Flyers can identify the most-warming flights when booking

When we think about the climate impact of airplanes, the image that usually comes to mind is the invisible trail of carbon dioxide gas left behind by burning jet fuel. This gas traps heat in the atmosphere and stays there for centuries, slowly warming the planet. However, there is a second, more immediate effect that happens high in the sky, often unnoticed by passengers looking out the window. When planes fly through cold, moist air, they leave behind long, white streaks of ice crystals known as contrails. These streaks can spread out and form thin, high clouds that act like a blanket, trapping heat radiating from the Earth below. Unlike carbon dioxide, which lingers for generations, these clouds can disappear in a matter of hours. If we could stop them from forming, the warming they cause would vanish just as quickly. The problem is that not every flight creates these heat-trapping clouds, and those that do are not all equally harmful. Some flights create a massive amount of warming, while others create very little or even have a slight cooling effect, depending on the time of day, the route, and the type of aircraft.

A team of researchers has now shown that it is possible to predict which flights will be the worst offenders for this type of warming, even before a passenger buys a ticket. By analyzing data from millions of commercial flights, they developed a method to identify the specific journeys that are most likely to create heat-trapping clouds using only the information available on a standard flight schedule. Their work suggests that if travelers and companies could simply choose to avoid these specific flights in favor of similar alternatives, they could significantly reduce the aviation industry's contribution to global warming without needing to change the aircraft or the routes themselves.

The researchers started by building a massive database of flight data from 2019 and 2021, covering over 52 million commercial flights. They used a sophisticated computer model that simulates how contrails form and evolve, feeding it real weather data to calculate exactly how much warming each individual flight caused. This process revealed a striking pattern: the warming effect is not spread evenly across all flights. Instead, it is highly concentrated. The study found that a tiny fraction of flights is responsible for the vast majority of the warming. Specifically, the worst 5% of flights accounted for roughly 95% of the total warming caused by contrails. The remaining flights contributed very little, and some even had a net cooling effect because they formed during the day when the ice crystals reflected sunlight back into space.

The key to this discovery was understanding what makes a flight "bad" for the climate. The researchers identified that the most harmful flights tend to share specific characteristics that are visible on a flight booking website. Flights that travel long distances, especially those that fly at high latitudes like over Europe or the northern United States, are more likely to create warming clouds. The time of departure matters immensely; flights that take off in the afternoon or evening are far more damaging than morning flights. This is because afternoon and evening flights leave trails that persist into the night, when the ice crystals can only trap heat without reflecting any sunlight. Morning flights, by contrast, usually form trails that exist only during the day, where the cooling effect of reflecting sunlight can offset the warming. The type of aircraft also plays a role, with certain older or less efficient engines producing more soot, which leads to the formation of more persistent ice crystals.

With these patterns in mind, the team trained a computer program to act as a predictor. They taught the program to look only at the basic details available when a ticket is purchased: the origin and destination, the scheduled departure time, the aircraft type, and the distance of the trip. They deliberately excluded any real-time weather data, because that information is not available weeks or months in advance when people book their travel. The goal was to see if the schedule alone could predict which flights would be the worst. The results were surprisingly effective. The model successfully identified the top 10% of flights that would cause the most warming, capturing about 68% of the total warming effect. Even more impressively, it could pinpoint a smaller group of just 5% of flights that were responsible for nearly half of all the warming. This means that a traveler does not need to know the complex science of atmospheric physics to make a better choice; they simply need to know which specific flight number to avoid.

To test how this would work in the real world, the researchers applied their model to the travel records of two large corporations. They looked at the actual flights taken by employees of these companies and calculated how much warming those flights caused. Then, they simulated a scenario where the companies avoided the flights their model flagged as the worst. If the companies had simply swapped those bad flights for other flights on the same route that were predicted to be less harmful, they could have reduced their total contrail warming by between 14% and 29%. This reduction was achieved even with a practical constraint: the replacement flight had to depart within 16 hours of the original. If the companies had been willing to accept any flight on the same route regardless of the time, the reduction could have been as high as 54%. The study showed that for many busy routes, there are often multiple flights available at similar times, making it easy to swap a high-warming flight for a low-warming one without changing the itinerary significantly.

The researchers also explored why some flights are worse than others by looking at the specific features that their model relied on. The most important factor was the distance of the flight, followed closely by the type of aircraft and the amount of daylight the flight would encounter. Flights that spend a large portion of their journey in darkness, particularly over high latitudes during the winter, are the most likely to create persistent, heat-trapping clouds. The model also highlighted that the fraction of the flight path that goes over land versus water was the least important factor. This suggests that the geography of the route matters less than the timing and the machinery of the plane. The study confirmed that the model's predictions held true across different years and different types of weather patterns, giving confidence that these are reliable, structural patterns in how aviation affects the climate.

This work challenges the idea that all flights are equally bad for the climate and that the only solution is to fly less or wait for new technology. Instead, it suggests that immediate, significant reductions in warming are possible through smarter choices at the moment of booking. While airlines are already exploring ways to reroute planes in real-time to avoid bad weather, this research shows that passengers and corporate travel managers can play a role weeks or months in advance. By choosing a flight that departs in the morning or selecting a different aircraft type, travelers can avoid the specific conditions that lead to the most damaging clouds. The researchers have even begun developing tools to make this information available to the public, such as a browser extension that could show the warming potential of a flight directly on booking websites.

The study does have its limits. The predictions are based on simulations and historical data, not on direct measurements of every single flight's future impact. The model works best when there are many alternative flights to choose from, which is true for major routes but might be harder for smaller, less frequent connections. Additionally, the researchers note that the warming from contrails is temporary, lasting only as long as the flight continues to create them, whereas carbon dioxide emissions last for centuries. This means that while avoiding contrails provides an immediate cooling benefit, it does not replace the need to reduce carbon emissions. However, the ability to identify and avoid the worst flights offers a powerful, immediate lever to reduce the aviation industry's total impact on the climate. It turns the act of booking a flight into an opportunity to make a specific, measurable difference, proving that the most warming flights are not a mystery, but a predictable part of the schedule that can be avoided.