2.4.1 Introduction and Summary of the Third Assessment Report
The TAR categorised aerosol RFs into direct and indirect effects. The direct effect is the mechanism by which aerosols scatter and absorb shortwave and longwave radiation, thereby altering the radiative balance of the Earth-atmosphere system. Sulphate, fossil fuel organic carbon, fossil fuel black carbon, biomass burning and mineral dust aerosols were all identified as having a significant anthropogenic component and exerting a significant direct RF. Key parameters for determining the direct RF are the aerosol optical properties (the single scattering albedo, ωo, specific extinction coefficient, ke and the scattering phase function), which vary as a function of wavelength and relative humidity, and the atmospheric loading and geographic distribution of the aerosols in the horizontal and vertical, which vary as a function of time (e.g., Haywood and Boucher, 2000; Penner et al., 2001; Ramaswamy et al., 2001). Scattering aerosols exert a net negative direct RF, while partially absorbing aerosols may exert a negative top-of-the-atmosphere (TOA) direct RF over dark surfaces such as oceans or dark forest surfaces, and a positive TOA RF over bright surfaces such as desert, snow and ice, or if the aerosol is above cloud (e.g., Chylek and Wong, 1995; Haywood and Shine, 1995). Both positive and negative TOA direct RF mechanisms reduce the shortwave irradiance at the surface. The longwave direct RF is only substantial if the aerosol particles are large and occur in considerable concentrations at higher altitudes (e.g., Tegen et al., 1996). The direct RF due to tropospheric aerosols is most frequently derived at TOA rather than at the tropopause because shortwave radiative transfer calculations have shown a negligible difference between the two (e.g., Haywood and Shine, 1997; Section 2.2). The surface forcing will be approximately the same as the direct RF at the TOA for scattering aerosols, but for partially absorbing aerosols the surface forcing may be many times stronger than the TOA direct RF (e.g., Ramanathan et al., 2001b and references therein).
The indirect effect is the mechanism by which aerosols modify the microphysical and hence the radiative properties, amount and lifetime of clouds (Figure 2.10). Key parameters for determining the indirect effect are the effectiveness of an aerosol particle to act as a cloud condensation nucleus, which is a function of the size, chemical composition, mixing state and ambient environment (e.g., Penner et al., 2001). The microphysically induced effect on the cloud droplet number concentration and hence the cloud droplet size, with the liquid water content held fixed has been called the ‘first indirect effect’ (e.g., Ramaswamy et al., 2001), the ‘cloud albedo effect’ (e.g., Lohmann and Feichter, 2005), or the ‘Twomey effect’ (e.g., Twomey, 1977). The microphysically induced effect on the liquid water content, cloud height, and lifetime of clouds has been called the ‘second indirect effect’ (e.g., Ramaswamy et al., 2001), the ‘cloud lifetime effect’ (e.g., Lohmann and Feichter, 2005) or the ‘Albrecht effect’ (e.g., Albrecht, 1989). The TAR split the indirect effect into the first indirect effect, and the second indirect effect. Throughout this report, these effects are denoted as ‘cloud albedo effect’ and ‘cloud lifetime effect’, respectively, as these terms are more descriptive of the microphysical processes that occur. The cloud albedo effect was considered in the TAR to be an RF because global model calculations could be performed to describe the influence of increased aerosol concentration on the cloud optical properties while holding the liquid water content of the cloud fixed (i.e., in an entirely diagnostic manner where feedback mechanisms do not occur). The TAR considered the cloud albedo effect to be a key uncertainty in the RF of climate but did not assign a best estimate of the RF, and showed a range of RF between 0 and –2 W m–2 in the context of liquid water clouds. The other indirect effects were not considered to be RFs because, in suppressing drizzle, increasing the cloud height or the cloud lifetime in atmospheric models (Figure 2.10), the hydrological cycle is invariably altered (i.e., feedbacks occur; see Section 7.5). The TAR also discussed the impact of anthropogenic aerosols on the formation and modification of the physical and radiative properties of ice clouds (Penner et al., 2001), although quantification of an RF from this mechanism was not considered appropriate given the host of uncertainties and unknowns surrounding ice cloud nucleation and physics.
Figure 2.10. Schematic diagram showing the various radiative mechanisms associated with cloud effects that have been identified as significant in relation to aerosols (modified from Haywood and Boucher, 2000). The small black dots represent aerosol particles; the larger open circles cloud droplets. Straight lines represent the incident and reflected solar radiation, and wavy lines represent terrestrial radiation. The filled white circles indicate cloud droplet number concentration (CDNC). The unperturbed cloud contains larger cloud drops as only natural aerosols are available as cloud condensation nuclei, while the perturbed cloud contains a greater number of smaller cloud drops as both natural and anthropogenic aerosols are available as cloud condensation nuclei (CCN). The vertical grey dashes represent rainfall, and LWC refers to the liquid water content.
The TAR did not include any assessment of the semi-direct effect (e.g., Hansen et al., 1997; Ackerman et al., 2000a; Jacobson, 2002; Menon et al., 2003; Cook and Highwood, 2004; Johnson et al., 2004), which is the mechanism by which absorption of shortwave radiation by tropospheric aerosols leads to heating of the troposphere that in turn changes the relative humidity and the stability of the troposphere and thereby influences cloud formation and lifetime. In this report, the semi-direct effect is not strictly considered an RF because of modifications to the hydrological cycle, as discussed in Section 7.5 (see also Sections 2.2, 2.8 and 2.4.5).
Since the TAR, there have been substantial developments in observations and modelling of tropospheric aerosols; these are discussed in turn in the following sections.