PONE-D-23-32708Carbon System State Determines Warming Potential of EmissionsPLOS ONE

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Reviewer #1: Yes

Reviewer #2: Partly

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2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: I Don't Know

Reviewer #2: Yes

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Reviewer #1: Yes

Reviewer #2: No

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5. Review Comments to the Author

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Reviewer #1: Thanks for writing this paper. It is a relevant and overlooked issue, and something that has worried me for some time. It is nice to see an analysis of this. As you show, the path dependence is much stronger for the carbon response, compared to the temperature response (TCRE). The way you frame the paper, it feels like there is a big path dependence on TCRE. There is a counteracting path dependency in the forcing to temperature response as well. This is something that could be explored or brought out a little more.

Overall, I don’t have any major problems with the paper, but I have lots of clarifications, minor comments, and requests.

1. Abstract: “total anthropogenic carbon emissions”. To make sure there is no ambiguity, probably makes sense to put cumulative in there. “total anthropogenic cumulative carbon emissions”, or similar

2. The text in Supporting Information Text S1 is good, and if possible, I would try and integrate it into the main text. You mention many of these issues in the main text, so it is a matter of just layering in any differences.

3. Table 1: Is 5 years averaging long enough to get the interannual variability out?

4. Figure 1c, shows the [CO2] versus cumulative CO2. It would be worth putting in the equivalent figure for T versus cumulative CO2. I see you have this in Figure S2. Figure S2 shows a much stronger linearity, and it is this linearity that is referred to in the TCRE relationship. There is still a state dependence, but it is not as big. However, there are really two state dependencies here, which are playing out in opposite directions: the [CO2] versus cumulative CO2 and the temperature versus [CO2]. Presumably, the state dependence of temperature versus [CO2] is completely opposite to the equivalent for [CO2]? Is it possible to somehow shows this. I do not know the best way to plot this, but at each year is it possible to calculate something like T/[CO2] versus cumulative CO2? Not sure it makes sense. There is the style of figure in Matthews et al 2009, but that is with time and not cumulative emissions. Perhaps this would mean an extra figure with subpanels in the paper, but it would be really nice to have a set of figures showing the CO2 sensitivity, the temperature sensitivity, and how they offset each other to produce Figure S2. I think this would greatly strengthen the paper. Though, it shows that the state dependence is less at the aggregate, important at the more disaggregated level.

5. I did not get how you calculated the TCRE, you write “TCRE is estimated using the conventionally used linear regression method7”. But they seem to take the last 20 years, and I don’t see they did a regression. A regression of the entire timeseries will give a different number, compared to the last 5-20 years. I think it is more conventional to get the TCRE using a 1% CO2 experiment, and the temperature when emissions go to zero. Just clarify how you did this, and note if it is important, as the slope may not be the same as last 5-20 years average temperature divided by cumulative emissions. Does the regression goes through zero?

6. Figure 1a. It was a little bit ambiguous on the 100 and 200 years. You either distribute the emissions over 100 years, or over 200 years? Basically, the 100 years version does everything at twice the rate? Perhaps try and make that more clear.

7. Figure 2. The vegetation dynamics. These simulations just use preindustrial vegetation? I assume there is no LUC? So the difference in carbon in forests, etc, is purely due to feedbacks on the land system? If you have age dynamics, it will be important how the system is initialised? Also, you do this >60N, to take out LUC, is that a robust choice?

8. Figure 3, panel a and b. Again, showing the temperature equivalent of this would be useful. This would be ZEC? Basically, the ZEC is path dependent (already clear in ZECMIP). But the temperature response must somehow offset the variation in the carbon response? (also in ZECMIP)

9. Figure 3, panel c and d. I did not get how you constructed the figure. You did an additional simulation? How could you constrain CMIP6 to 2.2ppm/yr, etc? Or was this different SSPs? If so, write it down.

10. Sink efficiency. Line 196. Is this defined the same as Raupach et al, equation 14. Can you write this as an equation to make it less ambiguous?

11. The slow and fast time scales, are based on the Figures 3a and b? And this is after zero emissions? After zero, the models generally only run to 100 years? How then can you have confidence in fitting time scales of 750 years or so? There is very little knowledge to fit that time scale. I imagine that you could force 200 years as the long time scale, and maybe get a similar fit?

12. Lines 15-17: Though, this is really referring to Figure S2, where the path dependence seems much weaker?

13. Line 57: “Business as usual scenario for 2C” What is that? Perhaps just say a 2C scenario.

14. Lines 70-71: If forcing is something like log(CO2), then as emissions and concentrations decline, the forcing will also decline. So that declining concentration phase, must be offset by the forcing change? Basically, Figure 1c and Figure S2 have to balance somehow.

15. Lines 77+. You say you get the TCRE by a regression. So would you put a linear fit through the green circles in Figure S2c? Or do you get the last five years temperature? If anything, Figure S2 shows how really important it becomes on how you estimate TCRE. I would think this is a point that really needs to be highlighted.

16. Line 128+. You are showing that the ZEC is path dependent (yes, you see this in MacDougall et al)

17. Lines 143+: I have really low confidence in these long time scales here, given that they are so far outside the simulation domain.

18. Lines 222-224: Well, I think Figure S2 sort of shows the opposite. Despite the different pathways, they all generally show that T is proportional to cumulative CO2, the difference is sort of in the second digit?

19. Lines 225: “It is not the amount emitted…” Well, CO2 = cumulative emissions - cumulative land sink - cumulative ocean sink. I guess I could plot temperature as a function of either of the three cumulative terms (Figure 4, Figure S7). It would be worth doing Figure S7 for each of the cumulative terms, which one is more robust description of T? I am not sure why in Figure 4 you make the choices on which you will use for the x-axis. Presumably, Figure 4c and d would be similar with cumulative emissions?

20. Figure 1a. Can you give equations for the curves, in the methods or something.

21. One implication of your paper is that emission driven runs are important. This preprint just out is relevant in this context. https://egusphere.copernicus.org/preprints/2023/egusphere-2023-2127/

22. Particularly for your ZEC analysis and fast and slow time scales, Jenkins et al is a relevant paper. It is probably worth having a look and making the relevant connections if they exist. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2022GL101047

23. This paper may also be relevant for the ZEC: https://iopscience.iop.org/article/10.1088/1748-9326/acab1a

24. Overall, you are showing that TCRE and ZEC are path dependent, and that is an important point to make, and not sufficiently understood.

Reviewer #2: See attached pdf for all comments. The rest of this comment is filler to reach the minimum character count.

Filler Filler Filler Filler Filler Filler Filler Filler Filler Filler Filler Filler Filler

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Reviewer #1: No

Reviewer #2: No

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PONE-D-23-32708R1Carbon System State Determines Warming Potential of Emissions

PLOS ONE

Dear Dr. Winkler,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

In addition to R1's comments, please also consider and respond to the following comments sent to me directly from R2 during the 2nd round of review.

For what it's worth, I still think the authors need to do a better job of explaining how their conclusion fits with current knowledge. I don't think a paper based on a single model that says, "Throw away everything you thought you knew about TCRE being pathway independent", without explaining exactly how they come to such a strong conclusion, is justifiable. If they can say something like, "The TCRE appears pathway independent because all our simulations basically use the same sort of carbon cycle state", (or some other explanation of why it has appeared that the TCRE is pathway-independent for so long and no-one has noticed that it isn't), that would be great. Alternately, they might just say, "All the effects we're looking at here lead to deviations of ~10% from a pathway-independent TCRE", in which case their work is completely in line with previous work e.g. https://esd.copernicus.org/preprints/esd-2023-7/ and https://iopscience.iop.org/article/10.1088/1748-9326/ab83af 

Please submit your revised manuscript by Jun 14 2024 11:59PM. If you will need more time than this to complete your revisions, please reply to this message or contact the journal office at plosone@plos.org. When you're ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

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We look forward to receiving your revised manuscript.

Kind regards,

Yangyang Xu

Academic Editor

PLOS ONE

Journal Requirements:

Please review your reference list to ensure that it is complete and correct. If you have cited papers that have been retracted, please include the rationale for doing so in the manuscript text, or remove these references and replace them with relevant current references. Any changes to the reference list should be mentioned in the rebuttal letter that accompanies your revised manuscript. If you need to cite a retracted article, indicate the article’s retracted status in the References list and also include a citation and full reference for the retraction notice.

Additional Editor Comments:

In addition to Reviewer #1' comments, please also consider the following comments sent to me directly from Reviewer #2:

For what it's worth, I still think the authors need to do a better job of explaining how their conclusion fits with current knowledge. I don't think a paper based on a single model that says, "Throw away everything you thought you knew about TCRE being pathway independent", without explaining exactly how they come to such a strong conclusion, is justifiable. If they can say something like, "The TCRE appears pathway independent because all our simulations basically use the same sort of carbon cycle state", (or some other explanation of why it has appeared that the TCRE is pathway-independent for so long and no-one has noticed that it isn't), that would be great. Alternately, they might just say, "All the effects we're looking at here lead to deviations of ~10% from a pathway-independent TCRE", in which case their work is completely in line with previous work e.g. https://esd.copernicus.org/preprints/esd-2023-7/ and https://iopscience.iop.org/article/10.1088/1748-9326/ab83af

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #1: All comments have been addressed

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2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

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3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

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4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

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5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

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6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: I thank the authors for a very comprehensive response to comments! I think the article is ready to see the light of day, and I am happy to accept.

I do have a few minor comments, the authors may wish to take up. I do not need to review again.

• Fig 1d. I would prefer if this showed the delta T versus cumulative CO2 figure here, like 1c, but for T. The current 1d can go in the SI. You discuss Figure Sup 2 so much, and it would just be very useful to have the full TCRE figure here

• Fig 2b. I would write “Cumulative airborne fraction” to avoid confusion

• Fig 2. I see how you tried to use the arrows to show the panel connections, though, it took a while. You could write in each panel things like TCRE = b*c, etc. Or you could, maybe, do a 3x3 panel: TCRE, TCR, CAF in the first, TCR (yes repeat), d*e, etc. So if you can figure something out, as it was useful. Maybe even you could put the equations in the bottom right, in the white space? (Just ideas, you decide what works)

• Fig Sup S2. It could be useful, even if a little messy, to add the regression lines for each curve used to calculate the TCRE. I still feel uneasy with these regression fits, when the data may not be quite linear. It would be good to visualise if it was a problem or not…

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Reviewer #1: No

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Carbon System State Determines Warming Potential of Emissions

PONE-D-23-32708R2

Dear Dr. Winkler,

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

Within one week, you’ll receive an e-mail detailing the required amendments. When these have been addressed, you’ll receive a formal acceptance letter and your manuscript will be scheduled for publication.

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If your institution or institutions have a press office, please notify them about your upcoming paper to help maximize its impact. If they’ll be preparing press materials, please inform our press team as soon as possible -- no later than 48 hours after receiving the formal acceptance. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information, please contact onepress@plos.org.

Kind regards,

Yangyang Xu

Academic Editor

PLOS ONE

Additional Editor Comments (optional):

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #1: All comments have been addressed

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2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

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3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

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4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

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5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

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6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: (No Response)

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Reviewer #1: No

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