Please refer to the attached file.

Review on ‘Investigating Carbon and Nitrogen Conservation in Reported CMIP6 Earth System Model Data’

First of all, apologies for the delay in the review.

The paper investigates the carbon and nitrogen mass balance in simulations from the Coupled Model Intercomparison Project Phase 6 (CMIP6) with the published flux and pool variables. The authors find a significant mass imbalance in the reported data for both carbon and nitrogen in various experiments. They discuss the possible causes of this imbalance, arriving at the conclusion that it is likely due to missing fluxes and inconsistency between the reported data and the variable definitions. This leads to the conclusion that for the upcoming CMIP requests, more consideration should be given to consistency of variable requests between MIPs and incorporating mass conservation into the data validation process.

The analysis in the paper is well done and highly relevant considering the upcoming CMIP7 data request. However, the writing of the paper could be improved. There is an excessive use of both lists and parenthesis in the main text, hindering the flow of the text. At times the authors seem to become defensive and scared about not wanting to offend other people through misconceptions. This is most prevalent in the disclaimer found in the last paragraph of the introduction. A scientific paper should just focus on facts without unnecessary disclaimers, defenses or attacks. The authors should furthermore revise the paper regarding sub- and superscripts in formulas and variable names. Especially the long variable names would benefit from proper subscripting for more readability. It would also be nice to add a table for the variables with the variable name, the long name or description, as well as its source, such as which CMIP data request it comes from. This table could then be introduced in section 2 and later referenced without needing to introduce variables and their definitions over time. The footnotes found in the paper should be turned into references to be cited.

With some adjustments in the writing, this paper will become a valuable resource for the CMIP carbon community.

Minor comments:

* denotes where my comments overlap with those of referee #1

• line 23ff: 3 uses of ‘the reported data’ in one sentences seems excessive, consider rewording
• *line 30: ‘Given that CMIP6 data is no longer being reported, ...’ - reported can be misleading to readers and unclear. Do you mean that errata are no longer reported? No new data is published? Errata published but error not fixed? Please rephrase.
• line 34: ‘The Coupled Model Intercomparison Project (Phase 6 - the latest version, CMIP6)’ - mentioning Phase 6 in the parenthesis while not referring to it outside
• *line 36 ‘climate(Meehl …’ - missing space before parenthesis
• *line 39f: ‘(such as (Nicholls et al., 2022; Turnock et al., 2020; Stouffer et al.,
• 2017))’ - wrong citation style while within the brackets
• *line 47 (and others): ‘CO2’ - please subscript the 2 for CO₂. This occurs throughout the paper.
• *line 74: no description given for sections 2 and 3.
• line 80: ‘relevant stakeholders, including ...’ stakeholders seems an add choice of word in this context.
• *line 83ff: This paragraph reads overly defensive, while nothing written so far would suggest that you believe the mass balance to stem from the models themselves. The list of possible factors is also repeated in the analysis in further chapters and does not belong to an introduction. I would suggest to remove this paragraph as it just duplicates what is found in later chapters.
• *line 103ff: Instead of listing the models separately, you could refer to Figure A1.
• line 146: ‘ - one being greater than the other or vice versa’ - I doubt it is necessary to explain what an over- or underestimation means in this context, this part of the sentence can be removed.
• line 154: ‘nbp (a CMIP5 variable)’ - I believe nbp is also in the CMIP6 data request, so do you mean ‘introduced in CMIP5’?
• line 199ff: As long as you are talking about the same figure, it is unnecessary to end every sentence with (Fig. 3). This should be done when starting to talk about another figure or referring to another figure later on. This also occurs later on for other figures.
• line 235ff: ‘fCnetLitterSoil ∗=’ etc - please check all of your sub- and superscripting. This asterisk should be on the variable with a space before the equals sign, same with other variables. ‘∗=’ has another meaning in programming languages and would be confusing.
• *line 291: ‘Notebally’ - Notably
• line 386: ‘(time steps also vary within ESMs’ sub-modules)’ - unnecessary information at this point, can be removed
• line 533ff: ‘ ESGF may develop necessary data quality control tools considering mass conservation for data publication.’ - may need to develop
• *line 537ff: ‘At first, this may seem a difficult task to meet. However, we would ask a different question. What is the value of sharing data which does not pass the basic test of conserving mass?’ - Doesn’t seem to fit the writing style of this paper. I would suggest to remove these sentences and add the next sentence (‘Given … ecosystem’) on the previous paragraph, as it already contains everything you want to convey.
• line 545: ‘by the fLuc and fFire‘ - either drop the ‘the’ or include it for both
• line 591: ‘, and ‘nitrogen uptake’ (fNup) and ‘net mineralization’’ - remove the first ‘and’
• line 599: ‘gigatons of carbon/nitrogen’ - ‘carbon and nitrogen’
• line 605: ‘This would take effort, but ..’ - Remove
• line 621ff: ‘Text A1’ etc – Regular sections are also not named ‘Text 1’, so remove the ‘Text’ from the appendix section headings.
• *Figures 5, 6, A8, A9: As you don’t have any nitrogen data from the CanESM models, you should remove them from the figures and the legends, and mention this at the beginning of section 5.