Frequently asked questions

Contrails may look harmless — but they are one of the most underestimated climate effects of aviation. On this page, we answer the most important questions: how contrails form, how they affect the climate, and how they can be avoided in practice. This is about science, policy, costs and the question of why a comparatively straightforward solution is not yet standard practice. We are especially interested in the perspective of people working in flight planning, the cockpit, air traffic control, research or airline operations. Because contrail avoidance will only work if it is scientifically sound — and if it works in real-world aviation.

1. What are contrails — and why do some disappear while others stay?

Contrails form when hot, moist exhaust from aircraft engines meets very cold air at high altitude. The water vapor freezes onto tiny particles and becomes ice crystals — visible as a white line in the sky. Some contrails disappear after a few minutes. Others persist because the aircraft is flying through ice-supersaturated air, where the crystals can keep growing and spread into thin cirrus clouds.

2. Are contrails chemtrails?

No. Contrails are not a secret spraying program, but a well-understood physical phenomenon. They form through water vapor, cold temperatures, particles and specific humidity conditions at high altitude. Their climate impact has been studied for decades by atmospheric scientists, aviation researchers and climate scientists. That is exactly why this issue matters: not because something is being hidden, but because something visible has been politically overlooked for far too long.

3. Are contrails a new problem — or have they always been there?

Contrails have existed since aircraft started flying at high altitudes. What is new is not the white line itself. What is new is the scale of aviation — and our understanding of how strongly persistent contrails can warm the climate. What once looked like a side effect is now a measurable climate problem. And one we could address much faster than many other aviation emissions.

4. How much do contrails really warm the climate?

Contrails are among the most important non-CO₂ effects of aviation. Depending on weather, route, time of day and the metric used, their climate impact can be in the same range as aviation’s total CO₂ impact — and in some assessments even higher. The most problematic ones are persistent contrails that spread into artificial cirrus clouds. That may sound like a detail. But it is one of the biggest blind spots in aviation climate policy.

5. I work in aviation — how can I contribute?

If you work in flight planning, the cockpit, air traffic control, aviation weather, airline operations, software or climate science: we need you. The solution does not only depend on studies, but on practical knowledge — how routes are planned, how clearances work, which tools are actually usable, and where the system gets stuck. Our chapter brings together people who know these questions from their professional practice. The goal is a place for exchange, implementation and pressure in the right systems.

6. Why are night flights in winter especially problematic?

Contrails do not always have the same effect. During the day, they can reflect some incoming sunlight; at night, that cooling effect disappears. What remains is mainly the warming effect: artificial cirrus clouds trap heat in the atmosphere. In winter, the relevant cold and humid layers are more common. That is why winter night flights are especially important for targeted contrail avoidance.

7. How reliable are the “⅔ / ~2%” numbers?

The short version: The numbers are robust enough to take contrail avoidance seriously. The “two thirds” figure describes the broad finding that a large share of aviation’s climate impact does not come from CO₂, but from non-CO₂ effects such as contrails, nitrogen oxides and water vapor. The “around two percent” figure refers to the approximate contribution of contrails to human-made global warming. Scientific uncertainties remain — especially around the exact strength of individual effects. But these uncertainties do not change the fact that the overall scale is relevant and that targeted avoidance should be seriously tested and implemented.

8. Why don’t aircraft already fly differently?

Because aviation is optimized for safety, efficiency, cost and punctuality — not for contrail avoidance. Flight routes are planned according to many factors: weather, traffic, airspace, fuel, slots, air traffic control and airline requirements. For a long time, contrails were simply not a factor that was measured, priced or regulated. And what does not count in the system rarely gets priority. That is where the problem begins.

9. Do we need new technology — or is existing infrastructure enough?

We do not need to reinvent the aircraft. Many of the building blocks already exist — weather models, satellite data, flight planning software and operational procedures. What is missing is reliable integration into daily operations: better forecasts, clear responsibilities, regulatory incentives and tools that flight planners, pilots and air traffic control can trust. So this is less about new aircraft technology and more about better use of existing data, systems and decision processes.

10. What exactly are DLR, Google, TUI and American Airlines doing?

DLR has been researching non-CO₂ effects, contrail forecasting and climate-optimized flight routes for years. Google Research and American Airlines have shown in real-world tests that AI-based forecasts can help significantly reduce contrails. TUI is working on operational applications and tests in the European context. Together, these projects show that this is not just theory. It has already reached the testing stage.

11. For flight planners: How far along are routing tools like Flightkeys? What is still missing in practice?

Routing tools can already do a lot: process weather data, optimize costs, compare routes and plan vertical profiles. The next step is to treat contrail risk not as a research overlay, but as a practical planning factor in the normal workflow. That requires reliable forecasts, clear cost-benefit logic and decision support that is easy to understand. In the end, the question should be: Which small change brings a large climate benefit without making operations unnecessarily complicated?

12. For pilots: What changes in the cockpit? Do more detours mean more workload?

Ideally: very little. Contrail avoidance should not mean that pilots suddenly have to interpret climate models in real time. The better solution happens before the flight — in planning, dispatch and clear altitude or route recommendations. In the cockpit, it should come down to what is already routine: safe, cleared and understandable adjustments. The additional burden must not be pushed onto the person flying the aircraft.

13. For air traffic control: How do altitude changes fit into existing sectors and clearance processes?

This is one of the key practical questions. An altitude change of a few hundred meters may sound simple, but in dense airspace it can interact with sector capacity, separation, traffic flows and existing clearance processes. Contrail avoidance cannot be treated as an isolated request from a single flight. It has to fit into traffic management and airspace planning. That is why the air traffic control perspective is needed from the start — not as an afterthought.

14. Will tickets become more expensive?

Probably only slightly, if contrail avoidance is implemented in a targeted way. Studies and modeling often point to very small additional costs — roughly in the range of a few dollars per ticket. The reason is simple: not every flight needs to avoid contrails, only a small share at the right time and place. The key is to focus the measure on the cases where it has the greatest effect. That keeps costs and additional fuel use likely manageable.

15. More fuel means more CO₂. So what does the climate balance look like?

Yes, rerouting or changing altitude can burn slightly more fuel — and therefore cause slightly more CO₂. That is why avoiding every contrail blindly would be the wrong approach. The key is the balance: if a small route or altitude change avoids a large warming contrail effect, the small CO₂ increase can be clearly worth it. Good avoidance does not mean: just fly differently. It means: fly differently only where the climate benefit is clearly positive.

16. What is the EU doing? What does the reporting obligation from 2025 achieve?

Since 2025, airlines under the EU Emissions Trading System have had to gradually monitor and report non-CO₂ effects as well. This is not yet full regulation, but it is an important step: what gets measured becomes harder to ignore politically. The reporting obligation creates data, comparability and pressure. It does not solve the problem automatically. But it removes its most convenient hiding place: invisibility.

17. Why are airlines lobbying against regulation?

Because regulation means cost, responsibility and change. The airline industry often points to uncertainty, data gaps and operational complexity — and some of that is not made up. But uncertainty can also become a delay tactic. We know the pattern from other climate debates: first it is too early. Later, it is too expensive or operationally unrealistic. That is why public pressure matters.

18. What can I do as an individual?

First: talk about the issue. Contrails are visible, but almost nobody knows their climate impact. Second: support political action — through petitions, emails to elected representatives, conversations with media or civil society organizations. Third: be honest about flying. Flying less still matters. But when flights happen, aviation should at least avoid the damage that is avoidable.

19. Does this petition — or do petitions in general — really make a difference?

A petition does not solve the problem on its own. But it can send an important signal: people want to know why a comparatively low-cost climate measure is not already being taken seriously. Petitions can bundle attention, open conversations with politics and media, and move an issue out of the expert debate into the public sphere. For contrails, that matters a lot. Because the problem is not only technical — it is also political and communicative.

20. Almost philosophical: flying or never flying again? The mindset matters.

The simple answer would be: stop flying altogether. The more realistic one is: many people will continue to fly — for family, work, politics, science or personal reasons. That is why we need both: fewer unnecessary flights and better rules for the flights that still happen. Contrail avoidance does not replace broader climate policy for aviation. But it is a concrete way to significantly reduce an avoidable harm.