Most atmospheric methane produced photo oxidizes to CO2 after about 20 years though.
CH4 + 2O2 -photons---> C02 + 2H20
I'm not sure what time scale of heat per mass equivalencies are based on in your example; there is approximately 28 : 1 heat/mass eq. ratio at the 100 year scale, during which time the heating effect decreases exponentially with the decrease in methane concentration[0]. After 100 years the ratio is approximately 1:1 since almost all methane has decomposed.