Nervy flyers be warned: warming will boost turbulence

Brace yourself: global warming is going to be a bumpy ride ? literally. The amount of moderate to extreme turbulence affecting transatlantic flights could more than double by the middle of the century as carbon dioxide levels increase.

Turbulence can develop where clouds and storms create updraughts and downdraughts. More problematic for planes, though, is clear-air turbulence. This occurs where air at one altitude is travelling faster than the air immediately below, leading to atmospheric instabilities.

"Clear-air turbulence is invisible to the human eye and also to the electronics on planes, which makes it difficult to avoid," says Paul Williams at the University of Reading, UK. Current estimates suggest that, on average, around 1 per cent of a transatlantic flight is spent flying through moderate to extreme clear-air turbulence.

Williams and Manoj Joshi at the University of East Anglia in Norwich, UK, used climate models to work out whether that 1 per cent figure is likely to rise as atmospheric carbon dioxide levels increase. The models show that the high-altitude atmospheric jet streams will accelerate as CO₂ levels climb. This speeding up, relative to the air immediately below, will lead to more clear-air turbulence at exactly the cruising altitude of transatlantic flights.

Jet stream migration

What's more, the models also suggest that the jet streams will migrate slightly northwards. This will shift the patch of the most severe clear-air turbulence from the central Atlantic to the north Atlantic ? right into the path of many flights.

Williams and Joshi calculated that if CO₂ levels double relative to pre-industrial levels ? an event projected to occur by the middle of the century if current emission trends continue ? the twin effects on the jet stream could lead to the strength of flight turbulence increasing by between 10 and 40 per cent, and an increase in the frequency of moderate to extreme turbulence of between 40 and 170 per cent.

With perhaps five minutes of a typical 8-hour flight today subject to such turbulence, a 170 per cent increase ? to around 13 1/2 minutes ? might seem trivial to anyone but those with an extreme fear of flying. "You could argue a few more drinks will get knocked over. So what?" says Williams. But he says that the extra turbulence could cost the aviation industry dear, by accelerating aircraft wear and tear, for instance.

Robert Sharman at the National Center for Atmospheric Research in Boulder, Colorado, researches flight turbulence. He says it is difficult to predict it even on a daily basis, and there is no single forecasting technique that researchers can agree to use. Williams and Joshi tackled this problem, he says, by using several of the most popular turbulence forecasting techniques ? and almost all suggested clear-air turbulence would rise.

Michael Sprenger at the Swiss Federal Institute of Technology in Zurich says future technologies should make it easier to identify and avoid clear-air turbulence, so any rise in the level of turbulence might have little impact on flights. However, if planes begin taking more convoluted routes to avoid turbulence, flight times and fuel consumption will rise, say Williams and Joshi, which may only aggravate the problem by adding yet more CO₂ to the atmosphere.

Journal reference: Nature Climate Change, doi.org/k4m

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