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'''Global warming potential''' ('''GWP''') is a measure of how much heat a [[greenhouse gas]] traps in the atmosphere up to a specific time horizon, relative to [[carbon dioxide]]. It compares the amount of heat trapped by a certain mass of the [[gas]] in question to the amount of heat trapped by a similar mass of [[carbon dioxide]] and is expressed as a factor of carbon dioxide (whose GWP is standardized to 1).

A GWP is calculated over a specific time horizon, commonly 20, 100, or 500 years. User related choices such as the time horizon can greatly affect the numerical values obtained for carbon dioxide equivalents. In the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, [[methane]] has a lifetime of 12.4 years and with climate-carbon feedbacks a global warming potential of 86 over 20 years and 34 over 100 years in response to emissions. For a change in time horizon from 20 to 100 years, the GWP for methane therefore decreases by a factor of approximately 2.5.<ref>{{cite book |url=http://www.climatechange2013.org/report/full-report/ |contribution=Climate Change 2013: The Physical Science Basis |title=IPCC, 2013: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, Ch.8, p. 711-714, Table 8.7. |date=2013 |accessdate=2014-02-13 }}</ref>

The GWP depends on the following factors:
* the absorption of [[infrared radiation]] by a given species
* the spectral location of its absorbing wavelengths
* the [[atmospheric lifetime]] of the species
Thus, 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 | doi = 10.1021/ed076p1702| title = Greenhouse Warming Potentials from the Infrared Spectroscopy of Atmospheric Gases| journal = Journal of Chemical Education| volume = 76| issue = 12| pages = 1702| year = 1999| last1 = Elrod | first1 = M. J. | bibcode = 1999JChEd..76.1702E}}</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]].

The substances subject to restrictions under the [[Kyoto protocol]] are either rapidly increasing their concentrations in [[Earth's atmosphere]] or have a large GWP.

== Calculating the global warming potential ==
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 
|accessdate=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>\mathit{RF} = \sum_{n=1}^{100} \mathit{Abs}_i \cdot F_i / (\mathit{path length} \cdot \mathit{density})</math>

where the subscript ''i'' represents an 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.{{Verify source|date=September 2008}}

The [[Intergovernmental Panel on Climate Change]] (IPCC) provides the generally accepted values for GWP, which changed slightly between 1996 and 2001. An exact definition of how GWP is calculated is to be found in the IPCC's 2001 Third Assessment Report.<ref>https://web.archive.org/web/20160131050350/http://www.grida.no/climate/ipcc_tar/wg1/247.htm</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>\mathit{GWP} \left(x\right) = \frac{\int_0^{\mathit{TH}} a_x \cdot \left[x(t)\right] dt} {\int_0^{\mathit{TH}} a_r \cdot \left[r(t)\right] 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., CO<sub>2</sub>, 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 CO<sub>2</sub> 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 CO<sub>2</sub> 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 CO<sub>2</sub> that are not filled up (saturated) as much as CO<sub>2</sub>, so rising ppms of these gases are far more significant.

=== Use in Kyoto Protocol ===
Under the [[Kyoto Protocol]], the Conference of the Parties decided (decision 2/CP.3) that the values of GWP calculated for the [[IPCC Second Assessment Report]] are to be used for converting the various greenhouse gas emissions into comparable [[Co2e|CO<sub>2</sub> equivalents]] when computing overall sources and sinks.<ref>{{cite book
|url=http://unfccc.int/resource/docs/cop3/07a01.pdf 
|contribution=Methodological issues related to the Kyoto Protocol
|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 |accessdate=17 January 2011
|publisher=[[UNFCCC]] 
|author=Conference of the Parties }}</ref>
<ref>[https://search.proquest.com/docview/198550594 "Testing 100-year global warming potentials: Impacts on compliance costs and abatement profile"], "Climatic Change" Retrieved March 16, 2018</ref>


== Importance of time horizon ==
A substance's GWP depends on the timespan 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 34 over 100 years but 86 over 20 years; 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 (e.g., the [[California Air Resources Board]]).

== Values ==
[[Carbon dioxide]] has a GWP of exactly 1 (since it is the baseline unit to which all other greenhouse gases are compared). Values for other gases have been estimated on 
*p.714 of the 2013 IPCC AR5 Fifth Assessment Report,<ref name="ipcc2013">Myhre, G., D. Shindell, F.-M. Bréon, W. Collins, J. Fuglestvedt, J. Huang, D. Koch, J.-F. Lamarque, D. Lee, B. Mendoza, T. Nakajima, A. Robock, G. Stephens, T. Takemura and H. Zhang (2013) [http://www.climatechange2013.org/images/report/WG1AR5_Chapter08_FINAL.pdf "Anthropogenic and Natural Radiative Forcing"]. In: ''Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change''. Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.). Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. [http://www.climatechange2013.org/images/report/WG1AR5_Chapter08_FINAL.pdf Anthropogenic and Natural Radiative Forcing]</ref> Page 732 has many more compounds not shown here.
*p. 212 of the 2007 IPCC AR4 Fourth Assessment Report,<ref name="ipcc2007">Forster, P., V. Ramaswamy, P. Artaxo, T. Berntsen, R. Betts, D.W. Fahey, J. Haywood, J. Lean, D.C. Lowe, G. Myhre, J. Nganga, R. Prinn, G. Raga, M. Schulz and R. Van Dorland (2007) [http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter2.pdf "Changes in Atmospheric Constituents and in Radiative Forcing"]. In: ''Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change''. Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.). Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.</ref> This page has many more compounds not shown here.
*the 2001 IPCC TAR Third Assessment Report.<ref name="ipcc2001">[http://www.grida.no/climate/ipcc_tar/wg1/248.htm "6.12.2 Direct GWPs"] {{webarchive|url=https://web.archive.org/web/20070329000822/http://www.grida.no/climate/ipcc_tar/wg1/248.htm |date=2007-03-29 }} in ''IPCC Third Assessment Report – Climate Change 2001''. GRID-Arendal (2003)</ref> This page has many more compounds not shown here.

{| class="wikitable sortable"
! rowspan="2" |GWP values and lifetimes
!rowspan="2"|Lifetime in years
!colspan="3"|Global Warming Potential (GWP)
!rowspan="2"|Source with/without<br>climate-carbon feedbacks
|-
!20 years
!100 years
!500 years
|-
| [[Methane]] ||12.4|| 86|| 34||||2013 p714 with feedbacks<ref name="ipcc2013"/>
|-
| [[Methane]] ||12.4|| 84|| 28||||2013 p714 no feedbacks<ref name="ipcc2013"/>
|-
| [[Nitrous oxide]] (N<sub>2</sub>O) || 121.0|| 268|| 298||||2013 p714 with feedbacks<ref name="ipcc2013"/>
|-
| [[Nitrous oxide]] (N<sub>2</sub>O) || 121.0|| 264|| 265||||2013 p714 no feedbacks<ref name="ipcc2013"/>
|-
| [[HFC-134a]] ([[hydrofluorocarbon]])|| 13.4|| 3790|| 1550||||2013 p714 with feedbacks<ref name="ipcc2013"/>
|-
| [[HFC-134a]] ([[hydrofluorocarbon]])|| 13.4|| 3710|| 1300||||2013 p714 no feedbacks<ref name="ipcc2013"/>
|-
| [[CFC-11]] ([[chlorofluorocarbon]]) || 45.0|| 7020|| 5350||||2013 p714 with feedbacks<ref name="ipcc2013"/>
|-
| [[CFC-11]] ([[chlorofluorocarbon]]) || 45.0|| 6900|| 4660||||2013 p714 no feedbacks<ref name="ipcc2013"/>
|-
| [[Carbon tetrafluoride]] (CF{{sub|4}})|| 50000|| 4950|| 7350||||2013 p714 with feedbacks<ref name="ipcc2013"/>
|-
| [[Carbon tetrafluoride]] (CF{{sub|4}})|| 50000|| 4880|| 6630||||2013 p714 no feedbacks<ref name="ipcc2013"/>
|}

{| class="wikitable sortable"
! rowspan="2" |GWP values and lifetimes
!rowspan="2"|Lifetime in years
!colspan="3"|Global Warming Potential (GWP)
!rowspan="2"|Source
|-
!20 years
!100 years
!500 years
|-
| [[Perfluorotributylamine]] (PFTBA) ||  ||  || 7100|| ||2013 GRL<ref name="grl"/>
|-
| [[Methane]]||  || 96|| 32||  ||2018 Sci+2016 GRL<ref name="grl2016">{{Cite journal |last=Etminan |first=M. |last2=Myhre |first2=G. |last3=Highwood |first3=E. J. |last4=Shine |first4=K. P. |date=2016-12-28 |title=Radiative forcing of carbon dioxide, methane, and nitrous oxide: A significant revision of the methane radiative forcing: Greenhouse Gas Radiative Forcing |journal=Geophysical Research Letters |language=en |volume=43 |issue=24 |pages=12,614–12,623 |doi=10.1002/2016GL071930}}</ref><ref name="sci">{{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}}</ref>
|-
| [[Methane]]|| 12|| 72|| 25|| 7.6||2007 p212<ref name="ipcc2007"/>
|-
| [[Methane]]|| 12|| 62|| 23|| 7||2001<ref name="ipcc2001"/>
|-
| [[Nitrous oxide]]|| 114 || 289 || 298 || 153 ||2007 p212<ref name="ipcc2007"/>
|-
| [[Nitrous oxide]]|| 114|| 275|| 296|| 156||2001<ref name="ipcc2001"/>
|-
| [[HFC-134a]] ([[hydrofluorocarbon]])|| 14 || 3830 || 1430|| 435 ||2007 p212<ref name="ipcc2007"/>
|-
| [[HFC-134a]] ([[hydrofluorocarbon]])|| 13.8|| 3300|| 1300|| 400||2001<ref name="ipcc2001"/>
|-
| [[CFC-11]] ([[chlorofluorocarbon]]) || 45.0|| 6730|| 4750||1620||2007 p212<ref name="ipcc2007"/>
|-
| [[CFC-11]] ([[chlorofluorocarbon]]) || 45.0|| 6300|| 4600||1600||2001<ref name="ipcc2001"/>
|-
| [[Carbon tetrafluoride]] (CF{{sub|4}})|| 50000|| 5210|| 7390||1120||2007 p212<ref name="ipcc2007"/>
|-
| [[Carbon tetrafluoride]] (CF{{sub|4}})|| 50000|| 3900|| 5700||8900||2001<ref name="ipcc2001"/>
|-
| [[HFC-23]] ([[hydrofluorocarbon]])|| 270|| 12,000|| 14,800|| 12,200||2007 p212<ref name="ipcc2007"/>
|-
| [[HFC-23]] ([[hydrofluorocarbon]])|| 260|| 9400|| 12,000|| 10,000||2001<ref name="ipcc2001"/>
|-
| [[Sulfur hexafluoride]]|| 3200 || 16,300 || 22,800 || 32,600||2007 p212<ref name="ipcc2007"/>
|-
| [[Sulfur hexafluoride]]|| 3200|| 15,100|| 22,200|| 32,400||2001<ref name="ipcc2001"/>
|}

The values given in the table assume the same mass of compound is released; 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> → CO<sub>2</sub> + 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> If you started with 1 tonne of methane which has a GWP of 25, after combustion you would have 2.74 tonnes of CO<sub>2</sub>, 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).

The global warming potential of [[perfluorotributylamine]] (PFTBA) over a 100-year time horizon has been estimated to be approximately 7100.<ref name="grl">{{cite journal|last=Hong|first=Angela C.|author2=Cora J. Young |author3=Michael D. Hurley |author4=Timothy J. Wallington |author5=Scott A. Mabury |title=Perfluorotributylamine: A novel long-lived greenhouse gas|journal=Geophysical Research Letters|date=28 November 2013|volume=40|issue=22|pages=6010–6015|doi=10.1002/2013GL058010|bibcode=2013GeoRL..40.6010H}}<!--|accessdate=13 December 2013--></ref> It has been used by the electrical industry since the mid-20th century for electronic testing and as a heat transfer agent.<ref>[http://www.ibtimes.com/new-greenhouse-gas-discovered-pftba-has-higher-global-warming-impact-co2-1503600 New Greenhouse Gas Discovered, PFTBA Has Higher Global Warming Impact Than CO2]. Ibtimes.com (2013-12-10). Retrieved on 2014-04-23.</ref> PFTBA has the highest '''radiative efficiency''' (relative effectiveness of greenhouse gases to restrict long-wave radiation from escaping back into space<ref>[http://encyclopedia2.thefreedictionary.com/Radiative+efficiency Radiative efficiency definition of Radiative efficiency in the Free Online Encyclopedia]. Encyclopedia2.thefreedictionary.com. Retrieved on 2014-04-23.</ref>) of any molecule detected in the atmosphere to date.<ref>[https://www.theguardian.com/environment/2013/dec/10/new-greenhouse-gas-powerful-chemical-perfluorotributylamine Newly discovered greenhouse gas '7,000 times more powerful than CO2' | Environment]. theguardian.com. 10 December 2013.</ref> The researchers found an average of 0.18 parts per trillion of PFTBA in Toronto air samples, whereas carbon dioxide exists around 400 parts per million.<ref>[https://www.thestar.com/news/gta/2013/12/11/new_greenhouse_gas_discovered_by_u_of_t_chemists.html New greenhouse gas discovered by U of T chemists | Toronto Star]. Thestar.com (2013-12-11). Retrieved on 2014-04-23.</ref>

== Water vapour ==
[[Water vapour]] is one of the primary [[greenhouse_gas|greenhouse gases]], but some issues prevent its GWP to be calculated directly. It has a profound infrared absorption spectrum with more and broader absorption bands than CO<sub>2</sub>, and also absorbs non-zero amounts of radiation in its low absorbing spectral regions.<ref>These are normalized absorbance spectrum; these must be compensated for using the [[Beer–Lambert law]] for atmospheric concentrations, http://www.chem.arizona.edu/chemt/C21/sim/gh/ this plot provides a resultant application: [[Sunlight#Composition and power]]</ref> Next, its concentration in the atmosphere depends on air temperature and water availability; using a global average temperature of ~16&nbsp;°C, for example, creates an average humidity of ~18,000ppm at sea level (CO<sub>2</sub> is ~400ppm<ref>[[Carbon dioxide#In the Earth's atmosphere]]</ref> and so concentrations of [H<sub>2</sub>O]/[CO<sub>2</sub>] ~ 45x). Unlike other GHG, water vapor does not decay in the environment, so an average over some time horizon or some other measure consistent with "time dependent decay," q.v., above, must be used in lieu of the time dependent decay of artificial or excess CO<sub>2</sub> molecules. Other issues complicating its calculation are the Earth's temperature distribution, and the differing land masses in the Northern and Southern hemispheres.


== Other metrics: Global Temperature change Potential (GTP) ==
The ''Global Temperature change Potential'' is another way to quantify the ratio change from a substance relative to that of CO<sub>2</sub>, in global mean surface temperature, used for a specific time span.<ref>{{cite journal|url=https://www.ipcc.ch/pdf/assessment-report/ar5/wg1/WG1AR5_Chapter08_FINAL.pdf|title=IPCC AR5 - Anthropogenic and Natural Radiative Forcing (Chapter 8 / page 663)|date=2013}}</ref>


== See also ==
{{portal|Global warming}}
*[[List of refrigerants#List]]
*[[Carbon dioxide equivalent]]
*[[Emission factor]]
*[[Emission standard]]
*[[Radiative forcing]]
*[[Total equivalent warming impact]]

== References ==
{{Reflist}}

== External links ==

* 2007 IPCC Fourth Assessment Report (AR4) by [https://web.archive.org/web/20070501031449/http://ipcc-wg1.ucar.edu/wg1/wg1-report.html Working Group 1] (WG1) and Chapter 2 of that report ([http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter2.pdf Changes in Atmospheric Constituents and in Radiative Forcing]) which contains GWP information.
* [https://web.archive.org/web/20160131050350/http://www.grida.no/climate/ipcc_tar/wg1/247.htm 2001 IPCC Third Assessment Report (TAR) page on Global-Warming Potentials] and [https://web.archive.org/web/20070329000822/http://www.grida.no/climate/ipcc_tar/wg1/248.htm Direct GWP].
* [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 [[United States Environmental Protection Agency|U.S. EPA]]
* [http://www.realclimate.org/index.php?p=142 An overview of the role of H<sub>2</sub>O as a greenhouse gas] from [[RealClimate]]
* [http://www.darkoptimism.org/2008/09/03/the-climate-science-translation-guide/ GWP and the different meanings of CO<sub>2</sub>e explained]
{{Global warming}}

{{DEFAULTSORT:Global Warming Potential}}
[[Category:Greenhouse gas emissions]]
[[Category:Climate forcing]]
[[Category:Infrared spectroscopy]]