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On the other hand, if one limits infrared radiation from the greenhouse, especially at night, one can substantially increase the temperature of the greenhouse, or limit the amount of heating that is needed. Aluminized screens which reflect the infrared thermal radiation so that it cannot heat the greenhouse windows are used for this purpose <ref name="Kittas> C. Kittas, N. Katsoulas, and A. Baille (2003) "Influence of an aluminized thermal screen on greenhouse microclimate and canopy energy balance" ''Transactions of the ASAE 46'' pp. 1653-1663.</ref> As energy prices rise, similar screens are coming into increasing use in greenhouses.


In the atmosphere, as in the greenhouse, sunlight heats the surface. A [[radiative balance]] is achieved when the thermal energy radiated to space matches the energy absorbed from the sun. Without [[greenhouse gases]] all of the energy radiated from the surface would reach space at the speed of light. As a practical matter none of the energy radiated from the surface at wavelengths that can be absorbed by the principal [[greenhouse gases]] reaches space directly. Rather due to cycles of absorption and emission, radiation at those frequencies is trapped within the atmosphere and can only be emitted by greenhouse gases high in the [[troposphere]] where the [[lapse rate]] ensures that it is significantly colder. Since the rate of emission by the colder molecules is much slower, the entire earth system must heat up in order to restore a radiative balance. In this way, the greenhouse effect limits the emission from the earth by radiation.
In the atmosphere, as in the greenhouse, sunlight heats the surface. A [[radiative balance]] is achieved when the thermal energy radiated to space matches the energy absorbed from the sun. Without [[greenhouse gases]] all of the energy radiated from the surface would reach space at the speed of light. As a practical matter none of the energy radiated from the surface at wavelengths that can be absorbed by the principal [[greenhouse gases]] reaches space directly. Rather due to cycles of absorption and emission, radiation at those frequencies is trapped within the atmosphere and can only be emitted by greenhouse gases high in the [[troposphere]] where the [[lapse rate]] ensures that it is significantly colder. Since the rate of emission by the colder molecules is much slower, the entire earth system must heat up in order to restore a radiative balance. In this way, the greenhouse effect limits the emission from the earth by radiation.

Revision as of 21:19, 4 February 2010

A schematic representation of the exchanges of energy between outer space, the Earth's atmosphere, and the Earth's surface. The ability of the atmosphere to capture and recycle energy emitted by the Earth surface is the defining characteristic of the greenhouse effect.

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.[1] Thus, greenhouse gases trap heat within the surface-troposphere system.[2] 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.[3]

The black body temperature of the Earth is 5.5 °C.[4][5] Since the Earth's surface reflects about 28% of incoming sunlight[6], the planet's mean temperature would be far lower - about -18 or -19 °C - in the absence of the effect.[7][8] Because of the effect, it is instead much higher at about 14 °C.[9]

Global warming, a recent warming of the Earth's surface and lower atmosphere,[10] is believed to be the result of an "enhanced greenhouse effect" mostly due to human-produced increases in atmospheric greenhouse gases.[11] This human induced part is referred to as anthropogenic global warming (AGW).

Basic mechanism

See also: Idealized greenhouse model

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. Like all bodies with a temperature above absolute zero the Earth's surface radiates energy in the infrared range. Greenhouse gases in the atmosphere absorb most of the infrared radiation emitted by the surface and pass the absorbed heat to other atmospheric gases through molecular collisions. The greenhouse gases also radiate in the infrared range. Radiation is emitted both upward, with part escaping to space, and downward toward Earth's surface. The surface and lower atmosphere are warmed by the part of the energy that is radiated downward, making our life on earth possible.[8]

Greenhouse gases

Main article: Greenhouse gas

In order of volume, Earth's most abundant greenhouse gases are:

By their percentage contribution to the greenhouse effect[12][13] 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.[13]

Enhanced greenhouse effect

Main article: Global warming

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.[14] This increase in radiative forcing from human activity is contributed to mostly by increased atmospheric carbon dioxide levels.[15]

CO2 is produced by fossil fuel burning and other activities such as cement production and tropical deforestation.[16] Measurements of CO2 from the Mauna Loa observatory show that concentrations have increased from about 313 ppm [17] 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.[18] 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".[19]

Over the past 800,000 years,[20] 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.[21] Certain paleoclimatologists consider variations in carbon dioxide to be a fundamental factor in controlling climate variations over this time scale.[22]

The distinction between the greenhouse effect and real greenhouses

A modern Greenhouse in RHS Wisley

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.[23] Thus greenhouses work by a different mechanism, primarily by preventing convective cooling.[24][25]

In the atmosphere, as in the greenhouse, sunlight heats the surface. A radiative balance is achieved when the thermal energy radiated to space matches the energy absorbed from the sun. Without greenhouse gases all of the energy radiated from the surface would reach space at the speed of light. As a practical matter none of the energy radiated from the surface at wavelengths that can be absorbed by the principal greenhouse gases reaches space directly. Rather due to cycles of absorption and emission, radiation at those frequencies is trapped within the atmosphere and can only be emitted by greenhouse gases high in the troposphere where the lapse rate ensures that it is significantly colder. Since the rate of emission by the colder molecules is much slower, the entire earth system must heat up in order to restore a radiative balance. In this way, the greenhouse effect limits the emission from the earth by radiation.

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.[26][27][28]

A runaway greenhouse effect occurs if positive feedbacks lead to the evaporation of all greenhouse gases into the atmosphere.[29] A runaway greenhouse effect involving carbon dioxide and water vapor may have occurred on Venus.[30]

See also

Wikibooks has a book on the topic of: Climate Change
Wikiversity has learning resources about Topic:Climate change

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. ^ [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 [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.
  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. ^ a b 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. ^ a b 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)
  30. ^ Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1038/2261037a0, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi=10.1038/2261037a0 instead.

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