When aircraft fly through air that’s moist enough and cold enough, they can leave strips of cloud known as a contrails, short for ‘condensation trails’, that can have a slight warming effect on ground temperatures below. Finding a way to eliminate contrails in an effort to reduce the climate impact of aviation is an area of urgent research. A plan to reduce contrails is set out in a recent research document by the University of Cambridge and the Cambridge Institute for Sustainability Leadership.

Contrails don’t always appear behind aircraft. They form only when the atmosphere up at cruising altitude is cold enough and contains enough moisture (like it does ahead of a weather front) for the water vapour in the engine exhaust to freeze into ice crystals that persist in the sky. Besides CO2 and a whole bunch of particulates, exhaust gasses include plenty of hot water vapour, the gas form of water. This only forms into a human-made cloud, known officially as Cirrus homogenitus, when atmospheric conditions up there are conducive. And conditions vary greatly from place to place and day to day. When the aircraft is flying through a region of drier air, any droplets or ice crystals that form soon dissipate away. When conditions are cold and moist, water vapour in the exhaust forms ice crystals that persist and multiply in the aircraft’s wake. Such persistent and spreading contrails have a warming effect on temperatures on the ground below. This is why there are efforts to work out how to reduce them.

The way to do so is to dynamically set cruising altitudes based on moisture conditions so that aircraft fly at whichever levels happen to be less favourable to contrail formation on a particular day. This sort of dynamic approach to air traffic control is explored in a project by the University of Cambridge and the Cambridge Institute for Sustainability Leadership. Together, they wrote a report titled Five Years to Chart a New Future for Aviation that outlines four aviation goals to reach net zero by 2050, one of which involves contrail reduction. The report claims that reducing persistent contrail formation could reduce the climate impact of the aviation industry by around 40%.

You can read more about this ambitious plan by visiting the University of Cambridge’s website or by reading the full Five Years to Chart a New Future for Aviation report that also highlights the other ways in which the aviation industry could reach net zero by 2050.