Google modified over 100 flights to cut climate-warming contrails

A trial of thousands of flights between the United States and Europe found that planes produce fewer contrails if they follow flight paths recommended by artificial intelligence to reduce their impact on global warming.

Contrails triggered by soot particles produced by aircraft engines are thought to cause more warming than the carbon dioxide emitted by planes. Research has also shown that certain ice-rich regions of the upper atmosphere are more likely to form contrails when a plane passes through them, and that AI can predict where these regions will be using detailed weather forecasts.

Small-scale trials have shown that planes redirected to these regions produce fewer contrails, but the practice has not yet been applied to large-scale commercial flights.

Now, Google’s Dinesh Sanekommu and colleagues used a contrail forecasting tool to provide route guidance in a randomized controlled trial involving more than 2,400 real American Airlines flights.

The trial involved eastbound flights from the United States to Europe and lasted approximately 17 weeks, from January to May 2025. The direction was one-way because these flights would take place at night, during which time contrails were found to have a more obvious warming effect. During the day, contrails can have a cooling effect because they reflect sunlight back into space.

Each flight route between two cities was randomly assigned to one of two groups. For the first group, air traffic regulators had the option in their flight planning software to choose a low-drag route optimized by AI, but for the second, no alternative was suggested.

Although dispatchers in the first group were always given the option to choose a low-drag route, only 112 of the 1,232 flights in that group ended up taking the alternative route due to operational concerns, such as cost or safety, Sanekommu says.

According to an AI analysis of satellite imagery of flight paths, the number of visible contrails was 62% lower for flights taking the contrail-optimized route suggested to air traffic regulators. When all flights with the option to take a contrail-optimized route are included, the overall effective reduction in contrail formation was 11.6 percent compared to the control group.

“This validated the thesis that if we could find a way to properly and safely integrate into the flight planning process, then this would be a scalable route to avoid contrails on many flights,” says Sanekommu.

The team estimates that the warming effect of flights was reduced by 13.7 percent in the overall group given a suggested route and by 69.3 percent in flights that took the optimized route. There was no statistically significant difference in fuel consumption between groups.

“It’s probably the best you can do, at least with the tools we have now,” says Edward Gryspeerdt of Imperial College London. “This indicates that it is possible. The 62 percent reduction in contrails observed by satellite is unlikely to be a coincidence.”

However, it is unclear how much this 11.6 percent figure can be improved in real-world operations, due to the intricacies of flight planning, says Gryspeerdt. “You can’t necessarily increase that to achieve a 60 percent reduction in contrails on every flight, everywhere in the world, but even a 10 percent reduction in contrails is still a significant effect.”