The UK's independent Climate Change Committee has told the government that a third runway at Heathrow can only be considered compatible with the UK's legally binding Net Zero target if ministers first legislate policies requiring the aviation sector to abate 100% of its emissions by 2050, either through direct reductions or by paying for permanent engineered carbon removals. Short of that, the Committee's analysis is unambiguous: expanding Heathrow makes the UK's Seventh Carbon Budget and its 2050 Net Zero target harder to meet, not easier, and no amount of efficiency gains or demand management on their own can close the gap.
What Expansion Actually Costs, in Numbers
Heathrow is already the UK's largest airport, accounting for nearly a quarter of UK flights and around half of UK aviation emissions, largely because of its outsized share of long-haul international routes. The CCC's central estimate is that full Heathrow expansion, completing in 2054, would add 4.5 MtCO2e to UK emissions in that year, a figure the Committee notes is larger than the projected 2050 residual CO2 emissions of any other single sector of the UK economy. Measured against the UK's 2050 whole-economy residual CO2 emissions specifically, that represents a 6.9% increase.
The scale of that number matters because of what it's being measured against. By 2050, aviation is expected to make up the overwhelming majority of the UK's remaining, unavoidable CO2 emissions, since most other sectors, electricity, industry, agriculture, will have decarbonised far more completely by that point. The Committee's blunt framing: aviation cannot rely on emissions cuts happening elsewhere in the economy to cover for its own growth. It will have to reach Net Zero on its own terms, largely through a combination of SAF and engineered carbon removals.
Under current government policy, and even without Heathrow expansion at all, the Committee's modelling shows UK aviation emissions barely moving between now and 2050, projected to rise slightly from 37.2 MtCO2e in 2025 to 38.8 MtCO2e in 2050. Add full Heathrow expansion, and that figure rises to 43.9 MtCO2e by 2054, an 18% increase on today's already-elevated baseline. The Committee's own prior assessment, cited in this report, found that only 32% of the aviation emissions reductions required by the Sixth Carbon Budget period are currently backed by credible delivery plans, and just 1% of the engineered removals reductions needed over the same period.
Why the SAF Mandate Alone Won't Get the UK There
The UK's SAF Mandate is one of the two central pillars of current aviation decarbonisation policy, requiring jet fuel suppliers to progressively increase the proportion of SAF blended into aviation fuel, from 2% in 2025 rising to 17% by 2040, after which the mandate's requirement holds flat rather than continuing to climb. Actual uptake in 2025 stood at 2.3% of UK aviation fuel, essentially in line with the mandate's minimum requirement rather than exceeding it, though Heathrow itself has pushed further, with SAF making up 3.1% of fuel uplifted from the airport in 2025 and a stated ambition to reach 11% by 2030.
The Committee is explicit about the mandate's structural ceiling: because the requirement is only set through 2040 and then held flat, current policy assumes SAF uptake simply stops climbing after that point, precisely the period when the Committee's modelling shows aviation emissions beginning to rise again as demand growth outpaces the frozen SAF share. The mandate, as currently designed, was never built to carry the sector all the way to Net Zero on its own.
The report is also candid about SAF's technical limits, a detail worth flagging clearly since it runs somewhat against the more optimistic framing SAF often receives in industry communications: it is unlikely that bio-based SAF will reduce the lifecycle emissions impact of aviation fuel by 100% compared to fossil kerosene, given that biofuel carbon intensity varies widely depending on feedstock, indirect land-use change, and production technology. All UK SAF currently in use comes from biofuels made from waste fats and oils; while biomass could be used as a SAF feedstock in future, the Committee notes that availability will be constrained by land and competing uses, meaning that long-term, scalable SAF supply will likely have to come from synthetic fuels instead, produced by combining low-carbon hydrogen with captured carbon. Even synthetic fuels made using captured fossil carbon from industrial waste processes, rather than atmospheric carbon, are less carbon-intensive than fossil kerosene but are not carbon neutral on a full lifecycle basis.
The "Polluter Pays" Mechanism the Committee Wants
The Committee's preferred route to Net Zero aviation rests on a single organising principle: the cost of decarbonising aviation should be reflected directly in the cost of flying, with the industry itself funding both SAF and engineered removals rather than relying on public funding. Under the Committee's modelled Net Zero Aviation Pathway, this approach would let net aviation emissions fall to zero by 2050, with the abatement split across four sources: engineered removals contributing 36%, reduced demand growth 24%, efficiency improvements 20%, and SAF the remaining 20%.
The mechanism works, in the Committee's modelling, precisely because it does two things simultaneously: it makes flying somewhat more expensive, which naturally moderates demand growth, while the resulting revenue funds the SAF and removals capacity needed to abate what demand remains. The Committee estimates ticket prices could rise by around £150 for a return trip to Alicante and £400 for a return trip to New York by 2050 under this pathway, equivalent to roughly 6-9 pence per mile, comparable to current fuel duty on petrol and diesel.
Crucially, under this pathway, the Committee's modelling shows the net emissions impact of Heathrow expansion falling to zero by 2050, since the additional emissions generated by expanded capacity would be fully offset by the additional SAF and removals that the same polluter-pays revenue stream would fund. The policy mechanism, not the runway itself, is what determines whether expansion is compatible with Net Zero.
What the Public Actually Wants: The Citizens' Panel Findings
The Committee's advice draws on a citizens' panel convened as part of its earlier Seventh Carbon Budget work, and the panel's views add a layer of democratic legitimacy testing to the policy recommendations. Panel members mostly viewed flying as a choice rather than a necessity, and expressed a general preference for moderating the flying of frequent flyers, business travel and private jets specifically, rather than restricting occasional leisure travel. Most panel members found ticket price increases resulting from policy acceptable, with a strong emphasis on protecting the ability of families, including lower-income families, to take one annual holiday flight.
Panel members most strongly supported a frequent flyer levy or an emissions- and distance-based tax, and specifically preferred exemptions for a passenger's first flight, for children, and for people living on remote islands. They were notably less enthusiastic about a straightforward requirement for airlines to offset emissions through engineered removals, expressing discomfort about the efficacy, safety and monitoring of removals technology, and low trust in airlines to actually carry it out; where panel members did support removals, they wanted airlines to fund it directly and minimise costs passed to consumers. There was strong disapproval, specifically, of the idea of government- or taxpayer-funded removals, since panel members felt non-flyers shouldn't bear costs for a choice they hadn't made. Panel members were also broadly supportive of limiting airport expansion and capacity as a policy lever, viewing it as unfair for individual flight behaviour to be targeted while the industry itself continued expanding unchecked.
The Non-CO2 Problem Heathrow's Routes Make Worse
Beyond CO2, the report devotes specific attention to aviation's non-CO2 climate effects, primarily contrail formation, which are currently outside the scope of both international climate treaties and the UK's own Climate Change Act, despite contributing an estimated two-thirds of aviation's total warming effect globally. The Committee's analysis is specific about why Heathrow's route network makes this a particularly relevant issue for this airport: UK flights across the North Atlantic and to and from East Asia have been shown to produce a significantly higher share of persistent, warming contrails relative to their flight distance than other UK flights, and Heathrow accounted for 85% of UK passengers travelling to Canada and the United States and 81% of those travelling to East Asia in 2025. Because Heathrow's traffic mix is disproportionately weighted toward exactly the long-haul routes that generate the most contrail warming, the airport's non-CO2 climate footprint is likely higher, relative to its size, than the UK aviation average.
What the Committee Is Formally Asking the Government to Change
The report's most concrete, actionable recommendation concerns the government's own "climate test" for approving Heathrow expansion, currently defined narrowly as whether the increase in carbon emissions from any proposed scheme would have "a material impact on the UK's legally binding carbon budgets." The Committee argues this test needs to be strengthened in four specific ways: explicitly incorporating the 2050 Net Zero target rather than only carbon budget periods; assessing the emissions impact of the full scope of expansion currently planned for 2054, not just the initial opening phase; updating outdated evidence on surface access, construction and airport operations emissions, some of which the Committee found still relies on 2014 and 2018-era data that predates developments such as the UK's Zero Emission Vehicle mandate; and giving the Secretary of State for Energy Security and Net Zero, who holds responsibility for engineered removals policy specifically, a defined formal role in judging whether expansion is material to the UK's climate targets, alongside the Transport Secretary who currently owns that decision alone.
The Uncertainty the Committee Doesn't Paper Over
To its credit, the report doesn't present its Net Zero Aviation Pathway as a fixed, guaranteed outcome. A series of sensitivity scenarios modelling different technology cost assumptions shows that while net aviation emissions land at the same zero-by-2050 destination across every scenario tested, the actual mix of SAF versus engineered removals used to get there varies substantially, from roughly a quarter to as much as half of total abatement coming from engineered removals depending on relative costs. In a scenario assuming SAF and removals costs based on imported and internationally-sourced supply rather than domestic UK production, demand ends up higher and technology costs lower, illustrating that the UK's own domestic industrial choices, building SAF and removals capacity at home versus relying on imports, carry real consequences for both cost and strategic exposure that the modelling can bound but not resolve in advance.
The Committee is equally direct that SAF and engineered removals remain nascent technologies not yet available at the scale required, and that policy needs to build in contingency for delivery delays, since the long lead times involved mean the UK cannot simply wait and see whether the technology arrives on schedule before deciding whether Heathrow expansion is compatible with its climate commitments.
Source: Climate Change Committee (CCC), 16 September 2026.
