CO₂ equivalent (CO₂e) is a common unit for expressing emissions of different greenhouse gases so that they can be added up and compared. The conversion uses the global warming potential (GWP), which shows how many times more one ton of a gas warms the climate than one ton of CO₂ over a chosen period, usually 100 years.
Which gases are covered
Corporate accounting under the GHG Protocol and ISO 14064-1 covers the main greenhouse gases:
- carbon dioxide (CO₂): fuel combustion and a number of industrial processes;
- methane (CH₄): natural gas leaks, landfills, agriculture, coal mines;
- nitrous oxide (N₂O): fertilizers, certain chemical processes, fuel combustion;
- fluorinated gases (hydrofluorocarbons, perfluorocarbons, SF₆, NF₃): refrigeration equipment, electrical equipment, metallurgy.
For most businesses, the bulk of emissions comes from CO₂ from fuel combustion and electricity use, but methane and refrigerants can be significant even in small quantities because of their high GWP values.
GWP values: AR5 and AR6
GWP values are published by the Intergovernmental Panel on Climate Change (IPCC) in its assessment reports. In practice, two sets of 100-year values are in use:
| Gas | AR5 | AR6 |
|---|---|---|
| CO₂ | 1 | 1 |
| CH₄ (fossil) | 28 | 29.8 |
| CH₄ (non-fossil) | 28 | 27.0 |
| N₂O | 265 | 273 |
AR6 separates fossil methane (leaks from gas networks, extraction) from non-fossil, biogenic methane (waste, livestock), because the oxidation of fossil methane produces “new” CO₂.
Which set to use is determined by the program or reporting rule you report under. What matters most is to apply a single set consistently throughout the report and across all the years being compared, and to state your choice in the methodology.
Example: converting to CO₂e
Suppose that over a year a business estimated leaks of 2 t of fossil methane and emissions of 0.5 t of N₂O. Using AR6 values:
- methane: 2 t × 29.8 = 59.6 t CO₂e;
- nitrous oxide: 0.5 t × 273 = 136.5 t CO₂e;
- total: 196.1 t CO₂e.
Under AR5, the result would be 2 × 28 + 0.5 × 265 = 56 + 132.5 = 188.5 t CO₂e. The difference is about 4%, which is why you should not mix the sets when comparing years. If you switch to the new values, recalculate the base year as well; see Base year and base-year recalculation.
GWP and emission factors
In practice, you rarely multiply tons of gas by GWP by hand. Emission factors in reference databases are often already expressed in CO₂e: for example, the worked examples in this knowledge base use 2.04 kg CO₂e/m³ for natural gas. Factors in CO₂e usually cover CO₂, CH₄ and N₂O from combustion, already converted to an equivalent using a particular set of GWP values, so check the factor's description for its scope and source. For how to choose factors, see Emission factors.
How it works in the platform
The platform calculates emissions using methodologies from its reference catalog (IPCC 2006, DESNZ and other sources) and shows the results directly in CO₂e: in the records on the Data entry screen, in company GHG analytics and in the annual company report, which the administrator creates on the organization's GHG page in the Facilities section. Every record shows the methodology applied, so you can tell which value was used in the calculation.
Frequently asked questions
Which GWP set should we use, AR5 or AR6?
The one your reporting program or verifier requires. If there are no requirements, state the chosen set in your methodology and apply it consistently to all years and all sources.
Why does methane have two values in AR6?
When fossil methane oxidizes, it adds carbon to the atmosphere that was previously stored underground, so its GWP (29.8) is higher than that of non-fossil methane of biogenic origin (27.0).
Do we need to account for refrigerants?
Yes, if your facilities have refrigeration equipment or air conditioners that are refilled. Leaks of fluorinated gases fall under Scope 1 and, given their high GWP, can make a noticeable contribution.