Calculated Feedback Effects of Climate Change Caused by Anthropogenic Aerosols

Depending on their chemical composition, sizes and shapes, aerosol particles may scatter and absorb solar radiation and act as nuclei for condensation of water vapour and for freezing of water droplets. Availability of cloud condensation (CCN) and ice nuclei (IN) is responsible for the realized water vapour super-saturations in the troposphere. Human activity inadvertently produces aerosol particles. Production mechanisms include combustion of fossil fuels and biomass, leading to submicron particles containing sulphate, nitrate, black carbon and particulate organic matter. These compounds typically reside up to a week in the troposphere and the mixing ratios have considerable gradients. Depending on their composition as a function of size and shape, particles may scatter and absorb solar radiation and act as CCN. Anthropogenic changes in these properties may directly produce radiative forcing, or indirectly through changes in cloud properties. Considerable attention has been paid to the potential climate influence of anthropogenic particles (e.g. Charlson et al., 1987; Wigley, 1989; Charlson et al. 1991; Kiehl and Briegleb, 1993). There is considerable incertitude associated with its quantification, and in particular the indirect effect (Houghton et al., 2001).