Greenhouse effect

This article is about the atmospheric phenomenon causing planetary warming. For the general heating or cooling of Earth's surface, see Earth's energy budget. For other uses, see Greenhouse (disambiguation).

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.

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 33 °C (59 °F).

The greenhouse effect occurs when greenhouse gases 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 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 −18 °C (−0.4 °F) without the greenhouse effect,<ref>"Solar Radiation and the Earth's Energy Balance". The Climate System – EESC 2100 Spring 2007. Columbia University. Archived from the original on 4 November 2004. Retrieved 15 October 2010.</ref><ref name="IPCC4_ch012">Le Treut H, Somerville R, Cubasch U, Ding Y, Mauritzen C, Mokssit A, Peterson T, Prather M (2007). "Historical Overview of Climate Change Science" (PDF). In Solomon S, Qin D, Manning M, Chen Z, Marquis M, Averyt KB, Tignor M, Miller HL (eds.). Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge, UK and New York, NY: Cambridge University Press. p. 97. Archived from the original (PDF) on 26 November 2018. Retrieved 25 March 2014.</ref> compared to Earth's 20th century average of about 14 °C (57 °F), or a more recent average of about 15 °C (59 °F).<ref>"The Elusive Absolute Surface Air Temperature (SAT)". Goddard Institute for Space Studies. NOAA. Archived from the original on 5 September 2015. Retrieved 3 September 2008.</ref><ref name="clchgtrack">"Yearly average temperature". Climate Change Tracker.</ref> In addition to naturally present greenhouse gases, burning of fossil fuels has increased amounts of carbon dioxide and methane in the atmosphere.<ref name="ipcc-AR4WG1" /><ref name="budget">Rebecca, Lindsey (14 January 2009). "Climate and Earth's Energy Budget : Feature Articles". earthobservatory.nasa.gov. Archived from the original on 21 January 2021. Retrieved 14 December 2020.</ref> As a result, global warming of about 1.2 °C (2.2 °F) has occurred since the Industrial Revolution,<ref>Fox, Alex. "Atmospheric Carbon Dioxide Reaches New High Despite Pandemic Emissions Reduction". Smithsonian Magazine. Archived from the original on 10 June 2021. Retrieved 22 June 2021.</ref> with the global average surface temperature increasing at a rate of 0.18 °C (0.32 °F) per decade since 1981.<ref>Lindsey, Rebecca; Dahlman, Luann. "Climate Change: Global Temperature". NOAA Climate.gov.</ref>

The wavelengths of radiation emitted by the Sun and Earth differ because their surface temperatures are different. The Sun has a surface temperature of 5,500 °C (9,900 °F), 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 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 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">"What is Earth's Energy Budget? Five Questions with a Guy Who Knows". NASA.gov. 10 April 2017. Retrieved 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>Fourier, J. (1824). "Remarques Generales sur les Temperatures Du Globe Terrestre et des Espaces Planetaires". Annales de Chimie et de Physique (in français). 27: 136–167. Archived from the original on 2 August 2020. Retrieved 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>Foote, Eunice (November 1856). Circumstances affecting the Heat of the Sun's Rays. Vol. 22. pp. 382–383. Archived from the original on 30 September 2020. Retrieved 31 January 2016. {{cite book}}: |work= ignored (help)</ref><ref>Huddleston, Amara (17 July 2019). "Happy 200th birthday to Eunice Foote, hidden climate science pioneer". NOAA Climate.gov. Archived from the original on 30 September 2020. Retrieved 8 October 2019.</ref> The term greenhouse was first applied to this phenomenon by Nils Gustaf Ekholm in 1901.<ref>Easterbrook, Steve (18 August 2015). "Who first coined the term "Greenhouse Effect"?". Serendipity. Archived from the original on 13 November 2015. Retrieved 11 November 2015.</ref><ref>Ekholm N (1901). "On The Variations Of The Climate Of The Geological And Historical Past And Their Causes". Quarterly Journal of the Royal Meteorological Society. 27 (117): 1–62. Bibcode:1901QJRMS..27....1E. doi:10.1002/qj.49702711702.</ref>

Definition

The greenhouse effect on Earth is defined as: "The infrared radiative effect of all infrared absorbing constituents in the atmosphere. Greenhouse gases (GHGs), clouds, and some aerosols absorb terrestrial radiation emitted by the Earth’s surface and elsewhere in the atmosphere."<ref name="IPCC">IPCC, 2021: Annex VII: Glossary [Matthews, J.B.R., V. Möller, R. van Diemen, J.S. Fuglestvedt, V. Masson-Delmotte, C.  Méndez, S. Semenov, A. Reisinger (eds.)]. In 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>

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" />

Terminology

The term greenhouse effect comes from an analogy to greenhouses. 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>Worley, John. "Greenhouses: Heating, Cooling and Ventilation". University of Georgia Extension. Archived from the original on 12 March 2023. Retrieved 12 March 2023.</ref><ref name="ipccar4wg1ch1">Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, 2007, Chapter 1. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. p. 115. Retrieved 24 March 2023. 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

See also: History of climate change science

Eunice Newton Foote recognized carbon dioxide's heat-capturing effect in 1856, appreciating its implications for the planet.<ref>Foote, Eunice, 1856. 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>

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>Fourier, J. (1824). "Remarques Generales sur les Temperatures Du Globe Terrestre et des Espaces Planetaires". Annales de Chimie et de Physique (in français). 27: 136–167. Archived from the original on 2 August 2020. Retrieved 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>Foote, Eunice (November 1856). Circumstances affecting the Heat of the Sun's Rays. Vol. 22. pp. 382–383. Archived from the original on 30 September 2020. Retrieved 31 January 2016. {{cite book}}: |work= ignored (help)</ref><ref>Huddleston, Amara (17 July 2019). "Happy 200th birthday to Eunice Foote, hidden climate science pioneer". NOAA Climate.gov. Archived from the original on 30 September 2020. Retrieved 8 October 2019.</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, 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 Gustaf Ekholm in 1901.<ref>Easterbrook, Steve (18 August 2015). "Who first coined the term "Greenhouse Effect"?". Serendipity. Archived from the original on 13 November 2015. Retrieved 11 November 2015.</ref><ref>Ekholm N (1901). "On The Variations Of The Climate Of The Geological And Historical Past And Their Causes". Quarterly Journal of the Royal Meteorological Society. 27 (117): 1–62. Bibcode:1901QJRMS..27....1E. doi:10.1002/qj.49702711702.</ref>

Measurement

File:Climate Factor CO₂ (ZDF, Terra X).webm Matter emits thermal radiation in an amount that 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 −18 °C (−0.4 °F). However, because some of the radiation is absorbed, Earth's average surface temperature is around 15 °C (59 °F). Thus, the Earth's greenhouse effect may be measured as a temperature change of 33 °C (59 °F).

Thermal radiation is characterized by how much energy it carries, typically in watts per square meter (W/m²). 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"/><ref name="ipcc-ar6wg1-ch7"/><ref name="ravram1"/><ref name="Schmidt2010paper"/><ref name="Schmidt2010">Gavin Schmidt (1 October 2010). "Taking the Measure of the Greenhouse Effect". NASA Goddard Institute for Space Studies - Science Briefs. Archived from the original on 21 April 2021. Retrieved 13 January 2022.</ref> Currently, longwave radiation leaves the surface at an average rate of 398 W/m², but only 239 W/m² reaches space. Thus, the Earth's greenhouse effect can also be measured as an energy flow change of 159 W/m².<ref name="ipcc-ar6wg1-ch7"/><ref name="ipcc-ar6wg1-ch7"/> 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"/><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 articles: Climate change and Earth's energy budget

Earth's rate of heating (graph) is a result of factors which include the enhanced greenhouse effect.<ref>Joseph Atkinson (22 June 2021). "Earth Matters: Earth's Radiation Budget is Out of Balance". NASA Earth Observatory. Archived from the original on 10 April 2022. Retrieved 1 March 2022.</ref>

Strengthening of the greenhouse effect through additional greenhouse gases from human activities is known as the enhanced greenhouse effect.<ref name="IPCC" /> As well as being inferred from measurements by ARGO, CERES and other instruments throughout the 21st century,<ref>"IPCC AR6 WG1" (PDF). Archived (PDF) from the original on 9 August 2021.</ref> this increase in radiative forcing from human activity has been observed directly,<ref>Robert McSweeney (25 February 2015). "New study directly measures greenhouse effect at Earth's surface". Carbon Brief. Archived from the original on 18 April 2021. Retrieved 18 April 2021.</ref><ref>"Direct observations confirm that humans are throwing Earth's energy budget off balance". phys.org. Science X. 26 March 2021. Archived from the original on 18 April 2021. Retrieved 18 April 2021.</ref> and is attributable mainly to increased atmospheric carbon dioxide levels.<ref>"Enhanced Greenhouse Effect". Ace.mmu.ac.uk. Archived from the original on 24 October 2010. Retrieved 15 October 2010.</ref>

The Keeling Curve of atmospheric CO₂ abundance.

CO₂ is produced by fossil fuel burning and other activities such as cement production and tropical deforestation.<ref name="IPCC deforestation">IPCC Fourth Assessment Report, Working Group I Report "The Physical Science Basis" Archived 15 March 2011 at the Wayback Machine Chapter 7</ref> Measurements of CO₂ from the Mauna Loa Observatory show that concentrations have increased from about 313 parts per million (ppm)<ref>"Atmospheric Carbon Dioxide – Mauna Loa". NOAA. Archived from the original on 20 May 2019. Retrieved 8 December 2008.</ref> in 1960, passing the 400 ppm milestone in 2013.<ref>"Climate Milestone: Earth's CO₂ Level Passes 400 ppm". National Geographic. 12 May 2013. Archived from the original on 15 December 2013. Retrieved 10 December 2017.</ref> The current observed amount of CO₂ exceeds the geological record maxima (≈300 ppm) from ice core data.<ref>Hansen J. (February 2005). "A slippery slope: How much global warming constitutes "dangerous anthropogenic interference"?". Climatic Change. 68 (333): 269–279. Bibcode:2005ClCh...68..269H. doi:10.1007/s10584-005-4135-0. S2CID 153165132. Archived from the original on 1 August 2020. Retrieved 8 June 2020.</ref>

Over the past 800,000 years,<ref>"Deep ice tells long climate story". BBC News. 4 September 2006. Archived from the original on 23 January 2013. Retrieved 4 May 2010.</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>Hileman B (28 November 2005). "Ice Core Record Extended". Chemical & Engineering News. 83 (48): 7. doi:10.1021/cen-v083n048.p007. Archived from the original on 15 May 2019. Retrieved 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">Bowen, Mark (2006). Thin Ice: Unlocking the Secrets of Climate in the World's Highest Mountains. Owl Books. ISBN 978-1429932707. Archived from the original on 2 August 2020. Retrieved 1 June 2018.</ref><ref>"Temperature change and carbon dioxide change". U.S. National Oceanic and Atmospheric Administration Paleoclimatology. 20 August 2008. Archived from the original on 18 January 2017.</ref>

Energy balance and temperature

Incoming shortwave radiation

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 longwave radiation. Sunlight includes ultraviolet, visible light, and near-infrared radiation.<ref name="IPCC" />

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>Williams, David R. "Earth Fact Sheet". NSSDCA. Archived from the original on 25 December 2015. Retrieved 15 October 2010.</ref><ref name=harvard>Jacob, Daniel J. (1999). "7. The Greenhouse Effect". Introduction to Atmospheric Chemistry. Princeton University Press. ISBN 978-1400841547. Archived from the original on 3 October 2010. Retrieved 9 December 2009.</ref> and absorbs the rest (240 W/m²).<ref name="ipcc-ar6wg1-ch7" />

Outgoing longwave radiation

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" /> 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">Schmidt, G. A.; Ruedy, R. A.; Miller, R. L.; Lacis, A. A. (2010). "Attribution of the present-day total greenhouse effect" (PDF). Journal of Geophysical Research. 115 (D20). Bibcode:2010JGRD..11520106S. doi:10.1029/2010JD014287. Retrieved 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 cm⁻¹ (equivalent to a wavelength of 15 microns).<ref name="wijng">van Wijngaarden, W. A.; Happer, W. (2020). "Dependence of Earth's Thermal Radiation on Five Most Abundant Greenhouse Gases" (PDF). Atmospheric and Oceanic Physics. arXiv:2006.03098. Archived (PDF) from the original on 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

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 −18 °C (0 °F).<ref>"Solar Radiation and the Earth's Energy Balance". Eesc.columbia.edu. Archived from the original on 17 July 2012. Retrieved 15 October 2010.</ref><ref name="IPCC4_ch012"/>
• Emissions from surface: Based on thermal emissions from the surface, Earth's effective surface temperature is about 16 °C (61 °F),<ref name="ipcc-ar6wg1-ch7" /> which is 34 °C (61 °F) 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 15 °C (59 °F),<ref name="clchgtrack"/><ref>The elusive "absolute surface air temperature", see GISS discussion Archived 5 September 2015 at the Wayback Machine</ref> a bit lower than the effective surface temperature. This value is 33 °C (59 °F) 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/m², which is the number of joules 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, 5.1×10¹⁴ m² (5.1×10⁸ km²; 2.0×10⁸ sq mi).<ref name="utarea">"What is the Surface Area of the Earth?". Universe Today. 11 February 2017. Retrieved 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"/>

Radiative balance

The temperature of a planet depends on the 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">"Earth's Radiation Balance". CIMSS: University of Wisconsin. Retrieved 25 April 2023.</ref>

Earth's 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">"Don't Worry about CO2, Worry about the Earth's 'Energy Balance'". Scientific American. Retrieved 2 June 2023.</ref> A 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">von Schuckmann, K.; Palmer, M.; Trenberth, K.; Cazenave, A.; Chambers, D.; Champollion, N.; Hansen, J.; Josey, S. A.; Loeb, N.; Mathiew, P.P.; Meyssignac, B.; Wild, M. (2016). "An imperative to monitor Earth's energy imbalance" (PDF). Nature Climate Change. 6 (2): 138–144. Bibcode:2016NatCC...6..138V. doi:10.1038/nclimate2876.</ref>

Earth's energy imbalance (EEI) was about 0.7 W/m² 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" />

Over 90% of the retained energy goes into warming the oceans, with much smaller amounts going into heating the land, atmosphere, and ice.<ref>Hawkins, Ed (27 January 2016). "Earth's energy imbalance". Climate Lab Book. Retrieved 16 July 2023.</ref>

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">"RRTM Earth's Energy Budget". University of Chicago. Retrieved 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, 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>Michon Scott (24 April 2006). "Earth's Big Heat Bucket - Bad News, Good News". NASA Earth Observatory. Archived from the original on 4 December 2022. Retrieved 4 December 2022.</ref> Diurnal temperature changes decrease with height in the atmosphere.

Effect of lapse rate

Lapse rate

Further information: 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">Nugent, Alison; DeCou, David. "ATMOSPHERIC PROCESSES AND PHENOMENA: Chapter 5: Atmospheric Stability". Retrieved 31 May 2023.</ref>

On Earth, the air temperature decreases by about 6.5°C/km (3.6°F per 1000 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 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">Thomas, Gary E.; Stamnes, Knut (1999). Radiative Transfer in the Atmosphere and Ocean. Cambridge University Press. ISBN 0-521-40124-0.</ref>

Emission temperature and altitude

The temperature at which thermal radiation was emitted can be determined by comparing the intensity at a particular wavenumber to the intensity of a black-body emission curve. In the chart, emission temperatures range between T_min and T_s. "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 pressure broadening of absorption lines limits the wavelengths that gas molecules can absorb.<ref name="Plass1950">Strong, J.; Plass, G. N. (1950). "The Effect of Pressure Broadening of Spectral Lines on Atmospheric Temperature". Astrophysical Journal. 112: 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">Pierrehumbert, R. T. (January 2011). "Infrared radiation and planetary temperature" (PDF). Physics Today. American Institute of Physics. pp. 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

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">Mitchell, John F. B. (1989). "The "Greenhouse" effect and Climate Change" (PDF). Reviews of Geophysics. 27 (1): 115–139. Bibcode:1989RvGeo..27..115M. CiteSeerX 10.1.1.459.471. doi:10.1029/RG027i001p00115. Archived (PDF) from the original on 15 June 2011. Retrieved 23 March 2008.</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>Mann, Michael; Gaudet, Brian. "METEO 469: From Meteorology to Mitigation - Understanding Global Warming - Lesson 5 - Modelling of the Climate System - One-Layer Energy Balance Model". Penn State College of Mineral and Earth Sciences - Department of Meteorology and Atmospheric Sciences. Archived from the original on 31 October 2022. Retrieved 4 November 2022.</ref><ref name="Tziperman2022">Tziperman, Eli (2022). Global Warming Science. Princeton University Press. ISBN 9780691228808.</ref>

Infrared absorbing constituents in the atmosphere

Greenhouse gases

Main article: 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">Archer, David (2011). Global Warming: Understanding the Forecast, Chapter 4: Greenhouse Gases (PDF) (2 ed.). Wiley. ISBN 978-0470943410. Retrieved 14 June 2023.</ref> and act as greenhouse gases.

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

Gases with only one atom (such as argon, Ar) or with two identical atoms (such as nitrogen, N
₂, and oxygen, 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

Longwave absorption coefficients of water vapor and carbon dioxide. For wavelengths near 15 microns (15 μm in top scale), where Earth's surface emits strongly, CO₂ is a much stronger absorber than water vapor.

Greenhouse gases absorb and emit longwave radiation within specific ranges of wavelengths (organized as spectral lines or 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 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 (buoyant water vapor condensing into water droplets and releasing heat), thermals (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">Wallace, J. M.; Hobbs, P. V. (2006). Atmospheric Science (2 ed.). Academic Press. ISBN 978-0-12-732951-2.</ref><ref name="Manabe1964">Manabe, S.; Strickler, R. F. (1964). "Thermal Equilibrium of the Atmosphere with a Convective Adjustment". J. Atmos. Sci. 21 (4): 361–385. Bibcode:1964JAtS...21..361M. doi:10.1175/1520-0469(1964)021<0361:TEOTAW>2.0.CO;2.</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" /><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" /><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" />

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 and 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">"Cloud Radiative Effect". NOAA Geophysical Fluid Dynamics Laboratory. Retrieved 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>Liou, Kuo-Nan (1 June 1986). "Influence of Cirrus Clouds on Weather and Climate Processes: A Global Perspective". Monthly Weather Review. 114 (6): 1167–1199. Bibcode:1986MWRv..114.1167L. doi:10.1175/1520-0493(1986)114<1167:IOCCOW>2.0.CO;2. ISSN 1520-0493.</ref>

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">"Box 2: Solar and Earth Radiation and the Greenhouse Effect (adapted from Mackenzie, 2003)" (PDF). Myron B. Thompson Academy. 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

Increase in the Earth's greenhouse effect (2000-2022) based on NASA CERES satellite data.

The IPCC reports the greenhouse effect, G, as being 159 W m^-2, where 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" /><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, 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, g̃ = 0.40. In other words, 40 percent less thermal radiation reaches space than what leaves the surface.<ref name="ipcc-ar6wg1-ch7">"Chapter 7: The Earth's Energy Budget, Climate Feedbacks, and Climate Sensitivity". Climate Change 2021: The Physical Science Basis (PDF). IPCC. 2021. Retrieved 24 April 2023.</ref><ref name="ravram1">Raval, A.; Ramanathan, V. (1989). "Observational determination of the greenhouse effect". Nature. 342 (6251): 758–761. Bibcode:1989Natur.342..758R. doi:10.1038/342758a0. S2CID 4326910.</ref><ref name="ravram2">Raval, A.; Ramanathan, V. (1990). "Observational determination of the greenhouse effect". Global Climate Feedbacks: Proceedings of the Brookhaven National Laboratory Workshop: 5–16. Retrieved 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">Haberle, Robert M. (2013). "Estimating the power of Mars' greenhouse effect". Icarus. 223 (1): 619–620. Bibcode:2013Icar..223..619H. doi:10.1016/j.icarus.2012.12.022.</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 information: Earth's energy budget

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/m². Some emitted radiation is effectively cancelled out by downwelling radiation and so does not 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) energy imbalance (EEI) is the amount by which the power of incoming radiation exceeds the power of outgoing radiation:<ref name="UNeei">"The Earth's Energy Imbalance: Where does the energy go?" (PDF). United Nations Climate Change. Retrieved 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 mean solar irradiance incoming at the top of the atmosphere.

The 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">"Predicted Planetary Temperatures". ACS Climate Science Toolkit. American Chemical Society. Archived from the original on 26 March 2023. Retrieved 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 G or 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

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 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" /> 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 Callendar (1938) and Plass (1959) focused on the surface budget, but the work of Manabe in the 1960s clarified the importance of the top-of-atmosphere energy budget.<ref name="PierrehumbertTextbook">Pierrehumbert, Raymond T. (2010). Principles of Planetary Climate. Cambridge University Press. ISBN 978-0-521-86556-2.</ref>

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">"Global Warming Violates the Second Law of Thermodynamics: Brief Responses to Climate Change Denialism Statements". University of Maryland. Retrieved 13 June 2023.</ref><ref name="Halpern2010">Halpern, J. B.; Close, C. M.; Ho-Stuart, C.; Shore, J. D.; Smith, A. P.; Zimmermann, J. (2010). "Comment on "falsification of the Atmospheric CO2 Greenhouse Effects Within the Frame of Physics"". International Journal of Modern Physics B. 24 (10): 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">Modest, Michael F. (2021). Radiative Heat Transfer (4 ed.). Academic Press. ISBN 978-0323984065.</ref> Radiation heat flow occurs in the direction from the surface to the atmosphere and space,<ref name="budget" /> as is to be 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 radiation heat transfer.<ref name="Siegel1971">Siegel, R.; Howell, J. R. (1971). "Thermal Radiation Heat Transfer" (PDF). NASA. Retrieved 28 May 2023.</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">"The Earth's Energy Budget II -- Radiation Emitted by the Earth, the Greenhouse Effect, and the Overall Energy Balance". University of Arizona, Hydrology & Atmospheric Sciences. 2017. Retrieved 28 May 2023.</ref>

Simplified models

Further information: Idealized greenhouse model

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 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 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">"ACS Climate Science Toolkit - Atmospheric Warming - A Single-Layer Atmosphere Model". American Chemical Society. Retrieved 2 October 2022.</ref> Slightly more complex models add additional layers, or introduce convection.<ref name="acs-multi">"ACS Climate Science Toolkit - Atmospheric Warming - A Multi-Layer Atmosphere Model". American Chemical Society. Retrieved 2 October 2022.</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">Benestad, R. E. (2017). "A mental picture of the greenhouse effect". Theor Appl Climatol. 128 (3–4): 679–688. doi:10.1007/s00704-016-1732-y. S2CID 123923155.</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">"Radiation Balance of the Earth-Atmosphere System". Atmospheric and Oceanic Sciences. University of Wisconsin. Retrieved 1 June 2023. 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">Benestad, R. E. (2016). "What is the best description of the greenhouse effect?". RealClimate. Retrieved 1 June 2023.</ref><ref name="aosuw" /><ref name="Tziperman2022" />

Earth's overall equivalent emission altitude has been increasing with a trend of 23 m (75 ft)/decade, which is said to be consistent with a global mean surface warming of 0.12 °C (0.22 °F)/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">Schmithüsen, Holger; Notholt, Justus; König-Langlo, Gert; Lemke, Peter; Jung, Thomas (16 December 2015). "How increasing CO₂ leads to an increased negative greenhouse effect in Antarctica". Geophysical Research Letters. 42 (23). doi:10.1002/2015GL066749. ISSN 0094-8276. S2CID 131351000.</ref><ref name="sejas">Sejas, S.A.; Taylor, P. C.; Cai, M. (2018). "Unmasking the negative greenhouse effect over the Antarctic Plateau". npj Clim Atmos Sci. 1 (17): 17. Bibcode:2018npjCA...1...17S. doi:10.1038/s41612-018-0031-y. PMC 7580794. PMID 33102742.</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.

Bodies other than Earth

Greenhouse effect on different celestial bodies<ref name="ACSplanet">"Atmospheres and Planetary Temperatures". ACS. Retrieved 29 May 2023.</ref><ref name="McKay1991">McKay, C. P.; Pollack, J. B.; Courtin, R. (6 September 1991). "The greenhouse and antigreenhouse effects on Titan" (PDF). Science. 253 (5024): 1118–1121. Bibcode:1991Sci...253.1118M. doi:10.1126/science.11538492. ISSN 0036-8075. PMID 11538492. S2CID 10384331. Archived (PDF) from the original on 29 June 2023.</ref><ref name="nasatitan">"Titan". NASA. Retrieved 29 May 2023.</ref>

	Venus	Earth	Mars	Titan
Surface temperature, <math>T_\mathrm{observed}</math>	735 K (462 °C; 863 °F)	288 K (15 °C; 59 °F)	215 K (−58 °C; −73 °F)	94 K (−179 °C; −290 °F)
Greenhouse effect, <math>\Delta T}_\mathrm{GHE</math>	503 K (905 °F)	33 K (59 °F)	6 K (11 °F)	21 K (38 °F) GHE; 12 K (22 °F) GHE+AGHE
Pressure	92 atm	1 atm	0.0063 atm	1.5 atm
Primary gases	CO2 (0.965) N2 (0.035)	N2 (0.78) O2 (0.21) Ar (0.009)	CO2 (0.95) N2 (0.03) Ar (0.02)	N2 (0.95) Template:CH4 (~0.05)
Trace gases	Template:SO2, Ar	Template:H2O-nl, CO2	O2, CO	H2
Planetary effective temperature, <math>T_\mathrm{eff}</math>	232 K (−41 °C; −42 °F)	255 K (−18 °C; −1 °F)	209 K (−64 °C; −83 °F)	73 K tropopause; 82 K stratopause
Greenhouse effect, <math>G</math>	16000 W/m2	150 W/m2	13 W/m2	2.8 W/m2 GHE; 1.9 W/m2 GHE+AGHE
Normalized greenhouse effect, <math>\tilde g</math>	0.99	0.39	0.11	0.63 GHE; 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 735 K (462 °C; 863 °F). 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 232 K (−41 °C; −42 °F). 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">"ATM S - Climate and Climate Change". University of Washington. 10 October 2001. Retrieved 14 June 2023.</ref><ref>"Venus Fact Sheet". NASA Space Science Data Coordinated Archive. Retrieved 25 April 2023.</ref><ref>"5 Weird Facts About Venus". Britannica. Retrieved 25 April 2023.</ref>

Due to its high pressure, the CO₂ 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>Rasool, I.; De Bergh, C. (June 1970). "The Runaway Greenhouse and the Accumulation of CO₂ in the Venus Atmosphere" (PDF). Nature. 226 (5250): 1037–9. Bibcode:1970Natur.226.1037R. doi:10.1038/2261037a0. PMID 16057644. S2CID 4201521. Archived from the original (PDF) on 21 October 2011.</ref> this idea is still largely accepted.<ref name="McCarthy">McCarthy, Michael Cabbage and Leslie. "NASA climate modeling suggests Venus may have been habitable". Climate Change: Vital Signs of the Planet. Archived from the original on 11 August 2021. Retrieved 11 August 2021.</ref> The 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 900 m (3,000 ft) underwater on Earth. Venus may have had water oceans, but they would have boiled off as the mean surface temperature rose to the current 735 K (462 °C; 863 °F).<ref>Hashimoto, G. L.; Roos-Serote, M.; Sugita, S.; Gilmore, M. S.; Kamp, L. W.; Carlson, R. W.; Baines, K. H. (2008). "Felsic highland crust on Venus suggested by Galileo Near-Infrared Mapping Spectrometer data". Journal of Geophysical Research: Planets. 113 (E9): E00B24. Bibcode:2008JGRE..113.0B24H. doi:10.1029/2008JE003134. S2CID 45474562.</ref><ref>David Shiga (10 October 2007). "Did Venus's ancient oceans incubate life?". New Scientist. Archived from the original on 24 March 2009. Retrieved 17 July 2019.</ref><ref name="Jakosky">Jakosky, Bruce M. (1999). "Atmospheres of the Terrestrial Planets". In Beatty, J. Kelly; Petersen, Carolyn Collins; Chaikin, Andrew (eds.). The New Solar System (4th ed.). Boston: Sky Publishing. pp. 175–200. ISBN 978-0-933346-86-4. OCLC 39464951.</ref>

Mars

Mars has about 70 times as much carbon dioxide as Earth,<ref name="Crisp2012">Crisp, D. (2012). "CO₂ GREENHOUSE EFFECTS ON VENUS, EARTH, AND MARS" (PDF). Jet Propulsion Laboratory, California Institute of Technology. Retrieved 28 May 2023.</ref> but experiences only a small greenhouse effect, about 6 K (11 °F).<ref name="ACSplanet" /> The greenhouse effect is small due to the lack of water vapor and the overall thinness of the atmosphere.<ref>Brennan, John (2017). "Does Mars Have a Greenhouse Effect?". Sciencing.</ref>

The same radiative transfer calculations that predict warming on Earth accurately explain the temperature on Mars, given its atmospheric composition.<ref>"Does the CO₂ atmosphere of Mars disprove greenhouse warming?". metafact. Retrieved 28 May 2023.</ref><ref name="WeartVenus">Weart, Spencer. "The Discovery of Global Warming: Venus & Mars". Center for History of Physics. American Institute of Physics. Retrieved 29 May 2023.</ref><ref name="Haberle2013" />

Titan

Saturn's moon Titan has both a greenhouse effect and an anti-greenhouse effect. The presence of nitrogen (N₂), methane (CH₄), and hydrogen (H₂) in the atmosphere contribute to a greenhouse effect, increasing the surface temperature by 21 K (38 °F) over the expected temperature of the body without these gases.<ref name="McKay1991" /><ref name="astrobio2005">"Titan: Greenhouse and Anti-greenhouse". Astrobiology Magazine. 3 November 2005. Archived from the original on 27 September 2019. Retrieved 4 November 2019.{{cite web}}: CS1 maint: unfit URL (link)</ref>

While the gases N₂ and H₂ 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 9 K (16 °F).<ref name="McKay1991" /><ref name="astrobio2005" />

The net result of these two effects is a warming of 21 K − 9 K = 12 K (22 °F), so Titan's surface temperature of 94 K (−179 °C; −290 °F) 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" />

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">"The Atmosphere". Eastern Illinois University. Retrieved 30 May 2023.</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" />

See also

• Climate change feedback
• Climate model
• Global dimming
• Idealized greenhouse model
• Illustrative model of greenhouse effect on climate change
• Solar radiation management