Better constrained climate sensitivity when accounting for dataset dependency on pattern effect estimates

Modak, Angshuman; Mauritsen, Thorsten

doi:https://doi.org/10.5194/egusphere-2022-976

Preprints

https://doi.org/10.5194/egusphere-2022-976

Preprints

06 Oct 2022

| 06 Oct 2022

Better constrained climate sensitivity when accounting for dataset dependency on pattern effect estimates

Angshuman Modak and Thorsten Mauritsen

Abstract. Equilibrium climate sensitivity (ECS) constrained based on the instrumental record of the historical warming becomes coherent with other lines evidence when the dependence of radiative feedbacks on the pattern of surface temperature change (pattern effect) is incorporated. Pattern effect strength is usually estimated with atmosphere-only model simulations forced with observed historical sea-surface temperature (SST) and sea-ice change, and constant pre-industrial forcing. However, recent studies indicate that pattern effect estimates depend on the choice of SST boundary condition dataset, due to differences in the measurement sources and the techniques used to merge and construct them. Here, we systematically explore this dataset dependency by applying seven different observed SST datasets to the MPI-ESM1.2-LR model covering 1871–2017. We find that the pattern effect ranges from -0.01 ± 0.09 Wm^-2K^-1 to 0.42 ± 0.10 Wm^-2K^-1 (standard error), whereby the commonly used AMIPII dataset produces by far the largest estimate. When accounting for the generally weaker pattern effect in MPI-ESM1.2-LR compared to other models, as well as dataset dependency and inter-model spread, we obtain a combined pattern effect estimate of 0.30 Wm^-2K^-1 [-0.14 to 0.74 Wm^-2K^-1] (5–95 percentiles) and a resulting instrumental record ECS estimate of 3.1 K [1.7 to 9.2 K], which is slightly lower and better constrained than in previous studies.

Received: 23 Sep 2022 – Discussion started: 06 Oct 2022

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Journal article(s) based on this preprint

11 Jul 2023

Better-constrained climate sensitivity when accounting for dataset dependency on pattern effect estimates

Angshuman Modak and Thorsten Mauritsen

Atmos. Chem. Phys., 23, 7535–7549, https://doi.org/10.5194/acp-23-7535-2023,https://doi.org/10.5194/acp-23-7535-2023, 2023

Short summary

Angshuman Modak and Thorsten Mauritsen

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2022-976', Anonymous Referee #1, 03 Nov 2022

Please see attached comment.

Citation: https://doi.org/10.5194/egusphere-2022-976-RC1
- AC1: 'Reply on RC1', Angshuman Modak, 21 Mar 2023
  
  Please find the response to the comments in the attached file.
  
  Citation: https://doi.org/10.5194/egusphere-2022-976-AC1
RC2:
'Comment on egusphere-2022-976', Tim Andrews, 11 Nov 2022

Review of “Better constrained climate sensitivity when accounting for dataset dependency on pattern effect estimates” by Modak and Mauritsen.

This manuscript uses the MPI-ESM1.2-LR model to investigate the dependence of the pattern effect on underlying SST datasets used to force AGCMs with. By forcing the AGCM with 8 different reconstructions of historical SST patterns they find a substantial spread in the diagnosed pattern effect. By combining these results with published results on the inter-model spread the authors produce a new constraint on ECS from historical observations of past climate changes.

I found the manuscript well written and presented, and applaud the authors for tackling the important question of SST dataset dependency in methods that quantify the pattern effect. I think this manuscript will make a really useful contribution to the literature.

While there are a quite a comments here, all ought to be surmountable and are intended to be constructive.

Major Comments:

1. Presentation of SST patterns: the authors present a useful analysis of the geographical patterns of SST trends across the different datasets, and show how they differ (e.g. Fig 3), but are unable to find a geographical region or principal explanation for the variation in feedback seen in their experiments (e.g. Fig 4 and Section 4.3). This is done by calculating the SST trend as a function of time (i.e. K /century). However I wonder if this is the most appropriate method to tackle the question being investigated. I think a more appropriate calculation (of more relevance to feedback and patterns effects) is the SST change per global-mean dT (analogous to how the feedback is calculated as radiative response per global-mean dT), rather than a trend in time. This normalised pattern has the additional advantage of removing any global-mean dT differences between the datasets (as shown in Figure 2).

Hence I recommend the authors repeat the analysis of Figure 3, Figure 5, Figure 6 and discussion in Section 4.3 but this time calculating the SST patterns per global-mean dT (ideally calculated like the feedback parameter, i.e. with linear regression, in this case regressing dT(lat,lon,t) against global-mean dT(t)). It might turn out to make little difference, in which that is fine – but it would be good to know and comment on if so. On the other hand, as it ought to relate better to the feedback and pattern effect it might improve the relationships in Fig 6. If so, I would recommend simply dropping the K/century trends and replacing all this with K(lat,lon)/ were <> denotes global-mean in this case.

2. Calculation of lambda_4xCO2: the authors quote the 4xCO2 lambda for MPI-ESM1.2-LR as -1.48 +- 0.03 Wm-2 K-1 from regression in 150yrs of abrupt-4xCO2. However two other published papers (Andrews et al., 2022; https://doi.org/10.1029/2022JD036675) and Zelinka et al. (2020, GRL, updated to more models here https://zenodo.org/badge/latestdoi/259409949; see specifically https://github.com/mzelinka/cmip56_forcing_feedback_ecs/blob/master/CMIP6_ECS_ERF_fbks.txt) both report -1.39 Wm-2 K-1 for this model using similar linear regression on 150yrs of abrupt-4xCO2. This difference of ~0.1 Wm^-2 K^-1 will feed into all estimates of the pattern effect, and is a significant % of the reported pattern effect, and could even change the sign in some cases. I wonder if the difference is because this manuscript calculates the 4xCO2 changes relative to the “mean of the last 500 years of piControl simulations”, whereas both Andrews et al. and Zelinka et al. use the corresponding section of piControl and account for control drift? The authors ought to check, and I would recommend using the corresponding section and control drift method. If this is the cause of the difference, then this sensitivity to methodological choices ought to be discussed and acknowledged as a large source of uncertainty and potential bias in the pattern effect estimate.

3. Discussion of the limitations/assumptions in the approach: when coming up with a combined estimate of the pattern effect, I think the manuscript might be making an assumption that isn’t explicitly discussed. Specifically, if I’ve understood correctly, using the mean estimate and distribution from the observed-piForcing simulations is implicitly assuming that not only are all the various SST reconstructions independent (as the manuscript acknowledges, but clearly isn’t true), but also that they are equally plausible reconstructions of the truth. We do not know which dataset is ‘best’, right? The real world could have looked like any or none of them. If the authors have any thoughts on this, and how it could be taken forward – it might be useful to explicitly discuss. Related to this, I hesitate at statements such as line 223: “ … AMIP II dataset is an outlier suggesting that earlier studies may have overestimated the pattern effect…”. If gives the impression (perhaps not intended) that the AMIP II results are not trustworthy, whereas we have no idea if AMIP II SSTs are any less likely than any other SST reconstruction. They might be fine. For example, just as a counter argument, there are other situations where simulations with AMIPII SSTs have been shown to perform better than simulations with HadISST. As Andrews et al. (2022) discuss: “…Zhou et al. (2021; https://doi.org/10.1029/2022JD036675) showed that TOA radiative fluxes simulated by CAM5.3 correlated better with CERES observations when forced with AMIP II SSTs rather than HadISST SSTs, suggesting the results from may be more reliable…”. I do not think any major changes are required to address this comment, maybe just a simple change of wording or explicit acknowledgement of this point in the manuscript ought to suffice.

4. Structural (model) dependence on SST dataset sensitivity: the authors acknowledge that their results may depend on the model used (MPI-ESM1.2), and I appreciate the effort in trying to combine the model spread and adjusting the mean from previous intercomparisons to account for this. However the authors should update the analysis to use numbers from larger model intercomparison of Andrews et al. (2022), rather than the Andrews et al. (2018) study. For amip-piForcing the pattern effect across 14 models is 0.70 +- 0.47 Wm-2 K-1 in Andrews et al. (2022), compared to 0.64 +- 0.40 Wm-2 K-1 in Andrews et al. (2018). So the difference in the mean to ECHAM6.3 ought to be slightly larger than the authors used here, and the spread slightly larger too. But moreover, the method applied assumes that the ECHAM/MPI-ESM model is similarly different to other model in ALL the datasets as it is in the amip-piForcing simulation. But we know from Andrews et al. (2022) that this is not so, the ECHAM and MPI-ESM pattern effects under hadSST-piForcing are much further from the mean than under amip-piForcing. For example, the pattern effect for ECHAM6.3 and MPI-ESM2.1-LL is ~0.2 Wm^-2 K^-1 in hadSST-piForcing, whereas the model mean is 0.48 Wm-2 K-1 (Andrews et al. 2022; Table 2). I’m not sure exactly what the solution is here, but it seems adjusting for the mean difference to ECHAM6.3 of just 0.1 Wm-2 K-1 based on Andrews et al. (2018) is insufficient. It ought to be larger based on the larger ensemble of Andrews et al. (2022) and larger again given the hadSST-piForcing results where ECHAM6.3 pattern effect is further away from the rest of the models. At a minimum this potential structural dependence of the results on the ECHAM6.3/MPI-ESM model ought to be explicitly discussed.

Minor comments:

* Title: “Better constrained” does not particularly read well to me, how about “Improved constraints on..” ? But I’ll let the authors decide.

* Lines 45-49: I think it would be appropriate to include Andrews and Webb (2018; JCLIM, https://doi.org/10.1175/JCLI-D-17-0087.1) in the discussion of the mechanisms of the pattern effect.

* Lines 110: “-1.48 +- 0.03 Wm-2 K-1”, what is the level error being presented here and throughout, 5-95% or something?

* Line 116: “… account for land surface warming in the fixed-SST simulation to calculate the forcing..” – the literature has various ways of doing this, it would be good to briefly clarify which approach was used.

* Lines 126: “… pattern effect ranges from -0.01 +- 0.09 Wm-2 K-1…” – please clarify how the uncertainty is calculated here. Since it is difference between lambda_4xCO2 and lambda_piForcing have you added the errors in quadrature or something?

* Line 129: “… the dataset mean pattern effect is lower than both Andrews et al. (2018) and the values considered in IPCC AR6…” – this reads as if the value is outside the range. So it is not quite right that the value is lower than those “considered” by Andrews et al. and IPCC since it is within their uncertainty range.

* Line 200: “.. pattern effect estimate in MPI… averaged over all decades…” – I’m not exactly sure what this means, please clarify.

* Line 204: “bt” typo.

* Line 210-11: This sentence explains how the mean combined pattern effect estimate is arrived at, but it doesn’t explain how the uncertainty is combined between the pattern effect dataset dependence in this manuscript and the multi-model spread, i.e. how is the -0.14 to 0.74 Wm-2 K-1 on line 210 arrived at?

* Appendix: I found the Table summarising the datasets and Figure A1 showing the 30yr moving window lambda useful, and would recommend integrating them into the main text. One option would be to replace Figure 7 in the main text with Fig A1, since I find this figure more useful and interesting. However I do appreciate this is somewhat personal preference, so I do not demand it and leave the authors to choose.

I have signed the review.

Tim Andrews.

Citation: https://doi.org/10.5194/egusphere-2022-976-RC2
- AC2: 'Reply on RC2', Angshuman Modak, 21 Mar 2023
  
  Please find the response to the comments in the attached file.
  
  Citation: https://doi.org/10.5194/egusphere-2022-976-AC2

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2022-976', Anonymous Referee #1, 03 Nov 2022

Please see attached comment.

Citation: https://doi.org/10.5194/egusphere-2022-976-RC1
- AC1: 'Reply on RC1', Angshuman Modak, 21 Mar 2023
  
  Please find the response to the comments in the attached file.
  
  Citation: https://doi.org/10.5194/egusphere-2022-976-AC1
RC2:
'Comment on egusphere-2022-976', Tim Andrews, 11 Nov 2022

Review of “Better constrained climate sensitivity when accounting for dataset dependency on pattern effect estimates” by Modak and Mauritsen.

This manuscript uses the MPI-ESM1.2-LR model to investigate the dependence of the pattern effect on underlying SST datasets used to force AGCMs with. By forcing the AGCM with 8 different reconstructions of historical SST patterns they find a substantial spread in the diagnosed pattern effect. By combining these results with published results on the inter-model spread the authors produce a new constraint on ECS from historical observations of past climate changes.

I found the manuscript well written and presented, and applaud the authors for tackling the important question of SST dataset dependency in methods that quantify the pattern effect. I think this manuscript will make a really useful contribution to the literature.

While there are a quite a comments here, all ought to be surmountable and are intended to be constructive.

Major Comments:

1. Presentation of SST patterns: the authors present a useful analysis of the geographical patterns of SST trends across the different datasets, and show how they differ (e.g. Fig 3), but are unable to find a geographical region or principal explanation for the variation in feedback seen in their experiments (e.g. Fig 4 and Section 4.3). This is done by calculating the SST trend as a function of time (i.e. K /century). However I wonder if this is the most appropriate method to tackle the question being investigated. I think a more appropriate calculation (of more relevance to feedback and patterns effects) is the SST change per global-mean dT (analogous to how the feedback is calculated as radiative response per global-mean dT), rather than a trend in time. This normalised pattern has the additional advantage of removing any global-mean dT differences between the datasets (as shown in Figure 2).

Hence I recommend the authors repeat the analysis of Figure 3, Figure 5, Figure 6 and discussion in Section 4.3 but this time calculating the SST patterns per global-mean dT (ideally calculated like the feedback parameter, i.e. with linear regression, in this case regressing dT(lat,lon,t) against global-mean dT(t)). It might turn out to make little difference, in which that is fine – but it would be good to know and comment on if so. On the other hand, as it ought to relate better to the feedback and pattern effect it might improve the relationships in Fig 6. If so, I would recommend simply dropping the K/century trends and replacing all this with K(lat,lon)/ were <> denotes global-mean in this case.

2. Calculation of lambda_4xCO2: the authors quote the 4xCO2 lambda for MPI-ESM1.2-LR as -1.48 +- 0.03 Wm-2 K-1 from regression in 150yrs of abrupt-4xCO2. However two other published papers (Andrews et al., 2022; https://doi.org/10.1029/2022JD036675) and Zelinka et al. (2020, GRL, updated to more models here https://zenodo.org/badge/latestdoi/259409949; see specifically https://github.com/mzelinka/cmip56_forcing_feedback_ecs/blob/master/CMIP6_ECS_ERF_fbks.txt) both report -1.39 Wm-2 K-1 for this model using similar linear regression on 150yrs of abrupt-4xCO2. This difference of ~0.1 Wm^-2 K^-1 will feed into all estimates of the pattern effect, and is a significant % of the reported pattern effect, and could even change the sign in some cases. I wonder if the difference is because this manuscript calculates the 4xCO2 changes relative to the “mean of the last 500 years of piControl simulations”, whereas both Andrews et al. and Zelinka et al. use the corresponding section of piControl and account for control drift? The authors ought to check, and I would recommend using the corresponding section and control drift method. If this is the cause of the difference, then this sensitivity to methodological choices ought to be discussed and acknowledged as a large source of uncertainty and potential bias in the pattern effect estimate.

3. Discussion of the limitations/assumptions in the approach: when coming up with a combined estimate of the pattern effect, I think the manuscript might be making an assumption that isn’t explicitly discussed. Specifically, if I’ve understood correctly, using the mean estimate and distribution from the observed-piForcing simulations is implicitly assuming that not only are all the various SST reconstructions independent (as the manuscript acknowledges, but clearly isn’t true), but also that they are equally plausible reconstructions of the truth. We do not know which dataset is ‘best’, right? The real world could have looked like any or none of them. If the authors have any thoughts on this, and how it could be taken forward – it might be useful to explicitly discuss. Related to this, I hesitate at statements such as line 223: “ … AMIP II dataset is an outlier suggesting that earlier studies may have overestimated the pattern effect…”. If gives the impression (perhaps not intended) that the AMIP II results are not trustworthy, whereas we have no idea if AMIP II SSTs are any less likely than any other SST reconstruction. They might be fine. For example, just as a counter argument, there are other situations where simulations with AMIPII SSTs have been shown to perform better than simulations with HadISST. As Andrews et al. (2022) discuss: “…Zhou et al. (2021; https://doi.org/10.1029/2022JD036675) showed that TOA radiative fluxes simulated by CAM5.3 correlated better with CERES observations when forced with AMIP II SSTs rather than HadISST SSTs, suggesting the results from may be more reliable…”. I do not think any major changes are required to address this comment, maybe just a simple change of wording or explicit acknowledgement of this point in the manuscript ought to suffice.

4. Structural (model) dependence on SST dataset sensitivity: the authors acknowledge that their results may depend on the model used (MPI-ESM1.2), and I appreciate the effort in trying to combine the model spread and adjusting the mean from previous intercomparisons to account for this. However the authors should update the analysis to use numbers from larger model intercomparison of Andrews et al. (2022), rather than the Andrews et al. (2018) study. For amip-piForcing the pattern effect across 14 models is 0.70 +- 0.47 Wm-2 K-1 in Andrews et al. (2022), compared to 0.64 +- 0.40 Wm-2 K-1 in Andrews et al. (2018). So the difference in the mean to ECHAM6.3 ought to be slightly larger than the authors used here, and the spread slightly larger too. But moreover, the method applied assumes that the ECHAM/MPI-ESM model is similarly different to other model in ALL the datasets as it is in the amip-piForcing simulation. But we know from Andrews et al. (2022) that this is not so, the ECHAM and MPI-ESM pattern effects under hadSST-piForcing are much further from the mean than under amip-piForcing. For example, the pattern effect for ECHAM6.3 and MPI-ESM2.1-LL is ~0.2 Wm^-2 K^-1 in hadSST-piForcing, whereas the model mean is 0.48 Wm-2 K-1 (Andrews et al. 2022; Table 2). I’m not sure exactly what the solution is here, but it seems adjusting for the mean difference to ECHAM6.3 of just 0.1 Wm-2 K-1 based on Andrews et al. (2018) is insufficient. It ought to be larger based on the larger ensemble of Andrews et al. (2022) and larger again given the hadSST-piForcing results where ECHAM6.3 pattern effect is further away from the rest of the models. At a minimum this potential structural dependence of the results on the ECHAM6.3/MPI-ESM model ought to be explicitly discussed.

Minor comments:

* Title: “Better constrained” does not particularly read well to me, how about “Improved constraints on..” ? But I’ll let the authors decide.

* Lines 45-49: I think it would be appropriate to include Andrews and Webb (2018; JCLIM, https://doi.org/10.1175/JCLI-D-17-0087.1) in the discussion of the mechanisms of the pattern effect.

* Lines 110: “-1.48 +- 0.03 Wm-2 K-1”, what is the level error being presented here and throughout, 5-95% or something?

* Line 116: “… account for land surface warming in the fixed-SST simulation to calculate the forcing..” – the literature has various ways of doing this, it would be good to briefly clarify which approach was used.

* Lines 126: “… pattern effect ranges from -0.01 +- 0.09 Wm-2 K-1…” – please clarify how the uncertainty is calculated here. Since it is difference between lambda_4xCO2 and lambda_piForcing have you added the errors in quadrature or something?

* Line 129: “… the dataset mean pattern effect is lower than both Andrews et al. (2018) and the values considered in IPCC AR6…” – this reads as if the value is outside the range. So it is not quite right that the value is lower than those “considered” by Andrews et al. and IPCC since it is within their uncertainty range.

* Line 200: “.. pattern effect estimate in MPI… averaged over all decades…” – I’m not exactly sure what this means, please clarify.

* Line 204: “bt” typo.

* Line 210-11: This sentence explains how the mean combined pattern effect estimate is arrived at, but it doesn’t explain how the uncertainty is combined between the pattern effect dataset dependence in this manuscript and the multi-model spread, i.e. how is the -0.14 to 0.74 Wm-2 K-1 on line 210 arrived at?

* Appendix: I found the Table summarising the datasets and Figure A1 showing the 30yr moving window lambda useful, and would recommend integrating them into the main text. One option would be to replace Figure 7 in the main text with Fig A1, since I find this figure more useful and interesting. However I do appreciate this is somewhat personal preference, so I do not demand it and leave the authors to choose.

I have signed the review.

Tim Andrews.

Citation: https://doi.org/10.5194/egusphere-2022-976-RC2
- AC2: 'Reply on RC2', Angshuman Modak, 21 Mar 2023
  
  Please find the response to the comments in the attached file.
  
  Citation: https://doi.org/10.5194/egusphere-2022-976-AC2

Peer review completion

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

AR by Angshuman Modak on behalf of the Authors (26 Mar 2023) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (27 Mar 2023) by Matthew Lebsock

RR by Anonymous Referee #1 (10 Apr 2023)

Suggestions for revision or reasons for rejection

The authors have addressed my concerns and I support publication. The new version includes the needed details and justifications for the selected methods, and the results progress our understanding of the pattern effect and its relationship to ECS.

Minor language comments are at the end.

SUMMARY OF RESPONSE
In my first review, the comment on Seager et al. was phrased poorly, but the new Sec. 4.1 text fully addresses it.

Reviewer 2’s suggestion of regressing local T versus global T is a better approach than the nonlinear suggestion I made. I’m glad the authors adopted it for Figs 3 and 5.

I want to thank the authors in particular for the new appendix figures. They are all useful for judging the analysis assumptions (A1,A4) or making it easy to compare with other research (A2).

GRAMMAR/TEXT CHANGES
Sec. 5 could repeat that the post-1970s results are more negative in the mean, they don’t just show smaller spread. Part of the community cares more about the modern period and a reminder could be helpful for them.

Minor grammar should be caught by a final check and copy editing but pure grammar corrections could sometimes change the meaning, such as by picking singular when I believe you intend plural. Please check:

L48—50: “Whereas a colder tropical eastern Pacific and Southern Ocean, and a stronger tropical western Pacific warming is observed over the historical period, Atmosphere-Ocean General Circulation Model (AOGCM) simulates a long term climate response”
--> “Atmosphere-Ocean General Circulation Models (AOGCMs) simulate…”?

L87: "The seven different observed-reconstructed SST datasets applied as boundary condition are HadISST1"
--> “boundary conditions” plural?

L192 “To further investigate if any of the dataset bias the correlation,”
--> “any of the datasets” plural?

L202: “We find that the feedback from…”
--> I think you mean plural “feedbacks” since Fig. 7 shows the different components?

Some bits are clunky and could perhaps be rephrased:

L106—108: “The temperature anomaly in ERSSTv5 forced observedSST-piForcing simulation is lower compared to others, consistent with Fueglistaler and Silvers (2021), is related to ship SST bias corrections made to the temperature during the 1880-1940s and 1950-1960s (Huang et al., 2017)”
--> Consider splitting into two sentences or “,.. this is related to ship SST…”

L123—124: “we conduct a simulation in fixed-SST configuration, but evolving historical forcings to evaluate the effective radiative forcing (F) from 1851-2014.”
--> Consider after comma: “but with evolving historical forcings”

L178—179: “For instance, over the IPWP region, ERSSTv5 dataset shows this region is warming at the same rate as the globe but is true over the EEP region as well however with larger uncertainty.”
--> I think this sentence intends to say “this is also true over the EEP region”? Please check, and also convert to "the ERSSTv5 dataset" (added "the").

L231—233: “:Here, we apply their pattern effect based on AMIPII only as the number of models are larger which performed the amip-piForcing simulation.”
--> Suggest rephrasing the end bit to something like: “…based on AMIPII only as they had amip-piForcing simulations from more models.”

Hide

RR by Tim Andrews (21 Apr 2023)

Suggestions for revision or reasons for rejection

Review of “Better constrained climate sensitivity when accounting for dataset dependency on pattern effect estimates” by Modak and Mauritsen, revision #1.

The authors have done a good job addressing reviewer comments. I have some final suggestions for the authors to consider before publication:

1) Figure 5 and 6 and related discussion: I find it interesting that no strong correlation is found between the different SST pattern regions and the pattern effect. However I think the three most convincing SST metrics for explaining pattern effect variations haven’t being analysed, and these are based on relative differences rather that single regions. For example Zhou et al. (2016) use the difference in warming between tropical ascending and descending regions, Andrews and Webb (2018) use the difference in south east-Pacific minus west-Pacific SST pattern and Fueglistaler and Silvers (2021) use SST# defined as the warming of the warmest 30% SSTs relative to the tropical mean. I do not demand the authors include these, but I do think it would be a useful addition (at least one of them) as it might be the reason for the poor correlation.

2) Line 154-155 and Fig A2: When comparing the total pattern effect from the CAM5.3 Green’s function results (Lewis and Mauriten, 2021) to the MPI-ESM simulations it is simply concluded that they are “substantially different”, but I’m kind of left wondering “and so what”? What exactly is being said with this conclusion? At first I thought it implied an issue for the Green Functions method, but it’s a different models Green’s function and we know models simulate different pattern effects, so it is not entirely expected that the CAM5.3 green function results would give different estimates to the MPI simulations? So I guess I’m wondering what useful information is being drawn from this comparison? For example are the results substantially different in some interesting or unexpected way? Are the differences within the model-dependent uncertainty allowed for later on in the manuscript? If not, that is interesting and would suggest the uncertainty allowed for is not sufficient. I’m just throwing out ideas, but I think a couple more sentences are needed to explain how the authors interpret the “substantially different” result.

3) Figure 3: I got confused here. The pattern in dT(lat,lon) per dT ought to have a global-mean of unity, hence all the difference panels should have a global-mean of zero, right? But it doesn’t look that way. To eye some of them are strongly blue (negative). Is this just a visual thing and the global-mean really is zero? Or, is it a land effect, which is masked out here? Or something else?

Minor comments:

4) Line 1 Abstract: I think this opening sentence requires a qualifier that it is referring to the best estimate ECS, since the range/uncertainty in ECS is still bigger than that deduced from other lines of evidence (e.g. see Sherwood et al., 2020).

5) Line 245-247: “Figure A4 shows…..” – I did not follow this sentence. Please try again.

6) Line 280: append “… to address this outstanding concern”?

I have signed the review.
Tim Andrews.

Hide

ED: Publish subject to minor revisions (review by editor) (25 Apr 2023) by Matthew Lebsock

AR by Angshuman Modak on behalf of the Authors (24 May 2023) Author's response Author's tracked changes Manuscript

ED: Publish as is (30 May 2023) by Matthew Lebsock

AR by Angshuman Modak on behalf of the Authors (02 Jun 2023) Manuscript

Journal article(s) based on this preprint

11 Jul 2023

Better-constrained climate sensitivity when accounting for dataset dependency on pattern effect estimates

Angshuman Modak and Thorsten Mauritsen

Atmos. Chem. Phys., 23, 7535–7549, https://doi.org/10.5194/acp-23-7535-2023,https://doi.org/10.5194/acp-23-7535-2023, 2023

Short summary

Angshuman Modak and Thorsten Mauritsen

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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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We estimate Equilibrium Climate Sensitivity, a metric to constrain future global warming based on the instrumental temperature record including the corrections for the effects on radiation balance caused by the pattern of surface temperature change- pattern effect. We address the uncertainty caused by the underlying sea surface temperature datasets used in the estimates of pattern effect. This together with the intermodel spread lifts the corresponding IPCC AR6 estimate to 3.1 K [1.7 to 9.2 K].


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