cloud lifetime effect, observations of ship tracks show that polluted clouds have less liquid water, not more (Platnick et al., 2000; Coakley and Walsh, 2002). Thus, the enhancement of cloud water through the suppression of precipitation in polluted clouds is still not understood completely and therefore contributes to the significant uncertainty attributed to the aerosol indirect forcing.

Liu and Daum (2002) estimated that the magnitude of the first indirect aerosol effect can be reduced by 10 to 80 percent by including the influence that an increasing number of cloud droplets has on the shape of the cloud droplet spectrum. When this dispersion effect is taken into account in global climate models, the reduction is rather moderate and amounts to 15 to 35 percent (Peng and Lohmann, 2003; Rotstayn and Liu, 2003). Lower estimates of the indirect aerosol effect are in better agreement with inverse calculations based on historical climate record data of oceanic and atmospheric warming (Forest et al., 2002; Knutti et al., 2002; Anderson et al., 2003a).

The presence of ice in large-scale or convective clouds allows for further aerosol-cloud interactions. For large-scale, mixed-phase clouds,

FIGURE 2-2 Schematic of the warm indirect aerosol effect (blue arrows) and glaciation indirect aerosol effect (red arrows). SOURCE: Adapted from Lohmann (2002).

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