Uncertainty in Climate Change Caused by Aerosols. Stephen E. Schwartz and Meinrat O. Andreae. Science, 272, 1121-1122 (1996).

S. E. Schwartz is in the Environmental Chemistry Division, Brookhaven National Laboratory, Upton NY 11973 USA. Email: ses@bnl.gov. M. O. Andreae is in the Department of Biogeochemistry, Max Planck Institute of Chemistry, D-55020 Mainz, Germany, Email: moa@diane.mpch-mainz.mpg.de.

The National Research Council (NRC) recently issued a report "Aerosol Radiative Forcing and Climate Change" that underscores the importance of anthropogenic aerosols as agents of climate change. Atmospheric aerosols are suspensions of microscopic and submicroscopic particles; in industrial regions and over much of the Northern Hemisphere their sources are dominated by human activity. Anthropogenic aerosols influence climate directly, by scattering solar radiation, and indirectly, by modifying cloud properties. Of all atmospheric pollutants, aerosols are the most evident because they restrict visibility and whiten the otherwise deep blue of the sky, yet understanding of their influence on climate change is beset with uncertainty. Although the NRC report stresses the need to reduce these uncertainties, in our view the NRC panel seriously underestimates the research effort required to reduce the uncertainty in aerosol forcing to the specified level. The task of characterizing tropospheric aerosols, their spatial and temporal variability, their size-dependent chemical and physical properties, and their optical and cloud nucleating effects; of understanding the processes controlling these properties and effects; of representing these processes in models; of evaluating the performance of these models; and of representing these effects in climate models requires a research effort several fold greater than that outlined in the report. In the absence of this research knowledge of climate response to greenhouse forcing necessary for confident policy-making will be reliant entirely on climate models having little credible empirical confirmation.

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