Greenhouse effect

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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] Greenhouse gases are almost transparent to solar radiation but strongly absorb and emit infrared radiation. Thus, greenhouse gases trap heat within the surface-troposphere system.^[2][3][4][5] 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.^[6]

In the absence of the greenhouse effect and an atmosphere, the Earth's average surface temperature^[7] of 14 °C (57 °F) could be as low as −18 °C (−0.4 °F), the black body temperature of the Earth.^[8][9][10] Anthropogenic global warming (AGW), a recent warming of the Earth's lower atmosphere as evidenced by the global mean temperature anomaly trend,^[11] is believed to be the result of an "enhanced greenhouse effect" mainly due to human-produced increases in atmospheric greenhouse gases.^[12]

Basic mechanism

The Earth receives energy from the Sun mostly in the form of visible light. The atmosphere is almost transparent to visible light, so that about 50% of the sun's energy reaches the Earth and is absorbed by the surface. Like all bodies with a temperature above absolute zero the Earth's surface radiates energy in the infrared range. Greenhouse gases absorb infrared radiation 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

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

• water vapor
• carbon dioxide
• methane
• nitrous oxide
• ozone
• CFCs

When these gases are ranked by their contribution to the greenhouse effect, the most important are:

• water vapor, which contributes 36–70%
• carbon dioxide, which contributes 9–26%
• methane, which contributes 4–9%
• ozone, which contributes 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 greenhouse gases.^[13][14]

Anthropogenic greenhouse effect

Carbon dioxide is the human-produced greenhouse gases that contributes most of radiative forcing from human activity. CO₂ is produced by fossil fuel burning and other human activities such as cement production and tropical deforestation.^[15] Measurements of CO₂ from the Mauna Loa observatory show that concentrations have increased from about 313 ppm ^[16] 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.^[17] 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 will 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".^[18]

Over the past 800,000 years,^[19] 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.^[20] Certain paleoclimatologists consider variations in carbon dioxide to be a fundamental factor in controlling climate variations over this time scale.^[21]

Real greenhouses

The term "greenhouse effect" can be a source of confusion as actual greenhouses do not function by the same mechanism the atmosphere does. Various materials at times imply incorrectly that they do, or do not make the distinction between the processes of radiation and convection^[22].

The term 'greenhouse effect' originally came from the greenhouses used for gardening, but as mentioned the mechanism for greenhouses operates differently.^[23] 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.^[24]

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) that a "greenhouse" with a cover of rock salt heats up an enclosure similarly to one with a glass cover.^[25] Greenhouses thus work primarily by preventing convection; the atmospheric greenhouse effect however reduces radiation loss, not convection.^[26][23]

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.^[27][28][29]

A runaway greenhouse effect occurs if positive feedbacks lead to the evaporation of all greenhouse gases into the atmosphere.^[30] A runaway greenhouse effect involving carbon dioxide and water vapor may have occurred on Venus.^[31]

See also

Wikibooks has a book on the topic of Climate Change

Wikiversity has learning materials about Topic:Climate change

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

Footnotes

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."
3. ^ 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.
4. ^ E. Claussen, V. A. Cochran, and D. P. Davis, Climate Change: Science, Strategies, & Solutions, University of Michigan, 2001. p. 373.
5. ^ A. Allaby and M. Allaby, A Dictionary of Earth Sciences, Oxford University Press, 1999, ISBN 0192800795, p. 244.
6. ^ Annual Reviews (requires registration)
7. ^ The elusive "absolute surface air temperature," see GISS discussion
8. ^ ^{a b} Intergovernmental Panel on Climate Change Fourth Assessment Report. Chapter 1: Historical overview of climate change science page 97
9. ^ V1003 Science and Society - Solar Radiation
10. ^ Solar Radiation and the Earth's Energy Balance
11. ^ Merged land air and sea surface temperature data set
12. ^ The enhanced greenhouse effect
13. ^ Kiehl, J. T.; Kevin E. Trenberth (February 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. http://www.atmo.arizona.edu/students/courselinks/spring04/atmo451b/pdf/RadiationBudget.pdf. Retrieved on 2006-05-01. 
14. ^ "Water vapour: feedback or forcing?". RealClimate. 6 April 2005. http://www.realclimate.org/index.php?p=142. Retrieved on 2006-05-01. 
15. ^ IPCC Fourth Assessment Report, Working Group I Report "The Physical Science Basis" Chapter 7
16. ^ "Atmospheric Carbon Dioxide - Mauna Loa". NOAA. http://www.esrl.noaa.gov/gmd/ccgg/trends/co2_data_mlo.html. 
17. ^ Hansen, J., Climatic Change, 68, 269, 2005 ISSN 0165-0009
18. ^ IPCC Fourth Assessment Report Synthesis Report: Summary for Policymakers (p. 5)
19. ^ BBC NEWS | Science/Nature | Deep ice tells long climate story
20. ^ Chemical & Engineering News: Latest News - Ice Core Record Extended
21. ^ Bowen, Mark; Thin Ice: Unlocking the Secrets of Climate in the World's Highest Mountains; Owl Books, 2005.
22. ^ EPA Climate Change Site
23. ^ ^{a b} 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)." 
24. ^ GP 25 Web Book | Chapter 7
25. ^ 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
26. ^ * 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"
27. ^ ATM S 211 - Notes
28. ^ Titan: Greenhouse and Anti-greenhouse :: Astrobiology Magazine - earth science - evolution distribution Origin of life universe - life beyond :: Astrobiology is study of earth...
29. ^ SPACE.com - Pluto Colder Than Expected
30. ^ 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: 234-245, Commission on Engineering and Technical Systems (CETS). Retrieved on 2009. 
31. ^ Rasool, S. I.; De Bergh, C. (1970). "The Runaway Greenhouse and the Accumulation of CO2 in the Venus Atmosphere". Nature 226: 1037. doi:10.1038/2261037a0. http://pubs.giss.nasa.gov/docs/1970/1970_Rasool_DeBergh.pdf. Retrieved on 02/25/2009.  edit

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
• 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.