PCLM-D-22-00121

Dust as a solar shield

PLOS Climate

Dear Dr. Bromley,

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.

Please submit your revised manuscript by Nov 12 2022 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:

• 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,

Juan A. Añel

Academic Editor

PLOS Climate

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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 If you have cited papers that have been retracted, please include the rationale for doing so in the manuscript text, or remove these references and replace them with relevant current references.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 If you need to cite a retracted article, indicate the article’s retracted status in the References list and also include a citation and full reference for the retraction notice.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):

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

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: Yes

Reviewer #2: Yes

Reviewer #3: Yes

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

Reviewer #1: N/A

Reviewer #2: Yes

Reviewer #3: I don't know

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3. Have the authors made all data underlying the findings in their manuscript fully available (please refer to the Data Availability Statement at the start of the manuscript PDF file)?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception. 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

Reviewer #2: Yes

Reviewer #3: Yes

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

PLOS Climate 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

Reviewer #2: Yes

Reviewer #3: 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: Review of ‘Dust as a solar shield’ PCLM-D-22-00121

This paper analyzes a form of ‘astro-engineering’ as a possible means to reduce anthropogenic global warming. Specifically, it studies the composition, attenuation efficiency, and persistence of dust screens located at the L1 unstable equilibrium point between the Sun and the Earth for reducing the solar insolation. Although several other papers have considered this and related dust screen ideas, this is the most thorough analysis to date, especially as regards the scattering/attenuation efficiency of dust grains of different composition and shape. How to launch such grains from the Moon, which is much more cost effective than launching from Earth, is also briefly considered. Thus, the paper presents a number of new results in this area, and it is generally well-organized and well-written. Subject to a few minor corrections I think it is worthy and ready for publication. My detailed suggestions for revision follow.

1. I would suggest not using terms like ‘last-resort’ or ‘desperate’ to describe the space dust scenarios. I think terms like ‘not well-considered’ or ‘novel’ might be better. The more negative terms apply more to ideas like evacuating a remnant of humanity to the Moon or Mars before cataclysmic Earth warming. The dust-screen idea could actually be a cost effective short term mitigation, depending on how much damage it could prevent. It also has the advantage of cleaning itself up when not fed new dust, unlike many Earth-bound mitigations.

2. You might want to include the following reference along with references 11-13, since some of the latter are based on it, and it shows that dust screens have been considered for some time. 2007JBIS...60...82S

The Feasibility of Shading the Greenhouse with Dust Clouds at the Stable Lunar Lagrange Points

3. Bottom of page 9, ‘… physics of §.’ What section does this refer to?

4. At line 289: change ‘en’ to ‘an’

5. At the end of the ‘Particle Scattering’ section (or later) it might be worth summarizing that over a wide range of grain types and shapes the attenuation doesn’t vary by much more of an order of magnitude. This is probably far less than the cost to process the more exotic particle types on the Earth or Moon. Also the particle size spectrum is a far more important variable.

6. Plans for lunar mining do not anticipate launching processed material with rockets. Rather, mass drivers are envisioned to launch thousands of tons of mined material off the lunar surface. An array of thousands of such mass drivers could probably produce and maintain the L1 dust screen. To emphasize that dust screens are not wholly beyond the range of our technology, a little online research of this technology, and a mention in the last section might be worthwhile. There is also the possibility of transporting a small asteroid to L1 and using its material. Either pathway might be feasible in a few decades, and desirable if the global temperature is increasing above several degrees C.

Reviewer #2: The paper presents the theoretical background and factors influencing the performance of the L1 dust cloud in a very thorough way and expands on the understanding of the factors influencing how the L1 dust cloud will work.

Although the analysis of the behaviour of the L1 dust cloud released at that point is good, I find that a better focus for the paper would have been on orbits originating from the Lunar surface as this would have extended the L1 dust cloud science more than focussing on the release around the L1 point itself.

I would strongly recommend that there is a more direct comparison of the mass required to achieve the 6 attenuation days for the different materials and launch positions i.e. a table listing the mass for each as well as the masses from the reference papers for other space-based solar radiation methods.

A few other comments;

- particularly for the lunar dust, is there a known size distribution of particles to naturally occurring on the lunar surface? If so, a comparison between this and the optimal particle size would be useful as a guage of the engineering effort needed to mill the particles to the optimal size

- Line 193: a comparison is made to the mass of a sunshield with 10nm thickness. Most space-based geoengineering papers with solid sunshields rely on advanced manufacturing technologies to achieve reasonable masses. As another comparison, current thermal control foils used on spacecraft have an aluminium thickness of 100nm and then only when deposited on a supporting material. These foils are between 9-11g/m²

- Lines 194-196 do not read very well as if part of the sentence is missing, ". reports substantially less mass (about 7 million kilograms) is required for a thin film of silicon nitride with an engineered, non-uniform thickness [9]."

- Fig. 3 and 4: Here it would be useful to add in the first line of the caption that the simulations include the non-gravitational forces

Reviewer #3: General Impression:

The authors have used interesting approaches to discuss the potential of a dust cloud as an alternative space-based climate mitigation action, with the initial analyses offering the basic conclusions for dust cloud design based on lunar materials. In this regard, further analyses are needed specifically from a logistical perspective to discuss the orbital mechanical challenges in more detail. Specific review points are provided below:

Feedback:

+ At first glance, the estimated radii for the dust cloud seem comparable to other sunshade concepts.

+ Inclusion of first non-gravitational forces in addition to gravitational influences makes sense

+ Comparison of grain types over the attenuation-time makes sense

+ Clear conclusions in the discussion

- Minor: Currently still contains a few grammar and spelling mistakes

- Minor: Figures are not well resolved, especially Figure 1

- Major: Comparative calculations of the energy balance for lunar dust cloud should not appear in the conclusion, but in more detail in the main part.

Suggestions:

-> The input data for the generation of the charts should also be made available so that the results can be better understood

-> Supplementary table overview of mass requirements for various materials for Figure 1 useful, similar to that already listed in Table 1

-> List validation data for numerical experiments in the appendix.

-> A more detailed description of the lunar jet streams for the dust clouds would be useful, the corresponding calculation should be included

-> Continuous resupply requirement for dust cloud due to drift should be finally mentioned, since this is ultimately a non-controllable system.

-> In connection with the lack of controllability, the difficulty of a targeted distribution of the dust cloud to optimize the climatic effects should be mentioned, since simulations with controllable sunshade concepts have already shown that seasonal and regional variations require a non-uniform shading of Earth

Grammer / Spelling:

- In general “Earth” and “Sun” are capitalized without using the article, please double-check the overall paper

- Formatting in justified text

- Line 2-3: include some key references for critical tipping points

- Line 6: it is important to consider

- Line 9: sunlight reaching Earth

- Line 20: between Earth and Sun

- Line 41: on achievable orbits

- Equation (4): Was is meant by the symbol in front of (K)? function of (K)?

- Line 107: between Earth and Sun

- Line 108: solar radiance at Earth

- Line 120: solar photons

- Line 177: unit missing

- Line 194: Capitalization

- Line 289: at a

- Line 300: symbol?

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For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: Yes: Curtis Struck

Reviewer #2: No

Reviewer #3: No

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[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.

PCLM-D-22-00121R1

Dust as a solar shield

PLOS Climate

Dear Dr. Bromley,

Thank you for submitting your manuscript to PLOS Climate. After careful consideration, we feel that it has merit but it needs some minor changes before publication. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

Specifically, I would like that you address the following issues:

- In lines 3-5, you state that "changes...have tipped the balance toward the increased entrapment of solar energy and general rise in the global mean temperature". This language is strong, and I would say that excessive. The papers you cite deal with tipping points to the climate system, but it is not true that we can consider that our climate has reached any of the ones usually considered. Current levels of atmospheric CO2 have not gone to such points; others, well-known as the 1,5 degrees Celsius limit, the collapse of oceanic circulations, etc., have not happened. Therefore, I recommend you here state that climate change is acknowledged to be a severe problem. The previous sentence about being an existential threat is already clear enough.

- Next, in lines 5-6, you justify the need for research based on the lack of substantial action to avoid CO2 emissions. This is not a correct reasoning. First, even though greenhouse emissions and atmospheric concentrations continue raising, it is not true that substantial action has not been taken to avoid them. The word "substantial" is debatable, hard to quantify and lacks evidence supporting it. Moreover, you do not need this sentence to support the need for research, as preparedness or the aim to explore should be enough. Therefore, please remove the sentence.

- In the conclusions (and someway in the Introduction), you use the argument of non-nocive practices for the Earth's atmosphere to support non-Earth-based climate intervention techniques. Also, you mention that drift makes the dust in the L1 point need to be replenished. One issue of concern could be that if dust is deployed in the L1 point and eventually needs to be removed, it is not possible. In the meantime, additional intervention on the Earth's atmosphere could be possible, making, in this case, more desirable to intervene on Earth's atmosphere. I think it would be good if you address in a sentence or two if eventually all the dust particles are expected to be removed by drifting and the problem that could represent having to remove them.

Please submit your revised manuscript by Dec 31 2022 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:

• 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,

Juan A. Añel

Academic Editor

PLOS Climate

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):

[Note: HTML markup is below. Please do not edit.]

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

Reviewer #3: All comments have been addressed

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2. 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: (No Response)

Reviewer #3: Yes

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

Reviewer #1: (No Response)

Reviewer #3: I don't know

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4. Have the authors made all data underlying the findings in their manuscript fully available (please refer to the Data Availability Statement at the start of the manuscript PDF file)?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception. 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: (No Response)

Reviewer #3: Yes

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

PLOS Climate 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: (No Response)

Reviewer #3: Yes

**********

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)

Reviewer #3: Thanks for incorporating the comments. Great work!

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7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

Do you want your identity to be public for this peer review? If you choose “no”, your identity will remain anonymous but your review may still be made public.

For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: Yes: Curtis Struck

Reviewer #3: No

**********

[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.

Dust as a solar shield

PCLM-D-22-00121R2

Dear Professor Bromley,

We are pleased to inform you that your manuscript 'Dust as a solar shield' has been provisionally accepted for publication in PLOS Climate.

Before your manuscript can be formally accepted you will need to complete some formatting changes, which you will receive in a follow-up email from a member of our team. 

Please note that your manuscript will not be scheduled for publication until you have made the required changes, so a swift response is appreciated.

IMPORTANT: The editorial review process is now complete. PLOS will only permit corrections to spelling, formatting or significant scientific errors from this point onwards. Requests for major changes, or any which affect the scientific understanding of your work, will cause delays to the publication date of your manuscript.

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 climate@plos.org.

Thank you again for supporting Open Access publishing; we are looking forward to publishing your work in PLOS Climate.

Best regards,

Juan A. Añel

Academic Editor

PLOS Climate

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