Atmospheric methane (CH4) is second only to CO2 as an anthropogenic source of the greenhouse effect. Methane's overall contribution to global warming is large because it is 22 times more effective at trapping heat in the atmosphere than carbon dioxide over a 1OO-year time horizon when both the direct and indirect effects are accounted for. Furthermore, methane's concentration in the atmosphere has more than doubled over the last two centuries. Scientists have concluded that these atmospheric increases are largely due to increasing emissions from anthropogenic sources, such as landfills, agricultural activities, coal mining, fossil fuel combustion, the production and processing of natural gas and oil, and wastewater treatment.