This page has information on the two sources of methane and how they differ. It also has information on carbon dioxide and nitrous oxide.
Methane is a short-lived greenhouse gas. It degrades in the atmosphere over decades. Once in equilibrium, it can continue to be emitted at a stable rate without increasing its concentration in the atmosphere. All methane molecules behave the same in the atmosphere but they can come from two different types of sources: biological and fossil.
Biogenic methane is produced from biological (plant and animal) sources. This is carbon recently derived from carbon dioxide (CO2) present in the atmosphere. When the methane is emitted it causes additional warming (as methane is a more potent greenhouse gas than CO2). Over time it decays back to CO2 without adding to the concentration of CO2 in the long term. Biogenic methane is emitted by livestock, waste treatment and wetlands, for example.
Fossil methane emissions
Fossil methane emissions on the other hand return geological carbon to the atmosphere that has typically been stored underground for millions of years. Releasing this methane adds to the atmospheric concentration of CO2 as well as causing additional warming as methane. The Zero Carbon Amendment Bill aims to reduce our emissions of fossil methane together with other greenhouse gases, with the exception of biogenic methane, to net zero. Examples of fossil methane sources include coal mining, natural gas leakage and methanol production from natural gas.
Other major greenhouse gases
CO2 is a long-lived greenhouse gas. It stays in the atmosphere for hundreds to thousands of years. This means further emissions will increase its concentration in the atmosphere.
Nitrous oxide (N2O) has a lifetime between those of CO2 and methane. In the context of achieving the temperature goals of the Paris Agreement, it can be considered a long-lived gas.