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Climate-change turbulence risk for air carriers

Researchers at two UK universities have identified the likelihood of increased turbulence in aircraft over the coming half-century. But can we doing anything to stop it?

Climate change has been blamed for a dizzying array of problems – melting ice caps, severe storms, species migration, death and destruction – but one of the more specific phenomena to be attributed to it is bumpier flights.

Turbulence is the dread of most air passengers. Despite endless reassurances that modern planes can withstand the wildest turbulence the atmosphere has to offer, even the gentlest of bumps has some passengers grabbing their armrests in alarm.

The bad news for these nervy travellers is that by the middle of this century, according to research from the Universities of Reading and East Anglia, climate change will lead to bumpier transatlantic flights. Consequently, journey times could lengthen while fuel consumption and emissions look set to increase.

The modelling produced by the researchers suggests the average strength of transatlantic turbulence could increase by between 10 and 40 per cent, and the amount of airspace likely to contain significant turbulence by between 40 and 170 per cent, where the most likely outcome is around 100 per cent.

The research focused on clear-air turbulence in winter, which is particularly problematic to airlines, because clear-air turbulence is invisible to pilots and satellites, and winter is when it peaks. "It can be a problem because usually when you are flying through clouds it will be during take-off and descent so all the passengers should have their seatbelts on at that time," Dr Paul Williams from the University of Reading and an author of the report says. "While it may not be pleasant for the passengers it is not particularly dangerous. However, when you are flying at 11km up through clear blue skies, the seatbelt sign is off and people are moving around the cabin when you suddenly hit turbulence and that is when people can suffer injuries."

Refining predictions

Williams has been researching the effects of turbulence for five years, refining an algorithm to make aviation turbulence predictions that are valid for a period from an hour to over a day. Williams admits that these predictions are certainly not perfect. "They do not get it right 100 per cent of the time, I think the success rate is about 70 per cent, and so I worked on developing a new method for this a few years ago," he said.

Williams has also worked on climate-change problems and he realised there was an opportunity to bring together these two independent strands of research. "The climate science community and the aeroplane turbulence community are two very different groups of people. They usually do not talk to each other or attend the same conferences. I think this is why no one has ever looked at this before.

"It is shocking, really, that this is the first ever study to look at how climate change will affect the future of aviation turbulence. I was astonished when I went in search of literature and could find no previous work on it. I had worked with both communities before and because I had one foot in each camp I think I could see how to link them and bring them together."

There is anecdotal evidence from regular fliers that incidences of turbulence are increasing but Williams warns that we need to be wary of a psychological bias. He explains that when you hear something is getting worse you are highly vigilant towards it and notice it a lot more than you may have previously. "However, the independent data seems to back-up an increase," he says. "The two studies that have come out so far that claim to have some evidence of an increase have both removed themselves from their findings to some extent and come up with other explanations such as changes in the data, or the statistical significance perhaps not quite being there. I notice it too and other people I talk to have mostly suggested that they have noticed an increase as well."

Williams's original algorithm takes gridded atmospheric data from a computer model output; the same sort of model that would predict the weather. "That is used as a data source," Williams adds. "There are ways of taking that data set and estimating from it where and when the turbulence would be. I should say the resolution and grid spacing of the data is really far too coarse to explicitly predict if there is any turbulence in it."

Using a grid of tens of kilometres means that the data points are tens of kilometres apart, but now scientists have developed methods of taking data of this resolution and estimating where the turbulence will be using dozens of different measures to do these calculations. "In the paper we have used those exact same mathematical algorithms and instead of using data that corresponds to a weather forecast for tomorrow's weather as being our input, we take two different climates in relations and have looked at different turbulence statistics from the pre-industrial to the double CO2."

Trouble spots

Any location in which the wind speed changes with altitude – a wind shear effect – will always tend to be an unstable situation. There is a compensating, stabilising mechanism, which suppresses the instability – that is, the higher up you go in the atmosphere the less dense it gets – called stratification. In the same way that oil will rest on top of water in a stable situation, the stratification of the atmosphere is a stabilising effect that will try to inhibit the turbulence that is trying to be produced by the wind speed. The jet stream is a high wind-shear region, the wind is changing rapidly with the height, and that is why you tend to get this instability in the jet stream itself.

"Certainly, if you fly over a mountain range there are 'mountain waves', which result from the interaction of the airflow of the mountain itself that creates a 'gravity wave' that propagates up into the atmosphere and eventually breaks up," Williams adds. "It increases the shear locally; that is one reason you can get a lot of turbulence when you're flying over a mountain."

The effect that climate change is having on flying conditions boils down to warming by different amounts in different places. It is the difference the temperature between the North Pole and the equator at flight altitudes that drives the jet stream, and the difference is getting larger due to climate change. "At 10km up it is getting warmer more at the equator than it is at the North Pole," Williams explains. "That makes the jet stream flow faster, which is a well-known climate phenomenon. What we have looked at is the particular consequences of that for clear-air turbulence because a stronger jet stream will give you more turbulence.

"We have looked at these two computer simulations and tried to estimate the turbulence in both cases and indeed we have found that the stronger jet stream does mean more turbulence and we have been able to put some numbers to that to show exactly what the percentage increase is."

The concept seems self-evident, but someone is still needed to do the actual work. "I think it is consistent with our understanding of climate science and changes to the jet stream, but the increases are not small," Williams says. "We are looking at the most likely outcome as being a doubling of the amount of airspace containing significant clear-air turbulence – an enormous increase."

Levels of turbulence (light, moderate, severe and extreme) are all measured in terms of G-force that is felt by the plane. Light turbulence is anything up to 0.5 g, moderate is 0.5 g to 1 g, severe is 1-2 g and extreme is anything above 2 g.

Whenever a pilot passes through turbulence he or she is supposed to log it. The PIREP database hold records of not only turbulence but other hazards such as icing of the wings.

Advanced algorithms

The research itself hasn't helped the formulation of more accurate algorithms but has given extra motivation to the need to develop them. "If the atmosphere is going to contain twice as much airspace containing turbulence, the only way planes are going to be able to avoid a 100 per cent increase in the amount of time with the seatbelt sign on is if they know where the turbulence will be," Williams says. "In order to get that we need to improve these algorithms that are used and so I think one consequence of this paper is to make it even more clear that we have got to develop better algorithms pretty urgently."

The major airlines are wary about discussing the subject, and Williams himself has had no formal contact with any of the airlines although he has given a talk at an aviation turbulence symposium in the US where pilots and aviation industry representatives were present and he had "informal chats" with them.

"I think from a pilot's perspective it is not an issue of safety, but an issue of comfort," Williams says. "It makes the passengers' flight more of a nuisance and one in which drinks will get spilt, but that is not a huge hazard. There are hundreds of injuries each year, mainly to crew as they are out of their seats trying to serve passengers as the turbulence hits. So, while this is not a safety issue primarily, it does stand to reason that an increase in turbulence will lead to an increase in injuries."

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