Airlines have a big climate problem, and it’s not just CO2 emissions

A Virgin Atlantic flight from London to New York City grabbed the world’s attention last week when it successfully crossed the Atlantic Ocean without burning fossil fuels — a first for a major airline. But proving that planes can safely fly on 100 percent sustainable aviation fuel wasn’t the only purpose of the November 28 trip.

Researchers traveling on the Boeing 787 Dreamliner were also there to study another major climate problem caused by air travel: condensation trails, or ​“contrails.”

The wispy clouds form from the soot and water vapor that spews from jet engines. Most of the time, contrails quickly disappear with little effect. However, under certain conditions — such as when the atmosphere’s humidity is very high and the air is incredibly cold — contrails can act like a blanket, trapping the heat that radiates off Earth’s surface.

Although burning fossil jet fuel contributes about 2.4 percent of the world’s annual carbon dioxide emissions, flying may actually be responsible for 4 percent of total global warming when contrails and other non-CO₂ factors are taken into account, a 2021 study found.

If scientists can accurately predict where and when heat-trapping contrails are likely to form, then airlines could adjust a plane’s path to avoid flying through the riskiest regions. The challenge, however, is that research institutions and technology companies are only just beginning to develop the tools for making such predictions.

“If an air carrier intends to avoid a contrail, they need to have high confidence that those forecasts are indeed accurate,” said Joey Cathcart, a senior aviation associate in the Climate-Aligned Industries program at RMI, a clean energy think tank. (Canary Media is an independent affiliate of RMI.)

Cathcart was one of dozens of passengers who boarded last week’s transatlantic flight. As Virgin’s jet guzzled fuel made from used cooking oil and plant sugars, the pilots tested a new contrail-observation mechanism that Cathcart and his colleagues conceptualized and Virgin Atlantic developed.

The mechanism is fairly simple. If pilots spot a contrail left behind by another aircraft, they manually log the observation in an electronic form, noting details such as the time, altitude, temperature and humidity. That information is shared instantaneously with a dispatcher on the ground, similar to how pilots file turbulence reports during flight.

Researchers need this type of real-world data to help validate and calibrate their contrail-prediction models, Cathcart said. Satellite imagery is another important tool, though it can’t always detect contrail formations. Ground-based imagery is also somewhat limited because it generally doesn’t pick up contrails on cloudy days or over oceanic routes. The goal is to deploy in-flight reporting on many more planes to build up an industrywide database.

“The benefit of pilot reporting is that it’s easy to apply, and there’s very little cost,” Cathcart said the day after Virgin’s plane landed in New York City.

The contrail experiment has been about a year in the making. In November 2022, RMI and Breakthrough Energy launched the Contrail Impact Task Force to begin addressing the aviation industry’s ​“non-CO₂” effects. Over 50 organizations have since joined the initiative, including Virgin Atlantic and other major airlines, aircraft manufacturers and scientists from U.S. and U.K. research institutions.

Recently, Breakthrough Energy and two other task force members, American Airlines and Google Research, launched a separate effort to see whether pilots could avoid creating contrails by adjusting their altitudes during flight.

Over a six-month period, a group of pilots at American flew 70 test flights using contrail forecast maps, which were developed with Google’s artificial-intelligence-based predictions and Breakthrough Energy’s open-source contrail models. After the flights, researchers analyzed satellite imagery and found that pilots were able to reduce contrails by 54 percent, according to Google Research. They estimated that maneuvering around contrail-prone zones could require airlines to burn as little as 0.3 percent additional fuel.

Virgin Atlantic’s flight will, along with informing contrail models, help to address another crucial question facing contrail researchers: whether sustainable aviation fuel itself can reduce the formation of heat-trapping clouds.

When jet engines burn fuel, they generate carbon dioxide emissions as well as particulate matter, or soot. In certain circumstances, water vapor in the atmosphere attaches itself to the soot particles and forms ice crystals, which can then become cirrus clouds that fan out and persist — contributing to global warming. A growing number of studies suggest that burning sustainable aviation fuel, or SAF, produces fewer particulate-matter emissions than fossil jet fuel, which in turn lowers the potential to create warming contrails.

“If there are fewer particles, as we expect from the use of SAF, the contrail won’t last for as long and will therefore have a smaller climate effect,” Marc Stettler, who leads the Transport and Environment Laboratory at Imperial College London, said in a statement.

Stettler also boarded the London-to-NYC flight and is leading a project with Imperial and the University of Sheffield to assess the climate impact of last week’s voyage. That includes estimating how much soot the SAF-fueled flight released and tracking any contrails it might have formed en route from Heathrow Airport and John F. Kennedy International Airport. They’ll then analyze how Virgin’s trip compares to typical fossil-powered flights that take place in the same region at the same time.

“This provides us an unparalleled opportunity to track and analyze these contrails and study their likely climate effect,” said Roger Teoh, who is part of the Imperial project team.

Ultimately, the aim of all these research efforts is to prod airlines to do more to reduce their outsize climate impacts — not just by sourcing dramatically more lower-carbon jet fuel, but also by working to avoid creating heat-trapping contrails.

“Observation of contrails leads to an increase in accuracy…which will hopefully lead to an increase in adoption of contrail mitigation strategies,” Cathcart said.