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For many people, especially in rich countries, flying in planes amounts to the largest share of their personal carbon footprint. When I calculated my own footprint, flying amounted to about half of my emissions. A round trip from New York City to London emits around of carbon dioxide, more than many people in the world emit in a year.
Overall, global aviation makes up a much smaller proportion of total greenhouse gas emissions 鈥� in 2022. Yet cutting emissions from aviation is one of the most challenging aspects of the path to net zero. As demand for aviation fuel increases and other sectors become cleaner, flying is expected to make up an ever-increasing share of emissions.
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What鈥檚 to be done? Airlines everywhere are touting their use of 鈥渟ustainable aviation fuel鈥�, or SAF, as a more climate-friendly way of flying. Later this year, Virgin Atlantic the first transatlantic flight powered entirely by SAF, made mainly from used cooking oil.
These alternative fuels can help to reduce emissions compared with conventional jet fuel; Virgin Atlantic says its greasy intercontinental hop emit 70 per cent less CO鈧� than a normal flight. But the story isn鈥檛 as simple as all the publicity would have it.
The major advantage of all types of SAF is that they are 鈥渄rop in鈥� fuels that can power regular jet engines. That means airlines can use SAF to power their existing fleets. Hydrogen may eventually prove a better option, but there are no jets currently able to run on the clean-burning gas. Electric propeller planes powered by batteries or hydrogen fuel cells are also emerging, but are limited to smaller planes travelling short distances. Improving the also helps, but can only go so far. Aside from flying less, using more SAF is the main option now available to cut emissions from aviation.
SAF currently makes up a tiny fraction of all aviation fuel. According to the International Air Transport Association, of all types were produced in 2022, less than 0.2 per cent of all aviation fuel used. However, that was triple the amount produced the previous year, and the group estimates that annual production could see a 100-fold increase to 30 billion litres by 2030 based on expanding production and , as well as new .
Governments have also pushed to use more SAF. The UK is a rule that would require SAF make up 10 per cent of the aviation fuel blend by 2030. In the US, the Biden administration has an to produce more than 11 billion litres of SAF a year by 2030, and enough to supply all of US aviation by 2050. In April, the European Union went one further, that at least 2 per cent of jet fuel be SAF by 2025 and 70 per cent by 2050, a policy it estimates will reduce CO鈧� emissions from Europe鈥檚 aviation by two-thirds.
That all sounds good, but much depends on how all that fuel gets made. 鈥淣ot all SAF is created equal,鈥� says at European environmental group Transport & Environment. 鈥淭here are circumstances where SAF can be a cure that is worse than the disease.鈥�
Nearly all the SAF produced today is made using biomass, like vegetable oils, animal fat or waste. Another kind of SAF can also be or other alcohols. Over their life cycle, these can, in theory, substantially reduce emissions compared with conventional jet fuels. However, in practice, they may actually increase emissions by competing with other industries for biomass or requiring more land to grow crops. Even if this problem could be avoided, there isn鈥檛 enough biomass to go around to meet the aviation industry鈥檚 expected demand.
鈥淭here simply are not the scale of feedstocks available to produce the amount of biofuel they are asking for,鈥� says at Cranfield University in the UK, who co-authored a recent for the Royal Society detailing the pros and cons of different low-emissions aviation fuels.
Power to liquid
With biofuels in short supply, many are looking to what are known as 鈥渆lectrofuels鈥� or 鈥渟ynthetic fuels鈥� to meet future demand. These are mainly produced using captured CO鈧� and hydrogen made by splitting water molecules with clean electricity. They are also sometimes called power-to-liquid fuels because they transform electricity into liquid fuel.
Because this process isn鈥檛 limited by biomass, electrofuels are widely seen as a better option than biofuels for the very large scales needed to power aviation. 鈥淲e see a lot of potential for it, says Mirolo. He adds that another benefit of electrofuels is that they produce less air pollution, which could help reduce the significant climate warming effect of contrails produced by high-flying planes.
鈥淚n the long run, it may prove to be the only game in town,鈥� says Gratton. If it is made using CO鈧� captured directly from the atmosphere and some of this carbon is stored in the process, 鈥渋t is the only fuel really capable of being a true net-zero fuel,鈥� he says.
In August, I visited a plant in Montreal鈥檚 industrial East End that recently made some of the first litres of such synthetic jet fuel. Built by a company called the SAF+ Consortium, the demonstration plant was a squat building full of metal piping and valves set in the middle of a large petrochemical facility that smelled strongly of tar.
A poster attached to some pipes explained how the process works: CO鈧� is first converted to carbon monoxide (CO) under heat and pressure. This CO is mixed with hydrogen to produce something called 鈥渟yngas鈥�. Combining the syngas with more hydrogen and a catalyst, in what is known as the Fischer-Tropsch process, results in hydrocarbons that can then be refined to make jet fuel and other products, such as paraffinic wax.
鈥淲e are used to burning fuel to make power. Now, we are using power to make fuel,鈥� , one of the company鈥檚 engineers, told me during the visit. He showed me some of the final product, a clear liquid sloshing in a big glass jug.
The humble jug held some of the only litres of synthetic jet fuel that have been made so far anywhere. 鈥淵ou鈥檝e touched the future, because there鈥檚 nothing like it in North America,鈥� says , the company鈥檚 CEO.
He says the firm鈥檚 full-scale plant, which he hopes will start production in Quebec by 2029, will be able to produce 100 million litres of the fuel each year. It will rely on clean electricity from the region鈥檚 substantial hydropower and use 280,000 tonnes of CO鈧� each year from a still undisclosed industrial source. Paquin says the fuel will reduce emissions compared with conventional jet fuel by up to 90 per cent.
From and to US, and elsewhere in , a slew of other companies are also preparing to ramp up production. In Washington state, a company called Twelve (named for the atomic mass of carbon) on its first commercial electrofuel plant. The plant will use electricity from hydropower and CO鈧� trucked in from a nearby ethanol plant, relying on an efficient process to convert CO鈧� to CO, says at the firm. Twelve aims for the plant to be producing about 750 litres a day by 2024, and eventually millions of litres a year.
鈥淚t鈥檚 a very small contribution at the moment,鈥� says Stevenson. 鈥淏ut over time we see it as being one of the, if not the primary, ways to decarbonise aviation.鈥�
Clean energy crunch
But electrofuels face a big problem as they ramp up production: making all that hydrogen, converting CO鈧� to CO and powering the rest of the process requires vast amounts of clean electricity that鈥檚 already in limited supply.
It takes around 3 to 4.5 times as much energy to make a litre of electrofuel as the energy contained in the fuel itself, according to the Royal Society report. That makes producing the fuel a very inefficient way to use energy. The report found producing enough of the fuel to supply all of UK aviation would require 5 to 8 times the total renewable energy capacity of the UK in 2020.
A similar for the US by at the Pacific Northwest National Laboratory and his colleagues found that sufficient production in the country would require nearly as much new clean energy as all the solar, wind, hydropower and nuclear energy the US currently generates.
鈥淭he technology is there. Whether we can do it at scale isn鈥檛 so clear,鈥� says Weber. Access to enough CO鈧� could also be an issue in the long term. Capturing CO鈧� directly from the atmosphere would require even more energy.
鈥淚n some sense, aviation needs to be privileged, because there鈥檚 no real alternative,鈥� says Weber. But he argues it would initially make a bigger impact on emissions to use any new clean energy to replace fossil fuel generation on the grid, rather than to make cleaner jet fuel. 鈥淚f you鈥檙e using electricity to make that fuel, you鈥檙e not using electricity to power cars or houses,鈥� he says.
Mirolo, however, says that electrofuel production has no time to waste in starting to scale up if there is any hope of making enough fuel to meet climate targets. 鈥淎re the [e-fuel] projects going to be able to start production before 2030?鈥� he says. 鈥淭hat is going to determine whether or not the aviation sector is going to decarbonise by 2050.鈥�
In the meantime, he says, the best way to avoid emissions from flying is simple: fly less.
