The "greenhouse effect" is the process by which an atmosphere warms a planet. Mars, Venus and other celestial bodies with atmospheres (such as Titan) have greenhouse effects, but for simplicity the rest of this article will refer to the case of Earth.
The term greenhouse effect may be used to refer to two different things in common parlance: the natural greenhouse effect, which refers to the greenhouse effect which occurs naturally on earth, and the enhanced (anthropogenic) greenhouse effect, which results from human activities (see also global warming). The former is accepted by all; the latter is a matter of some dispute.
The natural greenhouse effect
The earth receives an enormous amount of solar radiation. Just above the atmosphere, the solar power flux density averages about 1367 watts/m2, or 1.28 * 1014 watts over the entire earth. This figure vastly exceeds the power generated by human activities.
The solar power hitting earth must be continually balanced by an equal amount of power radiating from the earth, or the earth's temperature would increase without limit. The radiation leaving the earth takes two forms: reflected solar radiation and thermal blackbody radiation.
Reflected solar radiation accounts for 30% of the earth's total radiation: on average, 6% of the incoming solar radiation is reflected by the atmosphere, 20% is reflected by clouds, and 4% is reflected by the surface.
The remaining 70% of the incoming solar radiation is absorbed: 16% by the atmosphere (including the almost complete absorption of shortwave UV over most areas by the stratospheric ozone layer); 3% by clouds; and 51% by the land and oceans. This absorbed energy heats the atmosphere, oceans and land.
Like the sun, the earth is a thermal blackbody radiator. But because the earth's surface is much cooler than the sun (287 K vs 5780 K), Wien's Displacement Law dictates that the earth must radiate its thermal energy at much longer wavelengths than the sun. While the sun's radiation peaks at a visible wavelength of 500 nanometers, earth's radiation peak is in the longwave (far) infrared at about 10 micrometres.
The earth's atmosphere is largely transparent at visible and near-infrared wavelengths, but not at 10 micrometres. Only about 6% of the earth's total radiation to space is direct thermal radiation from the surface. The atmosphere absorbs 71% of the surface thermal radiation before it can escape. The atmosphere itself behaves as a blackbody radiator in the far infrared, so it re-radiates this energy.
The earth's atmosphere and clouds therefore account for 91.4% of its longwave infrared radiation and 64% of the earth's total emissions at all wavelengths. The atmosphere and clouds get this energy from the solar energy they directly absorb; thermal radiation from the surface; and from heat brought up by convection and the condensation of water vapor.
Because the atmosphere is such a good absorber of longwave infrared, it effectively forms a one-way blanket over the earth's surface. Visible and near-visible radiation from the sun easily gets through, but thermal radiation from the surface can't easily get back out. In response, the earth's surface warms up. The power of the surface radiation increases by the Stefan-Boltzmann law until it compensates for the atmospheric absorption and a new equilibrium temperature is reached.
Any change to the earth's atmosphere that impedes or facilitates the transmission of longwave infrared radiation will upset this equilibrium, and the earth's surface will warm or cool until a new equilibrium temperature is reached.
The result of the greenhouse effect is that average surface temperatures are considerably higher than they would otherwise be if the earth's surface temperature were determined solely by the albedo and blackbody properties of the surface.
It is commonplace for over-simplistic descriptions of the "greenhouse" effect to assert that the same mechanism warms greenhouses (e.g. ), but this is an incorrect oversimplifacation: see below.
The degree of the greenhouse effect is dependent primarily on the concentration of greenhouse gases in the planetary atmosphere. The carbon dioxide-rich atmosphere of Venus causes a runaway greenhouse effect with surface temperatures hot enough to melt lead, the atmosphere of Earth creates habitable temperatures, and the thin atmosphere of Mars causes a minimal greenhouse effect.
The use of the term runaway greenhouse effect to describe the effect as it occurs on Venus emphasises the interaction of the greenhouse effect with other processes in feedback cycles. Venus is sufficiently strongly heated by the Sun that water is vaporised and so carbon dioxide is not reabsorbed by the planetary crust. As a result, the greenhouse effect has been progressively intensified by positive feedback. On Earth there is a substantial hydrosphere and biosphere which respond to higher temperatures by recycling atmospheric carbon more quickly (in geologic terms; the timescale for the ocean/biosphere to remove a CO2 perturbation is on the order of several hundred years). The presence of liquid water thus limits the increase in the greenhouse effect through negative feedback. This state of affairs is expected to persist for at least hundreds of millions of years, but, ultimately, the warming of an aging Sun will overwhelm this regulatory effect.
The greenhouse gases
Water vapor (H2O) causes about 60% of Earth's naturally-occurring greenhouse effect. Other gases influencing the effect include carbon dioxide (CO2) (about 26%), methane (CH4), nitrous oxide (N2O) and ozone (O3) (about 8%) . Collectively, these gases are known as greenhouse gases.
The wavelengths of light that a gas absorbs can be modelled with quantum mechanics based on atomic properties of different gas molecules. It so happens that tri- (and more) atomic gases absorb strongly in infra-red wavelengths, which is why H2O and CO2 are greenhouses gases but the major atmospheric constituents (N2 and O2) are not.
The term 'greenhouse effect' originally came from the greenhouses used for gardening, but it is a misnomer since greenhouses operate differently. A greenhouse is built of glass; it heats up primarily because the Sun warms the ground inside it, which warms the air near the ground, and this air is prevented from rising and flowing away. This can be demonstrated by opening a small window near the roof of a greenhouse: the temperature will drop considerably. It has also been demonstrated experimentally (Wood, 1909). Greenhouses thus work by preventing convection; the greenhouse effect however reduces radiation loss, not convection.
Effects of various gases
It is hard to disentangle the percentage contributions to the "GH" effect from different gases, because there are overlaps in the IR spectrum of the various gases. However, one can calculate the percentage of trapped radiation remaining, and discover:
|Species removed||% trapped radiation remaining|
|H2O, CO2, O3||50|
(Source: Ramanathan and Coakley, Rev. Geophys and Space Phys., 16 465 (1978))
Water vapour effects
Water vapor is the major contributor to Earth's greenhouse effect. Its effects vary due to localized concentrations, mixture with other gases, frequencies of light, different behavior in different levels of the atmosphere, and whether positive or negative feedback takes place. High humidity also affects cloud formation, which has major effects upon temperature but is distinct from water vapor gas.
The IPCC TAR (2001; section 2.5.3) reports that, despite non-uniform effects and difficulties in assessing the quality of the data, water vapour has generally increased over the 20th Century.
Estimates of the percentage of Earth's greenhouse effect due to water vapor:
- 36% (table above)
- 60-70% Nova. Greenhouse - Green Planet 
Including clouds, the table above would suggest 50%. For the cloudless case, IPCC 1990, p 47-48 estimate water vapour at 60-70% whereas Baliunas & Soon estimate 88%  considering only H2O and CO2. For a theoretical case if no other greenhouse gases were in the atmosphere, Richard Lindzen estimated 98% (Global warming: the origin and nature of the alleged scientific consensus. Regulation, Spring 1992 issue, 87-98 ).
Water vapour in the troposphere, unlike the better-known greenhouse gases such as CO2, is essentially passive in terms of climate: the residence time for water vapour in the atmosphere is short (about a week) so perturbations to water vapour rapidly re-equilibriate. In contrast, the lifetimes of CO2, methane, etc, are long (hundreds of years) and hence perturbations remain. Thus, in response to a temperature perturbation caused by enhanced CO2, water vapour would increase, resulting in a (limited) positive feedback and higher temperatures. In response to a perturbation from enhanced water vapour, the atmosphere would re-equilibriate due to clouds causing reflective cooling and water-removing rain. The contrails of high-flying aircraft sometimes form high clouds which seem to slightly alter the local weather.
In recent years researchers see the increasing greenhouse effect from increasing CO2 and other gases as a significant contributing factor to the current global warming.
- Kiehl, J.T., and Trenberth, K. (1997). Earth's annual mean global energy budget, Bulletin of the American Meteorological Society 78 (2), 197–208.
- Wood, R.W. (1909). Note on the Theory of the Greenhouse, Philosophical Magazine 17, p319–320. For the text of this online, see http://www.wmc.care4free.net/sci/wood_rw.1909.html
- Earth Radiation Budget, http://marine.rutgers.edu/mrs/education/class/yuri/erb.html