Aeroplanes could avoid forming planet-warming contrails by adjusting their altitude – in many cases, by just a few hundred metres. This could be one of the easiest ways to reduce the climate impacts of aviation.
“Ignoring the contrail question would be silly,” says Esther Roosenbrand at the Delft University of Technology in the Netherlands.
Aviation makes up a small but growing share of the emissions that drive global warming. As much as two-thirds of this comes in the form of “non-CO2” emissions from aircraft engines, such as water vapour and nitrogen oxides.
The most visible results of these emissions are the white contrails that often streak behind jets. They appear when water vapour and soot are injected into frigid, humid conditions and produce a net warming effect by trapping radiation within the atmosphere. Their climate impact may be equivalent to as much as half of CO2 emissions from aviation, says Joey Cathcart at RMI, an environmental non-profit in Colorado.
Roosenbrand and her colleagues looked at whether it would be feasible for millions of planes to prevent contrails by changing altitude to avoid the atmospheric conditions that create them. Such “altitude diversions” have long been proposed as a solution – but experts still need to answer many practical questions, including how much extra fuel changing altitude would require.
The researchers modelled how flight paths would have to change to avoid contrail-forming conditions. They used public data on nearly 6 million flights, along with measurements of temperature and humidity from a global network of weather balloons.
For about half of the flights that flew through contrail-forming conditions, they found the aircraft could have avoided these regions with altitude changes of 609 metres or less. Such changes are already common in air traffic control. The researchers also found this would require only a small amount of additional fuel and would cause few traffic issues. “Within the current structure of the airspace, a lot is possible,” says Roosenbrand.
Robert Sausen at the German Aerospace Center says the study is in line with earlier findings. However, he says relying on altitude diversions is “premature” without more accurate forecasts of contrails and their effects on the climate.
But some airlines are already changing flight plans to avoid contrails. Last year, American Airlines flew 70 test flights using contrail forecasts, and reported that the aircraft produced half as many contrails. This month, the airline Etihad Airways, based in the United Arab Emirates, said it would begin avoiding contrails by using weather models from UK-based start-up SATAVIA to plan flight paths.
Adam Durant at SATAVIA says more than 100 test flights have already used the company’s forecasts. Averaged across those flights, the changes provided a climate benefit about 100 times greater than the added emissions from the increased fuel requirements.
Article amended on 31 January 2024
We corrected the estimated climate benefit of SATAVIA’s altitude diversion tests