Editing Global warming potential (section)

== Calculation methods ==
[[File:1979-_Radiative_forcing_-_climate_change_-_global_warming_-_EPA_NOAA.svg|right|thumb|upright=1.6|The [[radiative forcing]] (warming influence) of long-lived atmospheric greenhouse gases has accelerated, almost doubling in 40 years.<ref name=NOAA_AGGI_2023>{{cite web |title=The NOAA Annual Greenhouse Gas Index (AGGI) |url=https://gml.noaa.gov/aggi/aggi.html |website=NOAA.gov |publisher=National Oceanic and Atmospheric Administration (NOAA) |archive-url=https://web.archive.org/web/20241005195609/https://gml.noaa.gov/aggi/aggi.html |archive-date=5 October 2024 |date=2024 |url-status=live }}</ref><ref>{{Cite web |title=Annual Greenhouse Gas Index |url=https://www.globalchange.gov/browse/indicators/annual-greenhouse-gas-index |url-status=live |archive-url=https://web.archive.org/web/20210421143115/https://www.globalchange.gov/browse/indicators/annual-greenhouse-gas-index |archive-date=21 April 2021 |access-date=5 September 2020 |publisher=U.S. Global Change Research Program}}</ref><ref name="butmon">{{Cite web |author=Butler J. and Montzka S. |year=2020 |title=The NOAA Annual Greenhouse Gas Index (AGGI) |url=https://www.esrl.noaa.gov/gmd/aggi/aggi.html |url-status=live |archive-url=https://web.archive.org/web/20130922035917/https://www.esrl.noaa.gov/gmd/aggi/aggi.html |archive-date=22 September 2013 |access-date=5 September 2020 |publisher=[[NOAA]] Global Monitoring Laboratory/Earth System Research Laboratories}}</ref>]]
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.<ref>[http://www.chem.tamu.edu/rgroup/north/ITS%20GWP%20Data.xls Matthew Elrod, "Greenhouse Warming Potential Model."] Based on {{Cite journal |last1=Elrod |first1=M. J. |year=1999 |title=Greenhouse Warming Potentials from the Infrared Spectroscopy of Atmospheric Gases |journal=Journal of Chemical Education |volume=76 |issue=12 |pages=1702 |bibcode=1999JChEd..76.1702E |doi=10.1021/ed076p1702}}</ref>

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.<ref>
{{cite web 
|url=http://www.eia.gov/tools/glossary/index.cfm?id=G 
|title=Glossary: Global warming potential (GWP) 
|publisher=U.S. Energy Information Administration 
|access-date=2011-04-26
|quote=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.
}}</ref>

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:

<math display="block">\mathit{RF} = \sum_{i=1}^{100} \text{abs}_i \cdot F_i / \left(\text{l} \cdot \text{d}\right)</math>
where the subscript ''i'' represents a [[wavenumber]] interval of 10 [[inverse centimeter]]s. Abs<sub>i</sub> represents the integrated infrared absorbance of the sample in that interval, and F<sub>i</sub> represents the RF for that interval.{{citation needed|date=September 2008}}

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.<ref>{{cite web |url=http://www.grida.no/climate/ipcc_tar/wg1/247.htm |title=Climate Change 2001: The Scientific Basis |website=www.grida.no |access-date=11 January 2022 |archive-url=https://web.archive.org/web/20160131050350/http://www.grida.no/climate/ipcc_tar/wg1/247.htm |archive-date=31 January 2016 |url-status=dead}}</ref> The GWP is defined as the ratio of the time-integrated radiative forcing from the instantaneous release of 1&nbsp;kg of a trace substance relative to that of 1&nbsp;kg of a reference gas:

<math display="block">\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}</math>
where TH is the time horizon over which the calculation is considered; a<sub>x</sub> is the radiative efficiency due to a unit increase in atmospheric abundance of the substance (i.e., Wm<sup>−2</sup> kg<sup>−1</sup>) 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|link=yes}}). The radiative efficiencies a<sub>x</sub> and a<sub>r</sub> 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}}, CH<sub>4</sub>, and N<sub>2</sub>O). 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.