Contrails are clouds formed when water vapor condenses on the soot emitted by jet engines.
Markus Meinka/Alamy stock photo
Planes flying at higher altitudes create longer-lasting vapor trails that could cause more global warming. Because private jets and modern fuel-efficient jetliners fly at higher altitudes than other commercial airliners, these aircraft may be causing even more warming than previously thought.
The findings could help airlines decide which routes to fly to minimise contrails, says Edward Griespirt of Imperial College London: “If we could predict accurately enough the areas in the atmosphere where contrails form, we could fly planes around them, reducing the impact of contrails.”
In some conditions, soot particles emitted by jet engines can spew ice particles in the aircraft’s wake, forming clouds called contrails, which contribute to the overall warming effect. It is estimated that half of the warming effect from aviation is due to contrails, rather than carbon dioxide emissions.
How long a contrail lasts determines how much warming it causes, but persistence is difficult to study. Grispiard and his team combined flight data and satellite observations to match contrails to specific aircraft and examine how the type of aircraft is related to persistence.
Previously, studies had been done manually and only on a small scale. But using artificial intelligence, the team was able to analyze 64,000 flights. They found that private jets and other fuel-efficient jetliners, which typically cruise at altitudes of about 12 kilometers (38,000 feet), one kilometer higher than other planes, are more likely to produce longer-lasting contrails. “That was unexpected,” Gris-Pierto says.
Not all soot particles emitted by aircraft turn into ice particles, he said: The team thinks that as planes fly higher, a higher proportion of soot particles become ice seeds, but the overall size of the ice particles gets smaller.
Because the tiny ice particles fall more slowly, they take longer to fall into areas of warmer air where they can sublimate back into water vapour, meaning the contrails linger for longer, causing even more warming.
But the team can’t say exactly how much warming contrails cause because the characteristics of high-altitude contrails are slightly different, so it’s unclear whether the additional warming caused by contrails lasting longer exceeds the warming avoided by modern planes’ lower fuel burn.
What’s clear is that the impact of private jets has been underestimated: “The climate impact per passenger is much larger than we thought,” Griespeert says.
Because contrails are more visible over oceans and the researchers only had data from a single geostationary satellite, they only looked at flights over the western Atlantic around Bermuda.
Griespielt said the findings may not apply to flights further north, such as over Greenland or Iceland, because the air at high altitudes is drier and contrails are less likely to form.
“This study highlights that high-altitude aircraft have a significant climate impact beyond carbon dioxide, mainly due to the persistent contrails they create,” said Christina Hentz of the European environmental group Transport and Environment.
High altitudes are primarily used for long-distance flights, which are excluded from European Union plans to reduce non-carbon-dioxide warming, and this shows the importance of switching to fuels that produce fewer soot particles, Hentz said.
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