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 refers to the change in the steady state temperature of a planet or moon by the presence of an atmosphere containing gas that absorbs and emits infrared radiation.[1] Greenhouse gases, which include water vapor, carbon dioxide and methane, 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.[2][3][4][5] This mechanism is fundamentally different from the mechanism of an actual greenhouse, which instead isolates air inside the structure so that the heat is not lost by convection and conduction, as discussed below. The greenhouse effect was discovered by Joseph Fourier in 1824, first reliably experimented on by John Tyndall in the year 1858 and first reported quantitatively by Svante Arrhenius in his 1896 paper.[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 increased concentrations of greenhouse gases in the atmosphere[12] and changes in the use of land.[13]

The greenhouse effect is one of several factors that 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 greenhouse effects according to their respective environments. In addition, Titan has an anti-greenhouse effect and Pluto exhibits behavior similar to the anti-greenhouse effect.[14][15][16]

Contents

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 and 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.[8]

Detailed explanation

Pattern of absorption bands generated by various greenhouse gases and their impact on both solar radiation and upgoing thermal radiation from the Earth's surface. Note that a greater quantity of upgoing radiation is absorbed, which contributes to the greenhouse effect.

The Earth receives energy from the Sun in the form of radiation. Most of the energy is in visible wavelengths and in infrared wavelengths that are near the visible range (often called "near infrared"). The Earth reflects 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

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

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.[17][18]

Positive feedback and runaway greenhouse effect

A runaway greenhouse effect occurs if feedback leads to the evaporation of all greenhouse gases into the atmosphere.[19] A runaway greenhouse effect involving CO2 and water vapor may have occurred on Venus.[20]

It is not possible that such an event will occur on Earth as a result of anthropogenic perturbations, but other potential runaway climate change effects involving Arctic methane release from permafrost have been identified.[21] Clathrates have also been identified as a major potential methane source capable of inducing a 'runaway' effect.[22]

A runaway greenhouse effect may be distinguished from runaway climate change, as the former is more commonly used to describe the complete and permanent transition of a climate system, such as on Venus.[19] The latter is typically used in non-scientific media to describe runaway features of anthropogenic global warming.

Anthropogenic greenhouse effect

Of the human-produced greenhouse gases, the one that contributes the bulk in terms of radiative forcing is carbon dioxide. CO2 production from increased industrial activity (fossil fuel burning) and other human activities such as cement production and tropical deforestation[23] has increased the concentrations in the atmosphere. Measurements of CO2 from the Mauna Loa observatory show that concentrations have increased from about 313 ppm (mole fraction in dry air[24]) in 1960 to about 375 ppm in 2005. The current observed amount of CO2 exceeds the geological record maxima (~300 ppm) from ice core data.[25]

The effect of combustion-produced carbon dioxide on the global climate, a special case of the greenhouse effect first demonstrated in the 1930s, may be 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".[26]

Over the past 800,000 years,[27] 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.[28] Certain paleoclimatologists consider variations in carbon dioxide to be a fundamental factor in controlling climate variations over this time scale.[29]

Responses to anthropogenic global warming fall into three categories:

Real greenhouses

A modern Greenhouse in RHS Wisley

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[30].

The term 'greenhouse effect' originally came from the greenhouses used for gardening, but as mentioned the mechanism for greenhouses operates differently.[31] 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.[32]

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.[33] Greenhouses thus work primarily by preventing convection; the atmospheric greenhouse effect however reduces radiation loss, not convection.[34][31]

See also

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. ^ Land Use, Land-Use change and Forestry, IPCC Special report SPM
  14. ^ ATM S 211 - Notes
  15. ^ Titan: Greenhouse and Anti-greenhouse :: Astrobiology Magazine - earth science - evolution distribution Origin of life universe - life beyond :: Astrobiology is study of earth...
  16. ^ SPACE.com - Pluto Colder Than Expected
  17. ^ 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. 
  18. ^ "Water vapour: feedback or forcing?". RealClimate. 6 April 2005. http://www.realclimate.org/index.php?p=142. Retrieved on 2006-05-01. 
  19. ^ a b 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. 
  20. ^ 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
  21. ^ Lawrence, David M. (2008). "Accelerated Arctic land warming and permafrost degradation during rapid sea ice loss". Geophysical Research Letters 35: L11506. doi:10.1029/2008GL033985.  edit
  22. ^ Archer, David (2005). "Time-dependent response of the global ocean clathrate reservoir to climatic and anthropogenic forcing". Geochemistry Geophysics Geosystems 6: Q03002. doi:10.1029/2004GC000854.  edit
  23. ^ IPCC Fourth Assessment Report, Working Group I Report "The Physical Science Basis" Chapter 7
  24. ^ "Atmospheric Carbon Dioxide - Mauna Loa". NOAA. http://www.esrl.noaa.gov/gmd/ccgg/trends/co2_data_mlo.html. 
  25. ^ Hansen, J., Climatic Change, 68, 269, 2005 ISSN 0165-0009
  26. ^ IPCC Fourth Assessment Report Synthesis Report: Summary for Policymakers (p. 5)
  27. ^ BBC NEWS | Science/Nature | Deep ice tells long climate story
  28. ^ Chemical & Engineering News: Latest News - Ice Core Record Extended
  29. ^ Bowen, Mark; Thin Ice: Unlocking the Secrets of Climate in the World's Highest Mountains; Owl Books, 2005.
  30. ^ EPA Climate Change Site
  31. ^ 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)." 
  32. ^ GP 25 Web Book | Chapter 7
  33. ^ 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
  34. ^ * 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"

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.

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