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?”
