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Bjorn’s Corner: Air Transport’s route to 2050. Part 25.
By Bjorn Fehrm
June 6, 2025, ©. Leeham News: We do a Corner series about the state of developments to improve the emission situation for Air Transport. We try to understand why development has been slow.
We examined alternative, lower-emission propulsion technologies four weeks ago and compared them the following week to the industry’s typical improvement in fuel consumption over time. Then, we examined the improvements that SAF can offer by 2050. Last week, we complemented the picture with the different Emission Trading Schemes (ETS) that exist globally.
Now, we discuss what warming contrail avoidance could achieve in reducing global warming.
Figure 1. A summary of the CO2 and non-CO2 Effective Radiative Forcing (ERF = warming effect) contributions from Air Transport. Source: The report “The contribution of global aviation to anthropogenic climate forcing for 2000-2018” by Lee et al. (2021)
What reduction in global warming can Contrail avoidance achieve?
In our previous articles, we have compiled the various emission-reducing actions that can contribute to reducing CO2 emissions. The warming effect of increased CO2 levels in the atmosphere is well-researched and widely accepted.
The warming effect from the specific contrails that are persistent and morph into new cirrus clouds, which contribute to trapping heat radiation from the Earth, is less widely accepted.
The study from which I have taken Figure 1 represents a significant step forward in this process, as it summarizes and further develops a large number of studies conducted over the past 30 years.
We know from these studies that few of the contrails we see from airliners are the ones that form persistent contrails and contribute to global warming (2% contribute 80% of the effect).
Trials have been conducted that demonstrate it’s possible to avoid generating warming contrails by adjusting route or altitude, with modest effects on total fuel consumption for the route (typically below 3%). The gain in CO2 from replanning 84 flights for a German airline resulted in an ERF effect equivalent to flying an additional 1,250 flights, a fantastic improvement. The improvement of extra CO2 produced for the flights versus CO2-equivalents saved was 700 times!
The limitation for what can be achieved is the Air Traffic Control (ATC) capacity once warming contrail prediction methods have matured and flight planning software fully supports flexible contrail avoidance flight planning.
Figure 2 illustrates the development of the different components in Figure 1 from 2000 to 2018, which is the last data year for the Lee et al. (2021) study.
Figure 2. The ERF (warming effect) development between 2000 and 2018 for contrails and other greenhouse gases. Source: Lee et al, 2021.
We see that contrails in total contribute about 50% of the total, with C02 at 30%, and with NOx, H20, and Aerosols from soot making up the rest.
How difficult will it be to get contrail avoidance to contribute as much as the other actions we discussed? We have observed that there are no significant changes in flight plans and that only a few routes are impacted of an airline’s daily flights. The number changes with the geographical position, as we saw in Part 15, Figure 3.
Figure 3. The net Radiative Forcing of flights during 2019. Source: The report “Global aviation contrail climate effects from 2019 to 2021” from 2024.
The developed world is more affected than the less developed parts. This is positive, as it should be possible for North America and Europe to achieve the Air Traffic Control capacity necessary for warming Contrail avoidance.
The next step would be to define how contrail avoidance is factored into, for instance, reduction in the airlines’ ETS CO2 tonnes. With a factor of many times the increase in extra fuel CO2 emissions for the flight, it can be a strong driver towards more aggressive progress in contrail avoidance compared to the slower and more challenging SAF blend schemes.
This is a really thought provoking piece. We could reduce the effects of aviation significantly without a radical fleet overhaul, and also leverage a smarter ATC network if the effort and money was put in. Seems like this should get some immediate focus given the other options will have a much longer implementation phase, and if done right this could improve ATC workload and safety at the same time. It’s also interesting even if we had hydrogen technology today if it resulted in contrails we would not be eliminating the bulk of warming effect.
Thanks Zoom.
Yes, I agree. We are breaking our necks when trying to implement alternative propulsion technologies (these still have potential long-term), at the same time as the really huge low-hanging fruit is there.
BTW, hydrogen burn engines don’t produce the soot needed to condensate the subtemp. saturated water vapor in the atmosphere, so these engines would not produce contrails, nor would fuel cell outputs. So long term, it’s a better solution as it gets rid of the CO2 and most of the NOx for the burn (all for the fuel cell version).
I realize the volume available would greatly reduce range but would it be theoretically possible to retrofit an a320/737 with hydrogen tanks where the fuel tanks are today and use hydrogen in a shorter range Neo/MAX for at least part of the fleet? So many of these fly sub 1200 mile flights it could be a “test” of infrastructure at key airports, especially ones like LGA or DCA where there are perimeter rules?
Hi,
no it’s not possible. While hydrogen is lighter than Jet fuel (almost 1/3 of the mass) for the same energy content it occupies four times the volume, but only if its cooled to -253°C. This means you need close to spherical thermos flask like tanks, which does not fit in an A320/737 wing.
The SAF burning airliners effect on forming contrails was missing, one could expecta more clean burning fuel (without sulfur or carbon particles) would have a positive effect. Data from trucks burning HVO100 and similar bio diesel should give data as well.
I have been idling along on the latest ones as I don’t have anything to contribute, so thank you Zoom for doing so.
I would disagree the developed nations are more affected. Pollution spans across the globe and everyone is affected by the impacts of Climate Change.
As my wife said, 100 Degrees + in Pendleton Oregon this time of the year is nuts, not a one off or occasional, normal now.
We just got snow on our mountains. Sometimes happens but heavy snow in June? And we are as close to undeveloped as any country (overall) in the world. 500,000 in a place almost 3x the size of Texas.
Undeveloped as measured by emissions per capita?? Guess not.
I think the “impact” is more that if it’s hot in the US, most (though not all) just turn up the AC whereas if it gets hotter in the developing world people don’t have that option and can die from it. I agree unless we think globally changes can be undone easily. I’m not a rabid environmentalist but I think it should be criminal to build new coal power plants knowing not only the warming effects but also the near-source impacts of air quality on people. I also think it’s irresponsible for the globe to not implement solutions to low hanging fruit like ATC routing. In the US we have chronic system issues as it is, and a revenue stream that can support modernization through navigation/landing fees. It’s not as flashy as a new aircraft but as I’ve flown a lot lately I see all the older gen aircraft and the scale of implementing things like that becomes clear. Not to mention smart routing would benefit us regardless in the end.
Seeing ABs latest forecasts, with growth mainly in Asia-Pacific, and Tim Clarks call for restarting the 380, even if it costs 15-20bn€, and growth of slot restrictions in megaports, combined with folding wings tech. This could be the killer app for 380: Make 380 fit in size E gates as 777x, & they can convert costly gate F to E, while getting the same capacity thoug in pax.
That can drive a positive investment case for the growing megaports outside the US, to make more profits from existing infrastructure. When flying between 2 megaports even more so. A 380 can then outbid big twin operators for slots, while the smaller gate make its more versatile. That drives a positive investment for the airlines too, then than again will drive more airports to adapt their E gates to dual-level bording & runways to 380s weight.
Make a slightly expanded 380 with efficient & folding wings, & UHBs.
Sometimes, a product is too early or lack the killler app/game-changing feature, due to immaturity of the needed tech.
Combined with new digital & production tech, it should make smaller series cheaper to produce. Say 550m€ for 300 planes.
Tim Clarks vision is likely true, but making the 380 fit into smaller E gates, will be the game changer that will make more megaports & airlines adapt to it, as it increases their revenue.This is the killer app.
AB should look into it, as its a small but growing & potentially very profitable market to serve the growing megahubs in China & India, Jpn, etc. It was not only TC that operated the 380 with profit, but also China Southern as ferry between the megahubs in China.
The rules that apply to US airports & airlines, are not true for megaports outside. And the US will not grow as much, growth is in Asia-Pacific & in megahubs serving megametropolitan areas as Seoul, Tokyo etc.