The greenhouse effect is a process by which thermal radiation from a planetary surface is absorbed by atmospheric greenhouse gases, and is re-radiated in all directions. Since part of this re-radiation is back towards the surface and the lower atmosphere, it results in an elevation of the average surface temperature above what it would be in the absence of the gases. http://en.wikipedia.org/wiki/Greenhouse_effect
Dr Iain Stewart explains the Greenhouse Effect, part of, ‘Earth: The Climate Wars’. https://www.bbc.co.uk/programmes/b00djvq9/episodes/guide
