Nature surprise: Aircraft contrails “may be causing more climate warming today than all the carbon dioxide emitted by aircraft since the start of aviation.”

A new study finds that all those aircraft condensation trails you see across the sky may, on any given day, be warming the planet more than all the CO2 emitted by all the planes since the Wright Brothers’ first flew over a century ago.

The study is “Global radiative forcing from contrail cirrus” (subs. req’d) in Nature Climate Change by Ulrike Burkhardt1 and Bernd K¤rcher of the DLR German Aerospace Center. That the climate forcing from airplanes is considerably greater than just that of their CO2 emissions has been known a long time.

What this study adds is an analysis of an “important but poorly understood component of this forcing,” namely contrail cirrus“””a type of cloud that consist of young line-shaped contrails and the older irregularly shaped contrails that arise from them.”  It turns out that “the radiative forcing associated with contrail cirrus as a whole is about nine times larger than that from line-shaped contrails alone.”  On the bright side, “contrail cirrus cause a significant decrease in natural cloudiness, which partly offsets their warming effect.”

Nature CC‘s news story has this explanation and satellite images:

Aircraft-engine emissions are mostly composed of carbon dioxide, water vapour, nitrogen oxides, sulphur oxides and aerosol particles. As well as the direct effect that these emissions have on climate, aviation has an added impact induced by the formation of condensation trails (contrails) in the wake of the aircraft. These line-shaped trails are formed by the mixing of hot, moist air coming out of the engine with cold ambient air. When the atmosphere is supersaturated with respect to ice, the line-shaped contrails can spread to form cirrus cloud, which has a warming effect on climate.  [Fig. 1, click to enlarge]

Satellite infrared images of contrails spreading into cirrus clouds over the UK.

The young contrails, which appear as a spring shape and sharp lines in the first image, gradually spread into cirrus clouds, which appear as bright white areas in the lower images. The time of each image and the satellite used to take it are shown in the inset of each frame. Burkhardt and K¤rcher used a model that simulates this spreading process to assess the warming effects of contrails and the cirrus clouds that form from them. Their results indicate that so-called spreading contrails cause an order of magnitude more climate warming than the line-shaped contrails alone, and are the largest single climate-forcing agent associated with aviation.

Here are some more details:

Burkhardt and K¤rcher developed a process-based model of how contrails form, grow (through the depletion of water vapour in the surrounding air), spread and finally disappear (through mixing and fall-out of the ice crystals). By tracking the fate of contrail and natural cirrus separately, the authors can quantify the radiative forcing from spreading contrails (including young line-shaped contrails), which they estimate to be 38 mW mˆ’2. This can be compared with a radiative forcing of 4 mW mˆ’2 from young contrails alone and 28 mW mˆ’2 from aviation carbon dioxide. Interestingly, spreading-contrail cirrus clouds cause a reduction in natural cirrus, because they modify the water budget in the upper troposphere; however, this reduction in natural cirrus is relatively small (-7 mW mˆ’2).

The lead author Ulrike Burkhardt told Reuters, “You can get rid of contrails very quickly. You can’t get rid of CO2 quickly.”  As Nature put it:

Overall, and despite their short lifetime, contrails may have more radiative impact at any one time than all of the aviation-emitted carbon dioxide that has accumulated in the atmosphere since the beginning of commercial aviation. It is important to note, however, that the emitted carbon dioxide would continue to exert a warming influence for much longer than contrails, should all aircraft be grounded indefinitely.

Yes, well, that ain’t gonna happen.

The question arises as to whether changing the flight pattern of aircraft or perhaps their engine technology could ameliorate this problem:

These findings are important, because if the calculations of Burkhardt and K¤rcher are correct, they provide a basis to develop mitigation strategies to reduce the impact of aviation on climate. For instance, it has been suggested that flight routes or flight altitudes could be planned and altered in real time to avoid parts of the atmosphere that are supersaturated with respect to ice8, 9. Even though this would help to reduce both young and spreading contrails, such a strategy is likely to lead to an increase in fuel consumption. It would be important to make sure that, given the large difference in atmospheric lifetime of carbon dioxide and contrails, the associated carbon dioxide penalty does not offset in the longer term the gain obtained by avoiding contrail formation10.

The results by Burkhardt and K¤rcher might also justify the development of a novel engine concept that seeks to condense a fraction of the water vapour in aircraft emissions in a cooling unit before it leaves the engine11. The condensed water could be vented in the form of large ice crystals or droplets that would fall quickly through the atmosphere. Reducing the content of water vapour in the engine exhaust would make contrail formation less likely.

Certainly this is an important area for continued work.


32 Responses to Nature surprise: Aircraft contrails “may be causing more climate warming today than all the carbon dioxide emitted by aircraft since the start of aviation.”

  1. catman306 says:

    About six months ago I asked the collective wisdom at RealClimate about the possibility that jet exhaust would increase the warming forcings of the CO2 because the exhaust was so high in the upper troposphere or lower stratosphere.

    I was assured, ‘No’, it won’t have any special effect. There was plenty of mixing up there, so the CO2 wouldn’t accumulate. Apparently the water vapor in the exhaust contrail doesn’t mix that fast.

    There’s plenty we still don’t know about the atmosphere and how our civilization has changed it.

  2. PeterW says:

    “should all aircraft be grounded indefinitely”

    Of all the frivolous things people do air travel has to be right up there. How many business trips are really necessary? Could a video conference have accomplished the same thing? Do you really need to take a vacation every year to the somewhere hot? I haven’t flown since 1993 and when I tell people this they are stunned. It really isn’t that much of a sacrifice considering the consequences.

    You may not be able to ground all aircraft but is it too much to ask people to really consider the importance of their trip before they decide to fly?

  3. Leif says:

    Ships can also leave a “contrail” in the lower atmosphere. I would think that as the Arctic shipping lanes get more use those “contrails” would in turn adversely affect the arctic weather.

  4. Same Ordinary Fool says:

    Nova’s 2006 “Global Warming” video:

  5. Same Ordinary Fool says:

    Nova’s 2006 “Global Warming” video: (corrected)

  6. Christopher S. Johnson says:

    #4 that NOVA video link is dead, but it is interesting to note NOVA did an episode around that time on global DIMMING, and contrails were heavily featured in that. In fact the episode suggested the OPPOSITE of this study above. It showed that particles in the air (unfortunately the toxic ones as well) dim the amount of sun reaching the Earth, which, besides retarding crops, was hiding some global warming. They were pretty clear about suggesting a COOLING effect from contrails and used temperature spike in the week after the 9/11 tragedy as example, because aircraft were grounded. Any takers on the discrepancy?

  7. Phil Marshall says:

    I had heard that too, Chris.

    Looking at the numbers in this post it also seems that the integrated impact of the contrails may turn out to be relatively small, despite the eye-catching headline. I see (38-7)=31 W/m^2 forcing from contrails/cirrus, compared to 28 W/m^2 from GHGs, “at any one time” – so contrails double the impact of aviation while they are there. But as far as global warming is concerned, isn’t it the total energy deposited in the atmosphere that is important? And if the contrails don’t last long, then isn’t the integral of the total forcing over time going to be dominated by the long-lived GHGs? I’d be interested to know the relative integrated impact of contrails and cirrus versus GHGs.

  8. Mark Shapiro says:


    You haven’t flown since 1993, which makes you a hero, but also an outlier.

    I beg, scream, and plead for clean energy, and read every ClimateProgress post, but unlike you and a few others, I buy gasoline, gas, coal (indirectly), and other products that support plutocrats. These guys have us pretty much where they want us — we scream but still buy their stuff.

    We have work to do.

  9. Mark says:

    Wow, timing….

    I just watched a BBC show on Global Dimming this morning. Part 1 of 5 is here

    It’s quite a tale of interconnected parts with a strong subplot about global warming. They seemed to think that the dimming is hiding the effect of AGW. One subplot was empirical observation after 9.11 showing an instant leap in range between daily high/low temps during three days of no air traffic. That was interpreted as contrails giving us cooler temps.

    I would look forward to hearing some sort of layman terms reconciliation between these studies.

  10. Christopher S. Johnson says:

    Concerning global dimming: There is a mean irony there, isn’t there? Cleaning up the cancer and asthma pollution may increase (reveal) more global warming.

    A pet theory of mine,for which I have no proof, is that the faster rate of rise in global temps (after 1980) correlates with when the Clean Air Act began to have an effect on clearing up air quality (catalytic converters, ect.), and reduction in particulates. Sigh.

  11. GFW says:

    > A pet theory of mine, for which I have no proof […]

    I’m under the impression that all the GCMs agree with you.

  12. Vic says:

    If we give the airlines enough rope perhaps they’ll hang themselves along with the rest of us.  

    Qantas’ cheif executive Alan Joyce explained yesterday why the company has cut flight services while slashing the company’s future growth plans. 
    “The Queensland floods had cost Qantas’ bottom line $60 million, cyclones Yasi and Carlos $20 million, the Christchurch earthquake $15 million and Japan’s earthquake and tsunami $45 million.”

  13. Tom Bennion says:

    I am with you PeterW @2

    A simple campaign from even a few key individuals (academics, Hollywood stars, scientists etc) taking a vow not to fly again expect for emergencies is blindingly obvious when you think about it.

  14. Alteredstory says:

    You KNOW this is going to be interpreted as “more warming than all CO2 emitted by all technologies,” right?

  15. Prokaryotes says:


    In the 1970’s when the first American satellite photos were taken of this vessel as it sat inland, it was believed to be a normal Jumbo jet sized aircraft under construction. When the vessel was later photographed sitting in water the questions started to fly “what is it ? ”

  16. Tom Bennion says:

    Is it just me, or does talk of stopping flying tend to bring conversations, even on this site, to a screeching halt.

    Is it fear of being called a hypocrite? Mark @8 seems to confront that when he says of PeterW:

    “You haven’t flown since 1993, which makes you a hero, but also an outlier.

    I beg, scream, and plead for clean energy, and read every ClimateProgress post, but unlike you and a few others, I buy gasoline, gas, coal (indirectly), and other products that support plutocrats. These guys have us pretty much where they want us — we scream but still buy their stuff.

    We have work to do.”

    Are we choosing not to use a blindingly obvious means of getting the message out because of potential social embarrassment and personal inconvenience?

    My impression is that many would prefer to lay down on railway tracks in front of a coal train than announce to the friends that they have given up flying. Am I wrong?

  17. I walked up to a pilot in Denver Airport last year, took him by surprise … they are not accustomed to being approached by customers and I asked him about all the “contrails” … the ones that to my way of looking at it no normal airplane could conceivably create and the patterns and the aluminum and barium measurements in water and plants … I could go on but I this subject is never okay to discuss openly… Anyway, he told me straight out that it was all about weather modification.

    Where I live in NorCal, on the spraying days, we get ‘contrails’ on a clear blue day which by evening is totally covered with clouds. We get a pick up in the wind. Mostly, we then get days of rain. When the local weather predicts rain and there are no ‘contrails’ the day before, the weather forcast changes. Then you’ll see contrails and a day or two later, rain.

  18. Overall, and despite their short lifetime, contrails may have more radiative impact at any one time than all of the aviation-emitted carbon dioxide that has accumulated in the atmosphere since the beginning of commercial aviation. It is important to note, however, that the emitted carbon dioxide would continue to exert a warming influence for much longer than contrails, should all aircraft be grounded indefinitely.

    Yet not all airplanes create the type of contrails which lead to cirrus like cloud cover. Not passenger jets flying into and out of SFO, thats for sure. The trails they leave (if they leave trails) fade quite quickly. The other trails don’t .. they actually create weather, shield the sun. morph into huge plumes…

  19. Barry says:

    The IPCC Special Report on Aviation reported a decade ago that the best climate science shows forcing from jet travel averages 2.7 times the CO2 emissions.

    This study confirms the contrail aspects of that science and perhaps makes the contrails a bit more forcing than was thought before…but not a lot more.

    Jet travel for fun is plain old barbaric in its impact on climate. It is the coal plants and tar sands of personal climate footprints.

    Consider that the average EU citizens emits about 1.2 tCO2e per year on all direct transportation except flying. Then consider that a first class ticket to a far flung vacation spot and back can be 20 tCO2e per person. We are talking decades of sustainable travel GHG from a single vacation flight.

    Frequent fliers?

    The aviation industry and consumers are in denial about the per-passenger stomp of this mode of transport.

    Also, biofuels won’t solve all those other non-CO2 forcings…just a portion of the CO2.

    The good news is that if you are a flier that worries about climate disruption you have a very easy and cheap way to do something about it. Get off the climate trashing jets. Save money. Feel good about your impact for a change.

  20. Jeandetaca says:

    The contrail warming effect is surely quite complicated to properly assess. In France, the Bilan Carbon Method assess its affect at the same level as the CO2 coming from fuel consumption.
    The basic rule I propose for the correct accounting is the following:
    *ask the company the number of litres of kerozene per average passenger (first class must be increased by a factor equal to the relative extra price)
    *adding 10% for carbon emission for kerozene delivery, plus 20% for carbon emission due to the manufactoring of the plane, these litres of kerozene are in fact kilos of carbone (forget the CO2 we count the carbon only)
    *multiply by 2 for the contrail effect and you get the carbon footprint of the flight per 2nd class passenger; if you can’t help to get rid of CO2, you multiply by 3,67 (=44/12).

    With the Parti de la Resistance, next sunday, we will organise a symbolic leaflet distribution in some french airports for warning air travellers they are committing kind of genocide.

  21. Sime says:

    Aah those airlines hum… here is a quote from a foul mouthed CEO of an airline you may have come across in your travels…

    “Do I believe there is global warming? No, I believe it’s all a load of bullshit. But it’s amazing the way the whole fucking eco-warriors and the media have changed. It used to be global warming, but now, when global temperatures haven’t risen in the past 12 years, they say ‘climate change’.”

    “… the United Nations as “one of the world’s most useless organisations”, its Intergovernmental Panel on Climate Change (IPCC) as “utter tosh”, and US politician Al Gore as someone who “couldn’t even get fucking re-elected” after a boom.”

    “The scientific community has nearly always been wrong in history anyway. In the Middle Ages, they were going to excommunicate Galileo because the entire scientific community said the Earth was flat… I mean, it is absolutely bizarre that the people who can’t tell us what the fucking weather is next Tuesday can predict with absolute precision what the fucking global temperatures will be in 100 years’ time. It’s horseshit.”

    – Michael O’Leary – CEO Ryan Air – 12/09/2010

    “Dark Side” or what, and completely “Gish Gallop” to boot!

  22. Colorado Bob says:

    The last week in Thailand –
    Weather data show that hard-hit Nakhon Si Thammarat was washed by rain amounting to about 28 inches between Wednesday and Monday. Elsewhere, rainfall of 8 to 16 inches was widespread east of the highlands that bulwark the Malay Peninsula, such as Surat Thani and the island of Ko Samui.

    At Nakhon Si Thammarat, normal rainfall during the month of March is about 2 inches, late winter to earliest spring being the driest time of the year.

  23. Colorado Bob says:

    Daniel Lamb, a meteorologist with the National Weather Service in Jackson, said officials went to Simpson County to determine if it was a tornado that hit the area. Three homes there were destroyed and 40 others were damaged, the Mississippi Emergency Management Agency said.

    “This damage was from straight line wind,” Lamb said.

  24. Joan Savage says:

    Planes often cruise with the jet stream on eastbound flights save fuel, and tack around the jet stream going west to avoid having to go head on against the stream. I can hope that climatologists might have access to information about aircraft altitude and the direction relative to jet stream to find out more about cloud formation in that regard.

  25. Joan Savage says:

    What do ClimateProgress folks know about a “Adverse Greenhouse Effect Index” as applied to air vehicles?

    A hybrid air vehicle (HAV) venture, sometimes called SkyCat, has a graph on environmental impact which uses an “Adverse Greenhouse Effect Index” in comparison to its diesel-fueled and prospectively hydrogen-fueled crafts.
    The accompanying text says “consume less fuel and operate at a lower altitude,” but I have to imagine that even a “small” use of diesel might leave a con trail, a propos Lief’s observation about shipping con trails.

  26. Prokaryotes says:

    WHO report for Europe
    Noise robs annually one million life years

    Aircraft noise rips residents from their sleep, the noise level near roads leading to irritability, high blood pressure, tinnitus. WHO researchers have calculated how much constant noise burden on the health of Europeans – and come to frightening numbers,1518,754206,00.html&

  27. Barry says:

    On the issue of jets flying different paths and altitudes to avoid the contrail effect, don’t hold your breath for a miracle.

    The airlines have known all about contrails for over a decade and have been studying alternatives. If there was an easy fix it would have been done by now.

    There was talk of flying lower but studies showed it burned too much extra fuel in the denser atmosphere. Ditto for re-routing. The cost of fuel is so high they can’t afford to burn more than their competitors. Plus more fuel = more CO2 which is what most consumers are focused on.

    Flying has been a known climate disaster for over a decade because of all the extra forcings. Nobody has a solution other than to switch to less damaging transportation options.

  28. Barry says:

    Tom Bennion (#17): “My impression is that many would prefer to lay down on railway tracks in front of a coal train than announce to the friends that they have given up flying. Am I wrong?”

    Exactly. I spent a couple years research, writing articles and talking to companies about reducing the level of flying climate pollution. I was even interviewed on NPR at one point about it.

    But I’ve pretty much stopped that line of climate work because not only was there no desire for action…but as you point out, it can lead to being socially ostracized even by friends and family.

    It is a very sad statement of human nature that even some of the most concerned people about climate and the environment refuse to cut their biggest and most non-essential climate stomp…or to even engage in discussions about it.

    Few things make me more pessimistic about the future of climate mitigation than this “I won’t change my luxury carbon stomp” attitude of folks who know better.

  29. Gestur says:

    I’ve long had an interest in identifying and empirically implementing an interesting comparison-metric, that is, a metric by which I could compare the carbon footprint of a flight I might take—which would be totally discretionary for me—with the carbon footprint of something meaningful in a simple way and also in a very non-discretionary way. I settled on comparing pounds of carbon from a flight to Rome and back with the pounds of carbon from just heating my house in the winter months of October-April. [Keep in mind I live in Minneapolis.] I thought I’d share it.

    I used my own natural gas bills for a whole year and got carbon factors for natural gas and from this and my monthly bills I could easily factor out my gas use for heating water and my stove. [I don’t have a clothes drier.] And I obtained good distance-sensitive carbon factors for air flight and factored in a stop in NYC. Turns out my household carbon footprint for heating-only is 2,302 lbs, or 1.15 tons. Since I’m a household of one, that is, alas, also my personal carbon footprint for heating-only. A round-trip flight to Rome would get me 1,114 lbs of carbon, or it would be an increase of my carbon footprint from just heating my house of 50%.

    But along comes this Burkhardt and Kärcher study and it says multiply that carbon-only footprint by (31+28)/28 ≈ 2.1. And that gets me 2,417 lbs of carbon-equivalent, or slightly more than I produce from heating my home in Minnesota the whole winter.

    But notice this is based on my personal one-man household. If a family of 4 lived in my house their per person carbon footprint would be one-quarter of this household one. [If they would settle for my low temps and wear long underwear, it would.] But if they or any subset of them flew to Roma they would still have that same per person carbon footprint from that flight. And thus any one of their flights to Roma would represent 4 times their carbon footprint compared to just heating their house. Again, I find that a pretty interesting comparison-metric.

    For the record, I haven’t flown anywhere, much less ‘a Roma’, since 2006 and have no plans to alter this commitment. In other words, I’m with ya Barry, cento percento!

  30. Morocco Bama says:

    Deborah, that could explain why many of these airlines have not gone under, despite the fact that, considering the Recession quickly becoming a Depression, airlines should now be going bankrupt left and right. They were already operating on hair-thin margins. We saw some bankruptcies prior to the downturn, but hardly any to speak of during this permanent downturn. It doesn’t add up…unless you consider other factors. Without the flights, the weather stops and we become a desert. Something to consider….with a grain of salt, of course.

  31. Mark H says:

    V. interesting this is a topic of much concern to me. Did my carbon calc for last year & predicted for this year – the biggest factor was flying – the calculator I used factors flying by 3x, somewhat in line with the above info. Currently, I have off-set this by buying trees, however, this ain’t sustainable long term as a practice for all flyers. I have been having this same conversation on TED regarding other ways of long distance transport which are considerably less damaging than jet planes, as yet no real alternative exists. Airships were mooted but are slow & air resistance increases dramatically as speed increases. The other question is what purpose is served by having jet transport… Any thoughts on this?