PCLM-D-21-00017Ending animal agriculture would stabilize greenhouse gas levels for 30 years and offset 70 percent of CO2 emissions this century

PLOS Climate

Dear Dr. Eisen,

Thank you for submitting your manuscript to PLOS Climate. After careful consideration, we feel that it has merit but does not fully meet PLOS Climate’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.

This paper has a clear potential to become a strong and impactful paper. The main issues that need to be addressed include a more rigorous discussion in relation to sensitivity tests for key assumptions that are fundamental to drive the results of the paper and the addition of corroboration and nuance in relation to the way that the results are presented. The latter includes discussing some of the trade-offs, barriers, and real plausibility in relation to eliminating animal agriculture.

Please submit your revised manuscript by Oct 17 2021 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 climate@plos.org. When you're ready to submit your revision, log on to https://www.editorialmanager.com/pclm/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

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Guidelines for resubmitting your figure files are available below the reviewer comments at the end of this letter.

We look forward to receiving your revised manuscript.

Kind regards,

Ana Maria Loboguerrero

Academic Editor

PLOS Climate

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1. Does this manuscript meet PLOS Climate’s publication criteria? Is the manuscript technically sound, and do the data support the conclusions? The manuscript must describe methodologically and ethically rigorous research with conclusions that are appropriately drawn based on the data presented.

Reviewer #1: Partly

Reviewer #2: Partly

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Reviewer #1: No

Reviewer #2: N/A

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Reviewer #1: Yes

Reviewer #2: Yes

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Reviewer #2: Yes

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5. 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: The manuscript is technically sound, and the data supports the conclusions. However, as I have detailed in my reviewer report, I think the authors need to more rigorously discuss/sensitivity test several of the assumptions in their methodology, and this is why I believe the manuscript currently only partly meets publication criteria. For this reason, and several other more minor comments detailed in my report where I think data used needs to be better justified or changed (e.g their analogy to driving, aquaculture), the statistical analysis is also partly satisfied at present. I have tried to be as helpful as possible in my comments regarding what the authors need to do to meet these criteria, and I am confident that once this is complete, they will have a strong and impactful paper.

Reviewer #2: The topic of the paper, how to tackle the climate emergency and how reduced or eliminated animal agriculture can contribute, is highly relevant.

The singular focus of the paper on animal agriculture as THE climate change solution should be nuanced, as many sectors and processes contribute to GHG emissions. It is not possible to tackle climate change by focusing on one single solution (or even one single sector). In addition, some of the trade-offs and barriers/challenges in relation to eliminating animal agriculture need to be brought more clearly to the readers' attention.

Most importantly, however, the results as presented (e.g. offset of 70% possible by eliminating animal agriculture) that could justify this strong focus, need to be corroborated and presented with much more nuance. The methodology followed in the manuscript combines many different calculations based on many assumptions and with large separate uncertainties associated with it.

More detailed comments (about methodology as well as the other parts of the manuscript) are attached.

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Reviewer #1: Yes: Caspar Donnison

Reviewer #2: No

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Attachments

Attachment

Submitted filename: Reviewer_report.docx

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Submitted filename: comments_20210901.docx

PCLM-D-21-00017R1

Rapid global phaseout of animal agriculture has the potential to stabilize greenhouse gas levels for 30 years and offset 68 percent of CO2 emissions

PLOS Climate

Dear Dr. Eisen,

Thank you again for your submission to PLOS Climate.

As relayed to you by email by Ana Maria, the reviewers identified some issues with the R1 version of your manuscript that need to be addressed before your paper can be editorially accepted. I understand that you have supplied a revised version of the manuscript incorporating responses to these points by email yesterday, but am issuing a Minor Revision to enable you to submit these new files into Editorial Manager. Once we have received the revised manuscript in the submission system, we will expedite a final editorial decision.

Thank you very much for your patience and understanding in this matter. We look forward to receiving the revised manuscript as soon as you are able.

All best wishes,

Jamie Males

Executive Editor

PLOS Climate

on behalf of

Ana Maria Loboguerrero

Section Editor

PLOS Climate

***

[Standard instructions follow]

Please submit your revised manuscript by Dec 14 2021 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 climate@plos.org. When you're ready to submit your revision, log on to https://www.editorialmanager.com/pclm/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

Please include the following items when submitting your revised manuscript:

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Guidelines for resubmitting your figure files are available below the reviewer comments at the end of this letter.

We look forward to receiving your revised manuscript.

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 (if provided):

The manuscript can be accepted for publication with PLOS Climate, upon addressing a small number of minor comments.

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Reviewers' comments:

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Attachments

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Submitted filename: Brown_Eisen_PLOS_Reviews_reviewer2 response.docx

Attachment

Submitted filename: Eisen_Brown_comments_Oct_21.docx

PCLM-D-21-00017R2

Rapid global phaseout of animal agriculture has the potential to stabilize greenhouse gas levels for 30 years and offset 68 percent of CO2 emissions this century

PLOS Climate

Dear Dr. Eisen,

Thank you again for submitting your manuscript to PLOS Climate and for your patience in waiting for this decision. After careful consideration, we feel that some minor adjustments are required to ensure the manuscript fully meets PLOS Climate’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the following points:

1. Could you please provide responses to the second set of revisions of the second reviewer? In your last document there were no responses provided for the second reviewer.

2. We note that your manuscript includes two references to non-peer-reviewed white papers (Bieker, 2021 and Heinrich-Boell-Stiftung and the Earth Europe, 2014). We would ask that you replace these references with references to appropriate peer-reviewed articles. If no suitable peer-reviewed references are available to supported the associated statements, please remove these statements from the manuscript.

Please submit your revised manuscript by 23 December, 2021 . If you will need more time than this to complete your revisions, please reply to this message or contact the journal office at climate@plos.org. When you're ready to submit your revision, log on to https://www.editorialmanager.com/pclm/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

Please include the following items when submitting your revised manuscript:

• A rebuttal letter that responds to each point raised by the editor and reviewer(s). You should upload this letter as a separate file labeled 'Response to Reviewers'.
• A marked-up copy of your manuscript that highlights changes made to the original version. You should upload this as a separate file labeled 'Revised Manuscript with Track Changes'.
• An unmarked version of your revised paper without tracked changes. You should upload this as a separate file labeled 'Manuscript'.

Guidelines for resubmitting your figure files are available below the reviewer comments at the end of this letter.

We look forward to receiving your revised manuscript.

Kind regards,

Jamie Males

Executive Editor

PLOS Climate

on behalf of

Ana Maria Loboguerrero

Academic Editor

PLOS Climate

Journal Requirements:

***

Note from journal office-- The reviewers’ comments on the first revision (R1) were transmitted to the authors outside of the Editorial Manager submission system. For completeness of the peer review record for this submission, we have appended to this letter copies of the reviewers' reports and Academic Editor's decision letter for the R1 version of the manuscript.

***

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.

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files.]

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email PLOS at figures@plos.org. Please note that Supporting Information files do not need this step.

***

Note from journal office-- The reviewers’ comments on the first revision (R1) were transmitted to the authors outside of the Editorial Manager submission system. For completeness of the peer review record for this submission, we have appended to this letter copies of the reviewers' reports and Academic Editor's decision letter for the R1 version of the manuscript.

R1 Reviewer comments:

Reviewer 1:

I commend the authors on the efforts taken to fully address comments from me and the other reviewer. It is good to see them thoroughly engaged with, including the addition of sensitivity testing in Figure 7, and extensively in the supplementary figures, which I think makes the study much more robust and emphasizes that the conclusion of the study withstands under sensitivity testing of the assumptions. As it appears in the new results, the assumption of what plant proteins replace animal products does not have a particularly sizeable impact on the results, which I think is a useful result.

Reviewing again the results, I particularly like the contribution this study makes in Figure 8, alongside the SSPs, where there has been a lack of radical scenarios concerning meat and dairy consumption.

I believe that this study now meets the necessary conditions for publication with PLOS Climate, upon addressing a small number of comments on the new version of the manuscript below.

1. Not sure I agree with the removal of “this century” from the title as it details what that 68% refers to.
2. I think you interchange between “phase out” and “phaseout” – consistency (also consistency of “plant-based” and “plant based”, e.g on PDF page 106).
3. PDF page 89: change to “often focus on only one factor”.
4. A thought on the Hayek et al. (2021) estimates: we should expect a rise in non-livestock ruminants under ecosystem and biodiversity recovery (e.g bison), though that would of course be hard to model and likely to have only a modest reduction in the potential GHG and warming savings of a plant-based diet transition. Had you also thought about this? It could be worth noting in the text.
5. Good to see the strong emphasis that this debate typically overlooks ongoing emissions, not the reversible warming impact (biomass recovery).
6. Regarding the use of Twine (2021), on my previous recommendation, I since spoke to the author of that paper and it appears he made an error, understandably given the inconsistent use of GWP weightings used by the FAO from which data was used. See Richard Twine’s blog post for more info. I asked him how best to reflect this debate in a paper and he agreed stating the following could be one way forward:

"estimated at 14.5% by the FAO, although this is based on outdated data and likely now represents an underestimate (Twine, 2021)"

Page 94 of PDF: “decay in the atmosphere on relevant timescales” – doesn’t seem worded correctly. They decay on a different timescale to CO₂ is the relevant point.

1. Page 94 of PDF: “How… would alter 2019 net anthropogenic emissions” – I don’t think the language quite works here. The impact of phaseout isn’t altering emissions in 2019, but you are comparing the impact to emissions in 2019.
2. Page 97 of PDF: “Coming from our not including”, suggest change to ‘our exclusion of’
3. Page 101 of PDF: I am trying to understand the final sentence here, and find myself confused. PHASE-POD has the same effect, through the end of the century, as a 68% cut in BAU CO2 emissions. This is comparable to eliminating all CO2 by 2050. It seems to me that CO₂ elimination by 2050 is a bigger impact than a 68% cut over the century, because after 2050 that CO₂ level is zero out to 2100 in the former case. I think this confusion is arising because of comparing emission totals over different time horizons? Also, eliminating all CO₂ by 2050 is essentially meeting the Paris Agreement but you state that your results find the phaseout of animal products alone would not be sufficient. Is this because you are only looking at eliminating CO2, not CH4 and N20. It is likely my misunderstanding, but please consider how you can improve clarity to the reader here.
4. Page 100 of PDF: sub-title refers to offsetting 65% but elsewhere this is written as 68%.
5. Page 102 of PDF: use of “percent” (“62 percent”), consistency needed, with ‘%’ used elsewhere. This also appears elsewhere (e.g “38 percent”, also on PDF page 102).
6. Page 103 of PDF: “2021 model year” – change to ‘2021 model’? ‘year’ seems unnecessary.
7. Much better discussion of the carbon recovery of land.
8. Page 105 of PDF: Change “from Hayek” to “from Hayek et al. (2021)”.
9. Page 106 of PDF: “emissions projected emissions” – typo here?
10. 0.58 to 1.47 – this is per year right? And 1.47 was the initial year of disturbance?
11. Page 109 of PDF: is that “400 million” in the Springmann et al (2018) reference, or does it need referencing separately? It’s a good and supportive statistic, as long as it is referenced.
12. Page 109 of PDF: Good to emphasize the conservative nature of your estimate regarding projected increase in meat consumption. If you wanted to cite it, Searchinger et al (2019) projects a 70% increase in animal-based foods between 2010 and 2050.
13. Page 111 of PDF: “they anticipate”: change to ‘the IPCC’. Later in this paragraph you state “Thus, all currently viable solutions” after having (very reasonably) questioned viability of BECCS at scale. Perhaps rephrase to “Thus, all potential solutions” to avoid contradiction.
14. Page 122 of PDF: “Mt” used and then “MT” later in sentence.

Reviewer 2:

• The topic of the paper, how to tackle the climate emergency and how reduced or eliminated animal agriculture can contribute, is highly relevant.
• The singular focus of the paper on animal agriculture as THE climate change solution should be nuanced, as many sectors and processes contribute to GHG emissions. It is not possible to tackle climate change by focusing on one single solution (or even one single sector). In addition, some of the trade-offs and barriers/challenges in relation to eliminating animal agriculture need to be brought more clearly to the readers' attention.

Although the focus of this paper is on animal agriculture, it was not our intention to suggest that it is the only solution. Indeed in both the text and analyses we emphasized that even if the full extent of the potential we highlight here were realized, it would still not be enough to solve the climate crisis. We have made this point clearer at multiple points in the manuscript.

*** This is highly appreciated

We also explore the challenges to eliminating animal agriculture in more detail.

*** Thanks

• Most importantly, however, the results as presented (e.g. offset of 70% possible by eliminating animal agriculture) that could justify this strong focus, need to be corroborated and presented with much more nuance. The methodology followed in the manuscript combines many different calculations based on many assumptions and with large separate uncertainties associated with it.

As discussed above, we have added a section exploring the sensitivity of our results to assumptions made in our analyses so that readers can get a better sense of the degree of uncertainty in our calculations and its sources.

*** I believe the section exploring the sensitivities to the assumptions adds value to the paper

• More detailed comments (about methodology as well as the other parts of the manuscript) are attached.

We have responded below to the reviewer’s comments and modified the manuscript where appropriate.

Introduction

• General:
  • The introduction is a bit too "one-sided".  It is important to also mention some of the positive aspects of animal production (e.g. contribution to economies, livelihoods, nutrition, soil fertility, …)  -- as these would need to be taken into account as "opportunity cost" if livestock production was to stop. This is very much in line with the concept of one of the key references cited in the introduction of the paper, Hayek et al., who also explicitly refer to the "carbon opportunity cost" of using the lands for extensive food production .

We have addressed this briefly in the introduction. 

*** Thanks

• A wide range of sectors and processes contribute to global emissions and there is thus not one single solution (or even sector to improve) in order to tackle climate change.  This point needs to be clearly stated.

In the introduction we reemphasize that eliminating animal agriculture alone would not solve the climate crisis.

*** Thanks

• line 4: I do not find these figures in Hayek et al. and Strassburg et al.  Can you specify page/line numbers? 

Both Hayek and Strassburg estimate that historical land conversation is responsible for the release of ~800 Gt of CO2. Direct CO2 emissions from fossil fuels are estimated at 1,650 Gt of CO2 (Friedlingstein et al., 2020). Hence ~⅓ of emissions have been due to land conversion.

*** I do not seem to find the estimate of historical land conversion in Hayek et al., but maybe I am overseeing it somewhere.  Line 4 in the manuscript refers specifically to “The historical reduction in terrestrial biomass as native ecosystems were transformed to support grazing livestock and the cultivation of feed and forage crops” while the Strassburg et al. paper explicitly considers all lands converted from natural ecosystems, including those that were converted to croplands (which in my reading of their paper would also include food crops). So, I’d suggest to edit the sentence accordingly

• p.5, last two lines: The figures mentioned in Hayek et al. and Strassburg et al. are lower.

Hayek, 3rd paragraph: Here we quantify the total carbon opportunity cost of animal agricultural production to be 152.5 (94.2–207.1) gigatons of carbon (GtC) in living plant biomass across all continents and biomes (Fig. 1 and Supplementary Table 3). We approximated the potential for CO2 removal in soil and litter as an additional 63GtC (Supplementary Table 4).

152.5 + 63 = 215.5 Gt C, which is 788 Gt CO₂.

***  Thanks for pointing this out.  I also appreciate the addition of “In the order of”…

Strassburg, see Figure 2b, which reports a maximum capacity of ~900 Gt CO₂ recovery. We do not use the Strassburg data directly, rather it serves as an independent confirmation of the value from Hayek.

*** As already noted above, Strassburg et al. explicitly consider all lands converted from natural ecosystems, including those that were converted to croplands. So, if you want to keep the last part of the sentence (i.e. “currentLY devoted to livestock production”), I’d suggest you do not cite Strassburg et al. here.

• p.5, line 4: Can you clarify how the figure of 1,400 Gt was arrived at?

800 Gt from carbon recovery + 80 years * 7.5 Gt / year  = 1,400 Gt.

***  Thanks for the clarification.

• p.5, line 6: The statement "warming is cumulative" needs a bit of nuancing, as it is only applicable to long-lived climate pollutions (i.e. not to methane).

We have rephrased for clarity. Our intention was to point out that because warming is cumulative, the timing of an increase or decrease in RF due to changes in emissions matters. This is true irrespective of the gas whose levels are being adjusted. For long-lived gases like CO2 the cumulative effect manifests with pulses of positive/negative emissions. For short-lived gases like CH4 it requires sustained changes to see the effect. But in either case the timing of when changes take place matters.

• p.5, line 5-8: Some more detailed explanation underpinning the statement that "this understates impact of dietary change on global warming"  would be helpful.

We have rephrased for clarity.

***  Thanks for rephrasing and for adding a few words explicitly explaining the short-lived nature of methane.

***  A few suggestions:

• “…which…decay in the atmosphere on relevant timescales, …”: the term “relevant timescales” is not very clear.  Did you mean “relative short timescales”?  If so, I’d have my reservations stating “relative short timescales” in relation to N₂O and would consider removing it.  But maybe you mean ““…decay in the atmosphere within the timescale (or time window) covered by our analysis”? Please, edit accordingly.
• I feel that the last part of the last sentence “having a greater cumulative effect on warming” is confusing in relation to short-lived GHGs; as it is the long-lived ones that have a cumulative effect on warming. I would thus suggest you edit the sentence as follows: “Of critical importance, many of the beneficial effects on greenhouse gas levels of eliminating livestock would accrue rapidly, via biomass recovery and decay of short-lived atmospheric methane CH₄. and therefore Their cooling influence would be felt for an extended period of time, having a greater cumulative effect on warming.”
• p.6, last line second last paragraph: "assuming that all other sources of emissions remain constant at 2019 levels" - this seems a fundamentally wrong assumption, as the reduced production of food/nutrition/manure/income as a result of eliminating animal agriculture would need to be compensated for.

We explicitly account for replacement diets in our model, and thus do not hold agricultural emissions constant. This sentence should have read “assuming that all non-agricultural emissions sources remain constant at 2019 levels” - we have made this correction.

***  Thanks

Results

***  Additional comments:

• “The dietary scenarios include the immediate replacement of all animal agriculture with a plant-only diet (IMM-POD), a more realistic gradual transition, over a period of 15 years” – consider editing as follows: “The dietary scenarios include the immediate replacement of all animal agriculture with a plant-only diet (IMM-POD), and a more realistic gradual transition, over a period of 15 years”
• “We updated estimates of global emissions from animal agriculture by scaling country-, species- and product-specific emission intensities… with country-specific data on primary production of livestock products”: I do not think the word “scaling” is the most appropriate here; maybe just use “multiplying”?

• General: This section also contains quite a bit of methodology. The manuscript would benefit from having all the details provided here integrated in the methodology section.

We have moved some additional methodological details to the methods where appropriate.

• second paragraph: Please indicate the data source of the total human emissions.

Done.

***  Thanks

• p.7 biomass recovery: This explanation is missing in the methods section.  Using this figure is also flawed, as Hayek et al calculates the potential sequestration associated with converting land to native/natural state while some of the land will have to be converted to cropland for plant-based food production.

Hayek accounts for land use for a replacement diet in their numbers. This was confirmed with the author.

***  This is indeed the case; thanks for pointing out and confirming.

***  I would still like to suggest to remove the reference to Strassburg et al. as they do not present results for land currently used in livestock production separately from all agricultural land.

Comment to the new section:

• “…recent modeling work by (Strassburg et al., 2020) that half of the biomass recovery potential of land currently used for animal agriculture could be realized by restoration of 25% of the relevant land”. I’d suggest to edit as follows: “…recent modeling work by (Strassburg et al., 2020) indicates that half of the biomass recovery potential of land currently used for animal agriculture could be realized by restoration of 25% of the relevant land”
   

Methods

• General:
  • It would be helpful to start the methods section with an overview, ideally with schematic.

We drafted schematics to try to capture what we did clearly but were unable to come up with something that we felt enriched the paper, so hope that the changes to the manuscript make it clearer.

***  Accepted

• There is a mismatch in the current methodology between steps that are "rough estimates" with steps that are worked out through complicated formulas that seem to imply high precision.  Kindly address.

We use exclusively data from published sources that we would classify not as rough estimates, but as best current measures of the extent of global animal agriculture, emissions due to animal and non-animal agriculture, and land use. We also explicitly stayed away from any complex formula in our analyses and believe the only thing that might be characterized as such is the RF calculation, which is the standard form used in the literature.

***  I acknowledge that the term “rough estimates” might not have been appropriate. The data used does, however, come with large uncertainties. Which might lead to a large variation in terms of e.g. resulting RF values associated with the min-max range of these estimates.  The new section on sensitivity deals with this in an acceptable way.

• The methodology combines many different steps, all associated with large uncertainties.  I believe the authors would need to address a few related points: (i) each of the separate uncertainties need to be clearly stated (methodology) and quantified (results); (ii) an estimate of the combined uncertainty needs to be included; (iii) this needs to be extensively discussed in the discussion session.

We have added a new section that explicitly addresses major areas of uncertainty and their impact on the results.

***  The inclusion of the sensitivity analysis associated with the assumptions (replacement of animal-sourced food & duration of biomass recovery) is highly appreciated. 

• p.18, last paragraph: Can you clarify to what the emission data was scaled?

Done.

***  OK

• p.20, emissions from agriculture: Please, state explicitly which sector(s)' data you used.

Clarified.

• p.19, diet-linked emissions:
  • Please indicate which of the soybean scenarios from Behnke et al. were used (and the actual figure) for replacing the emissions associated with livestock production with emission associated with soy bean production when modeling reductions in livestock consumption? As the treatments in Behnke et al. are somehow "best practices" with e.g. low fertiliser rates and very localised, I doubt it is realistic to use this as a global GHGe estimate. Please, use a more globally representative data source and confirm that the number used is a realistic global GHGe estimate, by comparing with a number of other sources (across different agro-ecologies, systems, geographies). 
  • Can you clarify if a similar replacement (animal product replaced by soy bean) for the land use estimates is also carried out? If not, I believe this is an adjustment that should be made, thereby also keeping in mind that in large areas used for animal feed production (e.g. the arid rangelands), plant-based protein production would require larger areas of land than animal-based protein production.

In response to your comments and those of Reviewer 1 we have switched from using the soy replacement diet, which we viewed as a limiting case, with emissions data from non-animal agriculture from FAOSTAT. This has a nominal impact on the results. In the sensitivity section we also include data  from Xu et al. 2021  for global plant-based diets based on a more comprehensive analysis. This reduced the projected positive impact by around 5%.

***  I believe this is an improvement.

***  New comment to the “replacement diets” section

• “…scaling non-livestock agricultural emission intensities for unit protein by protein required to match that provided by the livestock being replaced”  This sentence doesn’t flow

• p.20, Emissions projections:
  • BFD: write in full

This was a reference to an analysis not in the current version of the MS and has been deleted.

***  Thanks

•  BAU scenario: fixing emissions at 2019 level is not realistic, as "reductions are likely to be achieved through e.g. increasing agricultural efficiency, reducing food waste, limiting excess consumption, increasing yields, and reducing the emission intensity of livestock production" (as mentioned in the introduction).

We explicitly did not attempt to model any of these factors, which we agree could reduce the impact of animal agriculture in the future. We do not make any claim that emissions from either agriculture or non-agricultural sources will continue at their current rates. Rather we project current rates out to the future as a way to capture the current impact of animal agriculture cognizant of the fact that the benefits of its elimination will accrue over time. We have clarified this motivation in the introduction.

We also note that, while it is likely true that reductions in emissions from animal agriculture can be achieved, current projections are that there will be increases in global consumption of animal products, potentially offsetting increased efficiency. We address both points in the manuscript.

***  Accepted

• Can you provide some more detail about the carbon recovery rate (magnitude and assumptions; 30 vs 50 years)?

The 30 years is based on assumptions from Hayek, via Griscom but, as estimates vary widely, we have now included 50 and 70 years recovery periods in the sensitivity section.

***  The recovery period is now clear.

***  It can be deducted from the sensitivity analysis section (in results) that the magnitude of the carbon recovery rate is extracted from Hayek et al.  It would be good to mention that in the methods section too.

• Estimating global non-anthropomorphic emissions:
  • Aren't these emissions already taken into account somewhere in the FAOSTAT "Environment_Emissions_by_Sector_E_All_Data_(Normalized)"

No. The categories of emissions in this dataset are “Agriculture total”, “Agricultural land use”, “Energy”, “Industrial processes and product use”,  “Waste”, “International Bunkers” and “Other n.e.c.”. The “Other n.e.c.” values for the most recent year are 14 kT CO₂, 48 T CH₄, 565 kT N₂O, which are way too small to account for non-anthropogenic emissions.

***  OK

• Projections of atmospheric gas levels (p.25):
  • What is the data source of the starting levels?

The values in the original paper were from a database we maintain of historical GHG levels from a variety of sources. For clarity and data integrity we have updated this to single-source data from NOAA.

***  Clear, thanks

• Computing emission and land carbon opportunity cost, Factor of 2:
  • as the terrestrial sinks are already included in the calculation of atmospheric C concentration, isn't this double-counting?

It is just reversing the 2 used to go from emissions to atmospheric levels.

Because of terrestrial/oceanic sinks 1 Gt of CO2 emissions only yields an increase of 0.5 Gt atmospheric CO2, thus a decrease of 0.5 Gt of CO2 in the atmosphere is the equivalent of a reduction of 1 Gt of CO2 emissions, hence the factor of 2.

***  Thanks for the clarification

• Computing Carbon Emissions Budgets for RF 2.6 and 1.9:
• Please explain why RF 2.6 and 1.9.

In the IPCC’s Representative Concentration Pathway framework, 2100 RF values of 2.6 and 1.9 are used, respectively, as surrogates for 2.0C and 1.5C warming.

***  This is quite clear. I believe it would be worth explicitly referring to these warming targets in the manuscript.

https://sedac.ciesin.columbia.edu/ddc/ar5_scenario_process/RCPs.html

• "RF calculations used in climate models", which climate models are being referred to?

Updated to be clear that we are specifically referring to MAGICC6.

***  Thanks

• "the RF as calculated above" - which calculation exactly does this refer to (to "the complete RF output of MAGICC6" or to the calculations described in the Radiative Forcing section)?

Clarified in text.

***  Thanks

• aCO2eq:
  • first sentence: How were the CO2 emission equivalents computed?
  • "simulations described above" - please, specify where exactly is "above", i.e. which simulations are referred to?

Clarified in text.

***  Ok

• Product equivalents
  • line 25: "per protein" missing.
  • p.26: Please compare the calculated value of 470kg CO2 eq/kg beef with some values in the literature - e.g. the FAOSTAT data source you used for estimating the overall emissions from agriculture - and explain where the huge difference is coming from.

As noted above in response to a suggestion from Reviewer 1, we updated the calculation as described in the response to use emission reductions through 2100 (it was previously 2050 to highlight the short term impact of eliminating animal agriculture) for consistency with the rest of the manuscript and makes our estimates more directly comparable to those in the literature. This is a more conservative assumption and results in a lower value of 297kg CO2eq/kg beef.

We now compare that directly to the global mean estimate from Poore and Nemecek, and explain that the difference in magnitude comes primarily from our inclusion of carbon fixation on land taken out of agricultural use:

These product-specific aCO2eq’s can be interpreted on a per product unit (Figure 6B) or per protein unit (Figure 6C) as emissions intensities. Eliminating the consumption of a kilogram of beef, for example, is equivalent to an emissions reduction of 297 kg CO2. 38 percent (113 kg aCO2eq) comes from reduced emission, in line with the mean estimate of 99.5 kg CO2eq from a systematic meta analysis of GHG emissions from agricultural products (Poore and Nemecek, 2018), with the remaining 62 percent from biomass recovery.

***  OK; I can see that you have these new numbers in the results section

***  However, in the methods section, it still reads 470kg.

Discussion

• p.15: Apart from calories, protein and fat, it is also worth to say something about micro-nutrients.

All essential micronutrients are readily available at scale from non animal sources. Any reasonably balanced plant based diet can be counted on for everything but Vitamin B12 and sometimes iron. B12 can be produced very inexpensively at scale from microbial sources. Iron requires more attention, but a reasonable plant based diet can cover it.

***  This does not negate the fact that e.g. B12 deficiency is already prevalent in several developing countries where a considerable fraction of the population will not be in a position to source supplements or access a sufficiently diverse and healthy plant-based diet to compensate for the micro-nutrients they now get from low levels of consumption of animal products.  I understand you do not want to discuss this in lots of detail, but would suggest that you explicitly mention nutrition security in your section on the potential economic and social impacts. 

***  May I also suggest that you edit the last part of the last sentence in that section along the following lines “And, while it is expected that …, investment will also be required to prevent local food and nutrition insecurity in regions where wide-scale access to a diverse and healthy plant-based diet is lacking” - “heavily reliant on animal based foods” may be interpreted as having large intakes of animal sourced food.

***  I do appreciate very much that that section is there, by the way.  Global modeling efforts, like yours, have great value in outlining the theoretical potential of drastic interventions such as a rapid global phase-out of animal production, but it is also important to note that practical on-the-ground implementation of such strategies needs contextualization and that trade-offs with social/economic/health impacts do need to be taken into account. 

• treatment of methane: The long-and short-term warming effects of methane and CO2 are very different and there is an ongoing debate as to how to weigh the methane emissions; it would be worth saying something about that in the discussion.

The debate about methane involves, essentially, how to credit effects over different time horizons to emission pulses. By directly modeling methane levels from emissions and decay, and relating them to RF, we avoid this issue.

***  OK

***  New comment:

• Can you provide a reference for “an additional 4635 million km2 - … - would be needed to support the required growth in livestock populations”?

• perspectives: Please include a short discussion on the social and political feasibility of eliminating animal production completely.

We view this paper as being about the climate potential of eliminating animal agriculture. We expressly avoided offering what would essentially be an opinion about feasibility, as that is more a statement about politics and economics than anything else.

***  Accepted

Figure 1: The conversion factors for methane and nitrous oxide used here are not standard.  Please, adjust.

These are GWP₁₀₀ values used by FAO/GLEAM http://www.fao.org/gleam/model-description/en/ which are taken from the IPCC AR5,  Chapter 8, Table 8.7 https://www.ipcc.ch/site/assets/uploads/2018/02/WG1AR5_Chapter08_FINAL.pdf using the values that include climate-carbon feedback.

***  OK

R1 Decision Letter from Academic Editor (Minor Revision):

Dear Michael and Pat, I have sent a response for your article.

Please address the comments in the attached and you will be totally ready.

Best regards,

Ana Maria

Attachments

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Submitted filename: Brown_Eisen_PLOS_Reviews_reviewer2 response.docx

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Submitted filename: R1 review comments - Reviewer 1.docx

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Submitted filename: R1 review comments - Reviewer 2.docx

Rapid global phaseout of animal agriculture has the potential to stabilize greenhouse gas levels for 30 years and offset 68 percent of CO2 emissions this century

PCLM-D-21-00017R3

Dear Dr. Eisen,

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.

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