Have efforts to cut emissions from modern aircraft backfired? The planet-warming condensation trails, or contrails, left by modern commercial aircraft flying at high altitudes are longer-lived than the thin streaks of cloud left by older aircraft.
While the exact warming effect of contrails is uncertain, partly as these trails of ice particles also reflect sunlight back into space during the daytime, scientists believe it is greater than the warming caused by carbon emissions from burning jet fuel in engines.
Because private jets fly at even higher altitudes than commercial aircraft, up to 40,000 feet, they create long-lived, outsized contrails more often than previously thought, according to the research.
Led by scientists at Imperial College London, the study published today in Environmental Research Letters used a form of AI called a neural network to analyse satellite data on more than 64,000 contrails from a range of aircraft flying over the North Atlantic Ocean. Consisting of ice particles, the contrails stand out when using satellite instruments that can detect infra-red, or heat, radiation.
This analysis revealed that private jets and more fuel-efficient jets, which typically cruise at altitudes around a kilometre higher than other planes, are more likely to generate longer-lasting contrails.
Dr Edward Gryspeerdt, the lead author of the study who works at Imperial’s Grantham Institute, said: ‘Most people do not appreciate that contrails and jet fuel carbon emissions cause a double whammy warming of the climate.’
He added: ‘Carbon dioxide emissions from many sectors are decreasing, but air travel is still growing, so aviation will contribute more to future climate change.’
To reduce jet fuel consumption, modern aircraft are designed to fly at higher altitudes where the air is thinner with less aerodynamic drag, compared to older commercial aircraft, which usually fly at slightly lower altitudes (around 35,000 feet, or 11 km).
But the study showed that modern aircraft that fly at above 38,000 feet (roughly 12 km), such as the Airbus A350 and Boeing 787 Airliners, create more contrails than older passenger-carrying commercial aircraft, such as the Boeing 747 that can be seen in the museum’s Flight Gallery (and above) and these contrails take longer to dissipate – creating a warming effect for longer.
‘The unintended consequence of this is that these aircraft flying over the North Atlantic are now creating more, longer-lived, contrails, trapping additional heat in the atmosphere and increasing the climate impact of aviation,’ said Dr Gryspeerdt.
The study did confirm that modern aircraft engines that are designed to be cleaner, so they typically emit fewer soot particles, reduce the lifetime of contrails: soot seeds the formation of ice particles, so the less soot, the fewer and the smaller the contrail ice crystals, creating shorter-lived contrails.
‘The engines that produce particularly low soot emissions (so-called ‘lean-burn’ engines) are starting to come into service now and this work would suggest that they will deliver a benefit to the climate by reducing contrail lifetimes,’ added Dr Gryspeerdt, who has written a blog outlining the study in more detail.
Co-author Dr Marc Stettler commented: ‘Our study provides the first evidence that emitting fewer soot particles results in contrails that fall out of the sky faster compared to contrails formed on more numerous soot particles from older, dirtier engines.’
As for how clean the emissions have to be to eliminate contrails, it is unclear because other components of the engine emissions, such as lubricant oil, could also form ice crystals.