Altering flight plans to avoid contrail formation holds the promise of reducing aviation’s impact on climate change.
But the global airline industry, which has pledged to reach net-zero carbon emissions by 2050, wants to see more evidence before implementing such steps.
An IATA spokesperson said that “while it may be possible to avoid creating contrails, either by flying around the areas of super-saturated cold air in which they form or flying at a different altitude, this may create the unintended consequence of releasing more CO2 through increased fuel burn.”
Contrails, the wispy condensation trails that form when soot and water vapor emissions from jets mix with humid, cold air, often look beautiful when framed against a blue sky. But they also trap heat in the atmosphere, just as clouds do. A 2021 study published in the peer-reviewed journal Atmospheric Science found that aviation is responsible for approximately 3.5% of global climate change, with contrails responsible for 57% of that impact.
However, not all contrails are the same. Some form and then quickly dissipate, while others can linger for several hours. The long-lasting ones often take on the appearance of ordinary cirrus clouds and are known as contrail cirrus. During the day, contrail cirrus, like clouds, can actually reflect the sun’s radiation, cooling the atmosphere. But at night, they trap heat in the atmosphere. Overall, contrails trap more heat than they reflect.
Long-lasting contrails tend to form under certain circumstances, most notably in cold and humid regions. Conditions for contrail cirrus formation, said Christiane Voigt, head of the cloud physics department at the German Aerospace Center, tend to be particularly ripe just ahead of incoming storm systems.
That’s the good news, because weather forecasts could potentially be used to help flight planners steer aircraft clear of areas where contrail cirrus are likely to form, especially during evening flights.
The challenge, however, is that conditions agreeable for the formation of contrail cirrus tend to occur at between 30,000 and 40,000 feet, which is just about where commercial airlines like to fly to maximize fuel efficiency.
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A recent study conducted jointly by the German Aerospace Center and Eurocontrol, the European air traffic control coordinator, sought to determine how frequently flight paths would need to be altered to avoid regions that are ripe for contrail cirrus formation.
The study, which was conducted over 10 months in 2021, looked at the technical feasibility of contrail prevention, the accuracy of forecasts related to where contrail cirrus clouds are likely to form and the operational feasibility of altering flight plans to avoid contrail formation under various air traffic scenarios. All the flights in the study were performed after 6 p.m.
Voigt said the results were encouraging.
“Just 10% of the flights are in the areas where warming contrails form,” she said. “You would not have to do anything for 90% of flights.”
In cases in which flight routes would need to be altered, planes would typically need to fly just a few thousand feet higher or lower than normal, Voigt added, amounting to significantly less than 1% of fuel burn on a particular flight.
The German Aerospace/Eurocontrol study followed other research on the topic, including a 2019 paper that was published in the peer-reviewed Transport Research Interdisciplinary Perspectives journal, which found that just 15% of U.S. flights generate contrails and that increasing flight altitudes by 2,000 to 4,000 feet can reduce the net impact on climate warming by those flights by 92%.
Research results like that led the U.K.’s Royal Aeronautical Society in March 2021 to declare altering flight routes to avoid contrail creation to be “low-hanging fruit” in the aviation industry’s fight to reduce its climate impact.
But IATA isn’t alone in questioning that conclusion. In an essay last July, U.K.-based climate science professors Keith Shine and David Lee, both of who do work on aviation’s impact on climate change, cited findings of an October 2020 study in the peer-reviewed journal Aerospace to argue that forecast models are not yet good enough to reliably craft flight plans for contrail avoidance. When forecasts are wrong, the authors wrote, reroutes can actually divert aircraft into areas with contrail-forming conditions.
“An alternative approach that has been suggested is sometimes called ‘tactical avoidance,’ in which pilots react to contrail formation by, for example, changing their altitude,” Shine and Lee wrote. “But on current understanding, it would require guesswork to know whether the contrails being formed are short-lived or persistent or whether the new route is any less likely to lead to contrail formation.”
Uncertainties like those are why IATA says it will be cautious in adopting contrail avoidance measures.
“We welcome continued research into these issues until a scientific consensus emerges,” the spokesperson said.
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