7.5.3 Effects of Aerosols and Clouds on Solar Radiation at the Earth’s Surface

By increasing aerosol and cloud optical depth, anthropogenic emissions of aerosols and their precursors contribute to a reduction of solar radiation at the surface. As such, worsening air quality contributes to regional aerosol effects. The partially conflicting observations on solar dimming/brightening are discussed in detail in Section 3.4 and Box 3.2. This section focuses on the possible contribution by aerosols. The decline in solar radiation from 1961 to 1990 affects the partitioning between direct and diffuse solar radiation: Liepert and Tegen (2002) concluded that over Germany, both aerosol absorption and scattering must have declined from 1975 to 1990 in order to explain the simultaneously weakened aerosol forcing and increased direct/diffuse solar radiation ratio. The direct/diffuse solar radiation ratio over the USA also increased from 1975 to 1990, likely due to increases in absorbing aerosols. Increasing aerosol optical depth associated with scattering aerosols alone in otherwise clear skies produces a larger fraction of diffuse radiation at the surface, which results in larger carbon assimilation into vegetation (and therefore greater transpiration) without a substantial reduction in the total surface solar radiation (Niyogi et al., 2004; Section 7.2.6.2).

For the tropical Indian Ocean, Ramanathan et al. (2001) estimate an indirect aerosol effect of –5 W m^–2 at TOA and –6 W m^–2 at the surface. While the direct effect is negligible at TOA, its surface forcing amounts to –14 W m^–2 as a consequence of large atmospheric absorption in this region. In South Asia, absorbing aerosols may have masked up to 50% of the surface warming due to the global increase in greenhouse gases (Ramanathan et al., 2005). Global climate model estimates of the mean decrease in surface shortwave radiation in response to all aerosol effects vary between –1.3 and –3.3 W m^–2 (Figure 7.23). It is larger than the TOA radiation flux change because some aerosols like black carbon absorb solar radiation within the atmosphere (see also Jacobson, 2001; Lohmann and Feichter, 2001; Ramanathan et al., 2001; Liepert et al., 2004). As for the TOA net radiation, the decrease is largest over land, with values approaching –9 W m^–2. Consistent with the above-mentioned regional studies, most models predict larger decreases over land than over the oceans.

Figure 7.23. Global mean change in net solar radiation at the surface due to the total anthropogenic aerosol effect (direct, semi-direct and indirect cloud albedo and lifetime effects) from pre-industrial times to the present day and its contribution over the NH and SH, over oceans and over land and the ratio over oceans/land. Red bars refer to anthropogenic sulphate (Easter et al., 2004; Ming et al., 2005⁺), blue bars to anthropogenic sulphate and organic carbon (Quaas et al., 2004; Rotstayn and Liu, 2005⁺), turquoise bars to anthropogenic sulphate and black and organic carbon (Menon and Del Genio, 2005; Takemura et al., 2005; Johns et al., 2006; Storelvmo et al., 2006), dark purple bars to anthropogenic sulphate and black and organic carbon effects on water and ice clouds (Jacobson, 2006; Lohmann and Diehl, 2006), teal bars refer to a combination of GCM and satellite results (LMDZ/ECHAM plus MODIS, Quaas et al., 2006), green bars refer to results from coupled atmosphere/mixed-layer ocean (MLO) experiments (Feichter et al., 2004: sulphate and black and organic carbon; Kristjansson et al., 2005: sulphate and black carbon; Rotstayn and Lohmann, 2002⁺: sulphate only) and olive bars to the mean from all simulations. Vertical black lines refer to ±1 standard deviation.

⁺ refers to estimates solely from the indirect effects

Transient simulations (Roeckner et al., 1999) and coupled GCM-mixed-layer ocean equilibrium simulations (Feichter et al., 2004; Liepert et al., 2004) suggest that the decrease in solar radiation at the surface resulting from increases in optical depth due to the direct and indirect anthropogenic aerosol effects is more important for controlling the surface energy budget than the greenhouse-gas induced increase in surface temperature. There is a slight increase in downwelling longwave radiation due to aerosols, which in the global mean is small compared to the decrease in shortwave radiation at the surface. The other components of the surface energy budget (thermal radiative flux, sensible and latent heat fluxes) decrease in response to the reduced input of solar radiation. As global mean evaporation must equal precipitation, a reduction in the latent heat flux in the model leads to a reduction in precipitation (Liepert et al., 2004). This is in contrast to the observed precipitation evolution in the last century (see Section 3.3) and points to an overestimation of aerosol influences on precipitation. The simulated decrease in global mean precipitation from pre-industrial times to the present may reverse into an increase of about 1% in 2031 to 2050 as compared to 1981 to 2000, because the increased warming due to black carbon and greenhouse gases then dominates over the sulphate cooling (Roeckner et al., 2006).