Drew T. Shindell , Greg Faluvegi, Dorothy M. Koch , Gavin A. Schmidt , Nadine Unger and Susanne E. Bauer , “Improved Attribution of Climate Forcing to Emissions”, Science, 30 October 2009: Vol. 326 no. 5953 pp. 716-718: http://www.sciencemag.org/content/326/5953/716 and Shindell et al (2009), Fig.2: http://www.sciencemag.org/content/326/5953/716.figures-only .
The Global Warming Potential of CH4 is 21 times that of CO2 on a 100 year time frame but is 105 times greater than that of CO2 on a 20 year time frame and taking atmospheric aerosol impacts into account.