I think that's mostly the effect of methane oxidation in the stratosphere delivering water vapor to the stratosphere, plus reduction of the concentration of OH

I think that's mostly the effect of methane oxidation in the stratosphere delivering water vapor to the stratosphere, plus reduction of the concentration of OH radical in the atmosphere, and some effects on stratospheric ozone: > "Methane enhances its own lifetime through changes in the OH concentration, it leads to changes in tropospheric ozone, enhances stratospheric water vapour levels, and produces CO2. The GWP given in Table 2.14 includes the first three of these effects. The lifetime effect is included by adopting a perturbation lifetime of 12 years." - IPCC Report (That report indicates the radiative factor is more like 26:1 for CH4:CO2)