By Bjorn Fehrm

July 4, 2025, ©. Leeham News: We feature a Corner series on the state of actions to mitigate the global warming impact from Air Transport. We try to understand why different developments have been slow.

In the last Corner, we wanted to understand the relationship between Greenhouse gas emissions of CO2 and NOx and the effect of global warming from contrails. After some iterations, we arrived at the comparison shown in Figure 1, where we compare different warming effects using CO2 and CO2e (CO2 equivalents, i.e. the same warming effect as CO2).

Figure 1. The Global waring effect of CO2, NOx and warming contrails by 2050 as CO2 and CO2e. Source: Leeham Co. Click to enlarge.

The data behind Figure 1

In Figure 1 we have used the average annual traffic increase of 3.9% from Airbus’ and Boeing’s newly released Global Commercial Aircraft outlooks 2024 to 2045. These forecasts are very similar in their aircraft fleet changes between these years. They differ slightly in the traffic increase (i.e. the increase in daily flight hours and thus Available Seat Kilometers, ASKs, for these aircraft), it’s why we use the average between the two.

To understand what happens with the worldwide fleet of commercial aircraft we use the Boeing numbers as these include regional aircraft, Airbus doesn’t include aircraft below 100 seats.

The Boeing fleet data is summarized in Figure 2. If we add freighters to our earlier figure of 25,000 passenger airliners flying each day we arrive at 27,000 aircraft.

Figure 2. Worldwide fleets 2024, 2044, and 2050. Source: Boeing commercial aircraft market forecast.

By 2050, we will have 57,000 aircraft in the worldwide fleet. These generate 948 million tonnes of CO2 today when the fleet has 8,000 new aircraft (aircraft with GEnx, Trent1000/7000/XWB, LEAP, or GTF engines). It would generate 2,467 million tonnes of CO2 by 2050 (Figure 1) if the fleet had the same composition as today, an increase of 2.6 times.

However, Airbus and Boeing both predict the addition of 43,600 new-generation aircraft by 2045. Extrapolate to 2050 and we have 57,000 new deliveries, i.e., the old aircraft are gone and the early new aircraft have started their replacement cycle.

Global warming from traffic increase

In our previous Corners, we have predicted the reduction in CO2 and NOx from these new deliveries from 2025, and forward. When we deduce the mitigation effects 1 to 4, we have a total CO2 emission from burning jet fuel and SAF blends by 2050 of 1,733 million tonnes and a warming effect of the NOx that is emitted equivalent to 832 million CO2e tonnes.

This represents a 1.8 times increase over 2024, a reduction of 0.8 times of both CO2 and NOx through the actions outlined in Figure 1. The trouble is that the global warming from contrails is not reduced, as the actions defined by for instance EU do not include warming contrail avoidance. Warming contrails now have a CO2e of 4,336 million tonnes or 63% of the total warming effect.

It means that as the Global warming mitigation from Air Transport is defined today, two thirds of the problem is not addressed. Why is this so?

Many organizations engaged in the Global warming problem have not yet accepted and defined actions based on the research of warming from contrails, as it stand of less firm ground than CO2. It’s less scientifically researched and confirmed. It’s why we have a large uncertainty in these effects and figures in the Lee et al 2021 report.

Figure 3. Air transport warming from emissions and warming contrails. Source: The Lee et al 2021 study.

We discussed that uncertainty range (Figure 3, the black bars on top of the red bars) of the predictions in the study in the last Corner, We dwell deeper into this in the next Corner.