Annual Solar Geoengineering: Mitigating Yearly Global Warming Increases

Round 1

Reviewer 1 Report (Previous Reviewer 3)

Comments and Suggestions for Authors

I have read through the author's response to my previous review of this paper and note the improvements made.

I have the following comments.

(1) I remain concerned that in presenting annual estimates, the total summative needs over time are lost. The author needs to make it very clear that as the estimates provided are annual,  additions would have to be made annually (ie painting, etc) until such time as the annual increase in GHG driven radiative forcing was zero (or very small), and maintained until such time as the increase in GHG driven radiative forcing had reduced to an acceptable level. 

(2) Following (1) above, given that the author has raised RCPs, assuming we are on a a RCP4.5 trajectory, but wish to remain on a RCP2.6 trajectory, what would the peak implementation look like for the approaches (eg area), and how long would we need to maintain the approaches?

(3) I have previously made the observation that implementation and ongoing management of these approaches is a critical factor. For example, monitoring, maintenance (eg performance deterioration) and so on. The paper now addresses some of these, mainly implementation, although even this is only briefly touched on. I appreciate the primary purpose of the paper is to establish annual estimates of approaches for a given set of conditions, but it would be worth considering drawing some conclusion about which of these approaches has the least risk of failure over (a considerable) time, the least requirement for ongoing maintenance, and the least risk to political stability. I would guess that it is albedo modification. (The point about our current infrastructure driven albedo modification (eg roads) is well made. I remain completely sceptical about placing anything in space to reduce incoming solar radiation- the number of launches required to achieve even a small fraction of the area would likely be impossible to achieve annually.)

(4)  The author has not listed any negative consequences of the approaches. 

Author Response

Please see attached file.

Author Response File: Author Response.pdf

Reviewer 2 Report (New Reviewer)

Comments and Suggestions for Authors

Review of “Annual solar geoengineering: mitigating yearly global warming increases” by Alec Feinberg (Manuscript ID: climate-2755073)

 

This manuscript proposes a strategy of “annual solar geoengineering” (ASG) to mitigate yearly projected increases in global warming, rather than attempting a “full solar geoengineering” (full SG) approach to counter the entire amount of anthropogenic warming. The ASG approaches considered here include L1 space sunshading, Earth brightening (e.g., cool pavement/roof), and stratospheric aerosol injection. The author argues that ASG requires 50-150x less albedo modification than full SG, reducing costs and concerns about induced circulation and precipitation changes. 

 

This manuscript is not suitable for publication in Climate in its present form. The manuscript makes excessive use of jargon and abbreviations, without sufficient explanation/definition, making it very difficult to follow the methodology being employed. The advances included in this paper compared with previous cited works by the author are not clear. The notation used in the paper differs significantly from the common standard in the field. In particular, a clear distinction is not made between “forcings” and “feedbacks” leading to significant confusion in the notation used (e.g., delta-P with various subscripts). As such, I suggest that this paper be rejected from Climate. I include several specific comments on the manuscript below.

 

Abstract – Please clarify the distinction in the usage of “SG” versus “SRM”.

 

Introduction

 

• Please provide definitions for terms as they are first mentioned in this section (e.g., annual solar geoengineering, SG reverse forcing methods). The list of abbreviations provided above is insufficient. Complete definitions are needed to allow the reader to understand the approach. Currently, this section contains too much jargon presented without sufficient context.

• The statement “a 1oC rise is closer to 5.1 W m^-2” needs to be explained.

• The review of previous literature (esp. by the author himself) and of the advances contained in this work are insufficient.

 

Methods and data

 

• Results should not be presented in this section without context. Need more conceptual overview of methods, rather than just an exhaustive list of references to later sub-sections.

• Table 1 is extremely difficult to follow. Terms are presented here without sufficient context to understand what they represent, or in some cases what their units are.

• In the ASG temperature reversal estimate section, clarify that the discussion assumed ASG will be applied starting in 2023, and warming is measured relative to 1975 baseline.

• Define ZGWG.

 

Theory

• Equation (3) doesn’t account for any feedbacks, so does not represent the actual climate sensitivity (see, e.g., IPCC AR6 analysis).

• The delta-P_rev value of -5.1 W/m2 is much larger than the anthropogenic forcing. Need to differentiate between forcings and feedbacks in this discussion.

• The discussion in this section is extremely confusing and hard to follow. Many quantities (such as f, the “re-radiation”; SG reverse forcing, SG target reversal goal, UHI) are not defined at all.

• Need to explain more clearly why the water vapor feedback applies to SG but not to GHG removal. 

• The section on SRM area estimates lacks sufficient context/explanation.

Results

• Many of the results presented are unclear or misleading. For instance, it is claimed that the area needed for ASG is about 160x lower than that for full SG. But, this only refers to one year’s worth of ASG. If ASG were applied over any extended period of time, this “advantage” would decrease significantly. 

 

Discussion

• Can you provide an estimate for the amount of particulate mass that would be needed for L1 sunshading?

• Explain why cooling “natural” hotspots is disproportionately beneficial versus a more distributed cooling approach.

Comments on the Quality of English Language

The writing in the paper is sub-standard and would need to be improved significantly before publication. Jargon and abbreviations are used too extensively, without appropriate context or definitions. As a result, it is very difficult to follow and understand the methods used in the analysis.

Author Response

Please see attached file.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report (Previous Reviewer 3)

Comments and Suggestions for Authors

The paper can be accepted after making two final very minor corrections:

line 574: addition should be additional

Table 6: add units to col 3, (years).

 

Author Response

Dear Reviewer: Thank you for all your suggestions including the last few. In the acknowledgement, I provided a thank you.

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The title of this paper is impressive. But, I did not find any valuable quantitative results regarding geoengineering to mitigate climate change. 

Goal of the paper is not clear.  In order to introduce yourself into the problem I recommend you to read this relevant paper https://rmets.onlinelibrary.wiley.com/doi/full/10.1002/asl2.481

 

 

Author Response

Please see my response in the attached file.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

In this paper, the advantages of annual solar geoengineering (ASG) to mitigate yearly global warming increases are explored.

In my opinion the structure of the paper must be reviewed because information are not well-organized. The paper doesn't clearly define exactly what it will discuss. Moreover, it does not outline the order in which it will discuss each subtopic to give the reader any background information needed to understand the coming sections.

Author Response

Please see the attached file for my response to the reviewer's comments.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Although not identical, the author has previously presented similar work to this paper (ref 13,  https://doi.org/10.3389/fclim.2022.870071). The similarities between the current paper and ref 13 are sufficient to ask what is materially different in this paper? The major difference is that here the focus is suppression of the annual increase in temperature (annual GHG driven increase in the GHG  average global temperature- temperature anomaly), rather than previously where the focus was the total increase. 

It is questionable whether the difference between the two papers (as currently presented) is sufficient to warrant publication here. There are additional differences in the paper, but these are considered minor.

I note that the author has chosen as the target temperature anomaly the land-ocean data rather than just land data. Why? Given mixing between the ocean and atmosphere, how will the results calculated have the outcome suggested by the author? 

Transport of energy in the Earth's ocean-atmosphere-land system is complex. The claim by the author that they have taken a fundamental physics-based approach is overstated.  The calculations presented are overly simplistic. The author has not provided any discussion of the limitations of the model used and its outcomes. Given this, who is this paper directed at? How are the results to be used?

Furthermore, no indication is provided as to how the various SG approaches would actually be implemented and their outcomes managed to yield the target temperature reduction.  Research into SG (and similar) has been undertaken for some time, and it is now getting greater focus given our increasing failure to adequately respond to our changing climate. An important missing area is all the detail related to implementation, monitoring, and the obvious risks (use of SG as a political tool). This paper adds very little to this important missing area.

Finally, I question the logic of the paper. The author's main point appears to be that rather than mitigate the total temperature anomaly, we should aim to mitigate the annual increase. Does this not assume that the current anomaly is acceptable? There are plenty of climate scientists who would say it is not (see for example Hansen and co-author's recent paper http://www.columbia.edu/~jeh1/Documents/PipelinePaper.2023.05.19.pdf in this paper the authors note the need for more research into SG/SRM to  avoid unintended adverse effects). What then is an acceptable baseline? Further, given the moral hazard of SG (and similar), does not its use merely prolong GHG emissions? And if so, after a few years are we not then implementing SG at a scale that was proposed in the author's previous paper? If so, what does this paper add to our understanding of the mitigation problem?

Given these points, even supposing the author can address some of these points in a revision, it is would likely remain difficult to see what advance has been made in this paper over that previously published.

Author Response

Please see my response to reviewer 3 on the attached file.

Author Response File: Author Response.pdf