6.3.2 Methane and Nitrous Oxide
The SAR reported that several studies found a higher forcing due to CH_{4}
than IPCC (1990), up to 20%; however the recommendation was to use the same
value as in IPCC (1990). The higher radiative forcing estimates were obtained
using band models. Recent calculations using LBL and band models confirm these
results (Lelieveld et al., 1998; Minschwaner et al., 1998; Jain et al., 2000).
Using two band models, Myhre et al. (1998b) found the computed radiative forcing
to differ by almost 10%. This was attributed to difficulties in the treatment
of CH_{4} in band models since, given its present abundance, the CH_{4}
absorption lies between the weak line and the strong line limits (Ramanathan
et al., 1987). After updating for a small increase in concentration since the
SAR, the radiative forcing due to CH_{4} is 0.48 Wm^{2} since
preindustrial times. This estimate for forcing due to CH_{4} is only
for the direct effect of CH_{4}; for radiative forcing of the indirect
effect of CH_{4}, see Sections 6.5 and 6.6.
The problem mentioned above with the band models for CH_{4} does not
occur to the same degree in the case of N_{2}O, given the latter’s
present concentrations. Three recent studies, Myhre et al. (1998b) (two models),
Minschwaner et al. (1998) (one model), and Jain et al. (2000) (one model), calculated
lower radiative forcing for N_{2}O than reported in previous IPCC assessments,
viz., 0.13, 0.12, 0.11, and 0.12 Wm^{2}, respectively, compared to
0.14 Wm^{2} in the SAR. For N_{2}O, effects of change in spectroscopic
data, stratospheric adjustment, and decay of the mixing ratio in the stratosphere
are all found to be small effects. However, effects of clouds and different
radiation schemes are potential sources for the difference between the newer
estimates and the SAR. A value of 0.15 Wm^{2} is now suggested for
the radiative forcing due to N_{2}O, taking into account an increase
in the concentration since the SAR, together with a smaller preindustrial concentration
than assumed in IPCC (1996a; Table 2.2) (see Chapter 4).
