Contrails are not benign. Those wispy white contrails that criss-cross the skies are far less benign than their fluffy patterns might suggest.

     Until now, governments and industry have focused on cutting CO2 emissions from aircraft, because the aviation sector is responsible for around 2.4% of all global emissions. However, some scientists are now warning that aircraft condensation trails (contrails) could be even more significant than CO2 emissions. Contrails, they claim, may account for more than half (57%) of the entire climate impact of aviation.

     Contrails are water vapor that condenses as ice and latches on to the soot particles emitted from aircraft engines. These sooty ice drops trap and absorb heat from the engines that would otherwise escape into space. This phenomenon is enhanced at night, when it is colder and the contrail has a longer lifetime.

     Contrails don’t occur all the time. It requires certain atmospheric conditions: the air must be very cold, humid and “supersaturated” with water vapor for the ice to form. Contrails can last for seconds, hours or even a day, and this longevity determines the climate impact a particular contrail will have.

     Professor Marc Stettler, transport and environment lecturer at Imperial College London, says changing the altitude of fewer than 2% of flights could potentially reduce contrail-linked climate change by a staggering 59%. “Tweaking the flight elevation by just a thousand feet can stop some contrails from forming,” he explains. “A relatively small proportion of flights contribute to the majority of climate impact. So, if we can alter these flights, we can significantly reduce the effect.”

     Recent research from Professor Christiane Voigt, head of the cloud physics department at Mainz University, Germany, underlines this. She has been conducting trials with the German Aerospace Centre (DLR) to measure and mitigate the impact of contrails. Her team use high-altitude long-range (HALO) G550 research aircraft to gather their data. The aircraft carries wing-mounted instruments to measure contrail properties and the light scattered by radiation. “Our results have been really positive,” she said. “We were able to predict and avoid around 80% of the contrails, with little cost. We are at the beginning of a race to avoid them, and I have the impression that companies such as Lufthansa and Airbus, are really interested. It is low cost and effective.”

     Royal Aeronautical Society fellow, Professor Keith Hayward, is optimistic it may only need a software tweak to adjust many flight plans to avoid contrail creation, and that this could be done at a relatively low cost. Compared to the typical $200m cost of a passenger aircraft, or engine changes, which can run to $12m each, a software change is relatively inexpensive. Professor Hayward says the next challenge is for airlines to work out how altitude changes of a “few thousand feet” can be made mid-flight, while not disrupting passengers’ comfort. A pilot would need to spot these “contrail-forming pockets” in sufficient time for an aircraft to adapt gracefully”, he adds. But Professor Voigt does not believe this is necessarily a problem. She thinks flight comfort could improve, since these new flight paths would avoid some of the sky’s water vapor areas – which both form contrails, and cause bumpy turbulence.

     Professor Settler believes that people have hesitated to reduce contrails by adding flexibility to flight plans because of fears that all flights might have to be changed, or it would cause a huge increase in fuel consumption. This latest research shows this is not the case.

     Meanwhile atmospheric scientist Professor Ken Caldiera, from the Carnegie Institution for Science, makes a compelling case. He estimates that preventing most of the damaging climate impact of contrails, by software modifications to flights plans, would cost less than $1bn (£720m) a year. He believed the net value of the benefit would be more than a thousand times that figure. “We know of no comparable climate investment with a similarly high likelihood of success,” he wrote in the scientific magazine, Nature.

     I have to admit that I’ve always thought of contrails as somewhat decorative and benign, since I thought they were just water vapor. I certainly was not aware of their environmental impact, and even less aware of how potentially easy the problem is to fix.

     I’m sure it will take a while. It always does when governments are involved, especially on an international basis. However, it is an enlightened discovery, and solution, which one small step in the right direction. Please tell everyone you know.

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