Status: final response (author comments only)
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Stephanie Fiedler et al.
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