2.9.3 Global Mean Radiative Forcing by Emission Precursor

The RF due to changes in the concentration of a single forcing agent can have contributions from emissions of several compounds (Shindell et al., 2005). The RF of CH₄, for example, is affected by CH₄ emissions, as well as NO_x emissions. The CH₄ RF quoted in Table 2.12 and shown in Figure 2.20 is a value that combines the effects of both emissions. As an anthropogenic or natural emission can affect several forcing agents, it is useful to assess the current RF caused by each primary emission. For example, emission of NO_x affects CH₄, tropospheric ozone and tropospheric aerosols. Based on a development carried forward from the TAR, this section assesses the RF terms associated with each principal emission including indirect RFs related to perturbations of other forcing agents, with the results shown in Figure 2.21. The following indirect forcing mechanisms are considered:

• fossil carbon from non-CO₂ gaseous compounds, which eventually increase CO₂ in the atmosphere (from CO, CH₄, and NMVOC emissions);
• changes in stratospheric ozone (from N₂O and halocarbon (CFCs, HCFC, halons, etc.) emissions);
• changes in tropospheric ozone (from CH₄, NO_x, CO, and NMVOC emissions);
• changes in OH affecting the lifetime of CH₄ (from CH₄, CO, NO_x, and NMVOC emissions); and
• changing nitrate and sulphate aerosols through changes in NO_x and SO₂ emissions, respectively.

For some of the principal RFs (e.g., BC, land use and mineral dust) there is not enough quantitative information available to assess their indirect effects, thus their RFs are the same as those presented in Table 2.12. Table 2.5 gives the total (fossil and biomass burning) direct RFs for BC and organic carbon aerosols that are used to obtain the average shown in Figure 2.21. Table 2.13 summarises the direct and indirect RFs presented in Figure 2.21, including the methods used for estimating the RFs and the associated uncertainty. Note that for indirect effects through changes in chemically active gases (e.g., OH or ozone), the emission-based RF is not uniquely defined since the effect of one precursor will be affected by the levels of the other precursors. The RFs of indirect effects on CH₄ and ozone by NO_x, CO and VOC emissions are estimated by removing the anthropogenic emissions of one precursor at a time. A sensitivity analysis by Shindell et al. (2005) indicates that the nonlinear effect induced by treating the precursors separately is of the order of 10% or less. Very uncertain indirect effects are not included in Table 2.13 and Figure 2.21. These include ozone changes due to solar effects, changes in secondary organic aerosols through changes in the ozone/OH ratio and apportioning of the cloud albedo changes to each aerosol type (Hansen et al., 2005).

Table 2.12. Global mean radiative forcings since 1750 and comparison with earlier assessments. Bold rows appear on Figure 2.20. The first row shows the combined anthropogenic RF from the probability density function in panel B of Figure 2.20. The sum of the individual RFs and their estimated errors are not quite the same as the numbers presented in this row due to the statistical construction of the probability density function.

Notes: ^a For the AR4 column, 90% value uncertainties appear in brackets: when adding these numbers to the best estimate the 5 to 95% confidence range is obtained. When two numbers are quoted for the value uncertainty, the distribution is non-normal. Uncertainties in the SAR and the TAR had a similar basis, but their evaluation was more subjective. [15%] indicates 15% relative uncertainty, [2x], etc. refer to a factor of two, etc. uncertainty and a lognormal

distribution of RF estimates.

^b The TAR RF for halocarbons and hence the total LLGHG RF was incorrectly evaluated some 0.01 W m^–2 too high. The actual trends in these RFs are therefore more positive than suggested by numbers in this table (Table 2.1 shows updated trends).

Figure 2.21. Components of RF for emissions of principal gases, aerosols and aerosol precursors and other changes. Values represent RF in 2005 due to emissions and changes since 1750. (S) and (T) next to gas species represent stratospheric and tropospheric changes, respectively. The uncertainties are given in the footnotes to Table 2.13. Quantitative values are displayed in Table 2.13.