What does the “e” mean?
The “e” means “equivalent”.
- “CO2e” is a concept that allows us to express the impact of any greenhouse gas (GHG) by comparing it to some amount of CO2.
- There are two common ways of doing this.
1. “CO2e” is the mass of CO2 that would have an equal Global Warming Potential (GWP) to the mass of whatever greenhouse gas (GHG) is in question, over a defined period of time.
- Generally uses the units of tonnes.
- GWP is a measure of the effect of a greenhouse gas on the global mean temperature.
- It is calculated over fixed time interval, usually 100 years. This is because some GHGs have a finite lifetime in the atmosphere.
- For example methane (CH4) is reckoned to have about 25 times the impact on the climate over 100 years as the same mass of CO2, so the addition of one tonne of CH4 to the atmosphere would add 25 tonnes to the total CO2e.
2. “CO2e” is the concentration of CO2 in the atmosphere that would cause the same temperature impact on the climate as would be caused by the GHG in question.
- Generally uses the units of parts per million (ppm), calculated by volume rather than mass (sometimes called ppmv to be explicit).
- For example, the current concentration of CO2 in the atmosphere is 390 ppm (2012), but other GHGs push this up to a CO2e of 430 ppm, i.e. the greenhouse effect is as strong as if the only GHG in the atmosphere was CO2 at 430 ppm concentration.
Partially Updated 2019-09-23