Approved airport expansions already push UK aviation past its 2038-42 carbon budget share

The figure is a ratio, so start with the denominator. What the modelling compares is projected aviation emissions against aviation's allocated *share* of the Seventh Carbon Budget covering 2038 to 2042 ^[3]. How that share gets apportioned between aviation and everything else is a judgement, and the published summary does not state the allocation rule. A more generous split for flying would shrink the percentage without a single litre of jet fuel going unburned. That caveat cuts both ways, and it is the first thing I would want to see in the full report.

The range itself is more informative than the midpoint. Everything between 21 and 50 percent is driven by one variable: how much sustainable aviation fuel actually materialises ^[3]. Twenty-nine percentage points of overshoot therefore rest on a fuel supply chain ^[1]. The report's headline number of roughly 40 percent ^[4] sits above the arithmetic midpoint of 35.5 percent ^[3], which places the central case nearer the pessimistic fuel assumption than the optimistic one. What does not change anywhere across that span is the sign. On existing evidence, even optimistic SAF deployment leaves the sector above its share ^[6], and current production routes run into structural limits or compete with agriculture for land and water ^[7].

The removals side is where the two hedges stop being independent. No engineered greenhouse gas removal plants are yet operating in the UK, and global deployment is significantly off track ^[8]. Aviation is nonetheless the largest projected user of that future capacity, before any expansion is counted ^[9]. The scenarios also assume SAF pulls its weight, so a shortfall in fuel increases the call on removals ^[10]. Two backstops that fail in the same direction are one backstop.

Then there is what a carbon budget does not measure. Non-CO2 effects, including nitrogen oxides and contrails, account for around two-thirds of aviation's warming and sit outside the budgets entirely ^[12]. If two-thirds is outside, the accounted CO2 is about a third, and aviation's full warming effect is roughly three times what the budget books ^[2]. A plan can comply on paper and still be counting a minority of its own heat.

On provenance: Friends of the Earth commissioned this work ^[2], which is a reason to read the assumptions closely rather than a reason to discount them. The load-bearing check is that the government's own aviation scenarios overshoot the legislated budget too, before a third runway is considered ^[5]. What the modelling cannot tell you is how demand responds to a carbon price, what SAF costs at scale, or whether a removal technology surprises on the upside. The authors' recommendation to reject expansion ^[13] rests on timing evidence, and Pennington's own framing is narrower than the headline: the technologies can contribute, but there is no evidence they can support growth on this scale ^[14].