This is about complex atmospheric chemistry. Our previously simple understanding of the concentration of greenhouse gases in our atmosphere does not do justice to the complex atmospheric chemistry that goes on 24/7. The explanation is not so complex though. Methane reacts differently with different things in the atmosphere at different times. These different reactions tell us the strength of the greenhouse gas response. For example, the methane decomposes after a dozen or so years, but the decomposition byproducts are CO2 and ozone, both greenhouse gases. The consequences are much more far reaching and include water vapor, volatile organic compounds sulfur dioxide, carbon monoxide, aerosols, etc. In the past our view was much more simple. We just compared the methane straight to carbon dioxide to get a Global Warming Potential (GWP) But it is not that simple. Methane was listed in the Fourth Assessment report of the IPCC as having a GWP of 25. The new GWP is 34. All told, methane is much more powerful of a greenhouse gas than we once understood.
Shindell, et. al. October 30, 2009, Improved Attribution of Climate Change Forcing to Emissions, Science