What is CO2e or “carbon dioxide equivalent”?
By Anthony Capkun
May 5, 2021 – I am increasingly seeing the measurement CO2e used in the news when describing GHG (greenhouse gas) emissions, usually when quoting reductions in those emissions.
But what is CO2e? How is it measured? What is it based on?
I researched the term “CO2e” and found some helpful definitions on a website managed by the European Commission’s Directorate-General for Communication (an official website of the European Union).
Most of the words that follow come straight from their glossary (with some editing)…
A carbon dioxide equivalent or CO2 equivalent (a.k.a. CO2e) is a metric measure used to compare the emissions from various greenhouse gases (GHGs) on the basis of their global-warming potential (GWP) by converting amounts of other gases to the equivalent amount of carbon dioxide with the same GWP.
Global-warming potential, meantime, describes the relative potency—molecule for molecule—of a greenhouse gas, taking account of how long it remains active in the atmosphere.
The global-warming potentials currently used are those calculated over 100 years. Carbon dioxide is taken as the gas of reference and given a 100-year GWP of 1. The greenhouse gases identified in Kyoto Protocol include:
• carbon dioxide (CO2)
• methane (CH4)
• nitrous oxide (N2O)
• hydrofluorocarbons (HFCs)
• perfluorocarbons (PFCs)
• sulphur hexafluoride (SF6)
• nitrogen trifluoride (NF3)
Converting these gases to carbon dioxide equivalents makes it possible to compare them.
While I’ve not yet come across this, CO equivalents are commonly expressed as million metric tonnes of CO equivalents, abbreviated as MMTCDE.
So, the CO2 equivalent for a gas is derived by multiplying the tonnes of the gas by the associated GWP.
MMTCDE = (million metric tonnes of a gas) * (GWP of the gas)
For example, the GWP for methane is 25, while nitrous oxide is 298. This means that emissions of 1 million metric tonnes of methane and nitrous oxide, respectively, is equivalent to emissions of 25 and 298 million metric tonnes of carbon dioxide.
N.B. The information on these pages ranges in age: 2014 seems to be the oldest entry, with 2017 being the most recent.