Concepts
GWP sets and how one is chosen
A GWP set turns methane, nitrous oxide and fluorinated gases into CO₂e. Slate holds six, applies one per organisation, and pins it per reporting year.
On this page you'll learn
- Which set applies to a given row
- Why AR6 needs to know where methane came from
- What changes when an organisation switches set
- Default
- IPCC AR5, 100-year
- Available
- AR2, AR4, AR5, AR6 at 20, 100 and 500 years
- Values table
- GWP sets reference
Which set applies
- 1
The reporting year's pin
If anything in this reporting year has already been calculated with a GWP set, that set is pinned and used.
- 2
The organisation's choice
Otherwise, the set chosen in Settings → Emission Factors.
- 3
The system default
Otherwise, IPCC AR5 100-year.
The set is only applied to per-gas factors. Rows from publishers who supply CO₂e already carry their own vintage; see CO₂e and gas basis.
AR6 splits methane by origin
| Gas | AR5 100yr | AR6 100yr |
|---|---|---|
| CO₂ | 1 | 1 |
| CH₄, fossil | 28 | 29.8 |
| CH₄, non-fossil | 28 | 27.0 |
| N₂O | 265 | 273 |
Slate reads the origin from the publisher's own classification of each fuel (EPA prints its biomass fuels under separate headings), never from the fuel's name. 333 EPA activities are fossil, 17 biogenic, and none that reach a GWP set are unclassified.
Watch out
If a calculation under AR6 can't tell whether its methane is fossil or not, it stops with an error instead of choosing. The two values are 10% apart, and picking one silently would restate every methane figure with nothing in the audit trail.
Switching sets
Switching restates every methane and nitrous oxide figure. A reporting year that has already been calculated keeps its pinned set, so figures an assurance provider has signed off don't move. Only rows where a GWP set was actually applied create a pin; DEFRA rows, which arrive as CO₂e, don't.
When a total mixes vintages (for example AR6 on EPA rows and AR5 inside DEFRA rows), each affected row is flagged gwp_basis_mismatch. The flag is recorded whether or not the warning is switched on in settings.
Refrigerant blends
AR5 and AR6 don't list blends such as R-410A. Slate derives AR6 blend values from each blend's ASHRAE Standard 34 composition, checked against the blend's published AR5 figure first. 49 of 50 pass; R-424A doesn't, so it has no AR6 value and a leak of it under AR6 stops with the gas named.