particles; such clouds are more reflective and therefore have a negative radiative forcing. These smaller cloud droplets can also decrease the precipitation efficiency and prolong cloud lifetime; this is known as the second indirect aerosol effect. The so-called semidirect aerosol effect occurs when absorption of solar radiation by soot leads to an evaporation of cloud droplets. The IPCC Third Assessment Report gave an estimated range for the radiative forcing associated with the first indirect aerosol effect (0 to −2 W m−2); this range was larger than the uncertainty attributed to any of the other forcings, reflecting in large part the very low level of scientific understanding. Potential magnitudes of the second indirect effect and the semi-direct effect were not estimated in the report.

A number of research avenues hold promise for improving understanding of indirect and semidirect aerosol effects and better constraining estimates of their magnitudes. These include climate modeling, laboratory measurements, field campaigns, and satellite measurements. To improve the representation of the indirect effect in climate models, fundamental research is needed on the physical and chemical composition of aerosols, aerosol activation, cloud microphysics, cloud dynamics, and subgrid-scale variability in relative humidity and vertical velocity.

RECOMMENDATIONS:

• Conduct integrated and comprehensive field investigations to quantify indirect aerosol radiative forcings—for example, in closure experiments with redundant observational and modeling studies.

• Enhance the value of information derived from satellite instruments with targeted field campaigns and greater support for analysis of long-term surface records.

Improve understanding and parameterizations of the indirect aerosol radiative and nonradiative effects in GCMs using process models, laboratory measurements, field campaigns, and satellite measurements.

• Report the different indirect aerosol radiative forcings in climate change assessments and provide better estimates of the associated uncertainties.

Better Quantify the Direct Radiative Effects of Aerosols

Aerosols have direct radiative effects in that they scatter and absorb radiation. Knowledge of direct radiative forcing of aerosols is limited to a large extent by uncertainty in the global distribution and mixing states of the aerosols and by the role of different sources in contributing to atmospheric concentrations. Mixing states have major implications for aerosol optical properties that are not well understood and are difficult to parameterize in climate models. Another factor of uncertainty in representing



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