Activity data are data on the level of an activity that affects greenhouse gas (GHG) emissions or sequestration.
Emission factors are coefficients that specify the quantity of GHGs that is emitted or sequestered per unit of GHG-producing activity.
In the IPCC methodology, the general equation for estimating greenhouse gas emissions is:
GHG Emission/Removal = activity data X emission factor
Examples of activity data
- Number of livestock heads (i.e., 250 beef cattle)
- Quantity of nitrogen fertilizer applied to cropland soils (i.e., 100 kg of N-urea ha-1y-1) (see Esmitating Emissions Emissions from Managed Soils)
- Number of days of flooded irrigation of a paddy field (i.e., 80 days) (see Estimating Emissions Flooded Rice)
Examples of emission factors
- Methane emitted by beef cattle head (i.e., 55 kg of CH4 head-1 year-1) (see Estimating Emissions Manure Management and Enteric Fermentation)
- Percent of nitrogen applied to cropland emitted as N2O-N directly (i.e., 1%)
- Daily emissions from irrigated rice cultivation (i.e., 1.19 kg CH4 ha-1 day-1)
Global warming potential
GHG emissions are often measured in carbon dioxide equivalent (CO2e). To convert emissions to CO2e, the emissions are multiplied by their global warming potential (GWP), which takes into account that different greenhouse gasses are more efficient at capturing heat than CO2. Specifically, GWP is a measure of how much energy (i.e., heat) the emissions of 1 ton of gas will absorb over a given period of time relative to the emissions of 1 ton of CO2.
GWP was first introduced in the IPCC First Assessment Report, with the 100-year GWP (GWP100) adopted by the UNFCCC and its Kyoto Protocol and is now widely used as the default metric. The IPCC Fifth Assessment Report (AR5) recommends the use of 28 and 265 for methane and nitrous oxide GWPs, respectively.
GWP* is another variation of GWP. It allows the more consistent expression of emissions of short-lived climate pollutants (SLCPs) and long-lived climate pollutants (LLCPs) by equating an emission rate and stock change of an SLCP as equivalent to a single emissions pulse of an LLCP. The result is a single metric of carbon dioxide “warming equivalents,” or CO2-we. GWP* was first introduced by Allen et al. (2018), and subsequently updated in Cain et al. (2019) and Smith et al. (2021).
Under GWP*, emissions of LLCPs, defined here as those having an atmospheric lifetime longer than H, are still represented as a cumulative pollutant within time-horizon H, and therefore equivalent emissions for LLCPs are derived simply by multiplying those emissions by GWPH. Following contemporary emissions accounting approaches, H is defined as 100. GWPH is the conventional global warming potential (GWP100) over time-horizon H for a given SLCP.