Global Dimming and climate models

It is indeed true that the first climate change simulations were predominantly concerned with greenhouse gas forced climate change. Albeit very early papers argue that man-made aerosol changes might cause a cooling (Rasool and Schneider, Science 1971 and Bryson, Science 1974). And already in 1990, J. Hansen and A. Lacis (Science 1990) published a paper where they explicitly discuss the importance of anthropogenic aerosol forcing: “Sun and Dust versus Greenhouse Gas Forcing”. The authors list direct effects of increasing concentrations of light scattering sulfate aerosols, light absorbing carbonaceous aerosols like soot and even aerosol effects on cloud properties (indirect aerosol effects). Finally they conclude that “… solar variability will not counteract greenhouse warming and that future observations will need to be made to quantify the role of tropospheric aerosols … ”. The surface dimming effect was not yet considered an important climate factor. Back then, state-of-the-art climate models changed the reflectivity at the top of the atmosphere to account for the climate effect of increases in man-made aerosol emissions. Using cloud properties from independent climate simulations and weather forecast models to provide monthly mean water vapor and temperature fields, Kiehl and Briegleb (Science 1993) estimated a top of the atmosphere global mean human-related sulfate aerosol forcing of -0.3 W/m2 in contrast to the a +2.1 W/m2 greenhouse gas forcing. A year later, Jones, Roberts and Slingo (Nature 1994) added the indirect aerosol effect – the impact of increasing sulfate aerosol concentrations on cloud droplet sizes – which make look cloud darker. They used empirical relations to link the number of aerosol particles and number of cloud droplets to cloud droplet radii for their estimates. Other groups as well, started testing new prognostic cloud schemes for general circulation models that were able to capture the microphysical processes of cloud formation (e.g. MPI in Hamburg – Lohmann and Roeckner, Climate Dynamics 1996). These climate-modeling developments were compared (Wild et al., JC 1995) with the then available observational data of the surface solar radiation (incidentally the same data sets were used by Russak (Tellus 1990), Stanhill and Moreshet (Climatic Change 1992) and myself (Liepert et al., Contr. Atm. Physics 1994) to reconstruct the history of global dimming). One conclusion was that then-current models did not include enough aerosol absorbtion in the atmosphere (and Wild and I (GRL, 1998) wrote a paper on the “Excessive Transmission of Solar Radiation Through the Cloud-free Atmosphere in GCMs”). Note that at the end of 1990s these more complex climate models with a more physically based prognostic cloud scheme were run as equilibrium experiments hence transient 20th Century changes could not be used directly for comparison. But is has always been clear that anthropogenic aerosols are so temporally and spatially variable that long-term means are not adequate in assessing the actual aerosol forcing.

Several publications on model validations and improvements based on surface solar radiation records followed and I was involved in two of these studies. We analyzed the simulated multi-decadal changes in the direct tropospheric aerosol forcing in the NASA GISS GCM and utilized global dimming time series of the United States and Germany to assess the temporal change prognosed in the model (Liepert and Tegen, JGR 2002). Ina Tegen’s aerosol model was one of the first that added time variations in carbonaceous aerosol components (including black carbon – an absorbing aerosol). We concluded that increasing absorption might actually play a stronger role than expected.

Climate simulations are the primary tools for explaining and understanding observations that might otherwise seem counterintuitive. For instance, how global dimming can go hand in hand with global warming. In 2004, at the MPI in Hamburg my coauthors and I (Liepert et al. GRL 2004) analyzed output data from a brand new version of the ECHAM general circulation model (GCM) that incorporated a fully interactive aerosol module and aerosol-cloud-scheme. This model interactively calculates aerosol chemical transformation, aerosol transport, rainout and fallout processes and even aerosol formation for some species (e.g. sulfate). We showed that, in the model, global warming caused changing rainfall patterns that fed back on aerosol distribution and composition supressing the water cycle (i.e. evaporation) as had been observed. The key to explaining the apparent contradiction was that the surface forcing changes can be very large without affecting the top-of-the-atmosphere radiation as much.

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