Lohmann (2002) showed that if in addition to mineral dust, a fraction of the hydrophilic soot aerosol particles is assumed to act as contact ice nuclei at temperatures between 0° and −35°C, then a “glaciation indirect effect” results (Figure 2-2). Increases in contact ice nuclei in the present-day climate result in more frequent glaciation of clouds and increase the amount of precipitation via the ice phase. Observations in the presence of Saharan African dust indicate that mildly supercooled clouds at temperatures between −5 and −9°C are already glaciated (Sassen et al., 2003).

For convective mixed-phase clouds, Rosenfeld and Woodley (2000) analyzed aircraft data together with satellite data to show that pollution aerosols suppress precipitation. This hypothesis is supported by a modeling study with a cloud-resolving model by Khain et al. (2001). Taking these results to the global scale, Nober et al. (2003) evaluated the sensitivity of the general circulation to the suppression of precipitation by anthropogenic aerosols by implementing a simple warm cloud microphysics scheme into convective clouds. They found large instantaneous local aerosol forcings reducing the warm-phase precipitation (thermodynamic effects). Menon et al. (2002b) showed that absorbing aerosols over China change the atmospheric stability and vertical motion by heating the air and, thus, the large-scale circulation and the hydrological cycle.

The IPCC aviation report (Penner et al., 1999) identified the effects of aircraft on upper tropospheric cirrus clouds as a potentially important climate forcing. One aspect may be described as the direct effect due to the formation of condensation contrails as a result of supersaturated air from the aircraft. This effect can be nonnegligible as was found during the three-day grounding of all U.S. commercial aircraft following the September 11, 2001, terrorist attacks. An anomalous increase in the average diurnal temperature range over the United States was observed and partly attributed to the absence of contrails from jet aircraft (Travis et al., 2002). Aerosols emitted by the aircraft may also cause indirect effects associated with an increase in ice nuclei in the upper troposphere. Evidence of a climate effect of air traffic was first provided by Boucher (1999) who used ship-based measurements of cloud cover together with fossil fuel consumption data for aircraft to show that recent increases in air traffic fuel consumption are accompanied by an increase in cirrus cloudiness. Recent studies (Lohmann and Kärcher, 2002) suggest that the impact of aircraft sulfur emissions on cirrus properties via homogeneous freezing of sulfate aerosols is probably small. Hence the question has been raised whether aircraft-generated black carbon particles serving as heterogeneous ice nuclei, as found by Ström and Ohlsson (1998), may have a significant impact on cirrus cloudiness and cirrus microphysical properties.

Aerosols can also modify latent and sensible heat fluxes at the surface, thus exerting a nonradiative forcing on the hydrological cycle. Increasing

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