DEFRA 2024 emission factors in depth

What DEFRA / DESNZ publishes each year

The UK Department for Energy Security and Net Zero (DESNZ — the body that absorbed BEIS's GHG work in 2023, which in turn took over from DEFRA's inventory team) publishes three documents annually: a condensed factor set for typical reporters, a full set with every sub-category, and a methodology paper describing the derivation. All three are open and downloadable from gov.uk under the same publication identifier.

What each band actually means

The UK reporting distance bands are not the same as the aircraft-design short-/medium-/long-haul bands. DESNZ uses domestic for any flight beginning and ending within the UK, short-haul for international flights up to a great-circle distance of 3,700 km from the UK airport, and long-haul for anything further. Premium cabins do not exist on domestic UK routes, and "first" only exists on long-haul because few short-haul jets have a first-class cabin in the DESNZ-recognised sense.

The cabin-class multipliers, decoded

DESNZ's cabin multipliers reflect floor-area allocation per passenger, not actual per-seat fuel consumption — which on a single aircraft cabin is roughly equal across classes. Business class typically occupies about 2.9 × the floor area of an economy seat on a long-haul wide-body; first about 4 ×. The methodology paper acknowledges that this is a deliberate allocation choice and not a measurement.

The 1.9 × radiative-forcing uplift

The DESNZ headline numbers are kg CO₂e — they already include a 1.9 × multiplier that converts the underlying kg CO₂ into a CO₂-equivalent intended to reflect aviation's full warming effect. The methodology paper cites Lee et al.'s 2009 review as the base evidence and acknowledges scientific uncertainty: Lee 2021's updated decomposition pushes some analyses toward 1.7 ×, others toward values above 2.0 × depending on which time horizon and which GWP metric is used.

The net ERF in 2018 was 100.9 mW m⁻² and the CO₂ component alone was 34.3 mW m⁻², so the ratio of total-to-CO₂ is about 2.94 — not 1.9. The gap arises because DESNZ uses a GWP100-based uplift (long-term warming potential), while Lee 2021 reports ERF at a snapshot in time. Long-term, CO₂ accumulates while contrails decay, so the GWP100 ratio is smaller than the instantaneous-ERF ratio.

Mapping DEFRA factors outside the UK

The DESNZ factors are calibrated to UK aviation fuel mix and average load factors, but they are widely applied internationally for two reasons: they are open and well-documented, and no other jurisdiction publishes a comparable single-source factor set. Care is needed in two places: long- haul routes from low-load-factor regions (Africa, parts of Latin America) may run 10–20 % higher per pax-km, and very short hops from regional jets in the US can be closer to the DESNZ domestic figure than its short-haul.