The myth of net-zero aviation

In 1896, Swedish scientist Svante Arrhenius predicted that changes in atmospheric carbon dioxide levels will create a greenhouse effect that increases average global temperatures. In 1967, a computer model confirmed this conclusion which, over the next two decades, became established scientific truth.

The increasing acceptance of the threat of climate change led to the Kyoto Protocol of 1997 which was the first international agreement committing states to reducing greenhouse gas emissions. It was signed by 192 countries.

Yet this threat was not universally accepted. As late as 2017, climate sceptic Nigel Lawson claimed in a BBC radio interview that average temperatures were falling. In response to scientists responding furiously to this claim, the BBC defended its decision to interview Lawson on the basis that “it had a duty to inform listeners about all sides of a debate.”

It is difficult to imagine that the BBC would take that stance today, given that the latest YouGov poll showed that 71% believe the climate is changing due to human activity. As a result, it is now a commercial imperative for businesses and Government to show they care for the environment and explain how they propose to reduce emissions.

Climate Change Act

Under the 2008 Climate Change Act, Government has a legal duty to take action to reduce greenhouse gas emissions (GHG) which are generally referred to as carbon emissions, though this is not wholly accurate. The Act originally mandated an 80% reduction in the 1990 baseline emissions by 2025. In 2019, this was amended to 100% i.e., net-zero carbon emissions by 2050.

The UK is seemingly well on its way to net-zero as, since 1990, total GHG emissions have been reduced by 49%. However domestic transport emissions excluding aviation have only deceased by 14% and aviation emissions are more than twice those of 1990 (214%). These figures do not take account of non-CO2 effects from aviation emissions that reflect sunlight which the Climate Change Committee (CCC) estimates may double the warming effect from CO2 emissions.

The Act required the formation of the CCC, an expert body to advise Government, report on progress made in reducing emissions, and make recommendations for five-yearly carbon budgets. In 2019 the CCC produced its report ‘Net Zero – The UK’s contribution to stopping global warming’. This showed how it was technically possible for the UK to achieve net-zero emissions at a cost of 1-2% of GDP. It concluded that this could offer significant industrial opportunities and other benefits that could fully offset costs. However, it was recognised that some industries could suffer if policies were not in place for an effective transition.

This report considered that aviation emissions could be reduced by a combination of technology, airspace and operations management, alternative fuels, and managing demand. With all these measures, the CCC report considered that, at best, this would only reduce aviation emissions to 83% of their 2017 levels.

Batteries and hydrogen

A mode of transport that lifts passengers and goods over 10km into the air and propels them at 900 km/hr uses an enormous amount of energy. Hence a typical trans-Atlantic flight will use around 50 tonnes of jet fuel which is around 2,000 Megajoules (MJ) of energy. Trans-Atlantic aviation is only possible because of the jet fuel’s high energy density of 43 MJ/kg.

As with rail traction, the aviation industry is considering batteries and hydrogen as alternatives to jet fuel. Yet with a lithium battery having a typical specific energy of 0.7 MJ/kg, a transatlantic plane would require a battery weighing around 3,000 tonnes. Even with the most heroic assumptions about the development of battery technology, it is difficult to imagine that battery-powered transatlantic aviation will ever be feasible.

Yet a battery powered plane was certified for use in 2020. This is the Pipistrel Velis Electro which is a two-seater aircraft intended for pilot training. It offers 50 minutes of flight time with 20 minutes reserve and has a range of 185km. It is powered by a 58kW electric motor from a 70kg lithium battery.

Although it is claimed that this technology could eventually be scaled up to power domestic flights, the scaling factor must be considered.

In this respect the exponential scaling factor must be considered as objects are scaled up. For example, an Airbus A319 is five times the length of this battery plane but 95 times its weight. Thus, it is difficult to image that such flights could ever be battery powered.

Hydrogen, however, is a technically feasible option as Airbus has concluded from its ZEROe project. This has developed three concepts for hydrogen powered aircraft which can carry up to 200 passengers for over 3,500 km. Like the Saturn V moon rocket, these concept planes will be fuelled by liquid hydrogen which has an energy density of 8 MJ/kg which is 10 times greater than batteries, though still a fifth of jet fuel.

Airbus’s concept designs show that the hydrogen in these planes would occupy a third of the fuselage and so increase air fares by 50%. Furthermore, there would be the huge cost of installing liquid hydrogen fuelling and storage facilities at the world’s airports. The practicality of scaling up green hydrogen production also must be considered. A recent Royal Society report ‘Net zero aviation fuels: resource requirements and environmental impacts’ estimates that electricity required to produce enough hydrogen to replace UK aviation fuel consumption would require around 80% of the UK’s current electricity generation.

Thus, while technically feasible, practical and cost considerations would seem to make it unlikely that airlines will be using hydrogen-powered aircraft in the foreseeable future.

Carbon offsetting

Organisations can compensate for their carbon emission by investing in projects that reduce emissions elsewhere. For aviation, such offsetting is encouraged by the UK Emissions Trading Scheme (ETS) and the Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA).

Trading carbon reduction in this way is a key aspect of international climate change agreements. However, this is only effective if the offsetting investment creates emission reductions that are additional to those that were going to happen anyway. They must also be monitored and verified by independent third parties to verify the promised emission reductions.

A significant proposed investment in emission reductions outside the aviation industry is the £1 billion commitment in the previous Government’s Jet Zero strategy to develop Carbon Capture and Storage (CCS) clusters. Worldwide, there are currently relatively few such facilities almost all of which require Government support. There are plans to significantly increase CCS facilities including those with Direct Air Capture (DAC).

Outside of the petro-chemical industry, some environmental groups consider CCS to be an unproven, expensive technology that distracts from global decarbonisation efforts. Yet it would seem feasible to capture carbon as an integral part of an industrial process (e.g. producing green hydrogen from methane). However, it remains to be seen whether DAC is feasible at scale as the carbon dioxide it extracts only makes up 0.04% of the atmosphere.

The Jet Zero strategy estimates that, by 2050, 19 million tonnes of carbon (CO2E) will be abated outside the industry and that this will contribute 37% of aviation’s carbon reduction to achieve net-zero in 2050. This is greenhouse gas removal by technologies such as DAC which various sources estimate will cost between £100 to £500 per tonne of carbon. If DAC CCS was used to achieve this 37% reduction, using the lowest estimate, the annual cost would be £2 billion per annum. It is not clear who will pay for this.

Sustainable aviation fuels

Within the aviation industry, ‘drop-in’ sustainable aviation fuels (SAF) are seen as the solution for net-zero aviation. There are three ways of producing SAF: biofuels from crops, biofuels from waste, and synthetic fuels.

Biofuels are deemed carbon free as, providing that they are grown in a sustainable manner, they absorb more carbon dioxide when they are grown than they release when they are burned. However, as with hydrogen, scaling up production to meeting demand is problematic. The CCC’s net-zero report notes that “consideration should be given as to whether aviation is the most appropriate place to use biomass, given that it is likely to be a scarce resource with a range of alternative uses which may save more emissions.” As a result, it explains “a pragmatic planning assumption would be to aim for up to 10% biofuel use in aviation in 2050.”

The limitations of biofuel production are indicated in the Royal Society aviation’s fuel report. This concludes that over half the UK’s agricultural land would be needed to grow sufficient biofuel crops to replace the 12-million tonnes of aviation’s fuel consumed by the UK in 2019.

Biofuels are currently produced from waste include cooking oil. The Royal Society report considers that this fuel costs 50% more than jet fuel and could satisfy 0.3% to 0.6% of UK aviation fuel demand. It also notes that it may be possible to produce up to 10% of aviation fuel demand from municipal waste, though at a much higher price than fuel from cooking oil.

The production of carbon-neutral synthetic fuels is a process that has a low thermodynamic efficiency with multiple stages. Hence the energy required to produce such fuels can be far greater than their energy content. The Royal Society report estimates that between 468 and 660TWh of electricity would be needed to produce sufficient synthetic fuel to satisfy UK aviation demand. This is around five times the UK’s current electricity consumption. This report also shows that the cost of such synthetic fuels would be four times the cost of aviation fuel.

Given the importance of sustainable fuels for aviation decarbonisation, legislation has been enacted to mandate that aviation fuel suppliers blend a minimum percentage of SAF in their fuels. In 2025, 2030, and 2040 onwards this is respectively 2%, 10%, and 24%. 

Efficiencies

Much has been done to reduce aircraft fuel burn. For example, newer aircraft like the Boeing 787 Dreamliner and Airbus A350 are much more fuel efficient than previous generation aircraft. It has been estimated that halving fleet age by building more aircraft could reduce fuel burn by 11-14%.

Other measures to improve efficiency are designing aircraft to fly 15% slower to reduce fuel burn by 5-7% and ensuring aircraft are flown close to their design range which could lower fuel burn by 4-7%. There is also scope for increased efficiency in the management of airspace by giving aircraft the most direct routing and minimising level flight below an aircraft’s preferred cruising level.

Taking all these factors into account the CCC estimates that operational and air transport movement efficiencies could reduce the carbon intensity of flying by an average 1.3% per year between 2025 and 2050.

Managing demand

The CCC net-zero report proposes that the increase in aviation passenger demand should be limited to a 60% increase in passenger demand above 2005 levels by 2050. It notes that there could be policies to directly manage demand, and that demand may be lower in future if preferences or social norms change, especially as 80% of journeys are for leisure purposes.

This report shows the potential for modal shift from aviation to high-speed rail. However, it accepts that the potential for emissions saving from modal shift is limited to domestic flights which is a small percentage of air passenger km.

However, past and present governments have disregarded the CCC’s advice in this respect as they believe that the aviation sector can achieve net-zero without the need to limit aviation growth. In January, the Government announced that it is “committed to ensuring that the economic benefits of airport expansions are delivered in a way that considers and addresses environmental responsibilities” and that there have been “great strides in transitioning to greener aviation”.

This statement was justified with reference to the SAF mandate which was claimed to be “one of the key measures required to reach net-zero emissions from aviation by 2050.”

Net-zero aviation?

There are various estimates of how the UK will achieve net-zero aviation by 2050. The Jet Zero strategy considers that by 2050 the UK aviation sector will have abated 52 million tonnes of carbon dioxide equivalent (MtCO2E) as follows:

• Offsetting – 14 Mt.
• Abatement elsewhere (DAC) – 19 Mt.
• Fuel efficiencies – 8 Mt.
• Zero carbon aircraft – 2 Mt.
• SAF – 9 Mt.

Hence the actual planned reduction in aircraft emissions by 2050 is 19 Mt which is about the same as that from the expected growth in air travel. This will leave UK-fuelled aircraft emitting 33 MtCOE by 2050. This is 74% more than aviation emissions in the 1990 baseline year. Readers can judge for themselves whether this constitutes net-zero aviation.

Economy and the environment

The extent to which the need for carbon reductions conflicts with the requirement for economic growth depends on the sector concerned. The impression cost reduction in renewable power generation provided an incentive to decarbonise. Railway electrification also offers decarbonisation and a more cost-effective railway, though at a high initial capital cost. Innovative low-carbon technologies also provide significant business opportunities.

Yet innovation cannot change the laws of physics nor the properties of materials. Aircraft will always require large amounts of energy. Hence decarbonising aviation comes at a cost and is subject to resource constraints.

Air transport and aerospace makes a significant contribution to the UK economy by directly employing around 230,000 people and contributing around £20 billion to the nation’s GDP. This is a vital contribution to the UK economy and for many it is the only way to get to their destinations.

Hence it is clearly unrealistic to significantly reduce air travel. Yet the goal of net-zero by 2050 does require a transitional strategy which limits demand.

Although there is scope to reduce emissions by SAF and efficiencies, the Jet Zero strategy projections show that increasing air travel demand will cancel out any such savings. Therefore it is misleading to pretend that net-zero aviation is a possibility.

Minimising energy use should be a key aspect of the UK’s decarbonisation strategy. Between Glasgow and London, a 200-seat plane consumes four tonnes of jet fuel (176,000 MJ) while a 609-seat Class 390 Pendolino consumes 10MWh (36,000 MJ). Hence, the plane requires 15 times more energy per seat than a train.

In this year of Railway200, the fact that trains are engineered to be a particularly energy efficient and carbon friendly form of travel is something that should be both stressed and celebrated.

Image credit: David Shirres