J. E. Penner,* R. J. Charlson,+ J. M. Hales,** N. S. Laulainen,++ R. Leifer,# T. Novakov,@ Quantifying and Minimizing Uncertainty of Climate Forcing by Anthropogenic Aerosols J. Ogren,## L. F. Radke,@@ S. E. Schwartz,+ and L. Travis++ Abstract Anthropogenic aerosols are composed of a variety of aerosol types and components including water-soluble inorganic species (e.g., sulfate, nitrate, ammonium), condensed organic species, el- emental or black carbon, and mineral dust. Previous estimates oft he clear sky forcing by anthropogenic sulfate aerosols and by organic biomass-burning aerosols indicate that this forcing is of sufficient magnitude to mask the effects of anthropogenic greenhouse gases over large regions. Here, the uncertainty in the forcing by these aerosol types is estimated. The clear sky forcing by other anthropo- genic aerosol components cannot be estimated with confidence, although the forcing by these aerosol types appears to be smaller than that by sulfate and biomass-burning aerosols. The 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 situation with the clear sky forcing, estimates of the cloudy sky forcing by anthropogenic aerosols are little more than guesses, and it is not possible to quantify the uncertainty of the estimates. In view of present concerns over greenhouse gas-induced cli- mate change, this situation dictates the need to quantify the forcing by anthropogenic aerosols and to define and minimize uncertainties in the calculatedforcings.ln this article, a research strategy for improv- ing the estimates of the clear sky forcing is defined. The strategy encompasses five major, and necessarily coordinated, activities: surface-based observations of aerosol chemical and physical prop- erties and their influence on the radiation field; aircraft-based obser- vations of the same properties; process studies to refine model *Lawrence Livermore National Laboratory, Livermore, California. +University of Washington, Seattle, Washington. **ENVAIR, Richland, Washington. ++Pacific Northwest Laboratory, Richland, Washington. "Environmental Measurements Laboratory, New York, New York. @Lawrence Berkeley Laboratory, Berkeley, California. ••NOAA Climate Monitoring and Diagnostics Laboratory, Boulder, Colorado. @@National Center for Atmospheric Research, Boulder, Colorado. +Brookhaven National Laboratory, Upton, New York. ++NASA Goddard Institute for Space Studies, New York, New York. Corresponding author address: J. E. Penner, Lawrence Livermore National Laboratory, Atmospheric Microphysics and Chemistry, P .0. Box 808, L-262, Livermore, California 94550. In final form 31 August 1993. ©1994 American Meteorological Society Bulletin of the American Meteorological Society treatments; satellite observations of aerosol abundance and size distribution; and modeling studies to demonstrate consistency be- tween the observations, to provide guidance for determination 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 incorporated into climate models in the next several years. A similar strategy for defining the magnitude of the cloudy sky forcing should also be possible, but the less firm understanding ofthisforcing suggests that research of a more exploratory nature be carried out before undertak- ing a research strategy of the magnitude recommended for the clear sky forcing. 1. Introduction Recent applications of coupled atmospheric chemi- cal/radiative transfer models have demonstrated, by utilizing empirical aerosol scattering properties, that anthropogenic sulfate and biomass combustion aero- sols in the troposphere may cause a clear sky climatic forcing that is comparable in magnitude but opposite in sign to forcing by anthropogenic greenhouse gases (Charlson et al. 1990, 1991, 1992; Penner et al. 1991, 1992a; IPCC 1992, Kiehl and Briegleb 1993). Al- though these estimates are still quantitatively uncer- tain, the uncertainty is not great enough to allow the aerosol forcing to be negligible. Unlike greenhouse gases, however, these aerosol particles are not uni- formly distributed over the globe but are found mainly in regions influenced by industrial emissions and sporadically over areas where biomass is burned. This nonuniform distribution, in conjunction with the green- house forcing, leads to a differential spatial forcing with net heating in some areas and net cooling in others and with, as yet, unevaluated climate impact. The industrial haze is currently largely over the North- ern Hemisphere, causing a spatially nonuniform, present-day forcing that ranges from zero in clean areas of the Southern Hemisphere to perhaps -4 W m- 2 (annual average) across regions of one thousand to a few thousand square kilometers. It has been suggested that this aerosol forcing may have 375