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Tuesday, December 05, 2023

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."

Friday, August 09, 2024

Study on planet-warming contrails “a spanner in the works” for aviation industry

CONTRAILS NOT CHEMTRAILS

Imperial College London

Modern commercial aircraft flying at high altitudes create longer-lived planet-warming contrails than older aircraft, a new study has found.

The result means that although modern planes emit less carbon than older aircraft, they may be contributing more to climate change through contrails.

Led by scientists at Imperial College London, the study highlights the immense challenges the aviation industry faces to reduce its impact on the climate. The new study also found that private jets produce more contrails than previously thought, potentially leading to outsized impacts on climate warming.

Contrails, or condensation trails, are thin streaks of cloud created by aircraft exhaust fumes that contribute to global warming by trapping heat in the atmosphere.

While the exact warming effect of contrails is uncertain, scientists believe it is greater than warming caused by carbon emissions from jet fuel.

Published today in Environmental Research Letters, the study used machine learning to analyse satellite data on more than 64,000 contrails from a range of aircraft flying over the North Atlantic Ocean.

Modern aircraft that fly at above 38,000 feet (about 12km), such as the Airbus A350 and Boeing 787 Airliners, create more contrails than older passenger-carrying commercial aircraft, the study found.

To reduce jet fuel consumption, modern aircraft are designed to fly at higher altitudes where the air is thinner with less aerodynamic drag, compared to older commercial aircraft, which usually fly at slightly lower altitudes (around 35,000ft/11km).

This means these higher-flying aircraft create less carbon emissions per passenger. However, it also means they create contrails that take longer to dissipate – creating a warming effect for longer and a complicated trade-off for the aviation industry.

Double whammy of warming

Dr Edward Gryspeerdt, the lead author of the study and a Royal Society University Research Fellow at the Grantham Institute – Climate Change and the Environment, said: “It's common knowledge that flying is not good for the climate. However, most people do not appreciate that contrails and jet fuel carbon emissions cause a double whammy warming of the climate.

“This study throws a spanner in the works for the aviation industry. Newer aircraft are flying higher and higher in the atmosphere to increase fuel efficiency and reduce carbon emissions.

“The unintended consequence of this is that these aircraft flying over the North Atlantic are now creating more, longer-lived, contrails, trapping additional heat in the atmosphere and increasing the climate impact of aviation.

“This doesn’t mean that more efficient aircraft are a bad thing – far from it, as they have lower carbon emissions per passenger-mile. However, our finding reflects the challenges the aviation industry faces when reducing its climate impact.”

The study did confirm a simple step that can be taken to shorten the lifetime of contrails: reduce the amount of soot emitted from aircraft engines, produced when fuel burns inefficiently.

Modern aircraft engines are designed to be cleaner, typically emit fewer soot particles, which cuts down the lifetime of contrails.

While other studies using models have predicted this phenomenon, the study published today is the first to confirm it using real-world observations.

Co-author Dr Marc Stettler, a Reader in Transport and the Environment in the Department of Civil and Environmental Engineering, Imperial College London, said: “From other studies, we know that the number of soot particles in aircraft exhaust plays a key role in the properties of newly formed contrails. We suspected that this would also affect how long contrails live for.

“Our study provides the first evidence that emitting fewer soot particles results in contrails that fall out of the sky faster compared to contrails formed on more numerous soot particles from older, dirtier engines.”

Private jets the worst offenders of contrails

Even higher in the sky, the researchers found that private jets create contrails more often than previously thought – adding to concerns about the excessive use of these aircraft by the super-rich.

Despite being smaller and using less fuel, private jets create similar contrails to much larger commercial aircraft, the analysis found, which surprised the researchers.

Private jets fly higher than other planes, more than 40,000 feet above earth where there is less air traffic. However, like modern commercial aircraft creating more contrails compared to lower-flying older commercial aircraft, the high altitudes flown by private jets means they create outsized contrails.

Dr Gryspeerdt said: “Despite their smaller size, private jets create contrails as often as much larger aircraft. We already know that these aircraft create a huge amount of carbon emissions per passenger so the super-rich can fly in comfort.

“Our finding adds to concerns about the climate impact caused by private jets as poor countries continue to get battered by extreme weather events.”

Journal

Environmental Research Letters

DOI

10.1088/1748-9326/ad5b78

Article Title

Operational differences lead to longer lifetimes of satellite detectable contrails from more fuel efficient aircraft

Article Publication Date

7-Aug-2024

Friday, October 18, 2024

Why we should care about airplane contrails

Anne-Sophie Brändlin

DW

Condensation trails or contrails — the white, feathery lines behind airplanes — could have as big an impact on the climate as the aviation sector's CO2 emissions. Here's why, and what we can do mitigate the effects.

The climate impact of aviation could be three times higher than thought because of non-CO2 emissions like contrails

Image: Nicolas Economou/picture alliance

When thinking of flying's environmental impact, the CO2 emitted from burning jet fuel is usually what springs to mind. But there's another, lesser understood climate culprit hiding in plain sight: condensation trails.

The wispy, cloud-like formations left by airplanes as they traverse the skies may look innocuous, but the climate impact from 'contrails' could be similar to aviation CO2.

"I'm actually more worried about contrails at this point than I am about CO2 emissions because it is an impact that has not been internalized by the industry in any shape or form," said Jayant Mukhopadhaya, a lead aviation researcher for the International Council on Clean Transportation (ICCT), a US environment think tank.

A 2021 study suggests contrails and other non-CO2 emissions could account for up to two-thirds of aviation's total climate impact. Contrails could represent 57% of that impact — roughly the same as CO2 emissions from burning fuel.

The aviation industry is currently thought to be responsible for 2 to 3.5% of global CO2 emissions annually.

"But if you start taking into account these other pollutants that aviation is responsible for, the aviation sector is actually accounting for a far higher amount of warming than we usually ascribe to it," said Mukhopadhaya, adding that it could be three times greater than previously assumed.

How are contrails formed and why are they a problem?

Contrails form when airplanes fly through very cold, humid pockets of air in the upper atmosphere. When plane engines burn jet fuel, water vapor condenses on particles from the air and exhaust to form ice crystals. If there are a lot of ice crystals, they make cirrus clouds.

"Some of them persist for only a few seconds or minutes, others for hours or even days, depending on the amount of moisture and the temperature," said Patrick Minnis, a senior NASA scientist researching the climate impact of contrails and the behavior of cirrus clouds.

Contrail cirrus clouds trap heat in the atmosphere. The warming effects are worse at night when they're not also reflecting sunlight back into space.

"Producing contrails is basically like wrapping a blanket around Earth every year that traps heat and warms the planet," said Mukhopadhaya.
How much warming do contrails cause?

While scientists say contrails cause warming, there's less consensus regarding the degree and timescale, as well as what they mean for climate change.

"The exact magnitude of the warming impact of contrails is uncertain. The estimates range from 30% of the impact airplane of CO2 to as much as four times, so it's quite a large range," said Mukhopadhaya.

That's because there's a level of uncertainty in most contrail studies so far due to a lack of sufficient data. But it also comes down to the metrics scientists use to measure contrail impact. For instance, according to Mukhopadhaya, unlike for long-lasting CO2, it doesn't make sense to look at contrails over a 100-year period because they dissipate so quickly.

"What we're interested in is how much global mean surface temperature changes because of these pollutants," said Mukhopadhaya. "And contrails could be responsible for roughly 15% of our available carbon budget to reach the 1.5 degrees goal by 2050."

"There are direct incentives to reduce the climate impact of CO2 emissions. Those don't exist for contrails," said MukhopadhayaImage: Florian Gaertner/picture alliance
How to cut contrails' climate impact

As contrails are short-lived compared to CO2 emissions their warming impact would disappear quickly if efforts were made to minimize their formation, say experts.

One solution is switching to "cleaner" fuels with less sulfur, like hydrogen. This would reduce the amount of air pollutants released by jets and lower the life span of contrails, according to NASA's Patrick Minnis.

Last year, the European Union introduced legislation setting mandates for sustainable aviation fuels. By 2050, all jet fuels sold in European countries will have to consist of 70% "sustainable aviation fuels."

"That will help not only reduce the non-CO2 impact from aviation, it will also have a significant impact on the sector's CO2 emissions," Mukhopadhaya said.

Scientists have also found that not all flights create contrails in the first place. It all depends on weather conditions and the aircraft's trajectory. Rerouting less than 2% of flights in Japan could have reduced the warming effect of contrails by nearly 60%, a 2020 study found.

But predicting beforehand which flight routes will cause contrails and how much warming these contrails will cause is very difficult.

"You have to consider the atmosphere like a cake. The top layer of the cake is the upper atmosphere and within that there's all sorts of striations of moisture. So, knowing exactly where those layers are that have a lot of moisture in them is something that's been relatively difficult to predict," said Minnis.

Only a tiny propotion of the total flight distance is responsible for persistent contrails that cause warmingImage: Dreamstime/IMAGO

Proper prediction would require improvements in satellite work and a lot more expensive computer storage, added the NASA researcher.

Flying planes lower has been touted as another solution because contrails form at higher altitudes but the "problem is that if you go to lower levels, you're going to have more turbulence, and you're going to use more fuel," Minnis said. And that means higher CO2 emissions.

"But we estimate the impact of those additional CO2 emissions to be significantly lower than the impact from contrail production," said ICCT's Mukhopadhaya.
More data needed

A 2024 study by the International Air Transport Association (IATA), an industry trade body, suggests more data needs to be collected to understand the non-CO2 impacts of aviation so solutions can be found.

Improvements in satellite work could help mitigate contrail formation in the future

Image: ESA/dpa/picture alliance

Airlines such as Lufthansa, Air France, KLM and American Airlines have already started contrail avoidance test flights above or below at-risk areas with the help of satellite images, weather data, software models and AI prediction tools.

"That's a great first step, as about 50% of warming due to contrails happens over the US, EU and the North Atlantic — three regions with high aviation activities," said Mukhopadhaya.

A 2024 Cambridge University report suggests that accelerating the deployment of a global contrail avoidance system could reduce aviation's climate impact by 40%.

The EU has also agreed airlines will have to monitor and report the climate impact of contrails in a move opposed by the industry.

"The aviation industry has been delaying action for around 20 years now, driven by the lack of uncertainty regarding contrail science," said Mukhopadhaya. But the fact research is now being done in real life rather than on computer simulations is "very promising for avoidance measures in the future," he added.

Edited by: Jennifer Collins

Thursday, November 06, 2025

Long-lived contrails usually form in natural ice clouds

Research team identifies common environmental conditions for the formation of contrails and provides initial insights into their impact on the climate

Johannes Gutenberg Universitaet Mainz

image:
Contrails over Jülich, embedded in very thin and therefore barely visible cirrus clouds
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Credit: Photo/©: Andreas Petzold

Long-lived contrails form predominantly not in cloud-free skies, but within already existing ice clouds. This is the conclusion reached by a team of scientists from Forschungszentrum Jülich, the University of Cologne, the University of Wuppertal, and Johannes Gutenberg University Mainz (JGU). Using extensive observational data, the researchers were able, for the first time, to systematically determine the atmospheric conditions under which long-lasting contrails form – whether in cloudless skies, in very thin and barely visible ice clouds, or in more clearly visible ice clouds, known as cirrus clouds. The result: more than 80 percent of all persistent contrails form within pre-existing clouds, mostly within natural cirrus clouds. The effects of this on the climate are not yet clearly understood. The study, now published in Nature Communications, provides important insights for further research – and, beyond that, strong arguments for taking cloud cover into account when planning flight routes adapted to climate considerations.

Effect of natural and man-made ice clouds on the climate

Contrails are a visible signature of daily air traffic in the sky. They form when the hot exhaust gases from aircraft engines mix with the cold air at an altitude of around ten kilometers. In dry air, most contrails dissipate quickly. In cold and humid air, however, they can persist for several hours and develop into extensive ice clouds or cirrus clouds. Cirrus clouds are thin ice clouds that occur at altitudes of about eight to twelve kilometers and often appear as fine, fibrous veils in the sky. The overall climate impact of these cirrus clouds formed from contrails is greater than that of the direct CO₂ emissions produced by air traffic.

The decisive factor for their climate impact is whether the man-made clouds form in a blue, cloudless sky or within existing natural cirrus clouds. High ice clouds, whether natural or man-made, exist at cold temperatures below -40°C. Although they often appear optically very thin, they can act like a blanket that prevents heat from escaping from the atmosphere into space, thereby contributing to the greenhouse effect. Only when the clouds are very dense and the sun is barely visible does the amount of sunlight reflected back into space become large enough to produce a cooling effect on the climate.

Accordingly, artificial clouds formed by contrails affect the climate differently depending on their environment: Under clear conditions – such as blue skies or very thin cirrus clouds – they tend to contribute to warming, because they trap some of the Earth's radiation that would otherwise escape into space, while allowing sunlight to pass through. In dense, clearly visible cirrus clouds, however, the opposite effect can occur: Contrails reflect more sunlight than they absorb heat radiation, leading to a slight cooling effect. How exactly contrails and natural cirrus clouds influence each other is still poorly understood.

"Our results show that we need to take a more differentiated view of the climate impact of contrails in the future," says Professor Andreas Petzold from the Institute of Climate and Energy Systems – Troposphere (ICE-3) at Forschungszentrum Jülich. "If most persistent contrails occur within natural clouds anyway, it might be more effective to plan climate-relevant flight routes not only according to clear skies but also with regard to existing ice cloud structures."

For the study, the research team used measurement data on temperature and water vapor collected by commercial aircraft flying over the North Atlantic between 2014 and 2021. These aircraft are part of the European research infrastructure IAGOS (In-service Aircraft for a Global Observing System, https://www.iagos.org/), which is co-coordinated by Forschungszentrum Jülich. IAGOS aircraft are equipped with instruments that continuously collect atmospheric data during scheduled operations – something unique worldwide.

Mainz contribution to the study: model calculations of radiative forcing

The data evaluation was supplemented by model calculations on radiative forcing. "Our analysis shows that contrails in thick cirrus clouds actually have hardly any effect," says Professor Peter Spichtinger from JGU, who contributed this aspect to the study. "However, additional effects in more complex scenarios – such as those arising from multiple layers of contrails and cirrus clouds on top of each other – are difficult to estimate and will be investigated in more detail in the future."

The results of the study are being incorporated into ongoing international activities of the World Meteorological Organization (WMO), the International Civil Aviation Organization (ICAO), the European Aviation Safety Agency (EASA), and the aviation industry. The goal is to develop a sustainable flight-planning strategy to reduce climate-relevant contrails by designing flight routes with climate impact in mind. IAGOS aircraft will continue to play a key role in evaluating such strategies in the future.