Third Heathrow runway would break UK carbon budgets, scientists warn

A third runway at Heathrow Airport would make the United Kingdom’s legally binding climate targets unachievable, according to new modelling published by the Tyndall Centre for Climate Change Research at the University of Manchester. The report, commissioned by the environmental justice organisation Friends of the Earth, delivers the first detailed assessment of whether expanding Heathrow is compatible with the carbon budgets that Parliament has written into law under the Climate Change Act. Its conclusion is stark: there is no credible scenario in which the expansion can proceed without the aviation sector blowing a hole through the country’s statutory emissions limits. The findings land at a politically sensitive moment, arriving just as the public consultation on the government’s Heathrow Expansion National Policy Statement closed on 1 September, the same day that members of parliament returned from recess to scrutinise the proposal.

The central problem is arithmetic. UK aviation is already projected to exceed its share of the Seventh Carbon Budget, which covers the period from 2038 to 2042, by between 21 and 50 percent, depending on how much sustainable aviation fuel ultimately materialises. That overshoot exists before a single additional flight from a third Heathrow runway is counted. The report finds that even the government’s own modelling reaches a broadly similar conclusion, with the sector exceeding its allocated share of the budget by around 40 percent in the scenarios the government itself has published. In other words, the gap between what the law requires and what the aviation sector is expected to emit is not a marginal modelling artefact but a structural feature of every official projection currently on the table.

Dr Lois Pennington, a Research Fellow at the Tyndall Centre and one of the report’s lead authors alongside Professor Alice Larkin and Dr Chris Jones, said that every aviation scenario the UK Government has published overshoots the carbon budget Parliament has legislated, before a third runway at Heathrow is even taken into account. She argued that aviation cannot continue to grow if the sector is to make a proportionate contribution to meeting the UK’s carbon budgets, and that while sustainable aviation fuels and carbon removals can contribute to limiting aviation emissions, there is currently no evidence that these technologies can support the scale of expansion envisaged by the government. Further expansion, she warned, carries a significant risk of exacerbating climate impacts by relying on the growth of technologies that are unlikely to materialise in the timescale needed.

The report’s second key finding concerns sustainable aviation fuels, the technology most frequently cited by expansion advocates as the route to guilt-free flying. SAF encompasses a range of fuels intended to substitute for fossil kerosene, including biofuels produced from waste oils, agricultural residues and dedicated energy crops, as well as synthetic e-fuels manufactured by combining green hydrogen with captured carbon dioxide. The analysis concludes that, based on existing evidence, SAF would be unable to close the emissions gap: even optimistic deployment of the technology would leave aviation above its carbon budget share. Previous studies have shown that current SAF production methods face major structural constraints, or compete with agriculture for land and water, a particularly acute concern at a moment when farmers have experienced what could be the worst harvest on record and are already warning about the impact of the climate crisis on food production.

The third pillar of the expansion case rests on greenhouse gas removals, a family of techniques that extract carbon dioxide from the atmosphere and store it long-term, such as bioenergy with carbon capture and storage. The report highlights how global GGR deployment is significantly off track and that no engineered removal plants are operating yet in the UK. Despite this, aviation emissions modelling heavily relies upon future potential GGR to offset additional emissions. Even before any airport expansion is taken into account, aviation is the biggest projected user of GGR capacity in the government’s scenarios. The dependency compounds: all of these scenarios also assume that SAF pulls its weight, so if sustainable fuels fail to deliver, even more removal capacity will have to be deployed to compensate. The report concludes that building capacity for more aviation demand before large-scale GGR facilities are operational is inconsistent with taking climate targets seriously.

There is a further layer of accounting that the researchers argue must change. Aviation’s full warming effects are not captured within carbon budgets, which focus on carbon dioxide and a basket of other greenhouse gases. Non-CO2 impacts, including warming from nitrogen oxides emitted at altitude and from contrails, the ice clouds that aircraft leave in their wake, account for around two-thirds of aviation’s total climate warming. Because these effects fall outside the current accounting framework, the effective climate footprint of any expansion is substantially larger than the carbon budget figures alone suggest. The report recommends that these non-CO2e impacts should be incorporated into future climate targets, a change that would make the task of reconciling aviation growth with national emissions law considerably harder still.

The timing of the analysis gives it particular political weight. The UK has just experienced another exceptionally hot summer, while record-breaking severe wildfires have burnt across Western Europe. Pennington argued that the government must reassess the case for expansion and that climate policy must be grounded in what can be demonstrated and evidenced, rather than in what is hoped may become possible. The report’s authors recommend that decision-makers reject any proposed expansion at Heathrow Airport, positioning the findings as a direct challenge to the assumption, embedded in the government’s National Policy Statement, that passenger growth and climate law can be reconciled through future technological fixes.

Friends of the Earth framed the findings in even sharper terms. Mike Childs, the organisation’s head of policy, science and research, said that after a summer of record-breaking heat and nearly 3,000 excess deaths, a third runway is the equivalent of pouring petrol on a raging wildfire. He argued that the evidence is clear and that there is no route where expansion at Heathrow can be in line with the country’s legally binding climate targets, describing claims that sustainable aviation fuel and greenhouse gas removals can enable airport expansion as an attempt to pull the wool over people’s eyes. Childs presented the government with a binary choice: commit to taking action on the climate seriously, reject Heathrow expansion and constrain excessive flying, or abandon the UK’s climate targets and future generations by giving the third runway the go ahead.

The economic dimension of the debate, according to Childs, is weaker than expansion advocates suggest. He described the economic case for expansion as weak when set against the huge impact on the climate and communities, and called the decision an opportunity for the new Prime Minister to set a precedent by resisting corporate lobbying and saying no to a third runway at Heathrow. The framing reflects a broader tension in UK climate policy: the Climate Change Act obliges successive governments to meet carbon budgets set on the advice of the Climate Change Committee, yet individual infrastructure decisions are frequently assessed against narrower economic appraisals that do not fully internalise the cumulative emissions consequences across the transport system.

The report’s wider significance extends beyond a single airport. Recent approvals of airport expansion at Gatwick, Stansted, Luton and many other locations mean that UK aviation is already on a growth trajectory that exceeds its share of the Seventh Carbon Budget, so the Heathrow decision will determine whether that overshoot deepens further. Friends of the Earth and the Tyndall Centre are calling on the UK Government to reject Heathrow expansion and to ensure that future aviation policy remains consistent with the legally binding carbon budgets under the Climate Change Act. For climate scientists, the report is a case study in the risks of policy built on technologies that exist at pilot scale being asked to deliver at national scale within fifteen years. For the government, it presents an uncomfortable choice between the country’s climate legislation and one of its most prominent infrastructure commitments, with the evidence now suggesting that both cannot be delivered at once.

Subject of Research: Compatibility of Heathrow Airport expansion with the UK's legally binding carbon budgets

Article Title: Heathrow expansion incompatible with UK legal climate targets, major report finds

Article References: Heathrow expansion incompatible with UK legal climate targets, major report finds. (n.d.). Original publication

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Keywords: Heathrow, aviation emissions, carbon budgets, Climate Change Act, sustainable aviation fuel, greenhouse gas removals, Tyndall Centre, Friends of the Earth, airport expansion, climate policy, Seventh Carbon Budget, non-CO2 warming