How do you calculate greenhouse gas emissions?
While methods for estimating GHG emissions differ depending on the sector, source and available data, most calculations are based on the following formula and require three sets of data:
GHG emissions are usually measured and reported in kilograms or metric tons of “carbon dioxide equivalents”, denoted as kgCO2e or tCO2e (or CO2eq).
This single unit applies to all GHGs, including carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O).
Activity data
- This is the level of activity (amount) that results in emissions occurring (e.g. amount of fuel combusted/ electricity consumed/ waste sent to landfill).
- This could be measured in mass (kg or tonnes), volume (m3 or litres), or energy output (kWh or TJ).
- Once the activity data has been collected or calculated, it can be converted into different units using conversion factors.
Emission factors
- Converts activity data into emissions of GHGs.
- EFs can either be disaggregated by gas, or given as a total, depending on whether total GHG emissions or emissions of a specific gas are being calculated.
- EFs for grid electricity are always given as total tCO2e/ kWh (i.e. not disaggregated by gas).
Global warming potential
- GWP values are used to convert the mass of a GHG into the equivalent mass of CO2 (i.e. tCO2e).
- Converts emissions of other GHGs to a consistent unit → kg or tonnes of CO2 equivalent, I.e. the GWP unit could be kg CO2e/ kg specific greenhouse gas.
- The GWP value for CO2 will always be 1 (one), because it is the reference gas.
- Remember that some sources may have integrated the GWP values into the EF value – always check the units.
In summary, once the activity data and emission factors have been identified, GHG emissions can be calculated manually. After applying the GWP values to convert to tCO2e, the total emissions in tCO2e can be added up. Alternatively, these calculations can be performed in the CIRIS tool or Proxy Data Tool, which have embedded calculators to manage the conversions.