Burning a kilogram of jet fuel produces 3.15 kilograms of CO₂. If that were the whole story, flight emissions would be a matter of fuel burn and arithmetic. It isn’t, because an aircraft at cruising altitude also produces water vapour, nitrogen oxides and soot in air that is cold and thin. The result is a set of effects that are not greenhouse gases but still warm the climate.
What contrails do
The most important effect is the one you can see: condensation trails. When hot, humid exhaust meets air that is cold enough and supersaturated with ice, the water freezes on soot particles into ice crystals. Most trails disappear in seconds. Some persist for hours, spread out and become indistinguishable from natural cirrus.
These clouds reflect sunlight back to space (cooling) and trap heat radiating from the ground (warming). Which effect wins depends on the time of day, the surface below and the thickness of the cloud. Night-time contrails only warm. Averaged over the globe, the net effect is warming.
How much warming is the hard part. The most cited estimate (Lee et al., 2021) puts the non-CO₂ effects at roughly two thirds of aviation’s net climate forcing, but with error bars much wider than those for CO₂. Different studies, different years and different metrics give factors anywhere between well below 1.5 and above 2.
How the factor is used in practice
Because the uncertainty is large, calculation methods make a choice:
- Leave it out. The ICAO Carbon Emissions Calculator reports CO₂ only. So do many corporate reports, since the GHG Protocol counts greenhouse gases, and contrails are not one.
- A fixed uplift. The UK Government conversion factors offer a “with radiative forcing” variant that multiplies the CO₂ figure by 1.7 (reduced from 1.9 in earlier years). It is applied to every flight, long or short.
- Depending on altitude. This is what Carbon Compute does.
How Carbon Compute applies it
The physics only happens at altitude. A turboprop between Stockholm and Visby cruises far below the tropopause and produces no persistent contrails. A long-haul jet spends most of its flight in exactly the air where they form.
Our algorithm therefore applies the height factor only to the portion of the flight above roughly 8 000 metres. In the original published method the factor was 1.9, applied to the climb–cruise–descent phase but not to landing and take-off.¹ In the current implementation the factor builds up with altitude, so a short regional flight carries no uplift, medium-haul flights get part of it and long-haul flights approach ×1.9. Averaged over all flights worldwide the effect is about ×1.7.
We think this is the fair middle ground. A factor of 1 ignores the larger half of aviation’s climate impact. A flat 1.9 on every flight penalises a 45-minute turboprop for something it does not do.
What to put in your report
Whatever method you use, say so. Our recommendation:
- Report your CO₂e figure with the height factor and state the factor and how it is applied.
- Disclose the CO₂-only figure alongside, so that readers who follow a greenhouse-gas-only convention can compare.
- Keep the method constant between years, or restate earlier years when you change it.
Research on contrail avoidance is moving quickly. Rerouting a small share of flights by a few hundred metres could remove a large share of the warming. If that becomes routine, the height factor will fall, and so will the numbers in your report. We update the factor as the evidence changes.
¹ Schennings, A., Larsson, J. & Robèrt, M. (2019). Development and implementation of an emission optimization model for passenger flight bookings. Sustainable Environment Research 29:25. doi:10.1186/s42834-019-0024-5
Lee, D. S. et al. (2021). The contribution of global aviation to anthropogenic climate forcing for 2000 to 2018. Atmospheric Environment 244. doi:10.1016/j.atmosenv.2020.117834