Global warming potential

Global warming potential (GWP) is a measure of how much infrared thermal radiation a greenhouse gas added to the atmosphere would absorb over a given time frame, as a multiple of the radiation that would be absorbed by the same mass of added carbon dioxide (CO2). GWP is 1 for CO2. For other gases it depends on how strongly the gas absorbs infrared thermal radiation, how quickly the gas leaves the atmosphere, and the time frame being considered. The carbon dioxide equivalent (CO2e or CO2eq or CO2-e) is calculated from 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 climate effects of different gases. It is calculated as GWP times mass of the other gas.

The radiative forcing (warming influence) of long-lived atmospheric greenhouse gases has accelerated, almost doubling in 40 years.[1][2][3]

Methane has GWP (over 20 years) of 81.2[4] meaning that, for example, a leak of a tonne of methane is equivalent to emitting 81.2 tonnes of carbon dioxide. Similarly a tonne of nitrous oxide, from manure or paddy fields for example, is equivalent to 273 tonnes of carbon dioxide.[4]:7SM-24

Values

Carbon dioxide is the reference. It has a GWP of 1 regardless of the time period used. CO2 emissions cause increases in atmospheric concentrations of CO2 that will last thousands of years.[5] 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.[6] Earlier reports were the Second Assessment Report (1995),[7] Third Assessment Report (2001),[8] Fourth Assessment Report (2007)[9] and Fifth Assessment Report (2013).[10]

Though recent reports reflect more scientific accuracy, countries and companies continue to use SAR and AR4 values for reasons of comparison in their emission reports. AR5 has skipped 500 year values but introduced GWP estimations including the climate-carbon feedback (f) with a large amount of uncertainty.[10]

GWP values and lifetimes Lifetime
(years)
Global warming potential, GWP
20 years 100 years 500 years
Hydrogen (H2) 4–7[11] 33 (20-44)[11] 11 (6–16)[11]
Methane (CH4) 11.8[6] 56[7]
72[9]
84 / 86f[10]
96[12]
80.8 (biogenic)[6]
82.5 (fossil)[6]
21[7]
25[9]
28 / 34f[10]
32[13]
39f (biogenic)[14]
40f (fossil)[14]
6.5[7]
7.6[9]
Nitrous oxide (N2O) 109[6] 280[7]
289[9]
264 / 268f[10]
273[6]
310[7]
298[9]
265 / 298f[10]
273[6]
170[7]
153[9]
130[6]
HFC-134a (hydrofluorocarbon) 14.0[6] 3,710 / 3,790f[10]
4,144[6]
1,300 / 1,550f[10]
1,526[6]
435[9]
436[6]
CFC-11 (chlorofluorocarbon) 52.0[6] 6,900 / 7,020f[10]
8,321[6]
4,660 / 5,350f[10]
6,226[6]
1,620[9]
2,093[6]
Carbon tetrafluoride (CF4 / PFC-14) 50,000[6] 4,880 / 4,950f[10]
5,301[6]
6,630 / 7,350f[10]
7,380[6]
11,200[9]
10,587[6]
HFC-23 (hydrofluorocarbon) 222[10] 12,000[9]
10,800[10]
14,800[9]
12,400[10]
12,200[9]
Sulfur hexafluoride SF6 3,200[10] 16,300[9]
17,500[10]
22,800[9]
23,500[10]
32,600[9]

The IPCC lists many other substances not shown here.[10][6] Some have high GWP but only a low concentration in the atmosphere. The total impact of all fluorinated gases is estimated at 3% of all greenhouse gas emissions.[15]

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).[16] 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).

Use in Kyoto Protocol and UNFCCC

Under the Kyoto Protocol, in 1997 the Conference of the Parties standardized international reporting, by deciding (decision 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.[17][18]

After some intermediate updates, in 2013 this standard was updated by the Warsaw meeting of the UN Framework Convention on Climate Change (UNFCCC, decision 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 4th Assessment Report of the Intergovernmental Panel on Climate Change, which had been published in 2007.[19]

Those 2007 estimates are still used for international comparisons through 2020,[20] although the latest research on warming effects has found other values, as shown in the table above.

Greenhouse gas Chemical formula 100-year Global warming potentials
(2007 estimates, for 2013–2020 comparisons)
Carbon dioxide CO2 1
Methane CH4 25
Nitrous oxide N2O 298
Hydrofluorocarbons (HFCs)
HFC-23 CHF3 14,800
Difluoromethane (HFC-32) CH2F2 675
Fluoromethane (HFC-41) CH3F 92
HFC-43-10mee CF3CHFCHFCF2CF3 1,640
Pentafluoroethane (HFC-125) C2HF5 3,500
HFC-134 C2H2F4 (CHF2CHF2) 1,100
1,1,1,2-Tetrafluoroethane (HFC-134a) C2H2F4 (CH2FCF3) 1,430
HFC-143 C2H3F3 (CHF2CH2F) 353
1,1,1-Trifluoroethane (HFC-143a) C2H3F3 (CF3CH3) 4,470
HFC-152 CH2FCH2F 53
HFC-152a C2H4F2 (CH3CHF2) 124
HFC-161 CH3CH2F 12
1,1,1,2,3,3,3-Heptafluoropropane (HFC-227ea) C3HF7 3,220
HFC-236cb CH2FCF2CF3 1,340
HFC-236ea CHF2CHFCF3 1,370
HFC-236fa C3H2F6 9,810
HFC-245ca C3H3F5 693
HFC-245fa CHF2CH2CF3 1,030
HFC-365mfc CH3CF2CH2CF3 794
Perfluorocarbons
Carbon tetrafluoride – PFC-14 CF4 7,390
Hexafluoroethane – PFC-116 C2F6 12,200
Octafluoropropane – PFC-218 C3F8 8,830
Perfluorobutane – PFC-3-1-10 C4F10 8,860
Octafluorocyclobutane – PFC-318 c-C4F8 10,300
Perfluouropentane – PFC-4-1-12 C5F12 9,160
Perfluorohexane – PFC-5-1-14 C6F14 9,300
Perfluorodecalin – PFC-9-1-18b C10F18 7,500
Perfluorocyclopropane c-C3F6 17,340
Sulfur hexafluoride (SF6)
Sulfur hexafluoride SF6 22,800
Nitrogen trifluoride (NF3)
Nitrogen trifluoride NF3 17,200
Fluorinated ethers
HFE-125 CHF2OCF3 14,900
Bis(difluoromethyl) ether (HFE-134) CHF2OCHF2 6,320
HFE-143a CH3OCF3 756
HCFE-235da2 CHF2OCHClCF3 350
HFE-245cb2 CH3OCF2CF3 708
HFE-245fa2 CHF2OCH2CF3 659
HFE-254cb2 CH3OCF2CHF2 359
HFE-347mcc3 CH3OCF2CF2CF3 575
HFE-347pcf2 CHF2CF2OCH2CF3 580
HFE-356pcc3 CH3OCF2CF2CHF2 110
HFE-449sl (HFE-7100) C4F9OCH3 297
HFE-569sf2 (HFE-7200) C4F9OC2H5 59
HFE-43-10pccc124 (H-Galden 1040x) CHF2OCF2OC2F4OCHF2 1,870
HFE-236ca12 (HG-10) CHF2OCF2OCHF2 2,800
HFE-338pcc13 (HG-01) CHF2OCF2CF2OCHF2 1,500
(CF3)2CFOCH3 343
CF3CF2CH2OH 42
(CF3)2CHOH 195
HFE-227ea CF3CHFOCF3 1,540
HFE-236ea2 CHF2OCHFCF3 989
HFE-236fa CF3CH2OCF3 487
HFE-245fa1 CHF2CH2OCF3 286
HFE-263fb2 CF3CH2OCH3 11
HFE-329mcc2 CHF2CF2OCF2CF3 919
HFE-338mcf2 CF3CH2OCF2CF3 552
HFE-347mcf2 CHF2CH2OCF2CF3 374
HFE-356mec3 CH3OCF2CHFCF3 101
HFE-356pcf2 CHF2CH2OCF2CHF2 265
HFE-356pcf3 CHF2OCH2CF2CHF2 502
HFE-365mcfI’ll t3 CF3CF2CH2OCH3 11
HFE-374pc2 CHF2CF2OCH2CH3 557
– (CF2)4CH (OH) – 73
(CF3)2CHOCHF2 380
(CF3)2CHOCH3 27
Perfluoropolyethers
PFPMIE CF3OCF(CF3)CF2OCF2OCF3 10,300
Trifluoromethyl sulfur pentafluoride SF5CF3 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 (GWP100 = 25) but 86 over 20 years (GWP20 = 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.[21]

Commonly, a time horizon of 100 years is used by regulators.[5][22]

Water vapour

Water vapour does contribute to anthropogenic global warming, but as the GWP is defined, it is negligible for H2O: an estimate gives a 100-year GWP between -0.001 and 0.0005.[23]

H2O 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 towers, irrigation) are removed via precipitation within weeks, so its GWP is negligible.

Criticism and other metrics

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.[10] Calculation of GTP requires modeling how the world, especially the oceans, will absorb heat.[5] GTP is published in the same IPCC tables with GWP.[10]

GWP* has been proposed to take better account of short-lived climate pollutants (SLCP) such as methane, relating a change in the rate of emissions of SLCPs to a fixed quantity of CO2.[24] However GWP* has itself been criticised both for its suitability as a metric and for inherent design features which can perpetuate injustices and inequity.[25][26][27]

Calculating the global warming potential

The GWP depends on the following factors:

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.[28]

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.[29]

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:

where the subscript i represents a wavenumber interval of 10 inverse centimeters. Absi represents the integrated infrared absorbance of the sample in that interval, and Fi represents the RF for that interval.

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 his GWP almost doubled. An exact definition of how GWP is calculated is to be found in the IPCC's 2001 Third Assessment Report.[30] The GWP is defined as the ratio of the time-integrated radiative forcing from the instantaneous release of 1 kg of a trace substance relative to that of 1 kg of a reference gas:

where TH is the time horizon over which the calculation is considered; ax is the radiative efficiency due to a unit increase in atmospheric abundance of the substance (i.e., Wm−2 kg−1) 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). The radiative efficiencies ax and ar 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, CH4, and N2O). 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.

Carbon dioxide equivalent

Carbon dioxide equivalent (CO2e or CO2eq 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.[31] 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 CO2e of 200 tonnes, and 9 tonnes of the gas has CO2e 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. CO2e 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, CO2e 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.[32][33] 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.

CO2e calculations depend on the time-scale chosen, typically 100 years or 20 years,[34][35] since gases decay in the atmosphere or are absorbed naturally, at different rates.

The following units are commonly used:

  • By the UN climate change panel (IPCC): billion metric tonnes = n×109 tonnes of CO2 equivalent (GtCO2eq)[36]
  • In industry: million metric tonnes of carbon dioxide equivalents (MMTCDE)[37] and MMT CO2eq.[20]
  • For vehicles: grams of carbon dioxide equivalent per mile (gCO2e/mile) or per kilometer (gCO2e/km)[38][39]

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.

See also

References

Notes

  1. "The NOAA Annual Greenhouse Gas Index (AGGI)". NOAA.gov. National Oceanic and Atmospheric Administration (NOAA). Spring 2023. Archived from the original on 24 May 2023.
  2. "Annual Greenhouse Gas Index". U.S. Global Change Research Program. Archived from the original on 21 April 2021. Retrieved 5 September 2020.
  3. Butler J. and Montzka S. (2020). "The NOAA Annual Greenhouse Gas Index (AGGI)". NOAA Global Monitoring Laboratory/Earth System Research Laboratories. Archived from the original on 22 September 2013. Retrieved 5 September 2020.
  4. 7.SM.6 Tables of greenhouse gas lifetimes, radiative efficiencies and metrics (PDF), IPCC, 2021, p. 7SM-24
  5. "Understanding Global Warming Potentials". United States Environmental Protection Agency. 12 January 2016. Retrieved 2021-03-02.
  6. IPCC AR6 WG1 Ch7 2021
  7. IPCC SAR WG1 Ch2 1995, p. 121
  8. IPCC TAR WG1 Ch6 2001, p. 388
  9. IPCC AR4 WG1 Ch2 2007, p. 212
  10. IPCC AR5 WG1 Ch8 2013, p. 714;731
  11. Warwick 2022
  12. Alvarez 2018
  13. Etminan et al. 2016
  14. Morton 2020
  15. Olivier & Peters 2020, p. 12
  16. This is so, because of the reaction formula: CH4 + 2O2CO2 + 2 H2O. 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−1) will yield one molecule of carbon dioxide (molar mass = 44.01 g mol−1). This gives a mass ratio of 2.74. (44.01/16.04 ≈ 2.74).
  17. Conference of the Parties (25 March 1998). "Methodological issues related to the Kyoto Protocol". 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 (PDF). UNFCCC. Archived (PDF) from the original on 2000-08-23. Retrieved 17 January 2011.
  18. "Testing 100-year global warming potentials: Impacts on compliance costs and abatement profile", "Climatic Change" Retrieved March 16, 2018
  19. "Report of the Conference of the Parties on its 19th Session" (PDF). UNFCCC. 2014-01-31. Archived (PDF) from the original on 2014-07-13. Retrieved 2020-07-01.
  20. "Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2018, page ES-3" (PDF). US Environmental Protection Agency. 2020-04-13. Archived (PDF) from the original on 2020-04-14. Retrieved 2020-07-01.
  21. Regulation (EU) No 517/2014 of the European Parliament and of the Council of 16 April 2014 on fluorinated greenhouse gases Annex IV.
  22. Abernethy, Sam; Jackson, Robert B (2022-02-01). "Global temperature goals should determine the time horizons for greenhouse gas emission metrics". Environmental Research Letters. 17 (2): 024019. arXiv:2104.05506. doi:10.1088/1748-9326/ac4940. ISSN 1748-9326. S2CID 233209965.
  23. Sherwood, Steven C.; Dixit, Vishal; Salomez, Chryséis (2018). "The global warming potential of near-surface emitted water vapour". Environmental Research Letters. 13 (10): 104006. Bibcode:2018ERL....13j4006S. doi:10.1088/1748-9326/aae018. S2CID 158806342.
  24. Lynch, John; Cain, Michelle; Pierrehumbert, Raymond; Allen, Myles (2020-04-01). "Demonstrating GWP*: a means of reporting warming-equivalent emissions that captures the contrasting impacts of short- and long-lived climate pollutants". Environmental Research Letters. 15 (4): 044023. Bibcode:2020ERL....15d4023L. doi:10.1088/1748-9326/ab6d7e. ISSN 1748-9326. PMC 7212016. PMID 32395177.
  25. Meinshausen, Malte; Nicholls, Zebedee (1 April 2022). "GWP*is a model, not a metric". Environmental Research Letters. 17 (4): 041002. doi:10.1088/1748-9326/ac5930.
  26. Rogelj, Joeri; Schleussner, Carl-Friedrich (1 November 2019). "Unintentional unfairness when applying new greenhouse gas emissions metrics at country level". Environmental Research Letters. 14 (11): 114039. doi:10.1088/1748-9326/ab4928. hdl:10044/1/77353. S2CID 250668916.
  27. Rogelj, Joeri; Schleussner, Carl-Friedrich (1 June 2021). "Reply to Comment on 'Unintentional unfairness when applying new greenhouse gas emissions metrics at country level'". Environmental Research Letters. 16 (6): 068002. doi:10.1088/1748-9326/ac02ec.
  28. Matthew Elrod, "Greenhouse Warming Potential Model." Based on Elrod, M. J. (1999). "Greenhouse Warming Potentials from the Infrared Spectroscopy of Atmospheric Gases". Journal of Chemical Education. 76 (12): 1702. Bibcode:1999JChEd..76.1702E. doi:10.1021/ed076p1702.
  29. "Glossary: Global warming potential (GWP)". U.S. Energy Information Administration. Retrieved 2011-04-26. 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.
  30. "Climate Change 2001: The Scientific Basis". www.grida.no. Archived from the original on 31 January 2016. Retrieved 11 January 2022.
  31. "CO2e". www3.epa.gov. Retrieved 2020-06-27.
  32. "Atmospheric greenhouse gas concentrations - Rationale". European Environment Agency. 2020-02-25. Retrieved 2020-06-28.
  33. Gohar, L. K.; Shine, K. P. (2007). "Equivalent CO2 and its use in understanding the climate effects of increased greenhouse gas concentrations". Weather. 62 (11): 307–311. Bibcode:2007Wthr...62..307G. doi:10.1002/wea.103.
  34. Wedderburn-Bisshop, Gerard et al (2015). "Neglected transformational responses: implications of excluding short lived emissions and near term projections in greenhouse gas accounting". The International Journal of Climate Change: Impacts and Responses. RMIT Common Ground Publishing. Retrieved 16 August 2017.
  35. Ocko, Ilissa B.; Hamburg, Steven P.; Jacob, Daniel J.; Keith, David W.; Keohane, Nathaniel O.; Oppenheimer, Michael; Roy-Mayhew, Joseph D.; Schrag, Daniel P.; Pacala, Stephen W. (2017). "Unmask temporal trade-offs in climate policy debates". Science. 356 (6337): 492–493. Bibcode:2017Sci...356..492O. doi:10.1126/science.aaj2350. ISSN 0036-8075. PMID 28473552. S2CID 206653952.
  36. Denison, Steve; Forster, Piers M; Smith, Christopher J (2019-11-18). "Guidance on emissions metrics for nationally determined contributions under the Paris Agreement". Environmental Research Letters. 14 (12): 124002. Bibcode:2019ERL....14l4002D. doi:10.1088/1748-9326/ab4df4. ISSN 1748-9326.
  37. "Glossary:Carbon dioxide equivalent - Statistics Explained". ec.europa.eu. Retrieved 2020-06-28.
  38. "How Clean is Your Electric Vehicle?". Union of Concerned Scientists. Retrieved 2020-07-02.
  39. Whitehead, Jake (2019-09-07). "The Truth About Electric Vehicle Emissions". www.realclearscience.com. Retrieved 2020-07-02.

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Bibliography

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