Radiations from the sun have short wave length due to the high temperature of the sun. When this radiation, of short wave length passes through the water vapour and carbon dioxide gas in the lower layers of the atmosphere, it warms up the lower layers of the earth. The earth will then re-emit this radiation (infra-red radiation) as a black body radiation of long wavelength because of the low temperature of the earth. The black body radiation will therefore be trapped by the water vapor and carbon dioxide gas in the atmosphere thus preventing the radiation from escaping from the earth's atmosphere. The radiation which has been prevented from escaping from the earth's atmosphere will therefore cause global warming.
NOTE: If the amount of carbon dioxide in the atmosphere increases, the average temperature of the earth increases because more radiation, which is re-emitted, will be trapped. This will cause dramatic climatic and geographic change to occur.
