Scientists tend to talk about greenhouse gases' global warming potential (GWP). A common figure used for methane is it has the GWP 20x that of CO2 over 100 year period. This takes into account that the methane at first traps a lot of heat and then it breaks down to CO2 and traps less heat.

Another consideration is that as the concentration of methane in the atmosphere rises the rate at which it breaks down slows.