Stratospheric ozone projections under sulfur-based stratospheric aerosol injection: Insights from the multi-model G6-1.5K-SAI experiment

Bednarz, Ewa M.; Butler, Amy H.; Haywood, James M.; Henry, Matthew; Jones, Andy; Kravitz, Ben; Lee, Walker R.; MacMartin, Douglas G.; Maycock, Amanda C.; Sekiya, Takashi; Watanabe, Shingo; Visioni, Daniele

doi:10.5194/egusphere-2026-310

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https://doi.org/10.5194/egusphere-2026-310

Preprints

27 Jan 2026

| 27 Jan 2026

Stratospheric ozone projections under sulfur-based stratospheric aerosol injection: Insights from the multi-model G6-1.5K-SAI experiment

Ewa M. Bednarz, Amy H. Butler, James M. Haywood, Matthew Henry, Andy Jones, Ben Kravitz, Walker R. Lee, Douglas G. MacMartin, Amanda C. Maycock, Takashi Sekiya, Shingo Watanabe, and Daniele Visioni

Abstract. Owing to the crucial role of stratospheric ozone in shielding the Earth from harmful solar ultraviolet radiation, impacts of human activities on the ozone layer remain of interest. Here we provide an assessment of the potential impacts of Stratospheric Aerosol Injection (SAI), a proposed method to temporarily offset global warming, on stratospheric ozone projections over the 21^st century using the new multi-model GeoMIP G6-1.5K-SAI experiment. The experiment injects SO₂ at a pair of subtropical latitudes and utilizes a more plausible middle-of-the-road greenhouse gas emission pathway and SAI start date compared to earlier studies.

All three participating Earth system models show a decrease in global mean total column ozone of a few Dobson units (1–2 %) under SAI compared to no-SAI scenario. This decrease is dominated by heterogeneous halogen activation on sulfate aerosol, most clearly evident in the Southern Hemisphere mid- and high latitudes. This is unlike previous results using strategies injecting at the equator, which show increased global mean column ozone, partly due to larger ozone transport changes. As background halogen levels continue to decrease, the potential of SAI to deplete ozone is found to be a factor of ~2 larger in the earlier part of the 21^st century (2045–2064) than later (2065–2084). We further identify areas of model disagreement and sources of uncertainty, but also areas of more confidence and potential emergent constraints. Our results highlight the need to assess any projected SAI impacts in the wider strategy and scenario dimension using a multi-model framework.

Received: 19 Jan 2026 – Discussion started: 27 Jan 2026

Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics. The peer-review process was guided by an independent editor, and the authors also have no other competing interests to declare.

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.

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10 Sep 2026

Stratospheric ozone projections under sulfur-based stratospheric aerosol injection: Insights from the multi-model G6-1.5K-SAI experiment

Ewa M. Bednarz, Amy H. Butler, James M. Haywood, Matthew Henry, Andy Jones, Ben Kravitz, Walker R. Lee, Douglas G. MacMartin, Amanda C. Maycock, Takashi Sekiya, Shingo Watanabe, and Daniele Visioni

Atmos. Chem. Phys., 26, 12751–12770, https://doi.org/10.5194/acp-26-12751-2026,https://doi.org/10.5194/acp-26-12751-2026, 2026

Short summary

Ewa M. Bednarz, Amy H. Butler, James M. Haywood, Matthew Henry, Andy Jones, Ben Kravitz, Walker R. Lee, Douglas G. MacMartin, Amanda C. Maycock, Takashi Sekiya, Shingo Watanabe, and Daniele Visioni

Interactive discussion

Status: closed

RC1: 'Comment on egusphere-2026-310', Anonymous Referee #1, 25 Feb 2026

This study focuses on the global and regional (tropical, subtropical, and springtime polar) impacts of a new SAI experiment on stratospheric ozone changes and potential drivers. This experiment mainly differs from previous GeoMIP experiments by injecting SO₂ at symmetric subtropical latitudes instead of equatorial bands, and using a pathway of lower GHGs, namely SSP2-4.5 compared to SSP5-8.5, and reducing the warming to roughly present-day temperatures. It applies a multi-model approach to differentiate between inter-model and SAI experiment differences. Chemical and dynamical effects are separated using additional model runs that exclude heterogeneous chemistry on SAI aerosols. The simulated SAI experiments exhibits smaller lower stratospheric warming compared to equatorial injection scenarios, which translates into smaller stratospheric circulation changes.
The studies' main findings include the need for more accurate modelling of stratospheric transport, as inter-model differences appear to be driven by these. Generally, inter-model differences dominate over differences within injection scenarios in the one tested model. There seems to be an asymmetry between the polar responses. The authors attribute the decrease in column ozone to halogen catalysed chemical loss, and transport changes induced by SAI. As process importance differs between different latitudinal regions, the authors separate different contributions and attribute dynamical response differences to process level uncertainty in stratospheric transport.
The below-mentioned comments are not critical, but require some changes that go beyond simple re-wording. Thus, I suggest moderate revisions of the manuscript as outlined below.
Since heterogeneous halogen activation on sulfate aerosols is put forward as one of the major factors in decreasing ozone columns, a brief introduction of the mechanism, corresponding studies, and model differences would be helpful.
At multiple instances in the manuscript, the authors refer to either "heterogenous" or "heterogeneous" chemistry. Please check for consistency.
At multiple instances, the authors write TO3 or O3, please correct this to O₃.
Please check for consistency when referring to sulfate and sulfate aerosols.
l.26: Instead of, or additionally to, "more plausible middle-of-the-road", please mention the SSP.
l.29: "compared to [a] no-SAI"
l.35: While agree with the strategy dimension, I don't recall analysis of the scenario dimension. At least in case the latter refers to different SSPs. To keep this argument, some further analysis to isolate its influence should be included, which may already be present in previous G6-1.5K-SAI publications.
l.38: A reference for ozone's shielding role, and effects on life and ecosystems should be included here.
l.43: Since the impacts of climate intervention methods are highly uncertain, a more careful formulation, such as "to [potentially] temporarily offset", should be considered here.
l.52: This full sentence lacks a reference to acknowledge previous work.
ll.53-55: This statement also lacks at least one, or several references.
ll.64-66: This statement lacks a reference to heterogeneous chemistry of aerosols. At this stage, more details on halogen chemistry, and results from other models (e.g., SOCOL, MPI-ESM) with aerosol chemistry could be added.
l.68: Please reference the GeoMIP project properly here.
l.77: Consider hyphenating "middle-of-the-road".
l.79: Consider hyphenating "business-as-usual".
l.100: "each of the model[s']"
Figure 1 (a): This figure would benefit from a shading or similar representation of errors to better estimate model differences. This is especially the case for the runs with SAI. The authors may also consider a visualisation and brief discussion of the inter-model differences in the chosen base-line states, and their impacts on their findings.
l.115: Please clearly state the CESM version.
l.121: "including [the] most"
Table S1: Please write "2 Cl", "2 Br", ...; please also restructure the table's title, for instance moving "in each model" to the end of the title.
ll.160-162: This statement regarding the coarse mode may also deserve a citation.
ll.162-164: This statement regarding the SAD would pose as significantly increased uncertainty of chemical responses and would deem SAD as a much weaker constraint for this response. It would potentially also limit the extent to which chemical responses in UKESM imply responses in MIROC. This aspect deserves some further discussion in the corresponding section (section 7 or 8).
Figure 2: A visualisation of the uncertainties would be useful. Subfigure (c) uses a different period (2050-2069) on the right side compared to the left side. These differences should be discussed and evaluated. In the same subfigure, the relation of ozone and surface temperature change seems to be ambiguous and without clear physical connection. In case the authors want to use this metric, it should be more thoroughly introduced before.
l.180: lacks a reference to BDC changes following GHG emissions.
ll.184-186: The statement on dynamical effects on ozone lacks references, being itself not straightforward, nor directly related to the results of this study.
ll.186-188: The connection between global-mean total column ozone and surface temperature is ambiguous, and neglects the complex relationship between the two. It appears that the authors aim to connect the ozone column changes directly to potentially policy-relevant changes in surface temperature. By doing so, the underlying processes are blurred and physical understanding is lost. To my knowledge, no direct connection between these two quantities exists that would allow to link them in this way. I recommend to either exclude this metric (ozone column change per degree of warming), or justify its use and physical foundation more clearly. This oversimplification must be removed in the final version of the manuscript. This holds for all plots using this metric.
l.192: "larger earlier in the century than later" --> This doesn't really hold for CESM, except from the mid-2070s.
l.194: "progressively higher" --> the aerosol burden curve seems to flatten, whereas temperature difference increases roughly linearly. This may be discussed in the context of separating the earlier to later parts of the century, instead of mainly focussing on decreasing halogen concentration.
ll.224-226: I don't agree with this statement for the earlier period from 2045-2064. Please clarify this confusion, and also rather refer to the figure than, or in addition to, the original paper to clarify the comparison here.
l.229: Please reference the G6sulfur experiment.
ll.229-230: "and thus potentially less favourable outcome" --> I suggest to omit this part, being speculative and not adding information.
Figure 3: (a) is very busy, and may justify only showing the running mean (with uncertainty); (b) shows a jump (with change in sign) for CESM which may be discussed in more detail to ensure it being physical.
Figure 3 (c) uses 'DU' as the y-axis as physical quantity, whereas it should be 'TO₃' in [DU]. Similarly, Figure 6 confuses physical quantities and units. Please check all Figures regarding these.
Figure 4 lacks indications of the physical quantity (pressure) and its units (hPa) on the y-axis; please also add the diagnosed tropopause, since it is referred to at several instances in the text, but seems to be defined inconsistently.
ll.250-251: I only agree with the decrease in tropical ozone column from around 2050 in CESM and MIROC, and from around 2070 in UKESM.
l.252: Please reference the figure showing the change in the BDC here.
ll.252-254: The connection between lower stratospheric decrease in ozone and upper stratospheric ozone is not clear. Please reformulate (and split) this sentence to clarify the argument.
l.256: The difference in sign of the ozone response appears like an offset. A discussion of this effect between the models would be useful here. For instance, adding a factor to the UKESM response could shift it towards the observed CESM values. Within this framework, a brief discussion on the influence of the early phases of SAI on the following trajectory or sensitivity tests could be insightful, and further make the results more robust.
ll.257-258: MIROC seems to drift more significantly towards more negative values in Figure 3 (a). A short discussion of possible reasons and corresponding uncertainty would be useful.
l.267, l.276, l.280: The mentioning of the lower stratosphere and tropopause seems to use different definitions of these regions. Please ensure to consistently refer to the same regions, and annotate them within the corresponding figures.
ll.272-274: This statement requires a reference, or more elaborate evaluation.
l. 283: Please be more concrete in linking stratospheric warming to circulation changes. For instance, one may rather refer to "stratospheric circulation" and its specific features (e.g., BDC) rather than circulation in general.
ll.283-285: This statement is not new, and deserves acknowledgement of previous work.
ll.304-307: "The much lower climatological stratospheric water vapor in MIROC (Fig. 7h) might reduce aerosol water uptake and hence partially contribute to the model showing comparable amount of heterogenous halogen activation and halogen-catalyzed ozone depletion than the other two models (Section 3, also Sections 5-6), despite much larger aerosol SAD (Section 2, Fig. 1d)." --> Aerosol water uptake, or hygroscopic growth, mainly depends on Relative Humidity (RH) and not stratospheric water vapour. The former determines the thermodynamic equilibrium vapour pressure over binary solutions such as H₂SO₄-H₂O droplets. RH is generally very low in the stratosphere, resulting in large decrease in uptake sensitivity with height. The climatological absolute humidity in MIROC may be significantly lower than in the other two models, but so may saturation vapour pressure, resulting in relatively constant RH, and thus aerosol water uptake. I suggest to check RH for differences between MIROC and the other two models to constrain this statement.
Figure 5: In case no AoA figure is added for CESM, please change the naming of the subfigures to (a)-(e). This figure also deserves an annotated diagnosed tropopause.
Figure 6: Please write (a) and (b) in the caption, not (A) and (B). Write-out the abbreviation "TEM" in full please.
ll. 342-344: Increased transport by the BDC resulting in increased ozone lacks a reference.
Figure 8 would perhaps benefit from using relative instead of absolute units for ozone columns. Such a visualisation may also limit the model dependence on inter-model differences in climatology.
l. 350: I would agree with this statement for CESM and MIROC, but not for CESM.
ll. 352-354: Please provide some reference or figure for this statement. In this context, some justification for the presence (and absence) of these difference may be added.
ll.363-364: "higher climatological" than what?
ll.382-384: I agree that figures 9 and 8c illustrate the change in TO3, but don't see how they allow to attribute these to transport changes.
ll.384-385: Please show, or reference, the difference in the importance of chemistry vs. transport in a figure or similar to support this statement.
l.394: Figure 5d is non-existent.
ll.410-411: The projected increase in ozone is still debated and not as clear as stated here.
l.413: Just write CESM and MIROC here, without the later negation.
ll.416-419: This statement blurs climate sensitivity and ozone columns, and deserves justification or correction.
l.442: "Domaisen" -> "Domeisen"
ll.453-454: Please reference the corresponding figure here (10b).
ll.473-475: This statement deserves a reference.
ll.478-480: Neglecting differences in surface cooling and injection rate and deeming them less relevant deserves further justification.
ll.486-488: This statement lacks justification on why it has to be halogen catalyzed loss, and not other chemical processes, or chemistry-dynamics interactions.
l.487: "pearly" -> "clearly"?
ll.545-548: The direct comparison to Tilmes+2022 seems difficult since it uses a different "strategy" and scenario. I agree that mentioning these differences is relevant, but directly inferring conclusions from them obscures the differences and complexity.
l.579: I agree with the importance of mentioning the comparison alongside SSP2-4.5. However, its implications for the analysis should be further elaborated to justify its mentioning.
l.581: The "lagging" of the ozone response is not strictly shown here. Especially from 2060, the ozone responses rather seem to converge. This statement requires further justification.
ll.583-584: Please provide reference(s) for this "common assumption".
ll.584-586: I don't see the relevance or direct connection to the results and objectives of this study.

Citation: https://doi.org/10.5194/egusphere-2026-310-RC1
RC2: 'Comment on egusphere-2026-310', Anonymous Referee #2, 31 Mar 2026

The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2026-310/egusphere-2026-310-RC2-supplement.pdf

Citation: https://doi.org/10.5194/egusphere-2026-310-RC2
AC1: 'Authors response', Ewa Bednarz, 06 May 2026

The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2026-310/egusphere-2026-310-AC1-supplement.pdf

Citation: https://doi.org/10.5194/egusphere-2026-310-AC1

Peer review completion

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

AR by Ewa Bednarz on behalf of the Authors (06 May 2026) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (06 May 2026) by Peer Nowack

RR by Anonymous Referee #1 (23 May 2026)

Suggestions for revision or reasons for rejection

The first suggestion is essential in my opinion, whereas the remaining comments would improve the quality and readability of the manuscript.

Please make the used data available via zenodo or other means to ensure reproducibility.

l. 81: I meant quotations for "middle-of-the-road".

l. 83: I meant quotations for "business-as-usual".

ll. 121-122: Please indicate the CESM model (sub-) version used, e.g., 2.1.5. Bugs or similar issues may be found in the respective version which are easier to identify that way.

Figure 2: My point here is that it is possible to get/more less ozone w/o decreasing surface temperatures, and vice versa. See GHGs, halogens, …. I understand the motivation to connect the "cooling efficiency" of the respective method to the extent of ozone depletion. But in its current form, the different contributing processes are omitted from the reader. Ideally, the motivation for this metric would be more clearly justified and introduced in the beginning to avoid misunderstanding. The same holds for my previous comment to ll. 186-188. Here, a clear statement (as in the response to my comment) would improve the understanding of the manuscript and clarify the employed metrics. My point is, that I would like to see a justification of the used metric, being along the lines of comparing the "desired" reduction in Tsurf to the impact on O3. Not because they are physically justifyably connected but because the paper focusses on SAI and comparison of different approaches. Otherwise, the underlying physics may be understood blurred.

Previous comment to ll. 416-419:
This metric is focused on estimating how much O3 depletion one has to take into account per "achieved" degree of cooling, which is more of a policy/trade-off metric and has less physical meaning beyond that. Since the connection between Tsurf and O3 is not direct, differences in models (chemistry, radiation, resolution, …) could also lead to very different results. The metric (if it would be as straightforward as mentioned in the paper) would be great to have, but I find it more confusing/ambiguous than useful to understand the physics. I would rather just e.g., normalize the TO3 with respect to climatology.

Previous comment to ll.478-480:
I would imagine that the main difference comes from RH due to different cold point temperatures at the tropopause which affects stratospheric heterogeneous chemistry. So simply pointing to surface temperature normalization may not be sufficient.
Stratospheric meridional temperature differences, notably tropical lower stratospheric warming are previously found to show significant effects on stratospheric circulation and resulting changes in the hydrological cycle and ozone.

Hide

RR by Anonymous Referee #3 (01 Jul 2026)

Suggestions for revision or reasons for rejection

The manuscript by Bednarz et al. analyzes a stratospheric aerosol injection (SAI) experiment, using SSP2-4.5 as the control without SAI, for three chemistry-climate models that participated. Important differences from earlier SAI experiments include a more moderate greenhouse gas scenario as the control and specified sulfur injection at subtropical latitudes. The simulations are analyzed with a focus on the impacts on stratospheric ozone, looking at how chemical and dynamical changes induced by the SAI change the evolution of ozone over the rest of the 21st century.
I will note that I have been asked to review the article after it has already been through one round of reviews with a quite extensive set of comments and responses from the authors.
I find the manuscript in its current form to be generally well written and a clear presentation of the model results and the analysis. I believe the article should be published after addressing a couple of relatively minor aspects related to the presentation or interpretation of the results.
In the minor comments I point out I had trouble understanding (lines 140 – 157) how exactly the background SAD and SAI-enhanced SAD are treated in the UKESM experiments analysed here. If I have understood the setup correctly, heterogeneous chemistry on the specified SAD is turned off in both the SSP2-4.5 and G6-1.5K-SAI experiments for UKESM simulations used here? Given the dominance of the aerosol from SAI I am not concerned about the lack of heterogeneous chemistry on the background aerosols in the G6-1.5K-SAI simulation, but having zero aerosols in the SSP2-4.5 simulation seems to be a bit of an unrealistic case. With zero sulphate aerosol in the SSP2-4.5 simulation the attenuation of halogen effects on ozone by reactive nitrogen compounds will be stronger than they should be - and it is noted in the text that the SSP2-4.5 simulation used here has higher ozone than the standard SSP2-4.5 UKESM simulation with background SAD. As a consequence the impact of SAI on ozone estimated from the differences between G6-1.5K-SAI and SSP2-4.5 will be larger than it should be. I would argue this is likely part of the reason for the larger change in TCO in UKESM shown in Figure 2b even though the SAD from SAI in UKESM is not that large. This increased sensitivity to SAD in UKESM is then partly compensated for by the smaller increases in SAD to get the same cooling – Figure 1a and 1e – so that the impact on ozone per degree cooling comes out roughly the same as shown in Figure 2c. I would argue that this aspect of the UKESM results need to be put into context for the discussion.
In a few places the changes in the chemistry with SAI are attributed to chlorine activation on the increased sulphate (for example, at line 205 and at lines 307–310). I might be dated in my thinking, but I thought the explanation for Pinatubo effects on ozone was an increased sequestering of reactive nitrogen in nitric acid through N2O5 hydrolysis on sulphate aerosol, leaving less reactive nitrogen in forms that can tie up chlorine and bromine – at least in mid to low latitudes. Part of that mechanism is a result of the temperature dependence of reactions like HCl + ClONO2 on sulphate aerosols with very slow reaction rates (small gammas) much above 200K. See, for example, Kawa et al., Activation of chlorine in sulfate aerosols as inferred from aircraft observations, J. Geophys. Res, 102, 3921-3933, doi:10.1029/96JD01992, 1997. I would accept that certain features of the response are due to ‘chlorine activation’, like the larger response in TCO in the Southern Hemisphere (line 378) that is likely the hemispheric impact from increase Antarctic ozone depletion. But the more widespread low- to mid-stratosphere changes through the tropics and mid-latitudes I believe are more due to changes in chlorine as a result of changes to NOx.
Line 45: The phrase ‘to reduce the incoming solar radiation’ seems a bit imprecise if you consider incoming to be at the edge of the atmosphere.
Lines 140 – 157: The discussion of how UKESM treats heterogeneous reactions on background sulphate could use a single definitive statement of how sulphate reactions were treated in the two experiments used for this paper. I think I can find it, but it requires looking through the entire paragraph and parsing a bit of information from different sentences. From the text I understand that ‘a second revised set of both SSP2-4.5 and G6-1.5K-SAI simulations was carried out’ and that ‘in these revised UKESM simulations no heterogeneous chemistry occurs on the background imposed CMIP6-recommended stratospheric aerosol surface area density (SAD)’. It just seems a bit counterintuitive that to turn on heterogenous chemistry on the SAI sulphate it was decided to turn it off in both SSP2-4.5 and G6-1.5K-SAI. Being an important point it would help to see that stated explicity here.
Lines 524 – 527: In reference to the changes in TCO for the WITH_HET and NO_HET simulations shown in Figure 13, do the authors have any explanation for why the NO_HET TCO shows a clear increase between 2045-2064 and 2065-2084 in the Northern hemisphere, but the WITH_HET simulation shows almost identical changes for the two periods? This starts at about 30N and extends right to the North Pole.

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ED: Publish subject to minor revisions (review by editor) (02 Jul 2026) by Peer Nowack

AR by Ewa Bednarz on behalf of the Authors (16 Jul 2026) Author's response Author's tracked changes Manuscript

ED: Publish as is (26 Jul 2026) by Peer Nowack

AR by Ewa Bednarz on behalf of the Authors (04 Aug 2026)

Interactive discussion

Status: closed

RC1: 'Comment on egusphere-2026-310', Anonymous Referee #1, 25 Feb 2026

This study focuses on the global and regional (tropical, subtropical, and springtime polar) impacts of a new SAI experiment on stratospheric ozone changes and potential drivers. This experiment mainly differs from previous GeoMIP experiments by injecting SO₂ at symmetric subtropical latitudes instead of equatorial bands, and using a pathway of lower GHGs, namely SSP2-4.5 compared to SSP5-8.5, and reducing the warming to roughly present-day temperatures. It applies a multi-model approach to differentiate between inter-model and SAI experiment differences. Chemical and dynamical effects are separated using additional model runs that exclude heterogeneous chemistry on SAI aerosols. The simulated SAI experiments exhibits smaller lower stratospheric warming compared to equatorial injection scenarios, which translates into smaller stratospheric circulation changes.
The studies' main findings include the need for more accurate modelling of stratospheric transport, as inter-model differences appear to be driven by these. Generally, inter-model differences dominate over differences within injection scenarios in the one tested model. There seems to be an asymmetry between the polar responses. The authors attribute the decrease in column ozone to halogen catalysed chemical loss, and transport changes induced by SAI. As process importance differs between different latitudinal regions, the authors separate different contributions and attribute dynamical response differences to process level uncertainty in stratospheric transport.
The below-mentioned comments are not critical, but require some changes that go beyond simple re-wording. Thus, I suggest moderate revisions of the manuscript as outlined below.
Since heterogeneous halogen activation on sulfate aerosols is put forward as one of the major factors in decreasing ozone columns, a brief introduction of the mechanism, corresponding studies, and model differences would be helpful.
At multiple instances in the manuscript, the authors refer to either "heterogenous" or "heterogeneous" chemistry. Please check for consistency.
At multiple instances, the authors write TO3 or O3, please correct this to O₃.
Please check for consistency when referring to sulfate and sulfate aerosols.
l.26: Instead of, or additionally to, "more plausible middle-of-the-road", please mention the SSP.
l.29: "compared to [a] no-SAI"
l.35: While agree with the strategy dimension, I don't recall analysis of the scenario dimension. At least in case the latter refers to different SSPs. To keep this argument, some further analysis to isolate its influence should be included, which may already be present in previous G6-1.5K-SAI publications.
l.38: A reference for ozone's shielding role, and effects on life and ecosystems should be included here.
l.43: Since the impacts of climate intervention methods are highly uncertain, a more careful formulation, such as "to [potentially] temporarily offset", should be considered here.
l.52: This full sentence lacks a reference to acknowledge previous work.
ll.53-55: This statement also lacks at least one, or several references.
ll.64-66: This statement lacks a reference to heterogeneous chemistry of aerosols. At this stage, more details on halogen chemistry, and results from other models (e.g., SOCOL, MPI-ESM) with aerosol chemistry could be added.
l.68: Please reference the GeoMIP project properly here.
l.77: Consider hyphenating "middle-of-the-road".
l.79: Consider hyphenating "business-as-usual".
l.100: "each of the model[s']"
Figure 1 (a): This figure would benefit from a shading or similar representation of errors to better estimate model differences. This is especially the case for the runs with SAI. The authors may also consider a visualisation and brief discussion of the inter-model differences in the chosen base-line states, and their impacts on their findings.
l.115: Please clearly state the CESM version.
l.121: "including [the] most"
Table S1: Please write "2 Cl", "2 Br", ...; please also restructure the table's title, for instance moving "in each model" to the end of the title.
ll.160-162: This statement regarding the coarse mode may also deserve a citation.
ll.162-164: This statement regarding the SAD would pose as significantly increased uncertainty of chemical responses and would deem SAD as a much weaker constraint for this response. It would potentially also limit the extent to which chemical responses in UKESM imply responses in MIROC. This aspect deserves some further discussion in the corresponding section (section 7 or 8).
Figure 2: A visualisation of the uncertainties would be useful. Subfigure (c) uses a different period (2050-2069) on the right side compared to the left side. These differences should be discussed and evaluated. In the same subfigure, the relation of ozone and surface temperature change seems to be ambiguous and without clear physical connection. In case the authors want to use this metric, it should be more thoroughly introduced before.
l.180: lacks a reference to BDC changes following GHG emissions.
ll.184-186: The statement on dynamical effects on ozone lacks references, being itself not straightforward, nor directly related to the results of this study.
ll.186-188: The connection between global-mean total column ozone and surface temperature is ambiguous, and neglects the complex relationship between the two. It appears that the authors aim to connect the ozone column changes directly to potentially policy-relevant changes in surface temperature. By doing so, the underlying processes are blurred and physical understanding is lost. To my knowledge, no direct connection between these two quantities exists that would allow to link them in this way. I recommend to either exclude this metric (ozone column change per degree of warming), or justify its use and physical foundation more clearly. This oversimplification must be removed in the final version of the manuscript. This holds for all plots using this metric.
l.192: "larger earlier in the century than later" --> This doesn't really hold for CESM, except from the mid-2070s.
l.194: "progressively higher" --> the aerosol burden curve seems to flatten, whereas temperature difference increases roughly linearly. This may be discussed in the context of separating the earlier to later parts of the century, instead of mainly focussing on decreasing halogen concentration.
ll.224-226: I don't agree with this statement for the earlier period from 2045-2064. Please clarify this confusion, and also rather refer to the figure than, or in addition to, the original paper to clarify the comparison here.
l.229: Please reference the G6sulfur experiment.
ll.229-230: "and thus potentially less favourable outcome" --> I suggest to omit this part, being speculative and not adding information.
Figure 3: (a) is very busy, and may justify only showing the running mean (with uncertainty); (b) shows a jump (with change in sign) for CESM which may be discussed in more detail to ensure it being physical.
Figure 3 (c) uses 'DU' as the y-axis as physical quantity, whereas it should be 'TO₃' in [DU]. Similarly, Figure 6 confuses physical quantities and units. Please check all Figures regarding these.
Figure 4 lacks indications of the physical quantity (pressure) and its units (hPa) on the y-axis; please also add the diagnosed tropopause, since it is referred to at several instances in the text, but seems to be defined inconsistently.
ll.250-251: I only agree with the decrease in tropical ozone column from around 2050 in CESM and MIROC, and from around 2070 in UKESM.
l.252: Please reference the figure showing the change in the BDC here.
ll.252-254: The connection between lower stratospheric decrease in ozone and upper stratospheric ozone is not clear. Please reformulate (and split) this sentence to clarify the argument.
l.256: The difference in sign of the ozone response appears like an offset. A discussion of this effect between the models would be useful here. For instance, adding a factor to the UKESM response could shift it towards the observed CESM values. Within this framework, a brief discussion on the influence of the early phases of SAI on the following trajectory or sensitivity tests could be insightful, and further make the results more robust.
ll.257-258: MIROC seems to drift more significantly towards more negative values in Figure 3 (a). A short discussion of possible reasons and corresponding uncertainty would be useful.
l.267, l.276, l.280: The mentioning of the lower stratosphere and tropopause seems to use different definitions of these regions. Please ensure to consistently refer to the same regions, and annotate them within the corresponding figures.
ll.272-274: This statement requires a reference, or more elaborate evaluation.
l. 283: Please be more concrete in linking stratospheric warming to circulation changes. For instance, one may rather refer to "stratospheric circulation" and its specific features (e.g., BDC) rather than circulation in general.
ll.283-285: This statement is not new, and deserves acknowledgement of previous work.
ll.304-307: "The much lower climatological stratospheric water vapor in MIROC (Fig. 7h) might reduce aerosol water uptake and hence partially contribute to the model showing comparable amount of heterogenous halogen activation and halogen-catalyzed ozone depletion than the other two models (Section 3, also Sections 5-6), despite much larger aerosol SAD (Section 2, Fig. 1d)." --> Aerosol water uptake, or hygroscopic growth, mainly depends on Relative Humidity (RH) and not stratospheric water vapour. The former determines the thermodynamic equilibrium vapour pressure over binary solutions such as H₂SO₄-H₂O droplets. RH is generally very low in the stratosphere, resulting in large decrease in uptake sensitivity with height. The climatological absolute humidity in MIROC may be significantly lower than in the other two models, but so may saturation vapour pressure, resulting in relatively constant RH, and thus aerosol water uptake. I suggest to check RH for differences between MIROC and the other two models to constrain this statement.
Figure 5: In case no AoA figure is added for CESM, please change the naming of the subfigures to (a)-(e). This figure also deserves an annotated diagnosed tropopause.
Figure 6: Please write (a) and (b) in the caption, not (A) and (B). Write-out the abbreviation "TEM" in full please.
ll. 342-344: Increased transport by the BDC resulting in increased ozone lacks a reference.
Figure 8 would perhaps benefit from using relative instead of absolute units for ozone columns. Such a visualisation may also limit the model dependence on inter-model differences in climatology.
l. 350: I would agree with this statement for CESM and MIROC, but not for CESM.
ll. 352-354: Please provide some reference or figure for this statement. In this context, some justification for the presence (and absence) of these difference may be added.
ll.363-364: "higher climatological" than what?
ll.382-384: I agree that figures 9 and 8c illustrate the change in TO3, but don't see how they allow to attribute these to transport changes.
ll.384-385: Please show, or reference, the difference in the importance of chemistry vs. transport in a figure or similar to support this statement.
l.394: Figure 5d is non-existent.
ll.410-411: The projected increase in ozone is still debated and not as clear as stated here.
l.413: Just write CESM and MIROC here, without the later negation.
ll.416-419: This statement blurs climate sensitivity and ozone columns, and deserves justification or correction.
l.442: "Domaisen" -> "Domeisen"
ll.453-454: Please reference the corresponding figure here (10b).
ll.473-475: This statement deserves a reference.
ll.478-480: Neglecting differences in surface cooling and injection rate and deeming them less relevant deserves further justification.
ll.486-488: This statement lacks justification on why it has to be halogen catalyzed loss, and not other chemical processes, or chemistry-dynamics interactions.
l.487: "pearly" -> "clearly"?
ll.545-548: The direct comparison to Tilmes+2022 seems difficult since it uses a different "strategy" and scenario. I agree that mentioning these differences is relevant, but directly inferring conclusions from them obscures the differences and complexity.
l.579: I agree with the importance of mentioning the comparison alongside SSP2-4.5. However, its implications for the analysis should be further elaborated to justify its mentioning.
l.581: The "lagging" of the ozone response is not strictly shown here. Especially from 2060, the ozone responses rather seem to converge. This statement requires further justification.
ll.583-584: Please provide reference(s) for this "common assumption".
ll.584-586: I don't see the relevance or direct connection to the results and objectives of this study.

Citation: https://doi.org/10.5194/egusphere-2026-310-RC1
RC2: 'Comment on egusphere-2026-310', Anonymous Referee #2, 31 Mar 2026

The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2026-310/egusphere-2026-310-RC2-supplement.pdf

Citation: https://doi.org/10.5194/egusphere-2026-310-RC2
AC1: 'Authors response', Ewa Bednarz, 06 May 2026

The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2026-310/egusphere-2026-310-AC1-supplement.pdf

Citation: https://doi.org/10.5194/egusphere-2026-310-AC1

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AR by Ewa Bednarz on behalf of the Authors (04 Aug 2026)

Journal article(s) based on this preprint

10 Sep 2026

Stratospheric ozone projections under sulfur-based stratospheric aerosol injection: Insights from the multi-model G6-1.5K-SAI experiment

Ewa M. Bednarz, Amy H. Butler, James M. Haywood, Matthew Henry, Andy Jones, Ben Kravitz, Walker R. Lee, Douglas G. MacMartin, Amanda C. Maycock, Takashi Sekiya, Shingo Watanabe, and Daniele Visioni

Atmos. Chem. Phys., 26, 12751–12770, https://doi.org/10.5194/acp-26-12751-2026,https://doi.org/10.5194/acp-26-12751-2026, 2026

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Ewa M. Bednarz, Amy H. Butler, James M. Haywood, Matthew Henry, Andy Jones, Ben Kravitz, Walker R. Lee, Douglas G. MacMartin, Amanda C. Maycock, Takashi Sekiya, Shingo Watanabe, and Daniele Visioni

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https://doi.org/10.5194/egusphere-2026-310-supplement

Ewa M. Bednarz, Amy H. Butler, James M. Haywood, Matthew Henry, Andy Jones, Ben Kravitz, Walker R. Lee, Douglas G. MacMartin, Amanda C. Maycock, Takashi Sekiya, Shingo Watanabe, and Daniele Visioni

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An assessment of the potential impacts of Stratospheric Aerosol Injection, a proposed method to offset global warming, on stratospheric ozone projections over the 21^st century using the new multi-model GeoMIP G6-1.5K-SAI experiment. We discuss drivers of the responses, identify areas of model agreement and disagreement and sources of uncertainty. Our results highlight the need to assess any projected SAI impacts in wider strategy and scenario dimension using a multi-model framework.


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