Stretching out behind jets as they fly high in the atmosphere, contrails are not just a visual reminder of air travel’s impact on the environment. Reflecting heat radiation back towards Earth, they also make a considerable contribution to the warming caused by each flight.
A 2021 study in Atmospheric Environment found that contrail cirrus – both the trails and the clouds caused by them – could account for more than half (57%) of aviation’s climate impact.
Contrails, therefore, are a growing focus for research as industry tries to reduce its contribution to global warming. A study published earlier this year by Airbus, Rolls-Royce, the German Aerospace Centre (DLR) and sustainable aviation fuel (SAF) producer Neste found that using SAF could reduce contrails’ climate impact by 26% compared to standard jet fuel.
An in-flight experiment using an Airbus A350 powered by Rolls-Royce Trent XWB engines found a 56% reduction in the number of contrail ice crystals per mass of unblended SAF consumed, compared to reference Jet A-1 fuel.
“The results from the Eclif3 flight experiments show how the use of 100% SAF can help us to significantly reduce the climate-warming effect of contrails, in addition to lowering the carbon footprint of flying – a clear sign of the effectiveness of SAF towards climate-compatible aviation”, said Markus Fischer, DLR divisional board member for aeronautics.
The improvement could be due to reduced aromatics in the Neste fuel, said company vice-president Alexander Kueper.
Such a reduction in contrails’ climate impact “could be really useful,” said Dr Edward Gryspeerdt, a researcher from Imperial College London who was not involved in the work, to Professional Engineering. “Anything you can do to reduce the climate impact of contrails is clearly going to be important, and the use of sustainable fuels is one potential pathway for doing that, essentially a kind of drop-in replacement.”
Although the test flights had impressive results, the real challenge will be scaling up SAF. Made from waste, cooking oil and animal fats, production of the fossil fuel alternative is projected to only account for 0.53% of industry demand in 2024.
Flying high
While SAFs offer one potential avenue for improvement, other changes could have a negative impact. New research published this week by Dr Gryspeerdt and colleagues found that modern commercial aircraft create longer-lived contrails than older aircraft, thanks to higher flight paths.
Published yesterday (7 August) in Environmental Research Letters, the study used machine learning to analyse satellite data on more than 64,000 contrails from a range of aircraft flying over the North Atlantic Ocean.
To reduce fuel consumption, modern aircraft are designed to fly at 38,000 feet (about 12km), where air is thinner and has less aerodynamic drag. Older commercial aircraft usually fly at slightly lower altitudes (about 35,000 feet/ 11km).
This means these higher-flying aircraft create less carbon emissions per passenger – but it also means they create contrails that take longer to dissipate, creating a warming effect that lasts longer.
“There were also some optimistic bits that came out,” said Dr Gryspeerdt. “Reducing the amount of soot from the aircraft engine seemed to produce contrails that were detectable for a shorter period of time. They lived less long.
“So if you can clean up the soot emissions from aircraft engines, either by creating different types of engines, or potentially by using sustainable aviation fuels, that can then reduce the lifetime of contrails.”
Focusing on engines that produce less soot could therefore be one measure for companies to reduce their climate impact, he said, although further study is needed into the contrail formation that might happen with no soot at all.
Another solution could be dynamically adjusting flight altitude in response to contrail formation at certain heights.
“One of the things about the kind of air that supports persistent contrails is it typically forms in relatively vertically thin layers. If you know where those layers are, then you can divert or re-route an aircraft to fly over it or slightly under it, maybe only by 2,000 feet or so, and that stops your contrail forming,” said Dr Gryspeerdt.
“That has a small CO2 cost, because you're flying your aircraft a bit away from its most efficient route. But if you can predict the locations of contrails or these humid layers well enough, then that would be another part of the solution, as it were, to reducing this contrail impact.”
This would rely on better weather forecasts, supported by satellite data. Aircraft themselves could also collect data, using humidity sensors.
No solution will be straightforward – but with growing awareness and understanding of contrails’ impact on the environment, more investment could yield a relatively quick reduction in aviation’s climate impact.
Net zero measures should not just focus on commercial airliners, however. The new Imperial study found that private jets create contrails more often than previously thought, adding to concerns about their excessive use by the super-rich. Despite being smaller and using less fuel, they create similar contrails to much larger commercial aircraft.
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Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers.