Restoring Pre-Industrial CO₂ Levels While Achieving Sustainable Development Goals

Round 1

Reviewer 1 Report

The topic is interesting. The following comments can help the authors to impvoe the manuscript for a complete review: The abstract should be re-written. The following information should be inserted: problem, novelty, contributions, method and analysis, exact findings. Other infomration should be used and mentioned in the introudction. Line 78: Methods: Bracketing the choice between bioelectricity and biofuel: This section needs to clarify: what is the data set? is that a review paper using databases or using secondary data or it is based on original database? insert data profile, reliablity and validity tests/analysis. Line 206: all "Calculations, results, and discussion for each component" should be separated into differnt sections. Insert all authors affiliations.

Author Response

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Reviewer 2 Report

This is a robust analysis of the impact of adopting two main energy production technologies to meet CO2 emission goals. The work is interesting but there are questions;

1. How will authors convince the entire humanity to consume seafood?
2. What proportion of global food demand would be replaced by seafood and what are the carbon costs of this?
3. Access to coastal areas are limited, how would landlocked areas benefit from these pathways and what are the carbon costs of the envisaged demographic changes? What are the costs (economic and environmental) of transportation and distribution of seafood and biofuels?
4. The benefits of HTL over all other biomass/waste processing technologies have not been explained
5. Of the 60% biocarbon in the biocrude, what is the conversion to actual biofuel? Some data in the manuscript (20 million in Abstract and 22 million in Line 592) suggest that about 18.25% biocrude yield would be obtained from HTL of solid wastes; what would be the eventual biofuel yield?
6. It is unclear what the sources of the plastic wastes are. Would plastics from petrochemicals be considered as 'biocrude'? 
7. Considering that Allam Cycle uses gas, the liquid and solid feedstock would require gasification (maybe methanation) and separation. It is not clear if these additional costs have been considered?

Other comments

Lines 509-510: 1 billion tonne/year of seafood would give 342 g/person/day for 8 billion people

Line 10 (Abstract): Please consider to change 'The authors consider..' to 'This paper considers..' In any case, this line reads like part of the aim of the research.

Line 59: The starting line for the Introduction reads like part of an Abstract.

Line 273: It is not right to say that HTC produces 'ash'. Maybe call it solid residues

Lines 512-514: Different font sizes

Lines 627-635: Different font sizes

Figure 2 Caption: Consider to place 'during combustion' in brackets i.e. 'during combustion'.

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report

Reviewer comment

The article discusses the theoretical assumptions of increasing the energy use of biomass in order to reduce CO₂ production. The article also describes the methods of CO₂ sequestration.

The article provides an extensive and detailed calculation tool for estimating the costs, materials and energies for CO₂ removal from the atmosphere. In my opinion, the tool is very complex and not very intuitive for use by administrative staff but also by national experts.

No need to be corrected:

I recommend including in the calculations also the increased production of CO₂ by the mechanisms and technologies that are used for soil preparation, cultivation, collection and processing of biomass. An enormous increase in the energy use of biomass can also enormously increase the environmental impacts of these technologies and processes.

Research is focused only on the theoretical potential of biomass to reduce CO₂ production or CO₂ sequestration. There is a lot of space in the article devoted to the use of biomass for electricity production. Allam Cycle is also preferred. The use of renewable energy sources must be a complex process of a balanced ratio of locally available energy sources.

It is necessary to correct:

I cannot agree with the statement in line 75 - "Zero carbon electricity such as wind and solar stabilize, but do not reduce CO₂ levels".

Please correct or prove with published research results. Research confirms that increasing the share of wind and photovoltaic energy reduces CO₂ production.

Comments for author File: Comments.docx

Author Response

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Reviewer 4 Report

The paper by Capron et al. presents the author's vision on addressing climate change by the wide employment of three specific technologies: Allam Cycle power plants for energy production and CCS, hydrothermal liquefaction for biofuels production and total ecosystem aquaculture for the food and biomass production. Although the intention of this work is very appealing, I believe its scientific presentation should be significantly improved. The major concern is that this work is very different from the mainstream understanding of the potential solutions for climate change. Authors should make a strong argument why their approach is better than the solutions considered by mainstream science (and IPCC which relies on science). The technical clarity of the paper should be also significantly improved, a lot of information without a clear structure makes this paper difficult to read and validate. I do not feel the current paper condition is suitable for publication, it should be considerably improved. Please see some specific comments below.

 COMMENTS:

1. The title of the paper is misleading. The paper is discussing the scenarios for the net-zero carbon emissions with the Allam cycle, total aquaculture, and hydrothermal liquefaction, but there is not much discussion about the SDGs. In line 79 authors say “The approach of this paper is to solve the climate crisis coincident with achieving SDGs” but the spectrum of SDGs is much broader and cannot be solved simply by addressing one issue (even major on – climate change).
2. The affiliation section is not filled at all.
3. In graphical abstract black arrow is not explained, it is a carbon-positive process?
4. In the introduction, line 70 authors state that none of the influential literature reviews included three technologies focused on in this paper (total aquaculture, Allam Cycle, and hydrothermal liquefaction). Indeed, having an interest in NET I rarely hear about these technologies as meaningfully useful for CO₂ negative emissions. It would be great if the authors explain why it is so. Why high-level reviews ignore these technologies? What is the main reason they are not considered as necessary for wide employments? Are there any hidden disadvantages or barriers to wide employment?
5. The introduction should include a sufficiently detailed description of every technology under consideration. I believe most readers are not familiar with them.
6. Figure 1 should be improved: resolution should be sufficiently higher and all unnecessary information that does not serve to this paper should be removed. Please adapt (modify) the figures from the IPCC report with the given requirements and not just copy-paste.
7. I wonder how the overlay lines were put in Fig. 1. It is not appropriate to make overlay in graphical software (e.g. Paint) or Powerpoint. Please explain.
8. In Table 1, Per-capita electricity in P_fuel scenario is only 4 MWh/year is much lower than currently developed countries as given. According to the SDG7 by 2030 the access to electricity should be universal. Considering that currently, ~ 800 million people lack electricity, 4 MWh/year per capita seems to be the very low estimate for 2070.
9. In Table 1, Per-capita electricity in P_electric scenario is only 7 MWh/year seems to be fine from the SDG7 point of view. But such difference in scenarios raises the question about which one is actually satisfying the SDG7 targets for 2030? The targets of SDG are set for 2030 (and not 2070) and paper title points specifically to SDGs.
10. The sentence in lines 131-132 says “Developed countries might skip the first step while importing high-value seafood from developing countries” seems counterintuitive and counterproductive. First, most developing countries do not have access to the sea, and consequently high-value seafood production capability. Second, how some developed countries could possibly agree to follow this model? For instance, how would Japan – seafood consumer and seafood producer import it from elsewhere? This is just an example – I believe the manuscript should provide explanations fitting the reality of the global situation instead of vague and arbitrary statements.
11. The predicted total biomass needed for three technologies is given in Table 1 as 43 and 30 Gt/year seems quite far from practical, considering the estimated total biomass is ~ 105 Gt/year (https://science.sciencemag.org/content/281/5374/237). With such an estimate one can indeed assume that warming goals can be easily met. How would the authors explain this high amount of biomass can be provided every year?
12. Several times in the paper (e.g. line 140-141) authors mention that "each nation can plan for the path within the brackets" of the suggested scenarios. I keep thinking about what would be the author's suggestion for countries that cannot do any of suggested scenarios e.g. 10 countries located in the Sahara desert?
13. Diagram in Fig.5 of Supporting information is not legible – please replace it with an accurate image.
14. The cost of avoided CO2 is mentioned as $26/ton. How does this figure was obtained?
15. The approach used in this paper is different from what usually distinguishes the scientific paper from the “personal opinion” or “newspaper article”. For example, authors give many recommendations, like line 605-606: “Quickly build seafood-production infrastructure in developing countries. Convert solid waste to biocrude oil in developed and developing countries.”, there are many such recommendations in the paper. I believe the scientific publication should answer the question "how to do" rather than "what to do". Just saying “Stop using fossil fuels”, “Stop carbon emissions” does not belong to the scientific publication. We have to be very careful in statements and justify why one or another pathway is better and how much it is feasible in any given moment.
16. The said above is well described by Fig. 4. I have only one question to each bar in the diagram:
    -- “Pause all under-construction and planned power plants” à how it can be achieved? I believe if what authors suggest would be feasible, technically developed – a realistic economic alternative to plants under construction it would be done already.
    -- “Increase seafood production” à how? The demand for seafood if driven by elementary market drivers. How and who will have the power to decide the amount of production?
    -- “Build waste-to-biofuel systems” à It is easier said than done! Some companies are developing this technology for decades already and there is no operating power plants at the moment. How this can be achieved at any substantial scale from 2020 to 2040.

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

I am completely happy with the revised manuscript and the responses on the authors to my comments.

Author Response

Thank you. Your comments make for a vastly improved paper.

Author Response File: Author Response.docx

Reviewer 4 Report

Authors have improved the manuscript. It is still quite complex for the average reader, but I have no objections and leave the decision to the scientific editor.

Please make the figures of high resolution in the final version.

Author Response

Thank you. Your comments have vastly improved the paper.

Author Response File: Author Response.docx