March 21, 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.
After covering alternative propulsion concepts to lower CO2 and NOx emissions, we now study air transport’s non-CO2 effects on global warming. Of these, contrails have the largest impact.
Contrails form when aircraft gas turbine engines emit soot particles into low-temperature water vapor-saturated areas in the atmosphere. The soot particles form condensation nuclei, and the developed droplets freeze to ice crystals that form contrails.
Figure 1. The net Radiative Forcing of flights during 2019. Source: The report “Global aviation contrail climate effects from 2019 to 2021” from 2024.
Warming Contrails
Not all contrails contribute to Global Warming. Most contrails don’t last long, with only a few lasting so long that they convert to persistent Cirrus clouds, which change the amount of heat radiation into and out from the earth. Of all contrails, about 2% contribute 80% of the warming effect shown in Figure 1.
The figure shows the net Radiative Forcing, RF (i.e., the climate warming effect) from contrails. The color denotes the net RF in mW/m2 shown on the scale at the bottom of the figure. Observe that the study puts the major areas of warming contrails over Europe, the North Atlantic, and the eastern USA. These are the areas where the conditions for persistent contrails that convert to Cirrus are most common.
Recent research indicates that Global Warming from contrails can be larger than from CO2 (some reports say 60% from Contrails, 35% from CO2 and the rest from NOx and other emissions). Therefore, there has been a lot of interest in recent years in research into contrails’ formation, effects, and how to avoid warming contrails.
The importance of contrails for Global Warming is good news for Air Transport, as mitigating actions are much easier to achieve than the hard struggle to avoid CO2 emission by changing to SAF or new propulsion concepts.
Recent research has constructed methods to predict the parts of the atmosphere where there is a risk of warming contrail formation, using satellite information of the atmosphere combined with weather data. This information has then been used to verify that contrails form in these areas and that aircraft that change their trajectory avoid creating contrails.
Figure 2 shows how such avoidance can be made by changing the altitude of an airliner.
Figure 2. The principal of contrail avoidance. Source: Leeham Co.
The black line is the typical altitude profile of an airliner’s mission. Once the climb is finished at 33,000ft (FL330), the aircraft cruises for an hour at this altitude. When the fuel load has reduced so that a higher altitude is optimal, the aircraft cruise climbs to 35,000ft (FL350), where it stays until the Top Of Decent point is reached and the aircraft descends toward the destination.
The contrail prediction system has pinpointed a risk of warming contrails around 34,000 to 38,000ft along the cruise leg. Therefore, the aircraft avoids the cruise climb and stays at the original cruise altitude of 33,000ft. The non-optimal cruise altitude for the last part of the mission will result in an increased fuel burn.
Many trails have been made since 2021, when the first contrail avoidance trails started in Europe. Results indicate that contrail avoidance has a highly positive effect on Global Warming from air transport. We will look at some recent trials flown in Europe, where the positive impact of the contract avoidance outweighs the extra CO2 generated by a higher fuel burn by a factor of over 10 to one.
The low-hanging fruit in Global Warming mitigation
There is a lot of research and testing still to be done to get definitive knowledge on the scope of Global Warming from contrails, how to predict warming contrail areas reliably, what warming contrail avoidance costs in terms of extra fuel consumption, and how air traffic control shall handle needed route changes.
However, one has learned so far that warming contrail avoidance can be much easier to implement than CO2 emission avoidance and that small route changes can have a substantial climate effect. This is what makes the ongoing research and trials so interesting.
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