Most people have seen condensation trails (commonly known as “contrails”) before—the long, white clouds that appear behind airplanes in the sky. While contrails look pretty, scientific research has shown that they have a darker side. Contrails make a large contribution to climate warming—about as much as the carbon dioxide (CO2) from aircraft engines burning fossil fuel. The good news is that advances in data science, atmospheric modeling and computing now make it possible to predict when and where climate-warming contrails are likely to form, giving pilots the opportunity to take action to avoid them. As a result, contrail avoidance is becoming a near-term, low-cost way to reduce the aviation industry’s net climate impact. Since business travel is a challenging area of Scope 3 emissions to reduce, contrail mitigation is a key strategy for reducing air travel emissions and achieving net-zero goals.
What are contrails and how do they impact the climate?
Contrails are human-caused cirrus clouds resulting from aviation. These thin ice clouds often appear near airports and typically form at altitudes of 8–13 km (roughly 26,000–43,000 ft). Contrails warm the atmosphere by acting like a blanket, blocking outgoing heat from leaving the Earth. If contrails form during the day, this warming effect is partially compensated by the cooling that happens when they reflect some of the incoming sunlight back into space. Because cooling happens only during the day, and warming happens both day and night, contrails that form during the day might be slightly cooling, but contrails that form during the night are strongly warming (similar to the reason that cloudy nights are warmer than clear ones). Averaged over all aviation, contrails are strongly warming.
The science behind contrail formation
Contrails form when hot jet engine exhaust mixes with very cold air at high altitudes. The water vapor in the exhaust condenses on soot particles also in the exhaust, and freezes to form ice crystals. In most cases, these ice crystals quickly evaporate and the contrail disappears in a few minutes. However, if the ice crystals form in very cold, humid layers of air (called ice-supersaturated regions, or ISSRs) they can last for many hours and spread out to cover large areas. Although these “persistent” contrails only form in less than 10% of flights, they are the ones that are responsible for most of contrails' climate impact.
Policy developments on contrails
The European Union (EU) is starting to pay attention to contrails. In 2023, it passed Directive 2023/958, which requires aircraft operators to report their “non-CO2 emissions”, including contrails, annually starting in 2025. Airlines have begun to prepare for measuring and reporting their contrails impact, and a broad coalition of researchers and technology providers has begun building software tools to support this.
The European Commission will provide yearly information on reported non-CO2 emissions, followed by a final report in 2026. If appropriate, a legislative proposal for mitigating non-CO2 aviation emissions will be made by the end of 2027. The introduction of contrails reporting in the aviation industry could have important impacts on greenhouse gas accounting rules. Currently, these rules only require fuel-based emissions to be included, but companies may one day need to include contrails in their carbon accounting for business travel and air-based shipping.
What stakeholders can do about contrails
There are two basic ways to minimize the climate impact of contrails: (1) by adjusting flight paths to avoid ISSRs, and (2) by using sustainable aviation fuel (SAF), which can shorten contrail lifetimes. Because contrail formation is an active field of research, some uncertainties in the exact numbers remain. Nevertheless, the trends are clear. Avoiding ISSRs and using sustainable aviation fuel can have large impacts on reducing contrail-caused warming.
Adjusting flight paths to avoid ISSRs is known as “navigational avoidance”. Even though ISSRs typically cover large areas, they are thin, so small changes in altitude can often be enough to avoid them. Recent trials have demonstrated this in practice. While rerouting flights might lead to higher fuel consumption, one study estimates that using rerouting algorithms based on contrail predictions could avoid up to 65% of contrail-caused warming effects while increasing fuel consumption by less than 0.2%. From an operational perspective, this would require changes to the trajectories of fewer than 3% of flights, and would not require any changes to aircraft or fuel infrastructure.
Because sustainable aviation fuel has a higher hydrogen density than conventional jet fuel, it burns more efficiently and produces less soot. With less soot in the exhaust, the ice crystals that make up a contrail become larger, making the contrail heavier so that it sinks out of the ISSR faster and evaporates more quickly. A recent study estimates that contrail-caused warming could be reduced by up to 45% if 100% sustainable aviation fuel was applied to all flights. While SAF is only available in small quantities today, allocating the available fuel to contrail-prone flights could optimize its climate benefits. Simulation-based estimates suggest that up to 10% of contrail warming could be avoided by using a 50% SAF blend for just 2% of flights.
The bottom line
Flight contrails significantly contribute to global warming. Starting at the end of 2024, flight operators operating inside the European Economic Area will have to start reporting on their contrail emissions. This could drive changes in carbon accounting across other industries in the mid-to-long term. For those who fly, it is important to understand that your carbon footprint might be larger than you think and that incorporating contrails in your footprinting is essential to understanding your full carbon impact.
Carbon Direct believes that, in addition to purchasing sustainable aviation fuel, airlines should begin adopting navigational avoidance now or in the near future. This will have important climate benefits, and ensure that large, new emissions from contrails aren’t added to their Scope 1 emissions, and the Scope 3 emissions of their passengers, if greenhouse gas accounting rules are updated. Companies that frequently use air transportation—either passenger or cargo—should ask airlines about contrail avoidance and encourage them to take action.
Tags
Carbon Reduction
Climate Strategy
Carbon Accounting
