Editing Global warming potential (section)

== Applications ==

=== Carbon dioxide equivalent ===
Carbon dioxide equivalent ({{CO2}}e or {{CO2}}eq 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.<ref name="epadef">{{Cite web|title=CO2e|url=https://www3.epa.gov/carbon-footprint-calculator/tool/definitions/co2e.html|access-date=2020-06-27|website=www3.epa.gov}}</ref> 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 {{CO2}}e of 200 tonnes, and 9 tonnes of the gas has {{CO2}}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 an equivalent atmospheric concentration of {{CO2}}. {{CO2}}e 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, {{CO2}}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 {{CO2}} would warm it.<ref name="eea">{{Cite web |date=2020-02-25 |title=Atmospheric greenhouse gas concentrations – Rationale |url=https://www.eea.europa.eu/data-and-maps/indicators/atmospheric-greenhouse-gas-concentrations-6/assessment-1 |access-date=2020-06-28 |website=European Environment Agency |language=en}}</ref><ref name="rmets">{{cite journal |last1=Gohar |first1=L. K. |last2=Shine |first2=K. P. |title=Equivalent {{CO2}} and its use in understanding the climate effects of increased greenhouse gas concentrations |journal=Weather |date=November 2007 |volume=62 |issue=11 |pages=307–311 |doi=10.1002/wea.103 |bibcode=2007Wthr...62..307G |doi-access=free }}</ref> 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}}.

{{CO2}}e calculations depend on the time-scale chosen, typically 100 years or 20 years,<ref name="Wedderburn-Bisshop et al 2015">{{cite journal |last1=Wedderburn-Bisshop |first1=Gerard |last2=Longmire |first2=Andrew |last3=Rickards |first3=Lauren |title=Neglected Transformational Responses: Implications of Excluding Short Lived Emissions and Near Term Projections in Greenhouse Gas Accounting |journal=The International Journal of Climate Change: Impacts and Responses |date=2015 |volume=7 |issue=3 |pages=11–27 |id={{ProQuest|2794017083}} |doi=10.18848/1835-7156/CGP/v07i03/37242 }}</ref><ref name="OckoHamburg2017">{{cite journal |last1=Ocko |first1=Ilissa B. |last2=Hamburg |first2=Steven P. |last3=Jacob |first3=Daniel J. |last4=Keith |first4=David W. |last5=Keohane |first5=Nathaniel O. |last6=Oppenheimer |first6=Michael |last7=Roy-Mayhew |first7=Joseph D. |last8=Schrag |first8=Daniel P. |last9=Pacala |first9=Stephen W. |title=Unmask temporal trade-offs in climate policy debates |journal=Science |date=5 May 2017 |volume=356 |issue=6337 |pages=492–493 |doi=10.1126/science.aaj2350 |pmid=28473552 |bibcode=2017Sci...356..492O |s2cid=206653952 }}</ref> since gases decay in the atmosphere or are absorbed naturally, at different rates.

The following [[Units of measurement|units]] are commonly used:
* By the UN climate change panel ([[Intergovernmental Panel on Climate Change|IPCC]]): billion metric tonnes = n×10<sup>9</sup> [[tonne]]s of {{CO2}} equivalent (Gt{{CO2}}eq)<ref>{{cite journal |last1=Denison |first1=Steve |last2=Forster |first2=Piers M |last3=Smith |first3=Christopher J |title=Guidance on emissions metrics for nationally determined contributions under the Paris Agreement |journal=Environmental Research Letters |date=December 2019 |volume=14 |issue=12 |pages=124002 |doi=10.1088/1748-9326/ab4df4 |bibcode=2019ERL....14l4002D |doi-access=free }}</ref>
* In industry: million metric tonnes of carbon dioxide equivalents (MMTCDE)<ref>{{Cite web |title=Glossary:Carbon dioxide equivalent – Statistics Explained |url=https://ec.europa.eu/eurostat/statistics-explained/index.php/Glossary:Carbon_dioxide_equivalent |access-date=2020-06-28 |website=ec.europa.eu}}</ref> and MMT {{CO2}}eq.<ref name="epa20">{{Cite web |date=2020-04-13 |title=Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2018, p. ES-3 |url=https://www.epa.gov/sites/production/files/2020-04/documents/us-ghg-inventory-2020-chapter-executive-summary.pdf |url-status=live |archive-url=https://web.archive.org/web/20200414000128/https://www.epa.gov/sites/production/files/2020-04/documents/us-ghg-inventory-2020-chapter-executive-summary.pdf |archive-date=2020-04-14 |access-date=2020-07-01 |website=US Environmental Protection Agency}}</ref>
* For vehicles: grams of carbon dioxide equivalent per mile (g{{CO2}}e/mile) or per kilometer (g{{CO2}}e/km)<ref name="ucs-ev">{{Cite web |title=How Clean is Your Electric Vehicle? |url=https://evtool.ucsusa.org/ |access-date=2020-07-02 |website=Union of Concerned Scientists |language=en}}</ref><ref name="rcs">{{Cite web |last=Whitehead |first=Jake |date=2019-09-07 |title=The Truth About Electric Vehicle Emissions |url=https://www.realclearscience.com/articles/2019/09/07/the_truth_about_electric_vehicle_emissions_111097.html |access-date=2020-07-02 |website=www.realclearscience.com}}</ref>

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 ===
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.<ref>
{{cite book |author=Conference of the Parties |url=http://unfccc.int/resource/docs/cop3/07a01.pdf |title=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 |date=25 March 1998 |publisher=[[UNFCCC]] |contribution=Methodological issues related to the Kyoto Protocol |access-date=17 January 2011 |archive-url=https://web.archive.org/web/20000823193833/http://www.unfccc.int/resource/docs/cop3/07a01.pdf |archive-date=2000-08-23 |url-status=live}}</ref><ref>{{cite journal |last1=Godal |first1=Odd |last2=Fuglestvedt |first2=Jan |date=2002 |title=Testing 100-year global warming potentials: Impacts on compliance costs and abatement profile |journal=Climatic Change |volume=52 |issue=1/2 |pages=93–127 |doi=10.1023/A:1013086803762 |s2cid=150488348 |id={{ProQuest|198550594}}}}</ref>

After some intermediate updates, in 2013 this standard was updated by the Warsaw meeting of the [[United Nations Framework Convention on Climate Change|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.<ref name="unfccc19" /> Those 2007 estimates are still used for international comparisons through 2020,<ref name="epa20" /> 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)<ref name="sar" /> 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.<ref name="ar5" />