Notes 55about 23 for the 100 year time horizon. About seventy-five per cent of the
contribution of methane to the greenhouse effect is because of its direct
effect on the outgoing thermal radiation. The other twenty-five per cent
arises because of its influence on the overall chemistry of the atmosphere.
Increased methane eventually results in small increases in water vapour in
the upper atmosphere, in tropospheric ozone and in carbon dioxide, all of
which in turn add to the greenhouse effect.
11 Taking into account the loss processes due to reaction with OH in the tropo-
sphere, chemical reactions and soil loss lead to a lifetime of about ten years.
However, the effective lifetime of methane against a perturbation in con-
centration in the atmosphere (thenumber quoted here) is complexbecause
it depends on the methane concentration. This is because the concentration
of the radical OH (interaction with which is the main cause of methane
destruction), due to chemical feedbacks, is itself dependent on the methane
concentration (more details in Pratheret al., Chapter 4, in Houghtonet al.,
Climate Change 2001).
12 For more detail, seeScientific Assessment of Ozone Depletion: 1998. Global
Ozone Research and Monitoring Project – Report No 44. Geneva: World
Meteorological Organization, 732 pp.
13 Pratheret al., Chapter 4, in Houghtonet al.,Climate Change 2001.
14 More detail on this and the radiative effects of minor gases and particles
can be found in Ramaswamyet al., Chapter 6, in Houghtonet al.Climate
Change 2001.
15 More detail in Penner, J. E.et al. (eds.) 1999.Aviation and the Global
Atmosphere. An IPCC Special Report. Cambridge: Cambridge University
Press.
16 Pratheret al., Chapter 4, in Houghtonet al.,Climate Change 2001.
17 Penneret al., Chapter 5, in Houghtonet al.,Climate Change 2001.
18 For more complete information of the effects of aviation see Penner,Aviation
and the Global Atmosphere.