If your carbon calculator uses a GWP-100 value of 25 for methane, it's out of date by one full IPCC assessment cycle.
The problem
The IPCC Sixth Assessment Report (AR6), published 2021, updated GWP-100 values significantly:
| Gas | AR5 | AR6 | Change |
|---|---|---|---|
| CH4 (fossil) | 25 | 29.8 | +19.2% |
| CH4 (biogenic) | 25 | 27.9 | +11.6% |
| N2O | 265 | 273 | +3.0% |
| SF6 | 23,500 | 25,200 | +7.2% |
For a company with significant natural gas combustion, refrigerant leaks, or agricultural emissions, the difference between AR5 and AR6 is not cosmetic. It's a ~19% understatement of methane-related emissions.
Why it hasn't been fixed widely
The GHG Protocol Corporate Standard permits either AR5 or AR6. Most tools defaulted to AR5 when they launched and haven't updated. Many sustainability teams don't know which GWP basis their tools use — it's rarely displayed prominently.
What fossil vs biogenic methane means
AR6 distinguishes two GWP values for CH4:
- Fossil CH4 (29.8) — use for natural gas combustion, coal mining fugitives, oil & gas fugitive emissions
- Biogenic CH4 (27.9) — use for landfill gas, agricultural emissions, managed biological processes
Most tools use a single CH4 value and don't distinguish. This is technically incorrect under AR6.
How to check your tools
Look for a "methodology" or "about" page on any carbon calculator you use. It should state:
- Which IPCC assessment report the GWP values come from
- Whether fossil and biogenic CH4 are treated separately
- The source for emission factors (DESNZ, EPA, IPCC 2006, etc.)
If it doesn't disclose this, treat the output with caution.
What we built
I'm building GreenCalculus — a platform for carbon accounting calculators — and published the full AR6 GWP dataset openly on GitHub:
→ greencalculus/greencalculus-methodology
The data/gwp-values.json file has all 16 gases with AR5 and AR6 values side by side so you can audit your own tools. The METHODOLOGY.md explains exactly how we apply the GHG Protocol hierarchy.
Live demo
Scope 1 stationary combustion calculator using correct AR6 values, zero dependencies:
→ greencalculus.github.io/greencalculus-calculator-demo
Built in Vanilla JS — no frameworks, no bundler, ~300 lines. The calculation is:
Emissions (tCO₂e) = Activity Data × Emission Factor × GWP
Where GWP = 1 for factors already expressed in CO₂e (most fuel factors are pre-multiplied), and the AR6 value for raw gas quantities.
Bottom line
Check your GWP basis. If it says 25 for CH4, update it to 29.8 (fossil) or 27.9 (biogenic). Your Scope 1 inventory depends on it — and so does your CSRD disclosure.
The full open dataset is free to use and cite:
→ github.com/greencalculus/greencalculus-methodology
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