Why Gatwick’s approval does not settle Heathrow’s case
Gatwick is a smaller expansion assessed against an earlier climate budget. Heathrow’s full impact is scheduled to arrive after the 2050 net-zero deadline. Their emissions estimates also use different assumptions.
Gatwick has permission to bring its Northern Runway into routine use. Heathrow wants a third runway, and the Climate Change Committee says compatibility with the UK’s climate targets requires stronger legislation. It is tempting to read this as an inconsistency: if one London airport can expand, why should the other be different?
The comparison turns on scale, timing and the assumptions behind each assessment. The UK’s carbon budgets limit emissions across the economy over successive five-year periods. Permission for one airport does not reserve the same emissions allowance for another, or establish that any amount of expansion will fit.
This comparison accompanies The world is building airports. Britain is arguing about one., which examines the international picture and the policies behind aviation decarbonisation.
Gatwick’s additional flights also continue beyond 2050. Earlier expansion does not escape later climate constraints; it faces them on a different timetable.
Britain subjects expansion to carbon budgets without ruling it out. Gatwick’s Northern Runway project received development consent in September 2025. The same national carbon-budget framework that makes Heathrow contentious can accommodate airport expansion. The question is what a particular project must demonstrate, not whether Britain builds airports.
That approval was not a quick exemption from the rules. A provisional decision in February 2025 led to further consideration of conditions, including noise and public transport, before final consent and subsequent litigation.
“Another runway” hides the difference
Heathrow nevertheless represents a much larger expansion, with additional emissions that build later, closer to and beyond the UK’s net-zero deadline. Calling both projects “another runway” hides the physical difference. Gatwick is repositioning its existing standby Northern Runway to bring it into routine use alongside the main runway, for smaller departing aircraft only. Heathrow proposes a full new runway up to 3,500 metres long.
Potential additional flights per year
Timing changes the carbon test
Timing makes that larger increment harder to accommodate. Heathrow’s additional emissions ramp up around and after 2050, when the UK must balance its much smaller remaining emissions with removals. Gatwick has not simply used up a pot of carbon-budget space that otherwise belonged to Heathrow: the quoted assessments concern different periods, and neither airport owns a reserved share of the national budget.
Different assumptions, as well as different dates: Gatwick’s assessment uses Jet Zero’s high-ambition scenario. Heathrow’s figures use the current-policy assumptions in DfT’s Current Trends scenario. They are not directly comparable forecasts.
- Gatwick · 2033–37 average
- About 1.12 million tonnes CO₂e per year in additional aviation emissions.
- Heathrow · additional aviation emissions
- 2038–42 average: 0.4 million tonnes CO₂e per year.
- 2050: 2.4 million tonnes CO₂e.
- 2054, full expansion: 4.5 million tonnes CO₂e.
These labelled figures show the assessed periods, not a continuous forecast between them. Heathrow’s full impact arrives after the net-zero deadline; Gatwick’s earlier assessment does not mean its additional flights disappear afterwards.
The percentages measure different things
Gatwick’s percentage compares its added emissions with the UK’s entire five-year emissions allowance in the 2030s. Heathrow’s compares its eventual annual impact with the much smaller amount the UK is projected still to emit in 2050, before carbon removals. The percentages therefore cannot be used to compare the airports’ relative impact. Nor does a small budget percentage establish that there is spare capacity to absorb it.
Methodology: figures, assumptions and sources
Gatwick’s annual aviation figure: the decision letter reports 5.577 million tonnes CO₂e of additional aviation emissions across the five-year Sixth Carbon Budget, 2033–37. Dividing by five gives 1.1154 million tonnes per year, rounded here to 1.12. The assessment uses Jet Zero’s high-ambition assumptions; the Secretary of State accepted this approach while acknowledging technological uncertainty. See Gatwick decision, paragraphs 154–157.
Gatwick’s 0.66%: paragraph 154 reports 6.344 million tonnes CO₂e of total additional project emissions during 2033–37, equivalent to 0.657% of the entire Sixth Carbon Budget, rounded here to 0.66%. This broader total includes construction, airport buildings and ground operations, surface access and aviation. It is not the aviation-only annual figure above.
Heathrow’s annual figures: the CCC’s September 2026 advice, section 2.3 and Table 2.1, gives 0.4 million tonnes CO₂e as the annual average for 2038–42, 2.4 million in 2050 and 4.5 million for full expansion, scheduled for 2054. These use Current Trends aviation assumptions and exclude surface access, airport operations and construction. They are additional emissions relative to the model without Heathrow expansion, not the airport’s total footprint.
Heathrow’s 6.9%: this is the CCC’s reported comparison of the full annual expansion impact with whole-economy residual CO₂ emissions in 2050, before removals. Table 2.1 explicitly treats full expansion as if completed by 2050, or equivalently holds the economy-wide emissions reference unchanged between 2050 and 2054. It combines Current Trends for aviation with the CCC’s Balanced Pathway for other sectors. It is neither a share of a five-year budget nor a percentage of net-zero emissions. The figure is retained as reported rather than reconstructed from rounded table values.
Limits: the policy scenarios, years, emissions boundaries and denominators differ. CO₂e is greenhouse-gas accounting, not a measure of aviation’s full warming impact including contrails. No missing years have been interpolated, and no emissions ratio between the airports is implied by the capacity chart.
What the approval actually establishes
The carbon reasoning matters more than the headline approval. The government accepted that Gatwick would add emissions, but concluded that these could be managed within its aviation-wide Jet Zero strategy and its plans for meeting national targets. It relied on projected decarbonisation, acknowledging uncertainty about the technologies involved. The budgets constrain the economy as a whole; they do not automatically prohibit every project that increases emissions. (Court of Appeal’s account of the decision)
The legal distinction was between a significant emissions increase and one that would materially undermine meeting the UK’s targets. Those are not the same finding. A decision surviving legal challenge does not demonstrate that the forecast reductions will actually arrive. (Analysis of the High Court judgment)
That does not give an earlier project a free pass: Gatwick’s additional flights will continue into later, tighter budget periods too. Expansion depends on how much capacity is added, when its emissions arrive, and what reductions the wider economy can deliver.
That continuing reliance on decarbonisation is easy to misunderstand. It does not mean long-haul aircraft stop emitting CO₂. Jet Zero’s high-ambition scenario assumed improvements in fuel efficiency, increased use of sustainable aviation fuel and some zero-emission aircraft, mainly on shorter routes, while leaving residual emissions to be addressed outside aviation. SAF still produces exhaust CO₂; its benefit depends on the fuel’s carbon origin and production. (DfT analytical annex, pp. 9–10; DfT SAF guidance)
The CCC’s September 2026 advice recommends stronger policy, including permanent engineered removals for residual emissions. That is a different pathway from the older Jet Zero assumptions used for Gatwick. The companion article’s chart and accounting notes explain the distinction; buying an emissions allowance or a generic offset is not the same as permanent removal.
Gatwick’s approval tells us that a binding carbon budget can accommodate expansion. It does not settle how much larger, later expansion can fit, or prove that the forecast reductions will arrive. The question for Heathrow is what government requires before granting the additional capacity, and how much confidence in future decarbonisation is enough to approve it now.
The comparison was originally part of the airport-climate article published on 16 September 2026. Drafting and research were carried out with the help of ChatGPT Astra.