forcing relates to the direct interaction of aerosol particles with the incoming solar radiation in, essentially, non-cloudy conditions. Indirect forcing relates to forcing under cloudy conditions, during which the aerosol and the cloud may interact in a number of ways: the aerosol may increase the cloud droplet concentration, thereby influencing the cloud albedo; the aerosol may influence cloud persistence; or it may reduce the possibility of precipitation. Even modestly confident quantification of indirect forcing is not yet possible.

CHARGE TO THE PANEL

In their letter to the chair of the Board on Atmospheric Sciences and Climate (BASC) of the National Research Council, the director of the Environmental Sciences Division, Department of Energy; the chief of the Climate and Hydrologic Systems Branch, National Aeronautics and Space Administration; the director of the Office of Global Programs, National Oceanic and Atmospheric Administration; and the director of the Division of Atmospheric Sciences, National Science Foundation, requested that

In view of the potential significance of climate forcing by both man-made and natural aerosols and the relatively undeveloped ability to describe this forcing in climate models, we are writing to request that BASC advise the government on development of a strategy and potential program plan for a U.S. research effort. Given current agency priorities concerning climate and global change, we feel that the appropriate scientific focus for the U.S. program of aerosol research is the climatic effects of aerosol particles …

(see Chapter 1 for the complete Statement of Task). This report summarizes current understanding of the effects of aerosols on climate change and recommends a research program to identify and prioritize the research required to determine the effects of aerosol forcing on the atmosphere's energy balance.

FINDINGS

The panel's main findings are that (1) anthropogenic aerosols reduce the amount of solar radiation reaching the Earth's surface, (2) anthropogenic aerosols provide a negative climate forcing function for large regions, (3) global models suggest that sulfate aerosols produce a direct forcing in the Northern Hemisphere of the same order of magnitude as that from anthropogenic greenhouse gases, but opposite in sign, and (4) there is substantial uncertainty about the magnitude and spatial distribution of the radiative forcing by aerosols. Reduction in this uncertainty requires a scientifically and administratively integrated research program that could organize the



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