Most of the focus of this topic has been directed to the effects of increasing sulphate aerosol which is thought to occur as a by product of the anthropogenic increase of greenhouse gases. These aerosol produce their own radiative influence that may be both comparative in magnitude to the greenhouse forcing and also of opposite sign Kiehl and Breigleb, [1993]; Taylor and Penner, [1994]; Jones et al., [1994]. The real global impact of these aerosol is unfortunately difficult to determine [e.g. Stephens, 1994] for a number of reasons. These aerosol particles directly influence the radiative balance of the planet by reflecting solar radiation to space. A number of attempts to estimate the magnitude of the globally averaged radiative forcing introduced directly by scattering solar radiation exist [e.g. Kiehl and Breigleb, 1993; Charlson et al., 1991]. The major limitation of these studies is the assumptions of how much anthropogenic aerosol exists in the atmosphere (above normal background amounts) and where these aerosols are concentrated around the globe.
Perhaps the most important impact arises via an indirect path associated with the effect of aerosol on the microphysical properties of low stratiform clouds. We currently think of two processes that produce indirect effects [ Stephens, 1994]. One is the so-called Twomey effect that is a mechanism by which cloud reflectivity is enhanced due to increased concentrations of cloud droplets associated with the increased condensation nuclei associated with polluted air. The general idea of the Twomey effect has been demonstrated only in limited ways via study of ship tracks [e.g., King et al., 1993]. Claims that this effect is also apparent in Earth Radiation Budget Experiment (ERBE) data [ Kim and Cess, 1993] are ambiguous and unfortunately inconclusive. This area of research will maintain a high profile in the coming years especially with upcoming field experiments devoted to the topic. The second indirect effect is also related to the impact of aerosol as Cloud Condensation Nuclei (CCNs) on cloud microphysics. By shifting the droplet distributions towards smaller sizes, polluted clouds are less likely to produce drizzle and thus are less likely to rain-out. Shutting off the drizzle formation leads to an enhancement of cloud lifetimes and to an increase in cloud cover and thus reflection from these regions of low cloud.
Recent climate modelling studies have attempted to quantify the forcings associated with these indirect effects [e.g., Boucher et al., 1994; Jones et al., 1994]. Although uncertain, if correct these studies suggest that the indirect aerosol effect is larger than the direct effect.