Keep the barf bags handy

Air passengers face a bumpier ride due to climate change

DR PAUL WILLIAMS

Air passengers face various irritations when flying, from lost luggage to unappetising food. But one problem – turbulence – is not only unsettling for passengers but potentially dangerous too. What’s more, it is expected to worsen in future.

Evidence is growing that air turbulence is becoming stronger and more frequent because of climate change. In a 2013 study, we found that the amount of significant “clear-air turbulence” over the North Atlantic in winter would generally increase as CO2 levels double.

In our new study, published this week in Advances in Atmospheric Sciences, we refine these calculations to look at different severities of turbulence. We find that that the most severe – the kind that can launch passengers out of their seats and cause serious injuries – is set to become twice or even three times as common by the latter half of the century.

Despite the importance of this field of research for aircraft operations, our study is the first investigation into the future of severe turbulence, and it is the first time individual predictions have been made for all the different turbulence strengths.

Why turbulence occurs

Clear-air turbulence is caused by wind shear – changes in wind speed or direction in a relatively short vertical space – at altitude. The contrasting wind speed causes the different layers of the atmosphere to flow over each other, which can lead to instabilities and swirling air. It’s the most hazardous form of turbulence experienced by passengers.

As its name suggests, clear-air turbulence is invisible and so it cannot easily be detected and avoided. It tends to occur in and around jet streams, mountains, weather fronts, and thunderstorms.

Jet streams can be a pilot’s friend as much as their enemy. Jet streams are tubes of fast-flowing air that wrap around the globe. They can be thousands of miles long, but they are usually relatively narrow and only a few miles deep. Flying in jet streams can create an ideal tailwind, but also a headwind that’s best avoided.

The uneven temperature changes in the upper atmosphere associated with climate change – with the tropics warming faster than the poles at flight cruising altitudes – appear to be making the North Atlantic jet stream stronger at those altitudes. (This is different from the changes that are occurring in the lower atmosphere.) 

While the stronger jet stream is good news for getting us from North America to the UK faster, it also means round trips could take longer and clear-air turbulence encounters could become more frequent.

Increasing turbulence

Higher wind speeds clashing in the atmosphere create the potential for increasingly common episodes of turbulence over large parts of the North Atlantic.

We use a supercomputer and 21 different turbulence models to simulate how clear-air turbulence at an altitude of around 12km (39,000 feet) will be affected by climate change.

You can see the projected changes for the North Atlantic in the figure below. It shows projected increases (red shading) and decreases (blue) in turbulence for a doubling of atmospheric CO2 compared to pre-industrial levels. The darker shading shows where more of the models come up with the same result.

The number of clear-air turbulence models (out of 21 studied) to show an increase (red shading) or decrease (blue) in the amount of light-or-greater turbulence over the North Atlantic in winter when the CO2 level is doubled. Source: Williams (2017).

From our simulations, most of the models show large increases in turbulence of all strengths from light to severe. This level of agreement between the models gives us confidence in the results, although there is also a significant amount of variation from one model to the next.

On average, across the 21 turbulence models, we find that the amount of light turbulence increases by 59%, moderate by 94%, and severe by 149% for a doubling of CO2. You can see the results from the specific models in the chart below.

Percentage increase in transatlantic winter clear-air turbulence in five strength categories according to 21 different mathematical models of turbulence when the CO2 level is doubled. Source: Williams (2017).

It is important to note that severe turbulence is relatively rare, and airlines rightly point out that flights are as safe as ever during periods of turbulence. Modern aircraft are designed to deal with the forces encountered, albeit with the added risk of injury to passengers and crew caused by the unpredictable movements.

The risk of injuries, although small, is real. Serious injuries and fatalities can be caused by severe turbulence, which propels people and objects around with a force stronger than gravity. 

For “general aviation,” where aircraft tend to be smaller and therefore more susceptible, turbulence is blamed for around 40 fatalities a year in the US alone.

What impact could this have?

Aside from the risk of injuries, there are wider implications too. Diverting plane routes to attempt to avoid turbulence uses up more fuel and therefore pushes up fuel bills. Diversions also cause delays at airports, while injuries on board may force emergency landings and yet more delays.

According to the USA’s National Center for Atmospheric Research (NCAR), turbulence costs commercial airlines $150-$500 million a year. If turbulence increases, these figures could increase. 

Higher costs for airlines tend to be passed on to passengers, which could mean more expensive tickets.

The added risk of injury could also cause a knock-on impact for the insurance industry. The fact that one of the world’s busiest flight routes, the transatlantic corridor between Europe and North America, is in the region of the jet stream, means this is not an issue only affecting niche routes. This is literally the mainstream, where millions of passengers fly every year.

What does the future hold?

Aircraft CO2 emissions are known to contribute to climate change, but what our latest study reinforces is that this is a two-way relationship.

The CO2 produced from burning jet fuel is aviation’s biggest contributor, but aviation affects the climate in other ways, too. For example, the condensation trails (contrails) produced by flights can modify atmospheric temperatures. 

This means that, as well as making flights more efficient, airlines can explore other ways of reducing their impact on the climate. For example, airlines could reduce their contribution to climate change by making small changes to flight paths at relatively low cost.

A small but growing body of academic research suggests that the aviation sector will be adversely affected by climate change. On the plus side, this means the sector has much to gain from reducing its emissions.

Author Dr Paul Williams is an associate professor and Royal Society University Research Fellow at the University of Reading, specialising in atmospheric science. Published in: Williams, P. D. (2017) Increased light, moderate, and severe clear-air turbulence in response to climate change, Advances in Atmospheric Sciences, doi:10.1007/s00376-017-6268-2.