Reference

GWP sets

A global warming potential says how much warming one kilogram of a gas causes compared with one kilogram of CO₂ over a set period. It is how methane, nitrous oxide and refrigerants end up in a single CO₂e total, and the choice of set changes that total.

What a GWP set is

Each IPCC assessment report publishes a fresh set of GWPs as the science improves. Slate stores each one as a set you can select, and every calculated record names the set it used.

For example, a boiler that burned enough natural gas to emit 1,000 kg CO₂, 0.1 kg CH₄ and 0.01 kg N₂O reports 1,000 + 0.1 × 28 + 0.01 × 265 = 1,005.45 kg CO₂e under AR5, and 1,000 + 0.1 × 29.8 + 0.01 × 273 = 1,005.71 kg CO₂e under AR6. The difference is small for combustion and large for anything dominated by methane, such as livestock or landfill.

The sets in Slate

AR6 splits methane

AR6 is the first report to give methane two values: 29.8 for fossil and 27.0 for non-fossil (100-year). Oxidising fossil methane adds carbon to the atmosphere that was locked underground; oxidising methane from cattle or a landfill returns carbon that was recently in the air.

So under AR6 Slate needs to know where the methane came from. It reads that from the publisher, which marks its biomass fuels separately, never from a fuel's name. If a calculation can't tell, it stops and says so instead of picking a value: the two are 10% apart.

GWPs inside publishers' figures

Some publishers apply GWPs themselves and publish CO₂e directly. Slate never applies another set on top, which would count methane twice. Those figures stay on the publisher's vintage:

Choosing and changing a set

AR5 100-year is the default. The GHG Protocol and national inventories report on a 100-year horizon; the 20-year set is for scenario work, where methane cuts look very different, and isn't meant for an inventory.

Switching sets restates every methane and nitrous oxide figure you've published. To protect figures an auditor has already signed off, the first calculation in a reporting year pins the set it used, and that year stays on it. Rows whose CO₂e came pre-calculated from a publisher don't pin anything, because no set was applied to them.

Refrigerant blends

Neither AR5 nor AR6 lists blended refrigerants such as R-410A or R-404A. Slate holds 51 blends. For AR6 it derives each blend's GWP from its ASHRAE Standard 34 composition, as the mass-weighted sum of its components' AR6 values, but only after checking that the same method reproduces the blend's published AR5 figure.

Sources

AR6 major gases come from Table 7.15, the only table with the fossil and non-fossil methane split; halocarbons come from Supplementary Table 7.SM.7. AR5 values come from Table 8.A.1, the same table EPA's Hub cites, so the two agree by construction.