On 1 December, a United Airlines passenger plane flew from Chicago to Washington powered solely by fuel made from cooking oils, agricultural waste and other materials rather than fossil fuels.
Billed by the airline as the world’s first fully-loaded passenger flight to run on 100% sustainable aviation fuel (SAF), it was United’s latest attempt to demonstrate its climate credentials. United’s CEO Scott Kirby called it “a significant milestone” for efforts to decarbonize the industry.
But to critics, it was another attempt to make sustainable aviation seem a lot closer than it is in reality and give the impression people can fly guilt-free. The global supply of SAF remains very limited and there are big obstacles to scaling it up. Yet despite these challenges, United Airlines is pursuing a growth strategy which includes increasing flight numbers, adding new routes and investing in supersonic aviation.
When Covid restrictions ease again and the world opens back up, airlines including United are hoping to see rapid recovery. In the US, the industry is already expecting pre-pandemic levels of travellers this holiday season despite the rise of the Omicron variant. At the same time, it is under increasing pressure to tackle its climate footprint. Aviation makes up about 2.5% of global emissions but the UN has projected that carbon emissions could triple by 2050.
While airlines have been slow to make net zero commitments – the industry is heavily reliant on fossil fuels, and technologies to decarbonize flying are a long way from scaling – some are starting to come out with pledges.
United has sought to position itself at the front of the industry’s efforts. In 2020, the airline promised to become “100% green” by 2050 – a commitment to reach net zero without using carbon offsets, which are widely used by the rest of the industry. Kirby, who has called climate change “the biggest issue facing our generation” has criticized offsets as a “fig leaf”.
Instead, “we’re taking our dollars, our time and our commitment and really focusing all of it on those solutions that can actually pull the emissions out of flying,” said Lauren Riley, managing director for global environmental affairs and sustainability at United.
The airline is making big investments in sustainable aviation fuels, including an investment in Fulcrum Bioenergy, a company that makes jet fuel from household garbage. SAFs can reduce emissions by 80% on a lifecycle basis compared with conventional fuel, according to aviation industry body IATA.
United is also funding projects that suck carbon directly from the air. “Direct air capture is going to play a massive role,” said Riley. United is planning to invest in the project 1PointFive, said Riley, which is building a direct air capture plant in Texas. “One of these plants will remove as much CO2 as 40m trees,” she said.
But the challenges to scaling up these technologies are immense.
Direct air capture is not yet operating at anything like the scale required. There are only 19 DAC plants in operation globally, which collectively remove about 10,000 tons of CO2 annually – a long way from the 85m tons a year the IEA estimates must be removed by 2030 as part of measures to keep to 1.5C of warming.
And when it comes to sustainable fuels, supplies remain limited. Globally SAF still makes up less than 0.1% of aviation fuel and it costs about three to four times more than conventional fuel. Riley readily acknowledged that United currently uses tiny amounts of SAF. The airline burns about 4bn gallons of fuel in a typical pre-pandemic year, only 1m gallons of which is SAF. “And we have the most supply so that’s just bananas,” Riley said.
A big challenge to scaling up SAF is finding the right feedstocks, ones which don’t themselves cause environmental damage. For bio-based fuels, using cooking oils and agricultural waste, can keep the waste from landfill or being left to rot, but this source has limited availability. Growing crops to produce fuel can bring problems of land clearing, deforestation, agricultural pollution and could compete with food production.
And while sources for synthetic fuels – made by combining hydrogen produced by renewable energy and recycled carbon – are technically abundant, the process remains very expensive and will require huge amounts of clean energy.
Airlines have been talking about SAFs since at least 2009, said Dan Rutherford, who directs the International Council on Clean Transportation’s aviation and marine programs. “The history actually is very clear that these things aren’t scaling.” While SAF mandates are being considered by governments including the US and the EU, he said take-up “is not going to be as fast as the airlines think it is”.
Yet United’s strategy involves scaling up flying. It has increased the number of daily flights to London and in October it announced several new routes, including flights to Amman, Jordan, and Tenerife in the Canary Islands.
Perhaps the airline’s biggest move – and to some experts, the most surprising – is its leap into supersonic travel. In June, United announced a deal with Denver-based company Boom Supersonic, agreeing to spend an estimated $3bn on 15 Overtures – Boom’s supersonic passenger jets.
Both companies say supersonic flight fits in with net zero commitments. The supersonic jets are “designed to run on 100% SAF”, said Riley. They will be “net zero carbon”, said a spokesperson for Boom Supersonic, who added “we’re highly optimistic about SAF supply by 2029, when Overture is slated to carry passengers”.
But despite this confidence, there are questions about where this fuel will come from and what it would mean for decarbonizing the rest of the industry. Supersonic jets can burn five to seven times more fuel than traditional aircraft. Boom said it was too early to give numbers on how much SAF the Overtures would use but said that fuel efficiency is “a top priority”.
“What we do have to recognise here,” Riley said, “is that innovation is going to continue but also core to that is that we’re committed to all of these innovations, doing them sustainably.”
Experts remain very skeptical. “Putting these fuels into supersonic jets … would be an insane use of scarce resource,” said Cait Hewitt, policy director of the UK non-profit the Aviation Environment Federation.
Rutherford said United’s bet on supersonics “is just completely baffling to me – why you would take pretty pro-climate commitments on one hand [and] on the other hand sign on to an aircraft that we know is going to be at least five times as carbon intensive as subsonic aircraft.” According to his calculations, United’s 1m gallons of SAF a year would operate 15 supersonic jets for two days.
For some experts, reducing flying is the only genuine path to achieving net zero, especially as emissions from industry growth pre-pandemic outpaced those saved by efficiency measures and other climate policies.
The “flight shame” movement has become more mainstream over the last few years, making frequent flying less socially acceptable. One airline even tried making flying less part of its marketing strategy. In 2019, KLM ran a campaign encouraging people to consider taking a train or having a video call instead of flying.
When asked if flying less would ever be part of United’s vision for flying sustainable, Riley said “I like to look at it as flying sustainably so it’s not an either or; it’s getting on an aeroplane that’s fully fueled by sustainable aviation fuel and perhaps it’s a zero-carbon flight. That’s what we’re working towards.”
But Hewitt dismisses the idea of delivering zero emissions commercial flight any time soon. “Given how far we are from knowing how to decarbonize flying, we really need to be downsizing aviation demand,” she said.
This article was amended on 30 December 2021. The 19 DAC plants globally remove 10,000 tons of CO2 annually, not 100,000 as an earlier version said.