CLIMATE MODEL STUDIES OF SULFATE AEROSOLS AND CLOUDS

It is generally believed that increases in the atmospheric concentrati ons 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 c limate system and ameliorate the warming. The sulphate cooling may be separated into the direct effect in cloud-free regions and the indirec t effect in cloudy regions. This paper summarizes recent work at the H adley Centre on the indirect effect. Results from two versions of the Hadley Centre Climate Model are shown, using various parameterizations linking the amount of sulphate aerosol and the number concentration o f droplets in water clouds. These results illustrate the considerable uncertainties in estimating the indirect effect. It is shown that othe r naturally occurring components of the aerosol population, in particu lar sea salt, may be important in reducing the magnitude of the indire ct effect.