Answer :
global warming is an uneven heating of the earth's temperature by increasing concentration of green house gases produced by human activities
Gases like CO2,CFCs ,O3,NO and water vapour absorb IR radiations reaching the earth reflect them back to the earth's surface.Due to this, the surface of earth gets heated.This is called Green house effect or Global warming.Global warming is the observed and projected increase in the avg. temp. of Earth's atmosphere and oceans.