Recall that all matter with a temperature above absolute zero emits electromagnetic radiation, and that the intensity of this radiation increases with temperature. To understand the effect of greenhouse gases in the atmosphere, two main parameters must be considered at any given altitude:
- the concentration of absorbing gases, which generally decreases with altitude;
- the temperature of the emitting layer, since colder layers emit less radiation than warmer ones.
These two parameters determine the intensity of infrared radiation emitted to space at each wavelength. In the terrestrial spectrum, greenhouse gases do not create true “holes,” but rather spectral regions in which the outgoing radiation corresponds to emission from higher and colder atmospheric layers instead of from the warm surface.
As a first approximation, the atmosphere can therefore be described in terms of effective emission altitudes. At wavelengths where the atmosphere is relatively transparent, infrared radiation escapes directly from the surface. At wavelengths where greenhouse gases absorb strongly, radiation escapes only from higher levels of the atmosphere, where the air is thinner and the temperature is lower. Consequently, the spectrum observed above the atmosphere is composed of emissions originating from different altitudes depending on wavelength [1].
This mechanism is central to the greenhouse effect. Because higher atmospheric layers are generally colder than the surface, the radiation emitted to space at absorbing wavelengths is weaker than it would be if it were emitted directly from the surface. The presence of greenhouse gases therefore reduces the outgoing infrared radiation for a given surface temperature.
As a result, the Earth system is temporarily pushed out of radiative equilibrium. This initial imbalance is described by positive radiative forcing, that is, a perturbation of the Earth’s radiation budget that tends to warm the climate system. In response, the surface and lower atmosphere warm until outgoing radiation once again balances absorbed solar radiation (figure 1).
Figure 1: First-order illustration of the greenhouse effect at different altitudes. The diagram shows the path of infrared radiation emitted by the Earth and the way in which greenhouse gases absorb and re-emit this radiation at different atmospheric levels. The observed outgoing spectrum depends on both the temperature and the concentration of gases at the effective emission altitude. The 50% absorption value shown at level n+2 is purely illustrative.
The analogy between atmospheric greenhouse gases and the glass walls of a greenhouse is therefore only partially valid. A real greenhouse limits heat loss primarily by suppressing convection, whereas atmospheric greenhouse gases act mainly by absorbing and re-emitting infrared radiation. For this reason, the scientifically correct term for their influence on the Earth’s energy budget is radiative forcing.
More precisely, positive radiative forcing corresponds to a change in the radiative balance of the Earth system, expressed in W/m², that tends to increase the planet’s temperature. In the case of anthropogenic greenhouse gases, this forcing arises because additional greenhouse gases reduce the efficiency with which the Earth loses infrared energy to space.
Bibliography
[1] Dufresne, J.-L., & Treiner, J. (2024). L’effet de serre atmosphérique : plus subtil qu’on ne le croit ! Ens-Lyon.fr. https://perso.ens-lyon.fr/fenril.montorier/fichiers/LP21%20BUP%20Profil%20temp%C3%A9rature%20atmosph%C3%A8re