Evaluating the Effectiveness of Natural Carbon Sinks Through a Temperature-Dependent Model

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This study addresses the significant scientific question of whether the effectiveness of natural carbon sinks is declining. By incorporating temperature-dependent natural emissions into an extended carbon sink model, it offers a novel perspective on explaining the observed increase in atmospheric CO₂ concentrations. The research demonstrates strengths in data analysis, model innovation, and literature review. Overall, the manuscript is well-structured and methodologically rigorous, with significant implications for climate studies and carbon cycle understanding. However, certain aspects, such as theoretical assumptions, data validation, and broader implications, require further refinement. Specific recommendations are provided below:

1. The current title, "Are the Natural Carbon Sinks failing?" could lead to misinterpretation of the study's scope. It is recommended to adopt a more precise title, such as "Evaluating the Effectiveness of Natural Carbon Sinks through a Temperature-Dependent Model."
2. Although the abstract adequately summarizes the study, it does not sufficiently highlight the scientific significance of the extended carbon sink model or its potential applications in climate modeling and policy-making. Greater emphasis on these aspects is advised.
3. The introduction places substantial focus on media reports, which diminishes its scientific rigor. Expanding the discussion on the role of natural carbon sink dynamics in climate modeling and policy would be beneficial. Additionally, providing a more balanced summary of the findings and limitations of Reference [3] would enhance objectivity.
4. The labeling of the y-axis in Figure 1 should be clarified to better distinguish between raw and deseasonalized data. For Figure 4, key trends in the effectiveness of carbon sinks should be annotated, particularly those during 2023–2024, to highlight significant changes and their potential implications.
5. The methodology for deseasonalizing CO₂ and temperature data is not described in sufficient detail, particularly regarding the specific algorithms used to remove seasonal variability. A detailed explanation in the main text or supplementary material is recommended.
6. The theoretical basis for introducing a time lag in the extended carbon sink model is reasonable but requires further validation. Sensitivity analyses exploring the impact of the lag parameter on model outcomes or references to supporting empirical studies would strengthen the robustness of this assumption.
7. The use of nuclear bomb test data as a validation tool for the extended model is an innovative approach but requires additional discussion. The limitations of using such data as a proxy for natural carbon sink dynamics should be explicitly addressed to enhance the validity of the findings.
8. While the inclusion of temperature-dependent natural emissions represents a significant innovation, the potential for non-linear effects and feedback mechanisms under scenarios of greater temperature variability remains underexplored. Expanding this discussion would provide a more comprehensive understanding of the model's implications.
9. Key references, such as [4] and [6], are cited but their relevance to the manuscript’s core conclusions is not fully articulated. Strengthening the connections between these studies and the current research is recommended. Additionally, incorporating more recent empirical studies on oceanic CO₂ outgassing and soil respiration would enrich the manuscript’s literature base.
10. The conclusion effectively summarizes the findings but does not sufficiently address their broader implications for climate modeling and policy-making. Including a discussion on how the study's results could inform climate predictions or mitigation strategies would enhance its practical relevance.
11. Certain expressions, such as "wild speculations," undermine the manuscript's scientific tone. Replacing such language with neutral, academic phrasing is recommended to maintain professionalism and credibility.
12. The technical complexity of the extended model may present challenges for non-expert readers. Simplified explanations or additional visual aids could improve accessibility and ensure broader comprehension of the study's core concepts.

Author Response

Thank you for your constructive and valuable comments. 

Due to your and the other reviewers comments, the paper has been thoroughly restructured. 

My response to your numbered points

1. You are right. While the emotional impulse to write the paper was doubtless linked to the alarm spread by several media last autumn, this should be detached from the actual scientific content.  I accept gratefully your suggestion for a new title.
2. Following your advice, I mentioned the temperature->CO2 concentration causality and its implications in the abstract. I make the suggestion to include this functionality in climate models.  I also extended the conclusions aiming at climate models and policies. 
3.  I took out the "Guardian" reference, only mentioning the public debate briefly, added several papers (2 from Nature) that deal with the recent findings on carbon sinks 
4. Mentioned the color in the caption of  figure 1 (now figure 4). Former figure 4 is not figure 7 - added explanatory text to caption. 
5. Both used methods of deseasonalization are now described in the appendix.  The actual model equations  in the pager have implicit deseasonalization, this is derived and explained in the main text of the paper. 
6. The time lag is now not only evaluated with the R^2 diagram, but also with a table that contains the parameter values for the relevant time shifts. In the end, the key criterion must be the goodness of fit (80% of the total variance are explained with the model). Another "validation" is the optical quality of the end result.    
7. In my understanding, C14 is not a proxy. My claim is, that the physical (downwelling) absorption processes (ocean and photosynthesis) are treating C14 exactly the same as C12. The difference is that C14 is a one-way process. Upwelling emissions (outgasing) contain no excess C14. Therefore, the C14 atmospheric decay measures absorptions. This makes it an interesting tool for validation, when the extended sink model ends up with exactly the same absorption parameter as the C14 atmospheric decay (after Suess effect correction).  It has turned out that this subject is hard to understand. Therefore I made the diagram which is now Figure 10. 
8. I added some statements at the end of section 2.1.2 explaining that local processes with global averaged data must be linear or linearized. Nonlinear aspects are out of scope. 
9. Added a more thorough discussion of Spencer's and Engelbeen's work in Material and Methods. Added more references to recent studies. 
10. Enhanced the conclusions by adding some climate policy relevant suggestions.
11. Changed the wording towards objective language
12. Figure 10 was made for this purpose. Besides this, the subject is inherently difficult, and certain knowhow about e.g. regression models is a requirement.

Reviewer 2 Report

Comments and Suggestions for Authors

General comments:

The manuscript makes the case that decreasing global sink estimates for anthropogenic CO₂ are not the reason for increasing CO₂ levels in the atmosphere, rather increasing natural CO₂ sources are behind the CO₂increase. The author also states that recent literature on this subject is focusing on the sink rather than the source side of the story. In general, I don’t think that the scientific community as a whole is following this notion, many observers in the earth system science community see the source versus sink discussion on a net basis and, in fact, many studies have shown that increasing “natural” release of CO₂ have taken place in the recent past. This is particularly true for the high northern latitudes, where a combination of wildfires and permafrost degradation have turned the tundra from a net sink to a net source for GHGs. So, in this respect, the author overstates that there is a misconception, I think the community is aware of the increasing natural CO₂ source problem.

The manuscript, however, is a valid contribution as it assigns numbers to the different processes and highlights the challenges with synchronizing field observations and numerical models. More communication needs to happen there.

Specific comments

It seems that some of the text ended up in the wrong place as far as the break down between introduction, methods and results are concerned.

Line 267:  Both CO₂ and bomb ¹⁴CO₂ enter the ocean as CO₂ from the atmosphere, but their Atmospheric concentrations vary independently and their equilibration time is different Regular CO₂ has an air-sea equilibration time of about 1 year and has been increasing exponentially in the atmosphere for more than 150 years. Bomb ¹⁴C has an air-sea equilibration time on the order of 10 years and was added to the atmosphere over two decades starting ~1945, increased rapidly until 1965, and has been declining exponentially since then. This might effect the use of bomb ¹⁴C in the context of the manuscript. See Key (2001) for reference.

Line 296: This sentence should be in the methods section

Lines 332 and 343: be consistent with the numbers, 6.8 or 6.9 GtC?

Line 348: If this is the main motivation we have a problem. I don’t thing this is real, the community at large does not see it this way!

Line 365: Roy Spencer

Author Response

General comments: The manuscript makes the case that decreasing global sink estimates for anthropogenic CO₂ are not the reason for increasing CO₂ levels in the atmosphere, rather increasing natural CO₂ sources are behind the CO₂increase

Answer: It is a fact that there are papers that make the case that natural sinks are failing, either land sinks or ocean sinks. It is these papers that I am contradicting. In the paper I am not talking about CO2 sources in general (that's outside the scope of the paper), but about a specific property of natural emission sources, their temperature dependency.  Besides measuring this effect the extended model (Table 2),   the paper makes the hopefully convincing point, that in fact the 2 parameters CO2-concentration and (sea surface) temperature besides measured anthropogenic emissions are sufficient to explain 80% of the variance of CO2 concentration growth. Compared to those major causes, everything else is of comparatively small influence, apparently due to the remarkable property of nature to keep the system in balance and, of course, the immense size of the atmospheric (+oceanic + biological) systems. 

 

Specific comments

It seems that some of the text ended up in the wrong place as far as the break down between introduction, methods and results are concerned.

Answer: Motivated by this comment, the document has been thoroughly re-strutured and parts are re-written. There are too many changes to describe each one individually.    The sections Introduction, Materials and Methods, Results and Discussion are now as they should be. The only calculations in Materials and Methods are those concerning C14 and the Suess effect (Section 2.2.1). That's because the C14 results are used as a validation of the main model of the paper and is therefore considered to be "Material" 

 

Line 267:  Both CO₂ and bomb ¹⁴CO₂ enter the ocean as CO₂ from the atmosphere, but their Atmospheric concentrations vary independently and their equilibration time is different Regular CO₂ has an air-sea equilibration time of about 1 year and has been increasing exponentially in the atmosphere for more than 150 years. Bomb ¹⁴C has an air-sea equilibration time on the order of 10 years and was added to the atmosphere over two decades starting ~1945, increased rapidly until 1965, and has been declining exponentially since then. This might effect the use of bomb ¹⁴C in the context of the manuscript. See Key (2001) for reference.

Answer: There seems to be a misunderstanding. In Line 267 I just state a straightforward physical fact, that photosynthesis and ocean absorption cannot differentiate between the isotope C14 and the isotope C12 (There seems to be a small deviation for C13 in photosynthesis).  Therefore, the absorption rate must necessarily be identical for both isotopes.  Obviously, C12 and C14 in general have different histories.  As you mentioned, from a historic point of view there has been a C14 emission pulse in the 1950s, that suddenly stopped in 1963 with the test ban treaty. Therefore, we have the unique situation of a single decaying delta pulse after 1963.  While C12 gets absorbed and outgassed from the ocean, there is no outgassing of C14..  Therefore the decay of the atmospheric C14 Concentration after 1963, that I describe in 2.2.1, measures the "pure" Absorption without outgassing emissions. Additionally there is the thinning effect of C14 due to anthropogenic emissions from fossil fuils, which contain no C14 at all. This is the socalled Suess-effect, which has corrected. I explain this in section 2.2.1. 

 

Line 296: This sentence should be in the methods section   done

Lines 332 and 343: be consistent with the numbers, 6.8 or 6.9 GtC? done

Line 348: If this is the main motivation we have a problem. I don’t thing this is real, the community at large does not see it this way!   Sentence is removed 

Line 365: Roy Spencer  corrected

Reviewer 3 Report

Comments and Suggestions for Authors

See attached file.

Comments for author File: Comments.pdf

Author Response

General comments: The article critically approaches a series of methods and mathematical models related to the carbon / carbon dioxide cycle in nature. An important part of the article is that it brings together the necessary information. The less good part is that there is no systematization of the information. The specific parts of an article are not clearly delimited. If it is a review? Then this should be stated from the beginning. The diagrams should be clearly associated with the sources, otherwise it gives the impression that they are obtained with the help of databases.

Answer: Following your advice, the manuscript has been thoroughly restructured and partly rewritten in order to delimit the specific parts of the article.   All diagrams, that are considered to be results of the paper, are now in the Results section. The only calculations in Materials and Methods are those on C14 (Section 2.2.1), because the C14 data are considered as material, but cannot be directly used "as is". 

Abstracts:  No comments. It explains quite well which is the purpose of this present article.

Introduction: The problem is that the Guardian is not a scientific paper. But another citations are quite well argued.   Removed Guardian reference,

Maybe “biological world” can be replaced by “biota”.   done 

Materials and Methods & Results and Discussions: This chapter and the next one are not clearly separated. It is also not clear which graphs are the result of the author's studies. There are many decadal diagrams that come from other works. A systematization with a clear specification of the provenance of the data on which these graphs were made is mandatory. Equations and mathematical models should be clearly delimited from graphs. Comments should be made only in the Results and Discussion section. Conclusions: The conclusions are general ideas, which are not associated with the equations and mathematical models used. The conclusions in the graphical parts are not commented on either. Thus, it is difficult to understand why all the previous information was needed.

These sections have been restructured and partly rewritten.  The graphs from downloaded data have the reference to their source in the caption.  When data are processed, their source is referred to in the text. Most graphs have been moved to the Results and Discussion section.  Equations are in Materials and Methods.  Only in 2.2.1 (C14) some calculations are done and graphs are displayed, because the C14 Data serve as validation of my model and are considered as material.  But it requires some processing before it is useful. 

The conclusions refer to various aspects of the model and the computed results. 

 

 

 

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

accept

Reviewer 3 Report

Comments and Suggestions for Authors

See attached file.

Comments for author File: Comments.docx