Review of “Multi-centennial climate change in a warming world beyond 2100” by Sun-seon Lee et al.

This paper describes the changes in the physical climate system as well as the biochemical responses in climate projections with CESM2 extended to 2500 under SSP2-7.0. In this extended scenario, CO2 emissions increase until 2100, then follow a path of CO2 emission reductions until 2250 when net-zero emissions of CO2 are reached. The simulations reach a warming approaching 12C by ~2300 and stabilise at that temperature by 2500.

The paper is very well written and nicely laid out, and the results are interesting and a nice addition to the literature on multi-century climate projections. It is in some places quite dense to get through, but overall, I recommend publication after minor revisions. The geochemistry sections in particular are a bit difficult to understand in some places for an interdisciplinary / non-specialist readership.



One point that I would like to see briefly commented on, is some context on the relevance of this scenario for future projections and its implications. In e.g. the section on rainfall seasonality changes in megacities, L299 states “potentially resulting in substantial social and economic losses”.  While this is of course true, I think this is a vast understatement of the situation/problem at +12C and we can no longer talk about “potential losses and impacts” in such a hugely different simulated future. Without looking up an exact definition, and while this is of course subjective and not an exact science, I think people usually consider projected changes >3-4C as “catastrophic” (if not sooner). 12C is far beyond that and would likely include fundamental changes to the climate system. How is the reader meant to interpret those projections? A bit of context here would be welcome.

I include specific comments and requests for clarifications below.

L227ff “the weakening of the AMOC is related to the southward shift of the ITCZ (references)”: the sentence here reads as if the ITCZ shift causes the AMOC weakening, but surely that is not what you mean? Is it the other way around? Do you have any evidence for this other than the references? Please rephrase this sentence to include more details, and perhaps something like “studies have linked this change to …” to indicate this is not something that you have shown here.

L235-236 “In addition to the weakening of the easterlies […], our simulations project a substantial reduction of the trade winds in the extended future […]”: isn’t a weakening in the easterlies the same as a reduction in the trade winds, so you are repeating the same thing twice? Or is there a difference?

L257, “disappearance of the SPCZ”: I think this statement is too strong without further evidence, but the Supplementary Material supports this statement. Can you include a reference to the SI? Figure 3e and f) alone are not enough, as they show the difference relative to the reference climatology, not a new climatology for the later periods.

Figure 4h: Title should be precipitation instead of Nino3.4 SST variability (it is the same title as panel d)

E.g. Figure 10: “Fractional changes” typically refer to changes relative to a reference period, e.g. (Extended future – historical)/historical, rather than just a ratio between two periods. Perhaps rephrase to “ratio between Period2/Period1? Also, fractional changes are normally unitless.

L339: this should be “availability of food” or “food security”, not both.

L389-390: I was wondering up to this point whether the simulations were concentration or emissions-driven. Please also include this information in the section describing the simulations

L414: Please be more explicit about the link between NPP and POC export for non-biochemistry specialists. Does export imply a direction (vertical downwards)?  

L453-454 “thermocline PO4 concentrations”: do you mean concentrations above the thermocline? Suggest adding a line highlighting the position of the thermocline to Figure 11.

L462 “Viewed this way”: what way? This is unclear – the previous sentence states that the change in the transfer efficiency is modest, yet here you state the changes in the remineralisation source are substantial. Please clarify

L468:  Are there any explanations for this partial buffering in the case of carbon?

L548: “unprecedented”: there are a handful of studies looking at very long future projections at different global warming levels with fully coupled models, though the experimental design is different. Not a criticism, just mentioning for awareness

L572-573 “indicating that the stoichiometric plasticity mechanism identified by Kwon et al. (2022) provides only moderate modulation into the deep future.” What is the mechanism identified by Kwon et al.? Moderate modulation of what?

Review of “Multi-centennial climate change in a warming world beyond 2100” by Lee et al.

In this study, a set of extended simulations under forcings equivalent to reduced and then net zero carbon emissions is generated for the CESM2 model and then these are analysed. Substantial long-term climate change signals are identified in terms of temperature and precipitation, climate variability, carbon fluxes, and ocean nutrients.

The study is comprehensive and a useful addition to the literature. I don’t have any major concerns, although I have quite a few comments and suggestions below. Most of the comments are around the framing of the analysis and some overly strong inferences in my opinion.

I did think that even though a selling point of the analysis is the ensemble of simulations, much of the analysis doesn’t really make the most of the large sample sizes available. This is a minor comment though and I would be keen to see follow up analyses where these simulations are used to look at climate extremes for example.

Specific comments:

Line 11: The first sentence could be clearer. Maybe just “Changes in the climate system are anticipated well beyond the 21^st century due to human influences.” Or something similar?

L18-19: I don’t think this sentence is very helpful without more clarification on the baseline and given likely influence of high climate sensitivity on these numbers. I’d suggest a more qualitative sentence about global mean temperature and precipitation remaining elevated under net zero emissions would be more useful.

L23: Please define PO₄.

L24-26: This is a very confusing sentence that appears to try and summarise quite different findings in one. Some editing of this would be helpful.

L29-30: You might want to note CMIP7 plans (van Vuuren et al., 2025) which have standard scenario runs out to 2125 and more emphasis on extensions partly in response to the issue you raise.

L45-54: Santana-Falcón et al., (2023) may also be a relevant paper to use for this point, albeit with stronger mitigation than studied in this paper.

L74-76: I think it’s worth noting that there are some single model sets of simulations in existence either under constant concentrations (Dittus et al., 2024; Fabiano et al., 2023) or net zero emissions (King et al., 2024) but these are with a single simulation for a given forcing. This study is unusual in having an initial conditions ensemble for a given scenario under net zero emissions which is a nice selling point of your paper.

L104-105: Could you clarify whether the simulations are run in emissions or concentration-driven mode? I’m assuming the extensions beyond 2100 are emissions driven but it’s not as clear as it could be. It’s also not clear if the SSP3-7.0 simulations are emissions or concentration driven. You might interested in Sanderson et al., (2024) that discusses the merits of emissions-driven simulations.

Figure 1c,d: I assume the red lines are observations? In c it doesn’t look like you’re plotting anomalies from the observational period but the caption suggests you are.

L149-153: Could you clarify if this is annual-average sea ice extent? Of course, there are strong seasonal cycles and you are likely reaching “ice free” conditions at some times of the year. I’m wondering if this might be contributing to the decreased ensemble spread shown in Figure 2.

L153-156: Could note that in ZECMIP the AMOC projections are highly model dependent and diverse (MacDougall et al., 2022).

L208-210: These are certainly alarming amounts of global and local warming which should be highlighted. I think it is worth noting though that CESM2 has quite a high ECS (Gettelman et al., 2019).

L222-239: It could be noted that the precipitation changes are likely to be quite model dependent but increases at high latitudes in Southern Hemisphere are likely also associated with the warming in the region (Grose & King, 2023).

Figure 4h: Title is a bit confusing because I think this is precipitation variability? In general figure 4 could be improved as some axis labels are missing and some colour bar labels may lead to misinterpretation.

L269-285: The projected changes in MJO are indeed very interesting. I would caution that the interpretation could be a bit more understated given this is a single model result. Certainly this finding should motivate related analysis with other models.

L310-312: I can see why this sentence is included but it reads like a non-sequitur, and I think is unnecessary.

L375: ZECMIP studies with a carbon cycle focus may be worth citing here too (e.g MacDougall et al., 2020).

L389-390: This answers my earlier question. This is a fairly large caveat to some of the analysis which should be noted earlier in the Data and Methods section.

Figure 10: Given the same colour scale is used throughout it would look better to have a single larger colour bar.

L555-562: Similarly to a previous comment, the wording here is a bit over-confident given the results are derived from a single model.

References

Dittus, A. J., Collins, M., Sutton, R., & Hawkins, E. (2024). Reversal of Projected European Summer Precipitation Decline in a Stabilizing Climate. Geophysical Research Letters, 51(6), e2023GL107448. https://doi.org/10.1029/2023GL107448

Fabiano, F., Davini, P., Meccia, V., Zappa, G., Bellucci, A., Lembo, V., Bellomo, K., & Corti, S. (2023). Multi-centennial evolution of the climate response and deep ocean heat uptake in a set of abrupt stabilization scenarios with EC-Earth3. Earth System Dynamics Discussions. https://esd.copernicus.org/preprints/esd-2023-15/

Gettelman, A., Hannay, C., Bacmeister, J. T., Neale, R. B., Pendergrass, A. G., Danabasoglu, G., Lamarque, J. F., Fasullo, J. T., Bailey, D. A., Lawrence, D. M., & Mills, M. J. (2019). High Climate Sensitivity in the Community Earth System Model Version 2 (CESM2). Geophysical Research Letters, 46(14), 8329–8337. https://doi.org/10.1029/2019GL083978

Grose, M. R., & King, A. D. (2023). The circulation and rainfall response in the southern hemisphere extra-tropics to climate stabilisation. Weather and Climate Extremes, 100577. https://doi.org/10.1016/J.WACE.2023.100577

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

MacDougall, A. H., Frölicher, T. L., Jones, C. D., Rogelj, J., DamonMatthews, H., Zickfeld, K., Arora, V. K., Barrett, N. J., Brovkin, V., Burger, F. A., Eby, M., Eliseev, A. V., Hajima, T., Holden, P. B., Jeltsch-Thömmes, A., Koven, C., Mengis, N., Menviel, L., Michou, M., … Ziehn, T. (2020). Is there warming in the pipeline? A multi-model analysis of the Zero Emissions Commitment from CO2. Biogeosciences, 17(11), 2987–3016. https://doi.org/10.5194/BG-17-2987-2020

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

Sanderson, B. M., Booth, B. B. B., Dunne, J., Eyring, V., Fisher, R. A., Friedlingstein, P., Gidden, M. J., Hajima, T., Jones, C. D., Jones, C. G., King, A., Koven, C. D., Lawrence, D. M., Lowe, J., Mengis, N., Peters, G. P., Rogelj, J., Smith, C., Snyder, A. C., … Zaehle, S. (2024). The need for carbon-emissions-driven climate projections in CMIP7. Geoscientific Model Development, 17(22), 8141–8172. https://doi.org/10.5194/GMD-17-8141-2024

Santana-Falcón, Y., Yamamoto, A., Lenton, A., Jones, C. D., Burger, F. A., John, J. G., Tjiputra, J., Schwinger, J., Kawamiya, M., Frölicher, T. L., Ziehn, T., & Séférian, R. (2023). Irreversible loss in marine ecosystem habitability after a temperature overshoot. Communications Earth & Environment 2023 4:1, 4(1), 1–14. https://doi.org/10.1038/s43247-023-01002-1

van Vuuren, D., O’Neill, B., Tebaldi, C., Chini, L., Friedlingstein, P., Hasegawa, T., Riahi, K., Sanderson, B., Govindasamy, B., Bauer, N., Eyring, V., Fall, C., Frieler, K., Gidden, M., Gohar, L., Jones, A., King, A., Knutti, R., Kriegler, E., … Ziehn, T. (2025). The Scenario Model Intercomparison Project for CMIP7 (ScenarioMIP-CMIP7)   EGUsphere, 30, 1–38. https://doi.org/10.5194/EGUSPHERE-2024-3765