10.2.1.2 Forcing by Additional Species and Mechanisms

The forcing agents applied to each AOGCM used to make climate projections are summarised in Table 10.1. The radiatively active species specified by the SRES scenarios are CO₂, CH₄, N₂O, chlorofluorocarbons (CFCs) and SO₂, which is listed in its aerosol form as sulphate (SO₄) in the table. The inclusion, magnitude and temporal evolution of the remaining forcing agents listed in Table 10.1 were left to the discretion of the individual modelling groups. These agents include tropospheric and stratospheric ozone, all of the non-sulphate aerosols, the indirect effects of aerosols on cloud albedo and lifetime, the effects of land use and solar variability.

The scope of the treatments of aerosol effects in AOGCMs has increased markedly since the TAR. Seven of the AOGCMs include the first indirect effects and five include the second indirect effects of aerosols on cloud properties (Section 2.4.5). Under the more emissions-intensive scenarios considered in this chapter, the magnitude of the first indirect (Twomey) effect can saturate. Johns et al. (2003) parametrize the first indirect effect of anthropogenic sulphur (S) emissions as perturbations to the effective radii of cloud drops in simulations of the B1, B2, A2 and A1FI scenarios using UKMO-HadCM3. At 2100, the first indirect forcing ranges from –0.50 to –0.79 W m^–2. The normalised indirect forcing (the ratio of the forcing (W m^–2) to the mass burden of a species (mg m^–2), leaving units of W mg^–1) decreases by a factor of four, from approximately –7 W mgS^–1 in 1860 to between –1 and –2 W mgS^–1 by the year 2100. Boucher and Pham (2002) and Pham et al. (2005) find a comparable projected decrease in forcing efficiency of the indirect effect, from –9.6 W mgS^–1 in 1860 to between –2.1 and –4.4 W mgS^–1 in 2100. Johns et al. (2003) and Pham et al. (2005) attribute the projected decline to the decreased sensitivity of clouds to greater sulphate concentrations at sufficiently large aerosol burdens.