By Bjorn Fehrm
March 21, 2025, ©. Leeham News: We do a Corner series about the state of developments to replace or improve hydrocarbon propulsion concepts for Air Transport. We try to understand why development has been slow.
Last week, we examined MTU’s WET engine concept, which utilizes a water capture process in a Turbofan to lower emissions, such as NOx, and reduce the water content in the exhaust, thereby reducing contrail risk. The week before, we looked at a similar concept, focused on a hydrogen burn process from Pratt & Whitney called HySIITE.
Both concepts capture water in the turbofan’s exhaust and can then release it in a way that avoids contrail generation. Contrails have garnered significant attention in recent years, as research has raised the possibility that persistent contrails can contribute as much or more to global warming than the CO2 released during the Turbofan combustion process in an airliner.
We will spend the next Corners digging deeper into the question of air transport’s non-CO2 effects that contribute to global warming. Contrails have the largest impact on these non-CO2 contributions.
Figure 1. The complex interaction of different heat radiation and blocking processes in our atmosphere. Source: The report “The contribution of global aviation to anthropogenic climate forcing for 2000-2018” by Lee et al.
Climate Impact from Air Transport
I show the graphic in Figure 1 to illustrate that once we go beyond the relatively simple discussion of the CO2 impact of Air Transport from burning hydrocarbon fuel (where 1kg of fuel generates 3.16kg of CO2), it gets complicated.
There are a number of processes that contribute positively to global warming and others that reduce the temperature of the atmosphere. In total, it’s about an energy balance between the heat radiating from the Sun to Earth and how this heat radiates back to space or gets trapped by greenhouse gases, contrails, or other processes, Figure 2.
Figure 2. A Climate model that lists incoming and outgoing heat radiation in W/m2. Source: Wikipedia.
We won’t delve deeper into these graphs or models except to illustrate that it’s all about maintaining a balance in the radiation of heat to the Earth and then back into Space. If this balance keeps more heat on the earth or in the atmosphere, we will experience global warming, if not, the global temperature will reduce.
Wikipedia has the graph in Figure 3, listing the influence of different non-CO2 greenhouse gases and Aerosols. We also have a small contribution from Aviation contrails in the graph, but it should be remembered that air Transport is presently estimated to contribute 2% to 3% of CO2 to this process. Contrails generated by Air Transport can more than double this contribution, according to recent research.
Figure 3. The CO2 and non-CO2 contributors to global warming from all processes (transport, industry, cement, etc). Source: Wikipedia.
How to quantify non-CO2 contributions to global warming
CO2 contribution is measured in tonnes of generated CO2 from burning hydrocarbon fuels. Non-CO2 contributions can be measured in CO2 tonnes equivalents, called CO2e, or in terms of the Effective Radiative Force (ERF), measured in W/m². The term Effective Radiative Force is a climate change term that describes a change in the radiative flux between Earth and Space. From Wikipedia:
Human-caused radiative forcing of 2.72 W/m2 in 2019 relative to 1750 has warmed the climate system. This warming is mainly due to increased GreenHouse Gas (GHG) concentrations, partly reduced by cooling due to increased aerosol concentrations. “
We will use both CO2e tonnes and ERF W/m² when describing the effects of persistent contrails and the impact of different contrail avoidance actions.
What can be done about the negative effects of Contrails
The knowledge that persistent contrails and their transformation into Cirrus clouds can contribute as much or more to global warming from Air Transport as CO2 has grown over the last decade.
Since 2021, several organizations and airlines have begun implementing contrail avoidance trials involving route or altitude adjustments to minimize the generation of contrails.
We will describe what such flight changes entail, how they impact mission fuel burn and duration, the net effect of burning more jet fuel while reducing the CO2e contribution, and how these scale against each other.
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