These indirect and direct effects make methane a large contributor, second only to carbon dioxide, to potential future warming of the earth. Global warming potentials provide a measure of the relative impacts of the emissions of each greenhouse gas. This measurement of GWP relies on carbon dioxide as the reference gas. Methane, which has a GWP of 21, raises particular concerns about global warming because over a 100-year period it is 21 times more effective at trapping heat in the atmosphere than carbon dioxide. U.S. EPA also uses 100-year GWPs for policy making and reporting purposes.
Hydroxyl production depends on the photolysis of ozone by solar ultraviolet radiation.
The strength and effectiveness of these sinks determine methane's atmospheric lifetime. Climatic conditions related to global warming and failure to adopt methane emissions reduction strategies could reduce the power of these sinks, increasing methane's lifetime. For example, the reaction rate of hydroxyl and the amount of hydroxyl in the troposphere determine methane's persistence. In the future, the abundance of methane could overwhelm the limited amount of hydroxyl in the troposphere. This relatively short lifetime makes methane an excellent candidate for mitigating the impacts of global warming because emissions reductions could lead to stabilization or reduction in methane concentrations within 10 to 20 years.