Greenhouse gases are components of the atmosphere that contribute to the greenhouse effect. Without the greenhouse effect the Earth would be uninhabitable; in its absence, the mean temperature of the earth would be about −19 °C (−2 °F, 254 K) rather than the present mean temperature of about 15 °C (59 °F, 288 K) . Greenhouse gases include, in order of relative abundance: water vapour, carbon dioxide, methane, nitrous oxide, and ozone. Greenhouse gases come from natural sources and human activity.