What Exactly is CO₂e?

30 September 2021


We talk about trees sequestering carbon and helping to mitigate climate change. But how does this tie up with the CO2e (Carbon Dioxide Equivalent) which we refer to elsewhere?


In practice, there are seven greenhouse gases (GHG) which the Kyoto Protocol identified as contributing to global warming. Carbon Dioxide (CO2) is the most prevalent, accounting for 80% of GHG emissions.


This is followed by Methane (CH4), which comprises 10%, and Nitrous Oxide (N2O) which makes up 7% of all GHG emissions. The remaining 3% is accounted for by the fluorinated gases, such as Hydrofluorocarbon, used in refrigerants and aerosol propellants.


Each of these GHGs absorb different levels of energy so have different effects on the earth’s warming. Scientists have therefore developed Global Warming Potential (GWP) to enable comparisons between the global warming impacts of different GHGs. Specifically, GWP is a measure of how much energy the emissions of one tonne of a gas will absorb over a given period, relative to the emissions of one tonne of CO2.


The UK government estimates the GWP of Methane and Nitrous Oxide to be 25 and 298 respectively. This standardised metric then provides an easy benchmark for companies and offsetting projects alike who are attempting to balance emissions and suppressions. A company emitting four tonnes of Methane must sequester (4 x 25) 100 tonnes of CO2e to neutralise its impact on the earth’s warming.


By using one tonne of CO2e as a benchmark against which we can measure the impact on global warming of other GHG emissions, companies and offsetting projects are able to work on the same, easily understood and readily exchangeable unit of offsetting currency.

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Pending Issuance Unit: A promise to deliver a Woodland Carbon Unit during a given period, based on the trees’ predicted growth Woodland Carbon Unit: A ton of carbon dioxide which has been sequestered in a scheme verified under the Woodland Carbon Code