fuels derived from resources other than petroleum, as defined by the Energy Policy Act of 1992, current alternative fuels include ethanol, natural gas, propane, hydrogen, biodiesel, electricity, methanol, and p-series fuels.

the period of restoration of atmospheric composition following a small increase in the concentration of greenhouse gases in the atmosphere, may cover very large periods of time (recovery from a large input of CO2 will take tens of thousands of years).