Summary: A doubling of the amount of methane in the atmosphere with its present composition would produce a warming equal to only about one thirtieth of the warming produced by a doubling of carbon dioxide.
At present rates of increase it would take about 360 years for atmospheric methane levels to double.
Molecule for molecule, methane is 7 times more effective at being a greenhouse gas than carbon dioxide.
Present changes in atmospheric methane levels pose no environmental risk whatsoever.
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