>>7
Methane has an atmospheric lifetime of less than 10 years. hydroxyl radicals (OH), eventually react with it. Methane doesn't last very long in habitable atmospheres, thats why its presence is considered a bio-indicator for exoplanets, similar to free oxygen. If we ever find a planet with Methane and Oxygen in the atmosphere, we know it must be continuously produced by biological processes.
The amount of Methane in human flatus is minuscule compared to it's CO2 content. And even if humans stopped eating and farting, the food we wouldn't eat would rot, releasing methane.