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Radiative forcing of aerosols compared between different climate models

Radiative forcing of aerosols compared between different climate models

The radiative forcing associated with anthropogenic aerosols varies between different climate models even when the spatial distribution of aerosols is prescribed to be the same in the different models.

Human action has increased the burden of atmospheric aerosols, thereby causing an increased reflection of solar radiation to space. This has resulted in a negative radiative forcing that acts to cool the climate, which has partly offset the warming effect of increased greenhouse gases. The aerosol radiative forcing is, however, still rather poorly known, which is a major uncertainty in the estimates of anthropogenic climate change.

Finnish Meteorological Institute (FMI) participated in a study, which compared the aerosol radiative forcing simulated by different climate models. The models were run in a setting where the spatial distribution and optical properties of anthropogenic aerosols, and their impact on the cloud droplet number concentration, was prescribed to be the same in the different models. The purpose was to evaluate inter-model differences in aerosol radiative forcing that originate from other properties of the climate models than the treatment of aerosol physics, for example, the distribution of simulated cloudiness. These differences proved to be significant: for an aerosol distribution corresponding to year 2005, the global mean radiative forcing varied between -0.9 and -0.4 W m-2.

The study also compared the aerosol radiative forcing for years 1975 and 2005. The distribution of air pollution was quite different in these years: in 1975, the maximum aerosol burden was located in Europe, while by 2005 it had shifted to Southeast Asia. Nevertheless, all models considered showed consistently nearly the same global-mean radiative forcing for the two periods, on average -0.54 W m-2 for year 1975 and -0.59 W m-2 for year 2005.

The research was coordinated by the Max Planck Institute, Germany. Researchers from Germany, Finland, the United Kingdom, Switzerland and the Netherlands participated in the study.  FMI contributed through experiments with the NorESM and EC-Earth climate models. The research was funded by the European Union and the Academy of Finland.

Further information:

Senior research scientist Petri Räisänen, Finnish Meteorological Institute, tel. +358 29 539 2224,

Fiedler, S., Kinne, S., Huang, W. T. K., Räisänen, P., O'Donnell, D., Bellouin, N., Stier, P., Merikanto, J., van Noije, T., Makkonen, R., and Lohmann, U.: Anthropogenic aerosol forcing – insights from multiple estimates from aerosol-climate models with reduced complexity, Atmos. Chem. Phys., 19, 6821-6841,, 2019

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