Ellen Phiddian
Cosmos science journalist
It’s referred to as “contrail avoidance” and it has nothing to do with debunked conspiracy theories about secret chemicals added to airplane exhausts.
But it’s likely to reduce the environmental impact of the aviation industry, according to a new study.
The aviation industry causes climate change in several different ways. Planes typically emit CO2 by burning jet fuel, but they also emit other things that warm the atmosphere – and the location and timing of these emissions can have different warming effects.
One of the most complicated is condensation trails, or contrails. These lines of white clouds are formed by water in the atmosphere condensing onto aerosols emitted by the plane.
Contrails have different effects on the atmosphere depending on the location and time of day, but they generally cause atmospheric warming. While they reflect some sunlight back into space, contrails also absorb a lot of infrared heat from the Earth and keep it in the atmosphere.
In fact, it’s been estimated that contrails are the largest source of global warming in the aviation industry.
Since the places contrails form in (called ice supersaturated regions) are relatively small, it’s often possible to reroute flights to avoid them. Some airlines have begun trialling contrail avoidance routes to lower their emissions.
But rerouting flights requires planes to burn more fuel and emit more CO2, so the efficacy of these schemes have been hotly debated by atmospheric scientists.
The latest study, done by France- and UK-based researchers and published in Atmospheric Chemistry and Physics, comes out in favour of contrail avoidance.
“Rerouting flights to avoid contrails could in theory reduce the climate impact of aviation and make air travel more sustainable,” says co-author Professor Nicolas Bellouin, a researcher at the University of Reading, UK.
“Our findings lift a major obstacle against implementing contrail avoidance, but we now need better forecasting and real-world trials to make this work in practice.”
The researchers examined all of the flights that cross the North Atlantic Ocean in 2019 – 477,923 flights in total, 55% of which had formed contrails.
The team calculated the total warming effect of all of these flights, and modelled hypothetical rerouted flights to compare them.
They found that, assuming rerouting used 1% more fuel, contrail avoidance still generally caused less warming.
But the reduction depended on the way of measuring warming, particularly the time scales in play. Since contrails have a more intense but shorter-lasting effect than CO2, avoiding them will cause bigger benefits in 20 years than it will in 100 years.
The researchers point out that predicting contrail formation and warming, and managing air traffic, are both likely to cause complexities in any official contrail avoidance scheme. Plus, their study focuses on the North Atlantic, and other regions might produce different results.
But the researchers suggest that targeting only flights which produce strongly warming contrails could help to offset some of this uncertainty.
“Any proposed contrail avoidance schemes should, in addition to considering the many scientific uncertainties inherent in such schemes, consider a range of metrics and time horizons,” conclude the researchers in their paper.
