Where They Come From To make the most of opportunities for reducing greenhouse-gas emissions, it's a good idea first to understand where the emissions come from.
Carbon dioxide is the principal greenhouse gas, especially at the household level.
But some fuels give off more carbon dioxide than others. For example, natural gas is the cleanest fossil fuel, emitting nearly 40 percent less CO 2 per unit of heat output than oil does. Electricity is not a fuel per se, but rather a carrier of energy. Because of conversion and grid losses, it typically takes three or four units of fuel to produce and deliver one unit of electricity. If you're interested in the CO 2 emissions from using electricity and various fuels, see conversion factors for more detail.