It is generally believed that increases in the atmospheric concentrations of greenhouse gases as a result of man's activities are leading to global warming. It is also believed that the same activities lead to increasing concentrations of sulphate aerosol, which act to cool the climate system and ameliorate the warming. The sulphate cooling may be separated into the direct effect in cloud-free regions and the indirect effect in cloudy regions. This paper summarizes recent work at the Hadley Centre on the indirect effect. Results from two versions of the hadley Centre Climate Model are shown, using various parameterizations linking the amountof sulphate aerosol and the number concentration of droplets in water clouds. These results illustrate the considerable uncertainties in estimating the indirect effect. It is shown that other naturally occurring components of the aerosol population, in particular sea salt, may be important in reducing the magnitude of the indirect effect.
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