THERE are few more delightful antidotes to stress than to lie on your back in warm grass and watch the clouds go by. As children, we love finding the outlines of animals and castles in the billowing shapes. As adults, there is something calming and comforting about those fluffy tufts of white drifting slowly past. Clouds are beautiful. Clouds are innocent.

With one exception. The streaky smears of cloud that criss-cross the sky in the wake of aeroplanes may look too wispy to cause any harm. But we now know that these condensation trails, or contrails, make an outsized contribution to global warming by trapping heat like a downy jacket. “They are one of the few manifestations of man-made climate change agents that you can actually observe,” says David Lee, an atmospheric scientist at Manchester Metropolitan University in the UK. As the evidence mounts to show how harmful contrails are, some engineers are reaching for an audacious solution: scrub them from the sky altogether.

Contrails are created when water condenses to form ice crystals around tiny particles of soot from aircraft exhausts. Yet there is no fundamental reason why this has to happen. Decades of experiments with spy planes, alternative engines and, most recently, with artificial intelligence have shown that it is possible to stop them forming. It won’t be easy: wiping the atmosphere clean of contrails may require nothing less than a wholesale reimagining of the traffic in our skies.

The effect of clouds on our climate is subtle because they can both reflect incoming sunlight, which has a cooling effect, and trap heat beneath them, which has a warming effect. However, contrails are a type of artificial cirrus – a thin, cold, high-altitude cloud – and we have known for a long time that these are a climate menace. Their wispiness means they let almost all sunlight through while also trapping heat below them highly effectively.

“One study found that 57 per cent of the warming caused by aviation was due to contrails”

For sure, cirrus clouds are a natural part of our skies. But aeroplanes are creating an increasing number of them and recent evidence has laid bare how damaging this is. One study conducted by Lee looked at the atmospheric warming caused by aviation between 2000 and 2018. It found that contrails cause significantly more warming than the carbon dioxide pollution produced by burning jet fuel: 57 per cent of warming was due to contrails and only 34 per cent was from CO2 . The other 9 per cent was down to other components of exhaust fumes. That may come as an unwelcome surprise to climate conscious travellers paying to offset the carbon emissions from their flights because such offsets overlook the impact of contrails.

Admittedly, comparisons between CO2 emissions and contrails are fraught. This is because the greenhouse gas can stay in the atmosphere for hundreds of years, whereas contrails hang around for a maximum of about 12 hours. Still, contrails are a massive and growing problem. The rise and rise of air traffic may have been stalled by the covid-19 pandemic, with air passenger numbers falling from 4.5 billion in 2019 to 1.8 billion in 2020. But assuming some sort of return to normality in air travel, passenger numbers could easily begin to increase again – and perhaps very rapidly once the pandemic is under control.

So how do we curb contrails? One method might be to have planes fly at high altitudes. The density of contrails depends on how moist air is, and humidity declines with altitude as the air thins. If aeroplanes were to fly at 18,000 metres, the cruising height of Concorde, there wouldn’t be enough moisture for contrails. However, a plane like the Boeing 777 cruises efficiently at about 10,000 metres and just isn’t built to fly significantly higher. In any case, atmospheric scientists don’t know enough about how exhaust pollutants such as nitrogen oxides would affect the atmosphere when released at such high altitudes. If they were to damage the ozone layer, which absorbs harmful ultraviolet radiation from sunlight, that might outweigh the benefits of contrail control.

Rear-view mirror

Perhaps we can learn a lesson from a time when contrails were a concern for a reason other than climate. In the 1950s in the US, the CIA was preparing for the high-altitude U-2 spy plane to fly over the Soviet Union. It was capable of flying much higher than Concorde, but didn’t do so all the time. A visible contrail would be easy for Russian interceptor jets to home in on. There was, it turned out, a way to avoid that risk.

Although humidity tends to decrease with increased altitude overall, there are layers of air throughout the atmosphere with higher and lower humidity thanks to local weather conditions. This meant that adjusting the U-2’s altitude by a few hundred metres would usually stop the formation of contrails, or “conning” to use the aviation slang. The only trouble was that pilots often couldn’t tell when they were leaving a trail. In the end, the CIA found a simple fix: fitting a rear-view mirror outside the U-2’s cockpit. That way, pilots could see their own trails and try switching altitude.

This low-tech solution wouldn’t be practical in the maelstrom of modern air traffic. And with contrails back in the spotlight for environmental reasons, the hunt is on for a better fix.

Aircraft engines are an obvious place to look. Today, most planes use jet engines known as single annular combustors that have one ring of burners. Double annular combustors, a less common sort of engine, have two rings and so burn fuel more completely, producing little soot. “The number of ice crystals is pretty much dependent on the number of soot particles,” says Marc Stettler at Imperial College London’s Transport and Environment Laboratory. “If we were to reduce them, that would reduce the contrail.” Stettler recently analysed methods of reducing contrails and found that fleet-wide adoption of double burning engines would cut the climate impact of contrails by more than 60 per cent.

Switching the engines of the world’s airliners is too expensive to be feasible in the short term. However, there is one idea that might provide a quicker fix. It involves a return to the CIA’s method of shifting altitudes so that planes fly in layers of air with low humidity – but this time guided by something a little more sophisticated than a rear-view mirror.

The idea is to use real-time atmospheric models to pinpoint in advance low-humidity layers of the atmosphere where contrails won’t form, then divert planes accordingly. Making such predictions is a huge challenge. But Adam Durant, the founder of SATAVIA, a company based in Cambridge, UK, says his firm can do it. SATAVIA has developed a planet-wide model of the atmosphere up to an altitude of 18,000 metres fed with detailed meteorological data and processed by AI. “We’re now able to run our complex atmospheric models in hyper-resolution and at global scale for the first time,” says Durant.

In principle, this would make it possible to route flights so that they form fewer contrails. If it can be done, there is evidence to suggest the effects will be important.

In the same study that examined engine changes, Stettler looked at a set of flights over Japan and found that a tiny number of them – which he calls “big hits” – accounted for the vast majority of the warming caused by contrails. The flights responsible tended to produce contrails that hung around for a long time. If these flights, accounting for just 2 per cent of traffic, were diverted so that they didn’t produce contrails, the warming effect on the climate would again be reduced by 60 per cent. Emirates airline, based in the UAE, has agreed to work with SATAVIA to test the idea in the coming months.

Fasten seat belts

It isn’t a perfect fix. Greg Thompson at SATAVIA points out that diverting planes to lower altitudes can increase fuel burn, while pushing them higher increases the risk of turbulence. “Do I ask passengers to wear seat belts due to possible turbulence in order that we avoid producing a contrail?” he asks.

Klaus Gierens at the German Aerospace Centre Institute for Atmospheric Physics near Munich has cast doubt on whether the strategy will be effective. He recently compared two methods of predicting contrails with real weather data and contrail observations. The results suggested that predicting where contrails will form is possible, but it is much harder to predict which will be long lasting – Stettler’s “big hits” – and it is these that do the damage. Gierens didn’t test the method used by SATAVIA, however; the full details haven’t been made public. The firm remains confident that it has cracked it.

The history books might record 2021 as the beginning of the end of contrails. But their demise is going to take a while. Until then, when we look up on a clear day, we will see a wispy reminder that the planet is slowly and steadily getting warmer.