As poetic as a contrail – or condensation trail – may be, the white trails left behind by high-altitude aircraft are not nearly as harmless as they appear.
Although they come from only a small fraction of flights, they contribute to global warming at a level comparable to aviation’s CO2 emissions.
A new report by Klima Consulting, commissioned by the environmental organization T&E, states that preventing the formation of these contrails could delay aviation-induced global warming by as much as 10 years.
Heat blanket
When airplanes fly through cold, humid layers of air at an altitude of approximately 8 to 14 km, the jet engine emits water vapor and soot particles, among other things, as it burns kerosene. If the air is roughly colder than -40°C and humid enough, the water vapor and soot particles in the exhaust gases condense into ice crystals. These form the famous white trail.
Most contrails disappear after a few minutes, but when flying through very cold and humid air, those crystals can grow into artificial veil clouds or cirrus clouds that can linger for hours or even days.
During the day, they reflect a small amount of sunlight or allow most of it to pass through, but at night they primarily absorb the Earth’s rising heat. On balance, contrails have a powerful warming effect on the Earth – in fact, as much or more than all the CO2 emitted by aviation.
The good news is that only a small fraction of all flights accounts for most persistent contrails, because most airplanes do not fly through such layers. In 2019, for example, just 3% of all flights accounted for approximately 80% of contrail-induced warming (primarily night flights and flights in the winter).
Impact is significant
However, the warming they cause is significant: the climate impact of non-CO2 effects is at least as great as that of CO2. And if aviation continues to grow unchecked, the sector’s global energy consumption could not only more than double by 2050 compared to 2019, but in 20 years, the warming effect of the contrails from an average flight will actually be greater than that of CO2.
Given current air traffic levels, halving the warming caused by contrails over a 100-year period could save between 67 and 225 Mt of CO2 equivalents annually. This corresponds to 7% to 23% of global CO2 emissions from aviation in 2025, or the annual emissions of 15 to 50 million gasoline-powered cars.
As a result, aviation’s contribution to global warming would not reach 0,08°C until 2050, rather than as early as 2040. If warming from contrails is halved starting in 2030, this could prevent approximately 0,025 °C of warming by 2050 and about 0,049 °C by 2070.
In other words: avoiding contrails would free up 3 to 5% points of the global temperature for 2025-2050 in a 2°C scenario – a budget that would otherwise be consumed by the aviation industry.
Contrail avoidance and slightly more expensive tickets
To help address the problem, T&E proposes implementing contrail avoidance. Under this approach, airplanes fly a slightly modified route or at a slightly different altitude to avoid very cold, humid layers of air. By adjusting just about 3% of flights, more than half of the warming caused by contrails could be avoided.
Scientists have warned, however, that rerouting is only worthwhile if the climate benefits of avoiding contrails outweigh the additional CO2, and that the choice of benchmark makes a big difference.
But rerouting flights consumes extra fuel: up to 5% more CO2 on a single flight, but less than 0,5% for the aviation sector. T&E therefore argues that the climate benefits are at least 10 times greater than that drawback, possibly 100 times greater. The organization estimates the cost at about €4 per Paris-New York ticket and €1 for a flight within Europe.
Cleaner fuels with fewer aromatics and new engines can also reduce soot and thus contrails, although the exact effect is still being studied.
Recommendations
T&E also points out that, despite the significant impact, there are currently no binding measures in place at the regional or global level to reduce aviation’s non-CO₂ climate impact.
In 2025, the European Commission began monitoring non-CO₂ emissions from aviation, and just recently, as part of the revision of the EU Emissions Trading System (EU ETS), it proposed granting free emission allowances to airlines that succeed in avoiding contrails.
And contrary to what the aviation sector used to claim, T&E also argues that the scientific evidence is now robust enough to justify policy measures based on the precautionary principle.
It therefore recommends that the EU expand its monitoring system for non-CO2 emissions to all flights by 2027 in order to increase scientific knowledge about non-CO2 emissions, and that the EU research programs focus on the uncertainties that most significantly affect the mitigation of contrails and hinder action against global warming caused by them, including the prediction of humidity and real-time verification of contrails via satellites.
This winter, a consortium – including the British government and air traffic control services – will conduct Operation Blue Sky, the world’s first trial on the scale of an oceanic airspace to reduce warming caused by contrails over the North Atlantic.
Tests and rerouting measures to avoid contrails are also being conducted as part of the EU projects A4 Climate and CICONIA.
