View source for Runaway greenhouse effect

{{Short description|Climatic effect causing a planet's atmosphere to trap heat and prevent cooling}}
{{Distinguish|Tipping points in the climate system}}
{{Use dmy dates|date=August 2022}}

A '''runaway greenhouse effect''' will occur when a planet's atmosphere contains [[greenhouse gas]] in an amount sufficient to block thermal radiation from leaving the planet, preventing the planet from cooling and from having liquid water on its surface. A runaway version of the [[greenhouse effect]] can be defined by a limit on a planet's [[outgoing longwave radiation]] which is asymptotically reached due to higher surface temperatures evaporating water into the atmosphere, increasing its [[optical depth]].<ref>{{cite encyclopedia |last=Kaltenegger |first=Lisa |chapter=Greenhouse Effect |date=2015 |pages=1018 |editor-last=Gargaud |editor-first=Muriel |publisher=Springer Berlin Heidelberg |doi=10.1007/978-3-662-44185-5_673 |isbn=9783662441848 |editor2-last=Irvine |editor2-first=William M. |editor3-last=Amils |editor3-first=Ricardo |editor4-last=Cleaves|editor4-first=Henderson James |title=Encyclopedia of Astrobiology}}</ref> This [[positive feedback]] means the planet cannot cool down through longwave radiation (via the [[Stefan–Boltzmann law]]) and continues to heat up until it can radiate outside of the [[absorption bands]]<ref name=":36">{{cite book |title=Atmospheric evolution on inhabited and lifeless worlds |last1=Catling |first1=David C. |last2=Kasting |first2=James F. |isbn=9780521844123 |location=Cambridge |oclc=956434982 |date=13 April 2017}}</ref> of the water vapour.

The runaway greenhouse effect is often formulated with water vapour as the condensable species. The water vapour reaches the [[stratosphere]] and escapes into space via [[hydrodynamic escape]], resulting in a desiccated planet.<ref name=":13">{{cite journal |last1=Nakajima |first1=Shinichi |last2=Hayashi |first2=Yoshi-Yuki |last3=Abe |first3=Yutaka |year=1992 |title=A Study on the "Runaway Greenhouse Effect" with a One-Dimensional Radiative–Convective Equilibrium Model |journal=J. Atmos. Sci. |volume=49 |issue=23 |pages=2256–2266 |bibcode=1992JAtS...49.2256N |doi=10.1175/1520-0469(1992)049<2256:asotge>2.0.co;2 |doi-access=free}}</ref> This likely happened in the early history of [[Venus]].

In 2012 study about climate change it was quoted that 'Earth presently absorbs around 240 W m<sup>−2</sup> of solar radiation. Increasing carbon dioxide concentration will make surface warmer with the same outgoing thermal flux. Following this theory, we are not near the threshold of a runaway greenhouse. However, the behaviour of hot, water-vapour-rich atmospheres is poorly understood, and an in-depth study of these is necessary.'

However, the authors cautioned that "our understanding of the dynamics, thermodynamics, radiative transfer and cloud physics of hot and steamy atmospheres is weak", and that we "cannot therefore completely rule out the possibility that human actions might cause a transition, if not to full runaway, then at least to a much warmer climate state than the present one".<ref name="PhilTransactionsRoyalSociety_20120913">{{cite journal |last1=Goldblatt |first1=Colin |last2=Watson |first2=Andrew J. |date=13 September 2012 |title=The Runaway Greenhouse: implications for future climate change, geoengineering and planetary atmospheres |journal=Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences |volume=370 |issue=1974 |pages=4197–4216 |arxiv=1201.1593 |bibcode=2012RSPTA.370.4197G |doi=10.1098/rsta.2012.0004 |pmid=22869797 |s2cid=7891446}}</ref> 

A runaway greenhouse effect similar to Venus appears to have virtually no chance of being caused by people.<ref name="IPCC2009">{{cite report |url=https://www.ipcc.ch/site/assets/uploads/2018/03/inf3-6.pdf |title=Scoping of the IPCC 5th Assessment Report Cross Cutting Issues |work=Thirty-first Session of the IPCC Bali, 26–29 October 2009 |url-status=live |archive-url=https://web.archive.org/web/20091109215503/http://www.ipcc.ch/meetings/session31/inf3.pdf |archive-date=9 November 2009 |access-date=24 March 2019}}</ref> A 2013 article concluded that runaway greenhouse "could in theory be triggered by increased greenhouse forcing", but that "anthropogenic emissions are probably insufficient".<ref name="NatureGeoscience_20130728">{{cite journal |last1=Goldblatt |first1=Colin |last2=Robinson |first2=Tyler D. |last3=Zahnle |first3=Kevin J. |last4=Crisp |first4=David |date=28 July 2013 |title=Low simulated radiation limit for runaway greenhouse climates |url=https://www.nature.com/articles/ngeo1892 |url-status=live |journal=Nature Geoscience |volume=6 |issue=8 |pages=661–667 |bibcode=2013NatGe...6..661G |doi=10.1038/ngeo1892 |s2cid=37541492 |archive-url=https://web.archive.org/web/20220920164843/https://www.nature.com/articles/ngeo1892 |archive-date=20 September 2022 |access-date=17 September 2022 |hdl-access=free |hdl=2060/20160002421}}</ref> Venus-like conditions on [[Earth]] require a large long-term forcing that is unlikely to occur until the sun brightens by some tens of percents, which will take a few billion years.<ref name="Hansen et al 2013">{{cite journal |last1=Hansen |first1=James |first2=Makiko |last2=Sato |first3=Gary |last3=Russell |first4=Pushker |last4=Kharecha |date=2013 |title=Climate sensitivity, sea level and atmospheric carbon dioxide |journal= Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences |volume=371 |issue=2001 |at=20120294 |bibcode=2013RSPTA.37120294H |doi=10.1098/rsta.2012.0294 |pmid=24043864 |pmc=3785813|arxiv=1211.4846 }}</ref> Earth is expected to experience a runaway greenhouse effect "in about 2 billion years as solar luminosity increases".<ref name="PhilTransactionsRoyalSociety_20120913" />

==History==
[[File:19021015 Hint to Coal Consumers - Svante Arrhenius - The Selma Morning Times - Global warming.jpg |thumb|upright=1.35| This 1902 article attributes to Swedish Nobel laureate (for chemistry) [[Svante Arrhenius]] a theory that coal combustion could eventually lead to a degree of global warming causing [[human extinction]].<ref name="Selma_Arrhenius_19021015">{{cite news |date=15 October 1902 |title=Hint to Coal Consumers |page=4 |work=The Selma Morning Times |location=Selma, Alabama, US |url=https://www.newspapers.com/image/570090173/}}<br />"[[Carbonic acid]]" refers to [[carbon dioxide]] when dissolved in water.</ref>]]While the term was coined by [[California Institute of Technology|Caltech]] scientist [[Andrew Ingersoll]] in a paper that described a model of the atmosphere of Venus,<ref name=":44">{{cite journal|last1=Ingersoll|first1=Andrew P.|date=1969|title=The Runaway Greenhouse: A History of Water on Venus|journal=[[Journal of the Atmospheric Sciences]]|volume=26|issue=6|pages=1191–1198|bibcode=1969JAtS...26.1191I|doi=10.1175/1520-0469(1969)026<1191:TRGAHO>2.0.CO;2|url=https://authors.library.caltech.edu/3987/1/INGjas69c.pdf}}</ref> the initial idea of a limit on terrestrial outgoing infrared radiation was published by [[George Simpson (meteorologist)|George Simpson]] in 1927.<ref name=":5">{{Cite journal|date=1929|title=G. C. SIMPSON, C.B., F.R.S., ON SOME STUDIES IN TERRESTRIAL RADIATION Vol. 2, No. 16. Published March 1928|journal=Quarterly Journal of the Royal Meteorological Society|volume=55|issue=229|pages=73|doi=10.1002/qj.49705522908|issn=1477-870X|bibcode=1929QJRMS..55Q..73.}}</ref> The physics relevant to the, later-termed, runaway greenhouse effect was explored by Makoto Komabayashi at [[Nagoya University]].<ref name=":62">{{Cite journal|last=Komabayasi|first=M.|date=1967|title=Discrete Equilibrium Temperatures of a Hypothetical Planet with the Atmosphere and the Hydrosphere of One Component-Two Phase System under Constant Solar Radiation|journal=Journal of the Meteorological Society of Japan |series=Series II|volume=45|issue=1|pages=137–139|doi=10.2151/jmsj1965.45.1_137|issn=0026-1165|doi-access=free}}</ref> 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.<ref name=":13" />

==Physics==
[[File:KomabayashiIngersollLimit.png|thumb|245x245px|Graph of tropopause optical depth by tropopause temperature, illustrating the Komabayashi–Ingersoll limit of 385 W/m<sup>2</sup> using equations and values from Nakajima et al. (1992) "A Study on the Runaway Greenhouse Effect with a One-Dimensional Radiative–Convective Equilibrium Model". The Komabayashi–Ingersoll limit is the value of outgoing longwave radiation (FIRtop) beyond which the lines do not intersect.]]
A ''runaway'' greenhouse effect occurs when greenhouse gases accumulate in the atmosphere through a [[positive feedback]] cycle to such an extent that they substantially block radiated heat from escaping into space, thus greatly increasing the temperature of the planet.<ref name="Kasting_1991">{{cite conference |last=Kasting |first=James F. |date=1991 |title=Runaway and moist greenhouse atmospheres and the evolution of Earth and Venus. |url=http://books.nap.edu/openbook.php?record_id=1790&page=234 |conference= |publisher=Commission on Engineering and Technical Systems (CETS) |pages=234–245 |archive-url=https://web.archive.org/web/20110607101819/http://books.nap.edu/openbook.php?record_id=1790&page=234 |archive-date=7 June 2011 |access-date=9 April 2017 |book-title=Planetary Sciences: American and Soviet Research/Proceedings from the U.S.-U.S.S.R. Workshop on Planetary Sciences |url-status=live}}</ref>

The runaway greenhouse effect is often formulated in terms of how the surface temperature of a planet changes with differing amounts of received starlight.<ref name=":04">{{cite journal|last1=Kasting|first1=J. F.|year=1988|title=Runaway and moist greenhouse atmospheres and the evolution of Earth and Venus|url=https://zenodo.org/record/1253896|journal=[[Icarus (journal)|Icarus]]|volume=74|issue=3|pages=472–494|bibcode=1988Icar...74..472K|doi=10.1016/0019-1035(88)90116-9|pmid=11538226}}</ref> 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.<ref name=":13" /> The water vapor is then lost to space through [[hydrodynamic escape]].<ref name="PhilTransactionsRoyalSociety_20120913"/> In radiative equilibrium, a planet's [[outgoing longwave radiation]] (OLR) must balance the incoming stellar flux.

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.<ref name=":36"/> Positive [[climate change feedback]]s amplify changes in the climate system, and can lead to destabilizing effects for the climate.<ref name=":36"/> 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.<ref name=":04"/> 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]].<ref name="PhilTransactionsRoyalSociety_20120913"/>

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").<ref name=":13"/> 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.<ref>{{Cite web|url=https://www.ncdc.noaa.gov/monitoring-references/faq/greenhouse-gases.php?section=watervapor|title=Greenhouse Gases {{!}} Monitoring References {{!}} National Centers for Environmental Information (NCEI)|website=www.ncdc.noaa.gov|access-date=6 June 2019}}</ref> 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.<ref name=":72">{{Cite journal|last1=Kopparapu|first1=Ravi Kumar|last2=Ramirez|first2=Ramses|last3=Kasting|first3=James F.|last4=Eymet|first4=Vincent|last5=Robinson|first5=Tyler D.|last6=Mahadevan|first6=Suvrath|author6-link=Suvrath Mahadevan|last7=Terrien|first7=Ryan C.|last8=Domagal-Goldman|first8=Shawn|last9=Meadows|first9=Victoria|date=26 February 2013|journal=The Astrophysical Journal|volume=765|issue=2|pages=131|doi=10.1088/0004-637X/765/2/131|issn=0004-637X|bibcode=2013ApJ...765..131K|arxiv=1301.6674|title=Habitable Zones Around Main-Sequence Stars: New Estimates|s2cid=76651902}}</ref> 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<ref name=":44"/><ref name=":62"/> and the [[Simpson–Nakajima limit]].<ref name=":52">{{Cite journal|date=1929|title=G. C. SIMPSON, C.B., F.R.S., ON SOME STUDIES IN TERRESTRIAL RADIATION Vol. 2, No. 16. Published March 1928|journal=Quarterly Journal of the Royal Meteorological Society|volume=55|issue=229|pages=73|doi=10.1002/qj.49705522908|issn=1477-870X|bibcode=1929QJRMS..55Q..73.}}</ref><ref name=":13"/><ref name=":04"/> 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.<ref name=":36"/>

The Komabayashi–Ingersoll limit was the first to be analytically derived and only considers a grey stratosphere in radiative equilibrium.<ref name=":44"/><ref name=":62" /> A grey stratosphere (or atmosphere) is an approach to modeling [[radiative transfer]] that does not take into account the frequency-dependence of absorption by a gas. In the case of a grey stratosphere or atmosphere, the [[Optical depth (astrophysics)#The Eddington approximation and the depth of the photosphere|Eddington approximation]] can be used to calculate radiative fluxes. This approach focuses on the balance between the outgoing longwave radiation at the [[tropopause]],<math display="inline">F^\uparrow_\text{IRtop}</math>, and the [[optical depth]] of water vapor, <math display="inline">\tau_\text{tp}</math>, 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<ref name=":13"/><math display="block">\begin{align}
\frac{1}{2}F^\uparrow_\text{IRtop}\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{align}</math>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.<ref name=":13"/> 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.<ref name=":13"/> 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 needed to cool the tropopause.<ref name=":36"/>

The Simpson–Nakajima limit is lower than the Komabayashi–Ingersoll limit, and is thus typically more realistic for the value at which a planet enters a runaway greenhouse state.<ref name="PhilTransactionsRoyalSociety_20120913"/> For example, given the parameters used to determine a Komabayashi–Ingersoll limit of 385 W/m<sup>2</sup>, the corresponding Simpson–Nakajima limit is only about 293 W/m<sup>2</sup>.<ref name=":13"/><ref name=":04"/> The Simpson–Nakajima limit builds off of the derivation of the Komabayashi–Ingersoll limit by assuming a convective [[troposphere]] with a surface temperature and surface pressure that determines the optical depth and outgoing longwave radiation at the tropopause.<ref name=":13"/><ref name=":04"/>

=== The moist greenhouse limit ===
Because the model used to derive the Simpson–Nakajima limit (a grey stratosphere in radiative equilibrium and a convecting troposphere) can determine the water concentration as a function of altitude, the model can also be used to determine the surface temperature (or conversely, amount of stellar flux) that results in a high water mixing ratio in the stratosphere.<ref name=":04"/> While this critical value of outgoing longwave radiation is less than the Simpson–Nakajima limit, it still has dramatic effects on a planet's climate. A high water mixing ratio in the stratosphere would overcome the effects of a [[cold trap (astronomy)|cold trap]] and result in a "moist" stratosphere, which would result in the photolysis of water in the stratosphere that in turn would destroy the ozone layer and eventually lead to a dramatic loss of water through hydrodynamic escape.<ref name=":36"/><ref name="PhilTransactionsRoyalSociety_20120913"/> This climate state has been dubbed the moist greenhouse effect, as the end-state is a planet without water, though liquid water may exist on the planet's surface during this process.<ref name=":04"/>

==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.<ref>{{Cite journal|last1=Kasting|first1=James F.|last2=Whitmire|first2=Daniel P.|last3=Reynolds|first3=Ray T.|date=January 1993|title=Habitable Zones around Main Sequence Stars|journal=Icarus|volume=101|issue=1|pages=108–128|doi=10.1006/icar.1993.1010|pmid=11536936|bibcode=1993Icar..101..108K}}</ref> 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 outgoing longwave radiation limit beyond which the runaway greenhouse process occurs (e.g., the Simpson–Nakajima limit). This is because a planet's distance from its host star determines the amount of stellar flux the planet receives, which in turn determines the amount of outgoing longwave radiation the planet radiates back to space.<ref name=":36"/> While the inner habitable zone is typically determined by using the Simpson–Nakajima limit, it can also be determined with respect to the moist greenhouse limit,<ref name=":72"/> though the difference between the two is often small.<ref>{{Cite journal|last1=Kopparapu|first1=Ravi Kumar|last2=Ramirez|first2=Ramses M.|last3=SchottelKotte|first3=James|last4=Kasting|first4=James F.|last5=Domagal-Goldman|first5=Shawn|last6=Eymet|first6=Vincent|date=15 May 2014|journal=The Astrophysical Journal|volume=787|issue=2|pages=L29|doi=10.1088/2041-8205/787/2/L29|issn=2041-8205|bibcode=2014ApJ...787L..29K|arxiv=1404.5292|title=Habitable Zones Around Main-Sequence Stars: Dependence on Planetary Mass|s2cid=118588898}}</ref>

Calculating the inner edge of the habitable zone is strongly dependent on the model used to calculate the Simpson–Nakajima or moist greenhouse limit.<ref name=":36"/> The climate models used to calculate these limits have evolved over time, with some models assuming a simple one-dimensional, grey atmosphere,<ref name=":13"/> and others using a full [[radiative transfer]] solution to model the absorption bands of water and carbon dioxide.<ref name=":04"/> These earlier models that used radiative transfer derived the absorption coefficients for water from the [[HITRAN]] database, while newer models<ref>{{Cite journal|last1=Crisp|first1=David|last2=Kevin J. Zahnle|last3=Robinson|first3=Tyler D.|last4=Goldblatt|first4=Colin|date=August 2013|title=Low simulated radiation limit for runaway greenhouse climates|journal=Nature Geoscience|volume=6|issue=8|pages=661–667|doi=10.1038/ngeo1892|issn=1752-0908|bibcode=2013NatGe...6..661G|hdl=2060/20160002421|s2cid=37541492|hdl-access=free}}</ref> use the more current and accurate HITEMP database, which has led to different calculated values of thermal radiation limits. More accurate calculations have been done using three-dimensional climate models<ref name=":102">{{Cite journal|last1=Leconte|first1=Jérémy|last2=Forget|first2=Francois|last3=Charnay|first3=Benjamin|last4=Wordsworth|first4=Robin|last5=Pottier|first5=Alizée|date=December 2013|title=Increased insolation threshold for runaway greenhouse processes on Earth-like planets|journal=Nature|volume=504|issue=7479|pages=268–271|doi=10.1038/nature12827|pmid=24336285|issn=0028-0836|bibcode=2013Natur.504..268L|arxiv=1312.3337|s2cid=2115695}}</ref> that take into account effects such as planetary rotation and local water mixing ratios as well as cloud feedbacks.<ref name=":112">{{Cite journal|last1=Yang|first1=Jun|last2=Cowan|first2=Nicolas B.|last3=Abbot|first3=Dorian S.|date=27 June 2013|journal=The Astrophysical Journal|volume=771|issue=2|pages=L45|doi=10.1088/2041-8205/771/2/L45|issn=2041-8205|bibcode=2013ApJ...771L..45Y|arxiv=1307.0515|title=Stabilizing Cloud Feedback Dramatically Expands the Habitable Zone of Tidally Locked Planets|s2cid=14119086}}</ref> The effect of clouds on calculating thermal radiation limits is still in debate (specifically, whether or not water clouds present a positive or negative feedback effect).<ref name=":36"/>

== Runaway greenhouse effect in the Solar System ==

===Venus===
[[Image:PIA23791-Venus-RealAndEnhancedContrastViews-20200608_(cropped).jpg|thumb|right|Venus' oceans may have boiled away in a runaway greenhouse effect.]]
A runaway greenhouse effect involving carbon dioxide and water vapor likely occurred on [[Venus]].<ref>{{cite journal |author=S. I. Rasoonl|author2=C. de Bergh|name-list-style=amp |title=The Runaway Greenhouse Effect and the Accumulation of CO<sub>2</sub> in the Atmosphere of Venus |journal=Nature |volume=226 |pages=1037–1039 |date=1970 |pmid=16057644 |issue=5250 |doi=10.1038/2261037a0 |bibcode=1970Natur.226.1037R|s2cid=4201521}}</ref> In this scenario, early Venus may have had a global ocean if the outgoing thermal radiation was below the Simpson–Nakajima limit but above the moist greenhouse limit.<ref name=":36"/> 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 evaporated.

This scenario helps to explain why there is little water vapor in the atmosphere of Venus today. If Venus initially formed with water, the runaway greenhouse effect would have hydrated Venus' stratosphere,<ref name=":04"/> and the water would have escaped to space.<ref name=":44"/> 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.<ref>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</ref><ref>. 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</ref>

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 while the oxygen recombines or bonds to iron on the planet's surface.<ref name=":36" /> The deficit of [[water on Venus]] due to the runaway greenhouse effect is thought to explain why Venus does not exhibit surface features consistent with plate tectonics,<ref>{{Cite journal|last1=Taylor|first1=Fredric W.|author-link=Fred Taylor (physicist)|last2=Svedhem|first2=Håkan|last3=Head|first3=James W.|author-link3=James W. Head|date=February 2018|title=Venus: The Atmosphere, Climate, Surface, Interior and Near-Space Environment of an Earth-Like Planet|journal=Space Science Reviews|volume=214|issue=1|pages=35|doi=10.1007/s11214-018-0467-8|issn=0038-6308|bibcode=2018SSRv..214...35T|doi-access=free}}</ref> meaning it would be a [[stagnant lid]] planet.<ref name=":123">{{Cite journal|last1=Driscoll|first1=P.|last2=Bercovici|first2=D.|author-link2=David Bercovici|date=November 2013|title=Divergent evolution of Earth and Venus: Influence of degassing, tectonics, and magnetic fields|journal=Icarus|volume=226|issue=2|pages=1447–1464|doi=10.1016/j.icarus.2013.07.025|bibcode=2013Icar..226.1447D|s2cid=122173586 }}</ref>

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 [[Subduction|subducted]] into the Earth by plate tectonics on geologic time scales through the [[carbonate–silicate cycle]],<ref>{{cite web|url=http://home.case.edu/~sjr16/venus.html|title=Venus|author=Nick Strobel|archive-url=https://wayback.archive-it.org/all/20070212164038/http://filer.case.edu/~sjr16/venus.html|archive-date=12 February 2007|url-status=dead|access-date=17 February 2009}}</ref> which requires [[precipitation]] to function.<ref>{{Cite journal|last1=Walker|first1=James C. G.|last2=Hays|first2=P. B.|last3=Kasting|first3=J. F.|date=1981|title=A negative feedback mechanism for the long-term stabilization of Earth's surface temperature|journal=Journal of Geophysical Research: Oceans|volume=86|issue=C10|pages=9776–9782|doi=10.1029/JC086iC10p09776|issn=2156-2202|bibcode=1981JGR....86.9776W}}</ref>

===Earth===

Early investigations on the effect of atmospheric carbon dioxide levels on the runaway greenhouse limit found that it would take orders of magnitude higher amounts of carbon dioxide to take the Earth to a runaway greenhouse state.<ref name=":04"/> This is because carbon dioxide is not anywhere near as effective at blocking outgoing longwave radiation as water is.<ref name=":44"/> Within current models of the runaway greenhouse effect, carbon dioxide (especially anthropogenic carbon dioxide) does not seem capable of providing the necessary insulation for Earth to reach the Simpson–Nakajima limit.<ref name=":04" /><ref name="PhilTransactionsRoyalSociety_20120913"/><ref name="IPCC2009" /><ref name="Hansen et al 2013" />

Debate remains, however, on whether carbon dioxide can push surface temperatures towards the moist greenhouse limit.<ref name=":8">Kunzig, Robert. [https://web.archive.org/web/20130801153801/http://news.nationalgeographic.com/news/2013/13/130729-runaway-greenhouse-global-warming-venus-ocean-climate-science "Will Earth's Ocean Boil Away?"] ''[[National Geographic (magazine)|National Geographic Daily News]]'' (29 July 2013)</ref><ref name=":9">{{cite web|url=http://www.technologyreview.com/view/426608/how-likely-is-a-runaway-greenhouse-effect-on-earth/|title=How Likely Is a Runaway Greenhouse Effect on Earth?|website=MIT Technology Review|access-date=1 June 2015|archive-date=22 April 2015|archive-url=https://web.archive.org/web/20150422072924/http://www.technologyreview.com/view/426608/how-likely-is-a-runaway-greenhouse-effect-on-earth/|url-status=dead}}</ref> Climate scientist [[John Houghton (physicist)|John Houghton]] wrote in 2005 that "[there] is no possibility of [Venus's] runaway greenhouse conditions occurring on the Earth".<ref>{{cite journal
|author=Houghton, J.
|journal=Rep. Prog. Phys.
|title=Global Warming
|volume=68
|issue=6
|pages=1343–1403
|date=4 May 2005
|doi=10.1088/0034-4885/68/6/R02
|bibcode = 2005RPPh...68.1343H |s2cid=250915571
}}</ref> However, climatologist [[James Hansen]] stated in ''[[Storms of My Grandchildren]]'' (2009) that burning coal and mining [[oil sands]] will result in runaway greenhouse on Earth.<ref>{{cite web|title=How Likely Is a Runaway Greenhouse Effect on Earth?|url=http://www.technologyreview.com/view/426608/how-likely-is-a-runaway-greenhouse-effect-on-earth/|website=MIT Technology Review|access-date=1 June 2015|archive-date=22 April 2015|archive-url=https://web.archive.org/web/20150422072924/http://www.technologyreview.com/view/426608/how-likely-is-a-runaway-greenhouse-effect-on-earth/|url-status=dead}}</ref> A re-evaluation in 2013 of the effect of water vapor in the climate models showed that James Hansen's outcome would require ten times the amount of CO<sub>2</sub> we could release from burning all the oil, coal, and natural gas in Earth's crust.<ref name=":8"/>

As with the uncertainties in calculating the inner edge of the habitable zone, the uncertainty in whether CO<sub>2</sub> can drive a moist greenhouse effect is due to differences in modeling choices and the uncertainties therein.<ref name="PhilTransactionsRoyalSociety_20120913"/><ref name=":36"/> The switch from using HITRAN to the more current HITEMP absorption line lists in radiative transfer calculations has shown that previous runaway greenhouse limits were too high, but the necessary amount of carbon dioxide would make an anthropogenic moist greenhouse state unlikely.<ref>{{Cite journal|last1=Goldblatt|first1=Colin|last2=Robinson|first2=Tyler D.|last3=Zahnle|first3=Kevin J.|last4=Crisp|first4=David|date=August 2013|title=Low simulated radiation limit for runaway greenhouse climates|journal=Nature Geoscience|volume=6|issue=8|pages=661–667|doi=10.1038/ngeo1892|issn=1752-0894|bibcode=2013NatGe...6..661G|hdl=2060/20160002421|s2cid=37541492|hdl-access=free}}</ref> Full three-dimensional models have shown that the moist greenhouse limit on surface temperature is higher than that found in one-dimensional models and thus would require a higher amount of carbon dioxide to initiate a moist greenhouse than in one-dimensional models.<ref name=":102"/>

Other complications include whether the atmosphere is saturated or sub-saturated at some humidity,<ref name=":102" /> higher CO<sub>2</sub> levels in the atmosphere resulting in a less hot Earth than expected due to [[Rayleigh scattering]],<ref name=":36"/> and whether cloud feedbacks stabilize or destabilize the climate system.<ref name=":112"/><ref name=":102" />

Complicating the matter, research on Earth's climate history has often used the term "runaway greenhouse effect" to describe large-scale climate changes when it is not an appropriate description as it does not depend on Earth's outgoing longwave radiation. Though the Earth has experienced a diversity of climate extremes, these are not end-states of climate evolution and have instead represented climate equilibria different from that seen on Earth today.<ref name=":36"/> For example, it has been hypothesized that large releases of greenhouse gases may have occurred concurrently with the [[Permian–Triassic extinction event]]<ref name="Benton">{{cite journal|last1=Benton|first1=M. J.|last2=Twitchet|first2=R. J.|year=2003|title=How to kill (almost) all life: the end-Permian extinction event|url=http://palaeo.gly.bris.ac.uk/Benton/reprints/2003TREEPTr.pdf|journal=Trends in Ecology & Evolution|volume=18|issue=7|pages=358–365|doi=10.1016/S0169-5347(03)00093-4}}</ref><ref name="HBRM96111">{{cite journal|last=Morante|first=Richard|date=1996|title=Permian and early Triassic isotopic records of carbon and strontium in Australia and a scenario of events about the Permian-Triassic boundary|journal=Historical Biology: An International Journal of Paleobiology|volume=11|issue=1|pages=289–310|doi=10.1080/10292389609380546|bibcode=1996HBio...11..289M }}</ref> or [[Paleocene–Eocene Thermal Maximum]]. Additionally, 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 [[glacier]]s on the planet, the levels of carbon dioxide and other [[greenhouse gas]]es (such as [[water vapor]] and [[methane]]) were high, and [[sea surface temperature]]s (SSTs) ranged from 40&nbsp;°C (104&nbsp;°F) in the [[tropics]] to 16&nbsp;°C (65&nbsp;°F) in the [[polar region]]s.<ref>{{cite journal|last=Price|first=Gregory|author2=Paul J. Valdes|author3=Bruce W. Sellwood|year=1998|title=A comparison of GCM simulated Cretaceous 'greenhouse' and 'icehouse climates: implications for the sedimentary record|journal=Palaeogeography, Palaeoclimatology, Palaeoecology|volume=142|issue=3–4|pages=123–138|bibcode=1998PPP...142..123P|doi=10.1016/s0031-0182(98)00061-3}}</ref>

====Distant future====
{{see also|Future of Earth}}

Most scientists believe that a runaway greenhouse effect is inevitable in the long term, as the Sun gradually becomes more luminous as it ages, and will spell the end of all life on Earth. As the Sun becomes 10% brighter about one billion years from now, the surface temperature of Earth will reach {{convert|47|°C}} (unless [[Albedo]] is increased sufficiently), causing the temperature of Earth to rise rapidly and its oceans to boil away until it becomes a greenhouse planet, similar to Venus today.

The current loss rate is approximately one millimeter of ocean per million years.<ref name=":0">Brownlee, David and Peter D. Ward, ''The Life and Death of Planet Earth,'' Holt Paperbacks, 2004, {{ISBN|978-0805075120}}</ref> This is due to the colder upper layer of the troposphere acting as a cold trap currently preventing Earth from permanently losing its water to space at present, even with manmade global warming (this is also the reason why climate change is only going to make extreme weather events worse in the near term, [[Clausius-Clapeyron relation | as a warmer atmosphere can hold more moisture]], as even with global warming, the cold trap ensures that the current atmosphere will still be too cold to allow water vapor to be rapidly lost to space). This is being overshadowed by shorter-term changes in sea level, such as the currently [[Sea level rise|rising sea level]] due to the melting of glaciers and polar ice. However, the rate is gradually accelerating, as the sun gets warmer, to perhaps as fast as one millimeter every 1000 years, by ultimately making the atmosphere so hot that the cold trap is pushed even higher up until it eventually fails to prevent the water from being lost to space.<ref name=":0" /> 

Ward and Brownlee predict that there will be two variations of the future warming feedback: the "moist greenhouse" in which water vapor dominates the [[troposphere]] and starts to accumulate in the [[stratosphere]] and the "runaway greenhouse" in which water vapor becomes a dominant component of the atmosphere such that the Earth starts to undergo rapid warming, which could send its surface temperature to over {{convert|900|°C}}, causing its entire surface to melt and killing all life, perhaps about three billion years from now. In both cases, the moist and runaway greenhouse states the loss of oceans will turn the Earth into a primarily-desert world. The only water left on the planet would be in a few evaporating ponds scattered near the poles as well as huge salt flats around what was once the ocean floor, much like the [[Atacama Desert]] in Chile or [[Badwater Basin]] in Death Valley. The small reservoirs of water may allow life to remain for a few billion more years.

As the Sun brightens, CO<sub>2</sub> levels should decrease due to an increase of activity in the carbon-silicate cycle corresponding to the increase of temperature. That would mitigate some of the heating Earth would experience because of the Sun's increase in brightness.<ref name=":36"/> Eventually, however, as the water escapes, the [[carbon cycle]] will cease as plate tectonics come to a halt because of the need for water as a lubricant for tectonic activity.<ref name=":123"/>

==Runaway refrigerator effect==

Mars and Earth during the [[Cryogenian]] period may have experienced the opposite of a runaway greenhouse effect: a ''runaway refrigerator effect''. Through this effect, a runaway feedback process may have removed much carbon dioxide and water vapor from the atmosphere and cooled the planet. Water condenses on the surface, leading to carbon dioxide dissolving and chemically binding to minerals. This reduced the greenhouse effect, lowering the temperature and causing more water to condense. The result was lower temperatures, with water being frozen as subsurface permafrost, leaving only a thin atmosphere.<ref>{{cite web |title=Mars |url=https://www.astronomynotes.com/solarsys/s10.htm |website=Astronomy Notes |access-date=28 May 2023}}</ref><ref>{{cite journal |last1=Kite |first1=E. S. |last2=Mischena |first2=M. A. |last3=Fan |first3=B. |last4=Morgan |first4=A. M. |last5=Wilson |first5=S. A. |last6=Richardson |first6=M. L. |title=Changing spatial distribution of water flow charts major change in Mars's greenhouse effect |journal=Science Advances |date=2022 |volume=8 |issue=21 |pages=eabo5894 |doi=10.1126/sciadv.abo5894|pmid=35613275 |pmc=9132440 |arxiv=2206.00036 |bibcode=2022SciA....8O5894K }}</ref> In addition, ice and snow are far more reflective than open water, with an albedo of 50-70% and 85% respectively. This means that as a planet's temperature decreases and more of its water freezes, its ability to absorb light is reduced, which in turn makes it even colder, creating a positive feedback loop.<ref>{{Cite web |last=Pavitt |first=Dane |date=August 17, 2023 |title=The albedo effect - how snow and ice cools the world |url=https://theaveragescientist.co.uk/2023/08/17/the-albedo-effect-how-snow-and-ice-cools-the-world/}}</ref> This effect, combined with the decrease in heat-retaining clouds and vapor, becomes runaway once snow and ice coverage reach a certain threshold (within 30 degrees of the equator), plunging the planet into a stable [[Snowball Earth|snowball state]].<ref>{{Cite web |date=2019-02-11 |title=How planets die: climate catastrophe! – planetplanet |url=https://planetplanet.net/2019/02/11/how-planets-die-climate-catastrophe/ |access-date=2025-01-25 |language=en-US}}</ref><ref>{{cite journal
| author = Kirschvink, J.L.
| year = 2002
| title = When All of the Oceans Were Frozen
| journal = [[La Recherche]]
| volume = 355
| pages = 26–30
| url = http://web.gps.caltech.edu/~jkirschvink/pdfs/laRechercheEnglish.pdf
}}</ref>

==See also==
{{portal|Climate change}}
* [[Atmosphere of Venus]], an example of a runaway greenhouse effect
* [[Greenhouse and icehouse Earth]]

== References ==
{{Reflist|30em}}{{global warming|state=collapsed}}

{{DEFAULTSORT:Runaway Greenhouse Effect}}
[[Category:Climate change feedbacks]]
[[Category:Climate forcing]]
[[Category:Atmosphere]]
[[Category:Climatology]]
[[Category:Global natural environment]]