The Buying Time Argument within the Solar Radiation Management Discourse

Round 1

Reviewer 1 Report

This article contains an interesting attempt to provide specific arguments on buying time in favor of Sulphur-Aerosol-Injection. The discussion looks logical according to the main aim of the research and the manuscript is properly structured. The conclusion is correctly presented. However, the connection to previous work discussed in the introduction should be completed. A minor comment: the author(s) and date of the references should be indicated in the text to improve the readability. I think it is a valuable contribution that should be considered for publication.

Author Response

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Reviewer 2 Report

“The Buying Time Argument within the Solar Radiation Discourse”

Reviewer comments

 

This paper provides the most detailed and compelling analysis to date of what is now a crucial argument in debates over solar radiation management (SRM). The analysis is strong. I recommend accepting the paper with minor revisions, based on the following comments. Some of these comments are more substantive than others, and some are more important than others. The authors should use their own judgment about whether and how to respond to them. Comments 5 and 7 are the most important.

 

1. I strongly recommend changing the title to “The Buying Time Argument within the Solar Radiation Management Discourse” or “The Buying Time Argument within the Solar Geoengineering Discourse,” as the current title will be missed in searches for “solar radiation management” or “solar geoengineering.”
2. On p. 2, ln. 85-89, the authors introduce the “reducing pressure” motivation for SRM. The meaning of “reducing pressure” becomes clearer later in the paper, but I would have found it helpful if the authors had said more about what it means here.
3. On p. 5, the authors use the term “temporal deployment,” but I think it would communicate the idea much more clearly if they called it “temporary deployment,” which also aligns their terminology more closely with others’.
4. On p. 5, the authors make the important point that “peak-shaving” deployments of SRM would need to be timed in relation to greenhouse gas (GHG) concentrations, rather than emissions. It might be clearer to revise Figure 1 to include emissions, SRM deployment, and concentrations, with SRM deployment located relative to concentrations. This would enable Fig. 1 to show the correct way of doing things while still making the authors’ point about emissions. In connection with this point, the authors might find it useful to cite MacDougall et al. 2020 (DOI 10.5194/bg-17-2987-2020) on the Zero Emissions Commitment Model Intercomparison Project, which summarizes the latest modeling work on how temperature changes after emissions reach net-zero.
5. The centerpiece of the paper is the authors’ reconstruction of the Buying Time Argument (BTA). In their reconstruction, premise BT5 reads: “CE in O does not lead to less abatement compared to mitigation in O without CE.” Given the explicitly consequentialist framework of the argument, this seems too strong. As Morrow 2014 (DOI 10.1098/rsta.2014.0062) argues, some delay in abatement is compatible with SRM improving overall climate outcomes. Thus, if it is impossible (or, more debatably, much more costly) to implement SRM without some delay in abatement, but that delay is small enough that the overall climate policy portfolio O still yields better results than the next best available option, then it seems to me that the BTA would still work. There are several easy ways to respond to this point, within the way the authors have set up the paper, and so I have no doubt that the authors can address it successfully.
6. On pp. 15-17, the authors discuss “moral constraints” on governance. I believe the section would benefit from engaging with Daniel Callies’ Climate Engineering: A Normative Perspective (2019), which discusses the moral constraints on governance of SRM. The approach taken in that book dovetails nicely with the authors’, and so I don’t expect this would cause any difficulties for the authors, but it would enrich and strengthen their discussion.
7. Twice the authors discuss what they call the “fatal irony” of SRM—that, according to the authors, SRM might be permissible only when it is least needed. Roughly, they mean that SRM can only meet the conditions attached to the BTA under an emissions scenario like RCP2.6, but that SRM would likely not be necessary in RCP2.6. I think they overstate their case here because (1) I don’t think they’ve shown conclusively that no SRM scheme to buy time would permissible under, say, RCP3.4 or RCP4.5, and more importantly, (2) the figure that they use to show that SRM would be impermissible under higher warming scenarios doesn’t account for the possibility of large-scale carbon dioxide removal (CDR). If you combine an RCP4.5 or even RCP6.0 trajectory with very large-scale CDR, I’d think you could do design a peak-shaving or rate-slowing deployment of SRM that satisfies the conditions for the BTA. Also, while I agree that SRM is least objectionable when it’s least needed, I’m more ambivalent than they are about its permissibility in high-warming scenarios. It’s true that SRM involves a very dangerous long-term commitment under very high warming scenarios, but I’m still tempted by the “lesser evil” arguments in those scenarios, despite Ott’s warning (elsewhere) that future generations would find themselves in a terrible dilemma if they faced the choice between, say, 5˚ of warming or multicentennial SRM. But the authors do say early on that they’re setting aside real dilemmas, so I don’t think it’s urgent to address this.
8. The authors close with a quick, final point that the hubris argument should count as a tie-breaker that tips the balance of reason against SRM. This move strikes me as made too quickly to be convincing. If the question is Should we proactively plan to use SRM as part of our climate policy?, then the hubris argument strikes me as carrying more weight. But if the world doesn’t succeed in rapid mitigation and a future generation finds itself facing, say, 2.5˚C of warming, so the question is Should we deploy SRM to buy ourselves time for further mitigation and adaptation, given they’ve our efforts to date have been? then it’s far less clear that epistemic humility should trump the humanitarian reasons for limiting warming with SRM.

Comments for author File: Comments.pdf

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