It is a colorless gas made up of molecules of four hydrogen atoms and one carbon atom. Methane is the main component of natural gas, a fossil fuel. It is released into the atmosphere when organic matter decomposes in environments with low levels of oxygen. It is one of the main gases that contribute to the greenhouse effect, although its lifetime in the atmosphere is on the order of a decade. Natural sources include swamps, salt marshes, termites and oceans. Synthetic sources include the extraction and burning of fossil fuels, digestive processes in ruminants such as cattle, rice paddies and landfills. Most of the methane emitted is decomposed in the atmosphere by reactions with hydroxyl radicals (OH).
Since the beginning of the Industrial Revolution, the atmospheric concentration of methane has more than doubled, contributing 20% to the increase in the greenhouse effect, second only to carbon dioxide.