You know the CO₂ effect of flying. You only know one third of the story.

Around two percent of total human-made global warming comes from contrails. Sounds small. It isn’t — it is the same order of magnitude as Germany’s contribution to global CO₂ emissions. And it is more than the effect of the CO₂ from all flights combined.

By the time this became clear to us, one thing was obvious: We need to talk about this. Because this climate impact is not happening somewhere in the distant future. It happens every day above our heads — from a few lines in the sky.

What comes out of the back of the turbine is hot, humid and full of soot particles. Outside: minus fifty degrees, thin air. Within seconds, the water vapour condenses on the particles and freezes into ice crystals. A contrail forms.

Some of these trails break up after just a few minutes. Others remain. They remain precisely when the aircraft flies through an ice-supersaturated region — cold layers where the air already contains more moisture than it can hold. There, the trails grow into broad cirrus clouds. For hours. Sometimes longer.

And these “persistent” contrails are exactly the problem. During the day, they let some sunlight through. Their most climate-damaging effect occurs especially at night, when they trap the Earth’s heat like a lid.

When you hear aviation, you think CO₂. Makes sense — that’s what gets talked about, and that’s what gets measured. But it is only one third of the story.

The rest is called “non-CO₂ effects”: contrail cirrus, nitrogen oxides, water vapour at high altitude. Leading the pack, by far: contrails. Not a calculation error, but scientific consensus — most recently confirmed in the IPCC’s Sixth Assessment Report.

The problem: they do not show up in any CO₂ balance sheet.

Three reasons. All solvable.

1. First: The EU Emissions Trading System only covers CO₂. What is not measured costs nothing. What costs nothing is not avoided. Contrails do not appear in any balance sheet — and therefore in no pricing system.

2. Second: airline lobbying. The industry is playing for time. More studies, more data, more uncertainty — the standard playbook. Transport & Environment has documented this in detail.

3. Third: non-CO₂ effects are more complicated than CO₂. They act regionally, not globally. Only some flights, only certain routes, only certain weather conditions. Not every contrail is equally harmful. That is exactly why the climate impact disappeared into the small print for years — AND why the topic is too complex for the headlines.

But: complex does not mean unsolvable. It means you have to address the problem precisely.

Today, forecasting models can predict with useful accuracy where the problematic air layers are. Around two to three percent of flights hit these zones. For those flights, an altitude change of around 600 metres is usually enough — up, down, around.

Routine stuff in the cockpit. The result, according to model calculations and field trials: up to 80 percent less climate impact from contrails.

The first real-world test ran in 2023 — with Google Research and American Airlines. 54 percent fewer persistent contrails on the tested routes. Since then: Lufthansa with DLR. TUI. And Flightkeys.

Let’s do the maths. A flight that has to avoid a critical zone uses around one percent more fuel. Per ticket — averaged across all flights, not just the affected ones — that means an extra 1 to 5 US dollars.

Per tonne of avoided CO₂ equivalent, this is one of the cheapest climate measures out there. In some cases hundreds of times cheaper than synthetic fuels. Or fleet renewal. Or any offset that can be made to look good on paper.

It is not a cost problem. It is a policy problem — and an attention problem. That is why this page exists.