Airlines know by now that carbon dioxide emissions aren’t their only planet-warming problem. Condensation trails, which form in an aircraft’s wake, can also contribute to climate change in outsize ways. Yet while companies say they’re working to curb CO2 — including by developing cleaner fuels and more efficient jet engines — the industry has done relatively little to confront the wispy, white “contrails” that streak the sky.
A new aviation initiative aims to kick-start those lagging efforts.
The Contrail Impact Task Force, announced on Monday, joins major airlines and aircraft makers together with leading contrail researchers. Alaska Airlines, American Airlines, Southwest Airlines, United Airlines and Virgin Atlantic — along with Airbus and Boeing — are partnering with scientists from Imperial College London, Google Research and software development firm Flightkeys.
RMI, a clean-energy nonprofit, and Bill Gates–founded Breakthrough Energy are spearheading the task force, which met for the first time in October at Southwest’s Dallas headquarters. (Canary Media is an independent affiliate of RMI.)
“Contrails have been the elephant in the room for the aviation sector for decades,” said Andrew Chen, who heads aviation decarbonization in RMI’s Climate-Aligned Industries program.
It has long been unclear just how much contrails contribute to climate change. But recent research has shed more light on the issue. A 2020 study by the European Union found that contrails and other non-CO2 aircraft emissions warm the planet twice as much as the carbon dioxide released by airplanes. And a 2021 study found that, while aviation contributes about 2.4 percent of global annual CO2 emissions, flying is actually responsible for 4 percent of global warming when all factors are included.
“We’re now at a place where we know the impact [of contrails] is quite significant,” Chen said from RMI’s New York City office. “That means we have to do something about it.”
Participants in the new task force have two main goals. First, they will seek to improve scientific models that predict when and where heat-trapping contrails are most likely to form in the atmosphere. Second, the task force will work to develop flight-planning tools that allow planes to maneuver around those particular regions. The research groups will use satellite imagery, aircraft sensors and computer algorithms to gather data and verify their results, while the airlines have agreed to share key information about their flight routes and operations.
“Most of the research today has been focused on exploring how flight-planning tools might be able to avoid contrail formation in theory,” Helen Giles, Southwest’s director of environmental sustainability, told Canary Media.
“But the part that’s been missing is how that theory could actually be implemented in a complex operational setting,” she said, adding that Southwest hopes its participation “will yield really practical mitigation solutions.”
A handful of other airlines are also starting to confront contrails, though the RMI-led task force is the latest and largest collaboration of its kind. Etihad Airways, the national airline of the United Arab Emirates, is working with U.K. technology firm Satavia to run dozens of “contrail prevention flights.” In early October, Delta Airlines and MIT’s Department of Aeronautics and Astronautics announced a partnership to test and develop tools to eliminate harmful contrails, with plans to make their results publicly available.
Pam Fletcher, Delta’s chief sustainability officer, said the project “has the potential to make a major impact on our environmental footprint within just a few years.”
The contrail initiatives come as airlines and other large corporations are reportedly struggling to meet the ambitious goals they’ve set for tackling climate change.
Delta, for instance, failed to meet its corporate emissions-reduction target in 2021. Instead, the airline bought $137 million worth of carbon offsets to make up the difference, The Washington Post recently reported — a common industry practice that critics say does little to benefit the climate. Delta says it’s now working to directly slash emissions instead, including by electrifying ground equipment at airports, researching jet-fuel alternatives and collaborating with MIT on contrail avoidance.
Contrails may be easier to clean up than CO2, but they are harder to understand
To deeply cut CO2 emissions, the aviation industry will need to revamp the aircraft models and fuel sources it uses today to haul passengers and cargo through the sky. The steps needed to avoid heat-trapping contrails are less dramatic.
As RMI’s Chen and other experts see it, the industry can start tackling this particular form of climate pollution today — using existing aircraft and fuels — as it pursues the longer, costlier mission of decarbonizing aviation. Replacing fossil jet fuel with lower-carbon alternatives such as sustainable aviation fuel, hydrogen and batteries will likely take decades to accomplish.
Yet while curbing CO2 from air travel is relatively harder to do, carbon emissions are relatively easier to monitor and quantify compared to contrails. Whenever jet engines burn fossil fuels, they release CO2 into the atmosphere, where the greenhouse gas can persist for decades or centuries. It doesn’t matter where or when the fuel is burned; it’s how much fuel a plane consumes that determines the CO2 impact of a given flight.
Contrails are far more finicky. A cloudy white line slicing through the sky isn’t always a climate threat — but when it is, it’s especially potent.
Put simply, contrails form from the soot and water vapor that spews from jet engines. The hot, humid exhaust mixes with the cold, low-pressure air found high in the sky, creating white trails of condensation. Most of the time, contrails quickly disappear.
But at certain altitudes and in certain atmospheric conditions, the water vapor can attach itself to soot particles and form ice crystals. Rather than dissipate, these crystals become cirrus clouds that fan out and persist — trapping the heat radiating off the Earth’s surface and contributing to global warming. This isn’t necessarily a problem during the day. Clouds also reflect sunlight back into space and cool the Earth’s surface, balancing or even canceling contrails’ heat-trapping effect. At night, however, that doesn’t happen.
“It’s those persistent nighttime contrails that are most important, in terms of the climate effects,” Marc Stettler, who leads the Transport and Environment Laboratory at Imperial College London, said in an interview. The plane’s fuel blend and engine type can also influence contrail formation.
It turns out that persistent contrails aren’t very common. They primarily form when a plane flies into an “ice-supersaturated region” where the atmosphere’s humidity is very high and the air is incredibly cold. In a 2020 study, Stettler and co-authors found that just 2 percent of flights in Japan’s airspace were responsible for 80 percent of warming from contrails. In simulations, the researchers found that flying planes 2,000 feet higher or lower than their actual flight paths would’ve reduced contrails’ warming effects by nearly 60 percent.
The team later studied contrails in the North Atlantic region, which included more long-haul flights. They found that roughly 10 percent of North Atlantic flights were responsible for the majority of warming.
“It’s quite an important message for the industry,” Stettler said. “You don’t have to avoid contrails on all flights.”
Scientific uncertainty has slowed progress on contrails — until now
If planes can avoid flying in ice-supersaturated regions, they can avoid creating persistent warming contrails, according to the latest research. But this seemingly simple fix is tricky to achieve in practice.
That’s because scientists can’t always accurately predict where those icy regions will occur. Certain factors like relative humidity are difficult to forecast in the upper atmosphere. This makes it challenging for flight planners to know how they should adjust a plane’s path — or whether their decision has the intended effect of reducing global warming. A key concern within the task force is that planes could wind up burning more fuel in an effort to avoid contrails, emitting more carbon overall and creating a potential trade-off between addressing CO2 and non-CO2 emissions.
“Uncertainty is the biggest problem with this field,” said Marc Shapiro, the contrails project lead at Orca Sciences, a research group that’s now part of Breakthrough Energy.
Because of that uncertainty, aviation regulators and airline executives have been slow, if not reluctant, to formally address contrails in their sustainability strategies.
Last month, when 193 countries adopted a global agreement to curb air-travel emissions, they didn’t include contrail formation or other non-CO2 effects. Instead, members of the International Civil Aviation Organization agreed to achieve net-zero emissions by 2050 for carbon dioxide only.
In July, Google quietly removed contrails from the model it uses to calculate the climate impacts of flight choices. The tech giant said it worried it was showing inaccurate information to consumers, given the complexity in determining whether a contrail is benign or a climate culprit.
A senior manager for United Airlines recently acknowledged the company has done “relatively nothing” to address contrail formation because it’s waiting for the “right tools” to emerge. “The challenge here, and really it’s common for a lot of airlines, is we’re waiting for the science to harden,” Aaron Robinson said during a virtual workshop hosted by the NASA Aeronautics Research Institute in September.
The emerging partnerships between scientists and aviation companies are an attempt to overcome that inertia, Shapiro said.
“When you talk to the industry, they’re like, ‘Look, we recognize this is a problem, but we’re not going to do anything about this until the dust has settled,’” he said. “We’re taking the approach of: ‘OK, let’s settle the dust.’”
Designing a “drop-in” solution for a complex problem
Three participants in the RMI-led task force — Breakthrough Energy, Google Research and Imperial College London — have already been collaborating closely on contrails. The organizations are advancing research into questions such as when contrails have the biggest warming effect, based on factors like time of day, season, region and latitude.
Separately, MIT is leading efforts to observe actual contrails using NASA satellite imagery and 3D laser scanning tools. The university, which is partnering with Delta, then uses the view from space to develop algorithms that predict the altitude and locations where persistent contrails are likely to form next.
Ultimately, the task force plans to develop the equivalent of a new weather layer in flight-planning software, said Chen of RMI. Airlines use flight-planning systems before and during flights to keep planes from flying into storms, hitting air pockets or crossing paths with another plane. The goal is to create another layer that adjusts planes’ cruising altitude and keeps them from passing through icy regions where contrails are likely to form, he said.
As part of the task force, Southwest will help design flight trails to ensure the tools are not only accurate but also don’t interfere with airlines’ other competing priorities, such as safety, customer comfort and reaching destinations on time, said Giles.
“We need to make sure they work operationally, and not just on paper,” she said of contrail tools. The task force hasn’t yet announced a timeline for launching real-world trials, though participants plan to meet soon with the Federal Aviation Administration to get the agency’s perspective on the project.
While developing contrail-avoiding measures is a big challenge, getting airlines to actually use them will also likely require a major push.
At the NASA workshop in September, United Airlines and Etihad Airways both spoke of the need for financial incentives or mandatory schemes. Without a carrot or a stick, they suggested, airlines aren’t likely to invest the time and effort to reduce contrail formation on their own. The same has been true of the industry’s sluggish efforts to use sustainable aviation fuel, which is only now starting to ramp up owing to new tax incentives in the United States and emissions regulations in the European Union.
The hesitation reflects a broader tension simmering beneath industry discussions around contrails, one that’s likely kept the issue on the back burner for decades, Chen said. So long as contrails remain complex and uncertain, airlines face less pressure to deal with them.
“In showing that you’re developing a solution to a problem,” he said, “you have to acknowledge that problem.”