The gases which are transparent to solar radiation but retain and partially reflect back long wave heat radiations are called greenhouse gases (GHGs). The phenomenon is called greenhouse flux. Because of greenhouse flux, the mean annual temperature of earth is 15°C. In its absence it will fall to -18°C. Hence, greenhouse gases keep the earth warm and hospitable. The various greenhouse gases are CO2 (warming effect 60%), CH4 (effect 20%), chlorofluorocarbons or CFCs (14%) and nitrous oxide (N2O, 6%). Others of minor significance are water vapours and ozone.