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{{short description|Unavoidable future climate change due to inertial effects}}
[[File:1979- Radiative forcing - climate change - global warming - EPA NOAA.svg |thumb|right|upright=1.5 |The ongoing buildup of [[IPCC list of greenhouse gases|long-lived greenhouse gases]] in Earth's atmosphere, whose warming influence has nearly doubled since 1979, shows mankind's influence on the global climate.<ref name=NOAA_AGGI_2023>{{cite web |title=The NOAA Annual Greenhouse Gas Index (AGGI) |url=https://gml.noaa.gov/aggi/aggi.html |website=NOAA.gov |publisher=National Oceanic and Atmospheric Administration (NOAA) |archive-url=https://web.archive.org/web/20241005195609/https://gml.noaa.gov/aggi/aggi.html |archive-date=5 October 2024 |date=2024 |url-status=live }}</ref><ref>{{Cite web |url=https://www.globalchange.gov/browse/indicators/annual-greenhouse-gas-index |title=Annual Greenhouse Gas Index |publisher=U.S. Global Change Research Program |archive-date=21 April 2021 |archive-url=https://web.archive.org/web/20210421143115/https://www.globalchange.gov/browse/indicators/annual-greenhouse-gas-index |url-status=live }}</ref><ref>{{Cite web |url=https://www.esrl.noaa.gov/gmd/aggi/ |title=The NOAA Annual Greenhouse Gas Index (AGGI) - An Introduction |publisher=[[NOAA]] Global Monitoring Laboratory/Earth System Research Laboratories |access-date=2 March 2023}}</ref>]]
'''Climate commitment''' describes the fact that Earth's [[climate]] reacts with a delay to influencing factors ("[[climate forcing]]s") such as the growth and the greater presence of [[greenhouse gas]]es. Climate commitment studies attempt to assess the amount of future [[global warming]] that is "committed" under the assumption of some constant or some evolving level of forcing. The constant level often used for illustrative purposes is that due to [[carbon dioxide|{{CO2}}]] doubling or quadrupling relative to the pre-industrial level; or the present level of forcing.
{{clear}}

== Definition ==
Climate commitment is the "unavoidable future climate change resulting from
inertia in the geophysical and socio-economic systems".<ref name="IPCC AR6 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> Different types of climate change commitment are discussed in the literature. These include the "constant composition commitment"; the "constant emissions commitment" and the "zero emissions commitment".<ref name="IPCC AR6 glossary" />{{rp|2222}}

== Basic idea ==
[[File:Earth's Heat Accumulation.png|thumb|right|upright=1.2|The accumulation of [[ocean heat content|excess heat in the ocean]], at ever greater depths, measures global warming that has already become "irreversible" in the near term<ref name=EarthSysSciData_20200907>{{cite journal |last1=von Schuckmann |first1=K. |last2=Cheng |first2=L. |last3=Palmer |first3=M. D. |last4=Hansen |first4=J. |last5=Tassone |first5=C. |last6=Aich |first6=V. |last7=Adusumilli |first7=S. |last8=Beltrami |first8=H. |last9=Boyer |first9=T. |last10=Cuesta-Valero |first10=F. J. |display-authors=4 |title=Heat stored in the Earth system: where does the energy go? |journal=Earth System Science Data |date=7 September 2020 |doi=10.5194/essd-12-2013-2020 |doi-access=free |volume=12 |issue=3 |page=2013-2041 |bibcode=2020ESSD...12.2013V |hdl=20.500.11850/443809 |hdl-access=free }}</ref>]]
If a [[wiktionary:perturbation|perturbation]] — such as an increase in greenhouse gases or [[solar variation|solar activity]] — is applied to Earth's [[climate system]] the response will not be immediate, principally because of the large [[heat capacity]] and [[thermal inertia]] of the [[ocean]]s.<ref>{{cite journal |last1=Abraham |first1=John |last2=Cheng |first2=Lijing |last3=Mann |first3=Michael E. |last4=Trenberth |first4=Kevin |last5=von Schuckmann |first5=Karina |title=The ocean response to climate change guides both adaptation and mitigation efforts |journal=Atmospheric and Oceanic Science Letters |volume=15 |number=100221 |pages=1-9 |date=1 July 2022 |doi=10.1016/j.aosl.2022.100221 |doi-access=free}}</ref>
:''As an analogue, consider the heating of a thin metal plate (by the sun or by a flame): the plate will warm relatively quickly. If a thick metal block is heated instead, it will take much longer for the entire block to reach [[Thermodynamic equilibrium|equilibrium]] with the imposed heating because of its higher heat capacity.''

Land only stores [[heat]] in the top few meters.
Ocean water, by contrast, can move vertically and store heat within the ocean's depth ([[convection]]).
This is why the land surface is observed to warm more than the oceans. It also explains the large difference in [[climate sensitivity|global surface temperature response]] between
* "[[Transient state|transient]]" climate simulations in which the planet's incoming/outgoing energy flows are substantially out-of-balance and only a shallow ocean model might be utilized, and
* "[[Equilibrium state|equilibrium]]" climate simulations in which the energy flows approach a new balance and a full ocean model is needed.<ref>{{cite arXiv | eprint = 1307.6821|title=The Earth's Equilibrium Climate Sensitivity and Thermal Inertia|date=25 July 2013| first1 = B. S. H. | last1 = Royce | first2 = S. H. | last2 = Lam |class=physics.ao-ph}}</ref><ref>{{cite journal |last1=Hansen |first1=J. |last2=Russell |first2=G. |last3=Lacis |first3=A. |last4=Fung |first4=I. |last5=Rind |first5=D. |last6=Stone |first6=P. |url=https://pubs.giss.nasa.gov/docs/1985/1985_Hansen_ha09600g.pdf |title=Climate response times: Dependence on climate sensitivity and ocean mixing |journal=Science |volume=229 |pages=857–850 |year=1985 |issue=4716 |doi=10.1126/science.229.4716.857 |pmid=17777925 |bibcode=1985Sci...229..857H|s2cid=22938919 }}</ref>

The "commitment" can apply to variables other than temperature: because of the long mixing time for heat into the deep ocean, a given surface warming commits to centuries of [[sea level rise]] from thermal expansion of the ocean. Also once a certain threshold is crossed, it is likely that a slow melting of the Greenland ice sheet will commit us to a sea level rise of 5m over millennia.{{cn|date=November 2024}}

== Models ==
{{main|global climate model}}
Recent [[model (abstract)|model]]s forecast that even in the unlikely event of greenhouse gases stabilizing at present levels, the Earth would warm by an additional 0.5°C by 2100, a similar rise in temperature to that seen during the 20th century. In 2050, as much as 64% of that commitment would be due to ''past'' natural forcings. Over time, their contribution compared to the human influence will diminish. Overall, the warming commitment at 2005 greenhouse gas levels could exceed 1°C.<ref>{{cite journal |last=Wigley |first=T. M. L. |title=The Climate Change Commitment |journal=Science |volume=307 |issue=5716 |pages=1766–9 |date=17 March 2005 |doi=10.1126/science.1103934 |pmid=15774756 |url=ftp://ftp.soest.hawaii.edu/coastal/Climate%20Articles/Wigley_2005%20Sea%20level%20commitment.pdf |archive-url=https://web.archive.org/web/20171011182247/ftp://ftp.soest.hawaii.edu/coastal/Climate%20Articles/Wigley_2005%20Sea%20level%20commitment.pdf |archive-date=2017-10-11 |url-status=dead |bibcode=2005Sci...307.1766W }}</ref> As ocean waters expand in response to this warming, global [[sea level]]s would mount by about 10 centimeters during that time. These models do not take into account [[ice cap]] and [[glacier]] melting; including those [[climate feedback]] effects would give a 1–1.5°C estimated temperature increase.<ref>{{cite journal |url= http://www.nature.com/news/2005/050314/full/050314-13.html |title= Oceans extend effects of climate change |first=Deirdre |last=Lockwood |date= 2005-05-17 |journal=Nature News |doi=10.1038/news050314-13|url-access=subscription }} </ref>

== History ==
The concept has been discussed as far back as 1995 in the [[Intergovernmental Panel on Climate Change|IPCC]] [[IPCC Third Assessment Report|TAR]] [https://web.archive.org/web/20071231162245/http://www.grida.no/climate/ipcc_tar/wg1/008.htm] and in the [[IPCC_Second_Assessment_Report|SAR]].

== Misuse ==
Climate commitment studies span a range of [[Shared Socioeconomic Pathways|emissions scenarios]] which are intimately tied to past, present and future human choices. The "commitment" concept is misused when worst cases are asserted to be inevitable regardless of [[agency (sociology)|social agency]]. Models rather indicate that additional surface warming can be halted almost simultaneous with rapid emissions reductions.<ref>{{cite journal |url=https://www.science.org/cms/asset/7446d1ba-b4b9-4c2d-945b-ea04ca0547d6/pap.pdf |title=Irreversible Does Not Mean Unavoidable |last1=Mathews |first1=H. Damon |last2=Solomon |first2=Susan |journal=Science |publisher=American Association for the Advancement of Science |volume=340 |issue=6131 |pages=438–439 |date=26 April 2013 |doi=10.1126/science.1236372|pmid=23539182 |bibcode=2013Sci...340..438M |s2cid=44352274 }}</ref>

== See also ==
* [[Climate change scenario]]
* [[Climate inertia]]
* [[Climate sensitivity]]

== References ==
{{refbegin}}
*{{cite journal |last=Wetherald |first=Richard T. |first2=Ronald J. |last2=Stouffer |first3=Keith W. |last3=Dixon |title=Committed warming and its implications for climate change. |journal=Geophysical Research Letters |volume=28 |issue=8 |pages=1535–8 |year=2001 |doi= 10.1029/2000gl011786|url=https://www.gfdl.noaa.gov/bibliography/related_files/rw0101.pdf |bibcode=2001GeoRL..28.1535W|doi-access=free }}
*{{cite journal |last=Meehl |first=Gerald A. |last2=Washington |first2=Warren M. |last3=Collins |first3=William D. |last4=Arblaster |first4=Julie M. |last5=Hu |first5=Aixue |last6=Buja |first6=Lawrence E. |last7=Strand |first7=Warren G. |last8=Teng |first8=Haiyan |title=How Much More Global Warming and Sea Level Rise? |journal=Science |volume=307 |issue=5716 |pages=1769–72 |date=17 March 2005 |pmid=15774757 |doi=10.1126/science.1106663 |url=http://www.geo.utexas.edu/courses/387h/PAPERS/meehl2005.pdf |bibcode=2005Sci...307.1769M |doi-access=free }}
{{refend}}
{{reflist}}

== External links ==

{{Global warming}}
[[Category:Climatology]]

Climate commitment - Revision history

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