The TIPMIP Earth system model experiment protocol: phase 1

Jones, ﻿Colin; Bossert, Isaline; Dennis, Donovan P.; Jeffery, Hazel; Jones, Chris D.; Koenigk, Torben; Loriani, Sina; Sanderson, Benjamin; Séférian, Roland; Wyser, Klaus; Yang, Shuting; Abe, Manabu; Bathiany, Sebastian; Braconnot, Pascale; Brovkin, Victor; Burger, Friedrich A.; Cadule, Patrica; Castruccio, Frederic S.; Danabasoglu, Gokhan; Dittus, Andrea; Donges, Jonathan F.; Fröb, Friederike; Frölicher, Thomas; Georgievski, Goran; Guo, Chuncheng; Hu, Aixue; Lawrence, Peter; Lerner, Paul; Licón-Saláiz, José; Otto-Bliesner, Bette; Romanou, Anastasia; Shevliakova, Elena; Silvy, Yona; Swingedouw, Didier; Tjiputra, Jerry; Walton, Jeremy; Wiltshire, Andy; Winkelmann, Ricarda; Wood, Richard; Yokohata, Tokuta; Ziehn, Tilo

doi:10.5194/egusphere-2025-3604

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https://doi.org/10.5194/egusphere-2025-3604

Preprints

23 Sep 2025

| 23 Sep 2025

The TIPMIP Earth system model experiment protocol: phase 1

Colin Jones, Isaline Bossert, Donovan P. Dennis, Hazel Jeffery, Chris D. Jones, Torben Koenigk, Sina Loriani, Benjamin Sanderson, Roland Séférian, Klaus Wyser, Shuting Yang, Manabu Abe, Sebastian Bathiany, Pascale Braconnot, Victor Brovkin, Friedrich A. Burger, Patrica Cadule, Frederic S. Castruccio, Gokhan Danabasoglu, Andrea Dittus, Jonathan F. Donges, Friederike Fröb, Thomas Frölicher, Goran Georgievski, Chuncheng Guo, Aixue Hu, Peter Lawrence, Paul Lerner, José Licón-Saláiz, Bette Otto-Bliesner, Anastasia Romanou, Elena Shevliakova, Yona Silvy, Didier Swingedouw, Jerry Tjiputra, Jeremy Walton, Andy Wiltshire, Ricarda Winkelmann, Richard Wood, Tokuta Yokohata, and Tilo Ziehn

Abstract. We describe a new Earth system model (ESM) experiment protocol, as part of the international Tipping Points Modelling Intercomparison Project (TIPMIP) project. We propose this as a protocol for the Coupled Model Intercomparison Project 7 (CMIP7). The protocol requires ESMs to run in CO₂-emission mode, with atmospheric CO₂ a predicted variable. Forcing for the protocol consists solely of a constant emission of CO₂, based on each model’s transient climate response to cumulative emissions of carbon dioxide (TCRE) value, to give a common global mean warming rate of 2 °C per century. This positive emission experiment is started from the pre-industrial state of a given model. When the ramp-up run first exceeds a specified level of global warming (2 °C and 4 °C) relative to the model’s pre-industrial global mean surface air temperature (GMSAT), CO₂ emissions are set to zero and the positive emission run is branched into a zero-emission run. The zero-emission runs continue for 300 years. At 50 years into each zero-emission run, CO₂emissions are set to the negative of the positive emission rate and the model run until GMSAT cools below the original pre-industrial value. Additionally, when the negative emission run started from global warming level (GWL) = 4 °C first drops below GWL = 2 °C, a zero-emission run is branched off this, completing the set of experiments. Using this protocol, we are able to control the rate of global warming and cooling across participating models. TIPMIP experiments will support a range of analyses, including; an assessment of abrupt/rapid Earth system change, the long-term response to zero CO₂ emissions, the response to negative CO₂ emissions, the efficacy of negative emissions in driving cooling, and the reversibility of Earth system change under a pathway of positive (warming), zero (stabilization), and negative (cooling) CO₂ emissions.

How to cite. Jones, ., Bossert, I., Dennis, D. P., Jeffery, H., Jones, C. D., Koenigk, T., Loriani, S., Sanderson, B., Séférian, R., Wyser, K., Yang, S., Abe, M., Bathiany, S., Braconnot, P., Brovkin, V., Burger, F. A., Cadule, P., Castruccio, F. S., Danabasoglu, G., Dittus, A., Donges, J. F., Fröb, F., Frölicher, T., Georgievski, G., Guo, C., Hu, A., Lawrence, P., Lerner, P., Licón-Saláiz, J., Otto-Bliesner, B., Romanou, A., Shevliakova, E., Silvy, Y., Swingedouw, D., Tjiputra, J., Walton, J., Wiltshire, A., Winkelmann, R., Wood, R., Yokohata, T., and Ziehn, T.: The TIPMIP Earth system model experiment protocol: phase 1, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2025-3604, 2025.

Received: 25 Jul 2025 – Discussion started: 23 Sep 2025

Competing interests: At least one of the (co-)authors is a member of the editorial board of Geoscientific Model Development. 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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Journal article(s) based on this preprint

29 Jul 2026

The TIPMIP Earth system model experiment protocol: phase 1

Colin Jones, Isaline Bossert, Donovan P. Dennis, Hazel A. Jeffery, Chris D. Jones, Torben Koenigk, Sina Loriani, Benjamin Sanderson, Roland Séférian, Klaus Wyser, Shuting Yang, Manabu Abe, Sebastian Bathiany, Pascale Braconnot, Victor Brovkin, Friedrich A. Burger, Patrica Cadule, Frederic S. Castruccio, Gokhan Danabasoglu, Andrea Dittus, Jonathan F. Donges, Friederike Fröb, Thomas L. Frölicher, Goran Georgievski, Chuncheng Guo, Aixue Hu, Peter Lawrence, Paul Lerner, José Licón-Saláiz, Bette Otto-Bliesner, Anastasia Romanou, Elena Shevliakova, Yona Silvy, Didier Swingedouw, Jerry Tjiputra, Jeremy Walton, Andy Wiltshire, Ricarda Winkelmann, Richard Wood, Tokuta Yokohata, and Tilo Ziehn

Geosci. Model Dev., 19, 6941–6966, https://doi.org/10.5194/gmd-19-6941-2026,https://doi.org/10.5194/gmd-19-6941-2026, 2026

Short summary

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-3604', Anonymous Referee #1, 12 Nov 2025
Thank you for inviting me to review the paper: “The TIPMIP Earth system model experiment protocol: phase 1” by Jones et al.

With climate science under intense scrutiny, papers like these are extremely important. Such a manuscript demonstrates that the research community is open and transparent.

The role of description or methodological papers is just that, to set out proposed frameworks for simulations. And in that sense, the manuscript does well and fulfils the intended purpose. However, to provide some context and reasoning, it might be worthwhile to add a couple of sentences (at the beginning and in the discussion) explaining why there is such a rapidly growing interest in “overshoot” forcings. I would argue that there are four main reasons for modelling such scenarios.

Society is likely to find many aspects of substantial climate change difficult to manage, leading to an eventual strong push to lower global temperatures.

Society has set thresholds for global warming that it aims to stabilise at, notably around 1.5 or 2.0 degrees above pre-industrial levels. Since we are close to these benchmarks, achieving them will most likely involve a period of “overshoot”.

Studying the cooling period really helps understand the amount of negative emissions needed, raising challenges for others to question their feasibility (e.g., levels of geoengineering).

If climate change triggers regional tipping points, then the second less-discussed issue is that of hysteresis. By definition, this might become an issue with “overshoot” scenarios, and so things are not reset for falling temperatures. The TIPMIP projections described might be useful to drive offline impact models believed to potentially exhibit hysteresis.

Although the paper is generally clear throughout, my main concern is that the key assumptions underlying how the scenarios are constructed are spread over a very large number of paragraphs. Is there a way to present everything together in an easier form. Possibilities could include an extra “text box”, a schematic, or some other display item. It took me quite some time to fully grasp the assumptions, as I had to jump back and forth within the manuscript. Here are the points that should really stand out:

At the very top level, we set temperature profiles we wish our ESMs to follow. That includes ramp up, no warming, ramp down.

We cannot invert ESMs, to get out precisely the emissions to track these profiles. Hence, we need to estimate the emissions that get as close as possible to temperature profiles.

For the upward trajectory, we use the TCRE of each model.

For the stable part, we assume no net emissions into the atmosphere (ZEC). And maybe mention this clearly, because for most models, CO2 drawdown balances additional warming towards equilibrium. Hence, why temperatures are stable.

The ramp down assumes a negative value of emissions (i.e. “-X”) that were used in the ramp up.

I realise that the manuscript is mainly a presentation of protocol, but it might be worth adding a little more in the Discussion that [A] the analysis reconfirms with these ESMs that the ZEC is appropriate for Net Zero aims – getting net emissions down to zero does stabilise the climate. And [B], in the Allen et al. (2009) paper on TCRE, I think there was always an assumption that this was well-defined for positive emissions, specifically increasing temperatures. It is of much interest that the reversal also seems to give a good near-linear correspondence between -X and d_Temp / d_time, and of a similar ratio.

Small things

The paper can appear overly heavy on abbreviations (for instance, the left-hand column of Table 1). It may be worth reiterating at a few key points in the manuscript that these names are actually those associated with the metadata of the simulations themselves. Hence, their importance. One possibility is to put them in quotes, maybe?.

Please reiterate towards the end that the analysis is highly idealised. That is fine, and avoids more convoluted emissions scenarios (e.g. to match some sort of upturned parabola in global temperature). However, in reality, unless there is a massive global economic collapse, an instantaneous switch from business-as-usual to ZEC is unlikely.

Given the interest in Net Zero, then perhaps mention more strongly that this is often regarded as corresponding to the ZEC runs. Having these key buzzwords of phrases will enable more people to find this manuscript, and so appreciate that ESMs are now being configured for Overshoot and/or periods of Net Zero.

I am happy to see any further manuscript version.
Citation: https://doi.org/10.5194/egusphere-2025-3604-RC1
- AC1: 'Reply on RC1', Hazel Jeffery, 09 Feb 2026
  
  Please find attached the responses to reviewer comments.
  
  Citation: https://doi.org/10.5194/egusphere-2025-3604-AC1
RC2:
'Comment on egusphere-2025-3604', Anonymous Referee #2, 17 Dec 2025

Jones et al. propose a new Earth system modeling protocol for CMIP7. The protocol includes a set of CO2 emission-driven simulations that reach specified global warming levels (GWLs) with a common global warming rate. When the specified global warming level is reached, CO2 emissions are set to zero and, after 50 years, to the negative of the positive emissions rate and the experiments is continued until a model’s global mean temperature drops below the pre-industrial value. The paper justifies the modeling protocol, lays out the science questions the protocol is set to answer and provides a proof of concept with an initial set of ESM model simulations.
The proposed framework is compelling for its simplicity but also has disadvantages, which stem from the desire to answer a range of different science questions that, ideally, would entail different modeling protocols. While simulated ESM GMSAT trajectories have a common warming rate until specified GWLs are reached, these trajectories can be expected to diverge after this point due to a range of different GMSAT responses to zero emissions (as shown in Fig. 3b). The ramp-down GMSAT trajectories can be expected to diverge even more due to models’ different GMSAT responses to net negative CO2 emissions. Therefore, the only factor that is effectively the same across simulated GMSAT trajectories is the initial warming rate and the global warming level reached. The authors justify this approach with different science questions they intend to address: exploring tipping point behavior at different global warming levels, while quantifying the zero emissions commitment at different GWLs and reversibility in response to net negative emissions. However, the tipping point science question would be better answered with a set common GMSAT trajectories (as could be obtained with the AERAMIP protocol), which would allow to clearly explore the risk of tipping as a function of GMSAT. The ZEC and reversibility questions, on the other hand, would be better explored in a framework with common emission trajectories that entail the same rate of CO2 emissions, the same level of cumulative emissions and the same rate of net negative emissions (as in flat10mip; Sanderson et al., 2025). With the current design, it is challenging to attribute differences in GMSAT response to model differences as opposed to differences in emissions trajectory. It would be helpful if the authors could discuss why they chose not to use the AEAMIP protocol to derive a set of common GMSAT trajectories for the tipping point exploration, acknowledge the challenges of the current modeling protocol for the attribution of differences in results, and discuss how they are planning to isolate the effect of different GMSAT and emission trajectory characteristics on outcomes.
The paper is generally well written but some sections feel lengthy and somewhat repetitive and could be streamlined. Also, citations often seem to be limited to the authors’ own work and should include a wider body of literature on TCRE, ZEC and reversibility (see specific suggestions below).
Specific comments:
(numbers refer to line numbers)
76: ‘net’ negative CO2 emissions (here and elsewhere).

92: It would be helpful if the Introduction could include an overview of the MIP landscape to provide context for the proposed modeling protocol. You do this in section 3.4 but part of it should be in the Introduction as well.

165: Include reference to Matthews et al, 2009 (first paper to introduce TCRE).

166: ‘equation 21’: describe or reproduce equation in the text.

185: what is year ‘A’?

187-188: there will still be a strong transient after 50 years of zero emissions, which will influence the downward GMSAT trajectory.

188-189: the ramp down GMSAT trajectory will likely not mirror the ramp up trajectory.

364: except that GMSAT trajectories under the TIPMIP protocol will not stabilize exactly.

366: Keller et al.: clarify that reference is for CMIP6 CDR-MIP.

379: why ’risk’?

420: Section 4 feels long and would benefit form more concise writing.

433: Refer to 2025 tipping point report.

635: meaning of ‘connectivity’ unclear.

658: ‘net’ negative CO2 emissions

691: Include additional references on sink reversal under net negative emissions e.g. Tokarska et al (2015) https://iopscience.iop.org/article/10.1088/1748-9326/10/9/094013

695: This paragraph does not seem to fit here.

705: Move paragraph to section 4.2?

Citation: https://doi.org/10.5194/egusphere-2025-3604-RC2
- AC2: 'Reply on RC2', Hazel Jeffery, 09 Feb 2026
  
  Please find attached the responses to reviewer #2 comments.
  
  Citation: https://doi.org/10.5194/egusphere-2025-3604-AC2

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-3604', Anonymous Referee #1, 12 Nov 2025
Thank you for inviting me to review the paper: “The TIPMIP Earth system model experiment protocol: phase 1” by Jones et al.

With climate science under intense scrutiny, papers like these are extremely important. Such a manuscript demonstrates that the research community is open and transparent.

The role of description or methodological papers is just that, to set out proposed frameworks for simulations. And in that sense, the manuscript does well and fulfils the intended purpose. However, to provide some context and reasoning, it might be worthwhile to add a couple of sentences (at the beginning and in the discussion) explaining why there is such a rapidly growing interest in “overshoot” forcings. I would argue that there are four main reasons for modelling such scenarios.

Society is likely to find many aspects of substantial climate change difficult to manage, leading to an eventual strong push to lower global temperatures.

Society has set thresholds for global warming that it aims to stabilise at, notably around 1.5 or 2.0 degrees above pre-industrial levels. Since we are close to these benchmarks, achieving them will most likely involve a period of “overshoot”.

Studying the cooling period really helps understand the amount of negative emissions needed, raising challenges for others to question their feasibility (e.g., levels of geoengineering).

If climate change triggers regional tipping points, then the second less-discussed issue is that of hysteresis. By definition, this might become an issue with “overshoot” scenarios, and so things are not reset for falling temperatures. The TIPMIP projections described might be useful to drive offline impact models believed to potentially exhibit hysteresis.

Although the paper is generally clear throughout, my main concern is that the key assumptions underlying how the scenarios are constructed are spread over a very large number of paragraphs. Is there a way to present everything together in an easier form. Possibilities could include an extra “text box”, a schematic, or some other display item. It took me quite some time to fully grasp the assumptions, as I had to jump back and forth within the manuscript. Here are the points that should really stand out:

At the very top level, we set temperature profiles we wish our ESMs to follow. That includes ramp up, no warming, ramp down.

We cannot invert ESMs, to get out precisely the emissions to track these profiles. Hence, we need to estimate the emissions that get as close as possible to temperature profiles.

For the upward trajectory, we use the TCRE of each model.

For the stable part, we assume no net emissions into the atmosphere (ZEC). And maybe mention this clearly, because for most models, CO2 drawdown balances additional warming towards equilibrium. Hence, why temperatures are stable.

The ramp down assumes a negative value of emissions (i.e. “-X”) that were used in the ramp up.

I realise that the manuscript is mainly a presentation of protocol, but it might be worth adding a little more in the Discussion that [A] the analysis reconfirms with these ESMs that the ZEC is appropriate for Net Zero aims – getting net emissions down to zero does stabilise the climate. And [B], in the Allen et al. (2009) paper on TCRE, I think there was always an assumption that this was well-defined for positive emissions, specifically increasing temperatures. It is of much interest that the reversal also seems to give a good near-linear correspondence between -X and d_Temp / d_time, and of a similar ratio.

Small things

The paper can appear overly heavy on abbreviations (for instance, the left-hand column of Table 1). It may be worth reiterating at a few key points in the manuscript that these names are actually those associated with the metadata of the simulations themselves. Hence, their importance. One possibility is to put them in quotes, maybe?.

Please reiterate towards the end that the analysis is highly idealised. That is fine, and avoids more convoluted emissions scenarios (e.g. to match some sort of upturned parabola in global temperature). However, in reality, unless there is a massive global economic collapse, an instantaneous switch from business-as-usual to ZEC is unlikely.

Given the interest in Net Zero, then perhaps mention more strongly that this is often regarded as corresponding to the ZEC runs. Having these key buzzwords of phrases will enable more people to find this manuscript, and so appreciate that ESMs are now being configured for Overshoot and/or periods of Net Zero.

I am happy to see any further manuscript version.
Citation: https://doi.org/10.5194/egusphere-2025-3604-RC1
- AC1: 'Reply on RC1', Hazel Jeffery, 09 Feb 2026
  
  Please find attached the responses to reviewer comments.
  
  Citation: https://doi.org/10.5194/egusphere-2025-3604-AC1
RC2:
'Comment on egusphere-2025-3604', Anonymous Referee #2, 17 Dec 2025

Jones et al. propose a new Earth system modeling protocol for CMIP7. The protocol includes a set of CO2 emission-driven simulations that reach specified global warming levels (GWLs) with a common global warming rate. When the specified global warming level is reached, CO2 emissions are set to zero and, after 50 years, to the negative of the positive emissions rate and the experiments is continued until a model’s global mean temperature drops below the pre-industrial value. The paper justifies the modeling protocol, lays out the science questions the protocol is set to answer and provides a proof of concept with an initial set of ESM model simulations.
The proposed framework is compelling for its simplicity but also has disadvantages, which stem from the desire to answer a range of different science questions that, ideally, would entail different modeling protocols. While simulated ESM GMSAT trajectories have a common warming rate until specified GWLs are reached, these trajectories can be expected to diverge after this point due to a range of different GMSAT responses to zero emissions (as shown in Fig. 3b). The ramp-down GMSAT trajectories can be expected to diverge even more due to models’ different GMSAT responses to net negative CO2 emissions. Therefore, the only factor that is effectively the same across simulated GMSAT trajectories is the initial warming rate and the global warming level reached. The authors justify this approach with different science questions they intend to address: exploring tipping point behavior at different global warming levels, while quantifying the zero emissions commitment at different GWLs and reversibility in response to net negative emissions. However, the tipping point science question would be better answered with a set common GMSAT trajectories (as could be obtained with the AERAMIP protocol), which would allow to clearly explore the risk of tipping as a function of GMSAT. The ZEC and reversibility questions, on the other hand, would be better explored in a framework with common emission trajectories that entail the same rate of CO2 emissions, the same level of cumulative emissions and the same rate of net negative emissions (as in flat10mip; Sanderson et al., 2025). With the current design, it is challenging to attribute differences in GMSAT response to model differences as opposed to differences in emissions trajectory. It would be helpful if the authors could discuss why they chose not to use the AEAMIP protocol to derive a set of common GMSAT trajectories for the tipping point exploration, acknowledge the challenges of the current modeling protocol for the attribution of differences in results, and discuss how they are planning to isolate the effect of different GMSAT and emission trajectory characteristics on outcomes.
The paper is generally well written but some sections feel lengthy and somewhat repetitive and could be streamlined. Also, citations often seem to be limited to the authors’ own work and should include a wider body of literature on TCRE, ZEC and reversibility (see specific suggestions below).
Specific comments:
(numbers refer to line numbers)
76: ‘net’ negative CO2 emissions (here and elsewhere).

92: It would be helpful if the Introduction could include an overview of the MIP landscape to provide context for the proposed modeling protocol. You do this in section 3.4 but part of it should be in the Introduction as well.

165: Include reference to Matthews et al, 2009 (first paper to introduce TCRE).

166: ‘equation 21’: describe or reproduce equation in the text.

185: what is year ‘A’?

187-188: there will still be a strong transient after 50 years of zero emissions, which will influence the downward GMSAT trajectory.

188-189: the ramp down GMSAT trajectory will likely not mirror the ramp up trajectory.

364: except that GMSAT trajectories under the TIPMIP protocol will not stabilize exactly.

366: Keller et al.: clarify that reference is for CMIP6 CDR-MIP.

379: why ’risk’?

420: Section 4 feels long and would benefit form more concise writing.

433: Refer to 2025 tipping point report.

635: meaning of ‘connectivity’ unclear.

658: ‘net’ negative CO2 emissions

691: Include additional references on sink reversal under net negative emissions e.g. Tokarska et al (2015) https://iopscience.iop.org/article/10.1088/1748-9326/10/9/094013

695: This paragraph does not seem to fit here.

705: Move paragraph to section 4.2?

Citation: https://doi.org/10.5194/egusphere-2025-3604-RC2
- AC2: 'Reply on RC2', Hazel Jeffery, 09 Feb 2026
  
  Please find attached the responses to reviewer #2 comments.
  
  Citation: https://doi.org/10.5194/egusphere-2025-3604-AC2

Peer review completion

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

AR by Hazel Jeffery on behalf of the Authors (17 Feb 2026) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (18 Feb 2026) by Jinkyu Hong

RR by Anonymous Referee #1 (26 Feb 2026)

Suggestions for revision or reasons for rejection

Thank you for inviting me to re-review the paper: “The TIPMIP Earth system experiment protocol: phase 1”. I remain convinced this is an important manuscript, and especially given the combination of currently high GHG emissions in tandem with a desire to stabilise global warming at 1.5 or 2.0oC. These two factors imply a high likelihood of a period of global temperature overshoot.

It is during that overshoot period that climate components could potentially tip (depending on their level of inertia), and ESMs remain a key tool for answering that question. In that context, much of the 1.5oC tipping point report has now appeared as a peer-review paper titled: “The implications of overshooting 1.5oC on Earth system tipping elements – a review” – which feels like it should be cited. That paper (https://doi.org/10.1088/1748-9326/ae3cad) explicitly states that more research into the effect of overshooting on tipping is required. This paper should be cited, because it supports the TIPMIP work presented here.

Overall, the authors have responded very well to all comments and requests, making this re-review easy. The new paragraph on the rationale for investigating overshoot is appreciated, and the new display item, Box 1, showing the TIPMIP protocol, should make it easier for readers to fully understand the approach. Thanks for creating that.

The query about ZEC implying that Net Zero may be achieved by causing a subsequent constant temperature remains of much interest. Hence, the extra text the authors have added of noting this might not be applicable at higher temperatures is really important. That is, text “This spread in the ZEC….with few models suggesting the ZEC response becomes more positive with time”. It might be appropriate to re-iterate this important point in the Discussion part of the manuscript.

The extra paragraph noting that the experimental format is highly idealised is important. Thanks. This does not detract from the paper and will encourage others to discuss with policymakers what less-idealised (but non-SSP) profiles might look like.

At this final stage, I would recommend that the authors make one final sweep through to check for any ambiguities, grammar issues, or typos.

The format of the diagrams could be improved to bring them up to publication standard. For instance, in Figure 2, make the bounding box less bold, remove the dotted grid, and place the “y” labels (ppm and oC) next to the numbers, not as annotations within the panels. The legend “Global warming levels” looks wrong – these are “global warming thresholds”? Similar issues for the other figures.

Otherwise, I think the paper is broadly ready for publication.

Hide

RR by Anonymous Referee #2 (13 Apr 2026)

Suggestions for revision or reasons for rejection

I have reviewed the manuscript by Jones et al. for a second time. I appreciate the authors’ efforts in revising the manuscript and addressing a number of my previous comments. However, I found that the revisions in response to several key points remain incomplete. As a result, further revisions are needed before the manuscript can be considered for publication. Below, I summarize the main outstanding issues, many of which reiterate points raised in my initial review. Additional detail is provided in the annotated manuscript.
• The introduction would benefit from a clearer justification for why a new MIP is needed in addition to existing CMIP7 MIPs. I suggest expanding the final paragraph of the introduction to more explicitly describe what distinguishes the proposed MIP from others.
• As noted in my earlier review, a limitation of the proposed protocol is that it does not control for cumulative CO₂ emissions, which are the primary driver of temperature responses under zero and net negative emissions. This limitation should be clearly acknowledged and discussed.
• The manuscript continues to include statements suggesting that zero emissions result in stable temperatures. I recommend reviewing the text carefully and revising such statements for accuracy (several examples are flagged in the annotated manuscript). In Section 4, the newly introduced language relating net zero CO₂ emissions to global warming levels is difficult to follow and would benefit from clarification.
• In several instances, the manuscript refers to “TCRE reversibility.” As TCRE is a metric, it cannot itself be “reversed,” in contrast to changes in the Earth system. I suggest revising this terminology to refer either to the reversibility of global warming or to TCRE “symmetry,” i.e., the extent to which TCRE under net negative emissions mirrors TCRE under positive emissions.

Referee Report: PDF

Hide

RR by Carl-Friedrich Schleussner (17 Apr 2026)

Suggestions for revision or reasons for rejection

The manuscript by Jones et al. lays out the TIPMIP modelling protocol. The manuscript is very well written, fully contextualized within the body of the existing literature and other MIPs, and the research design and implementation are convincingly laid out. I commend the effort and am convinced that the TIPMIP experiments will provide important insights in a timely manner for the forthcoming IPCC AR7.

I nevertheless have some overarching comments on the modelling protocol. I think some comparably minor amendments can greatly enhance the policy relevance of the experiments. Given the advanced state of the Tier 1 model runs and the time pressure for AR7 it might be difficult to implement them. Yet I hope the authors see value in the feedback and will explore options on how to consider them:

1. The ramp up/down symmetry. Other experimental protocols, i.e. CDRMIP, also do this. Yet, I’ve not seen a real scientific justification on why it should be implemented like this (other than testing TCRE/TCRR but this doesn’t require forcing symmetry). There’s ample evidence implying that the ramp up/down phase in the real world will not be symmetric in terms of emissions. Removing around 40 Gt CO2 per year is well outside the range implied by IAMs (AR6 overshoot scenarios would cluster more around 10 Gt removal by the end of the century, and there are already considerable feasibility and sustainability concerns with that). Which means the ramp down segments of the protocol suggested here are skewed optimistic to an unrealistic level. This matters for the interpretation of the results. And there would be a simple fix, i.e. just halving the rate on the way down (still optimistic, but a big step towards a more ‘realistic’ overshoot trajectory). Ultimately, for Tier 2 it could also be interesting to explore the implications of different rates of decline.

2. I’d like to urge the authors to consider to add a 1.5°C GWL to the Tier 1 experiments, or as a Tier1b setup. I appreciate the beauty in symmetry in the protocol (0,2,4°C). But as a result, the protocol misses important policy benchmarks. In fact, to inform a current discourse, having a 3°C upper limit (close to current policy) arguably would have been more targeted. It seems that ship has sailed. But more importantly, I think a 1,5°C GWL should be added, including a 1.5°C stabilization run and a branching off from the existing 2°C ramp down at 1.5°C. This comes with limited extra effort given the protocol already in place, but would greatly enhance the policy relevance of the insights gained as well as increase the utility of the simulations i.e. for analyzing overshoot pathways with a 1.5°C reference point including in the context of the AR7.

3. As a general consideration for the discussion of results, the question of what plausible reversibility scenarios are needs to be reflected. Recently, Gidden et al. (2025) suggested a prudent limit for underground CO2 storage equivalent to 0.4-0.7°C of GMST reduction. Yes, there’s marine CDR. But reversing 4°C of warming is far outside this range. Also reversing 2°C is questionable. Another argument to include a 1.5°C stabilization level.

Minor comments:

L230: I agree simplicity is a sometimes a plus. But I’m not sure I’d claim it’s the “main positive aspect”.

L288: This problem goes well beyond, because the amount of CO2 emitted is model dependent. Which means that atmospheric CO2 concentrations and marine biogeochemistry outcomes will differ across the model ensemble. It might be useful to discuss the implications of this in a bit more detail. Because it also means that i.e. effects related to atmospheric CO2 concentrations will differ across the model ensemble – which are relevant i.e. for Amazon tipping.

L320: I think adding 1.5°C and 1°C as landing zones is crucial. I think 1.5°C should be elevated to a Tier 1, even. This would greatly increase the policy relevance of the protocol as well as the utility in the context of analysis of overshoot pathway outcomes beyond tipping points. For interdisciplinary overshoot pathways, the space between 1.5°C and 2.5/3°C is of particular interest. Less so 4°C.

L499: Indeed, it sets the objective to ‘hold warming well below 2C”. Which makes a 2C stabilization a questionable choice for policy purposes.

L504: In most ESM simulations, also AMOC is recovering under stabilization at least at lower levels of warming. Introducing a number of changes.

L498: The rationale presented in this section has a considerable blind spot. It fully applied to stabilization at 2°C. But 4°C? This is well above the current policy trajectory. Of course it could still happen, but then probably because of a stronger climate response. Links to ISIMIP and other communities are being made here. But why should they run an unrealistically high scenario, opening the climate community up once again to scaremongering criticism. I’d fully support a 3°C stabilization. But find it difficult to follow the argument of why 4°C stabilization is a good choice other than for understanding extreme earth system responses.

L546ff: Some key questions are missing here.
What are the benefits of pursuing long-term temperature decline compared to stabilization at higher levels?
Can long-term critical thresholds of tipping elements be temporarily exceeded without triggering irreversible dynamics? Differently put: can pursuing an overshoot pathway pull us back from the brink?

L655ff: The preliminary results presented in this section are already very interesting. A few suggestions: Consider plotting ZEC (50,100, longer?) relative to the realized warming. Of course, only two data points but could still be interesting. With Tier 2 experiments down the road this could allow for a more quantitative statement on ZEC dependency on peak warming.

L685: It’s nice to see that there are some first results here. But am not so convinced of the utility of showing single model results. I suspect that for the final manuscript you’ll have more models. Else I’d suggest reconsidering the inclusion of this figure and the related discussion in the manuscript.

L699: I’d caution to draw this conclusion. Yes, the signal is small compared to 4°C, but maybe not so small in relative terms. A warming/cooling of 0.2°C is 10% of the total signal, or 440 Gt CO2 emission/removals. The implications for mitigation pathway design would be far reaching (44 years of removals…).

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ED: Reconsider after major revisions (21 Apr 2026) by Jinkyu Hong

AR by Hazel Jeffery on behalf of the Authors (05 May 2026) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (10 May 2026) by Jinkyu Hong

RR by Kirsten Zickfeld (14 Jun 2026)

RR by Anonymous Referee #1 (16 Jun 2026)

ED: Publish subject to technical corrections (25 Jun 2026) by Jinkyu Hong

AR by Hazel Jeffery on behalf of the Authors (26 Jun 2026) Author's response Manuscript

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29 Jul 2026

The TIPMIP Earth system model experiment protocol: phase 1

Geosci. Model Dev., 19, 6941–6966, https://doi.org/10.5194/gmd-19-6941-2026,https://doi.org/10.5194/gmd-19-6941-2026, 2026

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We introduce a new Earth system model experiment protocol to help researchers understand how Earth might respond to positive, zero, and negative carbon emissions. This protocol enables different models to be compared following similar warming and cooling rates. Researchers use the models to explore how the Earth reacts to different climate futures, including the risk of tipping points being exceeded and whether changes can be reversed. The results will support improved long-term climate policy.


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The TIPMIP Earth system model experiment protocol: phase 1

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