greenhouse effect

The internal heating effect in a planetary atmosphere, caused by its opacity to infrared radiation. The names arises because the mechanism is essentially the way a greenhouse works, with glass playing the same role as the atmosphere. The primary source of heat for a planet's surface and atmosphere is energy radiated from the Sun in the visible and infrared regions of the spectrum. Longer-wavelength infrared radiation emitted by the warm planetary surface becomes trapped in the atmosphere, causing the equilibrium temperature of the atmosphere (and hence the planetary surface) to be higher than it would otherwise be. On the Earth, the increase in temperature amounts to about 33 K. On Venus, a "runaway" greenhouse effect raises the temperature by 500 K. Mars is warmed by a modest 5 K. How much heating the greenhouse effect causes depends on how opaque the atmosphere is to infrared radiation. Carbon dioxide is one of the main sources of the opacity, but water vapour and rarer gases also play a part. Concern has mounted that global warming of the Earth will result from increased concentrations of carbon dioxide and other so-called greenhouse gases released by human activities, particularly the burning of fossil fuels such as coal and oil.