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. Plants reduce the greenhouse effect by absorbing carbon dioxide rather than excess heat radiation from the air while human activities like manufacturing increase the greenhouse effect by adding various greenhouse gases.