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Global warming potential

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Comparison of global warming potential (GWP) of three greenhouse gases 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 an index to measure how much infrared thermal radiation a greenhouse gas would absorb over a given time frame after it has been added to the atmosphere (or emitted to the atmosphere). The GWP makes different greenhouse gases comparable with regard to their "effectiveness in causing radiative forcing".[1]: 2232  It is expressed as a multiple of the radiation that would be absorbed by the same mass of added carbon dioxide (CO2), which is taken as a reference gas. Therefore, the GWP has a value of 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.


For example, methane has a GWP over 20 years (GWP-20) of 81.2[2] meaning that, for example, a 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.[2]: 7SM-24 

The carbon dioxide equivalent (CO2e or CO2eq 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 climate effects of different gases. It is calculated as GWP times mass of the other gas.

Definition

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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 (CO2). The GWP thus represents the combined effect of the differing times these substances remain in the atmosphere and their effectiveness in causing radiative forcing."[1]: 2232 

In turn, radiative forcing is a scientific concept used to quantify and compare the external drivers of change to Earth's energy balance.[3]: 1–4  Radiative forcing is the change in energy flux in the atmosphere caused by natural or anthropogenic factors of climate change as measured in watts per meter squared.[4]

GWP in policymaking

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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 hydrofluorocarbons (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).[5] This allows policymakers to have one standard for comparison instead of changing GWP values in new assessment reports.[6] One exception to the GWP100 standard exists: 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.[5]

Calculated values

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See also: Greenhouse gas § Global warming potential (GWP) and CO2 equivalents

Current values (IPCC Sixth Assessment Report from 2021)

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Global warming potential of five greenhouse gases over 100-year timescale.[7]

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.[8] 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.[9]: Table 7.15  The 2021 IPCC report lists the GWP as 83 over a time scale of 20 years, 30 over 100 years and 10 over 500 years.[9]: Table 7.15  The decrease in GWP at longer times is because methane decomposes to water and CO2 through chemical reactions in the atmosphere. Similarly the third most important GHG, nitrous oxide (N2O), is a common gas emitted through the denitrification part of the nitrogen cycle.[10] 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:

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)[9]: Table 7.15 [11]

Radiative Efficiency

(Wm−2ppb−1, molar basis).[9]: Table 7.15 [11]

20 year GWP[9]: Table 7.15 [11] 100 year GWP[9]: Table 7.15 [11] 500 year GWP[9]: Table 7.15 [12]
Carbon dioxide CO2 (A) 1.37×10−5 1 1 1
Methane (fossil natural gas) CH
4 		12 5.7×10−4 83 30 10
Methane (pure non-fossil) CH
4 		12 5.7×10−4 81 27 7.3
Nitrous oxide N
2O 		109 3×10−3 273 273 130
CFC-11 (R-11) CCl
3F 		52 0.29 8321 6226 2093
CFC-12 (R-12) CCl
2F
2 		100 0.32 10800 10200 5200
HCFC-22 (R-22) CHClF
2 		12 0.21 5280 1760 549
HFC-32 (R-32) CH
2F
2 		5 0.11 2693 771 220
HFC-134a (R-134a) CH
2FCF
3 		14 0.17 4144 1526 436
Tetrafluoromethane (R-14) CF
4 		50000 0.09 5301 7380 10587
Hexafluoroethane C
2F
6 		10 000 0.25 8210 11100 18200
Sulfur hexafluoride SF
6 		3 200 0.57 17500 23500 32600
Nitrogen trifluoride NF
3 		500 0.20 12800 16100 20700
(A) No single lifetime for atmospheric CO2 can be given.

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.[9]

The IPCC lists many other substances not shown here.[13][9] [14] 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).[15] 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).

Greenhouse gas Lifetime
(years) 		Global warming potential, GWP
20 years 100 years 500 years
Hydrogen (H2) 4–7[16] 33 (20–44)[16] 11 (6–16)[16] —
Methane (CH4) 11.8[9] 56[17]
72[18]
84 / 86f[13]
96[19]
80.8 (biogenic)[9]
82.5 (fossil)[9] 		21[17]
25[18]
28 / 34f[13]
32[20]
39 (biogenic)[21]
40 (fossil)[21] 		6.5[17]
7.6[18]
Nitrous oxide (N2O) 109[9] 280[17]
289[18]
264 / 268f[13]
273[9] 		310[17]
298[18]
265 / 298f[13]
273[9] 		170[17]
153[18]
130[9]
HFC-134a (hydrofluorocarbon) 14.0[9] 3,710 / 3,790f[13]
4,144[9] 		1,300 / 1,550f[13]
1,526[9] 		435[18]
436[9]
CFC-11 (chlorofluorocarbon) 52.0[9] 6,900 / 7,020f[13]
8,321[9] 		4,660 / 5,350f[13]
6,226[9] 		1,620[18]
2,093[9]
Carbon tetrafluoride (CF4 / PFC-14) 50,000[9] 4,880 / 4,950f[13]
5,301[9] 		6,630 / 7,350f[13]
7,380[9] 		11,200[18]
10,587[9]
HFC-23 (hydrofluorocarbon) 222[13] 12,000[18]
10,800[13] 		14,800[18]
12,400[13] 		12,200[18]
Sulfur hexafluoride SF6 3,200[13] 16,300[18]
17,500[13] 		22,800[18]
23,500[13] 		32,600[18]

Earlier values from 2007

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The values provided in the table below are from 2007 when they were published in the IPCC Fourth Assessment Report.[22][18] These values are still used (as of 2020) for some comparisons.[23]

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

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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.[24]

Commonly, a time horizon of 100 years is used by regulators.[25][26]

Water vapour

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See also: Greenhouse gas § Special role of water vapor, and 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 H2O: an estimate gives a 100-year GWP between -0.001 and 0.0005.[27]

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.

Calculation methods

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The radiative forcing (warming influence) of long-lived atmospheric greenhouse gases has accelerated, almost doubling in 40 years.[28][29][30]

When calculating the GWP of a greenhouse gas, the value 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.[31]

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.[32]

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.[citation needed]

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.[33] 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.

Applications

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Carbon dioxide equivalent

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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.[34] 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.[35][36] 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,[37][38] 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)[39]
  • In industry: million metric tonnes of carbon dioxide equivalents (MMTCDE)[40] and MMT CO2eq.[23]
  • For vehicles: grams of carbon dioxide equivalent per mile (gCO2e/mile) or per kilometer (gCO2e/km)[41][42]

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

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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.[43][44]

After some intermediate updates, in 2013 this standard was updated by the Warsaw meeting of the 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.[22] Those 2007 estimates are still used for international comparisons through 2020,[23] 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)[17] 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.[13]

Other metrics to compare greenhouse gases

[edit]

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.[13] Calculation of GTP requires modelling how the world, especially the oceans, will absorb heat.[25] GTP is published in the same IPCC tables with GWP.[13]

Another metric called GWP* (pronounced "GWP star"[45]) 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.[46] 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.[47][48][45]

See also

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References

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  1. ^ a b IPCC, 2021: 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 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.
  2. ^ a b 7.SM.6 Tables of greenhouse gas lifetimes, radiative efficiencies and metrics (PDF), IPCC, 2021, p. 7SM-24.
  3. ^ National Research Council (2005). Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties. The National Academic Press. doi:10.17226/11175. ISBN 978-0-309-09506-8.
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  5. ^ a b 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").
  6. ^ "Understanding Global Warming Potentials". US EPA, Greenhouse Gas Emissions. August 8, 2024. Retrieved August 26, 2024.
  7. ^ "Global warming potential of greenhouse gases relative to CO2". Our World in Data. Retrieved 2023-12-18.
  8. ^ IPCC, 2021: 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 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.
  9. ^ a b c d e f g h i j k l m n o p q r s t u v w x y z aa ab 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: 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.
  10. ^ Yang, Rui; Yuan, Lin-jiang; Wang, Ru; He, Zhi-xian; Lei, Lin; Ma, Yan-chen (2022). "Analyzing the mechanism of nitrous oxide production in aerobic phase of anoxic/aerobic sequential batch reactor from the perspective of key enzymes". Environmental Science and Pollution Research. 29 (26): 39877–39887. doi:10.1007/s11356-022-18800-3. ISSN 0944-1344.
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  15. ^ 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).
  16. ^ a b c Warwick, Nicola; Griffiths, Paul; Keeble, James; Archibald, Alexander; John, Pile (2022-04-08). Atmospheric implications of increased hydrogen use (Report). UK Department for Business, Energy & Industrial Strategy (BEIS).
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