SAF does not change tailpipe CO₂. Whatever the feedstock, SAF is still a hydrocarbon kerosene substitute, so burning 1 kg of fuel releases the same ≈3.16 kg of CO₂ at the engine — that number is fixed by the carbon content of jet fuel, not by how the fuel was made.
What SAF changes is the lifecycle (well-to-wake) footprint: growing/capturing the feedstock absorbs carbon that fossil kerosene never did. ICAO/CORSIA's default baseline for conventional Jet-A is ≈89 gCO₂e/MJ; blending in SAF pulls the average down:
LifecycleFactor = (1-blend)·89 + blend·SAF_factor [gCO2e/MJ]
Energy = fuelBurned(kg) × 43 MJ/kg
Lifecycle CO2 = Energy × LifecycleFactor / 1000 [kg]
- HEFA (used cooking oil/fats, TRL 9) ≈ 24 gCO₂e/MJ → ~73% lower lifecycle footprint.
- Fischer-Tropsch Power-to-Liquid (captured CO₂ + green H₂, TRL 6-7) ≈ 9 gCO₂e/MJ → ~90% lower.
- Alcohol-to-Jet (fermented ethanol/isobutanol, TRL 7-8) ≈ 40 gCO₂e/MJ → ~55% lower.
Contrails form when the exhaust plume's mixing line crosses saturation over ice — roughly, when outside air is colder than about −40 °C, which on a standard atmosphere happens above ~9-10 km. SAF's lower aromatic/sulfur content also means far fewer soot particles, so even when a contrail forms it nucleates fewer, smaller ice crystals — this simulator scales a "soot index" (1.0 for Jet-A down to 0.1 for pure Fischer-Tropsch fuel) that sets how dense the visible trail is. Persistent contrail cirrus is believed to contribute a warming effect comparable to aviation's cumulative CO₂, so a thinner contrail is a real climate lever on top of the CO₂ number.