Greenhouse effect

The greenhouse effect, first discovered by Joseph Fourier in 1824, and first investigated quantitatively by Svante Arrhenius in 1896, is the radiative forcing process by which an atmosphere warms a planet. The name is from the similar effect which greenhouses utilize in order to facilitate plant growth.

Mars, Venus and other celestial bodies with atmospheres (such as Titan) have greenhouse effects, but for simplicity this article mostly refers to the case of Earth.

In common parlance, the term greenhouse effect may be used to refer either to the natural greenhouse effect, which is the effect which occurs naturally on Earth, or to the enhanced (anthropogenic) greenhouse effect, which results from gases emitted as a result of human activities (see also global warming). No-one disputes the former, or its magnitude; the latter is accepted by a large majority of scientists, although there is some dispute as to its magnitude (see scientific opinion on climate change and attribution of recent climate change).

The natural greenhouse effect

Process

The earth receives an enormous amount of solar radiation. Just above the atmosphere, the direct solar radiation flux averages about 1366 watts per square meter, or 1.740×10¹⁷ W after being distributed over the entire Earth. This figure greatly exceeds the power generated by human activities. The difference between the natural greenhouse effect and global warming is that global warming is anthropogenic whereas greenhouse effect is not.

The solar power hitting Earth is balanced over time by an equal amount of power radiating from the Earth (as the amount of energy from the Sun that is stored is small). Almost all radiation leaving the Earth takes two forms: reflected solar radiation and thermal black body 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 ultraviolet 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 and powers life on the planet. It should be noted that the surface of the Earth is in constant flux with daily, yearly and age long cycles and trends in temperature and other variables for a variety of causes; thus these percentages apply on average only.

Like the Sun, the Earth is a thermal radiator. Because the Earth's surface is much cooler than the Sun (287 K vs 5780 K), Wien's displacement law dictates that Earth radiates its thermal energy at 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 (this is, probably, not entirely coincidental: the transparency to "visible" wavelengths makes eyes adapted to seeing these wavelengths useful; and eyes that could see in a strongly-absorbed wavelength would not be so useful). 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 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 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 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, Earth's surface warms up. The power of the surface radiation increases by the Stefan-Boltzmann law until it (over time) compensates for the atmospheric absorption. Another, simpler, but essentially equivalent way of looking at this is that the surface is heated by two sources: direct solar radiation, and thermal radiation from the atmosphere; it is thus warmer than if heated by solar radiation alone. 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 simplistic descriptions of the "greenhouse" effect to assert that the same mechanism warms greenhouses (e.g. ^*), but this is an incorrect oversimplification: see below.

The above description (and many other simplified expositions of the greenhouse effect) may give the impression that radiation is the most important method for transmitting heat through the atmosphere. In the lower atmosphere, particularly in the tropics, convection and latent heat transport is very important in moving heat vertically upwards from the surface; the greenhouse effect dynamics described above do operate, but become important higher in the atmosphere.

Limiting factors

The degree of the greenhouse effect is dependent primarily on the concentration of greenhouse gases in the planetary atmosphere. The deep and carbon dioxide-rich atmosphere of Venus (combined with an orbit closer to the sun than that of Earth) causes 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.

A runaway greenhouse effect occurred on Venus because of an interaction of the greenhouse effect with other processes in feedback cycles. Venus is sufficiently strongly heated by the Sun that water vapour can rise much higher in the atmosphere and is split into hydrogen and oxygen by ultraviolet light. The hydrogen can then escape from the atmosphere and the oxygen combines . Therefore less carbon dioxide is reabsorbed by the planetary crust causing an even higher temperature. As a result, the greenhouse effect was progressively intensified by positive feedback ^*." target="_blank" > On Earth there is a substantial hydrosphere and biosphere which responds 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) [http://www.phys.lsu.edu/faculty/cjohnson/climate.html. 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 average surface temperature would be −18°C if the atmosphere played no role. In reality this temperature is closer to 15°C above zero due to the combination of the greenhouse effect and the convective flow of heat energy within the atmosphere. Because convection (vertical exchanges of unstably stratified air, predominantly by storm clouds) moves heat above much of the thermal IR absorbance of the atmosphere, the greenhouse effect on the surface is smaller than it would be in the absence of such convection ^*.

Recent measurements of carbon dioxide amounts from Mauna Loa observatory show that CO₂ has increased from about 313 ppm (parts per million) in 1960 to about 375 ppm in 2005. The current observed amount of CO₂ exceeds the geological record of CO₂ maxima (~300 ppm) from ice core data (Hansen, J., Climatic Change, 68, 269, 2005 ^*); however, carbon dioxide levels during the Cretaceous Period are believed to be much higher than they are now. CO₂ production rate from increased industrial activity (fossil fuel burning) and other human activities such as land-use changes has overwhelmed the normal feedback control mechanisms. Global climate model calculations indicate that the elevated CO₂ levels are likely to lead to global warming. There has been an observed global average temperature increase of about 0.5^oC since 1960 (Science 308, 1431, 2005). There is still some public controversy about the role of human activities and that of CO₂ and other greenhouse gas increases for global warming.

The greenhouse gases

Water vapour is the single most important absorber (between 36% and 66% of the greenhouse effect), and together with clouds makes up between 66% and 85%. CO₂ alone makes up between 9 and 26%, while the O₃ and the other minor GHG absorbers consist of up to 7 and 8% of the effect, respectively ^*. Collectively, these gases are known as greenhouse gases. The greenhouse effect due to carbon dioxide is specifically known as the Callendar effect.

The wavelengths of light that a gas absorbs can be modelled with quantum mechanics based on molecular properties of the different gas molecules. It so happens that heteronuclear diatomic molecules and tri- (and more) atomic gases absorb at infrared wavelengths but homonuclear diatomic molecules do not absorb infrared light. This is why H₂O and CO₂ are greenhouse gases but the major atmospheric constituents (N₂ and O₂) are not.

Between the absorptions of water vapor and those of carbon dioxide, there is an atmospheric window where, prior to the industrial era, no infrared radiation was trapped, lying between 8 and 15 micrometres. Compounds such as perflurocarbons (CF₄, C₂F₆ etc.), chlorofluorocarbons, halons and SF₆ absorb very strongly in this window. This means that they are extremely potent greenhouse gases, especially given the absence of natural sinks to remove them. Perfluorocarbons can have a lifetime of 50,000 years.

Effects of various gases

It is hard to disentangle the percentage contributions to the greenhouse effect by different gases, because their respective infrared spectrums overlap. 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)); see also ^*.

Water vapor 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 vapor 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% PBS tv show Nova. Greenhouse—Green Planet ^*

Including clouds, the table above would suggest 50%. For the cloudless case, IPCC 1990, p 47–48 estimate water vapor at 60–70% whereas Baliunas & Soon estimate 88% ^* considering only H₂O and CO₂. Water vapor in the troposphere, unlike the better-known greenhouse gases such as CO₂, is essentially passive in terms of climate: the residence time for water vapor in the atmosphere is short (about a week) so perturbations to water vapor 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 vapor would increase, resulting in a (limited) positive feedback and higher temperatures. In response to a perturbation from enhanced water vapor, 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.

Real greenhouses

The term 'greenhouse effect' originally came from the greenhouses used for gardening, but it is a misnomer since greenhouses operate differently convection; the greenhouse effect however reduces radiation loss, not convection. It is quite common, however, to find sources (e.g. ^*" target="_blank" >^*.

References

Earth Radiation Budget, http://marine.rutgers.edu/mrs/education/class/yuri/erb.html
Fleagle, RG and Businger, JA: An introduction to atmospheric physics, 2nd edition, 1980
Fraser, Alistair B., Bad Greenhouse http://www.ems.psu.edu/~fraser/Bad/BadGreenhouse.html
Giacomelli, Gene A. and William J. Roberts1, Greenhouse Covering Systems, Rutgers University, downloaded from: http://ag.arizona.edu/ceac/research/archive/HortGlazing.pdf on 3-30-2005.
Ann Henderson-Sellers and McGuffie, K: A climate modelling primer (quote: Greenhouse effect: the effect of the atmosphere in re-readiating longwave radiation back to the surface of the Earth. It has nothing to do with glasshouses, which trap warm air at the surface).
Idso, S.B.: Carbon Dioxide: friend or foe, 1982 (quote: ...the phraseology is somewhat in appropriate, since CO₂ does not warm the planet in a manner analogous to the way in which a greenhouse keeps its interior warm).
Kiehl, J.T., and Trenberth, K. (1997). Earth's annual mean global energy budget, Bulletin of the American Meteorological Society 78 (2), 197–208.
Piexoto, JP and Oort, AH: Physics of Climate, American Institute of Physics, 1992 (quote: ...the name water vapor-greenhouse effect is actually a misnomer since heating in the usual greenhouse is due to the reduction of convection)
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.wmconnolley.org.uk/sci/wood_rw.1909.html
IPCC assessment reports, see http://www.ipcc.ch/

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