Methane radiative forcing estimates

Table 14.4 summarizes the estimated total direct radiative forcing calculated for the period from preindustrial times to 1992 for C02, CH4, N20, and O, (IPCC, 1996). The estimate for CH4 includes the effects due to its impacts on tropospheric ozone levels or on stratospheric water vapor, both of which are generated during the oxidation of methane. That shown for 03 is based on the assumption that its concentration increased from 25 to 50 ppb over the Northern Flemi-sphere. The total radiative forcing due to the increase in these four gases from preindustrial times to the present is estimated to be 2.57 W m 2. 

Stevenson, D.S. Johnson, C.E. Collins, W.J. Derwent, R.G. and Edwards, J.M. (2000). Future estimates of tropospheric ozone radiative forcing and methane turnover The impact of climate change. Geophys. Res. Lett., 27(14), 2073-2076. 

Methane oxidation leads to a net loss of OH in the atmosphere, thereby lengthening the lifetime of CH4 itself (we will discuss this later in this chapter). It is estimated that this longer lifetime increases the radiative forcing of CH4 by 25-35% over that in the absence of this feedback effect. Methane oxidation also leads to tropospheric O3 this indirectly increases the greenhouse effect by another 30-40% through the effect of the added O3 itself. Finally, increases in CH4 also indirectly lead to further climate forcing by increasing stratospheric H20 (about 7% of CH4 is oxidized in the upper troposphere). 

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