These icy clouds are aviation’s No. 1 contributor to atmospheric warming
In its quest to bring back supersonic passenger travel, Boom Supersonic of Colorado knows that speed alone isn’t enough to guarantee success. In the decades since Concorde made its final flight in 2003, passengers and the broader air transportation industry have become more conscious of the noise and environmental impacts of soaring through the air faster than the speed of sound.
Boom believes it has solved the first portion of that equation, as demonstrated in the January and February flight tests of its subscale XB-1 demonstrator. During flights over the Mojave Desert in California, the pilot maintained the required speed and altitude for XB-1’s sonic boom to refract and never reach the ground. For its Overture airliners, Boom plans to develop a collection of software algorithms that would continuously pull in weather forecasts and other information to make those calculations. An autopilot setting on the flight deck would automatically speed up or slow down the plane, as well as adjust altitude as needed.
However, an executive of a previous supersonic venture sees another potential application for this Boomless Cruise technology that could help Boom reduce its environmental impact: predicting contrail formation. These clouds of ice crystals form when the warm soot particles and water vapor emitted during combustion meet the cold atmospheric air. They make up roughly one-third of aviation’s emissions, according to the journal Atmospheric Environment, because the clouds can linger in the stratosphere, trapping surface radiation.
“At Aerion, what we were looking to do was provide real-time management of the contrails to reduce that part of our environmental impact,’’ says Gene Holloway, the former head of sustainability at Aerion Supersonic, the Nevada-based company that explored its own boomless cruise concept for its planned AS2 supersonic business jets. Aerion declared bankruptcy in 2021 and never flew an aircraft, but contrail prediction was on the mind of Holloway and his colleagues, he told me in an interview. “It went hand in hand with the way we planned to operate our boomless cruise.”
Aerion was in talks to receive the required “real-time climate profiles” from Virginia satellite operator Spire Global, Holloway says, which provides “ the information you needed in order to manage your sonic boom as well as manage your contrails.’’
Boom said in response to written questions that it does not anticipate contrails being a large concern for Overture operations based on the scientific consensus that contrails do not form as easily at their planned cruising altitude of around 60,000 feet. “However, we do recognize that contrails are responsible for a significant share of aviation’s climate impacts,’’ a spokeswoman said.
It’s true that historically, aircraft cruising in the stratosphere were not thought to form contrails as often, because the thin air at those altitudes is also too dry and warm to be conducive to ice crystal formation. But this may no longer be the case: A 2024 study led by Imperial College London found that flights by private jets — which fly at similar altitudes as Overtures would — were forming contrails more frequently than expected. The researchers also determined that these contrails were lingering for a longer period in the stratosphere, increasing their warming potential. The researchers examined some 64,000 cases in which contrails were emitted by aircraft flying over the North Atlantic, using artificial intelligence software to match satellite images of contrails with air traffic data to determine which aircraft types were emitting the most persistent plumes.
Higher-altitude flight also comes with other environmental concerns: Because the thin stratospheric air is more stagnant, any water vapor emitted during combustion can linger for 10 to 15 times longer than similar amounts emitted at lower altitudes, says David Lee, a professor at Manchester Metropolitan University who has spent 20 years studying aviation’s climate impact.
“Just the fact of the emission of water vapor into a very, very dry background atmosphere will have a warming effect,’’ he says. “That effect is not present for normal civil aviation in any great magnitude.’’
And while Boom plans for the Overtures to spend very little time at lower altitudes — mainly during the ascent and descent phases — that might not be the case for other supersonic designs, Holloway notes.
“Our thought [at Aerion] was we’re not always going to be flying at those high altitudes. There will be short jaunts we will be taking” at lower altitudes where contrails were more likely to form,” he says. “The other piece was we wanted to do everything we can to minimize environmental impacts.’’
There is good reason for caution, given the lack of certainty about the precise range of conditions in which contrails form and how long they linger. Lab tests cannot perfectly recreate the atmospheric conditions of flight, and researchers can’t always differentiate between contrail cirrus clouds and naturally occurring cirrus clouds when viewing satellite imagery. Kshitij Sabnis, a scientist at Queen Mary University of London who researches high-speed aerodynamics, notes that huge strides could be made in closing this knowledge gap if Boom or other entities conducting supersonic flights would track their contrail formation and share that data publicly. It could even help operators of subsonic aircraft address their own emissions.
By determining where contrails are most likely to form and routing aircraft around those areas, “that could have quite a big impact on the environment,’’ Sabnis says. “But at the moment, we just don’t know what those trajectories should be, and we can’t compute them to be able to make that happen in real time.’’
Another possible factor in the amount of contrails that Overtures could produce is the choice of fuel, says Lee, the Manchester scientist. To reduce its carbon footprint, Boom previously pledged that all Overtures will be able to run on 100% sustainable aviation fuel. SAFs produce just as much carbon dioxide during combustion as conventional jet fuels, but those emissions are offset by their source — for instance, forest waste that drew carbon dioxide from the air before its residue became feedstock for SAF. Studies by NASA and the German Aerospace Center (DLR) indicate that burning SAFs forms fewer contrails than conventional jet fuel, perhaps because most SAFs contain fewer hydrocarbon aromatics. These molecules are among the components of jet fuel that produce the pure carbon soot particles that ice crystals form around.
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“So what we do know is that if you burn SAF in an aircraft engine, you get reduced soot numbers behind the aircraft,” Lee says, but he notes that “you do not get zero soot.’’
He adds that SAFs could also increase the amount of water vapor that Overtures emit.
For Aerion’s Holloway, the choice is clear. Even if contrails end up not being a major concern for supersonic aircraft, he says that Boom and other companies pursuing these designs should do everything they can to reduce their climate impact, particularly given the increased amounts of fuel that these aircraft will require.
“There’s no way around it, [supersonic] is a gas hog,’’ he says. “It’s going to put a lot of pollutants in the atmosphere.’’
Opener photo caption: NASA and the German Aerospace Center (DLR) measured the chemical composition of the contrails formed by this NASA DC-8 research aircraft burning a mixture of SAF and conventional jet fuel. NASA/Eddie Winstead
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