In terms of climate change it’s not a 10 years thing and then methane instantly swaps, it’s a second by second thing where a percentage of existing methane is constantly converted. In steady state new methane from say cow farts 1:1 replaces methane from past cow farts being transformed into CO2. The only difference is an atom of CH4 at 16.043 grams per mol weigh less than an atom of CO2 at 44.009 grams per mol. Though many sources of CH4 start by extracting atmospheric CO2, natural gas leaks don’t.
The critical difference is if we cut net CO2 and hold methane steady the atmosphere stops warming, where cutting methane and CO2 would actually cool the atmosphere.
Put another way NEW sources of methane are different because it takes multiple years to reach an equilibrium. In the first years they have a very big impact, but eventually it falls off until eventually a constant source of CO2 may actually have a larger impact.
