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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 a 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 steps which lead to the greenhouse effect can be summarized as follows.
To summarize: The greenhouse effect occurs because the atmosphere is more transparent to solar radiation than thermal radiation. Its net effect is to keep energy within the ground/atmosphere system, thus resulting in higher surface temperatures than would otherwise be the case.
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. Of the planets Venus, Earth, and Mars the dense, carbon dioxide-rich atmosphere of Venus causes a runaway greenhouse effect with surface temperatures hot enough to melt lead, the atmosphere of Earth causes a greenhouse effect of habitable temperatures, and the thin atmosphere of Mars causes a minimal greenhouse effect.
The use of the term runaway greenhouse effect to describe the situation obtaining 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 of 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.
Water vapor (H2O) causes about 60% of Earth's naturally-occurring greenhouse effect, with others carbon dioxide (CO2) (about 26%), methane (CH4), nitrous oxide (N2O) and ozone (O3) (about 8%) , collectively known as greenhouse gases.
The wavelengths of light that a gas absorbs stems from quantum mechanics, and 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.
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 |
|---|---|
| All | 0 |
| H2O, CO2, O3 | 50 |
| H2O | 64 |
| Clouds | 86 |
| CO2 | 88 |
| O3 | 97 |
| None | 100 |
(Source: Ramanathan and Coakley, Rev. Geophys and Space Phys., 16 465 (1978))
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:
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 gasses 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 some researchers see the greenhouse effect as a significant contributing factor to the current global warming, due to the increased concentration of some greenhouse gases in the atmosphere as a result of human activity. Such climatologists are concerned that increased levels of greenhouse gases in the atmosphere might cause more heat to be trapped. The hypothesis that a man-made increase in greenhouse gas concentration would lead to a higher global mean temperature was first postulated in the late 19th century by Swedish chemist and 1903 Nobel Laureate Svante Arrhenius (see global warming), although, his peers largely rejected that theory. The theory that human greenhouse gas emissions are connected with the observed heating of Earth's atmosphere in the 20th century has steadily gained adherents in the popular community since the 1980s, to the extent that many bodies around the world have strongly endorsed it. Automobile exhausts, coal-burning power plants, factory smokestacks, and other waste vents of the industrial age now pump six billion tons of carbon dioxide and other greenhouse gases into Earth's atmosphere each year. Concentrations of human-influenced greenhouse gases in the atmosphere are currently at approximately 25% above pre-industrial values. This is considerably higher than at any time during the last 420,000 years, the period for which reliable data (from ice cores) exists. From less direct geological evidence, it is believed that values this high were last attained 40 million years ago. Since the last Ice Age, the Earth has had a lower temperature than usual, so discussion about recent warming since the Little Ice Age continues.
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