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Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties
BOX 4-1 Two Case Studies of Regional Aerosol Forcing
South Asian Pollution Observed over the North Indian Ocean
The figure below shows the anthropogenic aerosol forcing over the North Indian Ocean averaged for January-April during 1996 to 1999 for available INDOEX observations (Ramanathan et al., 2001a). The bar chart shows the direct forcing, the indirect forcing, and the greenhouse forcing at the top of the atmosphere, the surface, and the atmosphere. The sum of the direct and the indirect forcing at the surface is as much as −20 W m−2, which amounts to a reduction of about 10 percent of the absorbed solar radiation at the surface. A correspondingly large positive forcing of 15 W m−2 is exerted on the atmosphere. The positive atmospheric forcing is due largely to the soot and dust absorption of solar radiation. The TOA forcing is a small difference (−5 W m−2) between two large competing terms at the surface and the atmosphere. If only the TOA radiative forcing is considered, one would conclude that the direct climate effect of Asian aerosols is near zero.
Four-year (1996 to 1999) average of direct and indirect aerosol forcing during the dry season (January to April) observed during the INDOEX campaign. The data are averaged over the North Indian Ocean, from the equator to 25°N and from 60°E to 90°E. SOURCE: Ramanathan et al. (2001a).
the order of −10 to −15 W m−2 and the atmospheric forcing on the order of +10 to +15 W m−2 (see Box 4-1). For such aerosols, the TOA forcing is an ineffective, if not erroneous, metric for the impact of aerosol forcing on the surface temperature.
One way to address this limitation of the traditional radiative forcing concept is to calculate the global mean radiative forcing at the surface along with that at the top of the atmosphere. Considering the surface radiative forcing may enable quantification of the effects of aerosols on the surface