CONTRAILS REAL;  CHEMTRAILS NOT SO MUCH

Study: More eyes on the skies will help planes reduce climate-warming contrails

Images from geostationary satellites alone aren’t enough to help planes avoid contrail-prone regions, MIT researchers report.

Massachusetts Institute of Technology

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Four research figures show how contrails appear in two satellite views (left) and two photographs taken from the MIT Green Building.

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Credit: Courtesy of Prakash Prashanth, Marlene Euchenhofer, et al

Aviation’s climate impact is partly due to contrails — condensation that a plane streaks across the sky when it flies through icy and humid layers of the atmosphere. Contrails trap heat that radiates from the planet’s surface , and while the magnitude of this impact is uncertain, several studies suggest contrails may be responsible for about half of aviation’s climate impact.

Pilots could conceivably reduce their planes’ climate impact by avoiding contrail-prone regions, similarly to making altitude adjustments to avoid turbulence. But to do so requires knowing where in the sky contrails are likely to form. 

To make these predictions, scientists are studying images of contrails that have formed in the past. Images taken by geostationary satellites are one of the main tools scientists use to develop contrail identification and avoidance systems.  

But a new study shows there are limits to what geostationary satellites can see. MIT engineers analyzed contrail images taken with geostationary satellites, and compared them with images of the same areas taken by low-Earth-orbiting (LEO) satellites. LEO satellites orbit the Earth at lower altitudes and therefore can capture more detail. However, since LEO satellites only snap an image as they fly by, they capture images of the same area far less frequently than geostationary (GEO) satellites, which continuously image the same region of the Earth every few minutes. 

The researchers found that geostationary satellites miss about 80 percent of the contrails that appear in LEO imagery. Geostationary satellites mainly see larger contrails that have had time to grow and spread across the atmosphere. The many more contrails that LEO satellites can pick up are often shorter and thinner. These finer threads likely formed immediately from a plane’s engines and are still too small or otherwise not distinct enough for geostationary satellites to discern.

The study highlights the need for a multiobservational approach in developing contrail identification and avoidance systems. The researchers emphasize that both GEO and LEO satellite images have their strengths and limitations. Observations from both sources, as well as images taken from the ground, could provide a more complete picture of contrails and how they evolve. 

“With more ‘eyes’ on the sky, we could start to see what a contrail’s life looks like,” says Prakash Prashanth, a research scientist in MIT’s Department of Aeronautics and Astronautics. “Then you can understand what are its radiative properties over its entire life, and when and why a contrail is climatically important.”

The new study appears today in the journal Geophysical Research Letters. The study’s MIT co-authors include first author and graduate student Marlene Euchenhofer, undergraduate Sydney Parke, Ian Waitz, the Jerome C. Hunsaker Professor of Aeronautics and Astronautics and MIT’s vice president of research, along with Sebastian Eastham of Imperial College London.

Imaging backbone

Contrails form when the exhaust from planes meets icy, humid air, and the particles from the exhaust act as seeds on which water vapor collects and freezes into ice crystals. As a plane moves forward, it leaves a trail of condensation in its wake that starts as a thin thread that can grow and spread over large distances, lasting for several hours before dissipating. 

When it persists, a contrail acts similar to an ice cloud and, as such, can have two competing effects: one in which the contrail is a sort of heat shield, reflecting some incoming radiation from the sun. On the other hand, a contrail can also act as a blanket, absorbing and reflecting back some of the heat from the surface. During the daytime, when the sun is shining, contrails can have both heat shielding and trapping effects. At night, the cloud-like threads have only a trapping, warming effect. On balance, studies have shown that contrails as a whole contribute to warming the planet. 

There are multiple efforts underway to develop and test aircraft contrail-avoidance systems to reduce aviation’s climate-warming impact. And scientists are using images of contrails from space to help inform those systems. 

“Geostationary satellite images are the workhorse of observations for detecting contrails,” says Euchenhofer. “Because they are at 36,000 kilometers above the surface, they can cover a wide area, and they look at the same point day and night so you can get new images of the same location every five minutes.”

But what they bring in rate and coverage, geostationary satellites lack in clarity. The images they take are about one-fifth the resolution of those taken by LEO satellites. This wouldn’t be a surprise to most scientists. But Euchenhofer wondered how different the geostationary and LEO contrail pictures would look, and what opportunities there might be to improve the picture if both sources could be combined. 

“We still think geostationary satellites are the backbone of observation-based avoidance because of the spatial coverage and the high frequency at which we get an image,” she says. “We think that the data could be enhanced if we include observations from LEO and other data sources like ground-based cameras.”

Catching the trail

In their new study, the researchers analyzed contrail images from two satellite imagers: the Advanced Baseline Imager (ABI) aboard a geostationary satellite that is typically used to observe contrails and the higher-resolution Visible Infrared Radiometer Suite (VIIRS), an instrument onboard several LEO satellites. 

For each month from December 2023 to November 2024, the team picked out an image of the contiguous United States taken by VIIRS during its flyby. They found corresponding images of the same location, taken at about the same time of day by the geostationary ABI. The images were taken in the infrared spectrum and represented in false color, which enabled the researchers to more easily identify contrails that formed during both the day and night. The researchers then worked by eye, zooming in on each image to identify, outline, and label each contrail they could see. 

When they compared the images, they found that GEO images missed about 80 percent of the contrails observed in the LEO images. They also assessed the length and width of contrails in each image and found that GEO images mostly captured larger and longer contrails, while LEO images could also discern shorter, smaller contrails. 

“We found 80 percent of the contrails we could see with LEO satellites, we couldn’t see with GEO imagers,” says Prashanth, who is the executive officer of MIT’s Laboratory for Aviation and the Environment (LAE). “That does not mean that 80 percent of the climate impact wasn’t captured. Because the contrails we see with GEO imagers are the bigger ones that likely have a bigger climate effect.” 

Still, the study highlights an opportunity.

“We want to make sure this message gets across: Geostationary imagers are extremely powerful in terms of the spatial extent they cover and the number of images we can get,” Euchenhofer says. “But solely relying on one instrument, especially when policymaking comes into play, is probably too incomplete a picture to inform science and also airlines regarding contrail avoidance. We really need to fill this gap with other sensors.”

The team says other sensors could include networks of cameras on the ground that under ideal conditions can spot contrails as planes form them in real time. These smaller, “younger” contrails are typically missed by geostationary satellites. Once scientists have this ground-based data, they can match the contrail to the plane and use the plane’s flight data to identify the exact altitude at which the contrail appears. They could then track the contrail as it grows and spreads through the atmosphere, using geostationary images. Eventually, with enough data, scientists could develop an accurate forecasting model, in real time, to predict whether a plane is heading toward a region where contrails might form and persist, and how it could change its altitude to avoid the region. 

“People see contrail avoidance as a near-term and cheap opportunity to attack one of the hardest-to-abate sectors in transportation,” Prashanth says. “We don’t have a lot of easy solutions in aviation to reduce our climate impact. But it is premature to do so until we have better tools to determine where in the atmosphere contrails will form, to understand their relative impacts and to verify avoidance outcomes.  We have to do this in a careful and rigorous manner, and this is where a lot of these pieces come in.”

This work was supported in part by the U.S. Federal Aviation Administration Office

of Environment and Energy. 

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Written by Jennifer Chu, MIT News

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Journal

Geophysical Research Letters

DOI

10.1029/2025GL118386 

Article Title

“Contrail observation limitations using geostationary satellites”

Changing flight paths could slash aviation’s climate impact, study suggests

 ‘The climate opportunities and risks of contrail avoidance.’ 

University of Cambridge

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Small changes to aircraft flight paths to avoid the atmospheric conditions that create condensation trails – known as contrails – could reduce aviation’s global warming impact by nearly half, a new study suggests.

The study, led by researchers at the University of Cambridge, suggests that changing cruising altitude by a few thousand feet, either up or down, could prevent contrails from forming. Reducing or avoiding contrail formation in this way would also be faster and cheaper than other climate mitigation measures for the aviation industry, since the practice can be adopted with existing aircraft and fuel.

However, the researchers say that time is of the essence, and that the sooner airlines adopt contrail avoidance policies, the bigger the positive climate impact will be. Their results are reported in the journal Nature Communications.

Contrails are the thin white streaks seen behind aircraft flying at high altitude, and form when hot exhaust gases mix with cold, humid air at cruising altitude. Under the right conditions, the water vapour freezes into ice crystals, forming clouds that can persist for hours.

Contrails also trap heat in the atmosphere. Aviation contributes around 2–3% of global carbon dioxide emissions, but its total climate impact is larger because of non-CO₂ effects such as contrails. Interest in contrail avoidance has grown rapidly in recent years as governments and airlines search for ways to reduce aviation’s climate impact while the sector transitions to lower-carbon fuels.

“Contrail avoidance can often be as simple as changing the flight paths,” said lead author Dr Jessie Smith, from Cambridge’s Department of Engineering. “Often it’s even simpler than that – just moving slightly to a higher or lower altitude to avoid the areas of the atmosphere where contrails form.”

Smith and her colleagues modelled how altitude adjustments for contrail avoidance could affect aviation’s overall climate footprint. They found that such a programme, phased in between 2035 and 2045, could recover around 9% of the temperature budget the world has left before breaching the Paris Agreement’s 2°C limit.

However, they also found that if no action is taken, by 2050 aviation contrails will have added around 0.054°C of warming — 36% more than the warming attributable to aviation CO₂ over the same period.

“What surprised me was how quickly the temperature saving could be made,” said Smith. “Over a decade, you can take a really big chunk of aviation’s warming impact out very rapidly. That's unusual in climate science, where most changes take a very long time.”

The researchers also found that while rerouting aircraft can increase fuel use slightly, the reduction in warming from fewer contrails would more than offset the extra carbon dioxide emissions.

Implementing contrail avoidance would require airlines and air traffic controllers to adjust routes dynamically based on atmospheric conditions. Some aviation experts have raised concerns about whether such changes could increase workload for air traffic management systems, but the researchers say the adjustments required may be relatively modest.

Flights already alter their routes or altitude to avoid turbulence or bad weather, meaning similar systems could potentially be used to avoid contrail-forming regions.

“It's an operational change, not a technological one,” said Smith. “You don't need to modify aircraft. You just need to work out how it will operate, and then the system is already built for it — pilots do these manoeuvres all the time. That’s why we have more hope for this than for other interventions like sustainable aviation fuels, which face enormous infrastructure and supply-chain hurdles.”

Using a climate model that tracks temperature responses across 10,000 simulated scenarios, the researchers found that beginning contrail avoidance in 2035 rather than 2045 produces a temperature reduction at 2050 that is equivalent to roughly a 78% improvement in effectiveness. “In other words, waiting a decade has roughly the same effect as making the programme almost five times less efficient,” said Smith.

While more work is needed to improve forecasts of the atmospheric conditions that cause contrails and to better understand their climate effects, the researchers say that imperfect avoidance — even at 25% effectiveness — still delivers a meaningful climate benefit, and that starting early matters more than waiting for the technology to be perfected.

Scaling up contrail avoidance will require coordination from pilots, air traffic controllers, weather forecasters and policymakers, however. “The first step is demonstrating this works on a large scale through testing,” said Smith. “Once that's done, the policy can follow. But the modelling shows clearly that you do not want to wait for perfect conditions before you begin.”

Smith said the findings show the approach could play a major role in aviation’s climate strategy. “We’re not saying it solves everything,” she said. “But it could make a very big difference.”

Reference:
Jessie R. Smith et al. ‘The climate opportunities and risks of contrail avoidance.’ Nature Communications (2026). DOI: 10.1038/s41467-026-68784-8

 

---

Journal

Nature Communications

DOI

10.1038/s41467-026-68784-8 

Article Title

The climate opportunities and risks of contrail avoidance

Article Publication Date

2-Mar-2026

NASA and Boeing chase jet contrails with

 science of climate impact in doubt

AKA CHEMTRAILS

Dominic Gates, The Seattle Times on Dec 3, 2023

Scientific debate is getting heated over what to do about airplane contrails — the wispy lines of water vapor you often see trailing behind a jet.

Those harmless-looking vapor trails sometimes spread out to form thin cirrus clouds. Environmental activists and nonprofits focused on climate change routinely assert contrails contribute more to global warming than the carbon dioxide emitted from jet engines.

The aviation industry, under pressure to do something, has stepped up research into contrails.

In October, Boeing and NASA conducted flight tests out of Everett with a NASA DC-8 research plane flying behind a 737 MAX 10 to sniff its exhaust and analyze its contrails to test if so-called sustainable aviation fuel, or SAF, may reduce their incidence.

And Google, in a partnership with researchers at Breakthrough Energy — the Seattle-based climate action research group founded by Bill Gates — conducted a small experiment with American Airlines to test whether commercial pilots could avoid regions of the atmosphere likely to induce contrails. A larger trial with multiple air carriers, including Alaska Airlines, is planned for next year.

But last week, the consensus that contrails are so bad for the planet that we need to quickly find ways to reduce them was shattered.

David Lee, author in 2021 of the most influential study of the impact of aviation on the climate and chair of the U.N. Intergovernmental Panel on Climate Change's aviation working group, published a new and trenchant assessment of the myriad scientific uncertainties around the subject.

Co-authored by prominent U.K. climate researchers, the paper concludes that "the fundamental premise" that contrails are important enough to mitigate "is not yet established."

Beyond the uncertainty in the scale of the contrail warming effect, Lee points to a related cooling impact that could potentially cancel it out.

Lee argues that the data on aviation's non-CO2 impacts — chiefly the contrails and the effect of engine emissions on cloud formation — is so uncertain that any action to mitigate them "may be of limited effect or have unintended consequences."

He notes that rerouting planes to avoid airspace susceptible to inducing contrails means burning more fuel and emitting more CO2, which remains in the atmosphere for centuries while contrails dissipate in days.

Complex science and possible approaches

Contrails form when water vapor condenses around aerosols emitted by jet engines — particles and droplets, mainly soot and sulfur — to form ice crystals in the high altitude cold.

Most contrails dissipate quickly. Some spread out and persist as high, thin cirrus clouds.

These clouds can partially cool the planet during the day as they reflect sunlight away from Earth. However, the high, cold clouds also trap surface heat that would otherwise escape into space — a "greenhouse effect" parallel to that of CO2.

The overall impact of the contrail-induced clouds is warming, especially at night when there is no solar reflection.

The science is so complex that the scale of the contrail impact on the climate has always been very uncertain.

It varies not only according to the time of day or night but to the type of surface below. The scientific models of cloud formation remain highly imperfect.

Yet in recent years environmental activists have settled on data in Lee's 2021 assessment as proof that the warming effect of contrails is greater than that of the CO2 spewed from the engines.

Although the cited data comes with a colossal margin of error, that assertion has been repeated in the press and become the accepted basis for efforts to reduce contrails.

In an interview, Marc Shapiro, director of Breakthrough Energy's contrails project, cited the most recent IPCC assessment, which is based on Lee's 2021 paper: that contrails contribute between 1% and 2% of total human-caused climate change.

"That's staggering," Shapiro said. "Most people are astounded to hear that."

So Breakthrough, in collaboration with Google, is focused on finding ways for airplanes to avoid making contrails.

For this year's initial trial, Google used artificial intelligence to analyze satellite imagery, weather and flight data, and develop contrail forecast maps on 35 American Airlines routes.

Going one way, pilots flew over or under regions where the models indicated atmospheric conditions favored contrail formation. On the reverse leg, they flew straight through, ignoring the possibility of contrails.

The avoidance measures resulted in a 54% reduction in contrail-induced clouds, Shapiro said.

Next year, they plan "a much larger trial with multiple airlines," repeating the experiment on thousands of flights.

Shapiro acknowledged the fuzziness in the data about the impact of contrails but said the uncertainty "is overstated by the academic community."

It tends to lead to a conclusion that there's nothing to be done, he said, "and I really disagree with that."

Echoing that view, Matteo Mirolo, sustainable aviation manager for Transport & Environment, Europe's leading clean transportation advocacy group, said in an interview it's "important not to cross the line, where uncertainty is taken as an excuse for inaction."

Mirolo said operational changes to reduce contrails, like those Google and Breakthrough are promoting, should be an easy and cheap way to reduce aviation's contribution to global warming compared with reducing CO2 emissions.

In a phrase often used by climate activists on the subject, he called contrail prevention "low-hanging fruit" for the aviation industry.

But Lee, a professor of atmospheric science at Manchester Metropolitan University in England, pointedly dismissed that notion in an article posted on his university's website highlighting the new paper.

"There are no simple silver bullets or low-hanging fruit to solve the problem," Lee said.

Smaller impact

Political pressure to reduce contrails has grown strongest in Europe.

Beginning in 2025, European airlines will be required to monitor, report and verify the non-CO2 climate effects of their flights. By 2028, after an impact assessment, the European Commission has to make a proposal to address these effects — with contrails as the most salient.

This political push is premised on the belief that contrails are much worse for the earth's atmosphere even than greenhouse gases, and that belief arose from data in Lee's major 2021 assessment.

The analysis is cited by the IPCC, environmentalists, Boeing and NASA, and remains the definitive study of aviation's climate impact.

But interpretations vary, hence Lee's new effort to clarify its meaning.

The 2021 paper had offered "best estimates" from an analysis of multiple research studies that pegged contrails as by far the largest of aviation's non-carbon impacts, with just 34% of aviation's total warming effect contributed by carbon emissions and fully 57% by contrails.

However, that quantitative estimate for contrails is exceptionally mushy. It comes with an error margin of plus-or-minus 70%.

Drew Shindell, a professor of Earth Science at Duke University who assessed climate impacts for the IPCC, said the lack of rigor in the analysis of clouds and their formation creates the "enormous uncertainty" around the impact of contrails.

The latest IPCC estimate of the contribution of contrails and aviation-induced cirrus clouds to total global warming from all human activity is 0.7% to 3.7%.

Shindell said recent studies suggest it is "likely towards the low end of the [IPCC] range."

"The overall assessment is that it's probably in fact, very small," he said. "I don't personally think that it is something we have to worry about more than the CO2. I don't think the science supports that."

Even Shapiro of Breakthrough, whose research focus is on reducing contrails, said that "to be totally frank, our numbers are coming up on the low end of David Lee's [2021] estimates as well."

"There is kind of a growing consensus, the numbers in that paper are too high," he said, though quickly insisting that "it's not like coming out as insignificant."

Efforts to reduce contrails questioned

Lee's new assessment sharply critiques the misreading of his data.

The new paper states that the uncertain warming effect of contrails may be offset because the aircraft aerosols that trigger contrails also change how clouds form in complex ways that have an even less clear, but possibly larger, cooling effect.

Because of the uncertainty around how aerosols interact with clouds, with numerous studies showing wildly different results, the 2021 assessment hadn't even ventured an estimate for that.

Lee's new paper concludes that these competing factors taken together "could have a net positive or net negative" effect on global warming.

Lee specifically criticizes contrail avoidance solutions such as Breakthrough's.

In an email, Lee expressed concern about the "seemingly large (and in my view, ignorant) enthusiasm for navigational avoidance of contrails" because of the extra CO2 emitted.

The warming effect of that additional CO2 "lasts virtually forever," Lee wrote.

In addition, his paper says forecasting of atmospheric conditions is currently not sufficiently accurate to provide reliable contrail predictions.

His paper argues for more research and "careful analysis in order that perverse outcomes are avoided, and strategic investment decisions are based on a solid evidence base."

Breakthrough's Shapiro, responding to the Lee paper's rejection of contrail avoidance, counters that flying above or below contrail regions injects the same aerosols into the air, and so their likely cooling impact on cloud formation will be unaffected.

And Google's account of the American Airlines trial cites research suggesting that with sophisticated contrail avoidance "the total fuel impact could be as low as 0.3%."

"I believe this caution comes with a real climate opportunity cost," Shapiro wrote in an email.

Shapiro said that even if contrails end up being half a percent of total human climate impact rather than 2%, "that's still astounding."

"I'm not sure that changes my motivation or my approach to the problem," he said.

David Fahey, director of the National Oceanic and Atmospheric Administration's Chemical Sciences in Boulder, Colo., and a co-author with Lee of the influential 2021 paper, called the newly published assessment by Lee and his collaborators "stunning."

Fahey concurred that the uncertainties in the scientific models undermine the case for rushing to implement solutions.

He said climate activists pushing for immediate action on contrails are "painting a target in the sky and saying, 'That's the enemy.'"

Because there are so many unknowns, "we need to not do that," Fahey said, adding that Lee's new paper is "an antidote to the Kool-Aid drinking that's going on."

Boeing and NASA team up to study contrails

In the NASA/Boeing contrail study in October, pilots were not avoiding contrails but hunting them to gather data.

NASA research pilot Andy Barry flew the four-engine DC-8 research plane loaded with emissions sensors and cameras behind the Boeing MAX 10, which was painted in the colors of its eventual owner, United Airlines.

For three weeks, they took off on 5-to-6-hour flights seeking the right atmospheric conditions for contrails.

On the MAX, instruments drew in outside air and analyzed it.

In the much larger, heavier NASA chase plane, more instruments drew in the air from the MAX engines' exhaust — whether contrails were visible or not — and analyzed its composition.

Since a big part of the study was to examine how fuel might reduce contrails, the MAX was fitted with partitioned fuel tanks. The wings held a low-sulfur version of regular kerosene-based jet fuel. The center tank held 100% sustainable aviation fuel produced from biomass, oils and waste. Both fuels burn cleaner than regular jet fuel, producing less soot and aerosols.

With eight to 10 engineers in the back monitoring the instruments, the pilots flew one flight segment burning one type of fuel, then turned back and repeated the maneuver in the same air space burning the second fuel.

For Barry, it was tricky and demanding flying.

Communicating via radio with the Boeing jet, the NASA pilots aimed for a target air space about 200 feet wide and high, some 4 to 5 miles behind the Boeing jet.

They had to avoid the wake coming off the Boeing jet's wingtips that generated whirling vortices of doughnut trails behind it. And on days when there were strong crosswinds, they had to adjust laterally to find the exhaust stream.

"It's a small piece of sky to be in a moving environment," said Barry, interviewed in the DC-8 cockpit. "It's a highly turbulent environment, and that's why everyone's strapped in with five-point harnesses."

"We're doing this for 400-mile legs, and hand-flying all of this. Let's just say it's fatiguing," he said. "No autopilot is going to put you in this sweet spot to sample the data."

The complexity and uncertainty around contrail formation was illustrated on days when the pilots would spot an airplane in a neighboring pocket of air with a 20-mile contrail.

Barry said he'd think "it's gonna be easy today" to find contrails — "then we start getting on condition and there's nothing or it's a half-mile contrail."

Lee's new paper, while it warns contrail avoidance may increase carbon emissions, states that reducing contrails by burning SAF that also reduces carbon "could potentially be a 'win — win.'"

The results from the October flight tests will be published next year.

Aside from the work on contrails, the outcome should confirm a separate, substantial public health benefit from SAF. The lower aerosol emissions from the fuel will reduce pollution around airports all over the globe.

Dilemma for aviation

The aviation industry's goal of reducing carbon emissions to "net zero" by 2050 is fraught with major costs and heavy uncertainty.

Developing new aircraft technologies while vastly scaling up production of SAF is both immensely expensive and perhaps unattainable.

In contrast, reducing contrails, while not easy, seems enticingly possible.

Lee asserts that reducing carbon emissions should remain the priority.

Via email, he summed up aviation's dilemma and urged caution.

"As environmental scientists, we operate routinely with uncertainty," Lee wrote. "However, what is far more difficult to deal with is the uncertainty of being completely 'wrong' and doing permanent damage to climate in the process."

(c)2023 The Seattle Times Distributed by Tribune Content Agency, LLC. ©2023 The Seattle Times. Visit seattletimes.com. Distributed by Tribune Content Agency, LLC.

NOT CHEMTRAILS THEY ARE; 

Contrails: How tweaking flight plans can help the climate


Beth Timmins - Business reporter, BBC News
Fri, October 22, 2021, 

Aircraft condensation trails can have a significant impact on the climate, say researchers

Those wispy white lines that crisscross the skies after an aeroplane flies overhead are far less benign than their fluffy patterns might suggest.

Until now governments and industry have firmly focused on cutting CO2 emissions from aircraft - with good reason, as the aviation sector is responsible for around 2.4% of global CO2 emissions and a single flight can emit as much CO2 as many people do in an entire year.

But some scientists are now warning that the impact of radiation caused by aircraft condensation trails (contrails) could even be more significant.

Contrails, which heighten the effect of global warming, may account for more than half (57%) of the entire climate impact of aviation.

Contrails are water vapour that condenses as ice onto soot particles emitted from aircraft engines. They don't always occur as it requires certain atmospheric conditions: the air must be very cold, humid and "supersaturated" for ice to form.

They trap and absorb outgoing heat which otherwise escapes into space. This worsens at night when it is colder and the contrail has a longer lifetime. They can also have a less significant cooling effect, blocking incoming sunlight - but only during daytime.
Contrails: What changes might help?

Flight to the moon


Adjust flight routes so aircraft can avoid areas where they form


Teach pilots how to change altitude mid-flight without disrupting passengers' comfort


Introduce a contrails tax, and give airlines a refund if their flights are contrail-free

Contrails can last for seconds, hours or even a day in the atmosphere - and this determines the climate impact of a particular contrail will have. But recent research shows there could be a solution for this overlooked issue.

Prof 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.

Adjustments would mean an aircraft could avoid some of the regions of the atmosphere that are cold and humid enough to create contrails in the first place.

The HALO research aircraft which measures the impact of contrails

"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 climate impact," says Prof Stettler.

Recent research from Prof 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 measuring contrail properties and the light scattered by radiation. This allows them to evaluate their forecasts' accuracy and investigate the impact of radiation.

"Our results have been really positive. We were able to predict and avoid around 80% of the contrails with little cost," she says. Prof Voigt adds that very few flights would have to be deviated to win a "large climate impact".

Contrail sensors and measuring apparatus on HALO

While there were some uncertainties, her team were able to track down most of the contrails and avoid the correct areas.

"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, as it is low cost and effective," she says.

Royal Aeronautical Society fellow, Prof 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, a software change is relatively inexpensive, he says.

Prof Hayward says the next challenge is for airlines to work out how altitude changes of a "few thousand feet" can be made mid-flight to avoid contrails while also not disrupting passengers' comfort. A pilot would need to spot these in "sufficient time for an aircraft to adapt gracefully", he adds.

A Flightkeys cockpit system prototype, visualising the cost-optimum speed and altitude range at every point along a flight

But Prof Voigt does not believe this is necessarily a problem. She thinks flight comfort could improve as flight paths would avoid some of the sky's water vapour areas - which both form contrails and cause bumpy turbulence.

Raimund Zopp, former pilot and co-founder of Austrian flight services software company, Flightkeys, is working on contrail visualisations to programme into flight plan technology. The company plans to include contrail avoidance in their airline customers' flight plan trajectories by 2023.

"Only a very small portion of flights are causing the problem so you only need to change a few flight plans to have a huge effect," he says.

As a former pilot, Mr Zopp says that from a flight procedure perspective, adding this information would be easy. "The flight plans get programmed into the navigation of the plane system, but pilots need training on this new contrail aspect of the flight plan profile".

Marc Settler: "Tweaking the flight elevation by just a thousand feet can stop some contrails from forming"
Reducing contrails

Any action on climate change that doesn't relate directly to cutting emissions is lower down the priority list for governments and industry, because CO2 is the most important greenhouse gas for most sectors.

Yet unlike other sectors, aviation also has very significant non-CO2 impacts.

Prof Settler believes that people have hesitated to reduce contrails by diverting flights because of fears it would be completely unfeasible - that all flights might have to be changed or it would hugely increase fuel consumption. This latest research shows this is not the case.

Dr Jarlath Molloy, senior environmental affairs manager at the air traffic service provider, NATS, agrees that up until now, there has been a lack of focus on non-CO2 problems from the entire industry.

Yet from an operational perspective, tackling contrails is "just one extra element the aircraft would have to compute", he says, and it could even be managed in a similar way to how authorities already orchestrate groups of flights to avoid big winter storms.

"We're exploring the feasibility of it and what we'd have to do to manage flights looking for the same routes" Dr Molloy adds.

Preventing most of the damaging climate impact of contrails could cost less than $1bn a year

The Department for Transport says it is "currently considering" a range of responses to its Jet Zero consultation on how to "make the sector cleaner and greener", and that this strategy "will aim to address" aviation's non-CO2 impact.

Meanwhile atmospheric scientist Prof Ken Caldiera, from the Carnegie Institution for Science, makes a compelling case. He estimates preventing most of the damaging climate impact of contrails would cost less than $1bn (£720m) a year and the net value of the benefit could be more than a thousand times that.

"We know of no comparable climate investment with a similarly high likelihood of success," he wrote in the scientific journal Nature.

So could a financial penalty focus minds in aviation more quickly? Dr Andrew Gettleman of the US National Center for Atmospheric Research says while more research is needed, a contrail tax or avoidance rebate could be introduced if a carbon tax were approved.

"We haven't seen the overall regulatory scheme yet for carbon in aviation, but once we get a larger regime in place for mitigating climate change and reducing carbon - then we can tackle the contrail problem properly."