Modern fuel-efficient jets can cause more warming than older planes

Aeroplanes that fly at higher altitudes can create longer-lasting vapour trails that are likely to cause more global warming. Since private jets and modern fuel-efficient jets fly higher than other passenger jets, these aircraft may be causing even more warming than previously thought.

The findings could help airlines work out which routes to fly to minimise contrails, says Edward Gryspeerdt at Imperial College London. “If we could predict the contrail-forming regions of the atmosphere well enough, you could route aircraft around them, which would reduce this effect.”

In some conditions, the soot particles emitted by jet engines can seed the formation of ice particles in the wake of aircraft, forming clouds known as contrails that have an overall warming effect. As much as half of the warming effect of aviation is estimated to be due to contrails, rather than the carbon dioxide emissions.

How long contrails persist largely determines how much warming they cause, but their persistence is difficult to study. Gryspeerdt’s team has combined flight data and satellite observations to match specific aircraft to contrails, and see how the type of aircraft relates to persistence.

This has only been performed on a small scale before because it was done manually. But by using artificial intelligence, the team could analyse 64,000 flights. This revealed that private jets and more fuel-efficient jets, which typically cruise at around 12 kilometres (38,000 feet), a kilometre higher than other planes, are more likely to generate longer-lasting contrails. “It was not what we expected,” says Gryspeerdt.

Not all the soot particles emitted by an aircraft turn into ice particles, he says. What the team thinks is happening is that when an aircraft flies higher, a higher proportion of soot particles seed ice particles, but the overall size of ice particles is smaller.

Smaller ice particles fall more slowly, so take longer to fall to regions where the air is relatively warmer and where they sublimate back into water vapour. This means contrails persist for longer and cause more warming.

However, because the properties of these higher-altitude contrails are a bit different, the team cannot say exactly how much warming they cause. So it is not clear if the additional warming caused by longer-lasting contrails outweighs the avoided warming due to the lower fuel use of modern planes.

What is clear is that the impact of private jets is being underestimated. “They are having an even more outsized impact on the climate per passenger than we thought,” says Gryspeerdt.

Because contrails are more visible over oceans, and because the team only had data from a single geostationary satellite, they also looked only at flights over the western Atlantic, around Bermuda.

The findings may not apply to flights further north, for instance over Greenland and Iceland, says Gryspeerdt, because the air at high altitudes is drier and contrails are less likely to form.

“The study highlights the significant non-CO2 climate impact of aircraft operating at high altitudes, primarily due to the persistent contrails they produce,” says Krisztina Hencz at Transport & Environment, an environmental advocacy organisation in Europe.

High altitudes are mainly used by long-haul flights, Hencz says, but long-haul flights have been excluded from a European Union scheme that aims to reduce the non-CO2 warming. It also shows the importance of switching to fuels that generate fewer soot particles, she says.