A way forward for contrail mitigation

Operational contrail mitigation: Avoid flying where and when warming contrails form.

Fuel-based contrail mitigation: Reduce contrail climate impact even when flying when and where warming contrails form.

Contrails are made of tiny ice crystals that form in the aircraft exhaust. Soot particles, produced when fuel does not burn completely, helps these ice crystals form.

Reducing soot emissions generally reduces contrail warming. However, when soot emissions become very low, for example with SAF, hydrogen, or some newer engine designs, other particles emitted or already present in the air can help contrails form instead. This means that there can be substantial contrail formation even when soot emissions are close to zero.

Other factors, such as fuel sulphur content and engine age, also affect contrail formation, but their influence is not yet well understood.

There is also uncertainty about what happens when contrails form in existing natural clouds. Although they are expected to add warming locally, accounting for them could decrease previous estimates of total contrail warming. Scientists do not yet know whether this is significant.

Low risk for navigational contrail avoidance: Reduces effectiveness but does not invalidate the mitigation strategy.

High risk for contrail management via fuel properties: We cannot yet confidently predict whether different fuels will reduce contrail warming, especially in newer jet engines.

Contrails can warm the climate by trapping heat that would otherwise escape to space, but they can also cool it by reflecting sunlight back into space. Because these two effects partly cancel each other out, even relatively small errors in estimating either one can lead to larger errors in the overall climate impact.

The shape of a contrail also matters. Estimating the warming impact of contrails is complex. For instance, their shape influences their cooling effect. Most models simplify these details.

Scientists do not yet have a standard set of data that combines realistic weather conditions, accurate physics, and realistic contrail shapes. As a result, it is difficult to properly test and compare different models.

Low risk for nighttime contrails: Without sunlight, only the warming term remains, eliminating the uncertainty around cooling contrails to a large extent.

Medium risk for daytime contrails: Whether a contrail is cooling or warming can vary with time of day and cloud shape, making daytime mitigation decisions less reliable.

Contrails need water in the air to grow. In busy flight corridors, many contrails can form in the same area and compete for the available water. Once much of that water has been used, any new contrails are likely to be smaller or disappear more quickly. They may also form in contrail clouds left by earlier flights.

This means that if one airline reroutes a flight to avoid creating a contrail, the climate benefit may be smaller than expected if several other aircraft are already creating contrails in the same patch of air.

Most models currently treat contrails as if they form independently, rather than accounting for how they affect each other.

No risk for airspace restrictions: They are an all-or-nothing policy by design.

Low risk in low-traffic regions: When there are few nearby contrails, there is less competition for the available water. Avoiding a contrail in these areas is therefore more likely to deliver the expected climate benefit.

Medium risk in dense-traffic regions: Even when one airline avoids forming a contrail, nearby flights may still create contrails in the same patch of air, potentially reducing the overall benefit of that avoidance.

Contrails can make the air where they form drier. As the ice crystals in a contrail fall, they take some water with them. This may reduce the formation of natural high clouds that also warm the climate, partly offsetting contrail warming. The size of this effect depends on the weather and remains uncertain because it can only be estimated using computer models, which give different results.

Even when this effect is taken into account, contrails that form high above the ground appear to warm the Earth's surface less than an equivalent amount of CO₂, which is spread throughout the atmosphere. Current contrail warming estimates account for this difference, but they remain uncertain.

Medium risk for shifting departure times: Contrail interactions with the atmosphere could decrease the effectiveness of this strategy.

High risk for interventions with a fuel burn penalty: This uncertainty affects the net climate benefit of contrail mitigation. But it does not invalidate avoidance: Even if contrail interactions with the atmosphere are large, flights producing strong contrail warming still justify rerouting. It does mean, however, that current CO₂-equivalent estimates carry significant model-dependent uncertainty.

Contrail formation: Follow up on the ECLIF3 and VOLCAN campaigns to test how SAF and hydrotreated fossil kerosene affect contrail formation in newer engines.

How much heat do contrails absorb: Build better observational datasets to improve estimates of contrail warming and cooling.

Contrail-contrail interactions: Improve models of how nearby contrails affect one another when they form in the same area.

Contrail-atmosphere interactions: Run coordinated comparisons of climate models to assess how contrails affect the atmosphere.