Greenhouse gases are those gaseous constituents of the atmosphere, both natural and anthropogenic, that absorb and emit radiation at specific wavelengths within the spectrum of infrared radiation emitted by the Earth’s surface, the atmosphere, and clouds. This property causes the greenhouse effect. Water vapor (H2O), carbon dioxide (CO2 ), nitrous oxide (N2O), methane (CH4 ), and ozone (O3 ) are the primary greenhouse gases in the Earth’s atmosphere. Moreover, there are a number of entirely human-made greenhouse gases in the atmosphere, such as the halocarbons and other chlorine and bromine-containing substances which are dealt with under the Montreal Protocol. Beside CO2 , N2O, and CH4 , the Kyoto Protocol deals with the greenhouse gases sulfur hexafluoride (SF6 ), hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs).
