Altitude Changes Cut Contrail Effects


A British study has found that relatively small changes in altitude for a fraction of airline flights could reduce one of aviation’s climate change impacts by 59 percent. Imperial College in London says about 1.7 percent of flights are responsible for almost two-thirds of the warming effect on the planet caused by radiative forcing, in which cloud-forming contrails reflect most light back into space but allow infrared radiation to reach the ground, causing a warming effect. The study says dropping those airplanes 2,000 feet lower to make their contrails less persistent would result in the 59 percent reduction in warming.

“According to our study, changing the altitude of a small number of flights could significantly reduce the climate effects of aviation contrails,” Imperial College civil engineer Marc Stettler told ScienceAlert. “This new method could very quickly reduce the overall climate impact of the aviation industry. A really small proportion of flights are responsible for the vast majority of contrail climate impact, meaning we can focus our attention on them.” Although airliners burn more fuel at lower altitudes, the overall benefit of reducing cloud-forming contrails more than makes up for the slight increase in emissions, he said.

Russ Niles
Russ Niles is Editor-in-Chief of AVweb. He has been a pilot for 30 years and joined AVweb 22 years ago. He and his wife Marni live in southern British Columbia where they also operate a small winery.

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    • Try telling that the the planet Venus. It’s the poster-child of runaway global warming due to the “greenhouse effect”.

      Clouds do reflect some of the sun’s rays. But what does reach the surface is reflected by the ground not as light, but as infrared heat. This is trapped very effectively by clouds. And it turns out high cirrus clouds (and contrails) are more effective at trapping heat than low altitude clouds. As a result, less heat is lost (especially at night), resulting in a net increase in temperature.

      • They block light that heats the surface.
        What is the measurable net increase in the the upper atmosphere in the last 30 years? I’ll wait.

    • Mark you may have missed these words in the article: “cloud-forming contrails reflect most light back into space but allow infrared radiation to reach the ground, causing a warming effect”

      • It’s utter nonsense.
        Water vapor strongly absorbs in the IR.
        Maybe civil engineers should not write studies that involve basic Chemistry?

        • Water vapour absorbs infrared radition, correct. So, infrared radiation from the heated earth surface radiates spacewards at night, and is absorbed by water vapour (clouds), instead of radiating onwards back into space and out of the atmosphere. Hence, greenhouse effect. Here’s a description: . The article is not nonsense at all.

        • I took the time to read all of the references and went deep with it all this AM because I AM interested in learning about such subjects. There’s one chart which shows that the reflective difference between a CO2 laced atmosphere compared to only the water vapor at the same 12 – 15 um wavelength isn’t as large as I might have thought. While CO2 does fill in (overlaps) some of the water vapor caused absorption at those wavelengths, conveniently buried in the footnotes is the statement that water vapor is the largest single contributor to greenhouse gases. Burt Rutan talks about that in his one hour youtube video. And doesn’t water vapor then cause clouds which both cause absorption of energy (bad) AND drop off a little rain now and again to sustain us all (good)?

          Elsewhere, they’re saying the earth’s average surface temp has risen ~0.6 to 0.9 deg over the last century with a likely rise of an additional 0.6 deg in response to the CO2 induced absorption of outgoing reflected energy in the next. Conveniently left out is the fact that warmer temperatures allow longer growing seasons which then allow feeding of the populations. It’s all tied together but the rocket “scientists” who write these papers seek only to stir up their parochial believers into a frenzy. And I have charts which shows that the concentration of CO2 was FAR greater thousands of years ago than it is now … why was that. Were too many cave people driving too many miles in their V8 Fords ?

          Frankly, I don’t see what everyone is worried about. AOC says that we (now) have less than 12 years to go so … no matter what we do … we’re already doomed. When someone of these technical paper writing geniuses writes a paper about reducing the population of the planet, then I’ll pay attention. Trying to make me believe that a few contrails will change everything … not so much. It’s much akin to reading a corporate earnings summary … it can be written two ways depending upon what the writer wants the reader to believe.

          • Of course the earth is in a very gradual warming period; however there is no warming trend discernible in the upper atmosphere. That’s interesting and important to any discussion on climate.

            “IF” you believe that the gradual warming is a problem today and “IF” you believe that man can have impact, then worry about the heat trap in concrete, asphalt, roofing materials, and the associated loss of green areas.

            The upper atmosphere is not warming so to quote Indiana Jones, “they are looking in the wrong place”.

          • Just in the last day or two I read something about Delta claiming that it was somehow going to make some changes such that it’s aircraft flight ops are carbon neutral. Talk about pandering! How the heck are they gonna do that? Turn off the turbines and glide half of the time?Plant 200 trees for every flight mile flown? Ridiculous!

    • The article talks about this. “Although airliners burn more fuel at lower altitudes, the overall benefit of reducing cloud-forming contrails more than makes up for the slight increase in emissions.” The top-level goal is to preserve the planet’s hospitality to human life. Burning more fossil fuels can be an acceptable trade-off if the net result is less forcing of climate change.

  1. Less humid air could be found at higher altitudes as well. Airliners need not necessarily fly lower. Depends on the weather on a given day.

  2. This article riles me up ! Based upon six weeks of study in Japan, a UK civil engineer is pontificating to the aviation industry (sic). I dug through my files and reminded myself of an interesting book, “Lukewarming: The New Climate Science that Changes Everything.”

    “When it comes to global warming, most people think there are two camps: “alarmist” or “denier” being their respective pejoratives. Either you acknowledge the existence of man‐​made climate change and consider it a dire global threat, or you deny it exists at all. But there is a third group: the “lukewarmers.” In Lukewarming: The New Climate Science that Changes Everything, Pat Michaels and Chip Knappenberger explain the real science and spin behind the headlines and come to a provocative conclusion: global warming is not hot—it’s lukewarm. Climate change is real, it is partially man‐​made, but it is clearer than ever that its impact has been exaggerated—with many of the headline‐​grabbing predictions now being rendered implausible or impossible.”

    In that book, I lifted numerous charts and discussions pertinent to the discussion here as well as in today’s political arena seeking to take our freedoms and liberties away based upon an inexact science and questionable climate data. Can you spell “Climate Gate?”

    Digging through my own data, I found some VERY interesting Antarctic ice core sample based charts going back — are ya ready — 740,000 years. Using todays mean temps as a datum, temps were FAR higher 400,000; 330,000; 240,000; and the highest was 125,000 years ago. Today’s warming trends are but a pimple on the chart and are trending steady, relative.

    In the book, they used the polar opposite examples of a small probability that a large and dangerous climate change justifies drastic action to the converse, that a large and dangerous effect of moving toward renewable energy sources, reduced economic growth through carbon taxation and geo-engineering likewise demands extreme caution. They said, “Pascal’s Wager Cuts Both Ways.” (google it). Great analogy.

    I was also reminded of the position of Dr. Judith Curry, a climatologist who left a Chair position at the Georgia Institute of Technology over issues involving “Climate Gate” and the non-scientific political impact upon the subject. In 1999, Dr. Curry co-wrote, “Thermodynamics of Atmosphere’s and Oceans” as well as 140 scientific papers on climate. She knows what she’s talking about and is a believer in ‘Lukewarming.’ She was unhappy with climate scientists diddling with the data. Rather than argue, she retired and left.

    As far as I am concerned, if you’re worried about the end times due to climate, reduce the population and find a way to stop the water vapor cycle from impacting radiactive forming. A few contrails here and there ain’t diddly. I wish there was a way I could attach some of my charts here but — alas — ain’t possible. Me … I’m a “Lukewarmer.”

  3. As does “Larry” above, I call “HOGWASH” on most of this Climate Change “stuff”.
    Yes, things are getting warmer, it is “time” in the cyclical earth events.
    Yes, humans do put out a lot of CO2 and other pollution that might add to the problem.
    Yes, humans have the smarts and the ability to use technology to work towards easing the CO2 issues. We can possibly, with sci-fi level advances (certainly possible) even reverse that issue.
    No, contrails are not the big issue and flying 2000 feet off altitude won’t change things a bit.
    I’ve got a ton of time in 4 motor machines droning around the planet. There is not enough moisture change in a simple 2K ft altitude change to prevent contrails because the moisture band thickness strictly depends upon the weather systems driving the moisture layer.
    I repat: HOGWASH

  4. Thinking about the predominant generator of CO2 — cars — we are SO much better off today than back a few decades back. Just the other day, I walked by a 60’s vintage car that was running and was almost instantly asphyxiated by the stink of the thing. NOW, you can’t smell a car running. And the difference is clearly visible in the massive reduction in smog in the LA Basin, too. I don’t mind changes that are smart but when blanket statements are made by people who know nothing about the climate or mans impact on same … I take MAJOR exception. Worse, when they say that if only we pay a higher “tax,” they’ll make it all better. Like David C says … HOGWASH !!