It is much the most visible reminder of the presence of aircraft for people on the ground below — the distinctive white condensation trail left, in certain conditions, by the engines of a passing aircraft. It also looks from the ground as if it should be harmless — a stream that looks like cloud or steam, and usually dissipates relatively fast.
Yet over the past 30 years evidence has emerged that condensation trails — or “contrails” — may present at least as much of a danger to the global climate as the carbon emissions from airliners burning fuel. The contrails can prevent vast amounts of heat escaping earth’s surface, via a process known as “radiative forcing”.
Contrails.org, a non-governmental organisation working on the issue, has estimated that historic CO₂ emissions from aviation have caused about 1.5 per cent of human-made global warming, and that contrails are responsible for another 1 to 2 per cent.
Edward Gryspeerdt, a researcher at Imperial College London’s Grantham Institute for Climate Change and the Environment, underlined the gravity of the issue last year in a study that suggested newer, more efficient aircraft might be creating more dangerous, harmful contrails than older ones. “Most people do not appreciate that contrails and jet fuel carbon emissions cause a double-whammy warming of the climate,” Gryspeerdt told an Imperial newsletter.
Yet there are also indications that airlines can find ways of flying their aircraft that avoid contrail formation or make them less harmful.
The question, according to Andrew Charlton, a Geneva-based aviation analyst, is whether airlines are willing to adapt their operations to tackle the problem.
“It’s something that we should be focusing on more,” Charlton says. “[But] it’s something we’re not investing in because it’s just too hard.”
What is the problem?
Contrails are generated when water vapour in an aircraft engine’s exhaust forms ice crystals around the soot expelled by the engine. The contrail reflects light and heat.
However, different contrails produce starkly different effects, and most dissipate relatively quickly.
This is not the case in regions where the air is already ice-supersaturated. That is where, in the typically sub-zero temperatures of the upper atmosphere, there is more water vapour in the air than would normally be needed to form ice.
When a plane passes through, the water vapour from the surrounding atmosphere adds to the contrail and turns it into a persistent — or “big hit” — contrail. Such trails, which can turn into cirrus clouds, can be several hundred kilometres long and last for hours.
How does it affect the climate?
Parts of the aviation industry insist the overall effects are still not clear. Airline association Iata says: “The scientific understanding of the non-CO₂ climate effects of aviation has grown but significant uncertainties exist in predicting contrail formation and climate impact.”
There is nevertheless evidence that big-hit contrails have an outsize warming effect.
A report published in September by Victor, a private jet broker, calculated that just 1 per cent of contrails — the most damaging big-hit contrails — generated 48 per cent of the warming effect from the phenomenon.
The issue is further complicated because of the differing lasting effects involved.
Transport and Environment, a campaigning group, says that over 20 years, the warming effect of a single flight’s contrails can be greater than the effect from its carbon dioxide emissions. However, because carbon dioxide can remain in the atmosphere for centuries while contrails dissipate quickly, a flight’s contrails will over 100 years have only a third of the warming effect of its carbon emissions.
What can be done about it?
In theory, there are ways to avoid a significant proportion of big-hit contrails. According to a 2023 paper by the Royal Aeronautical Society (RAS), the ice-supersaturation regions (ISSRs) where big-hit contrails tend to form are typically only around 1,500 metres deep. That opens up the possibility that aircraft can change course to avoid such layers.
It should also generally be relatively cheap to mitigate the problem. Most studies suggest the necessary rerouting would need relatively little extra fuel. The RAS calculated that, for a flight encountering ice-supersaturated air for 20 per cent of its journey, the extra fuel burnt would be only about 0.5 per cent.
So is the issue being addressed?
The challenge is that the formation of contrails depends heavily on changing weather conditions, while most aircraft follow preset schedules and regular routes.
Charlton points out that it can be difficult to accommodate the kind of “dynamic” short-notice changes to routing and flight height necessary to avoid contrail formation.
“Controllers aren’t really keen on aircraft making a dynamic change of direction,” Charlton says. “It’s just another operational complication for the controllers.”
He points out that introducing more sudden changes risks putting extra stress on both aircrews and controllers.
“We don’t want to overload the poor pilots or the poor controllers,” he says.
Yet the focus on the problem is intensifying, and routing and air traffic control systems are constantly improving. That has led to optimism that airlines will soon have better tools for predicting when the most damaging contrails are likely to form and better navigation and planning systems to avoid the places where the risk is greatest.
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