Greenhouse effect

The Greenhouse effect refers to the change in the thermal equilibrium temperature of a planet or moon by the presence of an atmosphere containing gas that absorbs infrared radiation. [ [http://www.ipcc.ch/pdf/assessment-report/ar4/syr/ar4_syr_appendix.pdf] IPCC AR4 SYR Appendix Glossary] Greenhouse gases warm the atmosphere by efficiently absorbing thermal infrared radiation emitted by the Earth’s surface, by the atmosphere itself, and by clouds. As a result of its warmth, the atmosphere also radiates thermal infrared in all directions, including downward to the Earth’s surface. Thus, greenhouse gases trap heat within the surface-troposphere system.A concise description of the greenhouse effect is given in the "Intergovernmental Panel on Climate Change Fourth Assessment Report," "What is the Greenhouse Effect?" [http://ipcc-wg1.ucar.edu/wg1/Report/AR4WG1_Print_Ch01.pdf IIPCC Fourth Assessment Report, Chapter 1] , page 105: "To balance the absorbed incoming [solar] energy, the Earth must, on average, radiate the same amount of energy back to space. Because the Earth is much colder than the Sun, it radiates at much longer wavelengths, primarily in the infrared part of the spectrum (see Figure 1). Much of this thermal radiation emitted by the land and ocean is absorbed by the atmosphere, including clouds, and reradiated back to Earth. This is called the greenhouse effect."] [Stephen H. Schneider, in "Geosphere-biosphere Interactions and Climate," Lennart O. Bengtsson and Claus U. Hammer, eds., Cambridge University Press, 2001, ISBN 0521782384, pp. 90-91.] [E. Claussen, V. A. Cochran, and D. P. Davis, "Climate Change: Science, Strategies, & Solutions," University of Michigan, 2001. p. 373.] [A. Allaby and M. Allaby, "A Dictionary of Earth Sciences," Oxford University Press, 1999, ISBN 0192800795, p. 244.] This mechanism is fundamentally different from the mechanism of an actual greenhouse, which instead isolates air inside the structure so that heat is not lost by convection and conduction. The greenhouse effect was discovered by Joseph Fourier in 1824 and first investigated quantitatively by Svante Arrhenius in 1896. [ [http://arjournals.annualreviews.org/doi/full/10.1146/annurev.energy.25.1.441 Annual Reviews (requires registration)] ]

In the absence of the greenhouse effect, the Earth's average surface temperature of 14 °C (57 °F) would be about -18 °C (–0.4 °F) [ [http://ipcc-wg1.ucar.edu/wg1/Report/AR4WG1_Print_Ch01.pdf Intergovernmental Panel on Climate Change Fourth Assessment Report. Chapter 1: Historical overview of climate change science] page 97] ] [ [http://data.giss.nasa.gov/gistemp/abs_temp.html The elusive absolute surface air temperature] ] (Black body temperature of the Earth). Anthropogenic Global warming (AGW), a recent warming of the Earth's lower atmosphere as evidenced by the global mean temperature anomaly trend [ [http://www.ncdc.noaa.gov/gcag/gcagmerged.jsp Merged land air and sea surface temperature data set] ] , is believed to be the result of an "enhanced greenhouse effect" mainly due to human-produced increased concentrations of greenhouse gases in the atmosphere [ [http://www.science.org.au/nova/016/016key.htm The enhanced greenhouse effect] ] and changes in the use of land [ [http://www.grida.no/climate/ipcc/land_use/003.htm Land Use, Land-Use change and Forestry, IPCC pecial report SPM] ]

The greenhouse effect is only one of many factors which affect the temperature of the Earth. Other positive and negative feedbacks dampen or amplify the greenhouse effect.

In our solar system, Mars, Venus, and the moon Titan also exhibit significant greenhouse effect.

Basic mechanism

The Earth receives energy from the Sun mostly in the form of visible light. The bulk of this energy is not absorbed by the atmosphere since the atmosphere is transparent to visible light. 50% of the sun's energy reaches the Earth which is absorbed by the surface as heat. Because of its temperature, the Earth's surface radiates energy in infrared range. The Greenhouse gases are not transparent to infrared radiation so they absorb infrared radiation. Infrared radiation is absorbed from all directions and is passed as heat to all gases in the atmosphere. The atmosphere also radiates in the infrared range (because of its temperature, in the same way the Earth's surface does) and does so in all directions. The surface and lower atmosphere are warmed because of the greenhouse gases and makes our life on earth possible.Fraser, Alistair B., http://www.ems.psu.edu/~fraser/Bad/BadGreenhouse.html]

Detailed explanation

about 30% of the incoming solar radiation. The remaining 70% is absorbed, warming the land, atmosphere and ocean.

For the Earth's temperature to be in steady state so that the Earth does not rapidly heat or cool, this absorbed solar radiation must be very closely balanced by energy radiated back to space in the infrared wavelengths. Since the intensity of infrared radiation increases with increasing temperature, one can think of the Earth's temperature as being determined by the infrared flux needed to balance the absorbed solar flux. The visible solar radiation mostly heats the surface, not the atmosphere, whereas most of the infrared radiation escaping to space is emitted from the upper atmosphere, not the surface. The infrared photons emitted by the surface are mostly absorbed in the atmosphere by greenhouse gases and clouds and do not escape directly to space.

The reason this warms the surface is most easily understood by starting with a simplified model of a purely radiative greenhouse effect that ignores energy transfer in the atmosphere by convection (sensible heat transport, Sensible heat flux) and by the evaporation and condensation of water vapor (latent heat transport, Latent heat flux). In this purely radiative case, one can think of the atmosphere as emitting infrared radiation both upwards and downwards. The upward infrared flux emitted by the surface must balance not only the absorbed solar flux but also this downward infrared flux emitted by the atmosphere. The surface temperature will rise until it generates thermal radiation equivalent to the sum of the incoming solar and infrared radiation.

A more realistic picture taking into account the convective and latent heat fluxes is somewhat more complex. But the following simple model captures the essence. The starting point is to note that the opacity of the atmosphere to infrared radiation determines the height in the atmosphere from which most of the photons are emitted into space. If the atmosphere is more opaque, the typical photon escaping to space will be emitted from higher in the atmosphere, because one then has to go to higher altitudes to "see" out to space in the infrared. Since the emission of infrared radiation is a function of temperature, it is the temperature of the atmosphere at this emission level that is effectively determined by the requirement that the emitted flux balance the absorbed solar flux.

But the temperature of the atmosphere generally decreases with height above the surface, at a rate of roughly 6.5 °C per kilometer on average, until one reaches the stratosphere 10-15 km above the surface. (Most infrared photons escaping to space are emitted by the troposphere, the region bounded by the surface and the stratosphere, so we can ignore the stratosphere in this simple picture.) A very simple model, but one that proves to be remarkably useful, involves the assumption that this temperature profile is simply fixed, by the non-radiative energy fluxes. Given the temperature at the emission level of the infrared flux escaping to space, one then computes the surface temperature by increasing temperature at the rate of 6.5 °C per kilometer, the environmental lapse rate, until one reaches the surface. The more opaque the atmosphere, and the higher the emission level of the escaping infrared radiation, the warmer the surface, since one then needs to follow this lapse rate over a larger distance in the vertical. While less intuitive than the purely radiative greenhouse effect, this less familiar "radiative-convective" picture is the starting point for most discussions of the greenhouse effect in the climate modeling literature.

Greenhouse gases

Quantum mechanics provides the basis for computing the interactions between molecules and radiation. Most of this interaction occurs when the frequency of the radiation closely matches that of the spectral lines of the molecule, determined by the quantization of the modes of vibration and rotation of the molecule. (The electronic excitations are generally not relevant for infrared radiation, as they require energy larger than that in an infrared photon.)

The width of a spectral line is an important element in understanding its importance for the absorption of radiation. In the Earth’s atmosphere these spectral widths are primarily determined by “pressure broadening”, which is the distortion of the spectrum due to the collision with another molecule. Most of the infrared absorption in the atmosphere can be thought of as occurring while two molecules are colliding. The absorption due to a photon interacting with a lone molecule is relatively small. This three-body aspect of the problem, one photon and two molecules, makes direct quantum mechanical computation for molecules of interest more challenging. Careful laboratory spectroscopic measurements, rather than "ab initio" quantum mechanical computations, provide the basis for most of the radiative transfer calculations used in studies of the atmosphere.

The molecules/atoms that constitute the bulk of the atmosphere: oxygen (O₂), nitrogen (N₂) and argon (Ar); do not interact with infrared radiation significantly. While the oxygen and nitrogen molecules can vibrate, because of their symmetry these vibrations do not create any transient charge separation. Without such a transient dipole moment, they can neither absorb nor emit infrared radiation. In the Earth’s atmosphere, the dominant infrared absorbing gases are water vapor, carbon dioxide, and ozone (O₃). The same molecules are also the dominant infrared emitting molecules. CO₂ and O₃ have "floppy" vibration motions whose quantum states can be excited by collisions at energies encountered in the atmosphere. For example, carbon dioxide is a linear molecule, but it has an important vibrational mode in which the molecule bends with the carbon in the middle moving one way and the oxygens on the ends moving the other way, creating some charge separation, a dipole moment, thus carbon dioxide molecules can absorb IR radiation. Collisions will immediately transfer this energy to heating the surrounding gas. On the other hand, other CO₂ molecules will be vibrationally excited by collisions. Roughly 5% of CO₂ molecules are vibrationally excited at room temperature and it is this 5% that radiates. A substantial part of the greenhouse effect due to carbon dioxide exists because this vibration is easily excited by infrared radiation. CO₂ has two other vibrational modes. The symmetric stretch does not radiate, and the asymmetric stretch is at too high a frequency to be effectively excited by atmospheric temperature collisions, although it does contribute to absorption of IR radiation. The vibrational modes of water are at too high energies to effectively radiate, but do absorb higher frequency IR radiation. Water vapor has a bent shape. It has a permanent dipole moment (the O atom end is electron rich, and the H atoms electron poor) which means that IR radiation can be emitted and absorbed during rotational transitions, and these transitions can also be produced by collisional energy transfer. Clouds are also very important infrared absorbers. Therefore, water has multiple effects on infrared radiation, through its vapor phase and through its condensed phases. Other absorbers of significance include methane, nitrous oxide and the chlorofluorocarbons.

Discussion of the relative importance of different infrared absorbers is confused by the overlap between the spectral lines due to different gases, widened by pressure broadening. As a result, the absorption due to one gas cannot be thought of as independent of the presence of other gases. One convenient approach is to remove the chosen constituent, leaving all other absorbers, and the temperatures, untouched, and monitoring the infrared radiation escaping to space. The reduction in infrared absorption is then a measure of the importance of that constituent. More precisely, define the greenhouse effect (GE) to be the difference between the infrared radiation that the surface would radiate to space if there were no atmosphere and the actual infrared radiation escaping to space. Then compute the percentage reduction in GE when a constituent is removed. The table below is computed by this method, using a particular 1-dimensional model of the atmosphere. More recent 3D computations lead to similar results.

(Source: GISS-GCM ModelE simulation) [RealClimate, [http://www.realclimate.org/index.php?p=142 "Water vapour: feedback or forcing?"] ]

By this particular measure, water vapor can be thought of as providing 36% of the greenhouse effect, and carbon dioxide 9%, but the effect of removal of both of these constituents will be greater than the total that each reduces the effect, in this case more than 45%. An additional proviso is that these numbers are computed holding the cloud distribution fixed. But removing water vapor from the atmosphere while holding clouds fixed is not likely to be physically relevant. In addition, the effects of a given gas are typically nonlinear in the amount of that gas, since the absorption by the gas at one level in the atmosphere can remove photons that would otherwise interact with the gas at another altitude. The kinds of estimates presented in the table, while often encountered in the controversies surrounding global warming, must be treated with caution. Different estimates found in different sources typically result from different definitions and do not reflect uncertainties in the underlying radiative transfer.

Positive feedback, runaway greenhouse effect and tipping point

The tipping point in global warming is the point at which change due to human activity brings about sufficient new processes in nature to make any human reversal of the change impossible. Some climate scientists believe this will be reached in about 2017 [ [http://abcnews.go.com/Technology/Story?id=3223473&page=1 NASA: Danger Point Closer Than Thought From Warming] 'Disastrous Effects' of Global Warming Tipping Points Near, According to New Study. By Bill Blakemore, ABC News, May 29, 2007. Accessed April 2008.] , while others, notably James Hansen, NASA's top climate scientist, believe it has already been reached. [ [http://www.physorg.com/news126761406.html Earth in crisis, warns NASA's top climate scientist] PhysOrg.com , April 07, 2008 . Accessed April 2008.]

When there is a loop of effects such as the concentration of a greenhouse gas itself being a function of temperature, there is a feedback. If the effect is to act in the same direction on temperature it is a positive feedback; and if in the opposite direction it is a negative feedback. Sometimes feedback effects can be on the same cause as the forcing but it can also be via another greenhouse gas or on other effects such as change in ice cover affecting the planet's albedo.

Positive feedbacks do not have to lead to a runaway effect. With radiation from the Earth increasing in proportion to the fourth power of temperature, in accordance with the Stefan-Boltzmann law, the feedback effect has to be very strong to cause a runaway effect. An increase in temperature from greenhouse gases leading to increased water vapour which is a greenhouse gas causing further warming is a positive feedback. This cannot be a runaway effect or the runaway effect would have occurred long ago. Positive feedback effects are common and can always exist while runaway effects are much rarer and cannot be operating at all times.

If the effects from the second iteration of the loop of effects is larger than the effects of the first iteration of the loop this will lead to a self perpetuating effect. If this occurs and the feedback only ends after producing a major temperature increase, it is called a runaway greenhouse effect. A runaway feedback could also occur in the opposite direction leading to an ice age. Runaway feedbacks are bound to stop, since infinite temperatures are not observed. They are allowed to stop due to things like a reducing supply of a greenhouse gas or a phase change of the gas or ice cover reducing towards zero or increasing toward a large size that is difficult to increase.

According to the clathrate gun hypothesis a runaway greenhouse effect could be caused by liberation of methane gas from hydrates by global warming if there are sufficient hydrates close to unstable conditions. It has been speculated that the Permian-Triassic extinction event was caused by such a runaway effect. It is also thought that large quantities of methane could be released from the Siberian tundra as it begins to thaw, methane being 21-times more potent a greenhouse gas than carbon dioxide. [ [http://environment.newscientist.com/channel/earth/mg19125713.300-climate-change-one-degree-and-were-done-for.html Climate change: 'One degree and we're done for' - earth - 27 September 2006 - New Scientist Environment ] ]

A runaway greenhouse effect involving CO₂ and water vapor may have occurred on Venus due to its closer proximity to the sun. On Venus today there is little water vapor in the atmosphere. If water vapor did contribute to the warmth of Venus at one time, this water is thought to have escaped to space. Venus is sufficiently strongly heated by the Sun that water vapor 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 recombines. Carbon dioxide, the dominant greenhouse gas in the current Venusian atmosphere, likely owes its larger concentration to the weakness of carbon recycling as compared to Earth, where the carbon dioxide emitted from volcanoes is efficiently subducted into the Earth by plate tectonics on geologic time scales. [ [http://home.case.edu/~sjr16/venus.html Venus - Stuart Robbins and David McDonald] ] [Notes (created by Nick Strobel) for an introductory astronomy courses he teaches.
* [http://www.astronomynotes.com/solarsys/s9.htm Nick's new site]
* [http://web.archive.org/web/20070324205501/http://astronomy.nju.edu.cn/astron/Astronomynotes/solsysb.htm Old site (The Wayback Machine)] ]

Even so, the high temperatures on Venus are only partially caused by carbon dioxide; a major contributor is the thick bank of clouds containing sulphuric acid. [ [http://www.aas.org/publications/baas/v33n3/dps2001/354.htm How Hot Can Venus Get?] ] Therefore, the radiating layer emitting infrared to balance the solar flux occurs high in the atmosphere. [ [http://www-ssc.igpp.ucla.edu/ssc/tutorial/pvo_mission.html The Pioneer Venus Mission] ] [ [http://www.boulder.swri.edu/~rafkin/bougher_et_al_PSS_2006.pdf Dynamics of the Venus upper atomsophere] (PDF)] [ IMCCE [http://www.imcce.fr/vt2004/en/fiches/fiche_n13_eng.html "Clouds and atmosphere of Venus"] ] Although these clouds give Venus a high reflectivity in the visible region, the Galileo probe showed that the clouds appear black at infrared wavelengths of 2.3 micrometers due to strong infrared absorption [NASA [http://www2.jpl.nasa.gov/galileo/slides/slide3.html Infrared Image of Low Clouds on Venus] ] .

Anthropogenic greenhouse effect

CO₂ production from increased industrial activity (fossil fuel burning) and other human activities such as cement production and tropical deforestationIPCC [http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter7.pdf Fourth Assessment Report, Working Group I Report "The Physical Science Basis"] Chapter 7] has increased the CO₂ concentrations in the atmosphere. 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 [http://www.springerlink.com/content/x283l27781675v51/?p=799ebc88193f4ecfa8ca76f6e28f45d7 ISSN 0165-0009] ]

Because it is a greenhouse gas, elevated CO₂ levels will increase global mean temperature; based on an extensive review of the scientific literature, the Intergovernmental Panel on Climate Change concludes that "most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations". [IPCC Fourth Assessment Report [http://www.ipcc.ch/pdf/assessment-report/ar4/syr/ar4_syr_spm.pdf Synthesis Report: Summary for Policymakers] (p. 5)]

Over the past 800,000 years, [ [http://news.bbc.co.uk/2/hi/science/nature/5314592.stm BBC NEWS | Science/Nature | Deep ice tells long climate story ] ] ice core data shows unambiguously that carbon dioxide has varied from values as low as 180 parts per million (ppm) to the pre-industrial level of 270ppm. [ [http://pubs.acs.org/cen/news/83/i48/8348notw1.html Chemical & Engineering News: Latest News - Ice Core Record Extended ] ] Certain paleoclimatologists consider variations in carbon dioxide to be a fundamental factor in controlling climate variations over this time scale.Bowen, Mark; Thin Ice: Unlocking the Secrets of Climate in the World's Highest Mountains; Owl Books, 2005.]

Real greenhouses

The term "greenhouse effect" is a source of confusion in that actual greenhouses do not warm by this mechanism. Popular discussions often imply incorrectly that they do; this error is sometimes made even in materials from scientific or governmental agencies (e.g., the U.S. Environmental Protection Agency [ [http://www.epa.gov/globalwarming/kids/greenhouse.html Redirection to EPA Climate Change Site ] ] ).

The term 'greenhouse effect' originally came from the greenhouses used for gardening, but the mechanism for greenhouses operates differently. [ [http://www.wmconnolley.org.uk/sci/wood_rw.1909.html R. W. Wood: Note on the Theory of the Greenhouse ] ] Cite book
author=Schroeder, Daniel V.
title=An introduction to thermal physics
year=2000
publisher=Addison-Wesley
location=San Francisco, California
isbn=0-321-27779-1
quote=... this mechanism is called the "greenhouse effect", even though most greenhouses depend primarily on a different mechanism (namely, limiting convective cooling).
pages=pp. 305-307] Many sources make the "heat trapping" analogy of how a greenhouse limits convection to how the atmosphere performs a similar function through the different mechanism of infrared absorbing gases. [ [http://pangea.stanford.edu/courses/gp025/webbook/07_clement.html GP 25 Web Book | Chapter 7 ] ] [ [http://www.ncdc.noaa.gov/paleo/globalwarming/what.html#greenhouse NOAA Paleoclimatology Global Warming - The Story ] ] A greenhouse is usually built of glass, plastic, or a plastic-type material. It heats up mainly because the sun warms the ground inside it, which then warms the air in the greenhouse. The air continues to heat because it is confined within the greenhouse, unlike the environment outside the greenhouse where warm air near the surface rises and mixes with cooler air aloft. 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): a "greenhouse" built of rock salt (which is also transparent to infrared radiation) heats up just as one built of glass does. Greenhouses thus work primarily by preventing "convection"; the atmospheric greenhouse effect however reduces "radiation loss", not convection.

See also

* Anti-greenhouse effect
* Climate change
* Climate forcing
* Earth's energy budget
* Earth's radiation balance
* Global dimming
* Global warming

Footnotes

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: PDFlink|http://ag.arizona.edu/ceac/research/archive/HortGlazing.pdf|271 KiB 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/
* How to kill (almost) all life: the end-Permian extinction event, Michael J. Benton and Richard J. Twitchett, Department of Earth Sciences University of Bristol UK, TRENDS in Ecology and Evolution Vol.18 No.7 July 2003, DOI|10.1016/S0169-5347(03)00093-4 (PDFlink| [http://palaeo.gly.bris.ac.uk/Benton/reprints/2003TREEPTr.pdf full reprint] |506 KiB )