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6.6.2 Indirect Forcings of Methane, Carbon Monoxide and Non-Methane Hydrocarbons
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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