The contrails and tribulations of aviation’s climate impact  - ZeroAvia

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    February 20, 2024

    The contrails and tribulations of aviation’s climate impact 

    “Most fuel cell contrails will be short and those persistent ones will be optically thinner and have on average a shorter lifetime than traditional persistent contrails. From a climate point of view, the introduction of fuel cells into aviation can be recommended.”

    It is to our collective detriment that mainstream discussion of how to reduce aviation’s climate impact is typically weighted so heavily towards carbon emissions. Decarbonising is, of course, vital: but focusing solely on that allows all sorts of other emissions and challenges to fly under the radar. 


    As well as emissions of nitrous oxides (NOx), soot, and water vapour, for example, contrails represent another crucial element of aviation’s climate impact equation. Yet these ‘condensation trails’ – the streamers of cloud sometimes observed behind an airplane flying in clear cold humid air – receive comparatively little attention, and public awareness of how they form and what impact they have is low.  


    We need to wise up on the facts, and inform and educate people, because there are plenty of misleading claims out there. And if we’re to successfully achieve truly clean flight, then we need accurate data, transparency regarding the claims made by companies in this sector, science-based policy and an informed public (the voters for that policy). 


    Already we see with the adroitly branded ‘Sustainable Aviation Fuel’ (SAF) the danger of assumptions based on that misnomer. SAF is not truly sustainable in the meaning of the word that most matters here, given that it still involves combustion, thus producing huge amounts of CO2 and NOx, and in biofuel form will require volumes of feedstock (whether ‘waste’ or energy crops) that aren’t plausibly…sustainable.  


    The synthetic equivalent that is proposed to solve the availability of these fuels requires production of green hydrogen and direct air capture of carbon, and then a further energy intensive process to meld them together into a combustible hydrocarbon fuel with similar levels of CO2, NOx and contrail impact.  


    It should raise a lot of questions, yet many people are assuming SAFs are ‘the answer’, rather than being one temporary part of a much more complex solution. 


    The same is at danger of happening with discussion around contrails, where there are claims in some quarters that other fuels and technologies (those which will be competing with SAF…) will exacerbate the contrail problem. Let’s look at whether those claims are founded in science or not. 


    Firstly, though, a quick reminder of what contrails are and why they matter.   


    For many, these white trails criss-crossing skies on crisp mornings seem a benign, photogenic phenomenon. But contrails may last a long time and turn into cirrus clouds that trap heat in the earth’s atmosphere. Although these persistent contrails also create a small cooling effect by reflecting solar radiation during the day, their net effect is to accentuate global warming.  


    In fact, one recent study suggested that between 1940 and 2018, the net global warming impact of contrails, measured in terms of radiative forcing, may have been as much as three-quarters of that of the total net radiative forcing generated by aviation(Lee et al. 2021). 

    The Hydrogen Problem: (data source: FlyZero Sustainability Report, 2022/3)

    Basic chemistry principles dictate that higher water emissions will result in the formation of more contrails.  


    Burning SAF produces more water vapour than fossil jet fuel (due to the higher hydrogen to carbon ratio) so tends to form slightly more contrails: another entry in the ‘cons’ box for SAF, unfortunately. 


    Combusting hydrogen generates 2.6 times as much water as kerosene jet fuel, so it will produce even more contrails, at lower altitude and at higher ambient temperatures.  


    Hydrogen fuel cells will form contrails most of the time due to their much lower exhaust temperatures. However, only a few of these will become persistent, if they form in ISSRs – ice supersaturated regions. 


    But what about hydrogen-based fuel cells?

    Well, the only emission from hydrogen fuel cell-powered aircraft is water. And so, while there is no doubt about hydrogen-electric’s ability to reduce CO2 and NOx to zero, the question as to whether fuel cell-based aviation will create a bigger contrail problem is a logical one. 


    Science offers a clear, and positive, answer.  


    Firstly, the fact that hydrogen-electric aircraft will eliminate particulates has a major impact on contrail formation. For just as water vapour condenses on the inside of a car’s windscreen on a cold day, ice particles in contrails form much more easily if they have a solid surface on which to nucleate. With conventional aircraft, the water vapour condenses on the particulates present at the exhaust. However, fuel-cell-powered electric engines, where nothing is being combusted, have no exhaust besides the water vapour. Meaning they are not producing any solid matter for the contrails to attach to. 


    This is why researchers have found that “despite the high frequency of contrail formation from fuel cells, their climate impact is lower than that of contrails from jet engines.” (Theory of Contrail Formation for Fuel Cells, Gierens, 2021 


    What’s true is that a fleet of aircraft equipped with fuel cells that use stored hydrogen to generate electricity will certainly release more water into the atmosphere – but no carbon dioxide, no nitrous oxide and no other particulate matter. 


    As Dr Klaus Gierens, of the Institute of Atmospheric Physics, Munich, has argued, “Most fuel cell contrails will be short and those persistent ones will be optically thinner and have on average a shorter lifetime than traditional persistent contrails. From a climate point of view, the introduction of fuel cells into aviation can be recommended.” 


    Most importantly of all, perhaps is that the lower altitude operations of turboprop aircraft (which will be the first fuel cell planes) will mean non-contrail forming flight operations. When the technology advances to support the 100+ seat planes, it is likely that solutions can be devised to capture and store water vapour release in contrail forming conditions given the low temperature and pressure of the exhaust, allowing for release at non-contrail forming altitudes.  


    Cleaning up aviation is immensely challenging. Achieving truly clean flight is the biggest challenge of all. The science is clear that hydrogen and fuel cells have a huge part to play. 


    To read more about how some of the first fuel cell powered-engines will deliver zero-emission flight as early as 2025, download our whitepaper on the Hydrogen-Electric Cessna Caravan.