Aerosol - cloud - climate effect : Study with a radiative transfer model

The indirect radiative forcing (through the enhancement of cloud albedo) due to anthropogenic sulphate aerosols was calculated using a three-dimensional radiative transfer model. The calculations were made with diurnal and seasonal cycles and three-dimensional distributions of sulphate aerosol concentration data. The globally averaged annual mean indirect forcing is estimated to be -1.13 Wm-2 which is comparable with the recent General Circulation Model calculations. This forcing of -1.13 Wm-2 is compared with 2.12 Wm-2 due to increase in greenhouse gases from the pre-industrial period to the present. This indirect forcing due to anthropogenic aerosol is substantially higher than the warming effect due to gases over the industrial regions in the northern hemisphere, so that the net radiative forcing over these regions is negative. i.e., cooling. Some uncertainties in the estimation of the indirect forcing that needs further research are pointed out.