Comment on “An approach to sulfate geoengineering with surface emissions of carbonyl sulfide” by Quaglia et al. (2022)

Marc von Hobe, Christoph Brühl, Sinikka T. Lennartz, Mary E. Whelan, and Aleya Kaushik

Download

Final revised paper (published on 15 Jun 2023)
Preprint (discussion started on 28 Feb 2023)
Corresponding article

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-268', Daniele Visioni, 01 Mar 2023

As the second author of the Quaglia et al. (2022) , I'm very happy to have been asked to review this, and of this comment in general (also useful to point this out at the very beginning for COI reasons).
The authors make a lot of excellent points (no doubt, given their expertises), and highlight multiple reasons why OCS as a substitute to SO₂ injections would be not feasible as a form of stratospheric aerosol sources for climate intervention. While true that this had been mentioned in the past literature in a few occasions, using climate models to test out the hypotesis is always a useful exercise. If the evidences start to pile up around its unfeasibility, that can only be good news, it's Science after all. I strongly reccomend publication of this piece - hoping the authors can address some of my comments below around Section 4.
Sections 2 and 3 are really informative and well written and I have no comments over them.
Section 4:
- point well taken on the IR differences between our estimates and the authors' (but I wish the authors explained more why they see such a discrepancies with our estimates in the LW. What in the radiative-convective model they use changes the results compared to ours?)
- in ULAQ-CCM, evaporation is indeed calculated at every vertical level as a function of H2SO4 vapor pressure, surface area density and kernel for condensation.
- while the authors correctly point out to Fig. S2, looking at Fig. S3 clearly highlights that the peak in extinction and SAD appears much lower than 35 km, at around 21-22km, so - while there might be discrepancies between the forcing we calculate and the authors to this piece calculate - the reason why ours is higher and theirs is lower is not the fact that the ULAQ-CCM aerosols are so higher up, or at least that's not the main factor.
- differences in forcing could be due to many factors, such as aerosol size (which is report in Table 1 for ULAQ-CCM), H2O changes, ozone etc.. If the overall value the authors obtain is -0.4 W/m2 that is absolutely fair, but in the conclusions to the section I would just say "which is less than a third than was estimated in Quaglia et al. (2022), and would not be able to fully compensate for the forcing by anthropogenic CO₂ at the suggested injection rates" rather than just "which cannot compensate for forcing by anthropogenic CO₂"

- My last comment is that it's my understanding that ACP does not accept "available from the authors upon request" as a data availability statement, and so that part should be amended to include a DOI where data could be publicly accessed.

Citation: https://doi.org/10.5194/egusphere-2023-268-RC1
- AC1: 'Reply on RC1', Marc von Hobe, 26 Apr 2023
  
  We thank Daniele for his openness towards the scientific discourse expressed in his opening statement. Exchanging scientific evidence and arguments, and taking them into consideration and allow conclusions to evolve, is indeed what good Science should be about. We also appreciate the important questions and comments on Section 4 of our paper, which we respond to below (in italics).
  - point well taken on the IR differences between our estimates and the authors' (but I wish the authors explained more why they see such a discrepancies with our estimates in the LW. What in the radiative-convective model they use changes the results compared to ours?)
  We believe that the discrepancy arises from the method chosen by Quaglia et al. to calculate the infrared forcing. For a steady state change of the mixing ratio profile of a greenhouse trace gas, the change in infrared forcing should be calculated directly and not via the GWP concept, which is designed for comparing the effect of emissions of two greenhouse gases. We have added a corresponding statement in the manuscript. We also added an additional reference that gives more detailed information on the radiative-convective model used.
  - in ULAQ-CCM, evaporation is indeed calculated at every vertical level as a function of H2SO4 vapor pressure, surface area density and kernel for condensation.
  We replaced “neglected” by “underestimated”.
  - while the authors correctly point out to Fig. S2, looking at Fig. S3 clearly highlights that the peak in extinction and SAD appears much lower than 35 km, at around 21-22km, so - while there might be discrepancies between the forcing we calculate and the authors to this piece calculate - the reason why ours is higher and theirs is lower is not the fact that the ULAQ-CCM aerosols are so higher up, or at least that's not the main factor.
  Still, the sulfate maximum at ~35 km is unrealistic and also not supported by observations (as noted in the revised manuscript). Overall, the different results of the two models are consistent with findings in the model intercomparison paper by Quaglia et al. (2023), where ULAQ-CCM is near the high end and EMAC more at the low side concerning stratospheric optical depth. We added a corresponding statement and a reference to the Quaglia et al. (2023) paper in the revised manuscript.
  - differences in forcing could be due to many factors, such as aerosol size (which is report in Table 1 for ULAQ-CCM), H2O changes, ozone etc.. If the overall value the authors obtain is -0.4 W/m2 that is absolutely fair, but in the conclusions to the section I would just say "which is less than a third than was estimated in Quaglia et al. (2022), and would not be able to fully compensate for the forcing by anthropogenic CO₂ at the suggested injection rates" rather than just "which cannot compensate for forcing by anthropogenic CO₂"
  We changed the sentence at the end of Section 4 to: “In our calculations we get only -0.4 W m^-2for the TTL scenario and present-day conditions (2017 to 2021), which can only partially compensate for forcing by anthropogenic CO₂ at the suggested injection rates.”
  In the conclusions, we changed “insignificant” to “insufficient” in the concluding statement on the expected net cooling from OCS addition.
  - My last comment is that it's my understanding that ACP does not accept "available from the authors upon request" as a data availability statement, and so that part should be amended to include a DOI where data could be publicly accessed.
  We have uploaded our data in zenodo (one record for each model) and include the appropriate references and DOIs in the revised manuscript.
  
  Citation: https://doi.org/10.5194/egusphere-2023-268-AC1
RC2:
'Comment on egusphere-2023-268', Anonymous Referee #2, 29 Mar 2023

Review of “Comment on “An approach to sulfate geoengineering with surface emissions of carbonyl sulfide” by Quaglia et al. (2022)”
This short article seeks to point out deficiencies or oversights in the Quaglia et al. paper. They look specifically at 4 topics: uptake by plants, uptake by oceans, implications for toxicity due to higher concentrations resulting from geoengineering scenarios envisaged and revisit radiative effects. The paper is clear and succinct in that it soundly points to the non-viability of OCS injection to counter radiative warming due to increasing atmospheric CO2 and points to yet to be studied though likely toxic effects to terrestrial biosphere and the marine ecosystem. It does this with existing, available models that show how sensitive the ecosystem and oceanic are to atmospheric OCS concentrations and that they will uptake excess OCS to their likely detriment.
Major Issues
None.
Minor Issues
None.
Reviewer general comment.
The authors note in the introduction, that in the original review process of the Quaglia et al. paper an objection was made for not investigating the effect of higher atmospheric concentrations on the terrestrial biosphere. The editors allowed a brief statement to sidestep the opportunity to address this critical point and apparently others. Although an author can make any statement they deem valid and escape thoroughness via caveats, this is quite an obvious oversight. Through this comment these points have been made and the record set on an improved path. Though we are left to wonder why a critical stance was not taken at the time of the original review to strengthen the Quaglia paper. That decision was the consequence of the editorial staff’s commitment to standards for scientific rigor or not.

Citation: https://doi.org/10.5194/egusphere-2023-268-RC2
- AC2: 'Reply on RC2', Marc von Hobe, 26 Apr 2023
  
  We thank the reviewer for the appreciation of our initiative to write this short comment.
  We are also grateful for the closing remark at the end of the review comment. We fully agree and have nothing to add, really.
  
  Citation: https://doi.org/10.5194/egusphere-2023-268-AC2

Peer review completion

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

AR by Marc von Hobe on behalf of the Authors (26 Apr 2023) Author's response Author's tracked changes Manuscript

ED: Publish as is (08 May 2023) by Simone Tilmes

AR by Marc von Hobe on behalf of the Authors (09 May 2023)

Short summary

Carbonyl sulfide plays a role in the climate system as a greenhouse gas and as the major non-volcanic precursor of particles reflecting sunlight. Here, we comment on a proposal to increase the number of particles by emitting extra carbonyl sulfide at the surface. We show that the balance between aerosol cooling and greenhouse gas warming may not be as favorable as suggested and also that much of the carbonyl sulfide emissions will actually be taken up by the biosphere and the oceans.