View source for Global warming potential

{{Short description|Potential heat absorbed by a greenhouse gas}}
[[File:Perfluorotributylamine-global-warming-potential.jpg|thumb|Comparison of global warming potential (GWP) of three [[greenhouse gas]]es over a 100-year period: [[Perfluorotributylamine]], [[nitrous oxide]] and [[methane]], compared to [[carbon dioxide]] (the latter is the reference value, therefore it has a GWP of one)]]

'''Global warming potential (GWP)''' is a measure of how much heat a [[greenhouse gas]] traps in the atmosphere over a specific time period, relative to [[carbon dioxide]] ({{CO2}}).<ref name=":2" />{{Rp|page=2232}} It is expressed as a multiple of warming caused by the same [[mass]] of [[carbon dioxide]] ({{CO2}}). Therefore, by definition {{CO2}} has a GWP of 1. For other [[gas]]es it depends on how strongly the gas absorbs thermal radiation, how quickly the gas leaves the atmosphere, and the time frame considered.

For example, [[Methane emissions|methane]] has a GWP over 20 years (GWP-20) of 81.2<ref name=":0">{{Citation |title=7.SM.6 Tables of greenhouse gas lifetimes, radiative efficiencies and metrics |date=2021 |url=https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter_07_Supplementary_Material.pdf |page=7SM-24 |publisher=[[IPCC]]}}.</ref> meaning that, a [[Fugitive gas emissions|leak]] of a tonne of methane is equivalent to emitting 81.2 tonnes of carbon dioxide measured over 20 years. As methane has a much shorter atmospheric lifetime than carbon dioxide, its GWP is much less over longer time periods, with a GWP-100 of 27.9 and a GWP-500 of 7.95.<ref name=":0" />{{Rp|page=7SM-24}}

The '''carbon dioxide equivalent''' ({{CO2}}e or {{CO2}}eq or {{CO2}}-e or {{CO2}}-eq) can be calculated from the GWP. For any gas, it is the mass of {{CO2}} that would warm the earth as much as the mass of that gas. Thus it provides a common scale for measuring the [[Effects of climate change|climate effects]] of different gases. It is calculated as GWP times mass of the other gas.

== Definition ==
{{See also|Radiative forcing}}

The global warming potential (GWP) is defined as an "index measuring the [[radiative forcing]] following an emission of a unit mass of a given substance, accumulated over a chosen time horizon, relative to that of the reference substance, carbon dioxide (CO<sub>2</sub>). The GWP thus represents the combined effect of the differing times these substances remain in the atmosphere and their effectiveness in causing radiative forcing."<ref name=":2">IPCC, 2021: Annex VII: [https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_AnnexVII.pdf Glossary] [Matthews, J.B.R., V. Möller, R. van Diemen, J.S. Fuglestvedt, V. Masson-Delmotte, C.  Méndez, S. Semenov, A. Reisinger (eds.)]. In [https://www.ipcc.ch/report/ar6/wg1/ Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change] [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 2215–2256, doi:10.1017/9781009157896.022.</ref>{{Rp|page=2232}}

In turn, ''radiative forcing'' is a scientific concept used to quantify and compare the external drivers of change to [[Earth's energy balance]].<ref name="nrcrf">{{cite book |author=National Research Council |title=Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties |publisher=The National Academic Press |year=2005 |isbn=978-0-309-09506-8 |doi=10.17226/11175}}</ref>{{rp|1–4}} Radiative forcing is the change in [[energy flux]] in the atmosphere caused by [[Climate variability and change|natural]] or [[Human impact on the environment#Impacts on climate|anthropogenic]] factors of [[climate change]] as measured in [[watt]]s per meter squared.<ref>{{cite web |last=Drew |first=Shindell |author-link= |year=2013 |title=Climate Change 2013: The Physical Science Basis – Working Group 1 contribution to the IPCC Fifth Assessment Report: Radiative Forcing in the AR5 |url=http://climate.envsci.rutgers.edu/climdyn2013/IPCC/IPCC_WGI12-RadiativeForcing.pdf |url-access= |url-status=live |archive-url=https://web.archive.org/web/20160304083735/http://climate.envsci.rutgers.edu/climdyn2013/IPCC/IPCC_WGI12-RadiativeForcing.pdf |archive-date=4 March 2016 |access-date=15 September 2016 |website=envsci.rutgers.edu |series= |publisher=[[Rutgers University]] |agency=[[IPCC Fifth Assessment Report|Fifth Assessment Report]] (AR5) |department=Department of Environmental Sciences, School of Environmental and Biological Sciences}}</ref>

=== GWP in policymaking ===
As governments develop policies to combat emissions from high-GWP sources, policymakers have chosen to use the 100-year GWP scale as the standard in international agreements. The [[Kigali Amendment]] to the [[Montreal Protocol]] sets the global phase-down of [[hydrofluorocarbon]]s (HFCs), a group of high-GWP compounds. It requires countries to use a set of GWP100 values equal to those published in the IPCC's Fourth Assessment Report (AR4).<ref name=":5">'''[https://treaties.un.org/doc/Treaties/2016/10/20161015%2003-23%20PM/Ch_XXVII-2.f-English%20and%20French.pdf Amendment to the Montreal Protocol on Substances that Deplete the Ozone Layer] (the "Montreal Protocol"), adopted at Kigali on October 15, 2016, by the Twenty-Eighth Meeting of the Parties to the Montreal Protocol (the "Kigali Amendment").'''</ref> This allows policymakers to have one standard for comparison instead of changing GWP values in new assessment reports.<ref>{{Cite web |date=August 8, 2024 |title=Understanding Global Warming Potentials |url=https://www.epa.gov/ghgemissions/understanding-global-warming-potentials |access-date=August 26, 2024 |website=US EPA, Greenhouse Gas Emissions}}</ref> One exception to the GWP100 standard exists: [[Government of New York (state)|New York state]]’s [[Climate Leadership and Community Protection Act]] requires the use of GWP20, despite being a different standard from all other countries participating in phase downs of HFCs.<ref name=":5" />

== Calculated values ==
{{See also|Greenhouse gas#Global warming potential (GWP) and CO2 equivalents}}

=== Current values (IPCC Sixth Assessment Report from 2021) ===
[[File:Global-warming-potential-of-greenhouse-gases-over-100-year-timescale-gwp (OWID 0525).png|thumb|Global warming potential of five greenhouse gases over 100-year timescale.<ref>{{Cite web |title=Global warming potential of greenhouse gases relative to CO2 |url=https://ourworldindata.org/grapher/global-warming-potential-of-greenhouse-gases-over-100-year-timescale-gwp |access-date=2023-12-18 |website=Our World in Data}}</ref>]]The global warming potential (GWP) depends on both the efficiency of the molecule as a greenhouse gas and its atmospheric lifetime. GWP is measured relative to the same mass of {{CO2}} and evaluated for a specific timescale.<ref name=":02">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> Thus, if a gas has a high (positive) radiative forcing but also a short lifetime, it will have a large GWP on a 20-year scale but a small one on a 100-year scale. Conversely, if a molecule has a longer atmospheric lifetime than {{CO2}} its GWP will increase when the timescale is considered. Carbon dioxide is defined to have a GWP of 1 over all time periods.

Methane has an atmospheric lifetime of 12 ± 2 years.<ref name="ar6 WG1 Ch 7" />{{rp|Table 7.15}} The [[IPCC Sixth Assessment Report|2021 IPCC report]] lists the GWP as 83 over a time scale of 20 years, 30 over 100 years and 10 over 500 years.<ref name="ar6 WG1 Ch 7" />{{rp|Table 7.15}} The decrease in GWP at longer times is because [[Atmospheric methane#Removal processes|methane]] decomposes to water and {{CO2}} through chemical reactions in the atmosphere. Similarly the third most important GHG, [[nitrous oxide]] (N<sub>2</sub>O), is a common gas emitted through the [[denitrification]] part of the [[nitrogen cycle]].<ref>{{Cite journal |last1=Yang |first1=Rui |last2=Yuan |first2=Lin-jiang |last3=Wang |first3=Ru |last4=He |first4=Zhi-xian |last5=Lei |first5=Lin |last6=Ma |first6=Yan-chen |date=2022 |title=Analyzing the mechanism of nitrous oxide production in aerobic phase of anoxic/aerobic sequential batch reactor from the perspective of key enzymes |url=https://link.springer.com/10.1007/s11356-022-18800-3 |journal=Environmental Science and Pollution Research |language=en |volume=29 |issue=26 |pages=39877–39887 |doi=10.1007/s11356-022-18800-3 |issn=0944-1344}}</ref> It has a lifetime of 109 years and an even higher GWP level running at 273 over 20 and 100 years.

Examples of the atmospheric lifetime and GWP relative to {{CO2}} for several greenhouse gases are given in the following table:
{| class="wikitable sortable" style="text-align: right"
|+Atmospheric lifetime and global warming potential (GWP) relative to {{CO2}} at different time horizon for various greenhouse gases (more values provided at global warming potential)
!Gas name
!Chemical 
formula
!Lifetime 
(years)<ref name="ar6 WG1 Ch 7" />{{rp|Table 7.15}}<ref name="TableOfWarmingPotentials5">{{cite book |title=Intergovernmental Panel on Climate Change Fifth Assessment Report |page=731 |chapter=Appendix 8.A |access-date=6 November 2017 |chapter-url=http://www.ipcc.ch/pdf/assessment-report/ar5/wg1/WG1AR5_Chapter08_FINAL.pdf |archive-url=https://web.archive.org/web/20171013100414/http://www.ipcc.ch/pdf/assessment-report/ar5/wg1/WG1AR5_Chapter08_FINAL.pdf |archive-date=13 October 2017 |url-status=live}}</ref>
!Radiative Efficiency 
(Wm{{sup|−2}}ppb{{sup|−1}}, molar basis).<ref name="ar6 WG1 Ch 7" />{{rp|Table 7.15}}<ref name="TableOfWarmingPotentials5" />
!20 year GWP<ref name="ar6 WG1 Ch 7" />{{rp|Table 7.15}}<ref name="TableOfWarmingPotentials5" />
!100 year GWP<ref name="ar6 WG1 Ch 7" />{{rp|Table 7.15}}<ref name="TableOfWarmingPotentials5" />
!500 year GWP<ref name="ar6 WG1 Ch 7" />{{rp|Table 7.15}}<ref name="TableOfWarmingPotentials">{{cite book |title=IPCC Fourth Assessment Report |page=212 |chapter=Table 2.14 |access-date=16 December 2008 |chapter-url=http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter2.pdf |archive-url=https://web.archive.org/web/20071215200559/http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter2.pdf |archive-date=15 December 2007 |url-status=live}}</ref>
|-
| style="text-align:left;" |[[Carbon dioxide]]
| style="text-align:center;" |{{CO2}}
|<sup>(A)</sup>
|{{val|1.37e-5}}
|1
|1
|1
|-
| style="text-align:left;" |[[Methane]] (fossil [[natural gas]])
| style="text-align:center;" |{{chem|CH|4}}
|12
|{{val|5.7e-4}}
|83
|30
|10
|-
| style="text-align:left;" |[[Methane]] (pure non-fossil)
| style="text-align:center;" |{{chem|CH|4}}
|12
|{{val|5.7e-4}}
|81
|27
|7.3
|- id="N2O"
| style="text-align:left;" |[[Nitrous oxide]]
| style="text-align:center;" |{{chem|N|2|O}}
|109
|{{val|3e-3}}
|273
|273
|130
|-
| style="text-align:left;" |[[CFC-11]] (R-11)
| style="text-align:center;" |{{chem|CCl|3|F}}
|52
|{{val|0.29}}
|8321
|6226
|2093
|-
| style="text-align:left;" |[[CFC-12]] (R-12)
| style="text-align:center;" |{{chem|CCl|2|F|2}}
|100
|{{val|0.32}}
|10800
|10200
|5200
|-
| style="text-align:left;" |[[HCFC-22]] (R-22)
| style="text-align:center;" |{{chem|CHClF|2}}
|12
|{{val|0.21}}
|5280
|1760
|549
|-
| style="text-align:left;" |[[HFC-32]] (R-32)
| style="text-align:center;" |{{chem|CH|2|F|2}}
|5
|{{val|0.11}}
|2693
|771
|220
|-
| style="text-align:left;" |[[HFC-134a]] (R-134a)
| style="text-align:center;" |{{chem|CH|2|FCF|3}}
|14
|{{val|0.17}}
|4144
|1526
|436
|-
| style="text-align:left;" |[[Tetrafluoromethane]] (R-14)
| style="text-align:center;" |{{chem|CF|4}}
|50000
|{{val|0.09}}
|5301
|7380
|10587
|-
| style="text-align:left;" |[[Hexafluoroethane]]
| style="text-align:center;" |{{chem|C|2|F|6}}
|10 000
|{{val|0.25}}
|8210
|11100
|18200
|-
| style="text-align:left;" |[[Sulfur hexafluoride]]
| style="text-align:center;" |{{chem|SF|6}}
|3 200
|{{val|0.57}}
|17500
|23500
|32600
|-
| style="text-align:left;" |[[Nitrogen trifluoride]]
| style="text-align:center;" |{{chem|NF|3}}
|500
|{{val|0.20}}
|12800
|16100
|20700
|-
! colspan="7" |<small><sup>(A)</sup> No single lifetime for atmospheric {{CO2}} can be given.</small>
|}
Estimates of GWP values over 20, 100 and 500 years are periodically compiled and revised in reports from the [[Intergovernmental Panel on Climate Change]]. The most recent report is the [[IPCC Sixth Assessment Report]] (Working Group I) from 2023.<ref name="ar6 WG1 Ch 7">Forster, P., T. Storelvmo, K. Armour, W. Collins, J.-L. Dufresne, D. Frame, D.J. Lunt, T. Mauritsen, M.D. Palmer, M. Watanabe, M. Wild, and H. Zhang, 2021: [https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter07.pdf Chapter 7: The Earth's Energy Budget, Climate Feedbacks, and Climate Sensitivity]. In https://www.ipcc.ch/report/ar6/wg1/ [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. 923–1054, doi:10.1017/9781009157896.009.</ref>

The IPCC lists many other substances not shown here.<ref name="ar5">{{Harvnb|IPCC AR5 WG1 Ch8|2013|pages=714, 731}}.</ref><ref name="ar6 WG1 Ch 7" /> <ref> {{cite web |title= IPCC Sixth Assessment Report: The Physical Science Basis Ch7.Supp Mat Table 7
|url=https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_FGD_Chapter07_SM.pdf
|archive-url=https://web.archive.org/web/20240630195210/https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_FGD_Chapter07_SM.pdf 
|archive-date=30 June 2024}}</ref> Some have high GWP but only a low concentration in the atmosphere.

The values given in the table assume the same mass of compound is analyzed; different ratios will result from the conversion of one substance to another. For instance, burning [[methane]] to carbon dioxide would reduce the global warming impact, but by a smaller factor than 25:1 because the mass of [[methane]] burned is less than the mass of [[carbon dioxide]] released (ratio 1:2.74).<ref>This is so, because of the reaction formula: CH<sub>4</sub> + 2O<sub>2</sub> → {{CO2}} + 2 H<sub>2</sub>O. As mentioned in the article, the oxygen and water is not considered for GWP purposes, and one molecule of methane (molar mass = 16.04 g mol<sup>−1</sup>) will yield one molecule of carbon dioxide (molar mass = 44.01 g mol<sup>−1</sup>). This gives a mass ratio of 2.74. (44.01/16.04 ≈ 2.74).</ref> For a starting amount of 1 tonne of methane, which has a GWP of 25, after combustion there would be 2.74 tonnes of {{CO2}}, each tonne of which has a GWP of 1. This is a net reduction of 22.26 tonnes of GWP, reducing the global warming effect by a ratio of 25:2.74 (approximately 9 times).

{| class="wikitable sortable"
! rowspan="2" | Greenhouse gas
! rowspan="2" data-sort-type="number" | Lifetime <br />(years)
! colspan="3" | Global warming potential, GWP
|-
! data-sort-type="number" | 20 years
! data-sort-type="number" | 100 years
! data-sort-type="number" | 500 years
|-
|[[Hydrogen]] (H<sub>2</sub>)
|4–7<ref name=":3">{{Cite report |url=https://www.gov.uk/government/publications/atmospheric-implications-of-increased-hydrogen-use |title=Atmospheric implications of increased hydrogen use |last1=Warwick |first1=Nicola |last2=Griffiths |first2=Paul |date=2022-04-08 |publisher=UK Department for Business, Energy & Industrial Strategy (BEIS) |last3=Keeble |first3=James |last4=Archibald |first4=Alexander |last5=John |first5=Pile |ref={{Harvid|Warwick|2022}}}}</ref>
| data-sort-value="33" | 33 (20–44)<ref name=":3" />
| data-sort-value="11" | 11 (6–16)<ref name=":3" />|| {{n/a}}
|-
|[[Methane]] ({{CH4}})
|11.8<ref name="ar6 WG1 Ch 7" />
| data-sort-value="70" |56<ref name="sar">{{Harvnb|IPCC SAR WG1 Ch2|1995|p=121}}.</ref><br />72<ref name="ar4">{{Harvnb|IPCC AR4 WG1 Ch2|2007|p=212}}.</ref><br />84 / 86f<ref name="ar5" /><br />96<ref>{{Cite journal |last=Alvarez |date=2018 |title=Assessment of methane emissions from the U.S. oil and gas supply chain |url=http://ws680.nist.gov/publication/get_pdf.cfm?pub_id=924889 |journal=Science |volume=361 |issue=6398 |pages=186–188 |bibcode=2018Sci...361..186A |doi=10.1126/science.aar7204 |pmc=6223263 |pmid=29930092}}</ref><br />80.8 (biogenic)<ref name="ar6 WG1 Ch 7" /><br /> 82.5 (fossil)<ref name="ar6 WG1 Ch 7" />
| data-sort-value="30" |21<ref name="sar" /><br />25<ref name="ar4" /><br />28 / 34f<ref name="ar5" /><br />32<ref>{{cite journal |last1=Etminan |first1=M. |last2=Myhre |first2=G. |last3=Highwood |first3=E. J. |last4=Shine |first4=K. P. |date=28 December 2016 |title=Radiative forcing of carbon dioxide, methane, and nitrous oxide: A significant revision of the methane radiative forcing |journal=Geophysical Research Letters |volume=43 |issue=24 |bibcode=2016GeoRL..4312614E |doi=10.1002/2016GL071930 |doi-access=free}}</ref><br />39 (biogenic)<ref name=":4">{{cite news |last1=Morton |first1=Adam |date=26 August 2020 |title=Methane released in gas production means Australia's emissions may be 10% higher than reported |work=The Guardian |url=https://www.theguardian.com/environment/2020/aug/26/methane-released-in-gas-production-means-australias-emissions-may-be-10-higher-than-reported}}</ref><br />40 (fossil)<ref name=":4" />
| data-sort-value="7" |6.5<ref name="sar" /><br />7.6<ref name="ar4" />
|-
|[[Nitrous oxide]] ({{N2O}})
|109<ref name="ar6 WG1 Ch 7" />
| data-sort-value="270" |280<ref name="sar" /><br />289<ref name="ar4" /><br />264 / 268f<ref name="ar5" /><br />273<ref name="ar6 WG1 Ch 7" />
| data-sort-value="300" |310<ref name="sar" /><br />298<ref name="ar4" /><br />265 / 298f<ref name="ar5" /><br />273<ref name="ar6 WG1 Ch 7" />
| data-sort-value="160" |170<ref name="sar" /><br />153<ref name="ar4" /><br />130<ref name="ar6 WG1 Ch 7" />
|-
|[[HFC-134a]] ([[hydrofluorocarbon]])
|14.0<ref name="ar6 WG1 Ch 7" />
| data-sort-value="4000" |3,710 / 3,790f<ref name="ar5" /><br />4,144<ref name="ar6 WG1 Ch 7" />
| data-sort-value="1500" |1,300 / 1,550f<ref name="ar5" /><br />1,526<ref name="ar6 WG1 Ch 7" />
| data-sort-value="435" |435<ref name="ar4" /><br />436<ref name="ar6 WG1 Ch 7" />
|-
|[[CFC-11]] ([[chlorofluorocarbon]])
|52.0<ref name="ar6 WG1 Ch 7" />
| data-sort-value="7000" |6,900 / 7,020f<ref name="ar5" /><br />8,321<ref name="ar6 WG1 Ch 7" />
| data-sort-value="5000" |4,660 / 5,350f<ref name="ar5" /><br />6,226<ref name="ar6 WG1 Ch 7" />
| data-sort-value="2000" |1,620<ref name="ar4" /><br />2,093<ref name="ar6 WG1 Ch 7" />
|-
|[[Carbon tetrafluoride]] (CF{{sub|4}} / PFC-14)
|50,000<ref name="ar6 WG1 Ch 7" />
| data-sort-value="5000" |4,880 / 4,950f<ref name="ar5" /><br />5,301<ref name="ar6 WG1 Ch 7" />
| data-sort-value="7000" |6,630 / 7,350f<ref name="ar5" /><br />7,380<ref name="ar6 WG1 Ch 7" />
| data-sort-value="11000" |11,200<ref name="ar4" /><br />10,587<ref name="ar6 WG1 Ch 7" />
|-
| [[HFC-23]] ([[hydrofluorocarbon]])
|222<ref name="ar5" />
| data-sort-value="11000" |12,000<ref name="ar4" /><br />10,800<ref name="ar5" />
| data-sort-value="13000" |14,800<ref name="ar4" /><br />12,400<ref name="ar5" />
| data-sort-value="12200" |12,200<ref name="ar4" />
|-
|[[Sulfur hexafluoride]] {{chem2|SF6}}
|3,200<ref name="ar5" />
| data-sort-value="17000" |16,300<ref name="ar4" /><br />17,500<ref name="ar5" />
| data-sort-value="23000" |22,800<ref name="ar4" /><br />23,500<ref name="ar5" />
| data-sort-value="32600" |32,600<ref name="ar4" />
|}

=== Earlier values from 2007 ===
The values provided in the table below are from 2007 when they were published in the [[IPCC Fourth Assessment Report]].<ref name="unfccc19">{{Cite web |date=2014-01-31 |title=Report of the Conference of the Parties on its 19th Session |url=http://unfccc.int/resource/docs/2013/cop19/eng/10a03.pdf |url-status=live |archive-url=https://web.archive.org/web/20140713005511/http://unfccc.int/resource/docs/2013/cop19/eng/10a03.pdf |archive-date=2014-07-13 |access-date=2020-07-01 |website=UNFCCC}}</ref><ref name="ar4" /> These values are still used (as of 2020) for some comparisons.<ref name="epa20" />

{| class="wikitable sortable" style="text-align:right"
!Greenhouse gas
!Chemical formula
! data-sort-type="number" |100-year Global warming potentials<br /> (2007 estimates, for 2013–2020 comparisons)
|-
| align="left" |Carbon dioxide
|CO<sub>2</sub>
|1
|-
| align="left" |Methane
|CH<sub>4</sub>
|25
|-
| align="left" |Nitrous oxide
|N<sub>2</sub>O
|298
|-
| colspan="3" align="left" |Hydrofluorocarbons (HFCs)
|-
| align="left" |HFC-23
|CHF<sub>3</sub>
|14,800
|-
| align="left" |[[Difluoromethane]] (HFC-32)
|CH<sub>2</sub>F<sub>2</sub>
|675
|-
| align="left" |[[Fluoromethane]] (HFC-41)
|CH<sub>3</sub>F
|92
|-
| align="left" |HFC-43-10mee
|CF<sub>3</sub>CHFCHFCF<sub>2</sub>CF<sub>3</sub>
|1,640
|-
| align="left" |[[Pentafluoroethane]] (HFC-125)
|C<sub>2</sub>HF<sub>5</sub>
|3,500
|-
| align="left" |HFC-134
|C<sub>2</sub>H<sub>2</sub>F<sub>4</sub> (CHF<sub>2</sub>CHF<sub>2</sub>)
|1,100
|-
| align="left" |[[1,1,1,2-Tetrafluoroethane]] (HFC-134a)
|C<sub>2</sub>H<sub>2</sub>F<sub>4</sub> (CH<sub>2</sub>FCF<sub>3</sub>)
|1,430
|-
| align="left" |HFC-143
|C<sub>2</sub>H<sub>3</sub>F<sub>3</sub> (CHF<sub>2</sub>CH<sub>2</sub>F)
|353
|-
| align="left" |[[1,1,1-Trifluoroethane]] (HFC-143a)
|C<sub>2</sub>H<sub>3</sub>F<sub>3</sub> (CF<sub>3</sub>CH<sub>3</sub>)
|4,470
|-
| align="left" |HFC-152
|CH<sub>2</sub>FCH<sub>2</sub>F
|53
|-
| align="left" |HFC-152a
|C<sub>2</sub>H<sub>4</sub>F<sub>2</sub> (CH<sub>3</sub>CHF<sub>2</sub>)
|124
|-
| align="left" |HFC-161
|CH<sub>3</sub>CH<sub>2</sub>F
|12
|-
| align="left" |[[1,1,1,2,3,3,3-Heptafluoropropane]] (HFC-227ea)
|C<sub>3</sub>HF<sub>7</sub>
|3,220
|-
| align="left" |HFC-236cb
|CH<sub>2</sub>FCF<sub>2</sub>CF<sub>3</sub>
|1,340
|-
| align="left" |HFC-236ea
|CHF<sub>2</sub>CHFCF<sub>3</sub>
|1,370
|-
| align="left" |HFC-236fa
|C<sub>3</sub>H<sub>2</sub>F<sub>6</sub>
|9,810
|-
| align="left" |HFC-245ca
|C<sub>3</sub>H<sub>3</sub>F<sub>5</sub>
|693
|-
| align="left" |HFC-245fa
|CHF<sub>2</sub>CH<sub>2</sub>CF<sub>3</sub>
|1,030
|-
| align="left" |HFC-365mfc
|CH<sub>3</sub>CF<sub>2</sub>CH<sub>2</sub>CF<sub>3</sub>
|794
|-
| colspan="3" align="left" |[[Perfluorocarbons]]
|-
| align="left" |[[Carbon tetrafluoride]] – PFC-14
|CF<sub>4</sub>
|7,390
|-
| align="left" |[[Hexafluoroethane]] – PFC-116
|C<sub>2</sub>F<sub>6</sub>
|12,200
|-
| align="left" |[[Octafluoropropane]] – PFC-218
|C<sub>3</sub>F<sub>8</sub>
|8,830
|-
| align="left" |[[Perfluorobutane]] – PFC-3-1-10
|C<sub>4</sub>F<sub>10</sub>
|8,860
|-
| align="left" |[[Octafluorocyclobutane]] – PFC-318
|c-C<sub>4</sub>F<sub>8</sub>
|10,300
|-
| align="left" |[[Perfluoropentane|Perfluouropentane]] – PFC-4-1-12
|C<sub>5</sub>F<sub>12</sub>
|9,160
|-
| align="left" |[[Perfluorohexane]] – PFC-5-1-14
|C<sub>6</sub>F<sub>14</sub>
|9,300
|-
| align="left" |[[Perfluorodecalin]] – PFC-9-1-18b
|C<sub>10</sub>F<sub>18</sub>
|7,500
|-
| align="left" |Perfluorocyclopropane
|c-C<sub>3</sub>F<sub>6</sub>
|17,340
|-
| colspan="3" align="left" |[[Sulfur hexafluoride]] (SF<sub>6</sub>)
|-
| align="left" |Sulfur hexafluoride
|SF<sub>6</sub>
|22,800
|-
| colspan="3" align="left" |[[Nitrogen trifluoride]] (NF<sub>3</sub>)
|-
| align="left" |Nitrogen trifluoride
|NF<sub>3</sub>
|17,200
|-
| colspan="3" align="left" |[[Hydrofluoroether|Fluorinated ethers]]
|-
| align="left" |HFE-125
|CHF<sub>2</sub>OCF<sub>3</sub>
|14,900
|-
| align="left" |[[Bis(difluoromethyl) ether]] (HFE-134)
|CHF<sub>2</sub>OCHF<sub>2</sub>
|6,320
|-
| align="left" |HFE-143a
|CH<sub>3</sub>OCF<sub>3</sub>
|756
|-
| align="left" |HCFE-235da2
|CHF<sub>2</sub>OCHClCF<sub>3</sub>
|350
|-
| align="left" |HFE-245cb2
|CH<sub>3</sub>OCF<sub>2</sub>CF<sub>3</sub>
|708
|-
| align="left" |HFE-245fa2
|CHF<sub>2</sub>OCH<sub>2</sub>CF<sub>3</sub>
|659
|-
| align="left" |HFE-254cb2
|CH<sub>3</sub>OCF<sub>2</sub>CHF<sub>2</sub>
|359
|-
| align="left" |HFE-347mcc3
|CH<sub>3</sub>OCF<sub>2</sub>CF<sub>2</sub>CF<sub>3</sub>
|575
|-
| align="left" |HFE-347pcf2
|CHF<sub>2</sub>CF<sub>2</sub>OCH<sub>2</sub>CF<sub>3</sub>
|580
|-
| align="left" |HFE-356pcc3
|CH<sub>3</sub>OCF<sub>2</sub>CF<sub>2</sub>CHF<sub>2</sub>
|110
|-
| align="left" |HFE-449sl (HFE-7100)
|C<sub>4</sub>F<sub>9</sub>OCH<sub>3</sub>
|297
|-
| align="left" |HFE-569sf2 (HFE-7200)
|C<sub>4</sub>F9OC<sub>2</sub>H<sub>5</sub>
|59
|-
| align="left" |HFE-43-10pccc124 (H-Galden 1040x)
|CHF<sub>2</sub>OCF<sub>2</sub>OC<sub>2</sub>F<sub>4</sub>OCHF<sub>2</sub>
|1,870
|-
| align="left" |HFE-236ca12 (HG-10)
|CHF<sub>2</sub>OCF<sub>2</sub>OCHF<sub>2</sub>
|2,800
|-
| align="left" |HFE-338pcc13 (HG-01)
|CHF<sub>2</sub>OCF<sub>2</sub>CF<sub>2</sub>OCHF<sub>2</sub>
|1,500
|-
| align="left" |
|(CF<sub>3</sub>)<sub>2</sub>CFOCH<sub>3</sub>
|343
|-
| align="left" |
|CF<sub>3</sub>CF<sub>2</sub>CH<sub>2</sub>OH
|42
|-
| align="left" |
|(CF<sub>3</sub>)<sub>2</sub>CHOH
|195
|-
| align="left" |HFE-227ea
|CF<sub>3</sub>CHFOCF<sub>3</sub>
|1,540
|-
| align="left" |HFE-236ea2
|CHF<sub>2</sub>OCHFCF<sub>3</sub>
|989
|-
| align="left" |HFE-236fa
|CF<sub>3</sub>CH<sub>2</sub>OCF<sub>3</sub>
|487
|-
| align="left" |HFE-245fa1
|CHF<sub>2</sub>CH<sub>2</sub>OCF<sub>3</sub>
|286
|-
| align="left" |HFE-263fb2
|CF<sub>3</sub>CH<sub>2</sub>OCH<sub>3</sub>
|11
|-
| align="left" |HFE-329mcc2
|CHF<sub>2</sub>CF<sub>2</sub>OCF<sub>2</sub>CF<sub>3</sub>
|919
|-
| align="left" |HFE-338mcf2
|CF<sub>3</sub>CH<sub>2</sub>OCF<sub>2</sub>CF<sub>3</sub>
|552
|-
| align="left" |HFE-347mcf2
|CHF<sub>2</sub>CH<sub>2</sub>OCF<sub>2</sub>CF<sub>3</sub>
|374
|-
| align="left" |HFE-356mec3
|CH<sub>3</sub>OCF<sub>2</sub>CHFCF<sub>3</sub>
|101
|-
| align="left" |HFE-356pcf2
|CHF<sub>2</sub>CH<sub>2</sub>OCF<sub>2</sub>CHF<sub>2</sub>
|265
|-
| align="left" |HFE-356pcf3
|CHF<sub>2</sub>OCH<sub>2</sub>CF<sub>2</sub>CHF<sub>2</sub>
|502
|-
| align="left" |HFE-365mcfI’ll t3
|CF<sub>3</sub>CF<sub>2</sub>CH<sub>2</sub>OCH<sub>3</sub>
|11
|-
| align="left" |HFE-374pc2
|CHF<sub>2</sub>CF<sub>2</sub>OCH<sub>2</sub>CH<sub>3</sub>
|557
|-
| align="left" |
|– (CF<sub>2</sub>)<sub>4</sub>CH (OH) –
|73
|-
| align="left" |
|(CF<sub>3</sub>)<sub>2</sub>CHOCHF<sub>2</sub>
|380
|-
| align="left" |
|(CF<sub>3</sub>)<sub>2</sub>CHOCH<sub>3</sub>
|27
|-
| colspan="3" align="left" |[[Perfluoropolyether]]s
|-
| align="left" |PFPMIE
|CF<sub>3</sub>OCF(CF<sub>3</sub>)CF<sub>2</sub>OCF<sub>2</sub>OCF<sub>3</sub>
|10,300
|-
| align="left" |[[Trifluoromethylsulfur pentafluoride|Trifluoromethyl sulfur pentafluoride]]
|SF<sub>5</sub>CF<sub>3</sub>
|17,400
|}

=== Importance of time horizon ===
A substance's GWP depends on the number of years (denoted by a subscript) over which the potential is calculated. A gas which is quickly removed from the atmosphere may initially have a large effect, but for longer time periods, as it has been removed, it becomes less important. Thus methane has a potential of 25 over 100 years (GWP<sub>100</sub> = 25) but 86 over 20 years (GWP<sub>20</sub> = 86); conversely [[sulfur hexafluoride]] has a GWP of 22,800 over 100 years but 16,300 over 20 years (IPCC Third Assessment Report). The GWP value depends on how the gas concentration decays over time in the atmosphere. This is often not precisely known and hence the values should not be considered exact. For this reason when quoting a GWP it is important to give a reference to the calculation.

The GWP for a mixture of gases can be obtained from the mass-fraction-weighted average of the GWPs of the individual gases.<ref>[http://eur-lex.europa.eu/legal-content/EN/TXT/?uri=uriserv:OJ.L_.2014.150.01.0195.01.ENG Regulation (EU) No 517/2014 of the European Parliament and of the Council of 16 April 2014 on fluorinated greenhouse gases] Annex IV.</ref>

Commonly, a time horizon of 100 years is used by regulators.<ref name=":1">{{cite web |date=12 January 2016 |title=Understanding Global Warming Potentials |url=https://www.epa.gov/ghgemissions/understanding-global-warming-potentials |access-date=2021-03-02 |publisher=[[United States Environmental Protection Agency]]}}</ref><ref>{{cite journal |last1=Abernethy |first1=Sam |last2=Jackson |first2=Robert B |title=Global temperature goals should determine the time horizons for greenhouse gas emission metrics |journal=Environmental Research Letters |date=February 2022 |volume=17 |issue=2 |pages=024019 |doi=10.1088/1748-9326/ac4940 |arxiv=2104.05506 |bibcode=2022ERL....17b4019A |s2cid=233209965 }}</ref>

=== Water vapour ===
{{See also|Greenhouse gas#Special role of water vapor|Climate change feedbacks#Water vapor feedback (positive)}}
[[Water vapour]] does contribute to anthropogenic global warming, but as the GWP is defined, it is negligible for H<sub>2</sub>O: an estimate gives a 100-year GWP between -0.001 and 0.0005.<ref>{{cite journal | doi=10.1088/1748-9326/aae018 | title=The global warming potential of near-surface emitted water vapour | year=2018 | last1=Sherwood | first1=Steven C. | last2=Dixit | first2=Vishal | last3=Salomez | first3=Chryséis | journal=Environmental Research Letters | volume=13 | issue=10 | page=104006 | bibcode=2018ERL....13j4006S | s2cid=158806342 | doi-access=free | hdl=1959.4/unsworks_57193 | hdl-access=free }}</ref>

H<sub>2</sub>O can function as a greenhouse gas because it has a profound infrared absorption spectrum with more and broader absorption bands than {{CO2}}. Its concentration in the atmosphere is limited by air temperature, so that radiative forcing by water vapour increases with global warming (positive feedback). But the GWP definition excludes indirect effects. GWP definition is also based on emissions, and anthropogenic emissions of water vapour ([[cooling tower]]s, [[irrigation]]) are removed via [[precipitation]] within weeks, so its GWP is negligible.

== Calculation methods ==
[[File:1979-_Radiative_forcing_-_climate_change_-_global_warming_-_EPA_NOAA.svg|right|thumb|upright=1.6|The [[radiative forcing]] (warming influence) of long-lived atmospheric greenhouse gases has accelerated, almost doubling in 40 years.<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 |title=Annual Greenhouse Gas Index |url=https://www.globalchange.gov/browse/indicators/annual-greenhouse-gas-index |url-status=live |archive-url=https://web.archive.org/web/20210421143115/https://www.globalchange.gov/browse/indicators/annual-greenhouse-gas-index |archive-date=21 April 2021 |access-date=5 September 2020 |publisher=U.S. Global Change Research Program}}</ref><ref name="butmon">{{Cite web |author=Butler J. and Montzka S. |year=2020 |title=The NOAA Annual Greenhouse Gas Index (AGGI) |url=https://www.esrl.noaa.gov/gmd/aggi/aggi.html |url-status=live |archive-url=https://web.archive.org/web/20130922035917/https://www.esrl.noaa.gov/gmd/aggi/aggi.html |archive-date=22 September 2013 |access-date=5 September 2020 |publisher=[[NOAA]] Global Monitoring Laboratory/Earth System Research Laboratories}}</ref>]]
When calculating the GWP of a greenhouse gas, the value depends on the following factors:
* the absorption of [[infrared radiation]] by the given gas
* the time horizon of interest (integration period)
* the [[atmospheric lifetime]] of the gas
A high GWP correlates with a large infrared absorption and a long atmospheric lifetime. The dependence of GWP on the wavelength of absorption is more complicated. Even if a gas absorbs radiation efficiently at a certain wavelength, this may not affect its GWP much, if the atmosphere already absorbs most radiation at that wavelength. A gas has the most effect if it absorbs in a "window" of wavelengths where the atmosphere is fairly transparent. The dependence of GWP as a function of wavelength has been found empirically and published as a graph.<ref>[http://www.chem.tamu.edu/rgroup/north/ITS%20GWP%20Data.xls Matthew Elrod, "Greenhouse Warming Potential Model."] Based on {{Cite journal |last1=Elrod |first1=M. J. |year=1999 |title=Greenhouse Warming Potentials from the Infrared Spectroscopy of Atmospheric Gases |journal=Journal of Chemical Education |volume=76 |issue=12 |pages=1702 |bibcode=1999JChEd..76.1702E |doi=10.1021/ed076p1702}}</ref>

Because the GWP of a greenhouse gas depends directly on its infrared spectrum, the use of [[infrared spectroscopy]] to study greenhouse gases is centrally important in the effort to understand the impact of human activities on global [[climate change]].

Just as [[radiative forcing]] provides a simplified means of comparing the various factors that are believed to influence the climate system to one another, global warming potentials (GWPs) are one type of simplified index based upon radiative properties that can be used to estimate the potential future impacts of emissions of different gases upon the climate system in a relative sense. GWP is based on a number of factors, including the radiative efficiency (infrared-absorbing ability) of each gas relative to that of carbon dioxide, as well as the decay rate of each gas (the amount removed from the atmosphere over a given number of years) relative to that of carbon dioxide.<ref>
{{cite web 
|url=http://www.eia.gov/tools/glossary/index.cfm?id=G 
|title=Glossary: Global warming potential (GWP) 
|publisher=U.S. Energy Information Administration 
|access-date=2011-04-26
|quote=An index used to compare the relative radiative forcing of different gases without directly calculating the changes in atmospheric concentrations. GWPs are calculated as the ratio of the radiative forcing that would result from the emission of one kilogram of a greenhouse gas to that from the emission of one kilogram of carbon dioxide over a fixed period of time, such as 100 years.
}}</ref>

The '''radiative forcing capacity''' (RF) is the amount of energy per unit area, per unit time, absorbed by the greenhouse gas, that would otherwise be lost to space. It can be expressed by the formula:

<math display="block">\mathit{RF} = \sum_{i=1}^{100} \text{abs}_i \cdot F_i / \left(\text{l} \cdot \text{d}\right)</math>
where the subscript ''i'' represents a [[wavenumber]] interval of 10 [[inverse centimeter]]s. Abs<sub>i</sub> represents the integrated infrared absorbance of the sample in that interval, and F<sub>i</sub> represents the RF for that interval.{{citation needed|date=September 2008}}

The [[Intergovernmental Panel on Climate Change]] (IPCC) provides the generally accepted values for GWP, which changed slightly between 1996 and 2001, except for methane, which had its GWP almost doubled. An exact definition of how GWP is calculated is to be found in the IPCC's 2001 Third Assessment Report.<ref>{{cite web |url=http://www.grida.no/climate/ipcc_tar/wg1/247.htm |title=Climate Change 2001: The Scientific Basis |website=www.grida.no |access-date=11 January 2022 |archive-url=https://web.archive.org/web/20160131050350/http://www.grida.no/climate/ipcc_tar/wg1/247.htm |archive-date=31 January 2016 |url-status=dead}}</ref> The GWP is defined as the ratio of the time-integrated radiative forcing from the instantaneous release of 1&nbsp;kg of a trace substance relative to that of 1&nbsp;kg of a reference gas:

<math display="block">\mathit{GWP} \left(x\right) = \frac{a_x}{a_r} \frac{\int_0^{\mathit{TH}} [x](t)\, dt} {\int_0^{\mathit{TH}} [r](t)\, dt}</math>
where TH is the time horizon over which the calculation is considered; a<sub>x</sub> is the radiative efficiency due to a unit increase in atmospheric abundance of the substance (i.e., Wm<sup>−2</sup> kg<sup>−1</sup>) and [x](t) is the time-dependent decay in abundance of the substance following an instantaneous release of it at time t=0. The denominator contains the corresponding quantities for the reference gas (i.e. {{CO2|link=yes}}). The radiative efficiencies a<sub>x</sub> and a<sub>r</sub> are not necessarily constant over time. While the absorption of infrared radiation by many greenhouse gases varies linearly with their abundance, a few important ones display non-linear behaviour for current and likely future abundances (e.g., {{CO2}}, CH<sub>4</sub>, and N<sub>2</sub>O). For those gases, the relative radiative forcing will depend upon abundance and hence upon the future scenario adopted.

Since all GWP calculations are a comparison to {{CO2}} which is non-linear, all GWP values are affected. Assuming otherwise as is done above will lead to lower GWPs for other gases than a more detailed approach would. Clarifying this, while increasing {{CO2}} has less and less effect on radiative absorption as ppm concentrations rise, more powerful greenhouse gases like methane and nitrous oxide have different thermal absorption frequencies to {{CO2}} that are not filled up (saturated) as much as {{CO2}}, so rising ppms of these gases are far more significant.

== Applications ==

=== Carbon dioxide equivalent ===
Carbon dioxide equivalent ({{CO2}}e or {{CO2}}eq or {{CO2}}-e) of a quantity of gas is calculated from its GWP. For any gas, it is the mass of {{CO2}} which would warm the earth as much as the mass of that gas.<ref name="epadef">{{Cite web|title=CO2e|url=https://www3.epa.gov/carbon-footprint-calculator/tool/definitions/co2e.html|access-date=2020-06-27|website=www3.epa.gov}}</ref> Thus it provides a common scale for measuring the climate effects of different gases. It is calculated as GWP multiplied by mass of the other gas. For example, if a gas has GWP of 100, two tonnes of the gas have {{CO2}}e of 200 tonnes, and 9 tonnes of the gas has {{CO2}}e of 900 tonnes.

On a global scale, the warming effects of one or more greenhouse gases in the atmosphere can also be expressed as an equivalent atmospheric concentration of {{CO2}}. {{CO2}}e can then be the atmospheric concentration of {{CO2}} which would warm the earth as much as a particular concentration of some other gas or of all gases and aerosols in the atmosphere. For example, {{CO2}}e of 500 parts per million would reflect a mix of atmospheric gases which warm the earth as much as 500 parts per million of {{CO2}} would warm it.<ref name="eea">{{Cite web |date=2020-02-25 |title=Atmospheric greenhouse gas concentrations – Rationale |url=https://www.eea.europa.eu/data-and-maps/indicators/atmospheric-greenhouse-gas-concentrations-6/assessment-1 |access-date=2020-06-28 |website=European Environment Agency |language=en}}</ref><ref name="rmets">{{cite journal |last1=Gohar |first1=L. K. |last2=Shine |first2=K. P. |title=Equivalent {{CO2}} and its use in understanding the climate effects of increased greenhouse gas concentrations |journal=Weather |date=November 2007 |volume=62 |issue=11 |pages=307–311 |doi=10.1002/wea.103 |bibcode=2007Wthr...62..307G |doi-access=free }}</ref> Calculation of the equivalent atmospheric concentration of {{CO2}} of an atmospheric greenhouse gas or aerosol is more complex and involves the atmospheric concentrations of those gases, their GWPs, and the ratios of their molar masses to the molar mass of {{CO2}}.

{{CO2}}e calculations depend on the time-scale chosen, typically 100 years or 20 years,<ref name="Wedderburn-Bisshop et al 2015">{{cite journal |last1=Wedderburn-Bisshop |first1=Gerard |last2=Longmire |first2=Andrew |last3=Rickards |first3=Lauren |title=Neglected Transformational Responses: Implications of Excluding Short Lived Emissions and Near Term Projections in Greenhouse Gas Accounting |journal=The International Journal of Climate Change: Impacts and Responses |date=2015 |volume=7 |issue=3 |pages=11–27 |id={{ProQuest|2794017083}} |doi=10.18848/1835-7156/CGP/v07i03/37242 }}</ref><ref name="OckoHamburg2017">{{cite journal |last1=Ocko |first1=Ilissa B. |last2=Hamburg |first2=Steven P. |last3=Jacob |first3=Daniel J. |last4=Keith |first4=David W. |last5=Keohane |first5=Nathaniel O. |last6=Oppenheimer |first6=Michael |last7=Roy-Mayhew |first7=Joseph D. |last8=Schrag |first8=Daniel P. |last9=Pacala |first9=Stephen W. |title=Unmask temporal trade-offs in climate policy debates |journal=Science |date=5 May 2017 |volume=356 |issue=6337 |pages=492–493 |doi=10.1126/science.aaj2350 |pmid=28473552 |bibcode=2017Sci...356..492O |s2cid=206653952 }}</ref> since gases decay in the atmosphere or are absorbed naturally, at different rates.

The following [[Units of measurement|units]] are commonly used:
* By the UN climate change panel ([[Intergovernmental Panel on Climate Change|IPCC]]): billion metric tonnes = n×10<sup>9</sup> [[tonne]]s of {{CO2}} equivalent (Gt{{CO2}}eq)<ref>{{cite journal |last1=Denison |first1=Steve |last2=Forster |first2=Piers M |last3=Smith |first3=Christopher J |title=Guidance on emissions metrics for nationally determined contributions under the Paris Agreement |journal=Environmental Research Letters |date=December 2019 |volume=14 |issue=12 |pages=124002 |doi=10.1088/1748-9326/ab4df4 |bibcode=2019ERL....14l4002D |doi-access=free }}</ref>
* In industry: million metric tonnes of carbon dioxide equivalents (MMTCDE)<ref>{{Cite web |title=Glossary:Carbon dioxide equivalent – Statistics Explained |url=https://ec.europa.eu/eurostat/statistics-explained/index.php/Glossary:Carbon_dioxide_equivalent |access-date=2020-06-28 |website=ec.europa.eu}}</ref> and MMT {{CO2}}eq.<ref name="epa20">{{Cite web |date=2020-04-13 |title=Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2018, p. ES-3 |url=https://www.epa.gov/sites/production/files/2020-04/documents/us-ghg-inventory-2020-chapter-executive-summary.pdf |url-status=live |archive-url=https://web.archive.org/web/20200414000128/https://www.epa.gov/sites/production/files/2020-04/documents/us-ghg-inventory-2020-chapter-executive-summary.pdf |archive-date=2020-04-14 |access-date=2020-07-01 |website=US Environmental Protection Agency}}</ref>
* For vehicles: grams of carbon dioxide equivalent per mile (g{{CO2}}e/mile) or per kilometer (g{{CO2}}e/km)<ref name="ucs-ev">{{Cite web |title=How Clean is Your Electric Vehicle? |url=https://evtool.ucsusa.org/ |access-date=2020-07-02 |website=Union of Concerned Scientists |language=en}}</ref><ref name="rcs">{{Cite web |last=Whitehead |first=Jake |date=2019-09-07 |title=The Truth About Electric Vehicle Emissions |url=https://www.realclearscience.com/articles/2019/09/07/the_truth_about_electric_vehicle_emissions_111097.html |access-date=2020-07-02 |website=www.realclearscience.com}}</ref>

For example, the table above shows GWP for methane over 20 years at 86 and nitrous oxide at 289, so emissions of 1 million tonnes of methane or nitrous oxide are equivalent to emissions of 86 or 289 million tonnes of carbon dioxide, respectively.

=== Use in Kyoto Protocol and for reporting to UNFCCC ===
Under the [[Kyoto Protocol]], in 1997 the Conference of the Parties standardized international reporting, by deciding (see decision number 2/CP.3) that the values of GWP calculated for the [[IPCC Second Assessment Report]] were to be used for converting the various greenhouse gas emissions into comparable {{CO2}} equivalents.<ref>
{{cite book |author=Conference of the Parties |url=http://unfccc.int/resource/docs/cop3/07a01.pdf |title=Report of the Conference of the Parties on its third session, held at Kyoto from 1 to 11 December 1997 Addendum Part Two: Action taken by the Conference of the Parties at its third session |date=25 March 1998 |publisher=[[UNFCCC]] |contribution=Methodological issues related to the Kyoto Protocol |access-date=17 January 2011 |archive-url=https://web.archive.org/web/20000823193833/http://www.unfccc.int/resource/docs/cop3/07a01.pdf |archive-date=2000-08-23 |url-status=live}}</ref><ref>{{cite journal |last1=Godal |first1=Odd |last2=Fuglestvedt |first2=Jan |date=2002 |title=Testing 100-year global warming potentials: Impacts on compliance costs and abatement profile |journal=Climatic Change |volume=52 |issue=1/2 |pages=93–127 |doi=10.1023/A:1013086803762 |s2cid=150488348 |id={{ProQuest|198550594}}}}</ref>

After some intermediate updates, in 2013 this standard was updated by the Warsaw meeting of the [[United Nations Framework Convention on Climate Change|UN Framework Convention on Climate Change]] (UNFCCC, decision number 24/CP.19) to require using a new set of 100-year GWP values. They published these values in Annex III, and they took them from the [[IPCC Fourth Assessment Report]], which had been published in 2007.<ref name="unfccc19" /> Those 2007 estimates are still used for international comparisons through 2020,<ref name="epa20" /> although the latest research on warming effects has found other values, as shown in the tables above.

Though recent reports reflect more scientific accuracy, countries and companies continue to use the [[IPCC Second Assessment Report]] (SAR)<ref name="sar" /> and IPCC Fourth Assessment Report values for reasons of comparison in their emission reports. The [[IPCC Fifth Assessment Report]] has skipped the 500-year values but introduced GWP estimations including the climate-carbon feedback (f) with a large amount of uncertainty.<ref name="ar5" />

== Other metrics to compare greenhouse gases ==

The ''Global Temperature change Potential'' (GTP) is another way to compare gases. While GWP estimates infrared thermal radiation absorbed, GTP estimates the resulting rise in average surface temperature of the world, over the next 20, 50 or 100 years, caused by a greenhouse gas, relative to the temperature rise which the same mass of {{CO2}} would cause.<ref name="ar5" /> Calculation of GTP requires modelling how the world, especially the oceans, will absorb heat.<ref name=":1" /> GTP is published in the same IPCC tables with GWP.<ref name="ar5" />

Another metric called GWP* (pronounced "GWP star"<ref name=Reply/>) has been proposed to take better account of short-lived climate pollutants (SLCPs) such as methane. A permanent increase in the rate of emission of an SLCP has a similar effect to that of a one-time emission of an amount of carbon dioxide, because both raise the radiative forcing permanently or (in the case of carbon dioxide) practically permanently (since the {{CO2}} stays in the air for a long time). GWP* therefore assigns an ''increase'' in emission rate of an SLCP a supposedly equivalent amount (tonnes) of {{CO2}}.<ref>{{cite journal |last1=Lynch |first1=John |last2=Cain |first2=Michelle |last3=Pierrehumbert |first3=Raymond |last4=Allen |first4=Myles |date=April 2020 |title=Demonstrating GWP*: a means of reporting warming-equivalent emissions that captures the contrasting impacts of short- and long-lived climate pollutants |journal=Environmental Research Letters |volume=15 |issue=4 |pages=044023 |bibcode=2020ERL....15d4023L |doi=10.1088/1748-9326/ab6d7e |pmc=7212016 |pmid=32395177}}</ref> However GWP* has been criticised both for its suitability as a metric and for inherent design features which can perpetuate injustices and inequity. Developing countries whose emissions of SLCPs are increasing are "penalized", while developed countries such as Australia or New Zealand which have steady emissions of SLCPs are not penalized in this way, though they may be penalized for their emissions of {{CO2}}.<ref>{{cite journal |last1=Meinshausen |first1=Malte |last2=Nicholls |first2=Zebedee |date=1 April 2022 |title=GWP*is a model, not a metric |journal=Environmental Research Letters |volume=17 |issue=4 |pages=041002 |bibcode=2022ERL....17d1002M |doi=10.1088/1748-9326/ac5930 |doi-access=free}}</ref><ref>{{cite journal |last1=Rogelj |first1=Joeri |last2=Schleussner |first2=Carl-Friedrich |date=1 November 2019 |title=Unintentional unfairness when applying new greenhouse gas emissions metrics at country level |journal=Environmental Research Letters |volume=14 |issue=11 |pages=114039 |bibcode=2019ERL....14k4039R |doi=10.1088/1748-9326/ab4928 |s2cid=250668916 |hdl-access=free |hdl=10044/1/77353}}</ref><ref name=Reply>{{cite journal |last1=Rogelj |first1=Joeri |last2=Schleussner |first2=Carl-Friedrich |date=1 June 2021 |title=Reply to Comment on 'Unintentional unfairness when applying new greenhouse gas emissions metrics at country level' |journal=Environmental Research Letters |volume=16 |issue=6 |pages=068002 |bibcode=2021ERL....16f8002R |doi=10.1088/1748-9326/ac02ec |doi-access=free}}</ref>

== See also ==
{{Portal|Global warming|Energy}}
*[[Carbon accounting]]
*[[Carbon footprint]]
*[[Emission intensity]]

==References==
{{reflist|25em}}

===Sources===
*{{Cite book  | ref= {{harvid|IPCC SAR WG1 Ch2|1995}}
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 |year= 1995
 |title= Climate Change 1995: The Science of Climate Change
 |series=Contribution of Working Group I to the [[IPCC Second Assessment Report|Second Assessment Report]] of the Intergovernmental Panel on Climate Change (IPCC SAR WG1)
 |pages= 65–132 
}}
<!-- ## -->
*{{Cite book  | ref= {{harvid|IPCC AR4 WG1 Ch2|2007}}
 |chapter= Chapter 2: Changes in Atmospheric Constituents and Radiative Forcing
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 |pages= 129–234
}}
<!-- ## -->
*{{Cite book  |ref= {{harvid|IPCC AR5 WG1 Ch8|2013}}<!-- ipcc:20190900 -->
 |chapter= Chapter 8: Anthropogenic and Natural Radiative Forcing
 |chapter-url= https://archive.ipcc.ch/pdf/assessment-report/ar5/wg1/WG1AR5_Chapter08_FINAL.pdf
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}}
<!-- ## -->

== External links ==
* [https://web.archive.org/web/20041112075806/http://www.epa.gov/nonco2/econ-inv/table.html List of Global Warming Potentials and Atmospheric Lifetimes] from the U.S. EPA
* [https://www.darkoptimism.org/2008/09/03/climate-science-translation-guide/ GWP and the different meanings of {{CO2}}e explained]

{{climate change}}

[[Category:Greenhouse gas emissions]]
[[Category:Climate forcing]]
[[Category:Infrared spectroscopy]]
[[Category:Carbon dioxide]]
[[Category:Equivalent units]]