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.