Natural gas is often described as the cleanest fossil fuel, producing  less carbon dioxide per joule delivered than either coal or oil[33] and  far fewer pollutants than other hydrocarbon fuels -- Natural gas  produces far lower amounts of sulfur dioxide and nitrous oxides than any  other hydrocarbon fuel -- "Because of this higher carbon efficiency of  natural gas generation, as the fuel mix in the United States has change  to reduce coal and increase natural gas generation carbon dioxide  emissions have unexpectedly fallen. Those measured in the first quarter  of 2012 were the lowest of any recorded for the first quarter of any  year since 1992.[35]" -- Other sources of methane, the principal  component of natural gas, include landfill gas, -- Experimental systems  were being proposed for use in parts of Hertfordshire, UK, and Lyon in  France. -- Gas generated in sewage treatment plants is commonly used to  generate electricity. -- Manure and farm animals.

From Wikipedia

"Because of this higher carbon efficiency of natural gas generation, as the fuel mix in the United States has change to reduce coal and increase natural gas generation carbon dioxide emissions have unexpectedly fallen. Those measured in the first quarter of 2012 were the lowest of any recorded for the first quarter of any year since 1992.[35]"