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By Sherry Listgarten

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About this blog: Climate change, despite its outsized impact on the planet, is still an abstract concept to many of us. That needs to change. My hope is that readers of this blog will develop a better understanding of how our climate is evolving a...  (More)

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Contrails and aviation emissions

Uploaded: Aug 6, 2023

Many of us enjoy flying to new places for vacation, to visit family and friends, or to connect with work colleagues. Those flights, particularly the longer, high-altitude flights, contribute significantly to our carbon footprint. Using estimates from, a single round-trip economy flight from SFO to Heathrow warms the atmosphere with the equivalent of 2.5 metric tons of CO2. That is the same as a year of driving an efficient car (14,000 miles at 45 mpg) or a year’s worth of household gas use (500 therms). Just two international flights can exceed a year’s worth of our transportation and home emissions. Yikes.

Will technology come to the rescue? Aircraft manufacturers are looking into electric and hydrogen-powered aircraft, but there are challenges. The relatively high weight-to-energy ratio of batteries means that battery-powered aircraft are suitable mainly for short (500-1000 mile) flights. Low density hydrogen fuel takes up significant volume, requires new infrastructure, and is subject to leaks. As an alternative, airports like SFO are looking to Sustainable Aviation Fuel, a product nearly identical to jet fuel but derived from biomass. Imagine processing food waste from our landfills into aviation fuel. Even though CO2 would still be released into the atmosphere when the fuel is burned in the jet engine, the fact that the food products absorbed CO2 from the atmosphere when they were grown means that their use reduces net emissions. Moreover, the food waste would have released some additional methane when decomposing in the landfill. That means this fuel could even be carbon negative when accounting for the avoided decomposition.

However, it is not easy to produce Sustainable Aviation Fuel and get it to the airports where it is needed at sufficient scale. We can blend it in small amounts, which will help, but it turns out we may be able to get bigger reductions with less effort by looking at contrails. The cloud-like condensation trails formed behind high-elevation flights may be the cause of most of aviation’s warming effect (though there is uncertainty around the amount), and there are several ways to reduce them.

Contrail cirrus may be the cause of most aviation warming, though uncertainty about the exact amount is high. Source: The contribution of global aviation to anthropogenic climate forcing for 2000 to 2018, Lee et al, Atmospheric Environment, Volume 244 (2021)

The “contrail cirrus” shown in the graph above refers to condensation trails that last for several hours and spread out to resemble cirrus clouds. Many of these will warm the planet because they trap infrared radiation (heat) that is coming from Earth, preventing it from leaving our atmosphere. These clouds also reflect some of the incoming solar radiation, a cooling effect, but only during the day, and even that doesn’t count if other surfaces below them would have reflected that light. On average, contrail cirrus have a substantial warming effect.

It turns out it might be easier to minimize warming contrails than to convert to a new kind of jet fuel, though ultimately we need to do both. We know that certain flight paths are more likely to form contrails than others. These tend to be at higher altitudes (e.g., 30,000 feet) in very wet and cold areas of the upper troposphere (“ice-supersaturated regions”).

The white areas represent flight regions that are very cool with high humidity, where persistent contrails are more likely to form. Source: Interactive from Breakthrough Energy.

Contrails form when soot in jet exhaust attracts water vapor, cools, and forms long-lasting ice crystals. If there is some wind, the contrails will spread out into cloud-like formations, and if it stays cool, they will last many hours. These contrail clouds can cover up to 10% of the sky over the course of a year in high-air-traffic regions such as Europe and the US east coast.

MIT scientists estimate contrail coverage over the US during 2018-2019. Source: Contrail coverage over the United States before and during the COVID-19 pandemic, Meijer et al, Environmental Research Letters, Volume 17, Number 3 (2022)

Researchers have been working to figure out which flights have the worst contrail problems and why, so they can understand how to minimize the effect. One approach would be to route planes away from these ice-supersaturated regions. An online interactive tool created by Breakthrough Energy allows you to visualize this idea by looking at routes of specific flights.

A flight leaving SFO in early August is estimated to create 1243 km of contrail cirrus. Source: Interactive from Breakthrough Energy in collaboration with Imperial College of London.

Since the potential contrail regions are generally not very “thick”, flights can adjust their altitude to produce fewer long-lasting contrails.

The flight route is estimated to generate contrail cirrus as it enters ice-supersaturated regions. Source: Interactive from Breakthrough Energy in collaboration with Imperial College of London.

Height is not the only factor that affects whether a contrail forms and how much it ultimately warms or cools the atmosphere. Researchers studying flights over the North Atlantic found that just 12% of flights contributed 80% of the warming. They compared the flights with the most warming contrails (in red below) with those with the most cooling contrails (in blue below). The warming contrails form most in winter (graph (a) below) and at night (graph (i)). So if we are going to reroute flights, it might make sense to focus first on winter, night-time flights. The results also show that if planes can avoid flying over low-lying clouds or other reflective surfaces (graph (j)), that can reduce the warming effect.

Researchers compare flights with the most warming contrails (red) with those with the most cooling contrails (blue) to understand which underlying factors may be related. The grey line shows all contrail-forming flights. Source: Aviation contrail climate effects in the North Atlantic from 2016 to 2021, Teoh et al, Atmospheric Chemistry and Physics, 22, 10919–10935 (2022).

It is also possible to reduce contrails by limiting the amount of soot produced by the engines, which would decrease the amount of seed material for the ice particles. The same study found that 43.4% of flights with strongly warming contrails are powered by one engine combustor type that produces a lot of soot. We can move away from that type of engine, and we can also clean up jet fuel by reducing the components (“aromatics”) that convert into soot. The “hydrotreatment” process that is used today to clean diesel fuel can do the job. It is an added expense, but it could be used selectively on the flights most prone to contrails. Biomass-derived jet fuel also has few of these aromatics, and could be used selectively as well. Researchers have estimated that using a cleaner fuel could reduce contrail formation by 50%, though again uncertainty is high.

One of the encouraging things about addressing global warming is that there is a good amount of low-hanging (high-flying?) fruit. Contrails are an example of that. Academics, airlines, and environmental organizations are working together to better understand the problem and possible solutions. I think we will see a lot of progress in the next five years.

Current Climate Data (June 2023)
Global impacts, US impacts, CO2 metric, Climate dashboard

Source: Twitter/X, August 6

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Posted by MichaelB, a resident of Pleasanton Meadows,
on Aug 6, 2023 at 9:18 am

MichaelB is a registered user.

"One of the encouraging things about addressing global warming is that there is a good amount of low-hanging (high-flying?) fruit. Contrails are an example of that."

Electric and hydrogen powered aircraft have fuel and endurance/range problems. Sustainable Aviation Fuel can't be produced in quantity and delivered where needed. Reducing soot requires engine redesign. None of this is a "good amount low hanging fruit". It's costly, disruptive, and may not end up working at all if one wants to maintain the current utilization (and associated operating costs) of aircraft.

Regardless of the success (or failure) of the technology, can you guess what comes next? Read the first paragraph of this article. It's just going to be a question of time before the government starts punishing/restricting/taxing the "unnecessary use" of private aircraft (and travel) to reduce the global "carbon footprint". Expect anything and anyone transported by air to cost substantially more as our "leaders" prioritize "saving the planet" over the economy of the nation and lives of the people.

Posted by Eddie, a resident of Fairmeadow,
on Aug 6, 2023 at 10:40 am

Eddie is a registered user.

Sherry -

Interesting article about something (contrails) that appears to be solvable with little impact (slight adjustments to flight paths) and measurable in a quantitative way.
It's a shame that many of your readers only wish to seek out negative aspects of your postings.

Posted by Sherry Listgarten, a Mountain View Online blogger,
on Aug 6, 2023 at 2:04 pm

Sherry Listgarten is a registered user.

@Eddie, I’m glad you like the post, and I agree that these are promising ideas with several ways to mitigate the risk and cost. I have also seen that sometimes I get more comments from people who are opposed to climate action than those who are for it. I think people who agree often have better things to do than write a comment saying “I agree,” so thank you for yours!

@MichaelB, I agree with you that we should prioritize peoples' lives. Given the risks to our planet and its inhabitants posed by climate change, I think it is important that we address it.

I also agree with you on doing that in a cost-effective way. That is why I advocate being more proactive and mitigating our emissions rather than just reacting and adapting, though we will have to do both.

From your comments, though, I understand that you believe that the risk of climate change is being overstated -- not only by journalists but also by the IPCC, the EPA, the IEA, the NOAA, NASA, leading universities around the globe, etc. I expect that you believe that the risks are being overstated partly because of a “group-think” bias and social pressure, and partly to increase the power of these governments and other institutions. The end result, in your view, will be that our freedoms are restricted (e.g., our freedom to fly at low cost or drive a gas-powered car) with nothing to show for it but wasted money and time.

Well, you are not the only one who believes that. The distrust and anger you feel with these institutions, even with our weathermen, is growing across segments of media and society even as the real-world impacts of climate change become more evident. That adds more complexity to this difficult problem and impedes our ability to address climate change. Honestly, sometimes I find myself thinking there is a conspiracy by fossil fuel companies and rogue nations to foment this distrust. We do see that people residing in oil-rich countries are the most inclined to be dismissive of climate change. So maybe we are both conspiracy theorists in our own ways.

All that said, relatively simple technical solutions like the ones covered in this blog post are broadly appealing and politically straight-forward to incentivize, so I think we will see big progress on these fronts. I am excited about that.

Posted by MichaelB, a resident of Pleasanton Meadows,
on Aug 6, 2023 at 3:29 pm

MichaelB is a registered user.

"All that said, relatively simple technical solutions like the ones covered in this blog post are broadly appealing and politically straight-forward to incentivize, so I think we will see big progress on these fronts. I am excited about that."

The items you addressed are also part of a broader plan/strategy as it relates to aviation emissions that are not relatively inexpensive or simple to implement.

Web Link

Calling for "net zero emissions" by a certain year is easy to do for the financially/politically well off at campaigns, press conferences, and public hearings. What happens if goals are not met? Will the government recognize they are not feasible, or will they double down and begin to restrict national aviation (or other) activity because there is a "climate emergency"? Some of us remember the 1970s when we were told "an ice age was imminent" and "we were going to run out of oil". None of these predictions from the "experts" came true.

As far as anger and distrust are concerned, look no further than this state not being to keep the lights on. No "conspiracy" was required. Politicians were too anxious to get rid of fossil fuels and they could not cover electricity demand during summer months. Some of us knew solar and wind were not reliable - and not going to be enough. Instead of admitting failure, they lectured residents trying to live normal lives to "flex your power" and "use less".

Posted by Kent, a resident of Greenmeadow,
on Aug 7, 2023 at 10:55 am

Kent is a registered user.

Sherry, I agree. Thank You.

Posted by Sherry Listgarten, a Mountain View Online blogger,
on Aug 7, 2023 at 5:17 pm

Sherry Listgarten is a registered user.

@Kent: Thanks :)

@MichaelB: It's true, science is not perfect, though it’s apparently convincing enough to push insurance companies away from low-lying and fire-prone areas. I also agree that our energy systems are complicated, though imo we have some fantastic people working on these.

But maybe your larger point is that all of this accelerated change is risky and you are not a huge fan of risk, particularly when it might impact your ability to live a “normal life”, as you put it. That is a fair point. In your case, that disturbance might be an ask to turn down your A/C for a few hours or pay more for a flight. You might prefer to just let the climate warm, while others might view the tradeoff differently. Regardless, if we are to transition our energy systems, it’s interesting to consider your point: Climate change itself has unequal impacts, but so can the transition. Where is some of the risk of this transition being unduly imposed on passive actors such as yourself? I might tackle an example of that in my next blog post.

Posted by MichaelB, a resident of Pleasanton Meadows,
on Aug 7, 2023 at 7:49 pm

MichaelB is a registered user.

"Regardless, if we are to transition our energy systems, it's interesting to consider your point: Climate change itself has unequal impacts, but so can the transition. Where is some of the risk of this transition being unduly imposed on passive actors such as yourself?"

Web Link

Posted by Derek G, a resident of College Terrace,
on Aug 8, 2023 at 3:57 pm

Derek G is a registered user.

I consider myself reasonably well informed on climate change, but contrails are something I've never thought of, let alone researched as deeply as you. I've been occasionally reading your columns, but I now consider them required reading for myself.

And I also appreciate your follow up work in the comment section.

Posted by Michael Austin , a resident of Pleasanton Meadows,
on Aug 8, 2023 at 6:27 pm

Michael Austin is a registered user.

Blogger update: I am deleting this post because it consists of unreferenced misinformation. Please include a reliable pointer if you want to repost. In the meantime, consider reading Web Link or one of many other similar write-ups.

Posted by MichaelB, a resident of Pleasanton Meadows,
on Aug 9, 2023 at 7:47 am

MichaelB is a registered user.

"But maybe your larger point is that all of this accelerated change is risky and you are not a huge fan of risk, particularly when it might impact your ability to live a “normal life", as you put it.

I'm not a huge fan of people who don't do their jobs. Or forget that the government answers to the people vs. the other way around. If the state wants to alter the power generation/supply/distribution system, it needs to work and be reliable/robust before it is rolled out. Not just do it "to be first", hope it works, and then dismiss the concerns of those who point out the flaws/are directly impacted by it. We live in a modern, industrialized society and should not be subject to perpetual energy shortages which could negatively affect the health/well being of residents.

It's not difficult to see the potential problems. If there is no wind blowing and/or sun shining, no power will be generated. If there's a chronic shortage of electricity in the summer months because of air conditioning usage, it's going to be happening year round. Local/state governments have already seen fit to guarantee an increase in consumption by mandating electric vehicles, furnaces, water heaters, etc. Others have also pointed out that older homes can't run all electric because of capacity/wiring issues.

I suspect this was yet another example of the cult like mentality of climate activists/politicians (in a one party rule state with no debate) who thought it was "crisis" and/or the "world was going to end" if we didn't get rid of fossil fuels now. But they never bothered with the details and/or were not personally affected by the outcomes. Or just simply don't care.

Posted by Jack, a resident of Barron Park,
on Aug 10, 2023 at 12:01 pm

Jack is a registered user.

This is a really interesting article, thank you! I love the detail.

Regarding re-routing flights to reduce contrail formation: Presumably an airplane uses more fuel if it's diverted out of its nominal flight path. Does it make sense to add more CO2 -- which persists and will increase warming for many years -- to mitigate a short-term warming effect like a contrail?

Also I'm curious how we might encourage carriers to adopt mitigations that make sense. It's the problem of externality costs; they need financial or regulatory motivations to change the way they operate.

Posted by Sherry Listgarten, a Mountain View Online blogger,
on Aug 10, 2023 at 10:39 pm

Sherry Listgarten is a registered user.

@MichaelB: I think you would really enjoy reading some of the power operators' recent pushback on the Clean Air Act proposed a few months ago, which relies heavily on CO2 capture and green hydrogen. It's hard to disagree with them. The docket is here, or you can look at this 153-page (!) writeup by APPA (which Palo Alto for example belongs to) if you are interested. They are very upset with the magical thinking that they say went into the proposal. Sammy Roth (LA Times) also wrote about it some here.

BTW, where I disagree with you is that I do think that California is paying a lot of attention to reliability. Recently they postponed not only the shutdown of Diablo, but also yesterday the shutdown of the coastal gas plants in So Cal (see here). I think the Governor is crystal clear about what reliability problems can do to his career. But what imo is much more difficult to fix than the supply side is the wildfire issue. The utilities are proactively shutting off power to prevent fire, and that is a huge issue (e.g., I wrote about here). I don't know how we get out of that mess any time soon. And maybe now places like Hawaii will be doing it too, though not sure what started those fires.

@Jack, that's a great question re short-term vs long-term tradeoff. I think the degree matters (these contrails have a *lot* of radiative forcing compared with the extra co2 emitted, which is only 2-3% more). And we discount the longer-term co2 because we think (hope) that at some point we'll have an answer for it, relative to the near-term, which could lead to irreversible tipping points. Reducing the short-term buys us time to address the long-term, though the relative amounts matter as well. It's something like 100:1 here.

Re motivation, I think the carriers are very motivated, at least judging from their participation in all of this. Many of their business customers have net-zero ambitions, and that includes some amount of air travel. The airlines view it as a competitive feature. There is also some existing policy for aviation emissions, CORSIA, though it is still ramping up.

Thanks for the good comments/questions.

Posted by Gwen Stickley, a resident of College Terrace,
on Aug 28, 2023 at 2:11 pm

Gwen Stickley is a registered user.

Interesting reading.

Three weeks ago, a tragic fire swept through West Maui and destroyed the historic town of Lahaina. The media were quick to blame “climate change" for the destruction and loss of life.

It turns out that this could have been avoided if not for the incompetence of local government officials (“we must allocate water according to ‘water equity'," and “poor disaster response alerts"), and the misplaced priorities of HECO (investing in green energy alternatives as opposed to equipment maintenance and clearing brush around electrical lines, among other things).

A number of these public officials subscribe to the same “climate change" goals that you do. Indeed, you are “kindred spirits" with them.

Do you feel any responsibility for the Maui fire?

Posted by Eric H, a resident of Evergreen Park,
on Aug 28, 2023 at 3:02 pm

Eric H is a registered user.

You may have seen this already but Google is working with airlines on a project to reduce contrails
Web Link

Posted by Sherry Listgarten, a Mountain View Online blogger,
on Aug 28, 2023 at 4:20 pm

Sherry Listgarten is a registered user.

@Gwen: Hmm, interesting question, thanks.

Around the world there has been an increasing number of challenging (and expensive) weather events that have been made more likely because of climate change. That is bad, and there will only be more until we stop warming up the planet (aka get to net-zero emissions).

In the case of the awful tragedy in Maui, I agree with you that several factors probably contributed (e.g., utility response to high winds, fire response to fires). Afaik an attribution study hasn't been done yet to determine the extent to which climate change made this event more likely.

As you suggest, we will get better at fighting fires, combatting floods, hunkering down during hurricanes, dealing with drought, etc. That is called "adaptation", and we have to do it as those events become more likely on a warming planet.

Since adaptation will be increasingly hard and expensive and unpleasant, and many will not be able to adapt at all (people and wildlife), a lot of people, including me, would really like us to get to net-zero.

I'm not sure where your question about personal responsibility comes from. We are where we are for many reasons. Fossil fuel companies, for example, played a much, much bigger part in getting us here than I did. The important thing is that, at least in my opinion, we have a collective responsibility, now that we understand where we are, to improve our situation.

If you want to ask me how I feel, up top for me would be "sad", "frustrated" and "trying to do my part to help". I'm guessing maybe you are "frustrated" and also "angry", but not sure what else. Maybe "disbelieving"?

Anyway, thanks for the comment.

@Eric, yes, I mention some of their work in the post. (They are working with Breakthrough Energy.)

Posted by Gwen Stickley, a resident of College Terrace,
on Aug 28, 2023 at 7:34 pm

Gwen Stickley is a registered user.

Hi Sherry,

Thanks for your response to my thoughts about the tragic fire in West Maui.

Here is a quick video clip of (the renowned man of science) Dr. Anthony Fauci blaming the tragedy on “climate change."

Web Link

The problem that many “normies" like me have with Climate Change experts is that most of the Covid (government, medical, educational, and media) “experts" made proclamations and issued guidance in 2020-2022 that turned out to be quite wrong. Why should we believe you anymore than we did them? Why should we turn our economy inside out to follow your advice?

Posted by Sherry Listgarten, a Mountain View Online blogger,
on Aug 28, 2023 at 9:19 pm

Sherry Listgarten is a registered user.

@Gwen, there are certainly people who feel betrayed by experts, who feel they know more than experts do, because experts have been wrong. It's not just covid. I mean, every single expert has been wrong about something, sometimes in a big way. I can promise you one of these blog posts will assert something that turns out to be pretty wrong.

Look at the uncertainty range on contrails, for example. The range is huge, but I write as if we know the effect is big. Maybe it's not.

Covid was a particularly tough case. It was very fast-moving and also pretty serious, decimating and unnerving hospitals and the people working there. Science had to evolve in real time. It was sometimes wrong. Sometimes the reporting about it was wrong. And sometimes misinformation was intentionally spread.

I am confident the same thing is happening to some degree with climate change. Experts will be wrong on some things, reporting about it will be wrong in some cases, and misinformation will intentionally be spread.

I guess I would just caution against throwing out the baby with the bath water. Mistakes will always be made. That is the nature of science. But when virtually all experts agree, and the only people who disagree are non-experts, I'd take another look and ask why that is. The basics of climate change have been known since the 60's or 70's, are based on simple science, and have been validated over and again in multiple ways. That is not fast-moving science, that is established. The IPCC reports are pretty conservative and also very explicit about what is well understood. The more recent stuff that is fast-moving will be bumpier, whether it's battery technology or methane capture or ocean circulations or cloud effects or ... Experts do not agree on these things, the science is evolving. If you are looking for errors, you will find them. You will be able to say "I told you so!" But errors on the fast-moving edge are different from errors on the foundations. And there is an awful lot that scientists have proven to be right about when it comes to climate change and to covid.

That said, everyone is free to disagree. You can take ivermectin if you get really sick with covid. You can move to a wood house in Paradise. You can encourage others to do the same. But science is real, and it works.

That is my 2c.

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