Editing Cloud feedback (section)

== Overview ==
[[File:McKim 2024 cloud formulae.png|thumb|Details of how clouds interact with shortwave and longwave radiation at different atmospheric heights<ref name="McKim2024">{{Cite journal |last1=McKim |first1=Brett |last2=Bony |first2=Sandrine |last3=Dufresne |first3=Jean-Louis |date=1 April 2024 |title=Weak anvil cloud area feedback suggested by physical and observational constraints |journal=Nature Geoscience |volume=17 |issue=5 |pages=392–397 |doi=10.1038/s41561-024-01414-4 |doi-access=free |bibcode=2024NatGe..17..392M }}</ref>]]
Clouds have two major effects on the [[Earth's energy budget]]: they reflect shortwave radiation from sunlight back to space due to their high [[albedo]], but the water vapor contained inside them also absorbs and re-emits the longwave radiation sent out by the Earth's surface as it is heated by sunlight, preventing its escape into space and retaining this heat energy for longer.<ref name="IPCC AR6 WG1 CH7" />{{rp|1022}} 

In [[meteorology]], the difference in the [[radiation budget]] caused by clouds, relative to cloud-free conditions, is described as the cloud radiative effect (CRE).<ref name="IPCC_annexVII_glossary">{{cite journal |last1=Matthews |title=Annex VII: Glossary of the Climate Change 2021 – The Physical Science Basis: Working Group I Contribution to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change |date=6 July 2023 |doi=10.1017/9781009157896.022 |doi-access=free }}</ref> This is also sometimes referred to as cloud [[radiative forcing]] (CRF).<ref>{{cite web |last = NASA |title = Clouds & Radiation Fact Sheet : Feature Articles | publisher = NASA | date = 2016 | url = https://earthobservatory.nasa.gov/Features/Clouds/ | access-date = 2017-05-29}}</ref> However, since cloud changes are not normally considered an external forcing of climate, CRE is the most commonly used term. 

At the top of the atmosphere, it can be described by the following equation<ref>{{cite book |last= Hartmann |first= Dennis L. |date = 2016 |title= Global Physical Climatology |location= Amsterdam |publisher= Elsevier |isbn= 978-0123285317}}</ref>

:<math>\Delta R_{TOA} = R_{Average} - R_{Clear}</math>

The net cloud radiative effect can be decomposed into its longwave and shortwave components. This is because net radiation is absorbed solar minus the outgoing longwave radiation shown by the following equations

:<math> \Delta R_{TOA} = \Delta Q_{abs} - \Delta OLR </math>

The first term on the right is the shortwave cloud effect (''Q''<sub>abs</sub> ) and the second is the longwave effect (OLR).

The shortwave cloud effect is calculated by the following equation

:<math> \Delta Q_{abs} = (S_o/4) \cdot (1 - \alpha_{cloudy}) - (S_o/4) \cdot (1 - \alpha_{clear}) </math>

Where ''S''<sub>o</sub> is the [[solar constant]], ''∝''<sub>cloudy</sub> is the [[albedo]] with clouds and ''∝''<sub>clear</sub> is the albedo on a clear day.

The longwave effect is calculated by the next following equation

:<math> \Delta OLR = \sigma T_z^4 - F_{clear}^{up}</math>

Where σ is the [[Stefan–Boltzmann constant]], T is the temperature at the given height, and F is the upward flux in clear conditions.

Putting all of these pieces together, the final equation becomes

:<math> \Delta R_{TOA} = (S_o/4) \cdot ((1 - \alpha_{cloudy}) - (1 - \alpha_{clear})) - \sigma T_z^4 + F_{clear}^{up} </math>

[[File:Attribution of individual atmospheric component contributions to the terrestrial greenhouse effect, separated into feedback and forcing categories (NASA).png|thumb|left|Attribution of individual atmospheric component contributions to the [[greenhouse effect]], separated into feedback and forcing categories (NASA)]]

Under dry, cloud-free conditions, water vapor in atmosphere contributes 67% of the [[greenhouse effect]] on Earth. When there is enough moisture to form typical cloud cover, the greenhouse effect from "free" water vapor goes down to 50%, but water vapor which is now inside the clouds amounts to 25%, and the net greenhouse effect is at 75%.<ref>{{cite journal |last=Schmidt |first=G.A. |title=The attribution of the present-day total greenhouse effect |journal=J. Geophys. Res. |volume=115 |issue=D20 |pages=D20106 |df=dmy-all |year=2010 |bibcode=2010JGRD..11520106S |doi=10.1029/2010JD014287 |author2=R. Ruedy |author3=R.L. Miller |author4=A.A. Lacis |author-link1=Gavin Schmidt |doi-access=free}}, D20106. [http://pubs.giss.nasa.gov/abs/sc05400j.html Web page ] {{Webarchive|url=https://web.archive.org/web/20120604034848/http://pubs.giss.nasa.gov/abs/sc05400j.html|date=4 June 2012}}</ref> According to 1990 estimates, the presence of clouds reduces the [[outgoing longwave radiation]] by about 31 W/m<sup>2</sup>. However, it also increases the global [[albedo]] from 15% to 30%, and this reduces the amount of [[solar radiation]] absorbed by the Earth by about 44 W/m<sup>2</sup>. Thus, there is a net ''cooling'' of about 13 W/m<sup>2</sup>.<ref>{{cite book |last=Intergovernmental Panel on Climate Change |title=IPCC First Assessment Report.1990 |publisher=Cambridge University Press |year=1990 |location=UK |author-link=Intergovernmental Panel on Climate Change}}table 3.1</ref> If the clouds were removed with all else remaining the same, the [[Earth]] would lose this much cooling and the global temperatures would increase.<ref name="IPCC AR6 WG1 CH7" />{{rp|1022}}

[[Climate change]] increases the amount of water vapor in the atmosphere due to the [[Clausius–Clapeyron relation]], in what is known as the water-vapor feedback.<ref>{{Cite journal |last1=Held |first1=Isaac M. |last2=Soden |first2=Brian J. |date=November 2000 |title=Water vapor feedback and global warming |journal=[[Annual Review of Energy and the Environment]] |language=en |volume=25 |issue=1 |pages=441–475 |citeseerx=10.1.1.22.9397 |doi=10.1146/annurev.energy.25.1.441 |issn=1056-3466 |doi-access=free}}</ref> It also affects a range of cloud properties, such as their height, the typical distribution throughout the atmosphere, and [[cloud physics|cloud microphysics]], such as the amount of water droplets held, all of which then affect clouds' radiative forcing.<ref name="IPCC AR6 WG1 CH7" />{{rp|1023}} Differences in those properties change the role of clouds in the Earth's energy budget. The name ''cloud feedback'' refers to this relationship between climate change, cloud properties, and clouds' radiative forcing.<ref name="IPCC glossary">IPCC, 2021: [https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_AnnexVII.pdf 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 [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|2224}} Clouds also affect the magnitude of internally generated [[Climate variability and change|climate variability.]]<ref>{{Cite journal |last1=Brown |first1=Patrick T. |last2=Li |first2=Wenhong |last3=Jiang |first3=Jonathan H. |last4=Su |first4=Hui |date=2015-12-07 |title=Unforced Surface Air Temperature Variability and Its Contrasting Relationship with the Anomalous TOA Energy Flux at Local and Global Spatial Scales |url=https://dukespace.lib.duke.edu/dspace/bitstream/10161/15913/1/2016_BrownLiJiangSu_JCLI.pdf |url-status=live |journal=Journal of Climate |volume=29 |issue=3 |pages=925–940 |bibcode=2016JCli...29..925B |doi=10.1175/JCLI-D-15-0384.1 |issn=0894-8755 |archive-url=https://web.archive.org/web/20180719171852/https://dukespace.lib.duke.edu/dspace/bitstream/10161/15913/1/2016_BrownLiJiangSu_JCLI.pdf |archive-date=2018-07-19 |doi-access=free}}</ref><ref>{{Cite journal |last1=Bellomo |first1=Katinka |last2=Clement |first2=Amy |last3=Mauritsen |first3=Thorsten |last4=Rädel |first4=Gaby |last5=Stevens |first5=Bjorn |date=2014-04-11 |title=Simulating the Role of Subtropical Stratocumulus Clouds in Driving Pacific Climate Variability |journal=Journal of Climate |volume=27 |issue=13 |pages=5119–5131 |bibcode=2014JCli...27.5119B |doi=10.1175/JCLI-D-13-00548.1 |issn=0894-8755 |s2cid=33019270 |hdl-access=free |hdl=11858/00-001M-0000-0014-72C1-F}}</ref>