Jet fuel is bad for the environment. Contrails are even worse.

Airplanes constantly emit the trails every time they fly, blasting pollutants into the atmosphere. The industry behind them doesn't know how to fix it.

The fact that airplanes are climate-­damaging fuel hogs—aviation accounts for two per cent of human-caused climate change—has been obvious to the travelling public for some time. What’s becoming increasingly clear, though, is that spending even more jet fuel may be necessary to deal with the sector’s bigger contributor to the heating climate: contrails. As the airline industry puzzles over how to decarbonize, researchers are rapidly gathering an understanding of how these anthropogenic cloud formations add to global warming, and how they might be avoided.

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Planes constantly emit a trail of substances, including carbon dioxide, water vapour and black carbon (soot). When aircraft pass through patches of cold, humid air, the water vapour and large soot particulates combine to form a long stream of ice particles. The ones that disappear quickly aren’t a problem, explains Sebastian Eastham, research scientist at MIT’s Laboratory for Aviation and the Environment. But the formations that persist for hours can form human-made cirrus clouds, which trap huge amounts of thermal radiation that would otherwise escape into space. With contrails, Eastham says, “you have this large, sudden contribution to global warming, where you have caused the Earth’s atmosphere system to retain a significant amount of additional energy.” Carbon dioxide, by comparison, has a less acute but more prolonged energy-trapping effect.


Much of aviation’s challenge, then, is figuring out how flights can avoid the patches of cool, humid air that are ripe for creating contrails. Their locations are hard to predict—varying hour to hour—so it’s an air traffic control and modelling problem. There is a theory that temporarily flying higher (or lower) for brief stretches of some flights can create huge savings in contrails at the cost of a relatively small amount of extra fuel burn and carbon emissions—emitting a bit more to save the planet, as it were. It’s the “low-hanging fruit” for slashing aviation’s climate impact, the Royal Aeronautical Society’s John Green said at a conference last May. The industry has begun turning simulations into real-world examples: last fall, United Arab Emirates’ Etihad Airways teamed with a U.K. flight analytic firm to adjust the path of a Boeing 787 travelling from Heathrow to Abu Dhabi and says it avoided producing the equivalent of 64 tonnes of CO₂ by emitting only 0.48 extra tonnes.

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Another contrail avoidance option—well, besides, flight avoidance altogether—is flying with alternative fuels. The National Research Council of Canada (NRC) has experimented with jets burning crop-based biofuels, which are less carbon-intensive over their life cycle than jet fossil fuels. They don’t necessarily produce lower in-flight carbon emissions than jet fossil fuels, says Anthony Brown, research pilot engineer with the NRC, but they substantially reduce the large soot particles that help create contrails. Given the unpredictability of when flights will hit contrail-prone skies, using different fuels is a more definitive way to tackle this problem than changing flight paths, Brown says.

But it will be years before either solution scales up to industry-wide usage. So while the conspiracy fanatics who baselessly fear “chemtrails” remain as wrong as ever, there is reason to look up, see lingering jet exhaust clouds and get a bit anxious.

This article appears in print in the March 2022 issue of Maclean’s magazine with the headline, “Menace in the mist.” Subscribe to the monthly print magazine here.