Changing the flight paths of just a few aircraft could slash the contribution of contrails to global warming by three-fifths, according to a new analysis.

Contrails, the white tracks sometimes seen behind planes in the sky, form when ice crystals condense around black carbon particles in jet engine exhaust. Usually, the clouds last only a matter of minutes. But in certain cold and humid patches of atmosphere, they can spread and merge with other contrails and natural cirrus clouds, and persist for up to 18 hours.

Contrails may have a cooling effect during the day, but at night they generally make the atmosphere warmer. Increasing evidence suggests that contrails contribute roughly as much to global warming as planes’ carbon emissions do.

Researchers in the UK and Germany analyzed six weeks’ worth of high-resolution data on the flight paths of individual planes through Japanese airspace – 61 million way points in all. They combined this with meteorological information and used a cutting-edge contrail model to simulate when and where contrails were likely to have formed, and how long they lasted.

About 18% of flights form contrails, the researchers report in the journal Environmental Science and Technology. Strikingly, though, just 2.2% of flights are responsible for 80% of the climate impact of contrails.

And if just 1.7% of flights changed their flight paths slightly, this could reduce the warming effect of contrails by 59.3%, the researchers calculated. This could be done with little effect on air traffic management systems.

Previous studies have suggested that this strategy of tweaking flight paths would require planes to burn more fuel, increasing carbon emissions from the flight enough to cancel out the benefit from reduced contrails.

Those past studies assumed that flights would be diverted laterally. But the patches of atmosphere where persistent contrails form tend to be wide and shallow. So the researchers instead evaluated the effect of a small change in cruising altitude – diverting planes roughly 2,000 feet up or down from their normal flight path.

This strategy comes with only a 0.27% increase in fuel consumption and carbon emissions for each flight, or a 0.014% increase for the fleet overall. The reduction in warming from contrails far outweighs this slight increase in carbon emissions.

“This demonstrates the potential to reduce aviation’s climate forcing immediately,” the researchers write. “Although this study is restricted to the Japanese airspace, the qualitative findings are likely valid for other mid-latitude regions.”

The researchers also evaluated what would happen if airline fleets adopted a type of cleaner engine that emits less black carbon than the engines that are standard today. The same number of flights would still form contrails, but they would disappear sooner and would have less climate impact.

Cleaner engines alone could reduce the climate impact of contrails by 68.8%. And the combination of cleaner engines and tweaking flight paths could reduce the climate impact of contrails by a whopping 91.8% — and cut the overall climate impact of aviation by more than half.

Source: Teoh R. et al. “Mitigating the climate forcing of aircraft contrails by small-scale diversions and technology adoption.” Environmental Science and Technology 2020.

Image: NASA on the Commons via Flickr.