Global warming is the warming of earth's atmosphere due to green house effect. Green house effect is caused by some gases called green house gases (GHGs) which form a layer in atmosphere and retain and partially reflect back the heat radiations that are emitted by earth's surface, causing increase in earth's temperature. GHGs indude CO2, CH4, NOx (oxides of nitrogen) but not nitrogen (N2). Water vapours and ozone also are GHGs but they have very minor significance.