By Bjorn Fehrm

June 27, 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 have explored various methods to mitigate global warming throughout the series. Over the last few weeks, we have summarized the practical results we can expect from the different alternatives available to reduce global warming in air transport. We looked at the following:

1. Alternative, lower-emission propulsion technologies.
2. The industry’s typical improvement in fuel consumption over time.
3. The improvements that SAF can offer by 2050.
4. The different Emission Trading Schemes (ETS) that exist globally.
5. And finally, what warming contrail reductions can achieve.

We have summarized what the first four actions can achieve by 2050 and presented the results in a table, Figure 1. Now we add what global warming contrail avoidance can do.

Figure 1. The effects of Actions 1 to 4 on CO2 and NOx counted as CO2e emissions by 2050. Source: Leeham Co.

Adding warming Contrails reduction

When we discussed the reductions in CO2 emissions, it was an item where it was accepted that it had a direct impact on global warming. The increase of CO2 in the atmosphere blocks heat radiation out from the Earth, thus causing global warming.

When we added NOx, we examined a different climatic process in which the presence of NOx affects the levels of Ozone and Methane in the atmosphere. The CO2e we show represents the warming effect (called ERF by scientists) of the NOx processes, measured in CO2 equivalents (CO2e), Figure 1.

Warming contrails do not alter any atmospheric process. Some of the contrails morph into persistent linear cirrus, which then later morph into area cirrus clouds. This process is less deterministic, as only a few of the generated contrails are persistent and contribute to a warming effect.

The determination of the combined effects of CO2, NOx, other emissions, and warming Contrails is done through modeling of heat radiation into and out from the Earth. Some of the radiation cools the Earth, while other stops the heat radiation from the Earth and thus has a warming effect. It involves numerous interacting processes, as shown in Figure 2.

Figure 2. The different interacting processes from air transport that create global warming. Source: The Lee et al 2021 study.

In the 2021 study by Lee et al., 21 climate scientists and their teams collaborated to compile the current knowledge on global warming related to air transport into a single report.

The key to adding it all up is to calculate the global warming effect, known as the Forcing (ERF), of each contribution. Figure 3 is a simplified summary of the warming effects of emissions and contrails from air transport between 1940 until 2018 (the more comprehensive graph includes minor contributors that can be ignored in our discussions).

Figure 3. A summary of the main global warming effects of air transport. Source: Lee et al, 2021.

In the figure, observe the black lines superposed on the red bars. This is the 5% to 95% uncertainty range of the estimate from the models. We will look at what this means in the next Corner.

For now, we use the most likely value (the red bar) to change our emission mitigation table. When we go from CO2 and NOx mitigation to include warming contrails we need to change the table. We first calculate the warming effects of emissions and contrails from last year where we have the fuel consumption (300 million tonnes) and thus can calculate the CO2 effect and extrapolate the NOx effect. Then we add the warming contrails effect from the 2024 traffic intensity using the relationships in Figure 3.

To compare mitigation effects bv 2050 we first calculate the total CO2 and CO2e effects of the air transport intensity by then. For the predicted air transport traffic growth we use the average of the Airbus and Boeing market forecasts 2024 to 2045, which is 3.9% Compound Average Traffic Growth (CATG).

Then we deduce the mitigation effects from Alternatives 1 to 4. We then get a total including CO2 and NOx mitigation effects, Figure 4. We have not discussed how warming contrail avoidance can impact the results in Figure 4; we do this in following Corners.

Figure 4. Total warming effect table in million tonnes CO2 and CO2e. Source: Leeham Co. Click the table to see better.

We can see that the warming effects from contrails outnumbers CO2 and NOx by 2050 with 63% versus 37% for CO2 and NOx. So if the estimate is correct in the study, warming contrails are more important to mitigate than both CO2 from Jet A1 burn in the airliner engines that fly every day by 2050, and the NOx produced in these engines.

In the following Corners we will look at what all this means, and we will also discuss the uncertainties tied to the results on NOx and Contrail warming.