the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 07 Nov 2025
Overshoot and (ir)reversibility to 2300 in two CO2-emissions driven Earth System Models
Abstract. Future climate scenario projections are usually run with prescribed atmospheric CO2 concentrations. However, by not allowing the carbon cycle to interactively respond to emissions in Earth System models, the role of carbon cycle feedbacks on contributions to differences in climate model projections may be undersampled. Here, we present the main findings of two Earth System Models (MPI-ESM1.2-LR and NorESM2-LM) run with CO2 emissions to 2300 for three scenarios, two of which are climate overshoot scenarios, that were part of the Coupled Model Intercomparison Project Phase 6 (CMIP6). These experiments serve three important purposes: (i) an increasing focus on emissions driven runs, supplementing scenarios produced for the Coupled Climate-Carbon Cycle (C4MIP) and Carbon Dioxide Removal (CDRMIP) contributions to CMIP6; (ii) a focus on overshoot scenarios; and (iii) an extension of results beyond 2100, the timescales at which some of the most significant differences play out. Of the two models, NorESM2-LM shows more asymmetry in its response to the same global mean temperature levels before and after peak warming, particularly in terms of its regional pattern of warming and Atlantic Meridional Overturning Circulation (AMOC) response, with a substantially weakened AMOC persisting for decades after peak warming that takes more than a century to recover. In contrast, MPI-ESM1.2-LR shows reversibility with AMOC strength and regional warming more closely following surface temperature, but with some climate signals such as sea-level rise and ocean deoxygenation essentially irreversible. This diversity in model responses highlights the need for further research with a larger model ensemble that focuses on long-term emissions-driven model runs, particularly for overshoot scenarios, for CMIP7.
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Status: final response (author comments only)
- CC1: 'Comment on egusphere-2025-5292', Pedro Roldán, 11 Nov 2025
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RC1: 'Comment on egusphere-2025-5292', Mitchell Dickau, 08 Dec 2025
GENERAL COMMENTS:
This manuscript by Smith et al. presents results from two Earth System Models (MPI-ESM1.2-LR and NorESM2-LM) run in emissions-driven mode until the year 2300 under three scenarios (SSP1-1.9, SSP2-4.5, and SSP5-3.4-over). These simulations allow for the analysis the carbon cycle and the climate responses to overshoot and net-negative emissions. By including SSP1-1.9 and SSP2-4.5, the study fills a gap in the literature, as previous emissions-driven experiments in CMIP6 were dominated by SSP5-8.5, representing a very high end of century forcing scenario, or SSP5-3.4-over scenario. Furthermore, the extension of these simulations to 2300 enables the examination of Earth system responses on multi-century timescales, revealing long-term hysteresis and irreversibility. The authors find that both models show similar carbon cycle responses in terms of distribution of carbon and the transition of carbon reservoirs from sources to sinks, similar irreversibility in sea-level rise and ocean deoxygenation, and similar precipitation responses to temperature change. In contrast, the two models diverge significantly in their AMOC responses, with NorESM2-LM exhibiting strong hysteresis and a secondary warming period driven by AMOC recovery and MPI-ESM1.2-LR showing a largely reversible AMOC response that is proportional to the global warming level. The authors show that these contrasting AMOC responses lead to diverging surface temperature changes.
Overall, this is a timely and valuable contribution to the literature. As the authors note, emissions-driven runs are rare in CMIP6, especially for scenarios widely accepted as being more socio-economically realistic. These runs are essential for understanding the long-term carbon cycle feedbacks that concentration-driven runs may miss, especially in the context of overshoot and net-negative emissions. The contrast between the two chosen models – one relatively reversible (MPI-ESM1.2-LR) and one showing complex hysteresis (NorESM2-LM) – provides a useful caution against relying on single-model outcomes for long-term adaptation planning and highlights the need for more emissions-driven Earth System Model experiments.
The manuscript is generally well-written and logically structured. With minor revisions to address the specific comments below, this paper would be a strong addition to the literature.
SPECIFIC COMMENTS:
- Definition and Evidence of Zero Emissions Commitment (ZEC): In Section 3.1 (lines 173-175), the authors state that “Both models also show a negative zero emissions commitment evidenced by the emissions-temperature plot curving downwards after net zero (Koven et al., 2023), highlighted by the darker points in fig. 4, and confirmed by idealised CO2-only experiments in MacDougall et al. (2020).” While previous idealized experiments (e.g., ZECMIP) have established that these specific ESMs possess a negative ZEC, I question whether Figure 4 explicitly confirms this since ZEC is standardly defined by the temperature evolution when net-zero emissions are reached and maintained (MacDougall et al., 2020). In the scenarios presented here (esm-ssp119 and esm-ssp534-over), upon reaching zero (or net-zero) the first time, CO2 emissions do not stabilize at zero but immediately transition to net-negative. If the authors are defining ZEC as when zero/net-zero CO2 emissions is reached the second time (~2200 in esm-ssp-119 or ~2160 in esm-ssp-345-over), NorESM2-LM exhibits a warming trend which contradicts negative ZEC claim. The current text implies Figure 4 serves as proof of negative ZEC. I recommend the authors clarify how fig. 4 shows a response consistent with negative ZEC, perhaps by clarifying the definition of ZEC they are using, or remove the statement altogether.
- Physical Mechanism of AMOC Overshoot: In Section 3.3, the authors attribute the rapid warming in NorESM2-LM in the late 22nd century to an AMOC recovery. The manuscript lacks a physical explanation for why the AMOC overshoots so strongly in this specific model. Adding a brief physical explanation for the strong AMOC recovery driving the reported secondary warming period in NorESM2-LM, either supported by a supplementary figure or a citation, would strengthen the manuscript.
- Contextualizing Regional Hysteresis against Concentration-Driven Runs: In the Introduction (Line 114), the authors cite Pfleiderer et al. (2024) regarding regional climate signals in concentration-driven overshoot scenarios. However, this comparison is not revisited in the Results section. I recommend the authors explicitly compare their regional temperature and precipitation hysteresis patterns (Section 3.4) with those reported in Pfleiderer et al. (2024). Discussing similarities or differences would help clarify whether the inclusion of interactive carbon cycle feedbacks (emissions-driven) significantly alters regional reversibility compared to prescribed concentration pathways.
- Coupling the ITCZ Shift to AMOC Dynamics: In Section 3.4.2, the discussion on precipitation attributes the observed ITCZ shift to the "general hypothesis that the ITCZ moves towards the hemisphere with greater warming". I recommend explicitly linking the precipitation response back to the AMOC evolution. Since the strong AMOC reduction in NorESM2-LM is the primary driver of the Northern Hemisphere cooling, I would argue that it is the physical mechanism forcing the interhemispheric temperature gradient that shifts the ITCZ. Explicitly stating that the AMOC collapse drives the southward shift would provide a more cohesive mechanistic explanation for the NorESM2-LM results.
- Substantiating the NPP atmospheric CO2 Relationship: In Section 3.4.3 (lines 298-299), the authors state that "NPP more closely follows the evolution of atmospheric CO2 than temperature which is related to the fertilisation effect". The manuscript does not explicitly present data to verify this correlation for these specific simulations. I suggest supporting this claim by adding a figure to the Supplementary Material (e.g., a time-series overlay of Global NPP vs. Atmospheric CO2, or a scatter plot).
- Nuancing the "Best Case" Scenario Framing: In the Conclusions (lines 342-345), the authors argue that these results represent a "best case" scenario because both models exhibit low climate sensitivity and negative ZEC. While this characterization may hold for the peak global temperature, it risks obscuring the severe regional risks identified in the study or other potential harmful Earth system responses not explored in the study. For instance, NorESM2-LM projects a severe AMOC collapse and significant regional cooling in Europe, which represents a challenging outcome for regional adaptation in the region rather than a “best case”. In this example, Nor-ESM2-LM may very well present a much worse outcome of SSP5-3.4-over than a model that has higher climate sensitivity but exhibits less hysteresis in the AMOC response. I therefore recommend explicitly qualifying the “best case” statement. Since not all Earth system responses scale with global temperature, the authors should expand this paragraph to distinguish between "best case for global temperature" vs. "best case for earth system response and for implications for mitigation/adaptation" to avoid misinterpretation.
TECHNICAL CORRECTIONS
- Typo (page 16, line 300) "MPI-ESM2-LM" should be "MPI-ESM1.2-LR"
- "hist-esm" is used in the figure title but "esm-hist" is used in the figure caption and text
Citation: https://doi.org/10.5194/egusphere-2025-5292-RC1 -
RC2: 'Comment on egusphere-2025-5292', Anonymous Referee #2, 29 Dec 2025
Review of “Overshoot and (ir)reversibility to 2300 in two CO2-emissions driven Earth System Models” by Smith et al.
This study uses two Earth System Models to run additional ensembles of emissions-driven simulations under higher and lower overshoot scenarios. The use of ensembles enables some robust identification of global and regional differences pre- and post-overshoot, albeit with model differences which are particularly evident on the local level.
This is a very nice study with insightful analysis, and it is well presented. I found the Conclusions and final remarks to be particularly well written, and the arguments put forward here are well backed up. I have a few comments and these are mostly around providing more context for the motivation for the study and the results.
Specific comments:
L9: Perhaps change “response to” to “climate response at”.
Introduction: Overall, this is very comprehensive. The summary of how ScenarioMIP has worked previously and the benefit of emissions-driven simulations is particularly clear and useful. One thing I thought was missing was discussion of the work around emissions-driven ESM simulations more generally which helps to motivate your focus on long emissions-driven simulations. Most of these studies have used more idealised simulations, which highlights a benefit of the SSP-type emissions-driven simulations used here, but include a focus on low/net-zero emissions pathways. Relevant references include (Fyfe et al., 2021; King et al., 2024; Murphy et al., 2014; Sigmond et al., 2020).
L102-105: While Pfleiderer et al. (2024) is a relevant paper it is odd to pick out one when there are a few examples that could be raised. These include (Roldán-Gómez, De Luca, et al., 2025; Roldán-Gómez, Ortega, et al., 2025).
L140-143: Similar behaviour is found in ACCESS-ESM-1.5 too (Chamberlain et al., 2024).
Figure 3: While this is generally very clear, the legends are odd and incomplete with none shown in (b).
L161-164: Might be better to break this into two sentences for clarity.
L175-179 and L214-215: A bit of a leap is made from GSAT change to AMOC change and a strong connection is drawn between the two. More discussion as to why AMOC change may strongly relate to GSAT change in the net-negative emissions phase would be useful, preferably with a reference.
L238-241: This is a similar finding to other studies including (Lacroix et al., 2024; Nauels et al., 2025).
Section 3.4.1. This section is well written but could maybe benefit from noting that there is diversity in post-net zero regional temperature changes with some models showing more of a hemispheric difference than others (Cassidy et al., 2023; MacDougall et al., 2022). By using two models you’re seeing some differences under net-negative emissions but one would expect there is a broad range of simulated changes in a larger ensemble.
Section 3.4.2. Again, this is a very nice section although feels a little brief. Some further context against other studies could be useful, including (Douglas et al., 2025; Kug et al., 2022).
L309-311: The Conclusions are very well written. I did wonder if some expansion on this comment about the number of ensemble members could be made. Could you even go as far as suggesting a minimum number to be run for relevant CMIP7 experiments? There is quite a big difference between running 10 or 3 as was done with the MPI and NorESM models respectively here.
References:
Cassidy, L. J., King, A. D., Brown, J., MacDougall, A. H., Ziehn, T., Min, S.-K., & Jones, C. D. (2023). Regional temperature extremes and vulnerability under net zero CO2 emissions. Environmental Research Letters. https://doi.org/10.1088/1748-9326/AD114A
Chamberlain, M. A., Ziehn, T., & Law, R. M. (2024). The Southern Ocean as the climate’s freight train - driving ongoing global warming under zero-emission scenarios with ACCESS-ESM1.5. Biogeosciences, 21(12), 3053–3073. https://doi.org/10.5194/BG-21-3053-2024
Douglas, H. C., Revell, L. E., King, A., Harrington, L. J., & Frame, D. J. (2025). Effects of temperature overshoot amplitude on regional climate. Environmental Research Letters, 20(11), 114043. https://doi.org/10.1088/1748-9326/AE114F
Fyfe, J. C., Kharin, V. V., Swart, N., Flato, G. M., Sigmond, M., & Gillett, N. P. (2021). Quantifying the influence of short-term emission reductions on climate. Science Advances, 7(10). https://doi.org/10.1126/SCIADV.ABF7133;JOURNAL:JOURNAL:SCIADV;WEBSITE:WEBSITE:AAAS-SITE;REQUESTEDJOURNAL:JOURNAL:SCIADV;WGROUP:STRING:PUBLICATION
King, A. D., Ziehn, T., Chamberlain, M., Borowiak, A. R., Brown, J. R., Cassidy, L., Dittus, A. J., Grose, M., Maher, N., Paik, S., Perkins-Kirkpatrick, S. E., & Sengupta, A. (2024). Exploring climate stabilisation at different global warming levels in ACCESS-ESM-1.5. Earth System Dynamics, 15(5), 1353–1383. https://doi.org/10.5194/ESD-15-1353-2024
Kug, J. S., Oh, J. H., An, S. Il, Yeh, S. W., Min, S. K., Son, S. W., Kam, J., Ham, Y. G., & Shin, J. (2022). Hysteresis of the intertropical convergence zone to CO2 forcing. Nat. Clim. Change, 12(1), 47–53. https://doi.org/10.1038/s41558-021-01211-6
Lacroix, F., Burger, F. A., Silvy, Y., Schleussner, C. F., & Frölicher, T. L. (2024). Persistently Elevated High-Latitude Ocean Temperatures and Global Sea Level Following Temporary Temperature Overshoots. Earth’s Future, 12(10), e2024EF004862. https://doi.org/10.1029/2024EF004862;WGROUP:STRING:PUBLICATION
MacDougall, A. H., Mallett, J., Hohn, D., & Mengis, N. (2022). Substantial regional climate change expected following cessation of CO2 emissions. Environmental Research Letters, 17(11), 114046. https://doi.org/10.1088/1748-9326/AC9F59
Murphy, J. M., Booth, B. B. B., Boulton, C. A., Clark, R. T., Harris, G. R., Lowe, J. A., & Sexton, D. M. H. (2014). Transient climate changes in a perturbed parameter ensemble of emissions-driven earth system model simulations. Climate Dynamics 2014 43:9, 43(9), 2855–2885. https://doi.org/10.1007/S00382-014-2097-5
Nauels, A., Nicholls, Z., Möller, T., Hermans, T. H. J., Mengel, M., Kloenne, U., Smith, C., Slangen, A. B. A., & Palmer, M. D. (2025). Multi-century global and regional sea-level rise commitments from cumulative greenhouse gas emissions in the coming decades. Nature Climate Change 2025 15:11, 15(11), 1198–1204. https://doi.org/10.1038/s41558-025-02452-5
Roldán-Gómez, P. J., De Luca, P., Bernardello, R., & Donat, M. G. (2025). Regional irreversibility of mean and extreme surface air temperature and precipitation in CMIP6 overshoot scenarios associated with interhemispheric temperature asymmetries. Earth System Dynamics, 16(1), 1–27. https://doi.org/10.5194/ESD-16-1-2025
Roldán-Gómez, P. J., Ortega, P., & Donat, M. G. (2025). Contribution of meridional overturning circulation and sea ice changes to large-scale temperature asymmetries in CMIP6 overshoot scenarios. Ocean Science, 21(5), 2283–2303. https://doi.org/10.5194/OS-21-2283-2025
Sigmond, M., Fyfe, J. C., Saenko, O. A., & Swart, N. C. (2020). Ongoing AMOC and related sea-level and temperature changes after achieving the Paris targets. Nature Climate Change, 10(7), 672–677. https://doi.org/10.1038/s41558-020-0786-0
Citation: https://doi.org/10.5194/egusphere-2025-5292-RC2
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This paper may be interesting for the discussion:
https://doi.org/10.5194/os-21-2283-2025
There we see the behavior of the AMOC up to 2300 in overshoot simulations of MRI-ESM2-0, CNRM-ESM2-1, IPSL-CM6A-LR and CanESM5. There is also a strong dispersion across these models.
For the discussion on the irreversibility of temperature and precipitation, this other paper may be also interesting:
https://doi.org/10.5194/esd-16-1-2025