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A runaway greenhouse effect is a state in which a net positive feedback between surface temperature and atmospheric opacity increases the strength of the greenhouse effect on a planet until its oceans boil away.^[1]An example of this is believed to have happened in the early history of Venus. On the Earth the IPCC states that "a 'runaway greenhouse effect'—analogous to [that of] Venus—appears to have virtually no chance of being induced by anthropogenic activities."^[2]

Other large-scale climate changes are sometimes loosely called a "runaway greenhouse effect" although it is not an appropriate description. For example, it has been hypothesized that large releases of greenhouse gases may have occurred concurrently with the Permian–Triassic extinction event^[3]^[4] or Paleocene–Eocene Thermal Maximum. Other terms, such as "abrupt climate change", or tipping points could be used when describing such scenarios,^[5] as well as the term hothouse state.^[6]

A hypothetical runaway greenhouse effect on the earth should not be confused with a greenhouse earth, which has happened in several well-known periods. During 80% of the latest 500 million years, the earth is believed to have been in a greenhouse state due to the greenhouse effect, when there were no continental glaciers on the planet, the levels of carbon dioxide and other greenhouse gases (such as water vapor and methane) were high, and sea surface temperatures (SSTs) ranged from 28 °C (82.4 °F) in the tropics to 0 °C (32 °F) in the polar regions.^[7]

History

While the term was coined by Caltech scientist Andrew Ingersoll in a paper that described a model of the atmosphere of Venus,^[8] the initial idea of a limit on terrestrial outgoing infrared radiation was published by George Simpson (meteorologist) in 1927.^[9] The physics relevant to the, later-termed, runaway greenhouse effect was explored by Makoto Komabayashi at Nagoya university.^[10] Assuming a water vapor-saturated stratosphere, Komabayashi and Ingersoll independently calculated the limit on outgoing infrared radiation that defines the runaway greenhouse state. The limit is now known as the Komabayashi-Ingersoll limit to recognize their contributions.^[11]

Physics of the runaway greenhouse

The runaway greenhouse effect is often formulated in terms of how the surface temperature of a planet changes with differing amounts of received starlight.^[12] If the planet is assumed to be in Radiative equilibrium, then the runaway greenhouse state is calculated as the equilibrium state at which water cannot exist in liquid form.^[11] The water vapor is then lost to space through hydrodynamic escape.^[13] In radiative equilbrium, a planet's Outgoing longwave radiation must balance the incoming stellar flux. Typically a planet's outgoing longwave radiation is equal to the planet's surface temperature to the fourth power due to the Stefan–Boltzmann law, though in a runaway greenhouse state this balance is broken.

The Stefan-Boltzmann law is an example of a Negative feedback that stabilizes a planet's climate system. If the Earth received more sunlight it would result in a temporary disequilibrium (more energy in than out) and result in warming. However, because the Stefan-Boltzmann response mandates that this hotter planet emits more energy, eventually a new radiation balance can be reached and the temperature will be maintained at its new, higher value.^[14] Positive feedbacks amplify changes in the climate system, and can lead to destabilizing effects for the climate.^[14] An increase in temperature from greenhouse gases leading to increased water vapor (which is itself a greenhouse gas) causing further warming is a positive feedback, but not a runaway effect, on Earth.^[12] Positive feedback effects are common (e.g. ice-albedo feedback) but runaway effects do not necessarily emerge from their presence. Though water plays a major role in the process, the runaway greenhouse effect is not a result of Water vapor feedback.^[13]

The runaway greenhouse effect can be seen as a limit on a planet's outgoing longwave radiation that, when surpassed, results in a state where water cannot exist in its liquid form (hence, the oceans have all "boiled away").^[11] A planet's outgoing longwave radiation is limited by this evaporated water, which is an effective greenhouse gas and blocks additional infrared radiation as it accumulates in the atmosphere.^[15] Assuming radiative equilibrium, runaway greenhouse limits on outgoing longwave radiation correspond to limits on the increase in stellar flux received by a planet to trigger the runaway greenhouse effect.^[16] Two limits on a planet's outgoing longwave radiation have been calculated that correspond with the onset of the runaway greenhouse effect: the Komabayashi-Ingersoll limit^[8]^[10] and the Simpson-Nakajima limit.^[9]^[11]^[12] At these values the runaway greenhouse effect overcomes the Stefan-Boltzmann feedback so an increase in a planet's surface temperature will not increase the outgoing longwave radiation.^[14]

The Komabayshi-Ingersoll limit was the first to be analytically derived and only considers a grey stratosphere in radiative equilibrium.^[8]^[10] This approach focuses on the balance between the the outgoing longwave radiation at the tropopause, ${\textstyle F_{\text{IRtop}}^{\uparrow }}$ , and the optical depth of water vapor, ${\textstyle \tau _{\text{tp}}}$ , in the tropopause, which is determined by the temperature and pressure at the tropopause according to the Saturation vapor pressure. This balance is represented by the following equations^[11] ${\begin{aligned}{\frac {1}{2}}F_{\text{IRtop}}^{\uparrow }\left({\frac {3}{2}}\tau _{\text{tp}}+1\right)&=\sigma T_{\text{tp}}^{4}\\\tau _{\text{tp}}&=\kappa _{v}p^{*}(T_{\text{tp}}){\frac {1}{g}}{\frac {m_{v}}{\bar {m}}}\end{aligned}}$ Where the first equation represents the requirement for radiative equilibrium at the tropopause and the second equation represents how much water vapor is present at the tropopause.^[11] Taking the outgoing longwave radiation as a free parameter, these equations will intersect only once for a single value of the outgoing longwave radiation, this value is taken as the Komabayashi-Ingersoll limit.^[11] At that value the Stefan-Boltzmann feedback breaks down because the tropospheric temperature required to maintain the Komabayashi-Ingersoll OLR value results in a water vapor optical depth that blocks the OLR that is needed to cool the tropopause.^[14]

Connection to habitability

The concept of a habitable zone has been used by planetary scientists and astrobiologists to define an orbital region around a star in which a planet (or moon) can sustain liquid water. Under this definition, the inner edge of the habitable zone (i.e., the closest point to a star that a planet can be until it can no longer sustain liquid water) is determined by the point in which the runaway greenhouse process occurs. For sun-like stars, this inner edge is estimated to reside at roughly 84% the distance from the Earth to the sun^[17] although feedback such as cloud-induced albedo increase could modify this estimate somewhat.

In The Solar System

Venus

A runaway greenhouse effect involving carbon dioxide and water vapor may have occurred on Venus.^[18] In this scenario, early Venus may have had a global ocean. As the brightness of the early Sun increased, the amount of water vapor in the atmosphere increased, increasing the temperature and consequently increasing the evaporation of the ocean, leading eventually to the situation in which the oceans boiled, and all of the water vapor entered the atmosphere. 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. Some evidence for this scenario comes from the extremely high deuterium to hydrogen ratio in Venus' atmosphere, roughly 150 times that of Earth, since light hydrogen would escape from the atmosphere more readily than its heavier isotope, deuterium.^[19]^[20] Venus is sufficiently strongly heated by the Sun that water vapor can rise much higher in the atmosphere and be 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, 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.^[21]

Earth

Earth's climate has swung repeatedly between warm periods and ice ages during its history. In the current climate the gain of the positive feedback effect from increased atmospheric water vapor, as well as Earth being too far away from the Sun at its current luminosity for such to occur is well below that which is required to boil away the oceans.^[22] Climate scientist John Houghton has written that "[there] is no possibility of [Venus's] runaway greenhouse conditions occurring on the Earth".^[23] However, climatologist James Hansen disagrees. In his Storms of My Grandchildren he says that burning coal and mining oil sands will result in runaway greenhouse on Earth.^[24] A re-evaluation in 2013 of the effect of water vapor in the climate models showed that James Hansen's outcome might be possible, but requires ten times the amount of CO₂ we could release from burning all the oil, coal, and natural gas in Earth's crust. ^[25] Further, Benton and Twitchett have a different definition of a runaway greenhouse;^[3] events meeting this definition have been suggested as a cause for the Paleocene-Eocene Thermal Maximum and the great dying.

Distant future

Most scientists believe that a runaway greenhouse effect is actually inevitable in the long term as the Sun gradually gets bigger and hotter as it ages. Such will potentially spell the end of all life on Earth. As the Sun becomes 10% brighter in about one billion years' time, the surface temperature of Earth will reach 47 °C (117 °F), causing the temperature of Earth to rise rapidly and its oceans to boil away until it becomes a greenhouse planet similar to Venus today.

According to astrobiologists Peter Ward and Donald Brownlee in their book The Life and Death of Planet Earth,^[26] the current loss rate is approximately one millimeter of ocean per million years, but this rate is gradually accelerating as the sun gets warmer, to perhaps as fast as one millimeter every 1000 years. Ward and Brownlee predict that there will be two variations of this future warming feedback: the "moist greenhouse" where water vapor dominates the troposphere while water vapor starts to accumulate in the stratosphere, and the "runaway greenhouse" where water vapor becomes a dominant component of the atmosphere that the Earth starts to undergo rapid warming that could send its surface temperature to over 900 °C (1,650 °F) as the atmosphere will be totally overwhelmed by water vapor, causing its entire surface to melt and killing all life, perhaps in about three billion years' time. Either way, the loss of oceans will inevitably turn the Earth into a primarily desert world with the only water left being a few evaporating ponds scattered near the poles, and huge salt flats around what was once the ocean floor, much like the Atacama Desert in Chile or Badwater Basin in Death Valley, where the last life may remain for a few billion more years. Because of this, in the former case the loss of oceans will save the last life instead of destroying it completely. However, complex life like plants and animals will be long extinct before this happens, as the loss of oceans will cause plate tectonics to come to halt; water is a lubricant for tectonic activity and the loss of all water will make the crust too hard and dry to be subducted, therefore causing the carbon cycle to cease altogether (there would be fewer volcanoes to return CO₂ into the atmosphere).

References

^ Rasool, I.; De Bergh, C. (Jun 1970). "The Runaway Greenhouse and the Accumulation of CO₂ in the Venus Atmosphere" (PDF). Nature. 226 (5250): 1037–1039. Bibcode:1970Natur.226.1037R. doi:10.1038/2261037a0. ISSN 0028-0836. PMID 16057644. Archived from the original (PDF) on 2011-10-21. {{cite journal}}: Unknown parameter |dead-url= ignored (|url-status= suggested) (help)
^ https://archive.ipcc.ch/meetings/session31/inf3.pdf
^ ^a ^b Benton, M. J.; Twitchet, R. J. (2003). "How to kill (almost) all life: the end-Permian extinction event" (PDF). Trends in Ecology & Evolution. 18 (7): 358–365. doi:10.1016/S0169-5347(03)00093-4.
^ Morante, Richard (1996). "Permian and early Triassic isotopic records of carbon and strontium in Australia and a scenario of events about the Permian-Triassic boundary". Historical Biology: An International Journal of Paleobiology. 11 (1): 289–310. doi:10.1080/10292389609380546.
^ Kennett, James; Kevin G. Cannariato; Ingrid L. Hendy; Richard J. Behl (2003). Methane Hydrates in Quaternary Climate Change: The Clathrate Gun Hypothesis. ISBN 978-0-87590-296-8.
^ "Domino-effect of climate events could move Earth into a 'hothouse' state". The Guardian. 2018.
^ Price, Gregory; Paul J. Valdes; Bruce W. Sellwood (1998). "A comparison of GCM simulated Cretaceous 'greenhouse' and 'icehouse climates: implications for the sedimentary record". Palaeogeography, Palaeoclimatology, Palaeoecology. 142 (3–4): 123–138. Bibcode:1998PPP...142..123P. doi:10.1016/s0031-0182(98)00061-3.
^ ^a ^b ^c Ingersoll, Andrew P. (1969). "The Runaway Greenhouse: A History of Water on Venus". Journal of the Atmospheric Sciences. 26 (6): 1191–1198. Bibcode:1969JAtS...26.1191I. doi:10.1175/1520-0469(1969)026<1191:TRGAHO>2.0.CO;2.
^ ^a ^b "G. C. SIMPSON, C.B., F.R.S., ON SOME STUDIES IN TERRESTRIAL RADIATION Vol. 2, No. 16. Published March 1928". Quarterly Journal of the Royal Meteorological Society. 55 (229): 73–73. 1929. doi:10.1002/qj.49705522908. ISSN 1477-870X.
^ ^a ^b ^c Komabayasi, M. (1967). "Discrete Equilibrium Temperatures of a Hypothetical Planet with the Atmosphere and the Hydrosphere of One Component-Two Phase System under Constant Solar Radiation". Journal of the Meteorological Society of Japan. Ser. II. 45 (1): 137–139. doi:10.2151/jmsj1965.45.1_137. ISSN 0026-1165.
^ ^a ^b ^c ^d ^e ^f ^g Nakajima, Shinichi; Hayashi, Yoshi-Yuki; Abe, Yutaka (1992). "A Study on the "Runaway Greenhouse Effect" with a One-Dimensional Radiative–Convective Equilibrium Model". J. Atmos. Sci. 49 (23): 2256–2266. Bibcode:1992JAtS...49.2256N. doi:10.1175/1520-0469(1992)049<2256:asotge>2.0.co;2.
^ ^a ^b ^c Kasting, J. F. (1988). "Runaway and moist greenhouse atmospheres and the evolution of Earth and Venus". Icarus. 74 (3): 472–494. Bibcode:1988Icar...74..472K. doi:10.1016/0019-1035(88)90116-9. PMID 11538226.
^ ^a ^b Goldblatt Colin; Watson Andrew J. (2012-09-13). "The runaway greenhouse: implications for future climate change, geoengineering and planetary atmospheres". Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences. 370 (1974): 4197–4216. doi:10.1098/rsta.2012.0004.
^ ^a ^b ^c ^d Catling, David (David Charles),. Atmospheric evolution on inhabited and lifeless worlds. Kasting, James F.,. Cambridge. ISBN 9780521844123. OCLC 956434982.{{cite book}}: CS1 maint: extra punctuation (link) CS1 maint: multiple names: authors list (link)
^ "Greenhouse Gases | Monitoring References | National Centers for Environmental Information (NCEI)". www.ncdc.noaa.gov. Retrieved 2019-06-06.
^ Kopparapu, Ravi Kumar; Ramirez, Ramses; Kasting, James F.; Eymet, Vincent; Robinson, Tyler D.; Mahadevan, Suvrath; Terrien, Ryan C.; Domagal-Goldman, Shawn; Meadows, Victoria (2013-02-26). "HABITABLE ZONES AROUND MAIN-SEQUENCE STARS: NEW ESTIMATES". The Astrophysical Journal. 765 (2): 131. doi:10.1088/0004-637X/765/2/131. ISSN 0004-637X.
^ Selsis, F.; Kasting, J. F.; Levrard, B.; Paillet, J.; Ribas, I.; Delfosse, X. (2007). "Habitable planets around the star Gliese 581?". Astronomy and Astrophysics. 476 (3): 1373–1387. arXiv:0710.5294. Bibcode:2007A&A...476.1373S. doi:10.1051/0004-6361:20078091.
^ S. I. Rasoonl; C. de Bergh (1970). "The Runaway Greenhouse Effect and the Accumulation of CO₂ in the Atmosphere of Venus". Nature. 226 (5250): 1037–1039. Bibcode:1970Natur.226.1037R. doi:10.1038/2261037a0. PMID 16057644. {{cite journal}}: Unknown parameter |last-author-amp= ignored (|name-list-style= suggested) (help)
^ T.M. Donahue, J.H. Hoffmann, R.R. Hodges Jr, A.J. Watson, Venus was wet: a measurement of the ratio of deuterium to hydrogen, Science, 216 (1982), pp. 630–633
^ . De Bergh, B. Bézard, T. Owen, D. Crisp, J.-P. Maillard, B.L. Lutz, Deuterium on Venus—observations from Earth, Science, 251 (1991), pp. 547–549
^ Nick Strobel. "Venus". Archived from the original on 12 February 2007. Retrieved 17 February 2009. {{cite web}}: Unknown parameter |dead-url= ignored (|url-status= suggested) (help)
^ Isaac M. Held; Brian J. Soden (November 2000). "Water Vapor Feedback and Global Warming". Annual Review of Energy and the Environment. 25 (1): 441–475. doi:10.1146/annurev.energy.25.1.441. On this basis, one might expect runaway conditions to develop eventually if the climate warms sufficiently. Although it is difficult to be quantitative, primarily because of uncertainties in cloud prediction, it is clear that this point is only achieved for temperatures that are far warmer than any relevant for the global warming debate {{cite journal}}: Unknown parameter |last-author-amp= ignored (|name-list-style= suggested) (help)
^ Houghton, J. (May 4, 2005). "Global Warming". Rep. Prog. Phys. 68 (6): 1343–1403. Bibcode:2005RPPh...68.1343H. doi:10.1088/0034-4885/68/6/R02. Retrieved August 26, 2009.
^ "How Likely Is a Runaway Greenhouse Effect on Earth?". MIT Technology Review. Retrieved 1 June 2015.
^ Kunzig, Robert. "Will Earth's Ocean Boil Away?" National Geographic Daily News (July 29, 2013)
^ Brownlee, David and Peter D. Ward, The Life and Death of Planet Earth, Holt Paperbacks, 2004, ISBN 978-0805075120