A natural process where infrared radiation is retained in the earth's atmosphere by heat-absorbing greenhouse gases (GHG). After absorbing the radiation, the GHGs re-radiate the energy in all directions, including back to earth, a process which is necessary to keep the earth warm and habitable.

Many scientists claim that the greenhouse effect has been artificially enhanced by an increase in man-made GHGs, through activities such as burning fossil fuels. The increased GHGs result in more absorption and re-radiation, thus enhancing the greenhouse effect and raising the earth's temperature, fueling the theory of global warming.