Methane is the second most abundant long-lived greenhouse gas, after carbon dioxide. Water vapor is the most abundant greenhouse gas overall, but it is usually treated separately because its atmospheric amount is controlled mainly by temperature and the water cycle. Among the major long-lived, well-mixed greenhouse gases, carbon dioxide comes first and methane comes next.
Methane, or CH₄, is present in far smaller concentrations than carbon dioxide, generally measured in parts per billion rather than parts per million. Despite its lower abundance, each methane molecule is highly effective at absorbing infrared radiation. Its atmospheric lifetime is much shorter than carbon dioxide’s—roughly a decade—but it has a much stronger warming effect per molecule over shorter time periods.
Major methane sources include wetlands, livestock, rice cultivation, landfills, coal mining, and oil and natural-gas systems. Methane is often confused with nitrous oxide, which is another important long-lived greenhouse gas but is less abundant. Methane’s short lifetime also means that reducing emissions can slow warming relatively quickly.