Very difficult. The atmosperic changes with more Methane or carbon dioxide lead to the athmosphere keeping infrared radiation (heat radiation) from escaping in space, while sunlight comes through. So sunlight absorbed by the earth is converted in heat and this heat can escape slower than before. To reverse it you may try different things. First, you could reverse the effect of the athmosphere keeping heat radiation on earth. Best bet would to artificially remove carbon dioxide and methane from the athmosphere. Secondly we could reduce the solar input, meaning reflecting more sunlight before it even reaches earth. Both ways are very difficult.