Science Matters – Are aircraft vapour trails harmful?

The familiar sight of aircraft trails in the skies. Photo: Nestek

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Although much reduced by the Coronavirus pandemic, everyone has seen the vapour trails produced by high-flying aircraft as they streak across the sky. But what are they? and are they harmful?

Officially called contrails (short for condensation trails), these ephemeral, cloud-like structures are primarily made of ice crystals. They form at altitudes over 8,000 metres from water vapour in the aircraft’s engine exhaust as it freezes almost instantaneously at temperatures below minus 36 degrees celsius. There are, however, plenty of other volatiles and solid components in engine exhaust, including soot particles that often act as a nucleus around which the ice crystals form.

Official figures show that air traffic contributes around five per cent of greenhouse gases. Yet these estimates only account for CO2 emissions. They do not include the effect of contrails, which can persist for hours and even expand into cirrus clouds several miles wide. These clouds, which are most abundant over North America and Europe, trap heat within Earth’s atmosphere, so do they improve our climate or contribute to global warming?

Air traffic has been steadily increasing year-on-year, so, assuming the coronavirus effect is a temporary one, estimates suggest that the distance flown by passengers will double every 15 years, and that air traffic will quadruple by 2050. Therefore, it is imperative to determine how the increase in clouds created by contrails might affect us.

In a recent study*, scientists used a climate model to analyse the future impact of contrails on climate change, taking into account their predicted increase balanced against any likely changes in engine soot emissions.

The results show a three-fold increase in contrail cirrus coverage by 2050, despite a 50 per cent reduction in soot emissions due to improved fuel and engine efficiency. These predictions cover all major air traffic routes and are mostly due to increased air traffic volume, although a shift to flying at higher altitudes also contributes. The area with the greatest increase will be Eastern Asia, where the largest rise in air traffic is expected.

This previously unaccounted for, non-CO2, climate impact of air traffic is now reckoned to have a more damaging effect on climate change than all the CO2 emitted from aircraft engines.

*Atmos. Chem. Phys., 19, 8163-74. 2019