[ThisIBereave]

As the poster above mentioned, Methane has a greater immediate impact, but it only stays in the atmosphere "for 11-12 years"[1], making it a relatively short term problem. CO2 stays in the atmosphere for "approximately 100 years"[2].

1: http://www.bbc.co.uk/climate/evidence/methane.shtml 2: http://www.bbc.co.uk/climate/evidence/carbon_dioxide.shtml