Air Pollution as a Climate Forcing: A Workshop
Day 2 Presentations
Indirect Aerosol Climate Forcing
Leon D. Rotstayn
CSTRO Atmospheric Research, Aspendale, Victoria, Australia
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The familiar "radiative forcing" bar chart from IPCC (2001) suggests that the indirect aerosol forcing is (a) highly uncertain, and (b) possibly substantial. A rview of evidence shows that the indirect aerosol forcing is probably substantial. This evidence includes in-situ observations, satellite retrievals that include nearly global coverage, and a hierarchy of model calculations. Two additional aspects are not even hinted at by the IPCC bar chart. These are (c) the composition specificity of the indirect aerosol forcing, and (d) the spatial variability of this forcing.
The present understanding of the composition specificity was discussed. Whereas most global model calculations have emphasized the effect of sulfate aerosol, there is mounting evidence that organic aerosols from the burning of fossil fuels and biomass are also important, though the details are very uncertain.
The spatial variability of the indirect aerosol forcing, and the response of the climate sys- tem to this forcing, has received little attention to date. Two recent studies using global climate models (GCMS) have suggested that the indirect effect of anthropogenic sulfate has the potential to provoke a broad southward shift of tropical circulation and rainfall, due to a stronger surface cooling in the Northern Hemisphere than the Southern Hemisphere (Rotstayn et al. 2000; Williams et al. 2001). The response of the climate system to this forcing was further investigated by Rotstayn and Lohmann (2002), using an atmospheric GCM coupled to a mixed-layer ocean model. They ran the model to equilibrium for both present-day (1985) and preindustrial sulfur emission scenarios, and then compared the modeled rainfall changes with observed rainfall trends. Direct aerosol effects were deliberately neglected, to isolate the response to the indirect effect. Both the model and observations show a broad southward shift in the tropics, although the shift is stronger in the model than the observations (Fig. 1).
Both in the modeled changes and the observed trends, a prominent feature was the drying of the Sahel in North Africa (not shown). Previous work has identified a near-global, quasi-hemispheric pattern of contrasting SST anomalies (cool in the Northern Hemisphere and warm in the Southern Hemisphere) associated with dry conditions in the Sahel. The results from Rotstayn and Lohmann (2002), combined with this earlier finding, suggest that the indirect effects of anthropogenic sulfate may have contributed to the Sahelian drying trend. An important question concerns the extent to which the pattern of forcing and response will be altered if the indirect effects of non-sulfate aerosols (such as organic aerosols from biomass burning) are properly accounted for.
A more general conclusion from the above is that spatially varying aerosol-related forcing can substantially alter low-latitude circulation and rainfall. In other words, the globally averaged view shown in the IPCC bar chart is an over-simplification with respect to the indirect aerosol forcing (and, by extension, also with respect to the other aerosol forcings).
- IPCC, 2001: Climate Change 2001: The Scientific Basis - Summary for Policymakers and Technical Summary of the Working Group I Report. Cambridge University Press, Cambridge, United Kingdom, 20 pp.
- Rotstayn, L. D., and U. Lohmann, 2002: Tropical rainfall trends and the indirect aerosol effect. J. Climate. In press.
- Rotstayn, L. D., B. F. Ryan, and J. E. Penner, 2000: Precipitation changes in a GCM resulting from the indirect effects of anthropogenic aerosols. Geophys. Res. Lett., 27, 3045-3048.
- Williams, K. D., A. Jones, D. L. Roberts, C. A. Senior, and M. J. Woodage, 2001: The response of the climate system to the indirect effects of anthropogenic sulfate aerosol. Climate Dyn., 17, 845-856.