QUANTIFYING AND MINIMIZING UNCERTAINTY OF CLIMATE FORCING BY ANTHROPOGENIC AEROSOLS

Anthropogenic aerosols are composed of a variety of aerosol types and components including water-soluble inorganic species (e.g., sulfate, n itrate, ammonium), condensed organic species, elemental or black carbo n, and mineral dust. Previous estimates of the clear sky forcing by an thropogenic sulfate aerosols and by organic biomass-burning aerosols i ndicate that this forcing is of sufficient magnitude to mask the effec ts of anthropogenic greenhouse gases over large regions. Here, the unc ertainty in the forcing by these aerosol types is estimated. The clear sky forcing by other anthropogenic aerosol components cannot be estim ated with confidence, although the forcing by these aerosol types appe ars to be smaller than that by sulfate and biomass-burning aerosols. T he cloudy sky forcing by anthropogenic aerosols, wherein aerosol cloud condensation nuclei concentrations are increased, thereby increasing cloud droplet concentrations and cloud albedo and possibly influencing cloud persistence, may also be significant. in contrast to the situat ion with the clear sky forcing, estimates of the cloudy sky forcing by anthropogenic aerosols are little more than guesses, and it is not po ssible to quantity the uncertainty of the estimates. In view of presen t concerns over greenhouse gas-induced climate change, this situation dictates the need to quantify the forcing by anthropogenic aerosols an d to define and minimize uncertainties in the calculated forcings. In this article, a research strategy for improving the estimates of the c lear sky forcing is defined. The strategy encompasses five major, and necessarily coordinated, activities: surface-based observations of aer osol chemical and physical properties and their influence on the radia tion field; aircraft-based observations of the same properties; proces s studies to refine model treatments; satellite observations of aeroso l abundance and size distribution; and modeling studies to demonstrate consistency between the observations, to provide guidance for determi nation of the most important parameters, and to allow extension of the limited set of observations to the global scale. Such a strategy, if aggressively implemented, should allow these effects to be incorporate d into climate models in the next several years. A similar strategy fo r defining the magnitude of the cloudy sky forcing should also be poss ible, but the less firm understanding of this forcing suggests that re search of a more exploratory nature be carried out before undertaking a research strategy of the magnitude recommended for the clear sky for cing.