View source for Greenhouse effect

{{Short description|Atmospheric phenomenon causing planetary warming}}
{{about|the atmospheric phenomenon causing planetary warming|the general heating or cooling of Earth's surface|Earth's energy budget|other uses|Greenhouse (disambiguation)}}
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{{Use dmy dates|date=March 2024}}
[[File:Climate Change Schematic.svg|thumb|upright=1.35|Greenhouse gases allow sunlight to pass through the atmosphere and heat the planet, but then absorb and redirect some of the longwave radiation (heat) the planet emits.]]
[[File:Greenhouse effect with energy flows shown by altitude.svg|thumb|upright=1.35 |Energy flows down from the sun and up from the Earth and its atmosphere. When greenhouse gases intercept radiation emitted by Earth's surface, they prevent that radiation from escaping into space, causing surface temperatures to rise by about {{Convert|33|C-change}}.]]

The '''greenhouse effect''' occurs when [[greenhouse gas]]es in a planet's atmosphere insulate the planet from losing heat to space, raising its surface temperature. Surface heating can happen from an internal heat source as in the case of [[Jupiter]], or from its host star as in the case of the [[Earth]]. In the case of Earth, the Sun emits [[shortwave radiation]] (sunlight) that passes through greenhouse gases to heat the Earth's surface. In response, the Earth's surface emits [[outgoing longwave radiation|longwave radiation]] (heat) that is mostly absorbed by greenhouse gases. That heat absorption reduces the rate at which the Earth can cool off in response to being warmed by the Sun. Adding to greenhouse gases further reduces the rate a planet emits radiation to space, raising its average surface temperature.

The Earth's average surface temperature would be about {{Convert|−18|C|F|sigfig=1}} without the greenhouse effect,<ref>{{cite web |title=Solar Radiation and the Earth's Energy Balance |url=http://eesc.columbia.edu/courses/ees/climate/lectures/radiation/ |url-status=dead |archive-url=https://web.archive.org/web/20041104033042/http://eesc.columbia.edu/courses/ees/climate/lectures/radiation/ |archive-date=4 November 2004 |access-date=15 October 2010 |work=The Climate System – EESC 2100 Spring 2007 |publisher=Columbia University}}</ref><ref name="IPCC4_ch012">{{cite book |title=Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change |vauthors=Le Treut H, Somerville R, Cubasch U, Ding Y, [[Cecilie Mauritzen|Mauritzen C]], Mokssit A, Peterson T, Prather M |publisher=Cambridge University Press |year=2007 |veditors=Solomon S, Qin D, Manning M, Chen Z, Marquis M, Averyt KB, Tignor M, Miller HL |location=Cambridge, UK and New York, NY |page=97 |chapter=Historical Overview of Climate Change Science |access-date=25 March 2014 |chapter-url=http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter1.pdf |archive-url=https://web.archive.org/web/20181126204443/http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter1.pdf |archive-date=26 November 2018 |url-status=dead}}</ref> compared to Earth's 20th century average of about {{Convert|14|C|F}}, or a more recent average of about {{Convert|15|C|F}}.<ref>{{cite web |title=The Elusive Absolute Surface Air Temperature (SAT) |url=http://data.giss.nasa.gov/gistemp/abs_temp.html |website=[[Goddard Institute for Space Studies]] |publisher=[[National Aeronautics and Space Administration|NOAA]] |access-date=3 September 2008 |archive-date=5 September 2015 |archive-url=https://web.archive.org/web/20150905093320/http://data.giss.nasa.gov/gistemp/abs_temp.html |url-status=live }}</ref><ref name="clchgtrack">{{cite web |title=Yearly average temperature |url=https://climatechangetracker.org/global-warming/yearly-average-temperature |website=Climate Change Tracker}}</ref> In addition to naturally present greenhouse gases, burning of [[fossil fuel]]s has increased amounts of [[carbon dioxide]] and [[methane]] in the atmosphere.<ref name="ipcc-AR4WG1" /><ref name="budget">{{cite web|last=Rebecca|first=Lindsey|date=14 January 2009|title=Climate and Earth's Energy Budget : Feature Articles|url=https://earthobservatory.nasa.gov/features/EnergyBalance/page1.php|url-status=live|archive-url=https://web.archive.org/web/20210121082936/https://earthobservatory.nasa.gov/features/EnergyBalance/page1.php|archive-date=21 January 2021|access-date=14 December 2020|website=earthobservatory.nasa.gov}}</ref> As a result, [[global warming]] of about {{convert|1.2|C-change|F-change}} has occurred since the [[Industrial Revolution]],<ref>{{Cite web|last=Fox|first=Alex|title=Atmospheric Carbon Dioxide Reaches New High Despite Pandemic Emissions Reduction|url=https://www.smithsonianmag.com/smart-news/atmospheric-carbon-dioxide-reaches-new-high-despite-pandemic-emissions-reduction-180977945/|access-date=22 June 2021|website=Smithsonian Magazine|language=en|archive-date=10 June 2021|archive-url=https://web.archive.org/web/20210610130852/https://www.smithsonianmag.com/smart-news/atmospheric-carbon-dioxide-reaches-new-high-despite-pandemic-emissions-reduction-180977945/|url-status=live}}</ref> with the global average surface temperature increasing at a rate of {{convert|0.18|C-change|F-change}} per decade since 1981.<ref>{{cite web |first1=Rebecca |last1=Lindsey |first2=Luann |last2=Dahlman |title=Climate Change: Global Temperature |website=NOAA Climate.gov |url=https://www.climate.gov/news-features/understanding-climate/climate-change-global-temperature#:~:text=Earth's%20temperature%20has%20risen%20by,0.18%C2%B0%20C)%20per%20decade.
}}</ref>

The [[Electromagnetic spectrum|wavelengths of radiation]] emitted by the Sun and Earth differ because their surface temperatures are different. The Sun has a surface temperature of {{convert|5,500|C|F|-2}}, so it emits most of its energy as ''[[shortwave radiation]]'' in near-infrared and visible wavelengths (as [[sunlight]]). In contrast, Earth's surface has a much lower temperature, so it emits ''[[outgoing longwave radiation|longwave radiation]]'' at mid- and far-[[infrared]] wavelengths (sometimes called [[thermal radiation]] or radiated heat).<ref name="budget" /> A gas is a [[greenhouse gas]] if it [[infrared spectroscopy|absorbs longwave radiation]]. Earth's atmosphere absorbs only 23% of incoming shortwave radiation, but absorbs 90% of the longwave radiation emitted by the surface,<ref name="eeb1">{{cite web |title=What is Earth's Energy Budget? Five Questions with a Guy Who Knows |url=https://www.nasa.gov/feature/langley/what-is-earth-s-energy-budget-five-questions-with-a-guy-who-knows |website=NASA.gov |date=10 April 2017 |access-date=24 April 2023}}</ref> thus accumulating energy and warming the Earth's surface.

The existence of the greenhouse effect, while not named as such, was proposed as early as 1824 by [[Joseph Fourier]].<ref>{{Cite journal |last=Fourier |first=J. |date=1824 |title=Remarques Generales sur les Temperatures Du Globe Terrestre et des Espaces Planetaires |url=https://books.google.com/books?id=1Jg5AAAAcAAJ&pg=PA136 |url-status=live |journal=Annales de Chimie et de Physique |language=fr |volume=27 |pages=136–167 |archive-url=https://web.archive.org/web/20200802195946/https://books.google.com/books?id=1Jg5AAAAcAAJ&pg=PA136 |archive-date=2 August 2020 |access-date=8 June 2020}}</ref> The argument and the evidence were further strengthened by [[Claude Pouillet]] in 1827 and 1838. In 1856 [[Eunice Newton Foote]] demonstrated that the warming effect of the sun is greater for air with water vapour than for dry air, and the effect is even greater with carbon dioxide.<ref>{{cite book |last=Foote |first=Eunice |url=https://books.google.com/books?id=6xhFAQAAMAAJ&pg=PA382 |title=Circumstances affecting the Heat of the Sun's Rays |date=November 1856 |work=The American Journal of Science and Arts |volume=22 |pages=382–383 |access-date=31 January 2016 |archive-url=https://web.archive.org/web/20200930234235/https://books.google.com/books?id=6xhFAQAAMAAJ&pg=PA382%2F |archive-date=30 September 2020 |url-status=live}}</ref><ref>{{cite web |last=Huddleston |first=Amara |date=17 July 2019 |title=Happy 200th birthday to Eunice Foote, hidden climate science pioneer |url=https://www.climate.gov/news-features/features/happy-200th-birthday-eunice-foote-hidden-climate-science-pioneer |url-status=live |archive-url=https://web.archive.org/web/20200930234240/https://www.climate.gov/news-features/features/happy-200th-birthday-eunice-foote-hidden-climate-science-pioneer/ |archive-date=30 September 2020 |access-date=8 October 2019 |website=NOAA Climate.gov}}</ref> The term ''greenhouse'' was first applied to this phenomenon by [[Nils Ekholm|Nils Gustaf Ekholm]] in 1901.<ref>{{cite web |last1=Easterbrook |first1=Steve |date=18 August 2015 |title=Who first coined the term "Greenhouse Effect"? |url=http://www.easterbrook.ca/steve/2015/08/who-first-coined-the-term-greenhouse-effect/ |url-status=live |archive-url=https://web.archive.org/web/20151113131713/http://www.easterbrook.ca/steve/2015/08/who-first-coined-the-term-greenhouse-effect/ |archive-date=13 November 2015 |access-date=11 November 2015 |website=Serendipity}}</ref><ref>{{cite journal |author=Ekholm N |year=1901 |title=On The Variations Of The Climate Of The Geological And Historical Past And Their Causes |journal=Quarterly Journal of the Royal Meteorological Society |volume=27 |pages=1–62 |bibcode=1901QJRMS..27....1E |doi=10.1002/qj.49702711702 |number=117}}</ref>

== Definition ==
The ''greenhouse effect'' on Earth is defined as: "The infrared radiative effect of all infrared absorbing constituents in the atmosphere. [[Greenhouse gas|Greenhouse gases]] (GHGs), [[Cloud|clouds]], and some [[Aerosol|aerosols]] absorb terrestrial radiation emitted by the Earth’s surface and elsewhere in the atmosphere."<ref name="IPCC">IPCC, 2021: Annex VII: [https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_AnnexVII.pdf Glossary] [Matthews, J.B.R., V. Möller, R. van Diemen, J.S. Fuglestvedt, V. Masson-Delmotte, C.  Méndez, S. Semenov, A. Reisinger (eds.)]. In [https://www.ipcc.ch/report/ar6/wg1/ Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change] [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 2215–2256, doi:10.1017/9781009157896.022.</ref>{{Rp|page=2232}}

The ''enhanced greenhouse effect'' describes the fact that by increasing the concentration of GHGs in the atmosphere (due to human action), the natural greenhouse effect is increased.<ref name="IPCC" />{{Rp|page=2232}}

== Terminology ==
The term ''greenhouse effect'' comes from an [[wikt:analogy|analogy]] to [[greenhouse]]s. Both greenhouses and the ''greenhouse effect'' work by retaining heat from sunlight, but the way they retain heat differs. Greenhouses retain heat mainly by blocking [[convection]] (the movement of air).<ref>{{cite web |first=John |last=Worley |title=Greenhouses: Heating, Cooling and Ventilation |website=University of Georgia Extension |url=https://extension.uga.edu/publications/detail.html?number=B792&title=greenhouses-heating-cooling-and-ventilation |access-date=12 March 2023 |archive-date=12 March 2023 |archive-url=https://web.archive.org/web/20230312234640/https://extension.uga.edu/publications/detail.html?number=B792&title=greenhouses-heating-cooling-and-ventilation |url-status=live }}</ref><ref name="ipccar4wg1ch1">{{cite book |title=Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, 2007, Chapter 1 |publisher=Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. |page=115 |url=https://www.ipcc.ch/report/ar4/wg1/ |access-date=24 March 2023 |quote=The glass walls in a greenhouse reduce airflow and increase the temperature of the air inside. Analogously, but through a different physical process, the Earth's greenhouse effect warms the surface of the planet.}}</ref> In contrast, the greenhouse effect retains heat by restricting [[radiative transfer]] through the air and reducing the rate at which heat escapes to space.<ref name="ipcc-AR4WG1" />

==History of discovery and investigation==
{{also|History of climate change science}}

{{multiple image
| align             = right
| total_width       = 650
| image1            = 18560823 Eunice Newton Foote - greenhouse effect.png
| caption1          = [[Eunice Newton Foote]] recognized carbon dioxide's heat-capturing effect in 1856, appreciating its implications for the planet.<ref>Foote, Eunice, 1856. [https://web.archive.org/web/20230223113925/https://ia800802.us.archive.org/4/items/mobot31753002152491/mobot31753002152491.pdf Circumstances affecting the heat of the Sun's rays]": Art. XXXI, ''The American Journal of Science and Arts,'' 2nd Series, v. XXII/no. LXVI, November 1856, p. 382-383.</ref>
| image2            = 191203 Furnaces of the world - Popular Mechanics - Global warming.jpg
| caption2          = The greenhouse effect and its impact on climate were succinctly described in this 1912 ''[[Popular Mechanics]]'' article, accessible for reading by the general public.
}}
The existence of the greenhouse effect, while not named as such, was proposed as early as 1824 by [[Joseph Fourier]].<ref>{{Cite journal |last=Fourier |first=J. |date=1824 |title=Remarques Generales sur les Temperatures Du Globe Terrestre et des Espaces Planetaires |url=https://books.google.com/books?id=1Jg5AAAAcAAJ&pg=PA136 |url-status=live |journal=Annales de Chimie et de Physique |language=fr |volume=27 |pages=136–167 |archive-url=https://web.archive.org/web/20200802195946/https://books.google.com/books?id=1Jg5AAAAcAAJ&pg=PA136 |archive-date=2 August 2020 |access-date=8 June 2020}}</ref> The argument and the evidence were further strengthened by [[Claude Pouillet]] in 1827 and 1838. In 1856 [[Eunice Newton Foote]] demonstrated that the warming effect of the sun is greater for air with water vapour than for dry air, and the effect is even greater with carbon dioxide. She concluded that "An atmosphere of that gas would give to our earth a high temperature..."<ref>{{cite book |last=Foote |first=Eunice |url=https://books.google.com/books?id=6xhFAQAAMAAJ&pg=PA382 |title=Circumstances affecting the Heat of the Sun's Rays |date=November 1856 |work=The American Journal of Science and Arts |volume=22 |pages=382–383 |access-date=31 January 2016 |archive-url=https://web.archive.org/web/20200930234235/https://books.google.com/books?id=6xhFAQAAMAAJ&pg=PA382%2F |archive-date=30 September 2020 |url-status=live}}</ref><ref>{{cite web |last=Huddleston |first=Amara |date=17 July 2019 |title=Happy 200th birthday to Eunice Foote, hidden climate science pioneer |url=https://www.climate.gov/news-features/features/happy-200th-birthday-eunice-foote-hidden-climate-science-pioneer |url-status=live |archive-url=https://web.archive.org/web/20200930234240/https://www.climate.gov/news-features/features/happy-200th-birthday-eunice-foote-hidden-climate-science-pioneer/ |archive-date=30 September 2020 |access-date=8 October 2019 |website=NOAA Climate.gov}}</ref>

[[John Tyndall]] was the first to measure the infrared absorption and emission of various gases and vapors. From 1859 onwards, he showed that the effect was due to a very small proportion of the atmosphere, with the main gases having no effect, and was largely due to water vapor, though small percentages of hydrocarbons and carbon dioxide had a significant effect.<ref>John Tyndall, ''[[iarchive:heatconsideredas00tynduoft|Heat considered as a Mode of Motion]]'' (500 pages; year 1863, 1873)</ref> The effect was more fully quantified by [[Svante Arrhenius]] in 1896, who made the first quantitative prediction of global warming due to a hypothetical doubling of atmospheric carbon dioxide.<ref name="anrev" /> The term ''greenhouse'' was first applied to this phenomenon by [[Nils Ekholm|Nils Gustaf Ekholm]] in 1901.<ref>{{cite web |last1=Easterbrook |first1=Steve |date=18 August 2015 |title=Who first coined the term "Greenhouse Effect"? |url=http://www.easterbrook.ca/steve/2015/08/who-first-coined-the-term-greenhouse-effect/ |url-status=live |archive-url=https://web.archive.org/web/20151113131713/http://www.easterbrook.ca/steve/2015/08/who-first-coined-the-term-greenhouse-effect/ |archive-date=13 November 2015 |access-date=11 November 2015 |website=Serendipity}}</ref><ref>{{cite journal |author=Ekholm N |year=1901 |title=On The Variations Of The Climate Of The Geological And Historical Past And Their Causes |journal=Quarterly Journal of the Royal Meteorological Society |volume=27 |pages=1–62 |bibcode=1901QJRMS..27....1E |doi=10.1002/qj.49702711702 |number=117}}</ref>

{{excerpt|History of climate change science#First calculations of greenhouse effect, 1896|paragraph=2|file=no}}

==Measurement==
[[File:Climate Factor CO₂ (ZDF, Terra X).webm|thumb|How {{CO2}} causes the greenhouse effect.]]
Matter emits [[thermal radiation]] in an amount that [[Stefan-Boltzmann law|is directly proportional to the fourth power of its temperature]]. Some of the radiation emitted by the Earth's surface is absorbed by greenhouse gases and clouds. Without this absorption, Earth's surface would have an average temperature of {{convert|−18|C|F|sigfig=1}}. However, because some of the radiation is absorbed, Earth's average surface temperature is around {{convert|15|C|F}}. Thus, the Earth's greenhouse effect may be measured as a ''temperature change'' of {{convert|33|C-change}}.

Thermal radiation is characterized by how much energy it carries, typically in watts per square meter (W/m{{sup|2}}). Scientists also measure the greenhouse effect based on how much more longwave thermal radiation leaves the Earth's surface than reaches space.<ref name="ipcc-ar6wg1-ch7"/>{{rp|968}}<ref name="ipcc-ar6wg1-ch7"/>{{rp|934}}<ref name="ravram1"/><ref name="Schmidt2010paper"/><ref name="Schmidt2010">{{cite web |author=Gavin Schmidt |date=1 October 2010 |title=Taking the Measure of the Greenhouse Effect |url=https://www.giss.nasa.gov/research/briefs/2010_schmidt_05/ |url-status=live |archive-url=https://web.archive.org/web/20210421003101/https://www.giss.nasa.gov/research/briefs/2010_schmidt_05/ |archive-date=21 April 2021 |access-date=13 January 2022 |publisher=NASA Goddard Institute for Space Studies - Science Briefs}}</ref> Currently, longwave radiation leaves the surface at an average rate of 398 W/m{{sup|2}}, but only 239 W/m{{sup|2}} reaches space. Thus, the Earth's greenhouse effect can also be measured as an ''energy flow change'' of 159 W/m{{sup|2}}.<ref name="ipcc-ar6wg1-ch7"/>{{rp|968}}<ref name="ipcc-ar6wg1-ch7"/>{{rp|934}} The greenhouse effect can be expressed as a fraction (0.40) or percentage (40%) of the longwave thermal radiation that leaves Earth's surface but does not reach space.<ref name="ipcc-ar6wg1-ch7"/>{{rp|968}}<ref name="ravram1"/><ref name="ravram2"/>

Whether the greenhouse effect is expressed as a change in temperature or as a change in longwave thermal radiation, the same effect is being measured.<ref name="ravram1"/>

== Role in climate change ==
{{Main|Climate change|Earth's energy budget}}
[[File:Earth's heating rate since 2005.jpg|thumb|right|upright=1.2|[[Earth's Energy Imbalance|Earth's rate of heating]] (graph) is a result of factors which include the enhanced greenhouse effect.<ref>{{cite web |author=Joseph Atkinson |date=22 June 2021 |title=Earth Matters: Earth's Radiation Budget is Out of Balance |url=https://earthobservatory.nasa.gov/blogs/earthmatters/2021/06/22/earths-radiation-budget-is-out-of-balance/ |url-status=live |archive-url=https://web.archive.org/web/20220410085015/https://earthobservatory.nasa.gov/blogs/earthmatters/2021/06/22/earths-radiation-budget-is-out-of-balance/ |archive-date=10 April 2022 |access-date=1 March 2022 |publisher=NASA Earth Observatory}}</ref>]]
Strengthening of the greenhouse effect through additional greenhouse gases from human activities is known as the ''enhanced greenhouse effect''.<ref name="IPCC" />{{Rp|page=2232}} As well as being inferred from measurements by [[Argo (oceanography)|ARGO]], [[Clouds and the Earth's Radiant Energy System|CERES]] and other instruments throughout the 21st century,<ref>{{Cite web |title=IPCC AR6 WG1 |url=https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Full_Report.pdf |url-status=live |archive-url=https://web.archive.org/web/20210809080054/https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Full_Report.pdf |archive-date=9 August 2021}}</ref>{{Rp|page=7–17}} this increase in [[radiative forcing]] from human activity has been observed directly,<ref>{{cite web |author=Robert McSweeney |date=25 February 2015 |title=New study directly measures greenhouse effect at Earth's surface |url=https://www.carbonbrief.org/new-study-directly-measures-greenhouse-effect-at-earths-surface |url-status=live |archive-url=https://web.archive.org/web/20210418151415/https://www.carbonbrief.org/new-study-directly-measures-greenhouse-effect-at-earths-surface |archive-date=18 April 2021 |access-date=18 April 2021 |publisher=Carbon Brief}}</ref><ref>{{cite web |date=26 March 2021 |title=Direct observations confirm that humans are throwing Earth's energy budget off balance |url=https://phys.org/news/2021-03-humans-earth-energy.html |url-status=live |archive-url=https://web.archive.org/web/20210418072159/https://phys.org/news/2021-03-humans-earth-energy.html |archive-date=18 April 2021 |access-date=18 April 2021 |website=phys.org |publisher=Science X}}</ref> and is attributable mainly to increased atmospheric carbon dioxide levels.<ref>{{cite web |title=Enhanced Greenhouse Effect |url=http://www.ace.mmu.ac.uk/eae/Global_Warming/Older/Enhanced_Greenhouse_Effect.html |url-status=dead |archive-url=https://web.archive.org/web/20101024121935/http://www.ace.mmu.ac.uk/eae/global_warming/Older/Enhanced_Greenhouse_Effect.html |archive-date=24 October 2010 |access-date=15 October 2010 |publisher=Ace.mmu.ac.uk}}</ref> 

[[File:Mauna Loa CO2 monthly mean concentration.svg|thumb|left|upright=0.7|The [[Keeling Curve]] of atmospheric CO<sub>2</sub> abundance.]]
{{CO2}} is produced by [[fossil fuel burning]] and other activities such as [[cement]] production and [[tropical deforestation]].<ref name="IPCC deforestation">IPCC [http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter7.pdf Fourth Assessment Report, Working Group I Report "The Physical Science Basis"] {{Webarchive|url=https://web.archive.org/web/20110315005004/http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter7.pdf|date=15 March 2011}} Chapter 7</ref>  Measurements of {{CO2}} from the [[Mauna Loa Observatory]] show that concentrations have increased from about 313 parts per million (ppm)<ref>{{cite web |title=Atmospheric Carbon Dioxide – Mauna Loa |url=http://www.esrl.noaa.gov/gmd/ccgg/trends/co2_data_mlo.html |url-status=live |archive-url=https://web.archive.org/web/20190520204843/https://www.esrl.noaa.gov/gmd/ccgg/trends/co2_data_mlo.html |archive-date=20 May 2019 |access-date=8 December 2008 |publisher=[[NOAA]]}}</ref> in 1960, passing the 400 ppm milestone in 2013.<ref>{{cite magazine |date=12 May 2013 |title=Climate Milestone: Earth's {{CO2}} Level Passes 400 ppm |url=http://news.nationalgeographic.com/news/energy/2013/05/130510-earth-co2-milestone-400-ppm/ |url-status=live |magazine=National Geographic |archive-url=https://web.archive.org/web/20131215081625/http://news.nationalgeographic.com/news/energy/2013/05/130510-earth-co2-milestone-400-ppm/ |archive-date=15 December 2013 |access-date=10 December 2017}}</ref>  The current observed amount of {{CO2}} exceeds the geological record maxima (≈300 ppm) from ice core data.<ref>{{cite journal |author=Hansen J. |date=February 2005 |title=A slippery slope: How much global warming constitutes "dangerous anthropogenic interference"? |url=https://zenodo.org/record/1232798 |url-status=live |journal=Climatic Change |volume=68 |issue=333 |pages=269–279 |bibcode=2005ClCh...68..269H |doi=10.1007/s10584-005-4135-0 |s2cid=153165132 |archive-url=https://web.archive.org/web/20200801065245/https://zenodo.org/record/1232798 |archive-date=1 August 2020 |access-date=8 June 2020}}</ref> 

Over the past 800,000 years,<ref>{{cite news |date=4 September 2006 |title=Deep ice tells long climate story |work=BBC News |url=http://news.bbc.co.uk/2/hi/science/nature/5314592.stm |url-status=live |access-date=4 May 2010 |archive-url=https://web.archive.org/web/20130123202651/http://news.bbc.co.uk/2/hi/science/nature/5314592.stm |archive-date=23 January 2013}}</ref> [[ice core]] data shows that carbon dioxide has varied from values as low as 180 ppm to the pre-industrial level of 270 ppm.<ref>{{cite journal |author=Hileman B |date=28 November 2005 |title=Ice Core Record Extended |url=http://pubs.acs.org/cen/news/83/i48/8348notw1.html |url-status=live |journal=Chemical & Engineering News |volume=83 |issue=48 |page=7 |doi=10.1021/cen-v083n048.p007 |archive-url=https://web.archive.org/web/20190515033556/http://pubs.acs.org/cen/news/83/i48/8348notw1.html |archive-date=15 May 2019 |access-date=6 September 2006}}</ref> [[Paleoclimatologists]] consider variations in carbon dioxide concentration to be a fundamental factor influencing climate variations over this time scale.<ref name="Bowen (2005), Thin Ice">{{cite book |last=Bowen |first=Mark |url=https://books.google.com/books?id=HGIcabCZHVUC |title=Thin Ice: Unlocking the Secrets of Climate in the World's Highest Mountains |publisher=Owl Books |year=2006 |isbn=978-1429932707 |access-date=1 June 2018 |archive-url=https://web.archive.org/web/20200802192430/https://books.google.com/books?id=HGIcabCZHVUC |archive-date=2 August 2020 |url-status=live}}</ref><ref>{{Cite web |date=20 August 2008 |title=Temperature change and carbon dioxide change |url=http://www.ncdc.noaa.gov/paleo/globalwarming/temperature-change.html |url-status=dead |archive-url=https://web.archive.org/web/20170118144841/https://www.ncdc.noaa.gov/paleo/globalwarming/temperature-change.html |archive-date=18 January 2017 |website=U.S. National Oceanic and Atmospheric Administration Paleoclimatology}}</ref>

== Energy balance and temperature ==
=== Incoming shortwave radiation ===
[[Image:Solar spectrum en.svg|thumb|upright=1.35|The [[solar radiation]] spectrum for direct light at both the top of Earth's atmosphere and at sea level]]
Hotter matter emits shorter wavelengths of radiation. As a result, the Sun emits [[shortwave radiation]] as sunlight while the Earth and its atmosphere emit [[Outgoing longwave radiation|longwave radiation]]. Sunlight includes [[UV|ultraviolet]], [[visible light]], and [[near-infrared]] radiation.<ref name="IPCC" />{{rp|2251}}

Sunlight is reflected and absorbed by the Earth and its atmosphere. The atmosphere and clouds reflect about 23% and absorb 23%. The surface reflects 7% and absorbs 48%.<ref name="eeb1" /> Overall, Earth reflects about 30% of the incoming sunlight,<ref>{{cite web |url=http://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html |title=Earth Fact Sheet |publisher=NSSDCA |first1=David R. |last1=Williams  |access-date=15 October 2010 |archive-date=25 December 2015 |archive-url=https://web.archive.org/web/20151225024603/http://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html |url-status=live }}</ref><ref name=harvard>{{cite book |chapter-url=http://acmg.seas.harvard.edu/people/faculty/djj/book/bookchap7.html |title=Introduction to Atmospheric Chemistry |first=Daniel J. |last=Jacob |publisher=Princeton University Press |year=1999 |chapter=7. The Greenhouse Effect |isbn=978-1400841547 |access-date=9 December 2009 |archive-date=3 October 2010 |archive-url=https://web.archive.org/web/20101003035132/http://acmg.seas.harvard.edu/people/faculty/djj/book/bookchap7.html |url-status=dead }}</ref> and absorbs the rest (240 W/m{{sup|2}}).<ref name="ipcc-ar6wg1-ch7" />{{rp|934}}

=== Outgoing longwave radiation ===
[[File:Spectral Greenhouse Effect.png|thumb|upright=1.35|The greenhouse effect is a reduction in the flux of outgoing longwave radiation, which affects the planet's radiative balance. The spectrum of outgoing radiation shows the effects of different greenhouse gases.]]
The Earth and its atmosphere emit ''longwave radiation'', also known as ''thermal infrared'' or ''terrestrial radiation''.<ref name="IPCC" />{{rp|2251}} Informally, longwave radiation is sometimes called ''thermal radiation''. [[Outgoing longwave radiation]] (OLR) is the radiation from Earth and its atmosphere that passes through the atmosphere and into space.

The greenhouse effect can be directly seen in graphs of Earth's outgoing longwave radiation as a function of frequency (or wavelength). The area between the curve for longwave radiation emitted by Earth's surface and the curve for outgoing longwave radiation indicates the size of the greenhouse effect.<ref name="Schmidt2010"/>

Different substances are responsible for reducing the radiation energy reaching space at different frequencies; for some frequencies, multiple substances play a role.<ref name="Schmidt2010paper">{{cite journal |last1=Schmidt |first1=G. A. |last2=Ruedy |first2=R. A. |last3=Miller |first3=R. L. |last4=Lacis |first4=A. A. |title=Attribution of the present-day total greenhouse effect |journal=Journal of Geophysical Research |date=2010 |volume=115 |issue=D20 |doi=10.1029/2010JD014287 |bibcode=2010JGRD..11520106S |url=https://pubs.giss.nasa.gov/docs/2010/2010_Schmidt_sc05400j.pdf |access-date=14 June 2023}}</ref> Carbon dioxide is understood to be responsible for the dip in outgoing radiation (and associated rise in the greenhouse effect) at around 667&nbsp;cm<sup>−1</sup> (equivalent to a wavelength of 15 microns).<ref name="wijng">{{cite journal |last1=van Wijngaarden |first1=W. A. |last2=Happer |first2=W. |title=Dependence of Earth's Thermal Radiation on Five Most Abundant Greenhouse Gases |journal=Atmospheric and Oceanic Physics |date=2020 |arxiv=2006.03098 |url=https://wvanwijngaarden.info.yorku.ca/files/2020/12/WThermal-Radiationf.pdf?x45936 |url-status=live |archive-url= https://web.archive.org/web/20230508080935/https://wvanwijngaarden.info.yorku.ca/files/2020/12/WThermal-Radiationf.pdf?x45936 |archive-date= 8 May 2023 }}</ref>

Each layer of the atmosphere with greenhouse gases absorbs some of the longwave radiation being radiated upwards from lower layers. It also emits longwave radiation in all directions, both upwards and downwards, in equilibrium with the amount it has absorbed. This results in less radiative heat loss and more warmth below. Increasing the concentration of the gases increases the amount of absorption and emission, and thereby causing more heat to be retained at the surface and in the layers below.<ref name="IPCC4_ch012" />

=== Effective temperature ===
[[File:Effective temperature for climate thermal radiation.svg|thumb|Temperature needed to emit a given amount of thermal radiation.]]
The power of outgoing longwave radiation emitted by a planet corresponds to the ''[[effective temperature]]'' of the planet. The effective temperature is the temperature that a planet radiating with a uniform temperature (a [[blackbody]]) would need to have in order to radiate the same amount of energy.

This concept may be used to compare the amount of longwave radiation emitted to space and the amount of longwave radiation emitted by the surface:

* ''Emissions to space:'' Based on its emissions of longwave radiation to space, Earth's overall ''effective temperature'' is {{convert|-18|C}}.<ref>{{cite web |url=http://eesc.columbia.edu/courses/ees/climate/lectures/radiation/ |title=Solar Radiation and the Earth's Energy Balance |publisher=Eesc.columbia.edu |access-date=15 October 2010 |archive-date=17 July 2012 |archive-url=https://archive.today/20120717025320/http://eesc.columbia.edu/courses/ees/climate/lectures/radiation/ |url-status=dead }}</ref><ref name="IPCC4_ch012"/>
* ''Emissions from surface:'' Based on thermal emissions from the surface, Earth's ''effective surface temperature'' is about {{convert|16|C}},<ref name="ipcc-ar6wg1-ch7" />{{rp|934}} which is {{convert|34|C-change}} warmer than Earth's overall effective temperature.

Earth's surface temperature is often reported in terms of the average near-surface air temperature. This is about {{convert|15|C}},<ref name="clchgtrack"/><ref>The elusive "absolute surface air temperature", see [http://data.giss.nasa.gov/gistemp/abs_temp.html GISS discussion] {{Webarchive|url=https://web.archive.org/web/20150905093320/http://data.giss.nasa.gov/gistemp/abs_temp.html |date=5 September 2015 }}</ref> a bit lower than the effective surface temperature. This value is {{convert|33|C-change}} warmer than Earth's overall effective temperature.

=== Energy flux ===
Energy [[flux]] is the rate of energy flow per unit area. Energy flux is expressed in units of W/m<sup>2</sup>, which is the number of [[joule]]s of energy that pass through a square meter each second. Most fluxes quoted in high-level discussions of climate are global values, which means they are the total flow of energy over the entire globe, divided by the surface area of the Earth, {{convert|5.1e14|m2|km2 mi2|abbr=on}}.<ref name="utarea">{{cite web |title=What is the Surface Area of the Earth? |url=https://www.universetoday.com/25756/surface-area-of-the-earth/ |website=Universe Today |date=11 February 2017 |access-date=1 June 2023}}</ref>

The fluxes of radiation arriving at and leaving the Earth are important because [[radiative transfer]] is the only process capable of exchanging energy between Earth and the rest of the universe.<ref name="Wallace2006"/>{{rp|145}}

=== Radiative balance ===

The temperature of a planet depends on the [[Earth's energy budget|balance]] between incoming radiation and outgoing radiation. If incoming radiation exceeds outgoing radiation, a planet will warm. If outgoing radiation exceeds incoming radiation, a planet will cool. A planet will tend towards a state of [[radiative equilibrium]], in which the power of outgoing radiation equals the power of absorbed incoming radiation.<ref name="cimss">{{cite web |title=Earth's Radiation Balance |url=https://cimss.ssec.wisc.edu/wxwise/homerbe.html |website=CIMSS: University of Wisconsin |access-date=25 April 2023}}</ref>

Earth's [[Earth's energy budget|energy imbalance]] is the amount by which the power of incoming sunlight absorbed by Earth's surface or atmosphere exceeds the power of outgoing longwave radiation emitted to space. Energy imbalance is the fundamental measurement that drives surface temperature.<ref name="SciAmEEB">{{cite web |title=Don't Worry about CO2, Worry about the Earth's 'Energy Balance' |url=https://www.scientificamerican.com/article/dont-worry-about-co2-worry-about-the-earths-energy-balance/ |publisher=Scientific American |access-date=2 June 2023}}</ref> A [[United Nations|UN]] presentation says "The EEI is the most critical number defining the prospects for continued global warming and climate change."<ref name="UNeei"/> One study argues, "The absolute value of EEI represents the most fundamental metric defining the status of global climate change."<ref name="vsh2016">{{cite journal |last1=von Schuckmann |first1=K. |last2=Palmer |first2=M. |last3=Trenberth |first3=K. |last4=Cazenave |first4=A. |last5=Chambers |first5=D. |last6=Champollion |first6=N. |last7=Hansen |first7=J. |last8=Josey |first8=S. A. |last9=Loeb |first9=N. |last10=Mathiew |first10=P.P. |last11=Meyssignac |first11=B. |last12=Wild |first12=M. |title=An imperative to monitor Earth's energy imbalance |journal=Nature Climate Change |date=2016 |volume=6 |issue=2 |pages=138–144 |doi=10.1038/nclimate2876|bibcode=2016NatCC...6..138V |url=http://nora.nerc.ac.uk/id/eprint/512751/1/vonSchuckmannPostprint.pdf }}</ref> 

Earth's energy imbalance (EEI) was about 0.7 W/m{{sup|2}} as of around 2015, indicating that Earth as a whole is accumulating thermal energy and is in a process of becoming warmer.<ref name="ipcc-ar6wg1-ch7" />{{rp|934}} 

Over 90% of the retained energy goes into warming the oceans, with much smaller amounts going into heating the land, atmosphere, and ice.<ref>{{cite web |last1=Hawkins |first1=Ed |title=Earth's energy imbalance |url=https://www.climate-lab-book.ac.uk/2016/earths-energy-imbalance/ |website=Climate Lab Book |access-date=16 July 2023 |date=27 January 2016}}</ref>
[[File:Outgoing radiation with and without Greenhouse effect.svg|thumb|upright=2.5|Comparison of Earth's upward flow of longwave radiation in reality and in a hypothetical scenario in which greenhouse gases and clouds are removed or lose their ability to absorb longwave radiation—without changing Earth's albedo (i.e., reflection/absorption of sunlight). Top shows the balance between Earth's heating and cooling as measured at the top of the atmosphere (TOA). Panel (a) shows the real situation with an active greenhouse effect.<ref name="rrtmeeb">{{cite web |title=RRTM Earth's Energy Budget |url=http://climatemodels.uchicago.edu/rrtm/index.html |publisher=University of Chicago |access-date=9 June 2023}}</ref> Panel (b) shows the situation immediately after absorption stops; all longwave radiation emitted by the surface would reach space; there would be more cooling (via longwave radiation emitted to space) than warming (from sunlight). This imbalance would lead to a rapid temperature drop. Panel (c) shows the final stable steady state, after the surface cools sufficiently to emit only enough longwave radiation to match the energy flow from absorbed sunlight.<ref name="rrtmeeb"/>]]

=== Day and night cycle ===
A simple picture assumes a steady state, but in the real world, the day/night ([[diurnal cycle|diurnal]]) cycle, as well as the seasonal cycle and weather disturbances, complicate matters. Solar heating applies only during daytime. At night the atmosphere cools somewhat, but not greatly because the [[thermal inertia]] of the climate system resists changes both day and night, as well as for longer periods.<ref>{{cite web |url=https://earthobservatory.nasa.gov/features/HeatBucket/heatbucket4.php |title=Earth's Big Heat Bucket - Bad News, Good News |publisher=NASA Earth Observatory |author=Michon Scott |date=24 April 2006 |access-date=4 December 2022 |archive-date=4 December 2022 |archive-url=https://web.archive.org/web/20221204153806/https://earthobservatory.nasa.gov/features/HeatBucket/heatbucket4.php |url-status=live }}</ref> [[Diurnal temperature variation|Diurnal temperature changes]] decrease with height in the atmosphere.

== Effect of lapse rate ==
=== Lapse rate ===
{{Further|Lapse rate}}
In the lower portion of the atmosphere, the [[troposphere]], the air temperature decreases (or "lapses") with increasing altitude. The rate at which temperature changes with altitude is called the ''[[lapse rate]]''.<ref name="Nugent5">{{cite web |last1=Nugent |first1=Alison |last2=DeCou |first2=David |title=ATMOSPHERIC PROCESSES AND PHENOMENA: Chapter 5: Atmospheric Stability |url=http://pressbooks-dev.oer.hawaii.edu/atmo/chapter/chapter-5-atmospheric-stability/ |access-date=31 May 2023}}</ref>

On Earth, the air temperature decreases by about 6.5°C/km (3.6°F per 1000&nbsp;ft), on average, although this varies.<ref name="Nugent5" />

The temperature lapse is caused by [[convection]]. Air warmed by the surface rises. As it rises, air [[adiabatic process|expands and cools]].  Simultaneously, other air descends, compresses, and warms. This process creates a vertical temperature gradient within the atmosphere.<ref name="Nugent5" />

This vertical temperature gradient is essential to the greenhouse effect. If the lapse rate was zero (so that the atmospheric temperature did not vary with altitude and was the same as the surface temperature) then there would be no greenhouse effect (i.e., its value would be zero).<ref name="ThomasStamnes1999">{{cite book |last1=Thomas |first1=Gary E. |last2=Stamnes |first2=Knut |title=Radiative Transfer in the Atmosphere and Ocean |date=1999 |publisher=Cambridge University Press |isbn=0-521-40124-0}}</ref>

=== Emission temperature and altitude ===

[[File:Spectral OLR.png|thumb|upright=1.35|The temperature at which thermal radiation was emitted can be determined by comparing the intensity at a particular wavenumber to the intensity of a [[Planck's law|black-body emission curve]]. In the chart, emission temperatures range between T<sub>min</sub> and T<sub>s</sub>. "Wavenumber" is frequency divided by the speed of light).]]
Greenhouse gases make the atmosphere near Earth's surface mostly opaque to longwave radiation. The atmosphere only becomes transparent to longwave radiation at higher altitudes, where the air is less dense, there is less water vapor, and reduced [[Spectral line|pressure broadening]] of absorption lines limits the wavelengths that gas molecules can absorb.<ref name="Plass1950">{{cite journal |last1=Strong |first1=J. |last2=Plass |first2=G. N. |date=1950 |title=The Effect of Pressure Broadening of Spectral Lines on Atmospheric Temperature |url=https://ui.adsabs.harvard.edu/abs/1950ApJ...112..365S/abstract#:~:text=Pressure%20broadening%20causes%20lines%20in,absorbed%20by%20the%20upper%20layers. |journal=Astrophysical Journal |volume=112 |page=365 |bibcode=1950ApJ...112..365S |doi=10.1086/145352}}</ref><ref name="Wallace2006" />

For any given wavelength, the longwave radiation that reaches space is emitted by a particular ''radiating layer'' of the atmosphere. The intensity of the emitted radiation is determined by the weighted average air temperature within that layer. So, for any given wavelength of radiation emitted to space, there is an associated ''effective emission temperature'' (or [[brightness temperature]]).<ref name="Pierrehumbert2011">{{cite web |last1=Pierrehumbert |first1=R. T. |date=January 2011 |title=Infrared radiation and planetary temperature |url=https://geosci.uchicago.edu/~rtp1/papers/PhysTodayRT2011.pdf |website=Physics Today |publisher=American Institute of Physics |pages=33–38}}</ref><ref name="Wallace2006" />

A given wavelength of radiation may also be said to have an ''effective emission altitude'', which is a weighted average of the altitudes within the radiating layer.

The effective emission temperature and altitude vary by wavelength (or frequency). This phenomenon may be seen by examining plots of radiation emitted to space.<ref name="Pierrehumbert2011" />

=== Greenhouse gases and the lapse rate ===
[[File:Greenhouse_Effect_Overview.svg|thumb|upright=1.35|Greenhouse gases (GHGs) in dense air near the surface intercept most of the longwave radiation emitted by the warm surface. GHGs in sparse air at higher altitudes—cooler because of the environmental [[lapse rate]]—emit longwave radiation to space at a lower rate than surface emissions.]]
Earth's surface radiates longwave radiation with wavelengths in the range of 4–100 microns.<ref name="Mitchell-1989">{{cite journal|doi=10.1029/RG027i001p00115|last=Mitchell|first=John F. B.|date=1989|title=The "Greenhouse" effect and Climate Change|journal=Reviews of Geophysics|volume=27|issue=1|pages=115–139|url=http://astrosun2.astro.cornell.edu/academics/courses/astro202/Mitchell_GRL89.pdf|access-date=23 March 2008|bibcode=1989RvGeo..27..115M|citeseerx=10.1.1.459.471|archive-date=15 June 2011|archive-url=https://web.archive.org/web/20110615024949/http://astrosun2.astro.cornell.edu/academics/courses/astro202/Mitchell_GRL89.pdf|url-status=live}}</ref> Greenhouse gases that were largely transparent to incoming solar radiation are more absorbent for some wavelengths in this range.<ref name="Mitchell-1989" />

The atmosphere near the Earth's surface is largely opaque to longwave radiation and most heat loss from the surface is by [[evaporation]] and [[convection]]. However radiative energy losses become increasingly important higher in the atmosphere, largely because of the decreasing concentration of water vapor, an important greenhouse gas.

Rather than thinking of longwave radiation headed to space as coming from the surface itself, it is more realistic to think of this outgoing radiation as being emitted by a layer in the mid-[[troposphere]], which is effectively coupled to the surface by a [[lapse rate]]. The difference in temperature between these two locations explains the difference between surface emissions and emissions to space, i.e., it explains the greenhouse effect.<ref>{{cite web |url=https://www.e-education.psu.edu/meteo469/node/198 |title=METEO 469: From Meteorology to Mitigation - Understanding Global Warming - Lesson 5 - Modelling of the Climate System - One-Layer Energy Balance Model |publisher=[[Penn State]] College of Mineral and Earth Sciences - Department of Meteorology and Atmospheric Sciences |last1=Mann |first1=Michael |last2=Gaudet |first2=Brian |access-date=4 November 2022 |archive-date=31 October 2022 |archive-url=https://web.archive.org/web/20221031022823/https://www.e-education.psu.edu/meteo469/node/198 |url-status=live }}</ref><ref name="Tziperman2022">{{cite book |last1=Tziperman |first1=Eli |title=Global Warming Science |date=2022 |publisher=Princeton University Press |isbn=9780691228808}}</ref>

== Infrared absorbing constituents in the atmosphere ==

=== Greenhouse gases ===
{{Main|Greenhouse gas}}A greenhouse gas (GHG) is a gas which contributes to the trapping of heat by impeding the flow of longwave radiation out of a planet's atmosphere. Greenhouse gases contribute most of the greenhouse effect in [[Earth's energy budget]].<ref name="IPCC" />

==== Infrared active gases ====
Gases which can absorb and emit longwave radiation are said to be ''infrared active''<ref name="Archer2011Ch4">{{cite book |last1=Archer |first1=David |title=Global Warming: Understanding the Forecast, Chapter 4: Greenhouse Gases |date=2011 |publisher=Wiley |isbn=978-0470943410 |edition=2 |url=http://forecast.uchicago.edu/chapter4.pdf |access-date=14 June 2023}}</ref> and act as greenhouse gases.

Most gases whose molecules have two different atoms (such as carbon monoxide, {{chem|CO}}), and all gases with three or more atoms (including {{H2O-nl}} and {{CO2}}), are infrared active and act as greenhouse gases. (Technically, this is because when these molecules [[molecular vibration|vibrate]], those vibrations modify the molecular [[Bond dipole moment|dipole moment]], or asymmetry in the distribution of electrical charge.   See [[Infrared spectroscopy#Number of vibrational modes|Infrared spectroscopy]].)<ref name="IPCC" />

Gases with only one atom (such as argon, Ar) or with two identical atoms (such as nitrogen, {{chem|N|2}}, and oxygen, {{chem|O|2}}) are not infrared active. They are transparent to longwave radiation, and, for practical purposes, do not absorb or emit longwave radiation. (This is because their molecules are symmetrical and so do not have a dipole moment.) Such gases make up more than 99% of the dry atmosphere.<ref name="IPCC" />

==== Absorption and emission ====
[[File:Longwave Absorption Coefficients of H2O and CO2.svg|thumb|upright=1.35|[[Outgoing longwave radiation|Longwave]] [[absorption coefficient]]s of water vapor and carbon dioxide. For wavelengths near 15 microns (15 ''μ''m in top scale), where Earth's surface emits strongly, CO<sub>2</sub> is a much stronger absorber than water vapor.]]Greenhouse gases absorb and emit longwave radiation within specific ranges of wavelengths (organized as [[spectral line]]s or [[absorption band|bands]]).<ref name="IPCC" />

When greenhouse gases absorb radiation, they distribute the acquired energy to the surrounding air as thermal energy (i.e., kinetic energy of gas molecules). Energy is transferred from greenhouse gas molecules to other molecules via [[elastic collision|molecular collisions]].<ref name="uaATMO336eeb" />

Contrary to what is sometimes said, greenhouse gases do not "re-emit" photons after they are absorbed. Because each molecule experiences billions of collisions per second, any energy a greenhouse gas molecule receives by absorbing a photon will be redistributed to other molecules before there is a chance for a new photon to be emitted.<ref name="uaATMO336eeb" />

In a separate process, greenhouse gases emit longwave radiation, at a rate determined by the air temperature. This thermal energy is either absorbed by other greenhouse gas molecules or leaves the atmosphere, cooling it.<ref name="uaATMO336eeb" />

==== Radiative effects ====
''Effect on air:'' Air is warmed by [[latent heat]] ([[virtual temperature|buoyant water vapor]] condensing into water droplets and releasing heat), [[thermal]]s (warm air rising from below), and by sunlight being absorbed in the atmosphere.<ref name="budget" /> Air is cooled radiatively, by greenhouse gases and clouds emitting longwave thermal radiation. Within the [[troposphere]], greenhouse gases typically have a net cooling effect on air, emitting more thermal radiation than they absorb. Warming and cooling of air are well balanced, on average, so that the atmosphere maintains a roughly stable average temperature.<ref name="Wallace2006">{{cite book |last1=Wallace |first1=J. M. |last2=Hobbs |first2=P. V. |title=Atmospheric Science |date=2006 |publisher=Academic Press |isbn=978-0-12-732951-2 |edition=2}}</ref>{{rp|139}}<ref name="Manabe1964">{{cite journal |last1=Manabe |first1=S. |last2=Strickler |first2=R. F. |title=Thermal Equilibrium of the Atmosphere with a Convective Adjustment |journal=J. Atmos. Sci. |date=1964 |volume=21 |issue=4 |pages=361–385 |doi=10.1175/1520-0469(1964)021<0361:TEOTAW>2.0.CO;2|doi-access=free |bibcode=1964JAtS...21..361M }}</ref>

''Effect on surface cooling:'' Longwave radiation flows both upward and downward due to absorption and emission in the atmosphere. These canceling energy flows reduce radiative surface cooling (net upward radiative energy flow). Latent heat transport and thermals provide non-radiative surface cooling which partially compensates for this reduction, but there is still a net reduction in surface cooling, for a given surface temperature.<ref name="Wallace2006" />{{rp|139}}<ref name="Manabe1964" />

''Effect on TOA energy balance:'' Greenhouse gases impact the top-of-atmosphere (TOA) energy budget by reducing the flux of longwave radiation emitted to space, for a given surface temperature. Thus, greenhouse gases alter the energy balance at TOA. This means that the surface temperature needs to be higher (than the planet's ''effective temperature'', i.e., the temperature associated with emissions to space), in order for the outgoing energy emitted to space to balance the incoming energy from sunlight.<ref name="Wallace2006" />{{rp|139}}<ref name="Manabe1964" /> It is important to focus on the top-of-atmosphere (TOA) energy budget (rather than the surface energy budget) when reasoning about the warming effect of greenhouse gases.<ref name="PierrehumbertTextbook" />{{rp|414}}
[[File:Atmospheric heat flow profile.svg|thumb|upright=2.5|Flow of heat in Earth's atmosphere, showing (a) upward radiation heat flow and up/down radiation fluxes, (b) upward non-radiative heat flow ([[latent heat]] and [[thermals]]), (c) the balance between atmospheric heating and cooling at each altitude, and (d) the atmosphere's temperature profile.]]

=== Clouds and aerosols ===
{{See also|Cloud feedback|Cloud forcing}}
Clouds and aerosols have both cooling effects, associated with reflecting sunlight back to space, and warming effects, associated with trapping thermal radiation.

On average, clouds have a strong net cooling effect. However, the mix of cooling and warming effects varies, depending on detailed characteristics of particular clouds (including their  type, height, and optical properties).<ref name="GFDLcre">{{cite web |title=Cloud Radiative Effect |url=https://www.gfdl.noaa.gov/cloud-radiative-effect/ |publisher=NOAA Geophysical Fluid Dynamics Laboratory |access-date=15 June 2023}}</ref> Thin cirrus clouds can have a net warming effect. Clouds can absorb and emit infrared radiation and thus affect the radiative properties of the atmosphere.<ref>{{Cite journal |last=Liou |first=Kuo-Nan |date=1 June 1986 |title=Influence of Cirrus Clouds on Weather and Climate Processes: A Global Perspective |journal=Monthly Weather Review |language=EN |volume=114 |issue=6 |pages=1167–1199 |doi=10.1175/1520-0493(1986)114<1167:IOCCOW>2.0.CO;2 |bibcode=1986MWRv..114.1167L |issn=1520-0493 |doi-access=free }}</ref>

{{excerpt|Particulates#Climate effects|file=no}}

== Basic formulas ==

=== Effective temperature ===

A given flux of thermal radiation has an associated ''effective radiating temperature'' or ''[[effective temperature]]''. Effective temperature is the temperature that a [[black body]] (a perfect absorber/emitter) would need to be to emit that much thermal radiation.<ref name="mbta">{{cite web |title=Box 2: Solar and Earth Radiation and the Greenhouse Effect (adapted from Mackenzie, 2003) |url=http://www.soest.hawaii.edu/mguidry/Unnamed_Site_2/Chapter%202/Figures/Box%202%20Solar%20and%20Earth%20Radiation%20Greenhouse%20Effect.pdf |website=Myron B. Thompson Academy |date=2006}}</ref> Thus, the overall effective temperature of a planet is given by

:<math>T_\mathrm{eff} = (\mathrm{OLR}/\sigma)^{1/4}</math>

where OLR is the average flux (power per unit area) of outgoing longwave radiation emitted to space and <math>\sigma</math> is the [[Stefan-Boltzmann constant]]. Similarly, the effective temperature of the surface is given by

:<math>T_\mathrm{surface,eff} = (\mathrm{SLR}/\sigma)^{1/4}</math>

where SLR is the average flux of longwave radiation emitted by the surface. (OLR is a conventional abbreviation. SLR is used here to denote the flux of surface-emitted longwave radiation, although there is no standard abbreviation for this.)<ref name="Haberle2013" />

=== Metrics for the greenhouse effect ===
[[File:Greenhouse Effect metrics time series.svg|thumb|upright=1.35|Increase in the Earth's greenhouse effect (2000-2022) based on NASA CERES satellite data.]]

The IPCC reports the ''greenhouse effect'', {{mvar|G}}, as being 159 W m{{sup|-2}}, where {{mvar|G}} is the flux of longwave thermal radiation that leaves the surface minus the flux of outgoing longwave radiation that reaches space:<ref name="ipcc-ar6wg1-ch7" />{{rp|968}}<ref name="ravram1" /><ref name="Schmidt2010" /><ref name="Schmidt2010paper" />

:<math>G = \mathrm{SLR} - \mathrm{OLR}\;.</math>

Alternatively, the greenhouse effect can be described using the ''normalized greenhouse effect'', {{mvar|g̃}}, defined as

:<math>\tilde g = G/\mathrm{SLR} = 1 - \mathrm{OLR}/\mathrm{SLR}\;.</math>

The normalized greenhouse effect is ''the fraction of the amount of thermal radiation emitted by the surface that does not reach space''.
Based on the IPCC numbers, {{mvar|g̃}} = 0.40. In other words, 40 percent less thermal radiation reaches space than what leaves the surface.<ref name="ipcc-ar6wg1-ch7">{{cite book |title=Climate Change 2021: The Physical Science Basis |date=2021 |publisher=IPCC |url=https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter07.pdf |access-date=24 April 2023 |chapter=Chapter 7: The Earth's Energy Budget, Climate Feedbacks, and Climate Sensitivity}}</ref>{{rp|968}}<ref name="ravram1">{{cite journal |last1=Raval |first1=A. |last2=Ramanathan |first2=V. |title=Observational determination of the greenhouse effect |journal=Nature |date=1989 |volume=342 |issue=6251 |pages=758–761 |doi=10.1038/342758a0 |bibcode=1989Natur.342..758R |s2cid=4326910 |url=https://www.nature.com/articles/342758a0}}</ref><ref name="ravram2">{{cite journal |last1=Raval |first1=A. |last2=Ramanathan |first2=V. |title=Observational determination of the greenhouse effect |journal=Global Climate Feedbacks: Proceedings of the Brookhaven National Laboratory Workshop |date=1990 |pages=5–16 |url=https://www.osti.gov/servlets/purl/6440147-fP92Pd/#page=10 |access-date=24 April 2023}}</ref>

Sometimes the greenhouse effect is quantified as a temperature difference. This temperature difference is closely related to the quantities above.

When the greenhouse effect is expressed as a temperature difference, <math>\Delta T_\mathrm{GHE}</math>, this refers to the effective temperature associated with thermal radiation emissions from the surface minus the effective temperature associated with emissions to space:

:<math>\Delta T_\mathrm{GHE} = T_\mathrm{surface,eff} - T_\mathrm{eff}</math>
:<math>\Delta T_\mathrm{GHE} = \left(\mathrm{SLR}/\sigma\right)^{1/4} - \left(\mathrm{OLR}/\sigma\right)^{1/4}</math>

Informal discussions of the greenhouse effect often compare the actual surface temperature to the temperature that the planet would have if there were no greenhouse gases. However, in formal technical discussions, when the size of the greenhouse effect is quantified as a temperature, this is generally done using the above formula. The formula refers to the effective surface temperature rather than the actual surface temperature, and compares the surface with the top of the atmosphere, rather than comparing reality to a hypothetical situation.<ref name="Haberle2013">{{cite journal |last1=Haberle |first1=Robert M. |title=Estimating the power of Mars' greenhouse effect |journal=Icarus |date=2013 |volume=223 |issue=1 |pages=619–620 |doi=10.1016/j.icarus.2012.12.022|bibcode=2013Icar..223..619H }}</ref>

The temperature difference, <math>\Delta T_\mathrm{GHE}</math>, indicates how much warmer a planet's surface is than the planet's overall effective temperature.

===Radiative balance===
{{Further|Earth's energy budget}}
[[File:Earth Energy Budget with GHE.svg|thumb|upright=1.35|The greenhouse effect can be understood as a decrease in the efficiency of planetary cooling. The greenhouse effect is quantified as the portion of the radiation flux emitted by the surface minus that doesn't reach space, i.e., 40% or 159 W/m<sup>2</sup>. Some emitted radiation is effectively cancelled out by downwelling radiation and so does not [[Heat transfer#Radiation|transfer heat]]. Evaporation and convection partially compensate for this reduction in surface cooling. Low temperatures at high altitudes limit the rate of thermal emissions to space.]]
Earth's top-of-atmosphere (TOA) [[Earth's energy budget|energy imbalance]] (EEI) is the amount by which the power of incoming radiation exceeds the power of outgoing radiation:<ref name="UNeei">{{cite web |title=The Earth's Energy Imbalance: Where does the energy go? |url=https://unfccc.int/sites/default/files/resource/EID%20Pres%20T1%20Mercator%20EEI.pdf |publisher=United Nations Climate Change |access-date=14 June 2023}}</ref>

:<math>\mathrm{EEI} = \mathrm{ASR} -\mathrm{OLR}</math>

where ASR is the mean flux of absorbed solar radiation. ASR may be expanded as

:<math>\mathrm{ASR} = (1-A) \,\mathrm{MSI}</math>

where <math>A</math> is the [[albedo]] (reflectivity) of the planet and MSI is the [[solar irradiance|mean solar irradiance]] incoming at the top of the atmosphere.

The [[Planetary equilibrium temperature|radiative equilibrium temperature]] of a planet can be expressed as

:<math>T_\mathrm{radeq} = (\mathrm{ASR}/\sigma)^{1/4} = \left[(1-A)\,\mathrm{MSI}/\sigma  \right]^{1/4} \;.</math>
 
A planet's temperature will tend to shift towards a state of radiative equilibrium, in which the TOA energy imbalance is zero, i.e., <math>\mathrm{EEI} = 0</math>. When the planet is in radiative equilibrium, the overall effective temperature of the planet is given by

:<math>T_\mathrm{eff} = T_\mathrm{radeq}\;.</math>

Thus, the concept of radiative equilibrium is important because it indicates what effective temperature a planet will tend towards having.<ref name="ACSPredPlanTemp">{{cite web |title=Predicted Planetary Temperatures |url=https://www.acs.org/climatescience/energybalance/predictedplanetarytemperatures.html |website=ACS Climate Science Toolkit |publisher=American Chemical Society |archive-url=https://web.archive.org/web/20230326030348/https://www.acs.org/climatescience/energybalance/planetarytemperatures.html| archive-date=26 March 2023|access-date=14 June 2023}}</ref><ref name="rrtmeeb" />

If, in addition to knowing the effective temperature, <math>T_\mathrm{eff}</math>, we know the value of the greenhouse effect, then we know the mean (average) surface temperature of the planet.

This is why the quantity known as the greenhouse effect is important: it is one of the few quantities that go into determining the planet's mean surface temperature.

=== Greenhouse effect and temperature ===

Typically, a planet will be close to radiative equilibrium, with the rates of incoming and outgoing energy being well-balanced. Under such conditions, the planet's equilibrium temperature is determined by the mean solar irradiance and the planetary albedo (how much sunlight is reflected back to space instead of being absorbed).

The greenhouse effect measures how much warmer the surface is than the overall effective temperature of the planet. So, the effective surface temperature, <math>T_\mathrm{surface,eff}</math>, is, using the definition of <math>\Delta T_\mathrm{GHE}</math>,

:<math>T_\mathrm{surface,eff} = T_\mathrm{eff} + \Delta T_\mathrm{GHE} \;.</math>

One could also express the relationship between <math>T_\mathrm{surface,eff}</math> and <math>T_\mathrm{eff}</math> using {{mvar|''G''}} or {{mvar|''g̃''}}.

So, the principle that a larger greenhouse effect corresponds to a higher surface temperature, if everything else (i.e., the factors that determine <math>T_\mathrm{eff}</math>) is held fixed, is true as a matter of definition.

Note that the greenhouse effect influences the temperature of the planet as a whole, in tandem with the planet's tendency to move toward radiative equilibrium.<ref name="Modest2021" />

== Misconceptions ==
[[File:Earth's Overall Heat Flow.svg|thumb|upright=1.35|Earth's overall heat flow. Heat (net energy) ''always flows from warmer to cooler'', honoring the [[Second Law of Thermodynamics]].<ref name="2ndlawviol" /> (This heat flow diagram is equivalent to NASA's [[:File:The-NASA-Earth's-Energy-Budget-Poster-Radiant-Energy-System-satellite-infrared-radiation-fluxes.jpg|earth energy budget]] diagram. Data is from 2009.)]]

There are sometimes misunderstandings about how the greenhouse effect functions and raises temperatures.

The ''surface budget fallacy'' is a common error in thinking.<ref name="PierrehumbertTextbook" />{{rp|413}} It involves thinking that an increased {{CO2}} concentration could only cause warming by increasing the downward thermal radiation to the surface, as a result of making the atmosphere a better emitter. If the atmosphere near the surface is already nearly opaque to thermal radiation, this would mean that increasing {{CO2}} could not lead to higher temperatures. However, it is a mistake to focus on the surface energy budget rather than the top-of-atmosphere energy budget. Regardless of what happens at the surface, increasing the concentration of {{CO2}} tends to reduce the thermal radiation reaching space (OLR), leading to a TOA energy imbalance that leads to warming. Earlier researchers like [[Guy Stewart Callendar|Callendar]] (1938) and [[Gilbert Plass|Plass]] (1959) focused on the surface budget, but the work of [[Syukuro Manabe|Manabe]] in the 1960s clarified the importance of the top-of-atmosphere energy budget.<ref name="PierrehumbertTextbook">{{cite book |last1=Pierrehumbert |first1=Raymond T. |title=Principles of Planetary Climate |date=2010 |publisher=Cambridge University Press |isbn=978-0-521-86556-2}}</ref>{{rp|414}}

Among those who do not believe in the greenhouse effect, there is a fallacy that the greenhouse effect involves greenhouse gases sending heat from the cool atmosphere to the planet's warm surface, in violation of the [[Second Law of Thermodynamics]].<ref name="2ndlawviol">{{cite web |title=Global Warming Violates the Second Law of Thermodynamics: Brief Responses to Climate Change Denialism Statements |url=https://www.geol.umd.edu/sgc/elevator/elevator34.html |publisher=University of Maryland |access-date=13 June 2023}}</ref><ref name="Halpern2010">{{cite journal |last1=Halpern |first1=J. B. |last2=Close |first2=C. M. |last3=Ho-Stuart |first3=C. |last4=Shore |first4=J. D. |last5=Smith |first5=A. P. |last6=Zimmermann |first6=J. |title=Comment on "falsification of the Atmospheric CO2 Greenhouse Effects Within the Frame of Physics" |journal=International Journal of Modern Physics B |date=2010 |volume=24 |issue=10 |pages=1309–1332 |doi=10.1142/S021797921005555X}}</ref> However, this idea reflects a misunderstanding. Radiation heat flow is the ''net energy flow'' after the flows of radiation in both directions have been taken into account.<ref name="Modest2021">{{cite book |last1=Modest |first1=Michael F. |title=Radiative Heat Transfer |date=2021 |publisher=Academic Press |isbn=978-0323984065 |edition=4}}</ref> Radiation heat flow occurs in the direction from the surface to the atmosphere and space,<ref name="budget" /> as is to be [[Second Law of Thermodynamics|expected]] given that the surface is warmer than the atmosphere and space. While greenhouse gases emit thermal radiation downward to the surface, this is part of the normal process of [[Heat transfer#Radiation|radiation heat transfer]].<ref name="Siegel1971">{{cite web |last1=Siegel |first1=R. |last2=Howell |first2=J. R. |title=Thermal Radiation Heat Transfer |url=https://ntrs.nasa.gov/api/citations/19710021465/downloads/19710021465.pdf |publisher=NASA |access-date=28 May 2023 |date=1971}}</ref> The downward thermal radiation simply reduces the upward thermal radiation net energy flow (radiation heat flow), i.e., it reduces cooling.<ref name="uaATMO336eeb">{{cite web |title=The Earth's Energy Budget II -- Radiation Emitted by the Earth, the Greenhouse Effect, and the Overall Energy Balance |url=http://www.atmo.arizona.edu/students/courselinks/spring17/atmo336s2/lectures/sec3/energybudget2.html |publisher=University of Arizona, Hydrology & Atmospheric Sciences |access-date=28 May 2023 |date=2017}}</ref>

== Simplified models ==
{{further|Idealized greenhouse model}}
[[File:Greenhouse Effect.svg|thumb|upright=1.35|right|Energy flows between space, the atmosphere, and Earth's surface, with greenhouse gases in the atmosphere absorbing and emitting radiant heat, affecting [[Earth's energy budget|Earth's energy balance]]. Data as of 2007.]]
Simplified models are sometimes used to support understanding of how the greenhouse effect comes about and how this affects surface temperature.

=== Atmospheric layer models ===
The greenhouse effect can be seen to occur in a [[idealized greenhouse model|simplified model]] in which the air is treated as if it is single uniform layer exchanging radiation with the ground and space.<ref name="acs-single">{{cite web |title=ACS Climate Science Toolkit - Atmospheric Warming - A Single-Layer Atmosphere Model |url=https://www.acs.org/climatescience/atmosphericwarming/singlelayermodel.html |accessdate=2 October 2022 |publisher=[[American Chemical Society]]}}</ref> Slightly more complex models add additional layers, or introduce convection.<ref name="acs-multi">{{cite web |title=ACS Climate Science Toolkit - Atmospheric Warming - A Multi-Layer Atmosphere Model |url=https://www.acs.org/content/acs/en/climatescience/atmosphericwarming/multilayermodel.html |accessdate=2 October 2022 |publisher=[[American Chemical Society]]}}</ref>

=== Equivalent emission altitude ===
One simplification is to treat all outgoing longwave radiation as being emitted from an altitude where the air temperature equals the overall effective temperature for planetary emissions, <math>T_\mathrm{eff}</math>.<ref name="Benestad2017">{{cite journal |last1=Benestad |first1=R. E. |date=2017 |title=A mental picture of the greenhouse effect |journal=Theor Appl Climatol |volume=128 |issue=3–4 |pages=679–688 |doi=10.1007/s00704-016-1732-y |s2cid=123923155 |doi-access=free}}</ref> Some authors have referred to this altitude as the ''effective radiating level'' (ERL), and suggest that as the {{CO2}} concentration increases, the ERL must rise to maintain the same mass of {{CO2}} above that level.<ref name="aosuw">{{cite web |title=Radiation Balance of the Earth-Atmosphere System |url=https://www.aos.wisc.edu/~aos121br/radn/radn/sld001.htm |access-date=1 June 2023 |website=Atmospheric and Oceanic Sciences |publisher=University of Wisconsin |quote=Based on the account in Horel & Geisler (1996) 'Global Environmental Change: An Atmospheric Perspective'}}</ref>

This approach is less accurate than accounting for variation in radiation wavelength by emission altitude. However, it can be useful in supporting a simplified understanding of the greenhouse effect.<ref name="Benestad2017" /> For instance, it can be used to explain how the greenhouse effect increases as the concentration of greenhouse gases increase.<ref name="Rasmus2016">{{cite web |last1=Benestad |first1=R. E. |date=2016 |title=What is the best description of the greenhouse effect? |url=https://www.realclimate.org/index.php/archives/2016/02/what-is-the-best-description-of-the-greenhouse-effect/ |access-date=1 June 2023 |website=RealClimate}}</ref><ref name="aosuw" /><ref name="Tziperman2022" />

Earth's overall equivalent emission altitude has been increasing with a trend of {{convert|23|m|abbr=on}}/decade, which is said to be consistent with a global mean surface warming of {{convert|0.12|C-change}}/decade over the period 1979–2011.<ref name="Benestad2017" />

== Related effects on Earth ==
===Negative greenhouse effect===
Scientists have observed that, at times, there is a negative greenhouse effect over parts of Antarctica.<ref name="hsch">{{Cite journal |last1=Schmithüsen |first1=Holger |last2=Notholt |first2=Justus |last3=König-Langlo |first3=Gert |last4=Lemke |first4=Peter |last5=Jung |first5=Thomas |date=16 December 2015 |title=How increasing {{CO2}} leads to an increased negative greenhouse effect in Antarctica |journal=Geophysical Research Letters |language=en |volume=42 |issue=23 |doi=10.1002/2015GL066749 |issn=0094-8276 |s2cid=131351000 |doi-access=free}}</ref><ref name="sejas">{{cite journal |last1=Sejas |first1=S.A. |last2=Taylor |first2=P. C. |last3=Cai |first3=M. |date=2018 |title=Unmasking the negative greenhouse effect over the Antarctic Plateau |journal=npj Clim Atmos Sci |volume=1 |issue=17 |page=17 |doi=10.1038/s41612-018-0031-y |pmc=7580794 |pmid=33102742|bibcode=2018npjCA...1...17S }}</ref> In a location where there is a strong temperature inversion, so that the air is warmer than the surface, it is possible for the greenhouse effect to be reversed, so that the presence of greenhouse gases increases the rate of radiative cooling to space. In this case, the rate of thermal radiation emission to space is greater than the rate at which thermal radiation is emitted by the surface. Thus, the local value of the greenhouse effect is negative. 

===Runaway greenhouse effect===
{{excerpt|Runaway greenhouse effect#Distant future|paragraphs=1|file=no}}

== Bodies other than Earth ==
{| class="wikitable"
|+ Greenhouse effect on different celestial bodies<ref name="ACSplanet">{{cite web |title=Atmospheres and Planetary Temperatures |url=https://www.acs.org/climatescience/energybalance/planetarytemperatures.html |access-date=29 May 2023 |publisher=ACS}}</ref><ref name="McKay1991">{{Cite journal |last1=McKay |first1=C. P. |last2=Pollack |first2=J. B. |last3=Courtin |first3=R. |date=6 September 1991 |title=The greenhouse and antigreenhouse effects on Titan |url=https://wray.eas.gatech.edu/physicsplanets2012/Assignments/McKay_etal1991_TitanAntiGreenhouse.pdf |journal=Science |language=en |volume=253 |issue=5024 |pages=1118–1121 |bibcode=1991Sci...253.1118M |doi=10.1126/science.11538492 |issn=0036-8075 |pmid=11538492 |s2cid=10384331 |s2cid-access=free |url-status=live |archive-url=https://web.archive.org/web/20230629174917/https://wray.eas.gatech.edu/physicsplanets2012/Assignments/McKay_etal1991_TitanAntiGreenhouse.pdf |archive-date= 29 June 2023 }}</ref><ref name="nasatitan">{{cite web |title=Titan |url=https://solarsystem.nasa.gov/moons/saturn-moons/titan/in-depth/#:~:text=Titan%27s%20atmosphere%20is%20mostly%20nitrogen,accelerated%20in%20Saturn%27s%20magnetic%20field. |access-date=29 May 2023 |publisher=NASA}}</ref>
|-
! !!Venus !! Earth !! Mars !! Titan
|-
| Surface temperature, <math>T_\mathrm{observed}</math> || {{convert|735|K|C F|0|abbr=on}} || {{convert|288|K|C F|0|abbr=on}} ||{{convert|215|K|C F|0|abbr=on}} || {{convert|94|K|C F|0|abbr=on}}
|-
| Greenhouse effect, <math>\Delta T}_\mathrm{GHE</math>|| {{convert|503|K-change}} || {{convert|33|K-change}} || {{convert|6|K-change}} || {{convert|21|K-change}} GHE;<br />{{convert|12|K-change}} GHE+[[Anti-greenhouse effect|AGHE]]
|-
| Pressure || 92 atm || 1 atm || 0.0063 atm || 1.5 atm
|-
| Primary gases || {{CO2}} (0.965)<br /> N{{sub|2}} (0.035) || N{{sub|2}} (0.78)<br /> O{{sub|2}} (0.21)<br />Ar (0.009) || {{CO2}} (0.95)<br />N{{sub|2}} (0.03)<br />Ar (0.02) || N{{sub|2}} (0.95)<br /> {{CH4}} (~0.05)
|-
| Trace gases || {{SO2}}, Ar || {{H2O-nl}}, {{CO2}} || O{{sub|2}}, CO || H{{sub|2}}
|-
| Planetary effective temperature, <math>T_\mathrm{eff}</math> || {{convert|232|K|C F|0|abbr=on}} || {{convert|255|K|C F|0|abbr=on}} || {{convert|209|K|C F|0|abbr=on}} || 73 K [[tropopause]];<br /> 82 K [[stratopause]]
|-
| Greenhouse effect, <math>G</math>|| {{Val|16000}} W/m{{sup|2}} || 150 W/m{{sup|2}} || 13 W/m{{sup|2}} || 2.8 W/m{{sup|2}} GHE;<br /> 1.9 W/m{{sup|2}} GHE+AGHE
|-
| Normalized greenhouse effect, <math>\tilde g</math>|| 0.99|| 0.39 || 0.11 || 0.63 GHE;<br /> 0.42 GHE+AGHE
|}

In the solar system, apart from the Earth, at least two other planets and a moon also have a greenhouse effect.

===Venus===
The greenhouse effect on [[Venus]] is particularly large, and it brings the surface temperature to as high as {{convert|735|K|C F|abbr=on}}. This is due to its very dense atmosphere which consists of about 97% carbon dioxide.<ref name="McKay1991"/>

Although Venus is about 30% closer to the Sun, it absorbs (and is warmed by) ''less sunlight'' than Earth, because Venus reflects 77% of incident sunlight while Earth reflects around 30%. In the absence of a greenhouse effect, the surface of Venus would be expected to have a temperature of {{Convert|232|K}}. Thus, contrary to what one might think, being nearer to the Sun is not a reason why Venus is warmer than Earth.<ref name="uwatmos211oct10">{{cite web |date=10 October 2001 |title=ATM S - Climate and Climate Change |url=https://atmos.washington.edu/academics/classes/2001Q4/211/notes_greenhouse.html |access-date=14 June 2023 |publisher=University of Washington}}</ref><ref>{{cite web |title=Venus Fact Sheet |url=https://nssdc.gsfc.nasa.gov/planetary/factsheet/venusfact.html |access-date=25 April 2023 |website=NASA Space Science Data Coordinated Archive }}</ref><ref>{{cite web |title=5 Weird Facts About Venus |url=https://www.britannica.com/list/5-weird-facts-about-venus#:~:text=Even%20though%20Venus%20is%20much,and%20reradiated%20as%20infrared%20radiation. |access-date=25 April 2023 |publisher=Britannica}}</ref>

Due to its high pressure, the CO<sub>2</sub> in the atmosphere of Venus exhibits ''continuum absorption'' (absorption over a broad range of wavelengths) and is not limited to absorption within the bands relevant to its absorption on Earth.<ref name="Pierrehumbert2011" />

A [[runaway greenhouse effect]] involving carbon dioxide and water vapor has for many years been hypothesized to have occurred on [[Venus]];<ref>{{cite journal |last1=Rasool |first1=I. |last2=De Bergh |first2=C. |date=June 1970 |title=The Runaway Greenhouse and the Accumulation of CO<sub>2</sub> in the Venus Atmosphere |url=http://pubs.giss.nasa.gov/docs/1970/1970_Rasool_DeBergh.pdf |url-status=dead |journal=Nature |volume=226 |issue=5250 |pages=1037–9 |bibcode=1970Natur.226.1037R |doi=10.1038/2261037a0 |pmid=16057644 |s2cid=4201521 |archive-url=https://web.archive.org/web/20111021110413/http://pubs.giss.nasa.gov/docs/1970/1970_Rasool_DeBergh.pdf |archive-date=21 October 2011}}</ref> this idea is still largely accepted.<ref name="McCarthy">{{Cite web |last=McCarthy |first=Michael Cabbage and Leslie |title=NASA climate modeling suggests Venus may have been habitable |url=https://climate.nasa.gov/news/2475/nasa-climate-modeling-suggests-venus-may-have-been-habitable |url-status=live |archive-url=https://web.archive.org/web/20210811030814/https://climate.nasa.gov/news/2475/nasa-climate-modeling-suggests-venus-may-have-been-habitable/ |archive-date=11 August 2021 |access-date=11 August 2021 |website=Climate Change: Vital Signs of the Planet}}</ref> The planet [[Venus (planet)|Venus]] experienced a runaway greenhouse effect, resulting in an atmosphere which is 96% [[carbon dioxide]], and a surface [[atmospheric pressure]] roughly the same as found {{convert|900|m|ft|abbr=on}} underwater on Earth. Venus may have had water oceans, but they would have boiled off as the mean surface temperature rose to the current {{convert|735|K|C F|abbr=on}}.<ref>{{cite journal |author=Hashimoto, G. L. |author2=Roos-Serote, M. |author3=Sugita, S. |author4=Gilmore, M. S. |author5=Kamp, L. W. |author6=Carlson, R. W. |author7=Baines, K. H. |date=2008 |title=Felsic highland crust on Venus suggested by Galileo Near-Infrared Mapping Spectrometer data |journal=[[Journal of Geophysical Research: Planets]] |volume=113 |issue=E9 |pages=E00B24 |bibcode=2008JGRE..113.0B24H |doi=10.1029/2008JE003134 |s2cid=45474562 |doi-access=free}}</ref><ref>{{cite web |author=David Shiga |date=10 October 2007 |title=Did Venus's ancient oceans incubate life? |url=https://www.newscientist.com/article/dn12769-did-venuss-ancient-oceans-incubate-life.html |url-status=live |archive-url=https://web.archive.org/web/20090324134332/https://www.newscientist.com/article/dn12769-did-venuss-ancient-oceans-incubate-life.html |archive-date=24 March 2009 |access-date=17 July 2019 |work=New Scientist}}</ref><ref name="Jakosky">{{cite book |last1=Jakosky |first1=Bruce M. |title=The New Solar System |date=1999 |publisher=Sky Publishing |isbn=978-0-933346-86-4 |editor1-last=Beatty |editor1-first=J. Kelly |edition=4th |location=Boston |pages=175–200 |chapter=Atmospheres of the Terrestrial Planets |oclc=39464951 |editor2-last=Petersen |editor2-first=Carolyn Collins |editor3-last=Chaikin |editor3-first=Andrew}}</ref>

===Mars===
[[Mars]] has about 70 times as much carbon dioxide as Earth,<ref name="Crisp2012">{{cite web |last1=Crisp |first1=D. |date=2012 |title={{CO2}} GREENHOUSE EFFECTS ON VENUS, EARTH, AND MARS |url=https://www.lpi.usra.edu/meetings/climatology2012/pdf/8083.pdf |access-date=28 May 2023 |publisher=Jet Propulsion Laboratory, California Institute of Technology}}</ref> but experiences only a small greenhouse effect, about {{convert|6|K-change}}.<ref name="ACSplanet" /> The greenhouse effect is small due to the lack of water vapor and the overall thinness of the atmosphere.<ref>{{cite web |last1=Brennan |first1=John |date=2017 |title=Does Mars Have a Greenhouse Effect? |url=https://sciencing.com/mars-greenhouse-effect-1914.html |website=Sciencing}}</ref>

The same radiative transfer calculations that predict warming on Earth accurately explain the temperature on Mars, given its atmospheric composition.<ref>{{cite web |title=Does the {{CO2}} atmosphere of Mars disprove greenhouse warming? |url=https://metafact.io/factchecks/77-does-the-co2-atmosphere-of-mars-disprove-greenhouse-warming |access-date=28 May 2023 |website=metafact}}</ref><ref name="WeartVenus">{{cite web |last1=Weart |first1=Spencer |title=The Discovery of Global Warming: Venus & Mars |url=https://history.aip.org/climate/Venus.htm |access-date=29 May 2023 |website=Center for History of Physics |publisher=American Institute of Physics}}</ref><ref name="Haberle2013" />

===Titan===
[[Saturn]]'s moon [[Titan (moon)|Titan]] has both a greenhouse effect and an [[anti-greenhouse effect]]. The presence of nitrogen ([[Dinitrogen|N<sub>2</sub>]]), methane ([[CH4|CH<sub>4</sub>]]), and hydrogen ([[Hydrogen|H<sub>2</sub>]]) in the atmosphere contribute to a greenhouse effect, increasing the surface temperature by {{convert|21|K-change}} over the expected temperature of the body without these gases.<ref name="McKay1991" /><ref name="astrobio2005">{{Cite web |date=3 November 2005 |title=Titan: Greenhouse and Anti-greenhouse |url=https://www.astrobio.net/retrospections/titan-greenhouse-and-anti-greenhouse/ |url-status=usurped |archive-url=https://web.archive.org/web/20190927031140/https://www.astrobio.net/retrospections/titan-greenhouse-and-anti-greenhouse/ |archive-date=27 September 2019 |access-date=4 November 2019 |website=Astrobiology Magazine |language=en-US}}</ref>

While the gases N<sub>2</sub> and H<sub>2</sub> ordinarily do not absorb infrared radiation, these gases absorb thermal radiation on Titan due to pressure-induced collisions, the large mass and thickness of the atmosphere, and the long wavelengths of the thermal radiation from the cold surface.<ref name="Pierrehumbert2011" /><ref name="McKay1991" /><ref name="astrobio2005" />

The existence of a high-altitude haze, which absorbs wavelengths of solar radiation but is transparent to infrared, contribute to an anti-greenhouse effect of approximately {{convert|9|K-change}}.<ref name="McKay1991" /><ref name="astrobio2005" />

The net result of these two effects is a warming of 21 K − 9 K = {{convert|12|K-change}}, so Titan's surface temperature of {{convert|94|K|C F|0|abbr=on}} is 12 K warmer than it would be if there were no atmosphere.<ref name="McKay1991" /><ref name="astrobio2005" />

=== Effect of pressure ===
One cannot predict the relative sizes of the greenhouse effects on different bodies simply by comparing the amount of greenhouse gases in their atmospheres. This is because factors other than the quantity of these gases also play a role in determining the size of the greenhouse effect.

Overall atmospheric pressure affects how much thermal radiation each molecule of a greenhouse gas can absorb. High pressure leads to more absorption and low pressure leads to less.<ref name="Pierrehumbert2011" />

This is due to "pressure broadening" of [[spectral lines]]. When the total atmospheric pressure is higher, collisions between molecules occur at a higher rate. Collisions broaden the width of absorption lines, allowing a greenhouse gas to absorb thermal radiation over a broader range of wavelengths.<ref name="PierrehumbertTextbook" />{{rp|226}}

Each molecule in the air near Earth's surface experiences about 7 billion collisions per second. This rate is lower at higher altitudes, where the pressure and temperature are both lower.<ref name="eiuatmo">{{cite web |title=The Atmosphere |url=https://www.ux1.eiu.edu/~cfjps/1400/atmos_struct.html#:~:text=Near%20sea%20level%2C%20an%20atom,is%20about%201%20each%20minute. |access-date=30 May 2023 |website=Eastern Illinois University}}</ref> This means that greenhouse gases are able to absorb more wavelengths in the lower atmosphere than they can in the upper atmosphere.<ref name="Plass1950" /><ref name="Wallace2006" />

On other planets, pressure broadening means that each molecule of a greenhouse gas is more effective at trapping thermal radiation if the total atmospheric pressure is high (as on Venus), and less effective at trapping thermal radiation if the atmospheric pressure is low (as on Mars).<ref name="Pierrehumbert2011" />

===Anti-greenhouse effect===
{{excerpt|Anti-greenhouse effect|paragraphs=1}}

== See also ==
{{portal|Global warming|Environment}}
*[[Climate change feedback]]
*[[Climate model]]
*[[Global dimming]]
*[[Idealized greenhouse model]]
*[[Illustrative model of greenhouse effect on climate change]]
*[[Solar Radiation Management|Solar radiation management]]
{{Clear}}

== References ==
{{Reflist|30em|refs=

<ref name="ipcc-AR4WG1">A concise description of the greenhouse effect is given in the ''Intergovernmental Panel on Climate Change Fourth Assessment Report,'' "What is the Greenhouse Effect?" [https://archive.ipcc.ch/publications_and_data/ar4/wg1/en/faq-1-3.html FAQ 1.3 – AR4 WGI Chapter 1: Historical Overview of Climate Change Science] {{Webarchive|url=https://web.archive.org/web/20190805034123/https://archive.ipcc.ch/publications_and_data/ar4/wg1/en/faq-1-3.html |date=5 August 2019 }}, 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."<br />{{cite book |first=Stephen H. |last=Schneider |chapter=Global Climate Change in the Human Perspective |title=Geosphere-biosphere Interactions and Climate |editor-first=Lennart O. |editor-last=Bengtsson |editor2-first=Claus U. |editor2-last=Hammer |publisher=Cambridge University Press |year=2001 |isbn=978-0-521-78238-8 |pages=90–91 |chapter-url=https://books.google.com/books?id=MsASkmCPDpcC&pg=PA90 |access-date=31 May 2018 |archive-date=2 August 2020 |archive-url=https://web.archive.org/web/20200802004048/https://books.google.com/books?id=MsASkmCPDpcC&pg=PA90 |url-status=live }}<br />{{cite book |editor-first=E. |editor-last=Claussen |editor2-first=V.A. |editor2-last=Cochran |editor3-first=D.P. |editor3-last=Davis |title=Climate Change: Science, Strategies, & Solutions |publisher=University of Michigan |year=2001 |page=373 |isbn=978-9004120242 |chapter=Global Climate Data |chapter-url=https://books.google.com/books?id=g85OJ76ufJoC&pg=PA373 |access-date=1 June 2018 |archive-date=18 May 2020 |archive-url=https://web.archive.org/web/20200518010607/https://books.google.com/books?id=g85OJ76ufJoC&pg=PA373 |url-status=live }}<br />{{cite book |first1=A. |last1=Allaby |first2=M. |last2=Allaby |title=A Dictionary of Earth Sciences |publisher=Oxford University Press |year=1999 |isbn=978-0-19-280079-4 |page=[https://archive.org/details/dictionaryofeart00alla/page/244 244] |url-access=registration |url=https://archive.org/details/dictionaryofeart00alla/page/244 }}</ref>

<ref name=anrev>
{{cite journal
 | journal = [[Annual Review of Energy and the Environment]]
 | title = Water Vapor Feedback and Global Warming
 |first1=Isaac M. |last1=Held |first2=Brian J. |last2=Soden | volume = 25
 | pages = 441–475
 | doi = 10.1146/annurev.energy.25.1.441| doi-access=free
 | date = November 2000
 | citeseerx = 10.1.1.22.9397
 }}</ref>
}}

{{sister project links|d=Q41560|v=Topic:Climate change|b=Climate Change|c=Category:Greenhouse effect|wikt=greenhouse effect|s=no|q=no|n=Category:Climate change|voy=no|m=no|mw=no|species=no}}
{{Global warming |state=expanded |expanded=Causes}}

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{{DEFAULTSORT:Greenhouse Effect}}
[[Category:Atmosphere]]
[[Category:Atmospheric radiation]]
[[Category:Climate forcing]]
[[Category:Effects of climate change]]
[[Category:Atmospheric chemistry]]