Fix the Planet newsletter

Adam Vaughan | New Scientist | 10 June 2021

Hi. It's fair to say the return of commercial supersonic flight wasn't high on my predictions for climate change saviours in 2021. Yet last week, US-based United Airlines made an audacious - some might say far-fetched - pitch that it would relaunch supersonic air travel as a green enterprise.

Announcing an agreement to buy at least 15 planes from US manufacturer Boom Supersonic that would carry passengers from 2029, United said it would be the "first large commercial aircraft to be net-zero carbon from day one". This rests on the firm running the planes on sustainable aviation fuel, or biofuel to you and me.

So, what's not to like about flying Mach 1.7 on green fuel, and cutting journey times from London to Newark, New Jersey, in half? Quite a lot, it turns out.

Isn't supersonic flight noisy and fuel guzzling?

Yes and yes. Breaking the speed of sound involves consuming at least five times as much fuel as a conventional plane, says Dan Rutherford at the International Council on Clean Transportation (ICCT), a US non-profit organisation. "Supersonic aircraft are real gas-guzzlers, because a lot of the energy released by burning the fuel goes into generating sound waves rather than forward thrust," says Paul Williams at the University of Reading, UK. And even with technology improvements since Concorde and Russia's Tu-144, supersonic planes are noisy at take-off and generate noise pollution with their inevitable sonic booms. United's answer to the fuel consumption issue is to burn sustainable aviation fuel (SAF), a catch-all name for fuel derived from lower carbon alternatives to kerosene, such as animal fat and cooking oil.

What's the problem with SAF?

Where to start? The main problem is scale. Pre-pandemic, SAF accounted for just 0.01 per cent of all civil aviation fuel consumption, a figure the aviation industry had hoped could reach 2 per cent by 2025. In 2020, the share crept up 0.025 per cent, the airlines' trade body IATA tells me. Which shows that scaling up production will be hard. United does at least seem aware of the issue, saying it will help increase production, but unfortunately, it was unable to offer me anyone to speak to on how exactly it plans to do that, and how much it would cost.

Critics of supersonic flight also say its return will be a distraction from the challenge of providing enough SAF for subsonic flight. "We are way behind in developing enough SAF to decarbonise subsonic, and so adding back in supersonic is going to make it even more challenging," says Andrew Murphy at Brussels-based non-profit Transport & Environment. "The EU will propose a SAF mandate next month, and the bloc reckons it can get to 5 per cent SAF by 2030. It is a very slow process to certify and deploy such fuels."

There are still issues even if scale can be overcome. The main one is that SAF is typically blended with kerosene, so usually only results in a 70 to 80 per cent reduction in carbon dioxide emissions. Boom has said its supersonic plane, called Overture, would be "capable" of running on 100 per cent SAF. But United hasn't explicitly said if it will use 100 per cent SAF or a kerosene blend and didn't answer my questions on that. Even wholly using SAF would require some form of CO2 removal, such as direct air capture, to meet the airline's net zero promise. "Current and proposed future SAF fuels are far from carbon neutral, even at 100 per cent blend," says Piers Forster at the University of Leeds, UK.

Finally, SAF is typically three times as expensive as conventional jet fuel and even producers of the fuel, such as Neste, think the price is unlikely to come down much. Still, the extra cost feeding through to pricier fares is less of an issue for a service catering to affluent travellers.

How much CO2 could supersonic flights emit?

"Using a typical biofuel will generally emit at least as much CO2 per passenger as a subsonic plane flying on fossil jet fuel," says Rutherford. The ICCT has estimated what would happen if supersonic flight returned in a big way. A global fleet of 2000 aircraft by 2035 - United is buying 15 of Boom's planes, with an option to increase that to 50 - would emit 96 million tonnes of CO2 a year. That may not sound huge in a world that emitted 39 billion tonnes of CO2 last year, but it would amount to 2.4 billion tonnes over 25 years of service.

What about the impact of flying at a higher altitude than normal planes?

Boom says the Overture will fly twice as high as subsonic planes, at about 60,000 feet. That will be good for reducing turbulence, says Williams, but it means the planes' impact on the climate will be much bigger. That's because of what are called non-CO2 effects - the warming effect of water vapour, particulates and nitrous oxide (NOx) emissions produced by flights. "The big difference between subsonic and supersonic aircraft from a climate point of view is the effect of water vapour emitted by the engines," says Keith Shine, also at the University of Reading, who points to a 1999 report by the Intergovernmental Panel on Climate Change and a 2021 study part-funded by Boeing. At the altitude of subsonic flights, water vapour hangs around for a few days. It can last much longer, potentially a couple of years, when emitted higher into the stratosphere by a supersonic plane.

Shine says a small positive is that supersonic planes are expected to form few contrails, the straight lines of cloud we see when a plane flies overhead today, which are the largest non-CO2 effect from subsonic flights. However, he says: "Any reduction in climate effect due to reduction in contrail formation is likely to be compensated by the emission of water vapour directly into the stratosphere."

Finally, burning SAF produces fewer particulates and NOx than normal jet fuel, but not zero, which concerns scientists. "We need further research to investigate the detailed climatic effects of emitting these pollutants so high in the atmosphere," says Williams.

I think we've probably established this isn't a Fix. Anything else?

The other obvious problem is the same one that faced Concorde: you can fit far fewer passengers on a flight despite the greater fuel consumption. Concorde used to burn 20 tonnes of fuel an hour, twice that of a Boeing 747 carrying four times as many passengers. I remain open to being convinced that United could somehow deliver on its net zero promises while breaking the speed of sound. But it has a lot of work to do, and a lot of questions to answer.