It is through the natural process of the ‘greenhouse effect’, a term first used by Fourier in 1827, that the earth is a habitable planet. Radiant energy from the sun efficiently penetrates the atmosphere in the visible and ultraviolet high end of the energy spectrum warming the Earth to a level that enables life to exist.

Part of this energy, mostly as thermal (infrared or long-wave ) radiation, that is reflected back towards space is absorbed by greenhouse gases (link to CO2 section) in the atmosphere, returning to the Earth and further increasing its temperature like a blanket. As with a normal blanket the inner layer is warmer and the outer surface colder. The amount of thermal radiation from any substance is proportional to the temperature of the emitting surface. Thus the upper layer in the atmosphere as it is colder emits some colder radiation out into space and the warmer inner layer returns warmer radiation back to the surface of the Earth.

It is the balance between incoming and outgoing radiation that determines the day-night differences in temperature and the changing temperature of the Earth through the seasons and over longer periods of time. In the absence of incoming radiation on a clear night in winter the effect of the outgoing radiation is clearly apparent when frost forms on the ground. Without a gaseous atmosphere comprised of greenhouse gases such as carbon dioxide the surface temperature of the Earth would be -18°C instead of the present day mean temperature of ~15°C. Convective processes within the atmosphere that redistribute the heat ensure that this temperatures is not even higher.

Water vapour and the different greenhouse gases absorb radiation in different bands of the spectrum and with different strengths. Methane for example is a much stronger absorber than carbon dioxide, but is much less abundant as a constituent of the atmosphere. Increases in the proportions of greenhouse gases as a result of human activities have perturbed and enhanced the natural greenhouse effect leading to global warming.

It should be noted that the term greenhouse effect is an inaccurate analogy as the glass in a greenhouse does not absorb infrared radiation as in the atmosphere. It is the confinement of the warmed air package within a greenhouse that increases its temperature by reduced convection and turbulence.