Global warming potential (GWP) is a measure of how much heat a greenhouse gas (GHG) traps in the atmosphere over a specific time period, relative to carbon dioxide (). It is a dimensionless quantity expressed as a multiple of warming caused by the same mass of . Therefore, by definition has a GWP of 1. For other gases, it depends on how strongly the gas absorbs thermal radiation, how quickly it leaves the atmosphere, and the time frame considered.
For example, methane (CH4) has a GWP over 20 years (GWP-20) of 81.2. meaning that, a leak of a tonne of methane is equivalent to emitting 81.2 tonnes of CO2, both measured over 20 years. As methane has a much shorter atmospheric lifetime than CO2, its GWP is much less over longer time periods, with a GWP-100 of 27.9 and a GWP-500 of 7.95.
Greenhouse gas emissions (GHG emissions) can be expressed in terms of carbon dioxide equivalent mass or just carbon dioxide equivalent (symbolized e or eq, also denoted -e or -eq) can be calculated from the GWP and emitted mass. For any gas, it is the mass of 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; it is typically expressed in gigatonnes (symbol Gt).
Definition
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 duration these substances remain in the atmosphere and their effectiveness in causing radiative forcing."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.
In turn, radiative forcing is a scientific concept used to quantify and compare the external drivers of change to Earth's energy balance. 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.
Importance of time scale A substance's GWP depends on the time scale (expressed as a 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. Commonly, a time scale of 100 years is used by regulators. e calculations depend on the time-scale chosen, typically 100 years or 20 years, since gases decay in the atmosphere or are absorbed naturally, at different rates.
Carbon dioxide equivalent Carbon dioxide equivalent mass or just carbon dioxide equivalent (symbol e or eq or -e) of a quantity of gas is calculated from its GWP. For any gas, it is the mass of which 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 multiplied by mass of the other gas. For example, if a gas has GWP of 100, two tonnes of the gas have e of 200 tonnes, and 9 tonnes of the gas has e of 900 tonnes.
On a global scale, the warming effects of one or more greenhouse gases in the atmosphere can also be expressed as a carbon dioxide equivalent concentration. It is the atmospheric concentration of 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, e of 500 parts per million would reflect a mix of atmospheric gases which warm the earth as much as 500 parts per million of would warm it. Calculation of the equivalent concentration 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 .
The following units are commonly used: By the UN climate change panel (IPCC): billion metric tonnes = n×109 tonnes of equivalent (Gteq) In industry: million metric tonnes of carbon dioxide equivalents (MMTCDE) and MMT eq.
Further derived quantities include carbon dioxide equivalent mass per distance, as used for vehicle travels. It has SI units of grams per kilometer (g/km), often denoted "grams of carbon dioxide equivalent per kilometer" (ge/km) or per mile (ge/mile).
For example, the table below 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.
Calculation methods
When calculating the GWP of a greenhouse gas, the value depends on the following factors: the absorption of infrared radiation by the 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.Matthew Elrod, "Greenhouse Warming Potential Model." Based on
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.
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 its GWP almost doubled. An exact definition of how GWP is calculated is to be found in the IPCC's 2027 Third Assessment Report. 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. ). 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., , 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 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 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 that are not filled up (saturated) as much as , so rising ppms of these gases are far more significant.
Mixtures The GWP for a mixture of gases can be obtained from the mass-fraction-weighted average of the GWPs of the individual gases.Regulation (EU) No 517/2014 of the European Parliament and of the Council of 16 April 2014 on fluorinated greenhouse gases Annex IV.
Water vapor
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.
H2O can function as a greenhouse gas because it has a profound infrared absorption spectrum with more and broader absorption bands than . 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.
Applications
Use in policymaking 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).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"). This allows policymakers to have one standard for comparison instead of changing GWP values in new assessment reports. 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.
Use in Kyoto Protocol and for reporting to UNFCCC