Climate Change Could Mean Bumpier Flights
Scientists predict the frequency of clear-air turbulence will double by mid-century.
Areas of predicted clear-air turbulence at cruising altitudes over the North Atlantic Ocean on a simulated winter day under doubled carbon dioxide conditions.
Image courtesy Paul Williams, University of Reading
Buckle up - thanks to climate change, airline passengers may be in for a bumpier ride.
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Those bumps could also become stronger due to the intensification of conditions that lead to a type of turbulence called clear-air turbulence, according to the study published online today in the journal Nature Climate Change.
Unlike the turbulence associated with storm clouds, clear-air turbulence is mainly associated with jet streams - large rivers of air in the atmosphere - and can occur in clear blue skies. (Related: "Severe Weather More Likely Thanks to Climate Change.")
"The pilot can't see it and the sensors onboard can't see it - that's why it's a particularly dangerous form of turbulence," said Paul Williams, an atmospheric scientist at the University of Reading in the United Kingdom and lead author of the new paper.
Turbulence occurs mostly because of a change in airspeed with respect to height, said Mitchell Moncrieff, an atmospheric scientist at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, who was not involved in the study.
It happens mostly in in frontal areas - places where air masses of different characteristics meet - and jet streams.
Since climate change will accelerate the jet stream over the North Atlantic, Williams said, that river of air will flow faster, making the atmosphere more susceptible to turbulence - much like a fast-running river develops white water.
Double Leads to Trouble
Climate models have shown that climate change will draw the jet stream over the North Atlantic even farther north, said Williams. He and his colleague, Manoj Joshi of the University of East Anglia in the United Kingdom, wanted to know what that would mean for clear-air turbulence.
So the researchers took a well-known climate model from the Geophysical Fluid Dynamics Laboratory in New Jersey and looked at the jet stream over the North Atlantic Ocean during the winter months, when jet stream strength peaks.
Williams and Joshi doubled the amount of atmospheric carbon dioxide with respect to pre-industrial conditions in their climate model and looked at how clear-air turbulence conditions compared with respect to a world in which carbon dioxide levels remained at pre-industrial levels.
They then took wind and temperature profiles from that future scenario and plugged the numbers into 21 models forecasters currently use to predict clear-air turbulence.
"All 21 were showing an increase in the strength of turbulence," Williams said, while 20 models showed an increase in the frequency of turbulence, with some models predicting at least a doubling in frequency.
These increases are for moderate turbulence, he added. "So your drink might spill over, you might lift out of your seat a little bit - certainly the pilot would have the seatbelt sign on."
The volume of different models predicting the same result was striking, Williams said.
Shake, Rattle, and Roll?
This doesn't mean that airplanes will start to shake apart over the North Atlantic by mid-century.
The most severe turbulence, more powerful than what Williams modeled in his study, exerts about 1.5 G forces on an airplane, said Bret Jensen, a spokesperson for Boeing, an aircraft manufacturer based in Seattle, Washington.
But airplanes are designed to withstand 2.5 G forces before taking damage, and 3.5 G forces before experiencing structural failures, he added.
The real danger lies with injuries, particularly to the flight crew, said Bruce Carmichael, director of the Aviation Applications Program at NCAR, who was not involved in the study.
Since flight attendants aren't usually buckled in, they can get thrown around the cabin, he said. "They're really the ones most at risk."
A Valid Approach
Though Carmichael thinks Williams and Joshi took a novel and legitimate approach to the problem of predicting future occurrences of clear-air turbulence, "you have to be a little careful about how you interpret the results," he said.
Climate models are meant to look at large trends over big areas, Carmichael said - on the scale of hundreds of kilometers. Clear-air turbulence, meanwhile, occurs over tens of kilometers.
It's hard to know how accurate a climate model would be at predicting atmospheric conditions over the relatively small area right around an airplane, he explained.
Moncrieff agrees and said he looks forward to seeing how the study's results stack up to other models. Each climate model has its own quirks and biases, he explained, and so they can come up with varying future atmospheres. (Learn about climate modeling.)
In the meantime, Williams said his work has changed his habits when he flies: "I used to not keep my seatbelt fastened, but now I always do."
© 1996-Thu Aug 25 08:26:34 EDT 2016 National Geographic Society.