Introducing a Comprehensive Set of Stratospheric Aerosol Injection Strategies

Zhang, Yan; MacMartin, Douglas G.; Visioni, Daniele; Bednarz, Ewa; Kravitz, Ben

doi:https://doi.org/10.5194/egusphere-2023-117

Preprints

https://doi.org/10.5194/egusphere-2023-117

Preprints

02 Mar 2023

| 02 Mar 2023

Introducing a Comprehensive Set of Stratospheric Aerosol Injection Strategies

Yan Zhang, Douglas G. MacMartin, Daniele Visioni, Ewa Bednarz, and Ben Kravitz

Abstract. Stratospheric aerosol injection (SAI) comes with a wide range of possible design choices, such as the location and timing of the injection. Different injection strategies can yield different climate responses; therefore, making informed future decisions on SAI requires an understanding of the range of possible climate outcomes. Yet to date, there has been no systematic exploration of a comprehensive set of SAI strategies. This limits the ability to determine which effects are robust across different strategies and which depend on specific injection choices, or to determine if there are underlying trade-offs between different climate goals.

This study systematically explores how the choice of SAI strategy affects climate responses. Here, we introduce four hemispherically-symmetric injection strategies, all of which are designed to maintain the same global mean surface temperature: an annual injection at the equator (EQ), an annual injection of equal amounts of SO₂ at 15° N and 15° S (15N+15S), an annual injection of equal amounts of SO₂ at 30° N and 30° S (30N+30S), and a polar injection strategy that injects equal amounts of SO₂ at 60° N and 60° S only during spring in each hemisphere (60N+60S). We compare these four hemispherically-symmetric SAI strategies with a more complex injection strategy that injects different quantities of SO₂ at 30° N, 15° N, 15° S, and 30° S in order to maintain not only the global mean surface temperature but also its large scale horizontal gradients. We find that the choice of SAI strategy notably affects the spatial distribution of aerosol optical depths, injection efficiency, and various surface climate responses. Among other findings, we show that injecting in subtropics produces more global cooling per unit injection, with the EQ and the 60N+60S cases requiring, respectively, 59 % and 50 % more injection than the 30N+30S case to meet the same global mean temperature target. Injecting at higher latitudes results in larger equator-to-pole temperature gradients. While all five strategies restore September Arctic sea ice, the high-latitude injection one is more effective due to the SAI-induced cooling occurring preferentially at higher latitudes.

Received: 28 Jan 2023 – Discussion started: 02 Mar 2023

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Journal article(s) based on this preprint

13 Mar 2024

| Highlight paper

Hemispherically symmetric strategies for stratospheric aerosol injection

Yan Zhang, Douglas G. MacMartin, Daniele Visioni, Ewa M. Bednarz, and Ben Kravitz

Earth Syst. Dynam., 15, 191–213, https://doi.org/10.5194/esd-15-191-2024,https://doi.org/10.5194/esd-15-191-2024, 2024

Short summary Chief editor

Yan Zhang, Douglas G. MacMartin, Daniele Visioni, Ewa Bednarz, and Ben Kravitz

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-117', Anonymous Referee #1, 11 Mar 2023

The title of the paper is very misleading. The study done is certainly not comprehensive. And why “Introducing?”
The abstract is poorly written. It does not explain what global warming scenario is used. It does not mention what climate models are used. It jumps right into SAI while ignoring the fact that it does not exist, and is only a proposed scheme. It ignores the need to assess a wide range of potential benefits and risks before it is ever implemented. It does not say what is being injected. In fact the experiments are injecting gas and not aerosol.
The scientific questions being addressed by this paper are obscure. The paper says it wants to examine the response to certain sulfur dioxide emissions with respect to one global warming scenario using one climate model and specified injection altitudes and temperature reduction goal. It is by no means comprehensive. But why are they doing it? Is it a technical exercise for one specific modeling group? If so, this should be a technical report and not a journal article. I am not convinced it should be otherwise.
The paper is very long and detailed, going through many variables from the climate model simulations they did. I lost interest before I got halfway through. Why would other readers find this interesting? The paper does not pose interesting scientific questions that are then answered by specific experiments. I could not find any interesting new science in the paper. Many of the results are what would be expected.
On line 65 the paper says, “The understanding that comes from the analysis of the differences between these strategies lays the foundation for future work.” That is what a technical report should be doing, not a journal article which needs new science to justify publication.
In several places, “We note that” is in the text and should be deleted. Every sentence should be noted or it should not be in the paper.
There are 45 additional comments in the annotated manuscript. If the authors chose to reply to this review, a response of “we addressed all the comments” would not be sufficient. Each comment should be listed with a specific response.

Citation: https://doi.org/10.5194/egusphere-2023-117-RC1
- AC1: 'Reply on RC1', Yan Zhang, 14 Jun 2023
  
  Thank you for your comments. Please see the attached pdf for our responses.
  
  Citation: https://doi.org/10.5194/egusphere-2023-117-AC1
RC2:
'Comment on Zhang et al. egusphere-2023-117', Anonymous Referee #2, 23 Apr 2023

The authors present a study of a small set of stratospheric SO2 injection scenarios using a climate model, CESM2(WACCM6-MA).
Like the other reviewer, I have very serious issues with (1) the way the paper is framed in the context of what work has taken place and been published in the past, and (2) with the lack of detail given with regards to the methods. While the manuscript is long and quite detailed in terms of the analyses presented and the results (perhaps this analysis is the "comprehensive" part, rather than the very small set of injection scenarios investigated?), and there are likely some valuable technical additions to the literature in there that are worth publishing eventually, this is all called into question by these other major issues. Since those issues could be fixed by an extensive re-write I am recommending major revisions rather than rejection.
Claiming that the investigation presented here is "comprehensive," in a way that no previous studies have been, is simply incorrect. The authors present 5 injection strategies. Four of these are actually the same strategy, but with different injection latitudes - so actually only 2 different strategies. This is very much in line with previous similar studies in the literature, including ones that share many of the same co-authors with this paper. So this choice of language is deeply puzzling. Responsible, accurate, and technically rigorous communication is always important in the scientific literature, so I would comment on this in any case, but it is especially critical in areas of intense societal interest such as SAI research. Note that this is not a simple problem that's going to be solved with superficial "find and replace" type wording changes. The whole way the value of the study is presented and what aspects of the study are emphasized needs to be revisited and rewritten.
Far too little information is given on the technical approach. This aspect of the paper reads like an internal report rather than a manuscript for the literature. The other reviewer also commented on this. More information is needed on the model and climate scenario underpinning the simulations. The authors are surprisingly vague about the "controller(s)" which are used to determine injection rates. Equations and parameters for this technical feature need to be shared - along with some discussion of how this would be implemented in any kind of practical sense. The authors are directed to another paper for these details - which would not be sufficient even if the reference trail were clear - but it is not at all clear what paper is being referenced here (after 10 minutes searching I did not find a Visioni et al. 2022 with this title). The other reviewer touched on many more technical issues which must be addressed but which I will not repeat here.

Citation: https://doi.org/10.5194/egusphere-2023-117-RC2
- AC2: 'Reply on RC2', Yan Zhang, 14 Jun 2023
  
  Thank you for your comments. Please see the attached pdf for our responses.
  
  Citation: https://doi.org/10.5194/egusphere-2023-117-AC2
RC3:
'Comment on egusphere-2023-117', Anonymous Referee #3, 02 May 2023

Please see the enclosed PDF for my suggestions and recommandations.

Citation: https://doi.org/10.5194/egusphere-2023-117-RC3
- AC3: 'Reply on RC3', Yan Zhang, 14 Jun 2023
  
  Thank you for your comments. Please see the attached pdf for our responses.
  
  Citation: https://doi.org/10.5194/egusphere-2023-117-AC3

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-117', Anonymous Referee #1, 11 Mar 2023

The title of the paper is very misleading. The study done is certainly not comprehensive. And why “Introducing?”
The abstract is poorly written. It does not explain what global warming scenario is used. It does not mention what climate models are used. It jumps right into SAI while ignoring the fact that it does not exist, and is only a proposed scheme. It ignores the need to assess a wide range of potential benefits and risks before it is ever implemented. It does not say what is being injected. In fact the experiments are injecting gas and not aerosol.
The scientific questions being addressed by this paper are obscure. The paper says it wants to examine the response to certain sulfur dioxide emissions with respect to one global warming scenario using one climate model and specified injection altitudes and temperature reduction goal. It is by no means comprehensive. But why are they doing it? Is it a technical exercise for one specific modeling group? If so, this should be a technical report and not a journal article. I am not convinced it should be otherwise.
The paper is very long and detailed, going through many variables from the climate model simulations they did. I lost interest before I got halfway through. Why would other readers find this interesting? The paper does not pose interesting scientific questions that are then answered by specific experiments. I could not find any interesting new science in the paper. Many of the results are what would be expected.
On line 65 the paper says, “The understanding that comes from the analysis of the differences between these strategies lays the foundation for future work.” That is what a technical report should be doing, not a journal article which needs new science to justify publication.
In several places, “We note that” is in the text and should be deleted. Every sentence should be noted or it should not be in the paper.
There are 45 additional comments in the annotated manuscript. If the authors chose to reply to this review, a response of “we addressed all the comments” would not be sufficient. Each comment should be listed with a specific response.

Citation: https://doi.org/10.5194/egusphere-2023-117-RC1
- AC1: 'Reply on RC1', Yan Zhang, 14 Jun 2023
  
  Thank you for your comments. Please see the attached pdf for our responses.
  
  Citation: https://doi.org/10.5194/egusphere-2023-117-AC1
RC2:
'Comment on Zhang et al. egusphere-2023-117', Anonymous Referee #2, 23 Apr 2023

The authors present a study of a small set of stratospheric SO2 injection scenarios using a climate model, CESM2(WACCM6-MA).
Like the other reviewer, I have very serious issues with (1) the way the paper is framed in the context of what work has taken place and been published in the past, and (2) with the lack of detail given with regards to the methods. While the manuscript is long and quite detailed in terms of the analyses presented and the results (perhaps this analysis is the "comprehensive" part, rather than the very small set of injection scenarios investigated?), and there are likely some valuable technical additions to the literature in there that are worth publishing eventually, this is all called into question by these other major issues. Since those issues could be fixed by an extensive re-write I am recommending major revisions rather than rejection.
Claiming that the investigation presented here is "comprehensive," in a way that no previous studies have been, is simply incorrect. The authors present 5 injection strategies. Four of these are actually the same strategy, but with different injection latitudes - so actually only 2 different strategies. This is very much in line with previous similar studies in the literature, including ones that share many of the same co-authors with this paper. So this choice of language is deeply puzzling. Responsible, accurate, and technically rigorous communication is always important in the scientific literature, so I would comment on this in any case, but it is especially critical in areas of intense societal interest such as SAI research. Note that this is not a simple problem that's going to be solved with superficial "find and replace" type wording changes. The whole way the value of the study is presented and what aspects of the study are emphasized needs to be revisited and rewritten.
Far too little information is given on the technical approach. This aspect of the paper reads like an internal report rather than a manuscript for the literature. The other reviewer also commented on this. More information is needed on the model and climate scenario underpinning the simulations. The authors are surprisingly vague about the "controller(s)" which are used to determine injection rates. Equations and parameters for this technical feature need to be shared - along with some discussion of how this would be implemented in any kind of practical sense. The authors are directed to another paper for these details - which would not be sufficient even if the reference trail were clear - but it is not at all clear what paper is being referenced here (after 10 minutes searching I did not find a Visioni et al. 2022 with this title). The other reviewer touched on many more technical issues which must be addressed but which I will not repeat here.

Citation: https://doi.org/10.5194/egusphere-2023-117-RC2
- AC2: 'Reply on RC2', Yan Zhang, 14 Jun 2023
  
  Thank you for your comments. Please see the attached pdf for our responses.
  
  Citation: https://doi.org/10.5194/egusphere-2023-117-AC2
RC3:
'Comment on egusphere-2023-117', Anonymous Referee #3, 02 May 2023

Please see the enclosed PDF for my suggestions and recommandations.

Citation: https://doi.org/10.5194/egusphere-2023-117-RC3
- AC3: 'Reply on RC3', Yan Zhang, 14 Jun 2023
  
  Thank you for your comments. Please see the attached pdf for our responses.
  
  Citation: https://doi.org/10.5194/egusphere-2023-117-AC3

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

ED: Reconsider after major revisions (10 Jul 2023) by Gabriele Messori

AR by Yan Zhang on behalf of the Authors (29 Aug 2023) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (31 Aug 2023) by Gabriele Messori

RR by Anonymous Referee #2 (31 Aug 2023)

RR by Anonymous Referee #3 (21 Nov 2023)

ED: Publish subject to minor revisions (review by editor) (21 Nov 2023) by Gabriele Messori

Dear Authors,

I apologise for the delay in returning this manuscript to you. One of the two reviewers of the revised submission still raises concerns on the lack of contextualisation of the uncertainties/limits of the significance testing in your results. In your replies, you argued for the fact that large enough differences are robust even between individual model ensemble members. This may be true, but I don't find the authors' replies on this point to be entirely satisfactory. If one takes a single CMIP6 model run, for example, some measure of variability around the mean would likely result in a narrower range than the uncertainty deriving from running 30 or more model ensemble members, thus giving statistical overconfidence in your results. Another example of perhaps overconfident statistical testing, is the use of Welch's t-test. The authors never specify whether this assumes that the data being tested follows a specific underlying distribution, and never verify whether their data satisfies this. They also quote Wilks, but never explicitly mention whether a multiple testing correction is implemented (as per the Wilks 2016 "Stippling" paper).
I do not agree with the Reviewer that the above is sufficient grounds for rejection, but it is definitely something that should be addressed with a combination of careful rewording - to acknowledge that there are, like in all studies, some caveats to the conclusions being drawn - and reconsideration or further justification of some of the adopted statistical testing approaches. I also ask the authors to provide point-by-point replies to the rest of the remaining Reviewer comments.

Best Regards,
Gabriele Messori

Wilks, D. S., 2016: “The Stippling Shows Statistically Significant Grid Points”: How Research Results are Routinely Overstated and Overinterpreted, and What to Do about It. Bull. Amer. Meteor. Soc., 97, 2263–2273

Hide

AR by Yan Zhang on behalf of the Authors (16 Dec 2023) Author's response Author's tracked changes Manuscript

ED: Publish subject to minor revisions (review by editor) (21 Dec 2023) by Gabriele Messori

Dear Authors,

I would like to thank you for a thorough set of replies to the Reviewer’s comments. Before accepting your study for publication in Earth System Dynamics, I would encourage you to consider the following suggestions:

1. Concerning the number of ensemble members, it would make for an interesting discussion point to mention the paper by Bittner et al. (2016) and explain – as you do in your replies – why those conclusions do not necessarily apply to your work. This can be very short, but may help attentive readers to better understand the rationale behind the robustness of your results.

2. In the abstract, you could consider removing or shortening the following passage: “therefore, understanding the range of possible climate outcomes is crucial to making informed future decisions on SAI, along with the consideration of other factors. Yet to date, there has been no systematic exploration of a broad range of SAI strategies. This limits the ability to determine which effects are robust across different strategies and which depend on specific injection choices, or to determine if there are underlying trade-offs between different climate goals.” This reads more like a passage from the introduction, justifying the novelty of the article, than an abstract. I however accept that this point is to some extent subjective.

3. As Wilks (2016) points out, multiple testing issues can lead to spurious spatial patterns of significance, which are impossible to evaluate a priori and which can in turn lead to incorrect interpretation of results. Since you cite Wilks and are clearly aware of the issue, simply mentioning it in a sentence but not correcting your figures is hard to justify scientifically. This is particularly relevant for the global geographical plots. I would warmly encourage you to update your testing for these. There are ready-to-use packages performing multiple testing correction in several programming languages, which cause only a modest computational overhead.

Best Regards,
Gabriele Messori

Hide

AR by Yan Zhang on behalf of the Authors (13 Jan 2024) Author's response Author's tracked changes Manuscript

ED: Publish subject to technical corrections (17 Jan 2024) by Gabriele Messori

AR by Yan Zhang on behalf of the Authors (24 Jan 2024) Manuscript

Journal article(s) based on this preprint

13 Mar 2024

| Highlight paper

Hemispherically symmetric strategies for stratospheric aerosol injection

Yan Zhang, Douglas G. MacMartin, Daniele Visioni, Ewa M. Bednarz, and Ben Kravitz

Earth Syst. Dynam., 15, 191–213, https://doi.org/10.5194/esd-15-191-2024,https://doi.org/10.5194/esd-15-191-2024, 2024

Short summary Chief editor

Yan Zhang, Douglas G. MacMartin, Daniele Visioni, Ewa Bednarz, and Ben Kravitz

Data sets

Data from: Introducing a Comprehensive Set of Stratospheric Aerosol Injection Strategies Yan Zhang, Douglas G. MacMartin, Daniele Visioni, Ewa Bednarz, and Ben Kravitz https://zenodo.org/record/7545452#.Y9QqLnbMK3A

Yan Zhang, Douglas G. MacMartin, Daniele Visioni, Ewa Bednarz, and Ben Kravitz

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Nov 2023	22	13	2	37
Dec 2023	70	25	3	98
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.

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This study provides an intercomparison of different Stratospheric Aerosol Injection strategies. The strategy choice affects the distribution of aerosol optical depth, the injection efficiency and, crucially, the surface climate response. These results lay the foundation for future trade-off analyses and design strategy optimization.

Short summary

Injecting SO₂ into the lower stratosphere can temporarily reduce the global mean temperature and mitigate some of the risks associated with climate change, but injecting at different latitudes and seasons would have different impacts. This study introduces a comprehensive set of SAI strategies and systematically explores the importance of the choice of SAI strategy, demonstrating that it notably affects the distribution of aerosol cloud, injection efficiency, and various surface climate impacts.

Introducing a Comprehensive Set of Stratospheric Aerosol Injection Strategies

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Interactive discussion

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