At cruising altitude, jet engines often leave behind contrails: puffy white streaks that briefly lace the sky before thinning into nothing. Formed when hot, moist exhaust meets frigid, humid air, these manmade clouds have long been more aesthetic curiosity than environmental concern. They have inspired wonder, and occasionally provoked debunked conspiracy theories about toxic chemicals; but for decades they were treated as little more than a fleeting byproduct of flight.

In recent years, however, researchers have documented contrails’ hidden climate connection. Aviation is responsible for roughly 2.5 percent of global carbon dioxide emissions, but that’s only part of the story. Aircraft also release nitrogen oxides, aerosols, and contrails—emissions that, taken together, may warm the planet as much as aviation’s CO₂ output alone, according to a 2025 National Academies of Science report. Most contrails dissipate quickly. But under the right atmospheric conditions, some linger and spread into thin cirrus clouds that trap heat, subtly but persistently altering the climate below.

This growing recognition has turned contrails from an atmospheric footnote into a target of serious scientific and policy interest. One of the people leading this work is Marc Shapiro, an engineer with a background in earth systems modeling and the director of Contrails.org, a project backed by Breakthrough Energy, Bill Gates’s climate initiative. Shapiro hopes to provide airlines with tools to prevent long-lasting contrails. His organization is developing AI models to predict when and where these persistent contrails will form, so that airlines can re-route flights effectively to reduce aviation’s climate footprint.

Shapiro’s interest was sparked five years ago by a striking finding in the scientific literature. A study of Japanese airspace found that just 2.2 percent of flights contribute roughly 80% of the contrail warming impacts in that region. Rerouting an even smaller share, just 1.7 percent of flights, by 2,000 feet could cut that warming impact by up to 60 percent. The implication was hard to ignore: A modest change in how planes move through the atmosphere could yield an outsized climate benefit.

At first, Shapiro and colleagues thought there must be a company they could build to help airlines deliver this climate benefit. After they looked into it and conducted some initial modeling, they found that the research wasn’t yet robust enough to support a commercial endeavor. Instead, they decided to build an NGO to do that research to underpin contrails management.

Shapiro and his team of aviation experts, atmospheric researchers, and software engineers have spent the last four years working with airlines and air traffic controllers to test whether contrail avoidance can work in practice. The problem involves both logistics and atmospheric nuance. If airlines know where contrails will form, could they route planes above or below the thin pockets of cold, humid air? Re-routing flights could mean burning additional fuel, generating more carbon dioxide in the process. But the potential payoff could be large. If airlines can learn to identify and sidestep the small fraction of flights responsible for most contrail warming, they could cut a significant slice of aviation’s climate impact at a cost measured in thousands of feet and tens of dollars per flight.

The Messy Middle

Contrails are tricky beasts. Their warming or cooling potential depends on a number of factors, chiefly how long they last in the upper atmosphere—which is largely determined by the amount of humidity present. If the air is dry or only slightly humid, contrails sublimate quickly. For long-lasting contrails to form, the atmosphere must hold more water vapor than normal, a state called ice supersaturation, in which there’s ample moisture for ice crystals to persist and expand into long-lasting cirrus clouds.

Timing matters, too. Contrails that form during the daylight hours can reflect incoming solar radiation back into space, sometimes producing a modest cooling effect. At night, though, contrails lose that reflective benefit. Instead, they trap outgoing heat, resulting in a net warming impact.

So how can researchers accurately predict which contrails will have a net warming effect? The answer, according to the 2025 National Academies report, hinges on better measurements of the atmospheric conditions at various flight altitudes. “The biggest challenge is confidently predicting the humidity in the upper atmosphere,” said Shapiro. But there are no sensors 20,000 feet up. Instead, researchers have to cobble together what data they can from different sources to feed into contrail formation models. Radiosondes are weather balloons that provide a vertical profile, collecting and transmitting atmospheric measures of humidity, temperature, and wind as they ascend. Geostationary satellite imagery is able to detect the location of ice supersaturation states. And instruments installed on commercial aircraft (dubbed IAGOS for In-service Aircraft for a Global Observing System) can collect flight-bound data on water vapor, nitrogen oxides, aerosols, and cloud particles.

Data limitations mean that the existing contrail models can struggle to accurately predict the persistence of individual contrails. Shapiro is collaborating with Google Research on a model that powers his online dashboard, Contrails Map, which he says does a good job of depicting the contrail outbreak regions. But the evolution of individual contrails—including their length, duration, and climate warming potential—is less accurate. The dashboard currently allows users to compare model output to satellite imagery, but Shapiro plans to also include other data sources on the map in the first half of 2026. Scientifically, he said, we need to be able to attribute contrails back to individual flights to understand the efficacy of interventions.

Some researchers worry that calculating the climate impact of every individual contrail is unnecessary—and even impossible from a cloud physics perspective. “We know that bunches of contrails will occur in specific areas when conditions are right, and that will create an additional impact,” says Andreas Petzold, lead atmospheric scientist at the Jülich Research Center in Germany.

Shapiro agrees that it is unrealistic to observe or measure every contrail formed. Still, he thinks we need some way of attributing contrail warming back to individual flights, and to have measurements across all scales—”from individual contrails to outbreak regions and global contrail coverage.”

Petzold’s recent research, for example, revealed a previously overlooked dynamic at the regional scale. In a study published in November 2025, Petzold analyzed seven years of IAGOS data and concluded that over 80 percent of persistent contrails form in existing cirrus clouds rather than in clear, humid skies. When contrails thicken those clouds, they actually make them more reflective—decreasing their warming potential and possibly having a greater net cooling effect than previously understood.

Other research published last year also suggested that contrails may not result in quite as much warming as originally expected, due to rapid atmospheric changes, for example, in low to mid-level clouds.

Petzold doesn’t want his work to dissuade contrail avoidance efforts. Rather, he said, “we need to get the scientific understanding right before we start any operational procedures.” He added, “Our intention was simply to put a puzzle piece together to avoid wrong detours.” Getting it wrong could not only impact airline regulations, business plans, and flight paths— it could even increase carbon dioxide emissions, if airlines end up spending more fuel to avoid contrails.

But avoiding contrails may ultimately prove to be worthwhile, especially if the details can be worked out. “Compared to other climate measures, contrail avoidance is relatively cheap,” explained Alexander Kunkel, a senior data analyst for clean fuels at Transport Environment, a clean energy advocacy organization in Brussels, Belgium. Kunkel rattles off the most pressing questions: “What will regulation look like? Who is responsible—airlines or air traffic controllers? How will this scale up?”

“We’re in the messy middle right now,” said Shapiro. But, he adds, “even if contrail climate impacts are at the low end of the scientific estimates, contrail avoidance could still have a really big impact.”

Tracking contrails via meteors

Luc Busquin has been a commercial pilot for over 30 years. But even he just learned a few years ago that contrails can impact climate. “I have yet to fly with another pilot who is aware of that,” he said.

Busquin knows that the upper atmosphere is a notoriously data-poor environment. The IAGOS and radiosonde data can identify whether conditions exist for a contrail to form, while geosatellites are able to indicate whether a contrail persisted. But none of these measures could identify which plane formed a contrail.

Two years ago, Busquin noticed a box on his neighbor’s roof. It was a nighttime camera—one of 1,600 in 44 countries that are currently part of the Global Meteor Network, formed in 2018 to detect meteors and analyze their tracks from the ground. Busquin realized immediately that ground-based cameras could reliably match observed contrails with their originating flights.

Busquin borrowed his neighbor’s camera unit. He modified the Global Meteor Network software to also record contrails—during the day as well as at night. “Then I approached Denis,” said Busquin. Denis Vida is the founder of the Global Meteor Network and a meteor physics researcher at Western University in London, Ontario. Contrails were never part of Vida’s research agenda, but he realized swiftly after Busquin contacted him that his network of meteor-surveying cameras were in a position to help researchers ground-truth which flights form contrails.

Vida likens the regions where contrails form to invisible, thin, floating pancakes in the upper atmosphere. These so-called ice super-saturated regions (ISSR) are typically found between 25,000 and 40,000 feet, at air temperatures between roughly -40°C and -60°C. “Contrail model prediction accuracy is very low for the persistent contrails that have an overwhelming climate impact,” said Vida. “For short-lived contrails, the accuracy is closer to 50 percent.” He believes atmospheric models will need to improve the accuracy to at least 80 percent to be perceived as reliable. “Whether or not we’ll be able to achieve this remains to be seen,” he said. “Large uncertainties remain and no one has demonstrated an accurate, fully validated, operational model for contrail mitigation—yet.” But, he added, it could happen in the next couple of years, given how fast the field is moving. Shapiro said he hopes to include Global Meteor Network imagery in his organization’s Contrails Map in the future.

Junzi Sun, who studies the science of air traffic management at Delft University of Technology in The Netherlands, cautions that the highest caliber research is necessary given the stakes for airlines. “If the science is not strong, airlines can push back against any penalties,” said Sun. He also believes it could take 15 years “to sort all this out,” and that the challenges of practically rerouting flights to avoid contrail formation will be particularly difficult in Europe, where it may be challenging to close such congested air space.

As airlines and European regulators seek answers, tensions are rising. In 2025, the European Union’s Monitoring, Reporting and Verification (MRV) system began requiring aircraft operators to monitor and report on aviation’s non-carbon dioxide climate effects, including the impact of contrails. The pilot phase of data collection will go through this year, and the European Commission will use the data to debate future policies in 2027.

Re-routing flight trials

American Airlines in the United States and TUI airlines in Europe are at the forefront of testing rerouting flights to avoid contrails. “This has the potential to be a low-cost, actionable strategy for aviation,” said Jill Blickstein, vice president of sustainability at American Airlines. “But it will be really complicated.”

In 2023, American conducted its first trials with pilots. Pilots already have iPads that display a turbulence screen so they can communicate with air traffic control to avoid bumpy air. Shapiro’s colleagues developed an identical contrails screen so it looked familiar. During 35 out-and back-flights conducted over many weeks to test altitude adjustments to avoid contrail formation, Blickstein said, “we didn’t have any additional fuel burn. Universally, we saw excitement and enthusiasm.”

Last year, American conducted additional trials with flight dispatchers. While the results haven’t yet been released, Blickstein said they learned a lot. For example, some of the proposed contrail-avoidance flight plans were unrealistic for a variety of reasons: either the new route would cross turbulence; or, on occasion, it would burn too much fuel; or it would simply get rejected by Air Traffic Control. Some proposed flight plans were impossible because they crossed NAT tracks—weather-optimized invisible highways in the sky for flights crossing the North Atlantic Ocean. “NAT tracks are set by Air Traffic Control, and once you get on one, you don’t go to another one,” said Blickstein.

Blickstein, Shapiro, and others are collaborating to find a systemwide approach that works for all the players. The flurry of recent studies and flight trials have only added to a growing sense of urgency among climate researchers, policymakers, and airlines to determine to what degree contrail avoidance can be a climate win.

“From all the evidence we’ve seen, this is the place to invest in right now—even if it takes 15 years to get to full scale,” said Shapiro. “Our goal in five years is to have at least a few airlines and a few airspaces doing this as routine practice.” The effort might only target clear skies in a few hot spot regions or below a certain added fuel cost. “But if we’re not doing that,” he said, “we will have failed our mission.”

Virginia Gewin is a freelance science journalist based in Portland, Oregon. Her stories have appeared in Nature, Science, Discover, Washington Post, Modern Farmer and others. See more of her work at www.virginiagewin.com.