In this study, the authors reveal an “inconsistency” in aviation emissions of the CMIP6 dataset. They then use the EMAC climate model to quantify the impact on simulated ozone radiative forcing and methane lifetime. That impact is small but, at a few percent, not negligible. The impact on aviation aerosol radiative forcing does not meet statistical significance criteria.

The study is short and to the point. It is well written, and the figures and analysis support the discussion well. In my comments, I mostly request clarification of the text in places. However, the aerosol discussion could benefit from being more detailed, and it would be useful to provide guidance on how to interpret past results based on the wrong emissions. For these reasons, I recommend minor revisions.

Main comments:

• Throughout the paper, the authors speak of an “inconsistency”. That is diplomatic of them, but their analysis makes it clear that it is in fact a “mistake”. They provide at least two lines of evidence that the zonal distribution of emissions was not supposed to be like that. Furthermore, I was surprised not to see any mention of the issue on the CEDS github website. There isn’t even a new, corrected version available for download. Why not? There is a CEDS co-author, so they must be aware.
• The paper quantifies the impact of the “inconsistency” for the year 2015, finding that it causes the ozone radiative forcing to be about 8% larger. Can that number be used as a rough correction factor for CMIP6-based studies that are already published? How does it vary in time?
• The impact on aerosol radiative forcing is discussed in a 5-line paragraph. That feels rushed. Granted, the impact on radiative forcing is affected by the large interannual variability in cloudiness. But can’t the authors say something about the impact on aviation aerosol burden or residence time? The northward shift in emissions would certainly affect those.

Other comments:

• Line 6: Why single out ozone and leave out the result on methane lifetime?
• Line 9: The abstract also lacks a concluding sentence. What are the implications for aviation radiative forcing derived from CMIP6 emissions?
• Lines 12-13: I suppose that statement tries to make the point that CMIP6 emissions are used beyond CMIP6. Perhaps then clarify that those studies used CMIP6 emissions despite not contributing to the CMIP6 database itself.
• Line 21: “differs by” is vague. I suggest clearly saying that CMIP6 distributions need to be multiplied by the given latitude-dependent factors to fix the problem discussed here. It would also be useful to elaborate on the coefficients. I suppose the 1.344 and 1.912 factors are related to the shape of the emission distribution along latitudes. Is it some kind of emission-weighted mean cosine (or cosine squared) of latitude? Finally, it would be useful to repeat here that the correction is time-independent because emission patterns do not vary with time in those datasets.
• Lines 29-30: I suggest spelling out the implication that CMIP6 emissions for those species also need correcting for the same factors as above.
• Lines 34-35: Could cite Bock and Burkhardt (2019), for example, https://doi.org/10.5194/acp-19-8163-2019 in support of that statement.
• Line 70: But again using emission data for 2015?
• Caption of Figure 2: The lines have different styles, but also different colours, which should be mentioned.
• I am missing a concluding statement, here. Something saying that the shift in mean emission latitude probably had a small impact on the RF of aviation NOx emissions, an impact that is often neglected in climate studies. In addition, it would be worth noting that emissions will probably shift again in the future, and that it would be good to have emission scenarios that allow the study of the climate impacts of that shift.

I have read the paper entitled “An inconsistency in aviation emissions between CMIP5 and CMIP6 and the implications for short-lived species and their radiative forcing” by Thor et al.

It covers unstated differences in the spatial distribution of emissions between the CMIP5 and CMIP6 inventories and the implications for short-lived climate forcing and subsequent impacts.

This paper represents an important novel contribution to the scientific community, and I recommend it for publication in the journal Geoscientific Model Development.

The paper is well-written, and the methods and results are well-described. As such, I have no major comments, but please find a few specific comments below.

General comments:

The biggest question that I have is why these emissions inventories are different in the first place. Is there a likely calculation error performed by the emissions inventory community that would have led to this inconsistency? If there is a methodological error, it might be important to highlight some of the conversions that could have caused this inconsistency. In addition, also show if this inconsistency changes with the CMIP emissions year. Importantly, if the inventory is different for aviation, there might be other sectors for which the same distributional inconsistency exists. A comment on what might have caused this inconsistency, and a comment in the conclusion of this paper indicating whether this inconsistency is found for other sectors would greatly help the scientific community to fix these types of differences in future inventories.

The paper shows that the difference is the largest for polar emissions (page 2, line 24). As a result, an additional difference in radiative impact may occur due to differences in the impact of black carbon on ice pack darkening. Since this is not mentioned, I assume it is not included in this study. If it is evaluated, please comment on it, or if not, please include a comment.  

Line-by-line

Line 34-35: The impact on contrails might be substantial too and it would be good to mention the differences could be significant. For support consider referencing:

• Bock, L., & Burkhardt, U. (2019). Contrail cirrus radiative forcing for future air traffic. Chem. Phys, 19(12), 8163–8174. https://doi.org/10.5194/acp-19-8163-2019
• Lund, M. T., Aamaas, B., Berntsen, T. K., Bock, L., Burkhardt, U., Fuglestvedt, J. S., & Shine, K. P. (2017). Emission metrics for quantifying regional climate impacts of aviation. Earth System Dynamics, 8(3), 547–563. https://doi.org/10.5194/esd-8-547-2017

Methods section [Page 2 & 3]: This section describes the coupled climate chemistry model (line 39 to 49), but in the next paragraph it seems that the model is run with fixed meteorology using “quasi-chemical transport model” in the case of ozone, and for aerosols, “nudged meteorology … using reanalysis data”. The methods section would be easier to follow by clarifying this earlier in the methods section, perhaps by stating earlier in the methods section how this model is applied (e.g. in line 41) before the description of the model continues.

Conclusion: I find this paper could elaborate on potential other implications of this difference in emissions distribution. For instance, would we also expect a difference in air quality impacts due to these differences?