| 16 Sep 2025
Climate impact of contrail cirrus from hydrogen combustion aircraft
Abstract. To mitigate the climate impact of aviation, combustion of hydrogen as a fuel is one possible future pathway. Hydrogen combustion leads to zero carbon emissions in the exhaust, representing a major step toward climate neutrality, although the non-CO2 effects, primary contrail cirrus remain uncertain. In this study, we simulate the climate impact, in terms of energy forcing, of contrail cirrus from hydrogen combustion aviation using a modified version of the Contrail Cirrus Prediction model (CoCiP).
Without soot in the exhaust from hydrogen combustion, contrail ice particles instead form on ambient aerosols entrained into the plume and on lubrication oil droplets in the exhaust. The formation of ice particles are modeled using an emulator developed from a theoretically based microphysical contrail formation model.
Following the Schmidt Applemann criterion, hydrogen combustion enables contrail formation at lower altitudes and higher temperatures than fossil jet fuel. However, we find a significant reduction in contrail energy forcing. This result holds across a wide range of assumptions, including different oil particle size distributions and properties, with a global average reduction of about 70 % using our base case assumptions. We conclude that hydrogen aircraft not only eliminates CO2 emissions but may also substantially reduce the climate impact of contrail cirrus, although the reduction and magnitude depend on engine design for lubrication oil handling.
The authors investigate the potential for hydrogen-fueled aircraft to reduce the climate impacts of contrails. They do so by developing an emulator for a sophisticated model of the early plume, incorporating the effects of lubrication oil and ambient particles. They consider a broad range of possible oil properties, and make reasonable assumptions regarding the performance of near-future hydrogen aircraft. Using the CoCiP model to simulate the mature plume and radiative forcing they find that hydrogen aircraft contrails are likely to produce reduced radiative forcing per flight meter compared to kerosene contrails, but that this is sensitive to the handling of lubrication oil.
The question posed by the authors is interesting and timely, given that there is significant uncertainty regarding the potential environmental benefits of a transition to hydrogen. However, the methods used are somewhat limited, and this in turn limits the degree to which the data can support the conclusions drawn. Given that the results may be strongly dependent on the choice of meteorological data, early plume representation and contrail model, it is striking that there is relatively little uncertainty quantification and no acknowledgement in the abstract of potential sources of error in the methods.
Despite this, I am broadly supportive of the manuscript’s publication pending these issues being addressed. The development of an early plume emulator is a particularly valuable methodological advance, and it will be interesting to see how these results compare to those from groups performing (eg) LES modelling of hydrogen-fueled contrails.
Major comments
The development of an emulator of the K15 model is a potentially significant advance for the community. However, quantification of the accuracy of this emulator is currently lacking. The authors state on lines 229 to 234 that the fit looks “very good” and that, even in the worst cases, “the fit is decent”; these are however fundamentally value judgements. I recommend that the authors pick a representative set of, say, 100 cases (including some extremes) and quantify the degree to which disagreement between the emulator and the parent model propagates to disagreement in their simulated impacts (i.e. contrail lifetime and EF, not just inputs to CoCiP).
On a similar basis, it would be useful to quantify the degree of (dis)agreement between results from CoCiP with the standard and K15 models when simulating fossil jet fuel contrails (lines 256-258). While S6 does show how the two models differ in terms of activation fraction, this difference is not propagated to an overall impact; furthermore the different formatting choices and lack of parity plotting between the results in S6 make it very difficult to establish whether there really are meaningful differences between the two.
I was surprised that homogeneous nucleation of water was not discussed. Given the potential for extremely high RH in the early plume of a hydrogen-fueled contrail, Zink et al. argue in their preprint (Contrail formation for aircraft with hydrogen combustion - Part 1: A systematic microphysical investigation) that homogeneous droplet nucleation (HDN) may be significant under some circumstances. It is not clear to me whether neglecting HDN is or is not likely to significantly change the conclusions of this study, but I would recommend that the authors at least clearly highlight whether or not this eventuality is covered by their modelling approach (since the base model by Kärcher et al. was developed for kerosene-based contrails and therefore does not appear to include HDN). Whether or not it is, I would also recommend at least a brief discussion of the implications of HDN for their study since this mechanism would be expected to be possible even in the absence of lubrication oil.
The authors make recommendations regarding engineering decisions which, while plausible, are not fully justified by this work. A recommendation that future engine design should avoid venting of lubrication oil into the main exhaust (line 472) seems premature given that only one aircraft/engine combination is assessed and given that there is as-yet almost no data on how difficult it might be to re-engineer an engine to avoid this (and therefore whether there may be efficiency penalties). I would therefore suggest that statements such as that in closing – that lubrication oil handling “should” be treated “as a critical parameter” – be instead tempered. A defensible statement might be that, for example, a 10 times reduction in lubrication oil emissions in the exhaust appears to reduce hydrogen contrail RF by 60% compared to a representative base case (this latter number read from figure 5 – clearly a better assessment is needed). Whether or not this is critical depends on balancing that against the alternatives, including the potential costs of doing so (which are not evaluated here).
Minor comments
The authors introduce a well-reasoned set of caveats in the discussion, which I found very encouraging. However, these caveats did not propagate to the abstract which surprised me. Given that the authors appear to have identified potentially significant limitations, I would recommend that the abstract include at least a statement to the effect that these findings need to be validated by both a more comprehensive modelling campaign (including e.g. a higher-fidelity aircraft representation, alternative meteorological/contrail models, and a larger dataset of flights) and of course measurements given that there are as-yet no fully hydrogen-fueled aircraft in service or testing.
It is often unclear whether the authors are referring to CoCiP (essentially a modelling approach, given that the original Fortran codebase is not – to my knowledge – publicly available) or pycontrails (a public, Python-based implementation of CoCiP). If the latter, as I expect given line 274, I would recommend that the authors specifically refer to “CoCiP as implemented in pycontrails”. For example, line 264 states that “CoCiP has a built in aircraft performance model”. From what I understand however, pycontrails includes an implementation of the Poll-Schumann model which it uses to initialize its implementation of CoCiP (but which can equally be used to initialize other models, such as a dry advection code).
Similarly, I was surprised to see that the gridded version of CoCiP was used (line 274) but that the paper describing that code (Engberg et al., 2025) was not cited.
A small request: the zonal plots (e.g. Figures 2, 3, 4) are rather confusingly oriented. It is more conventional for altitude to be the vertical axis, and latitude the horizontal. To ease reader interpretation I would recommend changing these figures accordingly.
There are some minor typographical, grammatical, and formatting errors throughout the document (e.g. malformed citation on line 71; “is” instead of “are” on line 196; “effects” instead of “affects” on line 195; missing space on line 74; incorrectly formatted citations on lines 86, 136, 138, 155, 207…). I would recommend that the authors review the document thoroughly to eliminate such errors.
Finally, I would suggest that the authors aim to make their assessment more quantitative. For example, section 4.2.1 compares different lubrication oil parameters but provides almost no quantitative assessment; almost all statements refer to high, low, small, large. It would be of great benefit to the reader to understand, for example, what the average EF per flight meter was across the 20 flights in the baseline case, and the percentage reduction (or increase) which was achieved when testing different factors. Such quantitative assessments enable subsequent researchers to compare their own models directly, and therefore provide outsized value in terms of advancing the field. If the authors could therefore augment their analysis with more quantitative evaluation (not just in 4.2.1 but throughout) I believe it would improve the utility of the manuscript for the community.