Airplane contrails are a tricky, and surprising, contributor to global warming
'This study throws a spanner in the works for the aviation industry.'

Commercial airplanes have made strides in reducing their carbon emissions, but the exhaust clouds trailing behind them can still have long-term impacts on the environment, a new study suggests.
Researchers from Imperial College London found that condensation trails, or contrails, created by aircraft exhaust fumes trap heat in the atmosphere. As a result, these thin cloud streaks have a greater impact on global warming than that of carbon emissions from combustion of jet fuel, according to the study.
"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," Edward Gryspeerdt, lead author of the study, said in a statement. "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."
Modern commercial aircraft are designed to fly at altitudes above 38,000 feet (about 12 kilometers), where the air is thinner and there is less aerodynamic drag in order to reduce jet fuel consumption (which creates less carbon emissions). Meanwhile, private jets fly more than 40,000 feet (12.2 km) above Earth, where there is less air traffic. This is higher than older commercial aircraft, which usually fly at altitudes around 35,000 feet (11 km).
Related: Human-caused global warming at all-time high, new report concludes
Using machine learning to analyze satellite data on more than 64,000 contrails from a range of aircraft flying over the North Atlantic, the researchers found that modern aircraft, both commercial and private, create more contrails than older aircraft and that these contrails take longer to dissipate, which influences current estimates of climate warming.
"This doesn't mean that more efficient aircraft are a bad thing — far from it, as they have lower carbon emissions per passenger-mile," Gryspeerdt said in the statement. "However, our finding reflects the challenges the aviation industry faces when reducing its climate impact."
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The study offers insights to lessen the impact of contrails by reducing the amount of soot emitted from aircraft engines, which is produced when fuel burns inefficiently. This, in turn, would cut down on the lifetime of contrails and the subsequent warming effect.
"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," Marc Stettler, co-author of the study, said in the statement. "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."
Their findings were published Aug. 7 in the journal Environmental Research Letters.
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Samantha Mathewson joined Space.com as an intern in the summer of 2016. She received a B.A. in Journalism and Environmental Science at the University of New Haven, in Connecticut. Previously, her work has been published in Nature World News. When not writing or reading about science, Samantha enjoys traveling to new places and taking photos! You can follow her on Twitter @Sam_Ashley13.
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COLGeek Not surprising at all. Post 9/11, when aircraft were grounded/severely limited, there were noticeable/measurable changes tracked then.Reply -
Unclear Engineer It would have been nice if the article had described how contrails increase temperatures on the surface.Reply
There seem to be a lot of competing effects, but clouds have generally been described as causing cooling on the surface by reflecting incoming solar radiation (sunlight) back into space without changing its wavelengths. Of course, they can also capture heat radiated from the earth's surface towards space and reradiate it back to the surface. We generally think of cloudy days as cooler than average and cloudy nights as warmer than average for those reasons.
Also, particulates in the stratosphere have been proposed to decrease global warming, but usually something like sulfuric acid, not carbon particles.
And, CO2 in the upper atmosphere has been modeled to decrease upper atmospheric temperatures, mostly due to CO2 migration there from the air below, not from jet contrails.
So, this article does not really do anything to explain how the net effect of contrails warms the planet's surface air. But, it needs to provide some explanation to attain credibility. With all of the doom-and-gloom biases in science reporting in the media these days, it needs to seem to make scientific sense to the lay person to have any effect on thinking in the general population.
And, including a "global warming" video that does not even mention the phenomena discussed in the article just makes it seem more likely to be propaganda than new scientific discovery.
If you want credibility, don't preach - do explain. -
Classical Motion “As a result, these thin cloud streaks have a greater impact on global warming than that of carbon emissions from combustion of jet fuel, according to the study.”Reply
That’s quite a statement. How low do we need to fly so that only the combustion carbon emitted goes to global warming?
What’s the combustion/contrail ratio at 20,000 ft? Where’s the least of both?
Pilots will need faster response times for in flight control problems. Less drop time. -
John Topham
The heating effects occur mostly at night when there is no sunlight to reflect.Unclear Engineer said:It would have been nice if the article had described how contrails increase temperatures on the surface.
There seem to be a lot of competing effects, but clouds have generally been described as causing cooling on the surface by reflecting incoming solar radiation (sunlight) back into space without changing its wavelengths. Of course, they can also capture heat radiated from the earth's surface towards space and reradiate it back to the surface. We generally think of cloudy days as cooler than average and cloudy nights as warmer than average for those reasons.
Also, particulates in the stratosphere have been proposed to decrease global warming, but usually something like sulfuric acid, not carbon particles.
And, CO2 in the upper atmosphere has been modeled to decrease upper atmospheric temperatures, mostly due to CO2 migration there from the air below, not from jet contrails.
So, this article does not really do anything to explain how the net effect of contrails warms the planet's surface air. But, it needs to provide some explanation to attain credibility. With all of the doom-and-gloom biases in science reporting in the media these days, it needs to seem to make scientific sense to the lay person to have any effect on thinking in the general population.
And, including a "global warming" video that does not even mention the phenomena discussed in the article just makes it seem more likely to be propaganda than new scientific discovery.
If you want credibility, don't preach - do explain. -
Unclear Engineer I understand the individual effects, as already stated in my previous post.Reply
The question remains why the net effect is heating, rather than cooling.
And, for that matter, what is the uncertainty in the calculations of the individual effects that are somewhat canceling each other? What level of certainty does that leave that the net effect is heating rather than cooling?
To explain my last question, if one parameter is 5 +/- 1 and another parameter that is subtracted from it is 4 +/- 1 the result looks like 1, but it could also be anything from 6 - 3 = 3 to 4 -5 = -1, with various probabilities for the numbers between. So, the question becomes what is the probability that the difference is greater than 0? How does that come out for the atmospheric temperature change calculation for contrails? (Also note that uncertainty calculations for model results are notoriously underestimated by nearly all modelers.) -
John Topham
This was my source. I am assuming that their reasoning and references are accurate. I don't have the time to follow all the links to be sure.Unclear Engineer said:I understand the individual effects, as already stated in my previous post.
The question remains why the net effect is heating, rather than cooling.
And, for that matter, what is the uncertainty in the calculations of the individual effects that are somewhat canceling each other? What level of certainty does that leave that the net effect is heating rather than cooling?
To explain my last question, if one parameter is 5 +/- 1 and another parameter that is subtracted from it is 4 +/- 1 the result looks like 1, but it could also be anything from 6 - 3 = 3 to 4 -5 = -1, with various probabilities for the numbers between. So, the question becomes what is the probability that the difference is greater than 0? How does that come out for the atmospheric temperature change calculation for contrails? (Also note that uncertainty calculations for model results are notoriously underestimated by nearly all modelers.)
How Airplane Contrails Are Helping Make the Planet Warmer By Fred Pearce • July 18, 2019 -
Unclear Engineer Thanks for the title. I searched and found the link: https://e360.yale.edu/features/how-airplane-contrails-are-helping-make-the-planet-warmer .Reply
The article is clear that it states that the heat trapping effect over-night exceeds the heat reflecting effect during the day. But, it doesn't provide anything about how certain that conclusion is, mathematically.
It does say "There are certainly uncertainties in the measurement of the climate impact of contrails," but does not say anything more about uncertainty. It quotes a government bureaucrat as saying "William Raillant-Clark, said it could only change tack 'on the basis of a technical/scientific consensus, which currently does not exist. There are no commonly accepted numbers' for the climate impact of contrails." It then criticizes that statement with "“Uncertainty is being used as an excuse to remain inactive,” by the International Civil Aviation Organization, a United Nations agency that has been coordinating the industry’s response to the climate emergency,
Trying to click on links in the article gave me "This site can't be reached" errors, so basically dead ends unless I search for each title to see if there is still something on the Web. Some links are not even titles, such as "retrospective studies", which links to https://www.aviationsystemsdivision.arc.nasa.gov/publications/2010/Sridhar_DASC2010_final.pdf , which is no longer available.
So, as reported, I can only file this as an opinion by a writer who is not even a researcher: "Fred Pearce is a freelance author and journalist based in the U.K. He is a contributing writer for Yale Environment 360 and is the author of numerous books, including The Land Grabbers, Earth Then and Now: Amazing Images of Our Changing World, and The Climate Files: The Battle for the Truth About Global Warming."
Maybe not an unbiased source?
As we often discuss at a professional level on another forum, government policy development does need to consider the uncertainties in the parameters that it is considering affecting. Those uncertainties are often so large that it is not clear whether any policy option has net positive or net negative effects. And, to make matters even more complicated, sometimes the benefits accrue to a different sub-population than the disbenefits harm, so there is an equity issue on top of the uncertainty issues.
This article does discuss 3 options for reducing the warming effects of contrails, but also discusses how they might not be net-beneficial. So, it seems somewhat hypocritical to criticize the government for not taking action to solve this problem - if it really is one. -
John Topham Unclear Engineer said:Thanks for the title. I searched and found the link: https://e360.yale.edu/features/how-airplane-contrails-are-helping-make-the-planet-warmer .
The article is clear that it states that the heat trapping effect over-night exceeds the heat reflecting effect during the day. But, it doesn't provide anything about how certain that conclusion is, mathematically.
It does say "There are certainly uncertainties in the measurement of the climate impact of contrails," but does not say anything more about uncertainty. It quotes a government bureaucrat as saying "William Raillant-Clark, said it could only change tack 'on the basis of a technical/scientific consensus, which currently does not exist. There are no commonly accepted numbers' for the climate impact of contrails." It then criticizes that statement with "“Uncertainty is being used as an excuse to remain inactive,” by the International Civil Aviation Organization, a United Nations agency that has been coordinating the industry’s response to the climate emergency,
Trying to click on links in the article gave me "This site can't be reached" errors, so basically dead ends unless I search for each title to see if there is still something on the Web. Some links are not even titles, such as "retrospective studies", which links to https://www.aviationsystemsdivision.arc.nasa.gov/publications/2010/Sridhar_DASC2010_final.pdf , which is no longer available.
So, as reported, I can only file this as an opinion by a writer who is not even a researcher: "Fred Pearce is a freelance author and journalist based in the U.K. He is a contributing writer for Yale Environment 360 and is the author of numerous books, including The Land Grabbers, Earth Then and Now: Amazing Images of Our Changing World, and The Climate Files: The Battle for the Truth About Global Warming."
Maybe not an unbiased source?
As we often discuss at a professional level on another forum, government policy development does need to consider the uncertainties in the parameters that it is considering affecting. Those uncertainties are often so large that it is not clear whether any policy option has net positive or net negative effects. And, to make matters even more complicated, sometimes the benefits accrue to a different sub-population than the disbenefits harm, so there is an equity issue on top of the uncertainty issues.
This article does discuss 3 options for reducing the warming effects of contrails, but also discusses how they might not be net-beneficial. So, it seems somewhat hypocritical to criticize the government for not taking action to solve this problem - if it really is one. -
John Topham After I wrote my reply I found this: https://www.sciencedirect.com/science/article/pii/S1352231020305689?via%3DihubReply -
Unclear Engineer Thanks for that link, too. I will need to get some time to look through it.Reply
2 things jumped out of the abstract for me:
1. "The best estimates of the ERFs from aviation aerosol-cloud interactions for soot and sulfate remain undetermined."
and
2. "The formation and emission of sulfate aerosol yields a negative (cooling) term."
I note that #2 was not discussed in the subject article on this Space.com forum.
Which gets me to wondering if the aviation industry will think of adding sulfur to their aviation fuel to try for net zero warming effects. I'm not a fan of adding a lot of sulfur dioxide to the upper atmosphere, because it will eventually get to the lower atmosphere and the hydrosphere, adding to the acidification of our waters.