A young child watches a plane create a contrail against a clear, blue sky.

Frequently Asked Questions

If you already know about contrails and how they influence the climate, learn more in the scientific literature.

1. What are condensation trails (contrails)?

Contrails are line-shaped ice clouds that form behind an aircraft when flying at high altitudes through regions with high humidity and low temperatures.

2. How are contrails made?

Condensation trails are created when soot and water vapor are emitted from an aircraft's jet engines in cold and humid atmosphere. Water condenses on soot particles (and other background aerosols) and freeze into ice crystals that make up the white lines behind the airplane.

Contrails are said to persist when they last for more than a few minutes. Contrails persist when the atmosphere stays cold and humid (technically, supersaturated with respect to ice) and ice particles can grow and spread with the wind.

3. How are contrails contributing to climate warming?

The current scientific consensus is that contrails contribute to a large net warming effect on the climate.

Historical CO2 emissions from aviation are responsible for about 1.5% of anthropogenic global warming today. Contrails globally are estimated to add another 1 - 2% of additional climate warming.

4. How big is the impact of contrails on global warming?

The current scientific consensus is that contrails contribute to a large net warming effect.

Historical CO2 emissions from aviation are responsible for about 1.5% of anthropogenic global warming today. Contrails globally are estimated to add another 1 - 2% of additional climate warming.

The climate impact of an individual contrail depends on how long it persists, the time of day, and seasonal effects. Contrails only form 35% of the time at cruise altitude.

When a contrail forms, it may be short-lived (dissipating in less than a few minutes) or persist into a contrail cirrus cloud (becoming indistinguishable from natural cirrus clouds). Only around 5% of contrails that form last longer than a couple of minutes.

5. What can be done to mitigate contrails?

Scientists, companies, and airlines worldwide are working together on solutions to reduce contrail impacts on the climate. The best solution right now is for airplanes to minimize flight time in areas where contrails will have a high climate impact. This can be achieved by re-routing flights, similar to avoiding turbulent air.

Using sustainable aviation fuel (SAF) and cleaner-burning engines reduces the number of soot particles in the aircraft exhaust, which can also be effective at reducing contrail climate impacts.

6. Why are airlines not avoiding contrails if they are so bad for climate change?

Precise contrail forecasting is a really challenging science problem.

Solutions for contrail mitigation are under active development, with many researchers and airlines conducting contrail avoidance trials today.

We are focused on getting the first easy-to-use solutions ready for broader testing in 2023.

7. Do all airplanes need to adjust their routes to avoid making contrails?

No, only about 5% of all planes are responsible for 80% of the annual contrail climate impacts. In many cases, we can adjust their flight paths slightly to significantly reduce their contrail climate impact.

8. Do all planes make contrails?

Contrail formation depends on the type of aircraft, cruising altitude, and the type of fuel it is burning.

Propeller planes and helicopters do not make contrails. Newer jet engines emit fewer soot particles, so the contrails they create are thinner, less persistent, and overall less harmful. The same goes for planes using SAF (sustainable aviation fuel) or low aromatic fuels, which overall produce less harmful contrails.

9. What are the side effects of contrail mitigation?

When planes are rerouted above, below, or around a contrail-forming weather area, they may need to use a little more fuel. In general, the few planes that need to be rerouted for have an added fuel burn of less than 1 - 2%.

Contrail forecasting uncertainty means that some of the planes rerouted will still create harmful contrails, in addition to the fuel penalties incurred.

While contrail forecasts are still improving, we can set a threshold for adding fuel to avoid contrails. This approach minimizes the risks of interventions and ensures the overall positive climate effect of contrail mitigation far outweighs the added emissions.

10. How are contrails regulated?

Contrails are not regulated in any form despite their significant climate impact.

11. Why are contrails only forming behind some flights and not others?

The air must be cold and humid for contrails to form behind a plane. If it is too warm or too dry, water vapor from the engine will not condense or freeze on the exhaust particles.

12. What is the difference between warming contrails and cooling contrails?

Contrails created under cloud-free conditions early in the day are often cooling because they reflect some of the light from the sun back into space before it reaches Earth.

Contrails have a warming effect on climate by absorbing heat that would otherwise escape to space. In a sense, they act like a blanket, and their effect is to ultimately raise the surface temperature on Earth.

The average net effect of all contrails is to be warming to the climate. In particular, contrails that form at dusk and nighttime, when there is no incoming sunlight, can have a more severe warming effect.

13. Do contrails matter for global warming if it is already cloudy or overcast?

Yes—in fact, the warming effects of contrails are often more prominent when they are formed above low-level water clouds. This is because the incoming solar radiation would have been reflected by the low-level clouds regardless of the contrails. Some of the heat reflected by the low clouds is reabsorbed by the higher contrail cirrus, increasing the overall net warming effect.

14. Do artificial contrail cirrus clouds behave differently than natural ones?

Naturally occurring cirrus clouds have the same significant influence on climate as contrail cirrus clouds. Both reflect the sun during the day and keep earth’s surface warmer at night. In general, both trap more heat than they reflect back into space and contribute to global warming.

However, researchers have noticed slight differences in the particle composition of natural and contrail cirrus clouds depending on the age of the cloud. These are topics requiring further scientific research.

15. What are chemtrails and are they the same as contrails?

Chemtrails describe a conspiracy theory claiming that chemical substances are secretly being sprayed from the sky for nefarious purposes. There is no evidence supporting the conspiracy theory of chemtrails. BBC wrote about chemtrails here.

16. What can I do?

The significant impact of contrails on climate is still a secret to many people. For change to happen, the issue needs to be known.

Contrail climate impact is yet another big climate problem, but the good news is that this one comes with a cheap and easy solution that will be soon implemented.