numbers, not adjectives — D. J. C. MacKay

The UK government's Jet Zero Strategy is not Net Zero

by Carl Edward Rasmussen, 2023-01-13

In July 2022 the UK Department for Transport published its Jet Zero Strategy for delivering net zero aviation by 2050. The document declares (on page 11) that, quote "Our target is clear: Net Zero aviation by 2050". Here, I take a look at the numbers.

In 2019 the UK aviation sector emitted 38.2 MtCO2e, or million tons of CO2 equivalents. According to the government strategy, in 2050 this will be reduced to Net Zero. Net Zero carbon means that there is a balance between CO2 emissions and removals over similar time scales. The strategy is based on a 70% increase in passenger numbers, and the notional business as usual CO2 estimate of 52.3 MtCO2e, broken down five ways (page 16) listed here together with their anticipated CO2 savings:

Fuel efficiency improvements: 8 MtCO2e. This figure represents the savings in CO2 by assuming continuous 2% pa fuel efficiency improvements, which is probably a little higher than has been historically achieved, but let's hope it can be done!

Zero carbon aircraft: 2 MtCO2e. This is technology which hasn't yet been demonstrated commercially, let's hope the engineers are up to it!

Sustainable aviation fuels: 9 MtCO2e. These fuels, also called SAF, are extracted from biological feedstocks, and have the advantage that they can be mixed in with existing aviation fuel, without any major modifications to the aircraft. According to the report (page 34), 5 million tons of SAF will be required annually, but it's unspecified where it might come from. Perhaps from waste biomass, but this source is already being used for other purposes, and 5 million tons is a lot. It would take about 1/3 of the total area available for crops in the UK to grow it, requiring a massive shift in the way UK is farmed!

Estimate the proportion of UK agricultural land necessary to grow 5 million tons of SAF annually. There are many practical hurdles which need to be overcome to produce large amounts of SAF. Here, I compute a lower bound on the area by requiring enough energy and ignoring any inefficiencies or process losses. Kerosene contains 42.8×109 J/ton (joules per ton) of energy, so the energy requirement is 5×106 tons × 42.8×109 J/ton = 214×1015 J. The most efficient energy crop in the UK is probably Miscanthus (commonly known as Elephant Grass), which grows with a power density of about 0.5 J/s/m2 (joules per second per square meter). A year has 60×60×24×365 = 31.536×106 s (seconds), so the energy density per year is 0.5 J/s/m2 × 31.536×106 s = 15.768×106 J/m2 (joules per square meter). Therefore the required area is 214×1015 J / 15.768×106 J/m2 = 13.6×109 m2 = 1.36 million hectares (a hectare is 104 m2). The total area of arable crops in the UK is 3.7 million hectares (2021 figure). ∴ the area of land needed is about 1/3 of the total available.

ETS and CORSIA: 14 MtCO2e. ETS is the UK government's trading scheme for carbon emission permits, and CORSIA is the carbon offsetting scheme of ICAO, the International Civil Aviation Organisation. It's not clear that ETS and CORSIA will even operate in 2050. Since the UK will be Net Zero for carbon in 2050, presumably that means that the UK won't have any CO2 emission permits to sell (that wouldn't be Net Zero). Even if the aviation sector could buy such permits, the CO2 would still be emitted. Can CORSIA provide real means of off-setting in 2050, when most countries in the world are approaching Net Zero? It seems likely that these 14 MtCO2e will actually end up in the atmosphere, rather than being abated.

Abatement outside the aviation sector: 19.3 MtCO2e to be offset or removed (page 15). Hang on a minute here. The Net Zero strategy for the aviation sector includes an unspecified 3rd party, to remove 19.3 million tons of CO2 annually from the atmosphere on its behalf, to be Net Zero? This is not actually a plan that adds up. Calling it Net Zero is intentionally deceptive. The government is investing in CCUS Carbon Capture, Utilisation and Storage which could perhaps help out? We would need some other sector to remove 19.3 MtCO2e annually and not count it towards their own sector (that would lead to double counting). Since all sectors are required to reach Net Zero, finding a willing sector may be difficult, and if we can't, then we don't have a plan.

So, the plan for 2050 does not actually achieve aviation without emitting CO2 in to the atmosphere, but will emit 19.3 plus 14 MtCO2e from the two paragraphs above, or 33.3 MtCO2e. This is a reduction compared to 38.2 MtCO2e in 2019, corresponding to an annual reduction just shy of 0.5% pa. Which is a very modest reduction indeed. The strategy does not propose a concrete balance between CO2 emissions and removals, and therefore it is not Net Zero.

To be absolutely sure there is no room for misunderstanding here: the government strategy is not a realistic or practical plan leading to Net Zero CO2 emissions from aviation. The strategy is not reconcilable with the Paris agreement. The strategy is intentionally misleading. It conceals the truth about what the real consequences of the strategy would be. In my view, the report should be withdrawn by the Department for Transport and the responsible minister, the then Transport Secretary Right Honourable Grant Shapps.

Constructive alternatives

It's all very well to be critical towards existing plans, but what would an implementable strategy for Net Zero aviation look like? The reality is that we don't currently have technology to fly at scale which doesn't result in CO2 emissions which cannot with current technology be removed. Small improvements in technology and small scale adoption of SAF may help a little bit, but as outlined above this will be nowhere near enough to support a 70% rise in passenger numbers by 2050 as assumed by the Jet Zero strategy.

Therefore, any realistic plan for aviation must acknowledge that a very substantial reduction in passenger numbers is necessary. It will be inconvenient for our society to implement. But it will surely be better than the chaos or environmental destruction inevitably associated with planning strategies which are not realisable in the real world.