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Climate change portends a turbulent future for airline passengers.

Climate change portends a turbulent future for airline passengers

5 September 2023

Severe turbulence that is difficult to detect will likely increase as the planet warms, satellite data and climate models suggest.

An airplane flies on a clear day.
An airplane streaks across the sky on a clear day. Credit: Oleg Elkov/Alamy Stock Photo

Most airline passengers have experienced in-flight turbulence. In some cases, it can be severe. From 2009 to 2022, the US National Transportation Safety Board reported that because of turbulence, there were 163 injuries requiring hospitalization on commercial airlines and 38 deaths on private planes. In the US alone, turbulence is estimated to cost $150 million to $500 million annually from structural damage to planes, flight delays, and injuries, according to the National Center for Atmospheric Research (NCAR).

Turbulence is often associated with the towering cumulonimbus clouds of storm systems. Air within the clouds mixes with outside air of different temperature, pressure, and velocity, triggering atmospheric instabilities. But such turbulence is usually detectable in advance so the worst impacts are avoided. What can be more dangerous is clear-air turbulence (CAT), which is generated by chaotic instabilities at aircraft cruising altitudes. Unlike turbulence linked to clouds or storms, CAT cannot be detected ahead of time with onboard radar equipment. And it can produce large enough vertical accelerations of an aircraft to lift passengers from their seats.

Now evidence is mounting that the frequency and severity of CAT have been increasing due to climate change and that those trends will continue in a warming world. A study published in June by atmospheric scientist Paul Williams and colleagues from the University of Reading in the UK provides strong evidence that there have been large increases in CAT over the last four decades. The research shows that over the North Atlantic, light CAT, which causes rapid changes in altitude, increased by 17% from 1979 to 2020, and severe CAT, in which the airplane may momentarily be out of control and occupants are forced violently against their seat belts, increased by 55%.

Along with bumpier flights, a continuing increase in CAT may mean longer trips and the consumption of more jet fuel to avoid dangerously unstable air.

Quantifying CAT

CAT is caused by vertical wind shear, sharp differences in wind velocity that occur with altitude. Commercial flights tend to cruise at an altitude of about 9–12 kilometers, in a region of the atmosphere that hosts narrow bands of intense wind known as the jet stream. The winds are fueled by the temperature difference between Earth’s colder polar regions and warmer equatorial regions and are bent in a west-to-east direction due to the planet’s rotation.

A simulation of increased carbon dioxide in the atmosphere depicts areas in bright colors of clear-air turbulence.
This simulation snapshot depicts areas of severe clear-air turbulence in a future world with an increased atmospheric concentration of carbon dioxide. Credit: Paul Williams

The greatest potential for CAT occurs at the upper and lower boundaries of those bands, where wind speeds can drop quickly. If the gradient is large enough, the boundary between the faster- and slower-moving air deforms, and winds that had been blowing horizontally attain an increasing vertical velocity component. The danger emerges when those winds overturn to form turbulent eddies. The effect is more pronounced in the Northern and Southern Hemisphere winters, when the poles cool and the temperature differential that fuels the jet stream strengthens.

In theory, climate change should make those instabilities more likely, due to differential warming across the globe. Whereas warming is amplified near the poles at ground level, Williams says, at high altitudes warming is more pronounced at lower latitudes because of the larger amounts of water vapor in the atmosphere above the equator. A greenhouse gas itself, the water vapor hastens more warming, which allows the air to hold more water, thus creating a feedback loop of warming. Ultimately that process leads to an increased temperature difference across the jet stream, which drives an increase in wind speeds, more shear at the boundaries, and more turbulence.

Williams became interested in modeling the climate’s effect on CAT after having pursued a PhD in fluid dynamics modeling of CAT and postdoctoral work in climate change. A decade ago, he and his Reading colleagues started using atmospheric climate models to simulate the 3D structure of the jet stream. Then, by averaging the results of multiple independent measures, they calculated the wind shear and turbulence at aircraft-relevant altitudes.

In 2013 the team concluded that increases in atmospheric carbon dioxide would affect turbulence. The researchers found that in the Northern Hemisphere winter, moderate-or-greater CAT, where the vertical acceleration of an aircraft exceeds 0.5 g and unsecured onboard objects would be dislodged, could increase by 40–170% by the middle of this century. In a study published this past March using more up-to-date models, the team suggests that for every 1 °C of global near-surface warming, moderate turbulence over the North Atlantic will increase by 9–14%.

Williams also wanted to explore how the influence of CAT has already been changing in a warming world. Measuring individual episodes of CAT over time was not possible; although modern aircraft record turbulence, those records are relatively recent, and the data are owned by the airlines. So Williams and colleagues examined historical 3D satellite data with location-specific readings of temperature, wind speed, pressure, and humidity. Combining those observations with fluid dynamics equations to fill in gaps, they were able to reconstruct the size and speed of the North Atlantic jet stream.

A map of the world showing the relative changes in moderate to severe turbulence from 1979 to 2020.
The probability of encountering moderate to severe clear-air turbulence increased considerably over much of the globe from 1979 to 2020, according to a recent study. The areas of increased turbulence include the US and North Atlantic (boxes). Credit: M. C. Prosser et al., Geophys. Res. Lett. 50, e2023GL103814 (2023)

In 2019 the Reading team reported a 15% increase in the amount of wind shear since 1979, when satellite observations became available. Its June examination over the Atlantic and the continental US, in which it employed a more rigorous analysis method, provides the best evidence yet that climate change has increased, and will continue to increase, the number of severe CAT events.

“There’s been a very clear, consistent message from these studies that we can expect two or three times as much severe turbulence by 2050 to 2080,” says Williams. And it’s likely this will not be limited to the Atlantic. Other work has modeled a similar impact across southeast China, the western Pacific, and northern India, with severe turbulence due to CAT in those regions projected to increase by about 15% by 2059.

Robert Sharman, an atmospheric scientist at NCAR, says that his team has found similar results for CAT increases. He adds that climate-related factors may lead to increases in other kinds of turbulence as well. Cloud-related turbulence is projected to increase, as is atmospheric instability due to rapid airflow over mountainous terrain.

Finding solutions

CAT’s invisibility to conventional radar-based weather monitoring systems makes developing early-warning systems for aircraft a challenge. Patrick Vrancken, from the Institute of Atmospheric Physics at the German Aerospace Center, has been part of several recent projects to remotely sense turbulence with lidar.

Working with French aerospace lab ONERA, Vrancken and his team detect Doppler shifts in the frequency of backscattered laser pulses that are caused by the rapid movement of air toward and away from an aircraft. The system spots turbulence by taking constant measurements at several points 100–300 meters above and below the aircraft’s flight path. If it detects rough air ahead, it can adjust the hinged ailerons and other components of the wings to counteract the lift forces and alleviate structural stresses on the plane. “The pilot doesn't even intervene,” Vrancken says. “It's really an automatic system.”

Boeing, together with the Japan Aerospace Exploration Agency and Mitsubishi, has also developed and tested a lidar-based system.

The tail of a glider aircraft is visible with the ground far below.
A glider soars high above New Mexico during a June 2021 flight to test an infrasound turbulence detection system developed by Stratodynamics. Credit: Stratodynamics Inc/UAVOS

The aviation company Stratodynamics has completed multiple flight tests of an infrasound system that is based on technology licensed from NASA. The system detects sub-20 Hz soundwaves that are emitted from regions of CAT due to the fluctuating pressures created in those regions. The company is testing its system on the tow plane that launches Airbus’s Perlan Mission II, a piloted stratospheric glider that is designed to reach up to about 27 kilometers in altitude, at the edge of space.

Whether such detection systems end up in commercial aircraft remains to be seen. Vrancken says his team is collaborating with a major European aircraft manufacturer and that he hopes the system will be ready for market in five to eight years. Given the evidence for increases in turbulence levels, he says, manufacturers are more interested in detection technologies than they were a decade ago, when these systems did not seem cost effective. He adds that airlines are interested in not only passenger safety but also efficiency gains to reduce carbon usage.

In the meantime, passengers should probably expect bumpier flights. Says Williams, “Since I started doing this work, I always wear my seat belt.”

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