The Earth's atmosphere functions in much the same way as a greenhouse. A real greenhouse has temperatures greater than the surrounding air. This is because solar radiation can pass through the glass and cause heating by being absorbed by the materials inside the greenhouse. The heat cannot escape easily because convection is prevented and because glass has very poor transmission characteristics for infrared radiation. Heat escapes mainly by conduction through the glass. The crop yields in commercial greenhouses are also boosted by the use of kerosene burners which make the carbon dioxide [CO₂] concentration about three times as much as in the normal atmosphere. CO₂ is the main nutrient for carbon-based plants.

The Earth behaves in a similar way, but the surface is cooled additionally by convection and by the evaporation of water. The first Figure shows a generalized energy budget for the planet in which the units of energy flux [energy flow] are watts per square metre [W/m² or W m^-2] averaged over the whole of the Earth's surface.

Long-term radiation balance

Some incoming solar radiation is reflected by clouds, aerosols and the gaseous components of the atmosphere, and some is absorbed by the atmosphere. That absorbed by the stratosphere (10-50 km above the surface) leads to the formation of the ozone layer which protects the surface from the more energetic of the ultraviolet radiation and that absorbed by the troposphere (0-15 km above the surface) leads to warming. 

When the transmitted solar energy strikes the Earth's surface some is reflected back to space and the remainder in absorbed by the materials of the surface which in consequence becomes heated. The heated surface is cooled by thermal transfer to the atmosphere by the processes of conduction and convection, the evaporation of water and by the emission of infrared radiation. Most of the latter is absorbed by the greenhouse gases in the atmosphere, a small proportion escaping directly to space through what is known as the infrared 'window'. This is the region of the spectrum with wavenumber values in the range 750-1250 cm^-1, which the normal components of the atmosphere do not absorb, except for ozone.

The absorption of terrestrial radiation by the greenhouse gases is a major contributor to the warming of the atmosphere and in turn allows the atmosphere to cool by emitting infrared radiation in all directions, the fraction directed downwards to the surface contributing to the so-called 'greenhouse effect' - the further warming of the surface. Towards space the atmosphere emits sufficient infrared radiation to ensure a long-term energy balance, in which 235 W m^-2 of solar radiation is absorbed by the atmosphere/surface system which also emits the same flux as infrared radiation to space. 

This figure perhaps gives a better indication of the greenhouse effect, the 'cycle' of longwave emission from the surface followed by absorption by the atmosphere of the majority of the emitted radiation and ending with the 'back' radiation to the Earth's surface.