By Bjorn Fehrm

July 18, 2025, ©. Leeham News: We have done a Corner series on the state of actions to mitigate the global warming impact from Air Transport. Now, we start to summarize what we’ve learned.

During the series, we compiled tables describing the warming effect of air transport in 2024 and a calculation of the effect during 2050. We made two tables, one with the most probable effects, Figure 1, and one where we downplayed the non-CO2 effects to the maximum given in the Lee et al. 2021 study, to a 5% probability, Figure 2.

Figure 1. The effects of Actions 1 to 4 on CO2 and NOx, represented as CO2e emissions during 2050. Source: Leeham Co. Click to enlarge.

Before we summarize by examining the tables, we will discuss the additive effects of CO2 and non-CO2 warming over a given time period, as the different components don’t have the same decay time of their warming effects.

The long-term global warming effect of CO2 and non-CO2 for air transport

CO2, when released into the atmosphere, will stay there for a very long time. In studies about the integrated effect (adding the emissions from the present year, next year, and so on) of global warming from CO2, its effects from different years do not decay. It means that the CO2 effects from year one remain the same when adding the CO2 emissions from the following years.

NOx warming effect decays by about two-thirds in 50 years and to below 20% in 100 years. The warming effects of contrails decay to about half in 50 years and then to around a quarter in 100 years.

Our tables list the warming effect of CO2 and non-CO2 components for the years 2024 and 2050. It provides us with an estimate of the warming effect during the year. For the integrated effect spanning 2024 to 2050, we would have to include the decay of earlier years’ warming in our addition of yearly effects. For our purposes, the work we’ve done lets us discuss the effectiveness of different actions for the year in question. We don’t have to delve into the complexities of integrated additions of CO2 and non-CO2 warming over a time period for this discussion.

Effects of Different Global Warming Mitigation Actions

If we examine the efforts made to reduce CO2 emissions between 2024 and 2050 (Figure 1), we see that the significant push to develop alternative propulsion systems yields almost no progress (-2.0%) by 2050. The lower fuel consumption from new aircraft replacing old delivers 52.0%, EU SAF 9.2% and ETS 0.4%.

NOx reductions are then 1.1%, 29.7%, 5.3% and 0.2% of the total of CO2 and NOx CO2e in Figure 1.

Warming Contrail reduction would address the dominant 63% of total warming from air transport during the year 2050.

We discussed that the NOx and contrail warming effects are less mature in their research and acceptance. When we do the very tough “let’s use the 5% effect probability,” NOx goes to 5% warming during 2050 (Figure 2), but Contrails remain significant with 42%.

Figure 2. The warming effects of Air Transport by 2050 with NOx and contrail effects at their lowest probable impact. Source: Leeham Co. Click to enlarge.

Conclusions

Our discussion has focused on the effect of different mitigation actions on global warming. It provides guidance on what we can do in the short term to mitigate global warming and what we need to do in the long term.

Short term:

• The most effective action is the fuel consumption reduction of new aircraft deliveries (which reduces both CO2 and NOx effects), and to focus on mitigating warming contrail generation. The fact that the warming contrail effect decays is good. It means that the decay accelerates any action taken.
• SAF is the third most effective action. The problem here is that only the EU has mandates, but the increase in SAF production can hardly keep pace with additional mandates unless the investment in SAF production is scaled beyond present plans.

Long term:

• The exchange of today’s 27,000 planes and the 57,000 of 2050 for low- or no-emission aircraft is a long-term objective. To halt the present escalation of global warming, it has almost no effect. This is a painful conclusion, but true. The aviation industry has invested considerable energy, time, and resources in alternative propulsion. We’d better take some of that money and spend it on warming contrail detection and a more flexible ATC that can handle the slightly changed tracks needed. The alternative propulsion work is for the long-term reduction of greenhouse gas emissions.