Source GWP100 values from IPCC Sixth Assessment Report (AR6, 2021) Annex II; the values are: CH4 fossil = 29.8 CO2e per kg CH4, CH4 biogenic = 27.9 CO2e per kg CH4.
Classify each methane emission source as fossil (natural gas, coal mine, oil extraction, landfill gas from fossil fractions) or biogenic (enteric fermentation, manure, wetlands, agricultural waste, biogenic fraction of landfill gas) before applying the factor.
Multiply measured CH4 mass (kg) by the appropriate GWP100 factor to obtain kg CO2e; sum across all sources and convert to tonnes CO2e as needed.
Document which GWP100 basis you are using in your methodology notes; AR6 values differ from AR5 (fossil CH4 was 36 including climate-carbon feedbacks, or 34 without) and frameworks may specify which AR to use.
Check your reporting framework's requirements: GHG Protocol, CDP, and some national regulations may require a specific AR vintage; use AR6 only when the framework permits or requires it.
For scope 3 supply-chain calculations involving upstream natural gas, apply the fossil CH4 factor of 29.8; for agricultural supply chains or biogas systems, apply the biogenic factor of 27.9.
Known gotchas
AR6 distinguishes fossil CH4 (29.8) from biogenic CH4 (27.9); using a single undifferentiated value (e.g., the AR5 figure of 28 or 34) will misstate emissions when sources are mixed.
The AR6 fossil CH4 GWP100 of 29.8 includes the indirect effect of methane oxidising to CO2 that adds new carbon to the atmosphere; biogenic CH4's oxidation CO2 does not add new net carbon, hence the lower factor of 27.9.
Some published tools and calculators still default to AR5 values; verify the AR version embedded in any third-party carbon accounting software before accepting its output.
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