Review Reports - Net Isotopic Signature of Atmospheric CO<sub>2</sub> Sources and Sinks: No Change since the Little Ice Age

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This study aims to test the hypothesis that observed changes in the isotopic signature of carbon dioxide (CO2) in the atmosphere are largely natural, given that the biosphere has become more productive and expanded, leading to a natural amplification of the carbon cycle. The study uses modern instrumental and proxy data to investigate this hypothesis and answer research questions posed. Additionally, the study presents a theoretical framework that is applied to data in a diagnostic and modelling manner.

Comments:

The presentation of results and conclusions in this study needs clarification. Demetris Koutsoyiannis suggests in his study that human CO2 emissions did not significantly impact the isotopic signature of carbon in the atmosphere. He also suggests that natural changes in the carbon cycle, such as increased biosphere productivity, may be primary factors in the observed changes. These conclusions do not necessarily support the argument against the need to reduce CO2 emissions, as they only suggest a minimal human impact on the isotopic composition of atmospheric carbon. It is important to note that this interpretation is not a direct conclusion of the study. Additionally, it is crucial to understand that these results do not contradict the fact that CO2 emissions contribute to global warming through the greenhouse effect. In this context, the author should clarify these points to avoid misinterpretations. Additionally, it is worth noting that while CO2 emissions may have a small impact on the isotopic signature of carbon, they are still associated with other forms of pollution, such as emissions of fine particles and nitrogen oxides, which can have harmful effects on human health and the environment. This could help avoid misinterpreting the study as a denial of the need to reduce emissions of CO2 and other pollutants. It may be beneficial to avoid using jargon in this study to make the content more accessible to a wider audience and to facilitate understanding of the concepts presented. Using simpler language and providing explanations for technical terms can aid non-expert readers in comprehending the study's findings and implications. These explanations should be clear and concise, integrated seamlessly into the text without overloading it. Additionally, a glossary of terms at the end of the study could effectively clarify any unfamiliar terms without disrupting the main flow of the text. For instance, technical terms like "isotopic signature" or "Keeling plot"could be clarified by providing definitions or using simpler language to enhance comprehensibility.

Comments on the Quality of English Language

It may be beneficial to avoid using jargon in this study to make the content more accessible to a wider audience and to facilitate understanding of the concepts presented. Using simpler language and providing explanations for technical terms can aid non-expert readers in comprehending the study's findings and implications. These explanations should be clear and concise, integrated seamlessly into the text without overloading it.

Author Response

Please see attached document, "Response to review comments"

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

In the present work, the author aimed to answer two research questions, by examining isotopic data in four most important observation sites. It was shown that the standard metric δ13C is consistent with an input isotopic signature that is stable in the entire period of observations, so it is not affected by the increasing human CO2 emissions.

This study possesses certain academic value and can be considered for publication, however there are some issues that must be addressed. Specifically, the paper is too long and can be shortened because the first part contains unnecessary details that can be removed, according with the following suggestions.

The introduction is too heavy and contains too many numerical details. For example, the rows 68-71 contain numerical values already provided in Figure 1, so these rows can be removed or modified.

Please avoid reporting sentences from other works. Try to explain rows 81-84 with your own words. A similar issue at lines 95-113.

The procedure provided from line 166 to line 196 is a rather standard and elementary mathematical demonstration, so it cannot be included in a scientific paper (at most, it could be included in Appendix). I suggest condensing in 3-4 relevant formulae.

In a similar manner, Figure 4 shows standard plots of elementary functions, and does not provide relevant information to the reader. Figure 4 can be removed.

Specific comments

L. 93: there are plenty of scientific publications reporting the evolution of carbon emissions. I recommend using data taken from peer-reviewed journals, rather than from “Our world in data”.

L. 247: change “in our case one year”, with “(one year, in the present case)”.

L 265: “By merely plotting the data we can obtain some useful insights”. This sentence does not provide any information and can be removed.

L. 265: You probably mean “Figure 6”.

L. 274: Why do you say that the behaviors of all series are similar? Similar in what?

L 284: How can you say that no sign of the influence of fossil is apparent? What could “a sign of the influence of fossil fuel” be?

L 301, 315: Add more information in the caption of Figures 8 and 9.

Comments on the Quality of English Language

Minor editing of English language required

Author Response

Please see attached document, "Response to review comments"

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This study discussed an interesting scientific issue, i.e., a unidirectional, potentially causal link between T as the cause and [CO₂] as the effect, and concluded the major role of the biosphere in the carbon cycle and a non-discernible signature of humans. In my opinion, the discussions on different opinions are necessary for the progress of science, although I am inclined to the conventional view that the correlation between [CO₂] and atmospheric T seems not to be unidirectional, i.e., the increase in [CO₂] is also likely one of factors causing the increase in atmospheric T, and vice versa. Overall, I recommend this manuscript to be published after further improving its persuasion, e.g., the final isotopic signature of atmospheric CO₂ results from the mixing processes of diverse souses? there is a key process that the emission of fossil fuel is one of major sources for natural respiration (including human, soil, flora and fauna, etc.)?

Comments on the Quality of English Language

none

Author Response

Please see attached document, "Response to review comments"

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The author has responded to the reviewers' comments.

From the responses, it is clear that the author has left room for discussion, which is a common challenge in the scientific world. While some of the author's views may be controversial, it may be more appropriate to express them in a book.

However, the author has the freedom to support his ideas in this paper, and the discussion of the results cannot be disputed solely through a review, so the author assumes his results and statements. While I may not entirely agree with the style of some of the author's statements, I consider this paper to be challenging and acceptable for publication.