Greenhouse Gas

A greenhouse gas (sometimes abbreviated GHG) is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect.^[1] The primary greenhouse gases in theEarth's atmosphere are water vapor, carbon dioxide, methane,nitrous oxide, and ozone. Greenhouse gases greatly affect the temperature of the Earth; without them, Earth's surface would average about 33 °C colder, which is about 59 °F below the present average of 14 °C (57 °F).^[2][3][4]

Source : http://en.wikipedia.org/wiki/Greenhouse_gas