There are many sources that emit Methane which goes into the atmosphere. Some are bodies of water (swamp gas) and the most numerous are living souls, both animal and human.
The amount of "natural" methane which goes into the atmosphere is rather enormous. All living animals and humans experience flatulence, or the "passing of gas," as a result of digesting their food intake.
All living animals, and humans, also produce copius quantities of Carbon Dioxide in the process of breathing.
It occurs to me that since the natural sources of what are now considered to be "greenhouse gases" are so numerous, perhaps there is an "agenda" at work to bring about some "change" in response to all the alarmist propaganda regarding "global warming."
I am far from being convinced that any naturally produced gases are harmful to the balance of the earthly environment.