the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Injection strategy – a driver of atmospheric circulation and ozone response to stratospheric aerosol geoengineering
Amy H. Butler
Daniele Visioni
Yan Zhang
Ben Kravitz
Douglas G. MacMartin
Abstract. Despite offsetting global mean surface temperature, various studies demonstrated that Stratospheric Aerosol Injection (SAI) could influence the recovery of stratospheric ozone and have important impacts on stratospheric and tropospheric circulation, thereby potentially playing an important role in modulating regional and seasonal climate variability. However, so far most of the assessments of such an approach have come from climate model simulations in which SO2 is injected only in a single location or a set of locations.
Here we use CESM2-WACCM6 SAI simulations under a comprehensive set of SAI strategies achieving the same global mean surface temperature with different locations and/or timing of injections: an equatorial injection, an annual injection of equal amounts of SO2 at 15° N and 15° S, an annual injection of equal amounts of SO2 at 30° N and 30° S, and a polar strategy injecting SO2 at 60° N and 60° S only in spring in each hemisphere.
We demonstrate that despite achieving the same global mean surface temperature, the different strategies result in contrastingly different magnitudes of the aerosol-induced lower stratospheric warming, stratospheric moistening, strengthening of stratospheric polar jets in both hemispheres and changes in the speed of the residual circulation. In conjunction with the differences in direct radiative impacts at the surface, these drive different impacts on the extratropical modes of variability (Northern and Southern Annular Mode), including important consequences on the northern winter surface climate, as well as on the intensity of tropical tropospheric Walker and Hadley Circulations, which drive tropical precipitation patterns. Finally, we demonstrate that the choice of injection strategy also plays a first-order role in the future evolution of stratospheric ozone under SAI throughout the globe. Overall, our results contribute to an increased understanding of the fine interplay of various radiative, dynamical and chemical processes driving the atmospheric response to SAI, as well as lay the ground for designing an optimal SAI strategy that could form a basis of future multi-model intercomparisons.
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Journal article(s) based on this preprint
Ewa M. Bednarz et al.
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2023-495', Ulrike Niemeier, 20 Apr 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-495/egusphere-2023-495-RC1-supplement.pdf
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RC2: 'Comment on egusphere-2023-495', Anonymous Referee #2, 07 Jun 2023
The article explores the effects of different injection strategies for stratospheric aerosol geoengineering on various aspects of the stratospheric and tropospheric circulation. The results are important to highlight the different possible impacts of different strategies which had not been explored before. The paper is well presented and interesting, but I have some concerns as listed below that should be considered before publication.
- Methods: I understand that only one simulation is analyzed for each type of injection strategy. This could be problematic when extracting the forced signal at high latitudes, where the large internal variability can dominate. I would recommend using several members and extracting the ensemble mean response in order to obtain more robust results. If this is not possible I would ask the authors to at least acknowledge the possible uncertainties due to this limitation, in particular for the NH polar vortex response and the surface response.
- Figure 1 and 2, L179-180: ‘increases equator-to-pole meridional temperature gradients near the tropopause and lower stratosphere and thus forces strengthening of stratospheric jets in both hemispheres’
This argument should be rephrased more carefully. In the lower stratosphere temperature is lowest in the tropics except in SH summer. As SAI warms this region, the eq-to-pole gradient becomes smaller in absolute value. This implies a reduction of the (negative) wind shear, thus an acceleration of the wind above the temperature perturbation and a deceleration below. This is seen clearly in Fig. 2 (top left) for EQ. However, the high latitude strengthening of the zonal wind is also linked to the structure of the cooling over the polar cap, which is present in both hemispheres in the lowermost stratosphere in all strategies except POLAR, and also in middle and high latitude in the SH. Could you explain why you get this cooling regions? In general the cooling of the middle and upper stratosphere is quite an outstanding feature in all strategies and it should be discussed as it can influence not only the winds but also ozone.
- Another question is why the warming in the lower stratosphere is weaker in the case of polar injection. Is it because the injected material is transported into the troposphere and removed from the atmosphere?
- It would be useful to add letters to the figure panels.
- L120: the confinement to the tropical pipe will only work above ~20 km, while below that level there is strong horizontal mixing. It could be interesting (beyond this work) to investigate that case.
- L110: ‘throughout a year’ → throughout each year
- L214-215: ‘Warming in the lower stratosphere also reduces the stability of the stratosphere itself, thereby accelerating the deep branch of the BDC.’The BDC is forced by wave driving, so I would expect that thermal changes induce wind changes which modify wave propagation conditions and this drive the BDC. In order to explore this mechanism, it would be good to include the Eliassen-Palm flux divergence in order to examine the associated changes. Also the heat flux plots could be extended in altitude to show the stratosphere.
- L381-382: These conclusions cannot be extracted from the analysis because you are not comparing to the past period as you do for the previous figures, so the reader does not know if the BDC is stronger for SAI than for SSP2-4.5. It would be necessary to include those maps too or at least mention how is the BDC in the reference simulation.
- L292: typo ‘an qualitatively’
- L329: typo ‘near-air surface’ should be near-surface air
- L375: the different sign above and below the climatological ozone maximum should be noted (due to opposite-sign vertical gradients)
Citation: https://doi.org/10.5194/egusphere-2023-495-RC2 -
RC3: 'Comment on egusphere-2023-495', Gabriel Chiodo, 27 Jun 2023
The paper by Bednarz et al. examines the dependence of the climate effects of different injection strategies (specifically geographic position) for SAI, by using the CESM2-WACCM6 model. In particular, they highlight the widely different outcomes in terms of atmospheric circulation and ozone changes, despite using the same surface temperature target. An interesting conclusions of the paper is that polar injections can lead to much smaller circulation changes, although for other metrics (e.g. ozone) the side effects can be more serious than for tropical injections. I think that the paper deserves to be published after some corrections as detailed below.
GENERAL COMMENTS
1. One of the major conclusions of the paper, in my view, is that some of the undesirable side-effects of SAI in the stratosphere that are well established for tropical injections, such as stratospheric moistening, perturbations to the Brewer-Dobson circulation and stratosphere-troposphere coupling as well as some effects in the troposphere (e.g., weakening of the Hadley Cell) are largely reduced with polar injections. A lot of these side effects come from stratospheric heating, which is effectively reduced in the case of polar injections, but the detailed reason why this is the case is only marginally mentioned in Section 3.1 (spatial distribution of the aerosol cloud). I think the discussion of this feature and the quantification of the direct (radiative) heating should be expanded.
2. When it comes to the discussion of the different ozone responses to the different strategies (section 6.), a lot of emphasis is given to the dynamical changes (leading to the changes in ozone documented in the paper), while chemical processes (halogen activation on S-aerosols) is only qualitatively discussed, in particular for the case of the polar injections. I think this aspect should be discussed more extensively and if possible, the authors could consider quantifying the chemical contribution to the ozone changes. Also, what about the interaction between SO2, the liquid H2SO4-H2O aerosols and PSC formation (in particular STS PSCs)? What about N2O5 hydrolysis? Why is the ozone response in the global stratosphere (in particular the upper stratosphere) so much smaller in the case of polar injections, according to Fig. 7? I don't think that dynamics can explain it all. Another key result that is not even mentioned in the text are the sizable tropospheric ozone changes, in particular for the case of polar injections - these can affect the cooling efficiency of the aerosols, too. I think these are all aspects that deserve somewhat more discussion, and they would aid the mechanistic understanding and can better motivate future similar studies with other models.
4. Most of the relevant literature is included in the paper, but some additional recommendations are given below to put the present paper (even) more into context, in particular with respect to the stratospheric water vapor feedback (e.g. Banerjee et al., 2019). Also, more comparisons with other papers showing the impacts of the injection location could be done (e.g., Weistenstein et al., 2022).
5. The paper never discussed microphysical changes arising from tropical vs polar injections. In particular, the size distribution changes depending on the injection latitude could be another very interesting aspect to document... as that could also help the reader understand the lifetime / cooling efficiency depending on the injection strategy. This is a model with interactive microphysics so I guess this aspect could be studied?
SPECIFIC COMMENTS
L26-36 General comment about the abstract: it reads a bit qualitative, especially towards the end. It would be nice to give a "sign" of the changes when it comes to the latitudinal dependency of the outcome. Could we provide some insight into the general pattern that is arising (i.e. polar injections leading to more of X, tropical injections leading to more of Y).
L64 Another paper that is worth citing here is Banerjee et al., 2019, as well as (most recently) Nowack et al., 2023; they provide a more up-to-date assessment of the sWV feedback across CMIP5 and CMIP6 models.
L87-88 It would be good to highlight the novely over the "feedback" mechanism documented for GLENS, as those papers are what most of the community (even the non-SAI crowd) is most familiar with.
L122 The 30N-30S injection points were also tested in Weistenstein et al., 2022 - who compared region vs point injections... and also came to similar conclusions, i.e. that injecting just outside of the tropical pipe leads to more uniform aerosol distributions in the global stratosphere... and this was tested across 3 fully independent aerosol CCMs. It might be worth highlighting the consistency with that study.
L126 I would have not expected that injecting at the equator or 30S/N would make such a difference in terms of "average sizes", given that the coagulation time-scales (and condensational growth) are quite a bit smaller than the typical transit times in the BDC (6-12 months). Unless the tropical injections lead to aerosols that are more "tropically" confined. Have the authors verified this?
L145 This is a crucial point which deserves more attention, as many of the downstream effects depend on this. I think this deserves a more detailed discussion, especially with respect to the radiative contribution to this signal.
L162 I strongly recommend using the more up-to-date value from Banerjee, of 0.22 W/m2 (Fig.5), which is based on many more models than the 2 papers cited here.
L264-265 could this teleconnection be related to an ENSO-like response to SAI in these runs?
L290 I would expect aerosols to have actually the smallest impact on albedo over Antarctica, given the very reflective underlying surface (high surface albedo all year around). Hence, I do not think polar injections really lead to any "reduced (net) summer isolation" changes over Antarctica... and if anything, these changes in insolation would be reflected by the snow/ice of the surface. Can the authors comment on this?
Equations 1-2 I think the mathematical symbols do not match those mentioned in the main text.
L325 Can the authors briefly explain why the Hadley Cell weakens under SAI? What is the underlying mechanism?
L403-405 as mentioned in one of the main comments, I think this could be expanded and perhaps be analyzed more quantitatively. Similarly, the sizable tropospheric ozone changes should be discussed, as they might have important implications for tropospheric chemistry and/or air-quality.
L474-479 I think it might be good to be a bit more quantitative here, or at least give some information concerning the "direction" of the changes and if some coherent pattern emerges, concerning the advantages/disadvantages of each injection strategy (polar vs tropical).
REFERENCES
Banerjee, et al. (2019); Stratospheric water vapor: an important climate feedback, Climate Dynamics, DOI:10.1007/s00382-019-04721-4
Weisenstein, et al. (2022); A Model Intercomparison of Stratospheric Solar Geoengineering by Accumulation-Mode Sulfate Aerosols , Atmospheric Chemistry and Physics, DOI:10.5194/acp-22-2955-2022
Nowack et al. (2023); Response of stratospheric water vapour to warming constrained by satellite observations, Nature Geoscience, DOI:10.1038/s41561-023-01183-6
Citation: https://doi.org/10.5194/egusphere-2023-495-RC3 -
AC1: 'Authors response to all reviewers comments', Ewa Bednarz, 29 Aug 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-495/egusphere-2023-495-AC1-supplement.pdf
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2023-495', Ulrike Niemeier, 20 Apr 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-495/egusphere-2023-495-RC1-supplement.pdf
-
RC2: 'Comment on egusphere-2023-495', Anonymous Referee #2, 07 Jun 2023
The article explores the effects of different injection strategies for stratospheric aerosol geoengineering on various aspects of the stratospheric and tropospheric circulation. The results are important to highlight the different possible impacts of different strategies which had not been explored before. The paper is well presented and interesting, but I have some concerns as listed below that should be considered before publication.
- Methods: I understand that only one simulation is analyzed for each type of injection strategy. This could be problematic when extracting the forced signal at high latitudes, where the large internal variability can dominate. I would recommend using several members and extracting the ensemble mean response in order to obtain more robust results. If this is not possible I would ask the authors to at least acknowledge the possible uncertainties due to this limitation, in particular for the NH polar vortex response and the surface response.
- Figure 1 and 2, L179-180: ‘increases equator-to-pole meridional temperature gradients near the tropopause and lower stratosphere and thus forces strengthening of stratospheric jets in both hemispheres’
This argument should be rephrased more carefully. In the lower stratosphere temperature is lowest in the tropics except in SH summer. As SAI warms this region, the eq-to-pole gradient becomes smaller in absolute value. This implies a reduction of the (negative) wind shear, thus an acceleration of the wind above the temperature perturbation and a deceleration below. This is seen clearly in Fig. 2 (top left) for EQ. However, the high latitude strengthening of the zonal wind is also linked to the structure of the cooling over the polar cap, which is present in both hemispheres in the lowermost stratosphere in all strategies except POLAR, and also in middle and high latitude in the SH. Could you explain why you get this cooling regions? In general the cooling of the middle and upper stratosphere is quite an outstanding feature in all strategies and it should be discussed as it can influence not only the winds but also ozone.
- Another question is why the warming in the lower stratosphere is weaker in the case of polar injection. Is it because the injected material is transported into the troposphere and removed from the atmosphere?
- It would be useful to add letters to the figure panels.
- L120: the confinement to the tropical pipe will only work above ~20 km, while below that level there is strong horizontal mixing. It could be interesting (beyond this work) to investigate that case.
- L110: ‘throughout a year’ → throughout each year
- L214-215: ‘Warming in the lower stratosphere also reduces the stability of the stratosphere itself, thereby accelerating the deep branch of the BDC.’The BDC is forced by wave driving, so I would expect that thermal changes induce wind changes which modify wave propagation conditions and this drive the BDC. In order to explore this mechanism, it would be good to include the Eliassen-Palm flux divergence in order to examine the associated changes. Also the heat flux plots could be extended in altitude to show the stratosphere.
- L381-382: These conclusions cannot be extracted from the analysis because you are not comparing to the past period as you do for the previous figures, so the reader does not know if the BDC is stronger for SAI than for SSP2-4.5. It would be necessary to include those maps too or at least mention how is the BDC in the reference simulation.
- L292: typo ‘an qualitatively’
- L329: typo ‘near-air surface’ should be near-surface air
- L375: the different sign above and below the climatological ozone maximum should be noted (due to opposite-sign vertical gradients)
Citation: https://doi.org/10.5194/egusphere-2023-495-RC2 -
RC3: 'Comment on egusphere-2023-495', Gabriel Chiodo, 27 Jun 2023
The paper by Bednarz et al. examines the dependence of the climate effects of different injection strategies (specifically geographic position) for SAI, by using the CESM2-WACCM6 model. In particular, they highlight the widely different outcomes in terms of atmospheric circulation and ozone changes, despite using the same surface temperature target. An interesting conclusions of the paper is that polar injections can lead to much smaller circulation changes, although for other metrics (e.g. ozone) the side effects can be more serious than for tropical injections. I think that the paper deserves to be published after some corrections as detailed below.
GENERAL COMMENTS
1. One of the major conclusions of the paper, in my view, is that some of the undesirable side-effects of SAI in the stratosphere that are well established for tropical injections, such as stratospheric moistening, perturbations to the Brewer-Dobson circulation and stratosphere-troposphere coupling as well as some effects in the troposphere (e.g., weakening of the Hadley Cell) are largely reduced with polar injections. A lot of these side effects come from stratospheric heating, which is effectively reduced in the case of polar injections, but the detailed reason why this is the case is only marginally mentioned in Section 3.1 (spatial distribution of the aerosol cloud). I think the discussion of this feature and the quantification of the direct (radiative) heating should be expanded.
2. When it comes to the discussion of the different ozone responses to the different strategies (section 6.), a lot of emphasis is given to the dynamical changes (leading to the changes in ozone documented in the paper), while chemical processes (halogen activation on S-aerosols) is only qualitatively discussed, in particular for the case of the polar injections. I think this aspect should be discussed more extensively and if possible, the authors could consider quantifying the chemical contribution to the ozone changes. Also, what about the interaction between SO2, the liquid H2SO4-H2O aerosols and PSC formation (in particular STS PSCs)? What about N2O5 hydrolysis? Why is the ozone response in the global stratosphere (in particular the upper stratosphere) so much smaller in the case of polar injections, according to Fig. 7? I don't think that dynamics can explain it all. Another key result that is not even mentioned in the text are the sizable tropospheric ozone changes, in particular for the case of polar injections - these can affect the cooling efficiency of the aerosols, too. I think these are all aspects that deserve somewhat more discussion, and they would aid the mechanistic understanding and can better motivate future similar studies with other models.
4. Most of the relevant literature is included in the paper, but some additional recommendations are given below to put the present paper (even) more into context, in particular with respect to the stratospheric water vapor feedback (e.g. Banerjee et al., 2019). Also, more comparisons with other papers showing the impacts of the injection location could be done (e.g., Weistenstein et al., 2022).
5. The paper never discussed microphysical changes arising from tropical vs polar injections. In particular, the size distribution changes depending on the injection latitude could be another very interesting aspect to document... as that could also help the reader understand the lifetime / cooling efficiency depending on the injection strategy. This is a model with interactive microphysics so I guess this aspect could be studied?
SPECIFIC COMMENTS
L26-36 General comment about the abstract: it reads a bit qualitative, especially towards the end. It would be nice to give a "sign" of the changes when it comes to the latitudinal dependency of the outcome. Could we provide some insight into the general pattern that is arising (i.e. polar injections leading to more of X, tropical injections leading to more of Y).
L64 Another paper that is worth citing here is Banerjee et al., 2019, as well as (most recently) Nowack et al., 2023; they provide a more up-to-date assessment of the sWV feedback across CMIP5 and CMIP6 models.
L87-88 It would be good to highlight the novely over the "feedback" mechanism documented for GLENS, as those papers are what most of the community (even the non-SAI crowd) is most familiar with.
L122 The 30N-30S injection points were also tested in Weistenstein et al., 2022 - who compared region vs point injections... and also came to similar conclusions, i.e. that injecting just outside of the tropical pipe leads to more uniform aerosol distributions in the global stratosphere... and this was tested across 3 fully independent aerosol CCMs. It might be worth highlighting the consistency with that study.
L126 I would have not expected that injecting at the equator or 30S/N would make such a difference in terms of "average sizes", given that the coagulation time-scales (and condensational growth) are quite a bit smaller than the typical transit times in the BDC (6-12 months). Unless the tropical injections lead to aerosols that are more "tropically" confined. Have the authors verified this?
L145 This is a crucial point which deserves more attention, as many of the downstream effects depend on this. I think this deserves a more detailed discussion, especially with respect to the radiative contribution to this signal.
L162 I strongly recommend using the more up-to-date value from Banerjee, of 0.22 W/m2 (Fig.5), which is based on many more models than the 2 papers cited here.
L264-265 could this teleconnection be related to an ENSO-like response to SAI in these runs?
L290 I would expect aerosols to have actually the smallest impact on albedo over Antarctica, given the very reflective underlying surface (high surface albedo all year around). Hence, I do not think polar injections really lead to any "reduced (net) summer isolation" changes over Antarctica... and if anything, these changes in insolation would be reflected by the snow/ice of the surface. Can the authors comment on this?
Equations 1-2 I think the mathematical symbols do not match those mentioned in the main text.
L325 Can the authors briefly explain why the Hadley Cell weakens under SAI? What is the underlying mechanism?
L403-405 as mentioned in one of the main comments, I think this could be expanded and perhaps be analyzed more quantitatively. Similarly, the sizable tropospheric ozone changes should be discussed, as they might have important implications for tropospheric chemistry and/or air-quality.
L474-479 I think it might be good to be a bit more quantitative here, or at least give some information concerning the "direction" of the changes and if some coherent pattern emerges, concerning the advantages/disadvantages of each injection strategy (polar vs tropical).
REFERENCES
Banerjee, et al. (2019); Stratospheric water vapor: an important climate feedback, Climate Dynamics, DOI:10.1007/s00382-019-04721-4
Weisenstein, et al. (2022); A Model Intercomparison of Stratospheric Solar Geoengineering by Accumulation-Mode Sulfate Aerosols , Atmospheric Chemistry and Physics, DOI:10.5194/acp-22-2955-2022
Nowack et al. (2023); Response of stratospheric water vapour to warming constrained by satellite observations, Nature Geoscience, DOI:10.1038/s41561-023-01183-6
Citation: https://doi.org/10.5194/egusphere-2023-495-RC3 -
AC1: 'Authors response to all reviewers comments', Ewa Bednarz, 29 Aug 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-495/egusphere-2023-495-AC1-supplement.pdf
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Ewa M. Bednarz et al.
Ewa M. Bednarz et al.
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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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