I'm not OP (and I think his comment sort of misses the point...) but even allowing for its shorter lifespan, methane is about 28 times more potent per kg over 100 years (the standard measure I believe).

People have actually argued that its "worse" in practise: enough CO2 to cause a 4 degree rise in 100 years takes 100 years to fully take effect. The equivalent in methane will means 4 degrees of warming in just 25ish years.