A040-0015
Insights in the reasons for the model spread in the radiative forcing of anthropogenic aerosols

Tuesday, 8 December 2020
Poster
Stephanie Fiedler1, Nicolas Bellouin2, Alf Kirkevag3, Ulrike Lohmann4, Risto J. Makkonen5, Joonas Merikanto6, Declan O'Donnell6, Petri Räisänen6, Adriana Sima7, Christopher James Smith8, Steven Smith9, Philip Stier10 and Twan van Noije11, (1)University of Cologne, Institute of Geophysics and Meteorology, Cologne, Germany, (2)University of Reading, Department of Meteorology, Reading, United Kingdom, (3)Norwegian Meteorological Institute, Section for Climate Modelling and Air Pollution, Oslo, Norway, (4)ETH Zurich, Zurich, Switzerland, (5)University of Helsinki, Helsinki, Finland, (6)Finnish Meteorological Institute, Helsinki, Finland, (7)Laboratoire de Météorologie Dynamique Palaiseau, Palaiseau Cedex, France, (8)University of Leeds, School of Earth and Environment, Leeds, LS2, United Kingdom, (9)Joint Global Change Research Institute, College Park, MD, United States, (10)University of Oxford, Department of Physics, Oxford, United Kingdom, (11)Royal Netherlands Meteorological Institute, De Bilt, Netherlands
Abstract:
Climate models still have a large spread in the effective radiative forcing of anthropogenic aerosols (ERF), despite decades of research on aerosol effects on climate and the importance for understanding past and future climate change. In this presentation, I give an overview on our ongoing works for better understanding the reasons for model diversity in ERF using a deliberate reduction of the aerosol complexity in climate models. The results are based on several hundred years of climate simulations with contemporary models like those used in the new assessment report of the Intergovernmental Panel on Climate Change. The anthropogenic aerosol optical properties and the associated effect on clouds are prescribed with the simple-plumes aerosol parameterisation (MACv2-SP), currently used in the coupled model inter-comparison project phase 6 (CMIP6). Implementing MACv2-SP across several climate models allows us to assess the impact of model differences on ERF due to other factors than the diversity in aerosol parameterisations. The highlights of our studies so far are: (1) a strong impact of the year-to-year model-internal variability on the ERF magnitude, which underlines the need for averaging over several decades to quantify the ERF of a climate model, (2) a persistent model spread in ERF even in the case of identical anthropogenic aerosol optical properties and an effect on clouds, pointing to significant contributions to the ERF spread from model components other than the aerosol parameterisations, and (3) a surprisingly small change in the global ERF with the substantially different aerosol patterns from the mid-1970s to the mid-2000s, despite model differences in the ERF magnitude and in the representation of clouds. Further research with MACv2-SP is currently carried out in the framework of the Radiative Forcing Model Intercomparison Project (RFMIP) and elsewhere, making use of the MACv2-SP scaling from 1850 to 2100 based on the CMIP6 aerosol emissions. This new line of research is promising to better understand model differences in anthropogenic aerosol forcing and the climate response.

Find out more:

Fiedler et al. (2019a) https://doi.org/10.5194/acp-19-6821-2019

Fiedler et al. (2019b) https://doi.org/10.5194/gmd-12-989-2019

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