Methane (CH4) is a natural ly occurring gas. One of its major causes is the decomposition of plant and animal matter by methane producing bacteria. Methane is a very powerful greenhouse gas - 21 times more potent than carbon dioxide. It is produced natural ly, but over the last two centuries its concentration in the atmosphere has more than doubled as a direct result of human activities, leading to serious concerns about its impact on global warming. The major man made sources of methane are natural gas extraction, waste disposal, livestock farming and coal mining.