Emissions across the value chain
What SAF saves depends on how it is made
Burning SAF releases about the same CO2 at the engine as fossil jet fuel. Its benefit comes from the rest of the value chain: the carbon in the feedstock, and the energy and hydrogen used to make it. That is why HEFA and power-to-liquid SAF can save very different amounts.
Scope 1: at the engine
For an airline, the CO2 from burning SAF is reported like fossil CO2, at about 3.16 t per tonne of fuel, with the biogenic or recycled carbon shown separately. The saving does not appear here.
Scope 2: the energy to make it
Electricity dominates power-to-liquid. Made with renewable power and CO2 from the air, its lifecycle can be about 7 gCO2e/MJ; made with grid electricity, around 279, three times worse than fossil jet.
Scope 3: feedstock and hydrogen
HEFA from used cooking oil is about 14 gCO2e/MJ; from soybean or rapeseed, about 63–70 once land-use change is included. Fossil hydrogen for hydrotreating adds more.
89
gCO2e/MJ: fossil jet fuel baseline
14–70
gCO2e/MJ: HEFA, waste oils to crop oils
7–279
gCO2e/MJ: power-to-liquid
What this means for you: SAF’s value is in its lifecycle, not its tailpipe, and it ranges from a reduction of over 90% to an increase. Every claim needs the pathway, the feedstock and the power source behind it.
Sources: ICAO CORSIA Default Life Cycle Emissions (June 2025): UCO 13.9; soybean 40.4 + ILUC 22.5; rapeseed 47.4 + ILUC 22.8; fossil baseline 89 gCO2e/MJ. Boeing Australia, SAF Rules of Thumb (May 2026): PtL with DAC CO2 7 (wind), 25 (solar), 279 (grid).
Measuring climate impact
Measure the warming as well as the emissions
Aviation warms the climate through CO2 and through effects that last hours or years: contrails, NOx and water vapour. How much those effects count depends on the metric. The reference assessment of aviation’s climate impact found its total effect was 1.0 to 4.0 times that of its CO2 alone, depending on the metric chosen.
tCO2e (GWP100)
Heat trapped over a century after a one-off release. The basis of today’s reporting and compliance (UNFCCC, EU and UK ETS, CORSIA). It treats a year of contrails as a brief pulse, so it counts them lightly: aviation ≈1.7× its CO2.
tCO2-WE (GWP*)
Tonnes of CO2-warming-equivalent: the CO2 that would cause the same change in temperature. It reflects what a sustained change in contrails does to warming: aviation ≈3× its CO2.
Declare the period
Warming-equivalent figures depend on the accounting period. We report in tCO2-WE over a declared 30 years, with tCO2e alongside, so every figure can be read in the unit a regulator uses.
Our position: tCO2e is right for accounting and compliance; tCO2-WE is more right for the question that matters, how much warming is avoided. Since 1 January 2025 the EU has required airlines to monitor non-CO2 effects, and the Commission reports by the end of 2027.
Sources: Lee et al. (2021), Atmospheric Environment, Table 5 (GWP100 1.7; GWP* 3.0; range 1.0–4.0 for 2018); Allen et al. (2018), Cain et al. (2019), Lynch et al. (2020) on GWP*; EU MRV of non-CO2 effects from 1 January 2025.
Contrails
The largest part of aviation’s warming needs a fuel-based answer
Contrail cirrus is the largest single part of aviation’s warming: about 57% of its net effect in 2018, against 34% for its CO2. Most of it comes from a few flights: in 2019, 2% of flights caused 80% of contrail warming.
Why avoidance alone is limited
Change the time
Moving departures to avoid contrail-forming conditions clashes with slots, crews and connections, and costs airlines most where airspace is busiest.
Change the altitude
Climbing or descending out of ice-supersaturated air works when weather and traffic allow; seasons, routes and air traffic control often do not.
Change the path
Re-routing worked in a 2023 trial: 54% fewer contrails over 70 flights, for about 2% more fuel on those flights. But forecasting the right air in advance is hard, and the extra fuel adds CO2.
~0.05%
of European flights can use dynamic avoidance today
80%
of contrail warming is caused by 2% of flights
Building the solution into the fuel
Contrail ice forms on particles, mostly soot. Fewer soot particles mean fewer, larger ice crystals that fall out sooner, and less warming. Published modelling shows an 80% cut in soot particle number cuts contrail warming by about 41%, and fleet-wide low-soot fuel by up to about 44%. Targeted at the high-impact flights, each tonne of cleaner fuel delivers 9 to 15 times the average climate benefit.
We are building that effect into specification-compliant jet fuel: tuned to cut soot and contrail ice, independently tested, and deliverable through today’s fuel supply to the flights that matter, including the 99.95% that avoidance cannot reach.
Our intention
Our aim is a system that evidences the cost of every lever in one unit, £ per tonne of CO2-warming-equivalent avoided: SAF, avoidance and tuned fuel side by side. We expect it to favour tuned fuel strongly for contrails, deployed alongside SAF, not instead of it: SAF for carbon, tuned fuel for contrails, together for warming.
Sources: Lee et al. (2021) (57.4 vs 34.3 mW/m², 2018); Teoh et al. (2024) (2% of flights, 80% of contrail energy forcing, 2019); Google, American Airlines and Breakthrough Energy trial (2023); Bier & Burkhardt (2022) (80% soot number → −41%); Teoh et al. (2022) (fleet-wide 100% SAF −44%; targeting 9–15×); Burkhardt et al. (2018); Voigt et al. (2021).
Contrail research programme
Cutting contrail warming at the fuel: a research programme
If you want to see whether a specification-compliant jet fuel, tuned at production, can cut contrail warming, this programme is where that is being physically tested, and where you can take part.
- Works for SAF and fossil jet fuel
- Works with today’s aircraft, engines and airport infrastructure
Why contrails, and why fuel
The biggest single factor
Contrail cirrus is the largest single contributor to aviation's warming effect: 57% of its net effective radiative forcing in 2018. Lee et al. 2021
Jet fuel soot seeds the ice
Contrail ice forms mainly on engine soot. Fewer particles give fewer, larger crystals, and so thinner, shorter-lived contrails. Kärcher 2018; Burkhardt et al. 2018
Concentrated in a few flights
2% of flights caused 80% of contrail forcing in 2019. Teoh et al. 2024
Avoidance reaches few flights
Predicting contrail formation continues to improve; dynamically changing take-off times, altitudes and flight paths remain a challenge. Gierens et al. 2020; EUROCONTROL MUAC 2024 The largest planned European avoidance programme would reach about 0.05% of annual flights (CirculAIRity calculation). A fuel lever acts on every flight an aircraft makes.
Policy is coming
Measurement has started
Since 1 January 2025, the non-CO2 effects of flights within EU ETS scope are monitored and reported. Directive (EU) 2023/958
Proposals are due
The European Commission must report by 31 December 2027 and, where appropriate, propose measures to mitigate non-CO2 effects. In the UK, the Jet Zero Taskforce has set out a strategic framework for contrail mitigation.
A price follows measurement
Our central scenario sees the EU pricing non-CO2 warming around 2030–32 and the UK around 2033 (CirculAIRity scenario). Fuels tested and validated before then will be ready when it lands.
How much can fuel do? The evidence
Fuel composition impacts contrail forcing
Cleaner-burning fuels cut soot and contrail ice by 50–70%. Voigt et al. 2021; Märkl et al. 2024
The limits
At very low soot, volatile particles from sulphur and organics nucleate the ice (Voigt et al. 2026). Modelling based on 100% SAF flights indicates that the reduction in contrail warming is up to about 44% (Teoh et al. 2022). SAF blends partially provide this benefit by reducing aromatic content.
Targeted
Putting cleaner fuel on the right flights multiplies climate benefit by 9–15 times. Teoh et al. 2022
Tuning the whole fuel
Tuning the whole fuel as one aims for that contrail benefit at speed, at scale and at lower cost to you.
What you gain by taking part
- Ready for the policy and the specification: fuels designed for emerging contrail policy and meeting today's specifications (ASTM D1655, DEF STAN 91-091), so your aircraft and airport work as they are.
- Results in one unit of warming: tonnes of CO2-warming-equivalent (GWP*, declared 30-year period), with GWP100 alongside for your reporting.
- Evidence you can test: every claim carries an evidence grade, and the work is being independently peer reviewed by Cranfield University.
- The commercial case alongside the chemistry: what a climate-tuned fuel is worth to you, as well as what it does.
How you can become involved
If this research touches your work, we would be glad to hear from you, particularly if you can help in one of these areas.
- Engine test beds: burn the candidate fuel against matched comparators for various flight profiles, to validate our expected combustion characteristics and emissions.
- Independent atmospheric impact testing: assess how readily the exhaust particles during various flight profiles form into ice, using an ice-nucleation chamber.
- Into warming: run the measured change through contrail models to give tonnes of warming avoided, route by route, then subsequent flight testing and validation.
What is at stake for you
≈£18bn
a year: EU and UK contrail warming valued at today's EU ETS price, still unpriced
£10–17bn
a year by 2050: contrail-abatement market, balanced policy (≈£90bn if strong)
CirculAIRity figures under policy scenarios.
Introductory Signal
As an introduction to our work, we will prepare an initial Signal on a defined question of your choosing, at our expense. It sets out our view, the reasoning and evidence behind it, and a range of outcomes we see, in the most common form we deliver to any client.
It allows you, and those you report to, to assess the standard of our work against a decision that matters to you, before any mandate is agreed.
Provided under a mutual non-disclosure agreement; one per organisation. All rights reserved, including copyright, our discretion over engagements and the preservation of our independence.
Requests to action@circulairity.co.
Sources
ICAO CORSIA Default Life Cycle Emissions 2025 · Boeing Australia SAF Rules of Thumb 2026 · Google, American Airlines and Breakthrough Energy 2023 · Lee et al. 2021 · Teoh et al. 2024 · Teoh et al. 2022 · Kärcher 2018 · Burkhardt et al. 2018 · Bier and Burkhardt 2022 · Voigt et al. 2021 · Märkl et al. 2024 · Voigt et al. 2026 · Gierens et al. 2020 · Hofer, Gierens and Rohs 2024 · EUROCONTROL MUAC 2024 · Sanekommu et al. 2024 · Allen et al. 2018 · Cain et al. 2019 · Lynch et al. 2020 · Directive (EU) 2023/958 · Rogelj and Schleussner 2019 · IPCC 2021, AR6 WG1
We help you with mitigation and assessment of tCO2e and tCO2-warming-equivalent (tCO2-WE)