The secret way airlines could immediately reduce their climate impact

Contrails could be responsible for more than half of aviation’s climate impact. Mitigating them could be a simple fix.

An airplane creates contrails at sunset.

Let's talk about contrails.

Photo: Frank Rumpenhorst/picture alliance via Getty Images

Among the biggest climate challenges for the aviation industry are the wispy contrails that follow airplanes across the sky.

Those innocuous-seeming puffs are responsible for more than 50% of flights’ climate impact and up to 2% of total global warming, and yet very little is known about them beyond academic circles. While airlines have largely focused their climate plans around reducing carbon dioxide emissions associated with air travel, some very simple tricks to reroute flights could cut down on contrails and have huge — and immediate — climate benefits.

Contrails are essentially clouds of ice crystals. They can form when hot water vapor from jet engines mixes with cold air or when soot from engine exhaust provides nuclei for crystals to form in humid air. Unlike other airplane emissions like carbon dioxide, which generally have a warming effect on the atmosphere, contrails can sometimes have a cooling effect. Their impact on the climate depends on a number of factors, including the time of day, the plane’s height, temperature and other factors.

“One of the major uncertainties in climate change is the role of clouds and water vapor in the atmosphere,” said Michel Gelobter, managing director of climate nonprofit Reflective Earth.

Contrails that are present during the day generally reflect more solar radiation, essentially acting as a shield for the planet. That has a net cooling effect. Those same contrails, however, could lead to warming a few hours later at night, when that solar radiation is absent. When contrails spread out, they create a hazy layer that traps outgoing heat; Ken Caldeira, senior scientist at the Carnegie Institution for Science, likens it to throwing a blanket over the planet.

Being able to accurately and consistently measure how contrails affect the climate would be huge simply because their climate impact is significant. Doing so could also open the door to simple steps that would reduce that impact and buy us a little climate breathing room while the world works to cut carbon emissions. Contrails can dissipate within a few hours, whereas carbon dioxide emitted by planes will remain in the atmosphere for centuries.

There’s also an economic incentive for airlines that want to get serious about protecting the climate. Flying higher or lower to avoid areas that encourage contrail formation is cheaper and easier than developing sustainable jet fuel.

“We could solve the contrail problem within a few years, whereas getting carbon emissions-free aviation fuel is a long way off,” Caldeira said.

Preliminary research supports this thinking. A study from a team of scientists at Imperial College London found that changing flight altitude by only a few thousand feet on fewer than 2% of all scheduled flights could reduce the climate damage of the entire aviation industry by as much as 59%.

Though the fix is simple, there’s a catch: Neither scientists nor the aviation industry know enough about how and when contrails form.

Better prediction requires better observation, which is currently lacking. “There's no good way to account for them yet, but it is progressing,” said Sola Zheng, an aviation researcher at the International Council on Clean Transportation.

To be able to better forecast contrails would require more sensors on airplanes. But unlike nitrogen oxides and carbon dioxide, contrails aren’t regulated by international aviation regulatory bodies, and understanding the humidity and other conditions that influence contrail formation at 30,000 feet hasn’t been a priority for weather forecasting systems, according to Zheng.

Still, some attempts are starting to pop up. Efforts are underway at Google to account for the climate impact of contrails in the flight emissions estimates it provides to users, according to people familiar with the research. A Google spokesperson wrote in an email to Protocol that though the company doesn’t have anything to share on its contrails research at the moment, “it’s definitely a topic of interest.”

Another project, a public contrail map developed by Orca Sciences in collaboration with Imperial College’s Transport & Environment Laboratory, shows roughly 30% of all global air traffic is responsible for creating contrails that have the largest impact on the climate in terms of both heating and cooling high-impact contrails. Caldeira, who wasn’t involved in the research, said the goal is to eventually create a system that would be integrated with the same system that routes planes around turbulence.

The aviation industry is also starting to do some self-examining. Airbus subsidiary Airbus UpNext recently announced a flight test program to study contrails produced by hydrogen combustion engines as part of the company’s zero emissions road map. Airbus UpNext CEO Sandra Bour Schaeffer said in a statement that "contrail characterization is of significant interest to Airbus” and that “understanding their composition will be key to support our decarbonization journey.” Etihad Airways and Boeing have also made commitments to combat contrails.

Still, contrails remain a secondary concern of the airline industry when it comes to reducing damage to the climate. That may in part be because, unlike carbon dioxide emissions, contrails have flown under the public’s radar. Even many of those employed in the aviation industry aren’t fully aware of the scope of their climate impact.

But the tide is starting to turn. In July, the British Airline Pilots’ Association issued a “red warning” for the U.K. government’s decarbonization strategy, arguing that it’s overly focused on carbon dioxide emissions. The union called on the government to “look more into the reduction of contrails and other non-CO2 effects of aviation.”

Ian Poll, a past president of the Royal Aeronautical Society, said in the BALPA statement, “Contrails can be avoided immediately by changing operating procedures and, unusually, the climate benefit is immediate. The gains are large and immediate, the costs are marginal, no new technology is needed and the action is ethical, what more do we need to know and what are we waiting for?”


This carbon capture startup wants to clean up the worst polluters

The founder and CEO of point-source carbon capture company Carbon Clean discusses what the startup has learned, the future of carbon capture technology, as well as the role of companies like his in battling the climate crisis.

Carbon Clean CEO Aniruddha Sharma told Protocol that fossil fuels are necessary, at least in the near term, to lift the living standards of those who don’t have access to cars and electricity.

Photo: Carbon Clean

Carbon capture and storage has taken on increasing importance as companies with stubborn emissions look for new ways to meet their net zero goals. For hard-to-abate industries like cement and steel production, it’s one of the few options that exist to help them get there.

Yet it’s proven incredibly challenging to scale the technology, which captures carbon pollution at the source. U.K.-based company Carbon Clean is leading the charge to bring down costs. This year, it raised a $150 million series C round, which the startup said is the largest-ever funding round for a point-source carbon capture company.

Keep Reading Show less
Michelle Ma

Michelle Ma (@himichellema) is a reporter at Protocol covering climate. Previously, she was a news editor of live journalism and special coverage for The Wall Street Journal. Prior to that, she worked as a staff writer at Wirecutter. She can be reached at

Sponsored Content

Great products are built on strong patents

Experts say robust intellectual property protection is essential to ensure the long-term R&D required to innovate and maintain America's technology leadership.

Every great tech product that you rely on each day, from the smartphone in your pocket to your music streaming service and navigational system in the car, shares one important thing: part of its innovative design is protected by intellectual property (IP) laws.

From 5G to artificial intelligence, IP protection offers a powerful incentive for researchers to create ground-breaking products, and governmental leaders say its protection is an essential part of maintaining US technology leadership. To quote Secretary of Commerce Gina Raimondo: "intellectual property protection is vital for American innovation and entrepreneurship.”

Keep Reading Show less
James Daly
James Daly has a deep knowledge of creating brand voice identity, including understanding various audiences and targeting messaging accordingly. He enjoys commissioning, editing, writing, and business development, particularly in launching new ventures and building passionate audiences. Daly has led teams large and small to multiple awards and quantifiable success through a strategy built on teamwork, passion, fact-checking, intelligence, analytics, and audience growth while meeting budget goals and production deadlines in fast-paced environments. Daly is the Editorial Director of 2030 Media and a contributor at Wired.

Why companies cut staff after raising millions

Are tech firms blowing millions in funding just weeks after getting it? Experts say it's more complicated than that.

Bolt, Trade Republic, HomeLight, and Stord all drew attention from funding announcements that happened just weeks or days before layoffs.

Photo: Pulp Photography/Getty Images

Fintech startup Bolt was one of the first tech companies to slash jobs, cutting 250 employees, or a third of its staff, in May. For some workers, the pain of layoffs was a shock not only because they were the first, but also because the cuts came just four months after Bolt had announced a $355 million series E funding round and achieved a peak valuation of $11 billion.

“Bolt employees were blind sided because the CEO was saying just weeks ago how everything is fine,” an anonymous user wrote on the message board Blind. “It has been an extremely rough day for 1/3 of Bolt employees,” another user posted. “Sadly, I was one of them who was let go after getting a pay-raise just a couple of weeks ago.”

Keep Reading Show less
Nat Rubio-Licht

Nat Rubio-Licht is a Los Angeles-based news writer at Protocol. They graduated from Syracuse University with a degree in newspaper and online journalism in May 2020. Prior to joining the team, they worked at the Los Angeles Business Journal as a technology and aerospace reporter.


The fight to define the carbon offset market's future

The world’s largest carbon offset issuer is fighting a voluntary effort to standardize the industry. And the fate of the climate could hang in the balance.

It has become increasingly clear that scaling the credit market will first require clear standards and transparency.

Kevin Frayer/Getty Images

There’s a major fight brewing over what kind of standards will govern the carbon offset market.

A group of independent experts looking to clean up the market’s checkered record and the biggest carbon credit issuer on the voluntary market is trying to influence efforts to define what counts as a quality credit. The outcome could make or break an industry increasingly central to tech companies meeting their net zero goals.

Keep Reading Show less
Lisa Martine Jenkins

Lisa Martine Jenkins is a senior reporter at Protocol covering climate. Lisa previously wrote for Morning Consult, Chemical Watch and the Associated Press. Lisa is currently based in Brooklyn, and is originally from the Bay Area. Find her on Twitter ( @l_m_j_) or reach out via email (


White House AI Bill of Rights lacks specific guidance for AI rules

The document unveiled today by the White House Office of Science and Technology Policy is long on tech guidance, but short on restrictions for AI.

While the document provides extensive suggestions for how to incorporate AI rights in technical design, it does not include any recommendations for restrictions on the use of controversial forms of AI.

Photo: Ana Lanza/Unsplash

It was a year in the making, but people eagerly anticipating the White House Bill of Rights for AI will have to continue waiting for concrete recommendations for future AI policy or restrictions.

Instead, the document unveiled today by the White House Office of Science and Technology Policy is legally non-binding and intended to be used as a handbook and a “guide for society” that could someday inform government AI legislation or regulations.

Blueprint for an AI Bill of Rights features a list of five guidelines for protecting people in relation to AI use:

Keep Reading Show less
Kate Kaye

Kate Kaye is an award-winning multimedia reporter digging deep and telling print, digital and audio stories. She covers AI and data for Protocol. Her reporting on AI and tech ethics issues has been published in OneZero, Fast Company, MIT Technology Review, CityLab, Ad Age and Digiday and heard on NPR. Kate is the creator of and is the author of "Campaign '08: A Turning Point for Digital Media," a book about how the 2008 presidential campaigns used digital media and data.

Latest Stories