Look up on a clear day and there is a good chance you will see them.
Thin white streaks stretching across the blue sky, sometimes disappearing within minutes, sometimes lingering long enough to spread into wispy clouds.
To most people, they are simply aircraft trails.
Scientists, however, see something more complicated.
These trails, known as contrails, could be contributing significantly to aviation’s impact on the climate. Now, researchers in the UK are preparing to test whether artificial intelligence can help aircraft avoid the atmospheric conditions that make the most harmful contrails.
The experiment, backed by about £5 million, could eventually change something as ordinary as the altitude at which a plane flies.
And passengers may never even notice.
The White Trails That Do More Than Mark An Aircraft’s Journey
Contrails form when hot, moist exhaust from a jet engine meets extremely cold air at high altitude.
The encounter produces tiny ice crystals that can create the familiar white trail behind an aircraft.
Often, the trails vanish almost as quickly as they appear.
But under particular atmospheric conditions — especially where the air is very cold and humid — they can persist, spread and form cloud-like layers.
Those clouds can trap heat in the atmosphere.
That is where the apparently harmless streak in the sky becomes a climate problem.
Dr Paul Hodgson, Google’s technical lead on the project, gave the phenomenon a more playful description during an appearance on the BBC’s Tech Life programme.
His five-year-old calls them “sky graffiti.”
But the science behind that graffiti is considerably more serious.
Hodgson said scientists believe contrail warming could account for roughly a third of aviation’s total climate warming, although he acknowledged that there remains uncertainty around the exact figure.
The central concern, he said, is no longer whether contrails matter, but how large their impact actually is.
Enter AI
This is where Operation Blue Skies comes in.
The 30-month project will bring together Google, the UK government, the Met Office, air traffic control provider NATS, the University of Cambridge and Imperial College London.
The target is a particular stretch of airspace over the North Atlantic known as Shanwick Oceanic airspace.
It is not an insignificant piece of sky.
The eastern section of the North Atlantic corridor is estimated to account for around 5% of global contrail warming.
The idea behind the trial is surprisingly simple: if you can predict where the worst contrails are likely to form, why not fly around them?
Google will combine satellite imagery and historical flight data to identify where aircraft have previously created contrails.
That information will then be matched with weather conditions to determine where similar atmospheric conditions are likely to occur in the future.
AI will essentially be asked to look at the sky before the aircraft gets there.
If the system identifies an area where a flight is likely to produce a persistent, warming contrail, the information can be passed to NATS.
Air traffic controllers could then instruct the aircraft to make a small altitude adjustment.
No Dramatic Detours
The solution does not involve sending aircraft hundreds of miles off course.
In most cases, the proposed adjustment would be around 2,000 feet.
That may sound like a significant change, but such altitude adjustments are already routine in commercial aviation when aircraft encounter turbulence or adverse weather.
Ian Jopson, NATS’ director of sustainability, described the approach as largely “business as usual” for operators flying across the North Atlantic.
That is part of what makes the idea attractive.
If a relatively small change in altitude can prevent a persistent warming contrail without disrupting passengers or schedules, the climate benefit could potentially be achieved without requiring an entirely new aviation system.
The trial will take place on selected evenings and nights during the winters of 2026-27 and 2027-28, when traffic levels are lower.
About 10,000 flights are expected to pass through the relevant airspace during the trial periods.
Only a fraction of those flights, however, are expected to require an altitude change specifically to avoid contrails.
But There Is A Catch: Fuel
There is an obvious question.
If a plane has to change altitude, could it burn more fuel and therefore produce more carbon dioxide?
That would create an uncomfortable paradox: what if trying to stop one climate problem makes another slightly worse?
Researchers involved in the project say the additional fuel requirement should be relatively small compared with the climate benefit of preventing the most harmful contrails.
Google’s Hodgson said previous trials indicated that the additional fuel consumption was less than 1% compared with the estimated climate impact associated with contrail warming.
Modelling for Operation Blue Skies suggests that a full deviation through the Shanwick airspace could require approximately 100kg of additional fuel per affected aircraft.
That calculation will be one of the issues scientists will be watching closely.
The trial is therefore not simply about proving that AI can predict contrails.
It must also demonstrate that avoiding them produces a genuine net climate benefit.
Early Trials Offer A Reason For Optimism
There are already some encouraging signs.
According to Hodgson, previous airline-level experiments have achieved reductions of around 60% to 70% in contrail formation or warming.
If similar results can be reproduced across a busy international air corridor, the implications could extend far beyond Britain.
Aviation is notoriously difficult to decarbonise.
Electric aircraft remain limited by battery technology, while sustainable aviation fuels are expensive and available in insufficient quantities to replace conventional jet fuel at scale.
Aircraft also produce climate effects beyond their carbon dioxide emissions.
That makes contrail avoidance an attractive area of research because it could potentially reduce part of aviation’s warming impact without waiting for an entirely new generation of aircraft.
Can Technology Keep Up With The Growth In Flying?
There is, however, a bigger question hanging over the experiment.
The UK government is simultaneously supporting the expansion of airports, while air travel demand is expected to continue growing.
That raises an uncomfortable issue: can technological improvements make aviation clean enough if the number of flights keeps increasing?
Operation Blue Skies may help reduce the climate impact of individual flights, but it does not eliminate the emissions produced by aviation.
And there is another environmental contradiction surrounding AI itself.
Google, one of the project’s major partners, is also investing heavily in artificial intelligence and the enormous data centres required to run it.
Those facilities consume substantial amounts of electricity, raising concerns about the environmental cost of the AI boom.
So the experiment represents a fascinating collision of two modern realities: aviation wants to become cleaner, while AI is being asked to help make that happen — even as the computing infrastructure powering AI creates environmental pressures of its own.
The Real Test Begins Above The Atlantic
The British government is contributing £2.65 million to Operation Blue Skies, while Google is providing around £1.4 million through AI research, engineering and computing resources.
Aviation Minister Keir Mather described the project as a world-first, saying relatively small changes to flight paths could help make flying cleaner.
But the most important result will not be whether a computer can spot a contrail.
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It will be whether airlines can repeatedly avoid the most damaging ones without creating greater environmental costs elsewhere.
That is why the winter trials matter.
For thousands of passengers crossing the Atlantic, the experiment may feel completely invisible. There will be no dramatic announcement in the cabin. No passenger will necessarily know that their aircraft climbed or descended by a couple of thousand feet because an AI system had spotted a patch of sky they were better off avoiding.
But somewhere above the clouds, that small change could become part of a much bigger experiment.
The results are expected to be published in academic journals and made available for wider scientific scrutiny.
And if the numbers hold up, the next time you look into the sky and see one of those familiar white lines, it may be worth remembering that what looks like a simple trail behind a plane is actually part of a much larger climate puzzle.
The future of cleaner flying might not always require building a completely different aircraft. Sometimes, it could simply mean teaching planes where not to fly.