Methane is a potent greenhouse gas that has more than 80 times the warming power of carbon dioxide over the first 20 years after it reaches the atmosphere. The largest sources of methane are agriculture, fossil fuels, and decomposition of landfill waste, as well as natural processes. Methane does not directly harm human or crop health, but it is a precursor gas that can react to form tropospheric (ground-level) ozone.
More information: Climate and Clean Air Coalition