Carbon factors, validated against the population: 75,000 vessel-years say 3.13–3.15

Carbon factors look like settled science until you assemble five years of EU MRV reports and notice the spread. The regulation fixes default factors in FuelEU Annex II — 3.114 t CO2 per tonne of HFO, well-to-wake — but operators submit their own where they can justify a bunker-specific value. Both paths are legitimate. Both can also be wrong.

What we check

Provenship carries the Annex II defaults as the primary source. Every operator-supplied carbon factor is also compared against the population of factors implied by the 2018–2023 EU MRV emission reports (75,000+ vessel-years after deduplication). The check is a two-line filter:

• If the operator's factor sits inside the 5th–95th percentile of the population for that fuel, accept it.
• If it falls outside, flag it for verifier review — do not block, but do not silently swallow.

What the population says

For HFO, the population median implied factor lands at 3.13–3.15 t CO2 per tonne fuel — slightly above the Annex II default. That offset is not an error; it reflects the mix of HFO grades actually burned (some heavier than the regulation's reference grade). The 5th–95th band is 3.05–3.22.

MDO median: 3.20–3.22 against Annex II 3.206. Tight. LNG implied factors are noisier — the methane-slip term swings the well-to-wake number by ±25% — which is why we treat LNG separately and discuss it in its own note.

Why this check earns its keep

An operator carbon factor that drifts outside the population band almost always means one of three things: a transcription error in the bunker delivery note, an incorrectly applied fuel grade, or an outlier bunker that the verifier needs to see anyway. Catching any of the three before the report goes to the verifier is the difference between a clean filing and a back-and-forth that runs the clock on the reporting deadline.

What this check is not

It is not a substitute for the Annex II default. The regulator's default remains the source of truth when the operator does not justify a custom factor. The population band is a tripwire, not an authority.