August 7, 2026 · Tags: climate, aviation, contrails
You have seen them on any clear day: the white streaks trailing behind jets, slowly spreading into thin, hazy clouds. They look harmless. But those contrails are actually the biggest single source of warming from flying, and the industry has spent decades treating them as a footnote.
The Numbers #
Aviation accounts for about 3.5% of human-caused climate forcing. That number gets cited a lot. What gets left out is that only about a third of that comes from CO2. The rest around two-thirds comes from non-CO2 effects, and contrail cirrus dominates by far.
The most recent global estimates put contrail cirrus effective radiative forcing at 57 mW/m² for 2018, with a range of 17 to 98 mW/m². A 2024 study using real ADS-B flight tracking data put the 2019 number at 62.1 mW/m². That is roughly three times the warming from all the CO2 aviation has ever emitted (Teoh et al., 2024).
How a Thin Line of Ice Traps Heat #
Contrails form when jet engines emit water vapor and soot particles into air that is cold and humid enough at cruising altitude (8-13 km up). In ice-supersaturated conditions, those particles grow into ice crystals that can persist for hours and spread into cirrus clouds.
The warming mechanism is straightforward. Contrails reflect incoming sunlight back to space, which cools the surface. But they also trap outgoing infrared radiation, which warms it. The warming effect wins. Nighttime contrails are especially bad because there is no sunlight to offset the trapping.
The 2% That Does 80% of the Damage #
Here is the part that makes this solvable. A 2024 analysis found that only 14% of flights produce contrails with a net warming effect. And just 2% of all flights are responsible for 80% of the total warming impact. The rest either produce no persistent contrails or produce ones that actually cool.
You do not need to fix every flight. You just need to fix the ones that matter. The concentration is even more extreme regionally. Europe sees contrail radiative forcing of 876 mW/m². The US sees 414 mW/m². Both are an order of magnitude above the global average (Lee et al., 2021).
What We Can Do About It #
Climate-optimized flight planning is the most promising near-term fix. Instead of routing every plane the same way, you reroute flights around ice-supersaturated areas only when the climate benefit is large. Studies show this can cut contrail climate impact by 12 to 21% with a cost increase of just 0.2 to 2%. The EU is already moving in this direction. Starting in 2025, operators must report non-CO2 impacts, and by 2028 the Commission plans to propose actual mitigation rules.
The main obstacle is uncertainty. The confidence interval on contrail forcing is wide. Predicting where ice-supersaturated regions will form is hard. And there is a real tradeoff: rerouting to avoid contrails can burn more fuel, which means more CO2. But the social cost ratios suggest that the benefits of avoiding the worst contrails outweigh the extra CO2.
Why This Matters #
Contrail cirrus is the aviation sector's blind spot. CO2 gets the attention because it is long-lived and measurable. But contrails are right now warming the planet about three times as much as the CO2 from the same flights. The difference is that contrails are short-lived. Stop making them, and the warming stops within hours. Air traffic is growing at 3.6 to 4% a year, and by 2050 contrail forcing could triple. The infrastructure, prediction tools, and regulatory framework need to catch up fast.