As the Boeing 747 plummeted for what seemed about 300-plus meters, Fung Wing-ho seized the armrests as the shock lifted him out of his seat. Only a few rapid heartbeats later, a second, more violent jolt hit the plane and Fung saw two fellow passengers sail aloft, crashing into the ceiling of the fuselage. Passengers shrieked. In the galley, a flight attendant thrown to the floor howled in pain.
"The second jolt was the most ravaging one. It just threw us into chaos," said Fung, a 36-year-old passenger on the 747.
On June 4, Flight KL887 from Amsterdam, operated by Royal Dutch KLM, had made smooth progress. Then, 30 minutes before it was scheduled to land at Hong Kong International Airport, it hit what aviation experts suspect to be clear-air turbulence. The captain was announcing their imminent arrival when a sharp jolt cut him off mid-sentence.
It was a sunny Sunday morning, and no one registered the turbulence in their path－not the pilots or the air traffic controllers in Hong Kong because clear-air turbulence cannot be detected by radar. It's invisible, yet every year it is one of the leading causes of injuries to airline passengers and crew, hitting planes like a slapping hand.
Fung's seat belt was unfastened and it was sheer luck that he was spared the sudden falls that befell some of his fellow passengers.
When the plane landed safely at Terminal 1, paramedics sprinted aboard to treat six passengers who had not kept their seat belts buckled and had been thrown out of their seats, sustaining severe bruising, and two crew members who had minor injuries, according to a statement issued by KLM.
Clear-air turbulence, "the invisible troublemaker" as meteorologists call it, happens when two jet streams collide, with one moving faster than the other, in a cloudless sky. Experts predict that the phenomenon could show up with greater frequency as a result of climate change, caused by a rise in emissions of greenhouse gases. That means air passengers and crews are in for bumpier rides that pose a greater risk of injury.
The World Health Organisation predicts that climate change, aka global warming, will kill a wide range of plant and animal species as a direct result of temperature-related illnesses such as malaria. It's also been associated with a recent decline in levels of male fertility, and a rise in aggressive behaviour in humans, according to a growing number of studies published by the National Center for Biotechnology Information, part of the United States National Library of Medicine. In addition, the phenomenon is expected to play a major role in the "sixth great extinction", the ongoing, irreparable loss of species which is largely due to human activities.
Meanwhile, global warming also presents a growing peril for air travellers, according to Paul D. Williams, author of a meteorological study published in Advances in Atmospheric Sciences, a scientific journal, in April. Williams is a professor in the department of meteorology at the University of Reading in England.
He said he has discovered a causal link between clear-air turbulence and carbon dioxide in the atmosphere, and higher concentrations of atmospheric CO2 are linked directly to the growing number of clear-air turbulence incidents.
He used climate model simulations to assess the occurrence of the turbulence in response to a doubling of CO2 concentrations, from a preindustrial value of 280 parts per million to 560 ppm. The model predicted that light clear-air turbulence will rise by 59 per cent, moderate by 94 per cent and severe by 149 per cent, leading to life-threatening conditions by the end of the century.
Aircraft passengers and crews experiencing light to moderate turbulence are jostled a little, but severe clear-air turbulence can throw an aircraft out of control, while objects and people are tossed around the cabin.
Now, the CO2 concentration around the globe is estimated to be about 400 ppm. Current projections say that will reach 500-1,000 ppm by the end of the century, which is likely to make high-altitude travel less stable.
CHALLENGES IN THE AIR
Hong Kong encounters an average 15 days of clear-air turbulence annually. In the past two years, the Hong Kong Observatory has seen a rise in reports of severe cases.
"Severe CAT events have risen from just one, to now, two days per year," said Shun Chiming, the observatory's director, in an exclusive interview with China Daily.
He said it's hard to attribute the rise in severe incidents to global warming, as there are many other factors at play. However, he noted that more cases have been reported recently, especially in the upper layers of air to the northeast of Hong Kong, where jet streams are relatively rougher.
Usually, Hong Kong experiences the phenomenon from December to February, when the difference between polar and tropical temperatures is greatest and the jet stream－the driving force of the turbulence－is most violent.
West to east jet streams are driven by the disparity in temperatures between the poles and the tropics, because temperatures in polar regions are naturally much lower than in the tropics. That's partly because the sun's rays have to travel farther to reach the poles, in comparison with the equator, according to Williams of Reading University.
"However, the atmosphere is not warmed uniformly in response to our CO2 emissions. At cruising altitudes of 30,000 to 40,000 feet (9 to 12 kilometers), the tropics are warming faster than the poles. This effect is increasing the north-south temperature difference, which is what drives the jet stream. That will make jet streams more sheared in response and lead to more turbulence," he said.
In addition, many pilots point out that the topographical conditions at Hong Kong International Airport create conditions that leave aircraft more prone to turbulence. The airport is perched on reclaimed land in the north of mountainous Lantau Island and southwest of the hilly New Territories West. High-velocity winds can shoot out suddenly from between mountain gaps, creating even more powerful wind shear－a sudden change in wind direction or velocity－that can produce changes in headwinds or tailwinds encountered by planes. A strong wind shear can blow a plane off it's intended flight path, upward, downward or sideways.
About 70 per cent of all reported cases of wind shear and turbulence at the airport come from the strong winds that blow across the hills of Lantau. One former pilot in Hong Kong said the prevailing conditions make the airport a "not pleasant" place for liftoff and touchdown.
Nigel Leung, founder of the Aero Institute and a former first officer for Dragon Airlines, now Cathay Dragon, in Hong Kong, has 5,200 hours of flight time. He experienced being hit by a violent downdraft when leaving the city's airport in 2010. Leung's airbus was jolted hard as if hit by a giant fist that came out of a large pocket of storm cloud above. Crew and passengers were nearly ejected from their seats, but seat belts saved them from injury.
Williams said there is no way to avoid turbulence. "A lot of the turbulence encountered by planes in tropical regions, like Hong Kong, will be convective turbulence from storms," he said, noting that storms gain energy from the high temperatures and rise to altitudes higher than planes can achieve.
Clear-air turbulence is believed to be an added threat to aviators flying in and out of Hong Kong. Leung said pilots are used to wind shear close to the airport, but the turbulence is invisible.
"We have no choice but to bite the bullet and endure the violent jarring," he said.
Increased threats from clear-air turbulence may be approaching rapidly, with the doubling of preindustrial CO2 concentrations seen as certain within the next 50 years.
Pilots can access predictive forecasting by weather centres when they take off and land, but many get their information from their peers flying the same routes.
Light detection and ranging technology is one of the developments the aviation industry hopes will provide advance warning of air turbulence. The device is mounted on the nose of the aircraft, where it emits ultraviolet light signals to detect air density 10 to 15 km ahead. The signals are reflected back to the airplane, revealing any rapid shifts in air density that could spell the possible presence of clear-air turbulence ahead.
However, the median speed of commercial jets is about 900 km/h, while the technology, which can only detect turbulence a short distance away, may not provide enough warning to allow pilots to take evasive action, according to experts.
Moreover, the technology is still at an experimental stage, and is heavy and expensive, so it hasn't been adopted by commercial airlines. Many experts believe it will find acceptance as an onboard technology, but first it will have to be miniaturized and made more cost-effective.
"The planes that will be in the air at the end of this century are probably being designed now," Williams said. He recommended that aircraft manufacturers put climate change above their fleet design horizons, and equip their aircraft for the long-term effects of global warming.
Shun, from the Hong Kong Observatory, said his meteorologists are making preparations, and have teamed up with Cathay Pacific, the city's largest airline, to garner flight data from its fleet. "We intend to have a better grasp of how clear-air turbulence reacts to the trend of global warming by interpreting the statistics," he added.
In 2015, 36.3 billion tons of CO2 were pumped into the atmosphere, a rise of 63 per cent from 1990. In the decades to come, CO2 levels will rise and so will temperatures.
According to Williams, the scientist from England, turbulence will worsen as the atmosphere gets hotter, and incidents are likely to become 2.5 times more common, if CO2 emissions are not brought under control.
Fung, who didn't fasten his seat belt during the incident involving Flight KL887, considers himself "very lucky" not to have been tossed around the cabin, unlike his wife who was jolted out of her seat.
"I'm not scared of flying, but the incident taught me to be better prepared for the worst, and I intend to keep my seat belt fastened all the time," he said.