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Global warming potential: Difference between revisions

Revision as of 15:40, 15 December 2021

Carbon dioxide equivalent (CO₂e or CO₂eq or CO₂-e) is calculated from GWP. It can be measured in weightorconcentration. 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 tonnes of the gas have CO₂e of 200 tonnes, and 1 part per million of the gas in the atmosphere has CO₂e of 100 parts per million.

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.^[1] Estimates of GWP values over 20, 100 and 500 years are periodically compiled and revised in reports from the Intergovernmental Panel on Climate Change:

• SAR (1995)^[2]
• TAR (2001)^[3]
• AR4 (2007)^[4]
• AR5 (2013)^[5]
• AR6 (2021)

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.^[5]

The IPCC lists many other substances not shown here.^[5] 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.^[10]

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).^[11] 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).

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.^[12][13]

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.^[14]

Those 2007 estimates are still used for international comparisons through 2020,^[15] although the latest research on warming effects has found other values, as shown in the table above.

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.^[16]

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

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.^[17] 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^[18] 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.

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.^[5] Calculation of GTP requires modeling how the world, especially the oceans, will absorb heat.^[19] GTP is published in the same IPCC tables with GWP.^[5]

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.^[20]

Calculating the global warming potential

The GWP depends on the following factors:

• the absorption of infrared radiation by a given gas
• the time horizon of interest (integration period)
• the atmospheric lifetime of the gas

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:

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

where the subscript i represents an interval of 10 inverse centimeters. Abs_i represents the integrated infrared absorbance of the sample in that interval, and F_i 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.^[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:

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

where TH is the time horizon over which the calculation is considered; a_x is the radiative efficiency due to a unit increase in atmospheric abundance of the substance (i.e., Wm⁻²kg⁻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 a_x and a_r 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 tonnes of the gas have CO₂e of 200 tonnes, and 9 tonnes of the gas has CO₂e of 900 tonnes.

As concentration, CO₂e 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]

CO₂e 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 CO₂ equivalent (GtCO₂eq)^[29]
• In industry: million metric tonnes of carbon dioxide equivalents (MMTCDE)^[30] and MMT CO₂eq.^[15]
• For vehicles: grams of carbon dioxide equivalent per mile (gCO₂e/mile) or per kilometer (gCO₂e/km)^[31][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

• Carbon accounting
• Carbon footprint
• Vehicle emission standard
• List of refrigerants
• Emission intensity
• Radiative forcing
• Total equivalent warming impact

References

Notes

Sources

IPCC reports

• Schimel, D.; Alves, D.; Enting, I.; Heimann, M.; et al. (1995). "Chapter 2: Radiative Forcing of Climate Change". Climate Change 1995: The Science of Climate Change. Contribution of Working Group I to the Second Assessment Report of the Intergovernmental Panel on Climate Change. pp. 65–132.
• Ramaswamy, V.; Boucher, O.; Haigh, J.; Hauglustaine, D.; et al. (2001). "Chapter 6: Radiative Forcing of Climate Change". Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change. pp. 349–416.
• Forster, P.; Ramaswamy, V.; Artaxo, P.; Berntsen, T.; et al. (2007). "Chapter 2: Changes in Atmospheric Constituents and Radiative Forcing" (PDF). Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. pp. 129–234.
• Myhre, G.; Shindell, D.; Bréon, F.-M.; Collins, W.; et al. (2013). "Chapter 8: Anthropogenic and Natural Radiative Forcing" (PDF). Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. pp. 659–740.

Other sources

• Alvarez (2018). "Assessment of methane emissions from the U.S. oil and gas supply chain". Science. 361 (6398): 186–188. Bibcode:2018Sci...361..186A. doi:10.1126/science.aar7204. PMC 6223263. PMID 29930092.
• Etminan, M.; Myhre, G.; Highwood, E. J.; Shine, K. P. (2016-12-28). "Radiative forcing of carbon dioxide, methane, and nitrous oxide: A significant revision of the methane radiative forcing: Greenhouse Gas Radiative Forcing". Geophysical Research Letters. 43 (24): 12, 614–12, 623. doi:10.1002/2016GL071930.
• Derwent, R.G. (2018-10-07). "Hydrogen for Heating: Atmospheric Impacts. A literature review" (PDF). BEIS Research Paper (2018:21).
• Morton, Adam (2020-08-26). "Methane released in gas production means Australia's emissions may be 10% higher than reported". The Guardian. ISSN 0261-3077. Retrieved 2020-08-26.
• Olivier, J.G.J.; Peters, J.A.H.W. (2020). Trends in global CO₂ and total greenhouse gas emissions (2020) (PDF) (Report). The Hague: PBL Netherlands Environmental Assessment Agency.

External links

• List of Global Warming Potentials and Atmospheric Lifetimes from the U.S. EPA
• GWP and the different meanings of CO₂e explained

Bibliography

• Gohar, L. K.; Shine, K. P. (November 2007). "Equivalent CO₂ and its use in understanding the climate effects of increased greenhouse gas concentrations". Weather. 62 (11). Royal Meteorological Society: 307–311. Bibcode:2007Wthr...62..307G. doi:10.1002/wea.103. ISSN 1477-8696.