2.10.3 Indirect GWPs
Indirect radiative effects include the direct effects of degradation products or the radiative effects of changes in concentrations of greenhouse gases caused by the presence of the emitted gas or its degradation products. Direct effects of degradation products for the greenhouse gases are not considered to be significant (WMO, 2003). The indirect effects discussed here are linked to ozone formation or destruction, enhancement of stratospheric water vapour, changes in concentrations of the OH radical with the main effect of changing the lifetime of CH4, and secondary aerosol formation. Uncertainties for the indirect GWPs are generally much higher than for the direct GWPs. The indirect GWP will in many cases depend on the location and time of the emissions. For some species (e.g., NOx) the indirect effects can be of opposite sign, further increasing the uncertainty of the net GWP. This can be because background levels of reactive species (e.g., NOx) can affect the chemical response nonlinearly, and/or because the lifetime or the radiative effects of short-lived secondary species formed can be regionally dependent. Thus, the usefulness of the global mean GWPs to inform policy decisions can be limited. However, they are readily calculable and give an indication of the total potential of mitigating climate change by including a certain forcing agent in climate policy. Following the approach taken by the SAR and the TAR, the CO2 produced from oxidation of CH4, CO and NMVOCs of fossil origin is not included in the GWP estimates since this carbon has been included in the national CO2 inventories. This issue may need to be reconsidered as inventory guidelines are revised.
Four indirect radiative effects of CH4 emissions have been identified (see Prather et al., 2001; Ramaswamy et al., 2001). Methane enhances its own lifetime through changes in the OH concentration: it leads to changes in tropospheric ozone, enhances stratospheric water vapour levels, and produces CO2. The GWP given in Table 2.14 includes the first three of these effects. The lifetime effect is included by adopting a perturbation lifetime of 12 years (see Section 7.4). The effect of ozone production is still uncertain, and as in the TAR, it is included by enhancing the net of the direct and the lifetime effect by 25%. The estimate of RF caused by an increase in stratospheric water vapour has been increased significantly since the TAR (see Section 2.3.7). This has also been taken into account in the GWP estimate for CH4 by increasing the enhancement factor from 5% (TAR) to 15%. As a result, the 100-year GWP for CH4 has increased from 23 in the TAR to 25.