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Runaway greenhouse effect

Not to be confused with Tipping points in the climate system.

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.[1] This positive feedback loop 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[2] 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.[3] This likely happened in the early history of Venus.

In a 2012 study on climate change, it was quoted stating that "Earth presently absorbs around 240 W m−2 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."[4]

A runaway greenhouse effect similar to Venus appears to have virtually no chance of being caused by anthropogenic activities.[5] A 2013 article concluded that a runaway greenhouse effect "could in theory be triggered by increased greenhouse forcing," but that "anthropogenic emissions are probably insufficient."[6] 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.[7] Earth is expected to experience a runaway greenhouse effect "in about 2 billion years as solar luminosity increases".[4]

  1. ^ Kaltenegger, Lisa (2015). "Greenhouse Effect". In Gargaud, Muriel; Irvine, William M.; Amils, Ricardo; Cleaves, Henderson James (eds.). Encyclopedia of Astrobiology. Springer Berlin Heidelberg. p. 1018. doi:10.1007/978-3-662-44185-5_673. ISBN 9783662441848.
  2. ^ Catling, David C.; Kasting, James F. (13 April 2017). Atmospheric evolution on inhabited and lifeless worlds. Cambridge. ISBN 9780521844123. OCLC 956434982.{{cite book}}: CS1 maint: location missing publisher (link)
  3. ^ 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.
  4. ^ a b Goldblatt, Colin; Watson, Andrew J. (13 September 2012). "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. arXiv:1201.1593. Bibcode:2012RSPTA.370.4197G. doi:10.1098/rsta.2012.0004. PMID 22869797. S2CID 7891446.
  5. ^ Scoping of the IPCC 5th Assessment Report Cross Cutting Issues (PDF). Thirty-first Session of the IPCC Bali, 26–29 October 2009 (Report). Archived (PDF) from the original on 9 November 2009. Retrieved 24 March 2019.
  6. ^ Goldblatt, Colin; Robinson, Tyler D.; Zahnle, Kevin J.; Crisp, David (28 July 2013). "Low simulated radiation limit for runaway greenhouse climates". Nature Geoscience. 6 (8): 661–667. Bibcode:2013NatGe...6..661G. doi:10.1038/ngeo1892. hdl:2060/20160002421. S2CID 37541492. Archived from the original on 20 September 2022. Retrieved 17 September 2022.
  7. ^ Hansen, James; Sato, Makiko; Russell, Gary; Kharecha, Pushker (2013). "Climate sensitivity, sea level and atmospheric carbon dioxide". Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences. 371 (2001). 20120294. arXiv:1211.4846. Bibcode:2013RSPTA.37120294H. doi:10.1098/rsta.2012.0294. PMC 3785813. PMID 24043864.
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