Small changes to aircraft flight paths to avoid contrails could reduce aviation’s global warming impact by nearly half, according to a study.

The study, led by researchers at the University of Cambridge, suggests that changing cruising altitude by a few thousand feet, either up or down, could prevent contrails from forming.

Contrails, or vapour trails, are line-shaped clouds produced by aircraft engine exhaust or changes in air pressure, typically at aircraft cruising altitudes several kilometres above the Earth’s surface. 

While the exact warming effect of contrails is uncertain, it is believed to be greater than warming caused by aviation’s CO₂ emissions. Although modern commercial aircraft emit less carbon than their predecessors, previous studies have shown they could be contributing more to climate change due to their longer-lived contrails.

As such, interest in contrail avoidance has grown rapidly in recent years as governments and airlines search for ways to reduce aviation’s climate impact.

The University of Cambridge study finds that while reducing or avoiding contrail formation offers a fast and cheap climate mitigation measure for the aviation industry, since the practice can be adopted with existing aircraft and fuel, time is of the essence.

Dr Jessie Smith, study lead author from Cambridge’s Department of Engineering, said: “Contrail avoidance can often be as simple as changing the flight paths. Often it’s even simpler than that – just moving slightly to a higher or lower altitude to avoid the areas of the atmosphere where contrails form.”

Smith and her colleagues modelled how altitude adjustments for contrail avoidance could affect aviation’s overall climate footprint. They found that such a programme, phased in between 2035 and 2045, could recover around 9% of the temperature budget the world has left before breaching the Paris Agreement’s 2°C limit.

However, they also found that if no action is taken, by 2050 aviation contrails will have added around 0.054°C of warming – 36% more than the warming attributable to aviation CO₂ over the same period.

Smith said: “What surprised me was how quickly the temperature saving could be made. Over a decade, you can take a really big chunk of aviation’s warming impact out very rapidly. That’s unusual in climate science, where most changes take a very long time.”

The researchers also found that while rerouting aircraft can increase fuel use slightly, the reduction in warming from fewer contrails would more than offset the extra CO₂ emissions.

Additionally, implementing contrail avoidance would not necessarily increase workload for air traffic management systems as the adjustments required would be relatively modest. Anyway, flights already alter routes or altitude to avoid turbulence or bad weather. These similar systems could potentially be used to avoid contrail-forming regions.

Smith said: “It’s an operational change, not a technological one. You don’t need to modify aircraft. You just need to work out how it will operate, and then the system is already built for it – pilots do these manoeuvres all the time. That’s why we have more hope for this than for other interventions like sustainable aviation fuels, which face enormous infrastructure and supply-chain hurdles.”

While the researchers conceded that more work is needed to improve forecasts of the atmospheric conditions that cause contrails, they argue that putting contrail avoidance policies in place now would offer meaningful change, rather than waiting for the technology to be perfected.

Smith said: “Our modelling shows clearly that you do not want to wait for perfect conditions before you begin.”

Their study – The climate opportunities and risks of contrail avoidance – has been published in the journal Nature Communications.