Aerosols: Whipping Boy, Monkey Wrench, or Knob? Stephen E. Schwartz

Presented at Gordon Research Conference, Solar Radiation and Climate, Plymouth, NH, June 14-19, 1998

Per rules of the Gordon Research Conferences, please do not cite this abstract.

Atmospheric aerosols are of keen current interest with respect to shortwave radiation because they are the major under-determined atmospheric constituent to which the shortwave budget is highly sensitive. Recent comparisons of measured and modeled diffuse downwelling irradiance at the surface under cloud-free conditions challenge even the long held assumption that aerosol optical depth can be determined from direct beam extinction after subtraction of known gaseous absorption and imply substantial unaccounted-for absorption in the direct beam. Because of their direct light scattering and their indirect influence on shortwave radiation through modification of cloud microphysical and radiative properties, anthropogenic aerosols are thought to substantially perturb the top-of-atmosphere shortwave radiation budget locally and globally and consequently to have exerted a major change in the shortwave radiation budget over the industrial period. This aerosol-induced perturbation, often referred to as a radiative "forcing" of climate change, is quite uncertain and is thought to be the major source of uncertainty in estimates of radiative forcing over the industrial period, precluding confident demonstration of warming due to incremental greenhouse gases and likewise precluding empirical inference of the global temperature sensitivity coefficient, although none of this has kept people from trying. Major sources of uncertainty arise from inability to confidently specify aerosol loading and optical properties from either models or measurement and high sensitivity to relative humidity. Recent model intercomparison suggests that the uncertainty in calculated radiative forcing, under cloud-free conditions, of aerosols having well characterized loading and optical properties, with well specified surface reflectance, contributes relatively little to the uncertainty in aerosol forcing, but even for such well specified aerosols there are substantial model-to-model differences under specific conditions.

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