Editing Global warming potential (section)

=== 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.