Editing Greenhouse effect (section)

== 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=Department of Earth and Environmental Sciences, Columbia University |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.