In this chapter we examine how aerosol and cloud fields undergo perturbations by anthropogenic activities. Recent surface observations and satellite remote sensing have detected signatures of large-scale changes in the atmospheric aerosol amounts, associated changes in the cloud fraction, and microphysical structures on a global scale. Models can simulate these signatures fairly well, but problems still remain. Fields of anthropogenic aerosol optical depth (AOD) from several atmospheric models have been found to be consistent with the spatial pattern obtained from satellite data. Further studies are needed to differentiate between natural and anthropogenic aerosols and to interpret observed temporal and regional trends in aerosol parameters. The strength of the cloud-aerosol interaction can be characterized by the regression of AOD or aerosol index (AI) on cloud droplet number (Nc). From recent studies, the corresponding slopes dlog(Nc)/ dlog(4) vary between 0.19 and 0.7. Further work is needed to see whether such variability is the result of methodological problems or differences in cloud environments for which the studies have established the cloud-aerosol relation.

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