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CH4 is oxidised primarily (>90%) in the troposphere through reaction with OH. Since the CH4 oxidation cycle provides a substantial fraction of the OH loss in the troposphere, there is strong interaction between OH and CH4. This causes the OH to decrease when CH4 increases, leading to a further increase in CH4. This chemical feedback is examined in Chapter 4. Lelieveld and Crutzen (1992), Brühl (1993), Lelieveld et al. (1993, 1998), Hauglustaine et al. (1994) and Fuglestvedt et al. (1996) estimated that this feedback added 25 to 35% to the direct CH4 forcing depending on the initial CH4 perturbation and the model used. These values are in line with the 30% contribution estimated by IPCC (1992) and the SAR. The tropospheric O3 increase associated with photochemical production from the CH4 oxidation cycle also contributes to enhancing the total CH4 radiative forcing. The forcing from enhanced levels of tropo-spheric O3 estimated by Lelieveld and Crutzen (1992), Lelieveld et al. (1993, 1998), Hauglustaine et al. (1994) and Fuglestvedt et al. (1996) contributes a further 30 to 40% on top of the CH4 forcing, due directly to CH4 emissions. IPCC (1992) estimated a lower contribution of O3 of approximately 20%. Lelieveld et al. (1998) have estimated a direct radiative forcing associated with CH4 increase since the pre-industrial era (1850 to 1992) of 0.33 Wm-2 and an additional forcing of 0.11 Wm-2 associated with OH feedback. The increased tropospheric O3 contributes an additional 0.11 Wm-2 and stratospheric H2O another 0.02 Wm-2. These authors found that the total CH4 forcing (0.57 Wm-2) is higher by 73% than the direct forcing. The CH4 radiative forcing given in Section 6.3 (0.5 Wm-2) and based on recorded CH4 concentration increase includes both the direct and OH indirect contributions. This updated forcing is higher by 14% than the 0.44 Wm-2 obtained by Lelieveld et al. (1998) on the basis of calculated present day and pre-industrial CH4 distributions. In addition to that, oxidation of CH4 also leads to the formation of CO2, providing a further indirect effect.
In contrast to CH4, the direct radiative forcing of CO is relatively small (Evans and Puckrin, 1995; Sinha and Toumi, 1996). Sinha and Toumi (1996) have estimated a clear sky radiative forcing of 0.024 Wm-2 for a uniform increase in CO from 25 ppbv to 100 ppbv. However, CO plays a primary role in governing OH abundances in the troposphere. As indicated by Prather (1994, 1996) and Daniel and Solomon (1998), CO emissions into the atmosphere may have a significant impact on climate forcing due to chemical impact on CH4 lifetime, and tropospheric O3 and CO2 photochemical production (see Chapter 4). The contribution of these indirect effects to the GWPs based on box model calculations are presented in Section 6.12. Similarly, NMHCs have a small direct radiative forcing. Highwood et al. (1999) estimated an upper limit of 0.015 Wm-2 (1% of the present day forcing due to other greenhouse gases) on the globally averaged direct forcing of sixteen NMHCs. As indicated by Johnson and Derwent (1996), the indirect forcing through changes in OH and tropospheric O3 is also small for each NMHC taken individually but can be significant taken as a family. The indirect forcings of NMHCs are still poorly quantified and require the use of global three-dimensional chemical transport models. Accurate calculations of these effects are a notoriously difficult problem in atmospheric chemistry.
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