Analysis: Government admits to a “small error” in the aviation modelling in the Carbon Budget and Growth Delivery Plan

3rd March, 2026

The Carbon Budget and Growth Delivery Plan (CBGDP) – the Government’s plan for delivering economy-wide emissions reductions during the Sixth Carbon Budget (2033-37) – is an important pillar of climate policy. It is relevant to aviation as the Plan’s sectoral projections indicate the progress that the industry is expected to make during this period.

As such, local campaign group, CAGNE, was keen to scrutinise the aviation assumptions and forecasts in the CBGDP as part of its Judicial Review hearing challenging the Government’s decision to grant permission for Gatwick expansion. AEF acted as an expert witness for CAGNE, providing a commentary on how the CBGDP differed from the Jet Zero Strategy (published in 2022), to which the DfT responded. 

Since the court hearing concluded on 23rd of January, the Department of Transport (DfT has identified an error in its own forecasts. The error, which amounts to 4MtCO₂ of over-estimated aviation emissions savings (see table below) over the course of the Sixth Carbon Budget (CB6), has been confirmed by the Secretary of State (SoS). But while the Government downplays the significance of the mistake, arguing that it only represents 0.5% of the required additional carbon savings across the economy in CB6, the increase in aviation emissions is more marked, representing 2.5% over the period.  

Year	Disclosed figures (MtCO2e)	Corrected figures (MtCO2e)	Difference (MtCO2e)
2033	32.9	33.3	0.4
2034	32.3	32.6	0.4
2035	31.6	32.3	0.6
2036	31.1	32.1	1.0
2037	29.9	31.6	1.7
Average	31.6	32.4	0.8

Source: Secretary of State’s statement to Parliament

How was this mistake made?

The error in calculating efficiency improvements occurred, according to the Secretary of State’s statement, because the incorrect fuel efficiency figures were allocated to new generation planes flying on routes beyond their standard operating range. This generated over-inflated emissions savings that are more pronounced over time as these aircraft became a growing proportion of the fleet: although the error is relatively small in 2033, at around 400,000tCO₂, it becomes more statistically significant in the year 2037, when it represented 1.7MtCO2 of claimed emissions savings.

Are we too reliant on modelling?

This error shows the sensitivity of the modelling, and raises the question: how reliant should we be on forecasts when so many of the carbon mitigation levers assumed by the Government – SAF, Greenhouse Gas Removals, carbon pricing and zero-emission technologies – are so dependant on assumptions and come with significant delivery risks?

Later this year, the Government must prepare a delivery plan for the Seventh Carbon Budget. If the error hadn’t been spotted these claimed reductions would have resulted in even bigger annual discrepancies from 2038 onwards. And with limited means of putting the sector back on track in a short space of time, it would raise questions about the government’s ability to guarantee that aviation growth will not put the UK’s legally binding climate targets at risk. 

What is the significance of the Government acknowledging this error?

Two issues emerge from the Government’s acknowledgement of the error. Firstly, this “small error” means that the Government can no longer state that the CBGDP is over-delivering compared to the trajectory set out in the Jet Zero Strategy, rather, it is just tracking its 2022 projections. This is a significant shift in language.

The second issue is the importance of disclosing the full data so it can be scrutinised, ideally by sharing assumptions at the input stage. The aviation information made available for the CBGDP is high level, often aggregated across measures, and in some cases, is left to the user to add up the totals. This is not a solid foundation for discussion. 2026 will be an important year for modelling, with forecasts expected in support of the ANPS review and the CB7 delivery plan. NGOs and communities want to engage with the review and a planned summer consultation, but they need transparency to make informed input.

Photo credit: Egor Komarov, Unsplash