CH4 (0.48 W m−2), N2O (0.15 W m−2), and halocarbons (0.34 W m−2) (IPCC, 2001). The estimated uncertainty associated with this forcing is 10 percent, with that for CO2 and N2O being less and that for the other gases being greater. The estimated uncertainty for halocarbons is 10-15 percent for those molecules that have been studied in detail and is not well characterized for other halocarbons. Recent research on well-mixed greenhouse gases focuses on refining the models used to do radiative transfer calculations (e.g., Evans and Puckrin, 1999), considering the small temporal and spatial variations in concentrations which can lead to errors up to about 5-10 percent (Forster et al., 1997; Myhre and Stordal, 1997; Freckleton et al., 1998), and accounting for the extent to which clouds reduce radiative forcing (e.g., Myhre and Stordal, 1997).

The IPCC estimate for CH4 forcing includes an observation-based estimate for both the direct forcing of CH4 and the indirect forcing due to changes in the hydroxyl radical (OH) and tropospheric ozone (O3) resulting from methane oxidation. The oxidation of CH4 leads to a net loss of OH in the atmosphere, thereby lengthening the CH4 lifetime. It is estimated that this indirect effect of CH4 increases its radiative forcing by 25-35 percent over the direct CH4 forcing (Lelieveld and Crutzen, 1992; Brühl, 1993; Lelieveld et al., 1993, 1998; Hauglustaine et al., 1994; Fuglestvedt et al., 1996). The oxidation of CH4 also leads to the formation of tropospheric ozone, indirectly increasing the CH4 forcing by 30-40 percent through the greenhouse effect of the additional tropospheric O3. In the stratosphere, oxidation of CH4 is a source of water vapor. In situ measurements of water vapor in the lower stratosphere indicate an increase of about 1 percent per year for 1954-2000 (Rosenlof et al., 2001), whereas satellite measurements of water vapor in the stratosphere in the 1990s showed no steady rate of change (Randel et al., 1996). A 1 percent annual increase in stratospheric water vapor would be associated with an estimated radiative forcing of 0.2 W m−2 since 1980 (Forster and Shine, 1999). The oxidation of CH4 can explain only a fraction of such a water vapor increase.

TROPOSPHERIC AND STRATOSPHERIC OZONE

Atmospheric ozone modifies the radiative budget of the Earth system by absorbing radiation both in the IR and in the ultraviolet (UV). It acts both as a radiative forcing agent and as a climate feedback. Ozone is produced and destroyed by solar UV radiation and by chemical reactions involving natural and anthropogenic gases. Changes in ozone driven by anthropogenic emissions represent a forcing. However, ozone concentrations also respond to changes in temperature and UV radiation, transport patterns, and natural emissions from lightning and vegetation; these responses represent climate feedbacks. In what follows, tropospheric and



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