My last post used a GWP100 for methane of 35; however, estimates of combined natural and anthropogenic methane emission rates through 2100 justify the use of the findings from Isaksen et al's 7 x CH4 case for calculating a revised GWP for methane, as follows:
As the radiative forcing in a 50-year time horizon for 4 x CH4 additional emission of 0.80 GtCH4/yr is 2.2 Wm-2, and as the radiative forcing for the current methane emissions is 0.48 Wm-2, thus an updated GWP for methane, assuming the occurrence of Isaksen et al's 4 x CH4 case in 2040, would be: 33 (per Shindell et al 2009) times (2.2/[0.8 + 0.48]) divided by (0.54/0.48) = 50 by 2100.
If the GWP of methane increases to 50 by 2100 then the RCP 8.5 scenario will significantly under-estimate global warming by the end of this century.
Isaksen, I. S. A., Gauss M., Myhre, G., Walter Anthony, K. M. and Ruppel, C., (2011), "Strong atmospheric chemistry feedback to climate warming from Arctic methane emissions", Global Biogeochem. Cycles, 25, GB2002, doi:10.1029/2010GB003845