Global warming potential (GWP) is the heat absorbed by any greenhouse gas in the atmosphere, as a multiple of the heat that would be absorbed by the same mass of carbon dioxide (CO2). GWP is 1 for CO2. For other gases it depends on the gas and the time frame.

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.

Methane has GWP (over 100 years) of 27.9[1] meaning that, for example, a leak of a tonne of methane is equivalent to emitting 27.9 tonnes of carbon dioxide. Similarly a tonne of nitrous oxide, from manure for example, is equivalent to 273 tonnes of carbon dioxide.[1]: 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.[2] Estimates of GWP values over 20, 100 and 500 years are periodically compiled and revised in reports from the Intergovernmental Panel on Climate Change:

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.[6]

(years)
Global warming potential, GWP
20 years 100 years 500 years
Methane (CH4) 11.8[7] 56[3]
72[5]
84 / 86f[6]
96 [8]
80.8 (biogenic)[7]
82.5 (fossil)[7]
21[3]
25[5]
28 / 34f[6]
32[9]
39f (biogenic)[10]
40f (fossil) [10]
6.5[3]
7.6[5]
Nitrous oxide (N2O) 109[7] 280[3]
289[5]
264 / 268f[6]
273[7]
310[3]
298[5]
265 / 298f[6]
273[7]
170[3]
153[5]
130[7]
HFC-134a (hydrofluorocarbon) 14.0[7] 3710 / 3790f[6]
4144[7]
1300 / 1550f[6]
1526[7]
435[5]
436[7]
CFC-11 (chlorofluorocarbon) 52.0[7] 6900 / 7020f[6]
8321[7]
4660 / 5350f[6]
6226[7]
1620[5]
2093[7]
Carbon tetrafluoride (CF4 / PFC-14) 50,000[7] 4880 / 4950f[6]
5301[7]
6630 / 7350f[6]
7380[7]
11,200[5]
10587[7]
HFC-23 (hydrofluorocarbon) 222[6] 12,000[5]
10,800[6]
14,800[5]
12,400[6]
12,200[5]
Sulfur hexafluoride SF6 3,200[6] 16,300[5]
17,500[6]
22,800[5]
23,500[6]
32,600[5]
Hydrogen (H2) 4–7[11] 33 (20-44)[11] 11 (6-16)[11]

The IPCC lists many other substances not shown here.[6][7] 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.[12]

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).[13] 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.[14][15]

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.[16]

Those 2007 estimates are still used for international comparisons through 2020,[17] 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 0
Methane CH4 25 0
Nitrous oxide N2O 298 0
Hydrofluorocarbons (HFCs)
HFC-23 CHF3 14800 0
Difluoromethane (HFC-32) CH2F2 675 0
Fluoromethane (HFC-41) CH3F 92 0
HFC-43-10mee CF3CHFCHFCF2CF3 1640 0
Pentafluoroethane (HFC-125) C2HF5 3500 0
HFC-134 C2H2F4 (CHF2CHF2) 1100 0
1,1,1,2-Tetrafluoroethane (HFC-134a) C2H2F4 (CH2FCF3) 1430 0
HFC-143 C2H3F3 (CHF2CH2F) 353 0
1,1,1-Trifluoroethane (HFC-143a) C2H3F3 (CF3CH3) 4470 0
HFC-152 CH2FCH2F 53 0
HFC-152a C2H4F2 (CH3CHF2) 124 0
HFC-161 CH3CH2F 12 0
1,1,1,2,3,3,3-Heptafluoropropane (HFC-227ea) C3HF7 3220 0
HFC-236cb CH2FCF2CF3 1340 0
HFC-236ea CHF2CHFCF3 1370 0
HFC-236fa C3H2F6 9810 0
HFC-245ca C3H3F5 693 0
HFC-245fa CHF2CH2CF3 1030 0
HFC-365mfc CH3CF2CH2CF3 794 0
Perfluorocarbons
Carbon tetrafluoride – PFC-14 CF4 7390 0
Hexafluoroethane – PFC-116 C2F6 12200 0
Octafluoropropane – PFC-218 C3F8 8830 0
Perfluorobutane – PFC-3-1-10 C4F10 8860 0
Octafluorocyclobutane – PFC-318 c-C4F8 10300 0
Perfluouropentane – PFC-4-1-12 C5F12 9160 0
Perfluorohexane – PFC-5-1-14 C6F14 9300 0
Perfluorodecalin – PFC-9-1-18b C10F18 7500 0
Perfluorocyclopropane c-C3F6 17340 0
Sulphur hexafluoride (SF6)
Sulphur hexafluoride SF6 22800 0
Nitrogen trifluoride (NF3)
Nitrogen trifluoride NF3 17200 0
Fluorinated ethers
HFE-125 CHF2OCF3 14900 0
Bis(difluoromethyl) ether (HFE-134) CHF2OCHF2 6320 0
HFE-143a CH3OCF3 756 0
HCFE-235da2 CHF2OCHClCF3 350 0
HFE-245cb2 CH3OCF2CF3 708 0
HFE-245fa2 CHF2OCH2CF3 659 0
HFE-254cb2 CH3OCF2CHF2 359 0
HFE-347mcc3 CH3OCF2CF2CF3 575 0
HFE-347pcf2 CHF2CF2OCH2CF3 580 0
HFE-356pcc3 CH3OCF2CF2CHF2 110 0
HFE-449sl (HFE-7100) C4F9OCH3 297 0
HFE-569sf2 (HFE-7200) C4F9OC2H5 59 0
HFE-43-10pccc124 (H-Galden 1040x) CHF2OCF2OC2F4OCHF2 1870 0
HFE-236ca12 (HG-10) CHF2OCF2OCHF2 2800 0
HFE-338pcc13 (HG-01) CHF2OCF2CF2OCHF2 1500 0
(CF3)2CFOCH3 343 0
CF3CF2CH2OH 42 0
(CF3)2CHOH 195 0
HFE-227ea CF3CHFOCF3 1540 0
HFE-236ea2 CHF2OCHFCF3 989 0
HFE-236fa CF3CH2OCF3 487 0
HFE-245fa1 CHF2CH2OCF3 286 0
HFE-263fb2 CF3CH2OCH3 11 0
HFE-329mcc2 CHF2CF2OCF2CF3 919 0
HFE-338mcf2 CF3CH2OCF2CF3 552 0
HFE-347mcf2 CHF2CH2OCF2CF3 374 0
HFE-356mec3 CH3OCF2CHFCF3 101 0
HFE-356pcf2 CHF2CH2OCF2CHF2 265 0
HFE-356pcf3 CHF2OCH2CF2CHF2 502 0
HFE-365mcfI’ll t3 CF3CF2CH2OCH3 11 0
HFE-374pc2 CHF2CF2OCH2CH3 557 0
– (CF2)4CH (OH) – 73 0
(CF3)2CHOCHF2 380 0
(CF3)2CHOCH3 27 0
Perfluoropolyethers
PFPMIE CF3OCF(CF3)CF2OCF2OCF3 10300 0
Trifluoromethyl sulphur pentafluoride (SF5CF3)
Trifluoromethyl sulphur pentafluoride SF5CF3 17 0

## 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.[18]

Commonly, a time horizon of 100 years is used by regulators.

## Water vapour

Water vapour does contribute to anthropogenic global warming, but as the GWP is defined, it is negligible for H2O.[19]

H2O is the strongest 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 heat 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.[6] Calculation of GTP requires modeling how the world, especially the oceans, will absorb heat.[20] GTP is published in the same IPCC tables with GWP.[6]

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.[21]

## 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.[22]

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.[23]

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:

${\displaystyle {\mathit {RF))=\sum _{i=1}^{100}{\text{abs))_{i}\cdot F_{i}/\left({\text{l))\cdot {\text{d))\right)}$

where the subscript i represents an 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.[verification needed]

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.[24] 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:

${\displaystyle {\mathit {GWP))\left(x\right)={\frac {a_{x)){a_{r))}{\frac {\int _{0}^{\mathit {TH))[x](t)\,dt}{\int _{0}^{\mathit {TH))[r](t)\,dt))}$

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.[25] 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.[26][27] 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,[28][29] 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)[30]
• In industry: million metric tonnes of carbon dioxide equivalents (MMTCDE)[31] and MMT CO2eq.[17]
• For vehicles: grams of carbon dioxide equivalent per mile (gCO2e/mile) or per kilometer (gCO2e/km)[32][33]

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.

## References

### Notes

1. ^ a b 7.SM.6 Tables of greenhouse gas lifetimes, radiative efficiencies and metrics (PDF), IPCC, 2021, p. 7SM-24
2. ^ "Understanding Global Warming Potentials". United States Environmental Protection Agency. 12 January 2016. Retrieved 2021-03-02.
3. IPCC SAR WG1 Ch2 1995, p. 121
4. ^ IPCC TAR WG1 Ch6 2001, p. 388
5. IPCC AR4 WG1 Ch2 2007, p. 212
6. IPCC AR5 WG1 Ch8 2013, p. 714;731
7. IPCC AR6 WG1 Ch7 2021
8. ^ Alvarez 2018
9. ^ Etminan et al. 2016
10. ^ a b Morton 2020
11. ^ a b c Warwick 2022
12. ^ Olivier & Peters 2020, p. 12
13. ^ This is so, because of the reaction formula: CH4 + 2O2 → CO2 + 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).
14. ^ 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.
15. ^ "Testing 100-year global warming potentials: Impacts on compliance costs and abatement profile", "Climatic Change" Retrieved March 16, 2018
16. ^ "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.
17. ^ a b "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.
18. ^
19. ^ 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.
20. ^ "Understanding Global Warming Potentials". US EPA. 2016-01-12. Retrieved 2020-07-04.
21. ^ 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.
22. ^ 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.
23. ^ "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.
24. ^ "Climate Change 2001: The Scientific Basis". www.grida.no. Archived from the original on 31 January 2016. Retrieved 11 January 2022.
25. ^ "CO2e". www3.epa.gov. Retrieved 2020-06-27.
26. ^ "Atmospheric greenhouse gas concentrations - Rationale". European Environment Agency. 2020-02-25. Retrieved 2020-06-28.
27. ^ 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.
28. ^ 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.
29. ^ 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.
30. ^ 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.
31. ^ "Glossary:Carbon dioxide equivalent - Statistics Explained". ec.europa.eu. Retrieved 2020-06-28.
32. ^ "How Clean is Your Electric Vehicle?". Union of Concerned Scientists. Retrieved 2020-07-02.
33. ^ Whitehead, Jake (2019-09-07). "The Truth About Electric Vehicle Emissions". www.realclearscience.com. Retrieved 2020-07-02.