20 years is a very relevant timescale since we need to prevent the tipping point of melting artcitc methane deposits and loss of reflection from permafrost areas.
A possible solution would be methane fixing bacteria (methanotrophs) in the guts of cows or in the ocean where > 75% of the current concentration atmospheric methane originates. The ocean also contains a significant amount in solid form that gets released as temps rise. [0]
[0]http://www.washington.edu/news/2014/12/09/warmer-pacific-oce...
https://en.wikipedia.org/wiki/Dairy_cattle lists a few sources for the 4 years figure.