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).