NicoMethane is a greenhouse gas with a warming effect 80 times greater than that of carbon dioxide, or CO2. In reality, the effective warming caused by emission of methane is not quite so much greater than CO2, since methane will break apart in the atmosphere after a few years.
Methane is the main component of natural gas.
Most methane emissions are from agriculture, leaks from the production gas, or the vented byproducts of other fossil fuels.