Methane causes around a fifth of the global warming attributable to mankind. This could increase in future as global warming triggers the release of methane from new sources.

Methane is the major constituent of natural gas. It is derived from the breakdown of plant and other organic remains in rocks deep within the Earth but it is also emitted by rotting vegetation (including food in landfill sites), marshes, cattle and even rice paddies. Methane is a greenhouse gas like carbon dioxide, nitrous oxide and some gases containing fluorine and chlorine which are used in fridges and some fire extinguishers.

Methane is the second most important greenhouse gas after carbon dioxide and is currently responsible for around 18% of the global warming attributable to mankind (carbon dioxide contributes 63%). Methane is often picked out as a significant greenhouse gas because, over a hundred years, it is actually 25 times more powerful than carbon dioxide but fortunately, in the atmosphere, it has a much shorter lifetime and is over 200 times less concentrated. However methane can also be emitted in the summer months by the decay of plant and other organic-rich material, as the sub-Arctic permafrost melts, and it is also contained as an ice-like solid in some ocean sediments. Some scientists are concerned that as global warming accelerates the warming permafrost, and eventually the ocean sediments, may cause the emission of huge quantities of methane with disastrous consequences for an already warming world.