Aviation: Blue-sky thinking on contrails

Aircraft condensation trails, or contrails, which sometimes criss-cross otherwise clear skies in favourable weather conditions, can form beautiful patterns. But they are also very visible signs of human intrusion into the natural environment.

Now a study at Reading University shows that the water droplets in contrails make a significant contribution to global warming over and above the carbon dioxide generated by the fuel burned in aircraft engines. This environmental impact is large enough to justify rerouting flight paths to avoid the places where contrails are likely to form, even at the cost of extra fuel consumption, say the authors, who report their findings in the journal Environmental Research Letters.

Contrails form in “ice supersaturated” regions of the atmosphere, where the air is very cold and moist – for example in the ascending air around anticyclones. Under certain conditions they can last for many hours, spreading out to resemble thin natural clouds.

Although contrails reflect some incoming solar radiation, giving a cooling effect, the researchers say this is more than counteracted by the warming effect of trapping some of the infrared radiation that would otherwise be reflected back into space from the ground.

“If we can predict the regions where contrails will form, it may be possible to mitigate their effect by routing aircraft to avoid them,” says Emma Irvine, study leader. “Our work shows that for a rounded assessment of the environmental impact of aviation, more needs to be considered than just the carbon emissions of aircraft.”

The calculations are complicated because they depend critically on how long the contrails last. Typically, for a large jet it would be worth making a detour up to 60km to avoid making a contrail 20km long. Of course, if a diversion is justified on environmental grounds, airlines might still need some compensation for their additional fuel costs and extended journey times, but the extra costs would be small in the overall context, says Irvine. An example in the study shows a deviation of 22km on a flight from New York to London – or just 0.4 per cent of the original route – to avoid a contrail over the Atlantic.