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The greenhouse effect is the heating of the surface of a planet or moon due to the presence of an atmosphere containing gases that absorb and emit infrared radiation. Thus, greenhouse gases trap heat within the surface-troposphere system. This mechanism is fundamentally different from that of an actual greenhouse, which works by isolating warm air inside the structure so that heat is not lost by convection. The greenhouse effect was discovered by Joseph Fourier in 1824, first reliably experimented on by John Tyndall in 1858, and first reported quantitatively by Svante Arrhenius in 1896.

The black body temperature of the Earth is 5.5 °C. Since the Earth's surface reflects about 28% of incoming sunlight, the planet's mean temperature would be far lower - about -18 or -19 °C - in the absence of the effect. Because of the effect, it is instead much higher at about 14 °C.

Global warming, a recent warming of the Earth's surface and lower atmosphere, is believed to be the result of an "enhanced greenhouse effect" mostly due to human-produced increases in atmospheric greenhouse gases. This human induced part is referred to as anthropogenic global warming (AGW).

Basic mechanism

The Earth receives energy from the Sun mostly in the form of visible light and nearby wavelengths. About 50% of the sun's energy is absorbed at the Earth's surface, the rest is reflected or absorbed by the atmosphere. The absorbed energy warms the surface. Simple presentations of the greenhouse effect, such as Idealized greenhouse model, show this heat being lost as thermal radiation. The reality is somewhat more complex: the atmosphere near the surface is largely opaque to thermal radiation (with important exeptions for "window" bands), and most heat loss from the surface is by sensible heat and latent heat transport. However, radiative effects become increasingly important higher in the atmosphere as the higher levels become progressively more transparent to thermal radiation, largley because the atmosphere is drier and water vapour - an important greenhouse gas - becomes less. It is more realistic to think of the greenhouse effect as applying to a "surface" in the mid-troposphere, which is effectively coupled to the surface by a lapse rate.

Within the region where radiative effects are important the presentation of the Idealized greenhouse model becomes a reasonable: a layer of atmosphere with greenhouses gases will radiate heat in all directions, both upwards and downwards, thereby warming the surface and simultaneously cooling the atmosphere by transmitting heat to deep space at 2.7K. Increasing the concentration of the gases increases the amount of radiation, and thereby warms the surface more .

Greenhouse gases

By their percentage contribution to the greenhouse effect the four major gases are:

• water vapor, 36–70%
• carbon dioxide, 9–26%
• methane, 4–9%
• ozone, 3–7%

The major non-gas contributor to the Earth's greenhouse effect, clouds, also absorb and emit infrared radiation and thus have an effect on radiative properties of the atmosphere.

Enhanced greenhouse effect

When it comes to the physical processes that produce the greenhouse effect, increases that are caused by human activities are known as the enhanced (or anthropogenic) greenhouse effect. This increase in radiative forcing from human activity is contributed to mostly by increased atmospheric carbon dioxide levels.

CO₂ is produced by fossil fuel burning and other activities such as cement production and tropical deforestation. Measurements of CO₂ from the Mauna Loa observatory show that concentrations have increased from about 313 ppm in 1960 to about 383 ppm in 2009. The current observed amount of CO₂ exceeds the geological record maxima (~300 ppm) from ice core data. The effect of combustion-produced carbon dioxide on the global climate, a special case of the greenhouse effect first described in 1896 by Svante Arrhenius, has also been called the Callendar effect.

Because it is a greenhouse gas, elevated CO₂ levels contribute to additional absorption and emission of thermal infrared in the atmosphere, which could contribute to net warming. In fact, according to Assessment Reports from the Intergovernmental Panel on Climate Change, "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".

Over the past 800,000 years, 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. Certain paleoclimatologists consider variations in carbon dioxide to be a fundamental factor in controlling climate variations over this time scale.

The distinction between the greenhouse effect and real greenhouses

The term "greenhouse effect" is used by analogy to greenhouses but is incorrect. A greenhouse works by preventing absorbed heat leaving the structure by sensible heat transport. In the case of the greenhouse effect the rate of radiation from the Earth to space is limited by the greenhouse gases. Both the greenhouse effect and the greenhouse limit the rate of thermal energy flowing out of the system; in that way they are similar.

A greenhouse is built of any material that passes sunlight, usually glass, or plastic. It mainly heats up because the sun warms the ground inside, 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 (R. W. Wood, 1909) that a "greenhouse" with a cover of rock salt (which is transparent to infra red) heats up an enclosure similarly to one with a glass cover. Thus greenhouses work by a different mechanism, primarily by preventing convective cooling.

In the greenhouse effect, rather than retaining (sensible) heat by physically preventing movement of the air, greenhouse gases act to warm the Earth by re-radiating energy back towards the surface. This process exists in real greenhouses, but is comparatively unimportant.

Bodies other than Earth

In our solar system, Mars, Venus, and the moon Titan also exhibit greenhouse effects. Titan has an anti-greenhouse effect, in that its atmosphere absorbs solar radiation but is relatively transparent to infrared radiation. Pluto also exhibits behavior similar to the anti-greenhouse effect.

A runaway greenhouse effect occurs if positive feedbacks lead to the evaporation of all greenhouse gases into the atmosphere. A runaway greenhouse effect involving carbon dioxide and water vapor may have occurred on Venus.

See also

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

Literature

• 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
• IPCC assessment reports, see http://www.ipcc.ch/
• 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.

References

1. IPCC AR4 SYR Appendix Glossary
2. A concise description of the greenhouse effect is given in the Intergovernmental Panel on Climate Change Fourth Assessment Report, "What is the Greenhouse Effect?" IIPCC Fourth Assessment Report, Chapter 1, page 115: "To balance the absorbed incoming 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.
3. Annual Reviews (requires registration)
4. Sun's Influence on Earth
5. V1003 Science and Society - Solar Radiation (disagreements with the more exact figure of 5.48 are due to the rounding of the Solar surface temperature)
6. Introduction to Atmospheric Chemistry, by Daniel J. Jacob, Princeton University Press, 1999. Chapter 7, "The Greenhouse Effect".
7. Solar Radiation and the Earth's Energy Balance
8. ^ Intergovernmental Panel on Climate Change Fourth Assessment Report. Chapter 1: Historical overview of climate change science page 97
9. The elusive "absolute surface air temperature," see GISS discussion
10. Merged land air and sea surface temperature data set
11. The enhanced greenhouse effect
12. "Water vapour: feedback or forcing?". RealClimate. 6 April 2005. Retrieved 2006-05-01.
13. ^ Kiehl, J. T. (1997). "Earth's Annual Global Mean Energy Budget" (PDF). Bulletin of the American Meteorological Society. 78 (2): 197–208. doi:10.1175/1520-0477(1997)078<0197:EAGMEB>2.0.CO;2. Retrieved 2009-12-23. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help) Cite error: The named reference "kiehl197" was defined multiple times with different content (see the help page).
14. http://www.science.org.au/nova/016/016glo.htm
15. http://www.ace.mmu.ac.uk/eae/Global_Warming/Older/Enhanced_Greenhouse_Effect.html
16. IPCC Fourth Assessment Report, Working Group I Report "The Physical Science Basis" Chapter 7
17. "Atmospheric Carbon Dioxide - Mauna Loa". NOAA.
18. Hansen, J., Climatic Change, 68, 269, 2005 ISSN 0165-0009
19. IPCC Fourth Assessment Report Synthesis Report: Summary for Policymakers (p. 5)
20. BBC NEWS | Science/Nature | Deep ice tells long climate story
21. Chemical & Engineering News: Latest News - Ice Core Record Extended
22. Bowen, Mark; Thin Ice: Unlocking the Secrets of Climate in the World's Highest Mountains; Owl Books, 2005.
23. Wood, R.W. (1909) "Note on the Theory of the Greenhouse," Philosophical Magazine, 17, pp 319–320. For the text of this online, see R. W. Wood: Note on the Theory of the Greenhouse "When exposed to sunlight the temperature rose gradually to 65 C., the enclosure covered with the salt plate keeping a little ahead of the other, owing to the fact that it transmitted the longer waves from the sun, which were stopped by the glass. In order to eliminate this action the sunlight was first passed through a glass plate." "it is clear that the rock-salt plate is capable of transmitting practically all of it, while the glass plate stops it entirely. This shows us that the loss of temperature of the ground by radiation is very small in comparison to the loss by convection, in other words that we gain very little from the circumstance that the radiation is trapped."
24. * 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"
25. Schroeder, Daniel V. (2000). An introduction to thermal physics. San Francisco, California: Addison-Wesley. pp. 305–307. ISBN 0-321-27779-1. ... this mechanism is called the greenhouse effect, even though most greenhouses depend primarily on a different mechanism (namely, limiting convective cooling).
26. ATM S 211 - Notes
27. Titan: Greenhouse and Anti-greenhouse :: Astrobiology Magazine - earth science - evolution distribution Origin of life universe - life beyond :: Astrobiology is study of earth...
28. SPACE.com - Pluto Colder Than Expected
29. Kasting, James F. (1991). "Runaway and moist greenhouse atmospheres and the evolution of Earth and Venus.". Planetary Sciences: American and Soviet Research/Proceedings from the U.S.-U.S.S.R. Workshop on Planetary Sciences. Commission on Engineering and Technical Systems (CETS). pp. 234–245. Retrieved 2009. {{cite conference}}: Check date values in: |accessdate= (help); Unknown parameter |booktitle= ignored (|book-title= suggested) (help)
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• Atmospheric radiation
• Climate change
• Climate forcing
• Atmosphere