On clear days, thin white streaks trace paths across the sky where planes have passed. Most people barely notice them. Those lines, called contrails, vanish within hours, but their warming effect can matter as much as the carbon dioxide jets pump out over centuries.
Researchers at Chalmers University of Technology and the University of Gothenburg, working with Imperial College London, put numbers to something climate scientists have known but struggled to price. Contrails account for roughly 15 percent of aviation’s total climate impact when measured in economic terms. The study, published in Nature Communications, analyzed nearly half a million flights over the North Atlantic and found the damage varies wildly depending on where and when planes fly.
Carbon dioxide lingers in the atmosphere for hundreds of years. Contrails are different. They form when hot jet exhaust hits frigid air at cruising altitude, creating ice crystal clouds that trap heat radiating from Earth’s surface. The clouds usually last only hours, but during that time they work like a blanket. When conditions align (cold temperatures, high humidity, specific atmospheric layers) the warming effect can be substantial.
What Ice Crystals Cost in Dollars
To compare emissions that behave so differently, the researchers translated climate effects into economic damage. They used a modified climate-economy model to estimate the social cost of both carbon dioxide and contrail cirrus clouds, meaning the monetary value of future harm caused by today’s flights. The calculations factored in different scenarios for warming trajectories, economic growth, and how heavily society should weigh near-term versus distant consequences.
The 15 percent figure represents a global average, but individual flights told a more complex story. About 38 percent of North Atlantic flights produced warming contrails. Others generated none. A small fraction even created contrails with a slight cooling effect, though these were rare. Because contrails form only in narrow bands of cold, humid air, a modest altitude change or slight route adjustment can sometimes prevent them entirely.
This variability creates an opening. The study suggests that avoiding contrails on warming flights would often deliver a net climate benefit, even if rerouting burned slightly more fuel and emitted marginally more carbon dioxide. The trade-off makes sense because a relatively small number of flights generate most of the contrail warming.
“Our calculations can be used for optimisation of flight routes where climate impact is considered alongside, for example, fuel cost and travel time,” Susanne Pettersson explains.
Pettersson, a postdoctoral researcher at Chalmers, notes the goal isn’t to ignore fuel consumption but to weigh multiple climate factors simultaneously. In the team’s main scenario, roughly one third of North Atlantic flights would benefit from rerouting to avoid contrails, assuming only a modest fuel penalty.
Policy Catches Up to Physics
The European Commission is developing rules requiring airlines to report and eventually reduce non-carbon climate impacts, including contrails. The researchers argue that treating all flights as equally harmful misses crucial opportunities. Policies recognizing when and where contrails do the most damage could cut aviation’s climate footprint faster and at lower cost than blanket approaches.
Uncertainties remain. Contrail formation depends on weather conditions that are difficult to predict precisely, and the exact climate response continues to be studied. The ice crystals behave differently depending on atmospheric composition, altitude, and temperature gradients that shift constantly. But the central finding holds: aviation’s climate impact extends beyond fuel burned to include the invisible clouds left behind.
The study offers airlines and air traffic management new tools for climate optimization, but implementation will require balancing climate impact against operational constraints like fuel costs and flight times. Whether the industry moves quickly enough to matter depends partly on regulatory pressure and partly on how accurately weather models can predict where contrails will form.
Nature Communications: 10.1038/s41467-025-64355-5
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