“Methane is thought to contribute around one fifth of the greenhouse gas warming effect.”

Globally averaged atmospheric methane in 2009 was 1803 parts per billion

http://www.reportingclimatescience.com/

Wow! 1.8 ppm CH4 is causing 20% of the greenhouse effect. Those are powerful molecules.

Let’s do the math :

90% H2O and clouds
30% CO2
20% CH4

So far we have accounted for 140% of the greenhouse effect. Ain’t climate science grand?