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.

 

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

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."

SOME FOLKS CALL THEM CHEMTRAILS

More Data Needed to Understand Contrails, their Climate Effect and to Develop Mitigation

The report highlights the complexity of contrail science, noting gaps in the understanding of how contrails form, or when they could persist, and how they impact the climate

SME News Service

Published on: 

05 May 2024

The International Air Transport Association (IATA) called for urgent action to deepen the understanding on the formation and climate impact of aviation contrails to develop effective mitigation measures.

The newly released IATA report Aviation Contrails and their Climate Effect: Tackling Uncertainties and Enabling Solutions calls for a strengthening of collaboration between research and technological innovation, coupled with policy frameworks to address aviation’s non-CO2 emissions through more atmospheric data.

The report highlights the complexity of contrail science, noting gaps in the understanding of how contrails form, or when they could persist, and how they impact the climate. The lack of high-resolution, real-time data on atmospheric conditions (particularly humidity and temperature at cruising altitudes) hinders precise contrail forecasting.

Willie Walsh, IATA’s Director General said: "The industry and its stakeholders are working to address the impact of non-CO2 emissions on climate change, particularly contrails. To ensure that this effort is effective and without adverse effects, we must better understand how and where contrails form and shrink the uncertainties related to their climate impact.

"Action now, means more trials, collection of more data, improvement of climate models, and maturing technologies and operations. Formulating and implementing regulations based on insufficient data and limited scientific understanding is foolish and could lead to adverse impacts on the climate.

"That is why the most important conclusion from this report is to urge all stakeholders to work together to resolve current gaps in the science so that we can take effective actions."

Recommendations

With current levels of understanding, the report made the following recommendations:

• In the immediate term (2024-2030), the priority for mitigating aviation’s climate change impact should be on reducing CO2 emissions over the uncertain gains that could stem from contrail detection and their mitigation. Over this time, increasing airline participation in sensor programs, continuing scientific research, and improving humidity and climate models should be the focus of work on contrail mitigation.

• Mid-term actions (2030-2040) should involve establishing standards for data transmission, continuous validation of models, and encouraging aircraft manufacturers to include provisions for meteorological observations, as well as selected avoidance.

• Longer-term actions (2040-2050): Aircraft should be continuously providing data and the models and infrastructure should be there and be reliable. The community will have at this point a more complete understanding of the non-CO2 effects of alternative fuels, with extended mitigation measures. These action items collectively aim to mitigate the climate impact of aviation while advancing scientific understanding and technological capabilities.

Background on Aviation’s Non-CO2 Emissions

Aviation's impact on climate extends beyond CO2 emissions, with non-CO2 effects such as contrails and nitrogen oxides (NOx) also contributing to global warming. Persistent contrails, formed in ice-supersaturated regions, can transform into cirrus clouds which reflect incoming solar radiation (during the day) as well as trap outgoing heat.

On balance, it is understood that contrails have a warming effect on the climate, with diurnal, seasonal, and geographical variations. However, despite extensive studies, significant uncertainties exist with respect to the capacity to predict individual contrail formation and their specific climate impact.

Initiatives and Trials: Recent collaborations among meteorologists, climate researchers, airlines, and aircraft manufacturers have yielded new insights that underscore the need for enhanced data collection and analyses of the likely air traffic network complications regarding any solutions.

Trials with modified flight paths and alternative fuels have shown potential yet limited efficacy due to the variability of atmospheric conditions and the localized nature of where contrails occur.

Technological Advances and Future Directions: Advancements in developing humidity sensors to be placed on aircraft are critical for contrail prediction and avoidance strategies. Current sensor technology on commercial aircraft lacks the required sensitivity and response time, and there are only a handful of such sensors in operation at altitude.

Ongoing research aims to develop more accurate, robust, and scalable solutions, and the use of sensors on a limited population of aircraft would allow the necessary improvement and validation of numerical weather prediction models.

Climate change: How can we make flying greener?


Gero Rueter

DW

April 25, 2023

Biofuels, alternative flight routes and newer, green aircraft technology can make flying better for the environment. How close are we to introducing these climate-friendly alternatives?

Before the COVID-19 pandemic, when the global aviation sector was flying high in 2019, it contributed almost 6% of the planet-warming greenhouse gases in our atmosphere. A year later, with the industry crippled by pandemic-related flight cancellations, that figure had dropped by 43%. Last year, it was still 37% lower.

But air traffic has been steadily increasing, according to industry body the International Air Transport Association.

Greenhouse gas emissions are rising too. In response, the European Parliament has announced a proposal to introduce environmental labels for air travel from 2025. The system would serve to inform passengers about the climate footprint of their flights.

Carbon dioxide accounts for only about a third of the global warming effect attributed to air travel. Two-thirds is caused by other factors, most significantly the condensation trails, or contrails, aircraft leave behind.

Alternative flight routes could prevent contrails

Contrails — those narrow, white clouds that trace an airplane's path through the sky — are formed when jet fuel, which contains kerosene, burns. At the average cruising altitude of between 8,000 to 12,000 meters (around 26,000 to 40,000 feet), low temperatures cause water vapor to condense around the soot and sulfur left behind by jet emissions. The resulting ice crystals can remain suspended in the air for hours.

Contrails trap heat in the atmosphere, much like in a greenhouse, greatly amplifying the impact of flying on the world's climate. Recent studies have shown that contrails are around 1.7 times more damaging than CO2 emissions, when it comes to global warming.

Contrails, which can linger for hours, trap heat in the atmosphere

Image: Ohde/Bildagentur-online/picture alliance

On the plus side, contrails are relatively easy to avoid. Using satellite data, flight planners can optimize aircraft routes to avoid weather patterns that favor the formation of contrails. Pilots can also fly their jets 500 to 1,000 meters lower, for example, where temperatures aren't as cold.

"It doesn't require much effort to make these changes," said Markus Fischer, divisional director at the German Aerospace Center, adding that it would mean between 1 to 5% more fuel and flight time. However, he told DW, that would result in a 30 to 80% reduction in the warming effect caused by factors other than CO2, he said.

The European Union aims to include these non-CO2 climate effects in future European emissions trading agreements. Airlines will have to start reporting such pollutants from 2025, according to a preliminary agreement in the European Parliament.

Producing e-kerosene with green energy

Burning kerosene derived from petroleum produces lots of CO2 and, at high altitudes, other greenhouse gases such as ozone. The CO2-free alternative is e-kerosene.

E-kerosene can be produced in a climate-neutral way using green electricity, water and CO2 extracted from the air. First, hydrogen is generated using a process involving electrolysis, and then CO2 is added to produce synthetic e-kerosene.

The problem is that to be cost-effective, e-kerosene needs to be made with plenty of solar and wind power — and so far, there isn't enough of this renewable energy. New production plants for green hydrogen, CO2 direct air capture and synthetic fuels must also be built.

The EU is pushing for at least 2% of aviation fuels to be environmentally friendly by 2025, increasing every five years to reach 70% by 2050. The proposal is yet to be passed.

Biokerosene can be produced from plants like rapeseed

Image: Julian Stratenschulte/dpa/picture alliance

Could planes be powered with cooking oil?

Another option for planes is to refuel with biokerosene, which can be made from rapeseed, jatropha seeds or old cooking oil. Small-scale production plants already exist, but producers would need to greatly expand capacity to keep up with demand. Intensive production of biokerosene is also limited by the scarcity of arable land — the use of which is itself controversial, as it prevents take space needed for growing food.

Under a European Commission proposal, biofuels and e-kerosene would be mixed with conventional fossil kerosene from 2025. The share of biofuels in the mix would then rise by 2% per year, to reach 63% by 2050. The European Parliament has set a goal of 85% by mid-century.
Battery-powered short-haul flights on the horizon

With electric engines and batteries, flights could avoid producing direct emissions or heat-trapping contrails. But current batteries are too heavy and have insufficient storage capacity, limiting planes to short distances of just a few hundred kilometers.

This all-electric jet from Eviation Aircraft is expected to have a range of 445 kilometers

Image: Eviation Aircraft

Several companies are in the process of tinkering with battery and aircraft optimization. Israeli manufacturer Eviation Aircraft, for example, is building an all-electric jet with seating for nine passengers. The private aircraft is expected to have a range of 445 kilometers and a top speed of 400 kilometers per hour (about 250 miles per hour).

Norway is aiming to launch the first regularly scheduled electric flight service in less than three years. The country plans to connect the coastal cities of Bergen and Stavanger, some 160 kilometers apart, with a flight served by a battery-powered aircraft with space for 12 passengers from 2026.

Electromobility revolution takes to the skies 01:55

Hydrogen aircraft show promise, but not ready

Smaller airplanes that run on hydrogen have recently been in the spotlight. These aircraft use hydrogen fuel cells to generate electricity and efficiently power the plane's propellers. The jet engines on long-haul aircraft can also run on hydrogen but would be less efficient.

European aircraft manufacturer Airbus is planning to launch a hydrogen-powered passenger plane by 2035. These aircraft could account for more than 30% of global air traffic by 2050, according to a study by global consulting firm McKinsey.

But hydrogen-powered aircraft continue to pose numerous challenges. The volatile gas only becomes liquid at minus 253 degrees Celsius (minus 423 Fahrenheit), and must be stored under high pressure in special tanks. That means extra space and weight requirements for airplanes, and those plans have yet to be developed. In addition, airports will need to develop new refueling infrastructure for hydrogen-powered aircraft.

Hydrogen — fuel of the future? 09:00

One sure way to reduce emissions: Fly less

Even in the most optimistic scenarios, air travel won't be completely free of emissions by 2050. Experts believe that if the industry implements ambitious restructuring plans — completely replacing standard jet fuel with green hydrogen and e-kerosene, and rerouting planes to prevent contrails — it could reduce greenhouse gas emissions by up to 90%.

However, a recent study in the scientific journal Nature noted that even a complete switch to e-kerosene would still result in a residual negative effect on the climate. Therefore, avoiding all but necessary flights and giving preference to climate-friendly modes of transportation remains key, said the UBA, Germany's federal environment agency.

Choosing climate-friendly transportation over flying can help reduce emissions

Image: Flighttracker

Aviation experts have also stressed the need for new, lightweight airplanes with optimized wings, the use of propellers instead of jet engines and reducing airspeed. They point out that these measures could reduce fuel use by around 50%, compared to today.

Integrating environmental costs into the price of airline tickets would help to implement all these measures, said European clean transport campaign group Transport and Environment (T&E). Airlines currently pay nothing to account for their contribution to the climate crisis. Including environmental costs in air fares would be a fair way to promote a restructuring of the aviation industry and would make it easier to switch to climate-friendly modes of transport, according to T&E.

This article was originally published in German.

Jet fuel is bad for the environment. 

Contrails are even worse.

Jason Markusoff 

MACLEANS

FEB. 28,2022

The fact that airplanes are climate-­damaging fuel hogs—aviation accounts for two per cent of human-caused climate change—has been obvious to the travelling public for some time. What’s becoming increasingly clear, though, is that spending even more jet fuel may be necessary to deal with the sector’s bigger contributor to the heating climate: contrails. As the airline industry puzzles over how to decarbonize, researchers are rapidly gathering an understanding of how these anthropogenic cloud formations add to global warming, and how they might be avoided.

© Used with permission of / © St. Joseph Communications.

 (Jostein Nilsen/EyeEm/Getty Images)

Planes constantly emit a trail of substances, including carbon dioxide, water vapour and black carbon (soot). When aircraft pass through patches of cold, humid air, the water vapour and large soot particulates combine to form a long stream of ice particles. The ones that disappear quickly aren’t a problem, explains Sebastian Eastham, research scientist at MIT’s Laboratory for Aviation and the Environment. But the formations that persist for hours can form human-made cirrus clouds, which trap huge amounts of thermal radiation that would otherwise escape into space. With contrails, Eastham says, “you have this large, sudden contribution to global warming, where you have caused the Earth’s atmosphere system to retain a significant amount of additional energy.” Carbon dioxide, by comparison, has a less acute but more prolonged energy-trapping effect.

Much of aviation’s challenge, then, is figuring out how flights can avoid the patches of cool, humid air that are ripe for creating contrails. Their locations are hard to predict—varying hour to hour—so it’s an air traffic control and modelling problem. There is a theory that temporarily flying higher (or lower) for brief stretches of some flights can create huge savings in contrails at the cost of a relatively small amount of extra fuel burn and carbon emissions—emitting a bit more to save the planet, as it were. It’s the “low-hanging fruit” for slashing aviation’s climate impact, the Royal Aeronautical Society’s John Green said at a conference last May. The industry has begun turning simulations into real-world examples: last fall, United Arab Emirates’ Etihad Airways teamed with a U.K. flight analytic firm to adjust the path of a Boeing 787 travelling from Heathrow to Abu Dhabi and says it avoided producing the equivalent of 64 tonnes of CO₂ by emitting only 0.48 extra tonnes.

Another contrail avoidance option—well, besides, flight avoidance altogether—is flying with alternative fuels. The National Research Council of Canada (NRC) has experimented with jets burning crop-based biofuels, which are less carbon-intensive over their life cycle than jet fossil fuels. They don’t necessarily produce lower in-flight carbon emissions than jet fossil fuels, says Anthony Brown, research pilot engineer with the NRC, but they substantially reduce the large soot particles that help create contrails. Given the unpredictability of when flights will hit contrail-prone skies, using different fuels is a more definitive way to tackle this problem than changing flight paths, Brown says.

But it will be years before either solution scales up to industry-wide usage. So while the conspiracy fanatics who baselessly fear “chemtrails” remain as wrong as ever, there is reason to look up, see lingering jet exhaust clouds and get a bit anxious.

This article appears in print in the March 2022 issue of Maclean’s magazine with the headline, “Menace in the mist.” Subscribe to the monthly print magazine here.

 

Bjorn’s Corner: The challenges of Hydrogen. Part 11. Emissions

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October 2, 2020, ©. Leeham News: In our series on Hydrogen as an energy store for airliners we look deeper at the emissions from a hydrogen airliner and compare it to the emissions from today’s carbon fueled aircraft.

Figure 1. The three Hydrogen concepts from Airbus. Source: Airbus.

Emissions of a Hydrogen aircraft

In Part 9 of the series, we wrote the emission from a hydrogen-fueled Turbofan or Turboprop take care of the CO2 problem (no CO2 emissions), it lowers NOx emissions and increases the emission of water, H2O, into the atmosphere.

Figure 2 gives a more detailed view of the emissions from a carbon fueled and hydrogen-fueled airliner.

Figure 2. Emissions from a kerosene-fueled turbofan and hydrogen turbofan. Source: Airbus Cryoplane study.

Today’s airliner that burns 1 kg of jet fuel emits 3.16kg of CO2, 1.24kg H20, Carbon Monoxide, Soot, Sulphuric Acid, 11.2kg of Nitrogen and air. This compares with no CO2, 2.6 times more water, one fifth the amount of NOx, and 9.4kg Nitrogen and air from a hydrogen-fueled engine (both burn the same amount of energy, producing the same thrust). A hydrogen-based airliner is a clear improvement in terms of emissions.

The only caveat is the increased amount of water vapor in the exhaust. Water vapor has a greenhouse effect in the atmosphere but it disappears 200 times faster than CO2, and studies show that water vapor in the atmosphere is not the key problem from the increase in water emissions.

It’s rather contrails (ice crystals that form from water vapor condensation on nuclei in the turbofan exhaust) that are contributing to an increase in the greenhouse effect. Though hydrogen-fueled engines put out more water vapor, the ice crystals formed when the conditions create contrails are larger. This changes the effects of the contrails so they are thinner and contribute less to the greenhouse effect than the same amount of water vapor from a carbon fueled engine.

The combined effect of the increase in water vapor and the formation of contrails, considering the different types of ice crystals formed, is a reduction in the greenhouse effect from hydrogen-fueled airliners by around 20%.

I have taken these results from both the Airbus Cryoplane study (from 2000) and the EU’s study, released in May 2020. Both documents say these results are according to the best knowledge but this subject needs more research.

Summary

To summarize these and other studies, hydrogen-fueled airliners, as Airbus’ ZEROe concepts in Figure 2, would:

• Reduce CO2 emissions by 100%
• Reduce NOx emission by 80%
• Reduce the greenhouse effects from emitted water vapor by 20%

The above assumes the same efficiency aircraft and engines, transporting the same amount of passengers the same distance, fueled by Jet A1 kerosene alternatively hydrogen.

This assumes both aircraft fly the same trajectory, meaning the hydrogen airliner is not adapting it’s mission profile to avoid contrail creation (for instance, change flight level to one that does not produce contrails in areas where conditions predict contrail creation).

Bjorn’s Corner: The challenges of Hydrogen. Part 11. Emissions - Leeham News and Analysis