Methane
Although methane has a shorter lifespan, and consequently, is not as abundant in the atmosphere as carbon dioxide, in terms of it effect as a greenhouse gas, it is much more potent. Methane is produced when organic matter breaks down and also when ruminants digest their food.
Human sources of methane
Human sources of methane include:
Decomposing organic matter: landfills, animal waste, sewerage and dams are all human sources of methane. For example, organic matter is trapped behind dam walls where it sinks to the bottom of the dam and decomposes. Bacteria produce methane in the decomposition process, which slowly escapes to the surface and is released into the atmosphere. Landfill gas operations and biodigesters provide a method of mitigation, and not only harness methane that would otherwise be released into the atmosphere, but also provide an alternative source of fuel.
Natural gas extraction: methane escapes during oil and gas extraction (e.g. fracking) operations and is released into the atmosphere.