Methane (CH4) is another greenhouse gas, and is produced primarily by anaerobic (oxygen-deficient) processes such as the cultivation of rice paddies or animal digestion. It is destroyed in the lower atmosphere (troposphere) by reactions with free hydroxyl radicals (OH):
CH4 + OH
CH3 + H2O
Like CO2, its concentration in the atmosphere is increasing due to anthropogenic activities such as agricultural practices and landfills (Prather et al., 1995).