Global warming potential

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. Some gases, like methane, have large GWP, since a ton of methane absorbs much more heat than a ton of CO2. Some gases, again like methane, break down over time, and their heat absorption, or GWP, over the next 20 years is a bigger multiple of CO2 than their heat absorption will be over 100 or 500 years. Values of GWP are estimated and updated for each time frame as methods improve.

Carbon dioxide equivalent (CO2e or CO2eq or CO2-e) is calculated from GWP. It can be measured in weight or concentration. For any amount of any gas, it is the amount of CO2 which would warm the earth as much as that amount of that gas. Thus it provides a common scale for measuring the climate effects of different gases. It is calculated as GWP times amount of the other gas. For example if a gas has GWP of 100, two tons of the gas have CO2e of 200 tons, and 1 part per million of the gas in the atmosphere has CO2e of 100 parts per million.

Estimates of GWP values over 20 and 100 years are periodically compiled in reports from the Intergovernmental Panel on Climate Change.[1]

Values

Carbon dioxide has a GWP of exactly 1 by definition (since it is the baseline unit to which all other greenhouse gases are compared). Most greenhouse gases have GWP more than CO
2, therefore more than 1. A few have values below 1, meaning they do warm the earth, but not as much as an equal amount of CO
2 would. Values have been estimated on:

  • p.714 of the 2013 IPCC AR5 Fifth Assessment Report,[1] Page 731 has many more compounds not shown here.
  • p. 212 of the 2007 IPCC AR4 Fourth Assessment Report,[2] That page has many more compounds not shown here.
GWP values and lifetimes Lifetime
(years)		 Global warming potential, GWP Source with/without
climate-carbon feedbacks
20 years 100 years 500 years
Methane 12.486342013 p714 with feedbacks[1]
84282013 p714 no feedbacks[1]
Nitrous oxide (N2O) 121.0 2682982013 p714 with feedbacks[1]
2642652013 p714 no feedbacks[1]
HFC-134a (hydrofluorocarbon) 13.4 379015502013 p714 with feedbacks[1]
371013002013 p714 no feedbacks[1]
CFC-11 (chlorofluorocarbon) 45.0 702053502013 p714 with feedbacks[1]
690046602013 p714 no feedbacks[1]
Carbon tetrafluoride (CF4) 50000 495073502013 p714 with feedbacks[1]
488066302013 p714 no feedbacks[1]
GWP values and lifetimes Lifetime
(years)		 Global warming potential, GWP Source
20 years 100 years 500 years
Perfluorotributylamine (PFTBA)71002013 GRL[3]
Hydrogen (H2)4–7 4.3Derwent (2018)[4]
Methane96322018 Sci+2016 GRL[5][6]
Methane (biogenic) 39 2016 GRL with feedback[7]
Methane ("fossil")402016 GRL with feedback[7]
Nitrous oxide1142892981532007 p212[2]
HFC-134a (hydrofluorocarbon)14383014304352007 p212[2]
CFC-11 (chlorofluorocarbon)45.06730475016202007 p212[2]
Carbon tetrafluoride (CF4)500005210739011202007 p212[2]
HFC-23 (hydrofluorocarbon)27012,00014,80012,2002007 p212[2]
Sulfur hexafluoride320016,30022,80032,6002007 p212[2]

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).[8] If you started with 1 tonne of methane which has a GWP of 25, after combustion you would have 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).

The global warming potential of perfluorotributylamine (PFTBA) over a 100-year time horizon has been estimated to be approximately 7100.[3] It has been used by the electrical industry since the mid-20th century for electronic testing and as a heat transfer agent.[9] PFTBA has the highest radiative efficiency (relative effectiveness of greenhouse gases to restrict long-wave radiation from escaping back into space[10]) of any molecule detected in the atmosphere to date.[11] 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.[12]

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.[13][14]

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.[15] Those standards are still in effect as of 2020.[16]

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 34 over 100 years (GWP100 = 34) 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.[17]

Commonly, a time horizon of 100 years is used by regulators (e.g., the California Air Resources Board).

Water vapour

Water vapour is one of the primary 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 CO2, and also absorbs non-zero amounts of radiation in its low absorbing spectral regions.[18] Next, its concentration in the atmosphere depends on air temperature and water availability; using a global average temperature of ~16 °C, for example, creates an average humidity of ~18,000ppm at sea level (CO2 is ~400ppm[19] and so concentrations of [H2O]/[CO2] ~ 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 CO2 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 (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.[1] 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.[1]

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.[21]

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.[22]

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 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.

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.[23] 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. CO
2). 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) is calculated from GWP. It can be measured in weight or concentration. For any amount of any gas, it is the amount of CO2 which would warm the earth as much as that amount of that gas. Thus it provides a common scale for measuring the climate effects of different gases. It is calculated as GWP times amount of the other gas.

As weight, CO2e is the weight of CO2 which would warm the earth as much as a particular weight of some other gas;[24] it is calculated as GWP times weight of the other gas. For example if a gas has GWP of 100, two tons of the gas have CO2e of 200 tons, and 9 tons of the gas has CO2e of 900 tons.

As concentration, CO
2e is the 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; it is calculated as GWP times concentration of the other gas(es). 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.[25][26]

CO
2e calculations depend on the time-scale chosen, typically 100 years or 20 years,[27][28] 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 CO
    2     equivalent (GtCO2eq)[29]
  • In industry: million metric tonnes of carbon dioxide equivalents (MMTCDE)[30] and MMT CO2 Eq.[16]
  • For vehicles: grams of carbon dioxide equivalent per mile (gCO2e/mile)[31] or per kilometer (gCO2e/km)[32]

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
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  3. Hong, Angela C.; Cora J. Young; Michael D. Hurley; Timothy J. Wallington; Scott A. Mabury (28 November 2013). "Perfluorotributylamine: A novel long-lived greenhouse gas". Geophysical Research Letters. 40 (22): 6010–6015. Bibcode:2013GeoRL..40.6010H. doi:10.1002/2013GL058010.
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  8. 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).
  9. New Greenhouse Gas Discovered, PFTBA Has Higher Global Warming Impact Than CO2. Ibtimes.com (2013-12-10). Retrieved on 2014-04-23.
  10. Radiative efficiency definition of Radiative efficiency in the Free Online Encyclopedia. Encyclopedia2.thefreedictionary.com. Retrieved on 2014-04-23.
  11. Newly discovered greenhouse gas '7,000 times more powerful than CO2' | Environment. theguardian.com. 10 December 2013.
  12. New greenhouse gas discovered by U of T chemists | Toronto Star. Thestar.com (2013-12-11). Retrieved on 2014-04-23.
  13. 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. Retrieved 17 January 2011.
  14. "Testing 100-year global warming potentials: Impacts on compliance costs and abatement profile", "Climatic Change" Retrieved March 16, 2018
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  16. "Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2018, page ES-3" (PDF). US Environmental Protection Agency. 2020-04-13. Retrieved 2020-07-01.
  17. Regulation (EU) No 517/2014 of the European Parliament and of the Council of 16 April 2014 on fluorinated greenhouse gases Annex IV.
  18. 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
  19. Carbon dioxide#In the Earth's atmosphere
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  21. 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.
  22. "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.
  23. https://web.archive.org/web/20160131050350/http://www.grida.no/climate/ipcc_tar/wg1/247.htm
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