the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 15 Jan 2026
Multi-centennial ocean biogeochemical responses to extended Shared Socioeconomic Pathways
Abstract. Despite their profound consequences for the global carbon cycle, multi-centennial ocean biogeochemical responses to anthropogenic greenhouse gas forcing remain poorly constrained. Here we investigate long-term oceanic responses to atmospheric CO2 forcing using the Model for Interdisciplinary Research on Climate, Earth System version 2 for Long-term simulations (MIROC-ES2L) Earth system model driven by extended Shared Socio-economic Pathway (SSP) scenarios through to the year 2500. Ocean–atmosphere pCO2 disequilibrium exhibits strong scenario dependence. Under the low-emission scenario SSP1–2.6, oceanic CO2 undersaturation gradually weakens and approaches near equilibrium by 2100. In contrast, under the high-emission scenario SSP5–8.5, rapid increase in atmospheric pCO2 results in increasing undersaturation of CO2 in the ocean over the 21st century. Thereafter, the ocean becomes increasingly supersaturated, with CO2 supersaturation expanding into the equatorial and subtropical oceans between the mid-23rd and the 25th century. The Southern Ocean remains persistently undersaturated throughout the simulations, consistent with sustained influence from deep waters that are weakly affected by anthropogenic perturbations. Across all scenarios, early oceanic pCO2 changes are primarily driven by increases in dissolved inorganic carbon, whereas alkalinity becomes an increasingly important control from the mid-22nd century onward under SSP5–8.5. A progressive decline in the oceanic CO2 buffering capacity, driven by cumulative CO2 uptake, increases the sensitivity of surface pCO2 to additional carbon, with the buffering capacity approaching its effective minimum levels by the late 22nd century. In parallel, a reduction in surface alkalinity further enhances the influence of alkalinity on surface pCO2 during this period. Notably, thermal stress and nutrient limitation persist in regions such as the Arctic Ocean until the late 25th century even under SSP1–2.6, indicating a centennial-scale lagged response of marine ecosystems to atmospheric CO2 forcing. In the surface ocean, marine ecosystem stress (integrating thermal and biogeochemical stressors) continues to intensify through the late 22nd century under SSP5–8.5, despite the stabilization of atmospheric pCO2, highlighting the long memory and limited reversibility of oceanic ecosystem stress.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-120', Anonymous Referee #1, 17 Feb 2026
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RC2: 'Comment on egusphere-2026-120', Anonymous Referee #2, 07 Jun 2026
Summary of the study
This study examines the long-term ocean biogeochemical response to anthropogenic CO₂ emissions using the MIROC-ES2L Earth System Model under extended SSP scenarios through 2500. While most climate projections focus on the period up to 2100, the authors examine the long-term adjustment of marine biogeochemistry until 2500.
The authors show that ocean–atmosphere CO₂ disequilibrium evolves differently depending on the emissions pathway. Under SSP1–2.6, the ocean gradually approaches equilibrium with the atmosphere, whereas under SSP5–8.5, continued CO₂ emissions initially enhance ocean carbon uptake before leading to widespread surface-ocean CO₂ supersaturation from the 23rd century onward. Dissolved inorganic carbon is the main driver of early pCO₂ changes, while declining alkalinity and reduced buffering capacity become increasingly important over longer timescales, limiting the ocean’s ability to absorb additional carbon.
The study also highlights the persistence of climate-induced stress on marine ecosystems long after atmospheric CO₂ stabilizes. Thermal stress, nutrient limitation, and other biogeochemical pressures continue for centuries, particularly in the Arctic Ocean. Under the high-emission scenario SSP5–8.5, combined ecosystem stress intensifies until the late 22nd century despite stabilization of atmospheric CO₂, demonstrating the strong inertia and limited reversibility of ocean ecosystem responses to anthropogenic forcing.
Overall Assessment and Recommendation
I appreciate the overall topic of this study, which addresses an important and timely question: the multi-centennial response of ocean biogeochemistry and marine ecosystems to anthropogenic CO₂ forcing. Long-term projections beyond 2100 remain relatively scarce, and the results presented here have the potential to provide valuable insights into both the future evolution of the ocean carbon sink and the persistence of ecosystem stressors. These are important issues that deserve attention and publication.
However, I find that the current manuscript suffers from significant organizational and conceptual weaknesses. First, the scientific objectives are not clearly defined or sufficiently justified. The Introduction combines several distinct themes—including ocean carbon uptake, low-latitude productivity, nutrient cycling, ocean acidification, and ecosystem stress—without developing a coherent argument that explains why the long-term response (2100–2500) should be investigated and which specific scientific questions are being addressed. As a result, the narrative lacks focus and the reader is left uncertain about the primary goals of the study.
The Results section reflects this lack of focus. It concentrates disproportionately on the evolution of DIC and NPP, while other potentially more impactful aspects of the simulations receive limited attention. More generally, the manuscript attempts to address two major questions—long-term carbon uptake and long-term ecosystem stressors—without clearly separating them or establishing strong links between them.
In its current form, I do not believe that the manuscript is ready for publication in Biogeosciences. I strongly recommend a major reorganization of the paper. The authors should either clearly separate the carbon uptake and ecosystem stress components into distinct parts of the manuscript, each with dedicated objectives and analyses, or alternatively choose to focus on only one of these themes. Such a restructuring would require a substantial rewriting of the Introduction, a clearer formulation of the scientific objectives, and a reconsideration of the results presented in the central sections of the paper. I believe that these changes would significantly improve the clarity, impact, and overall scientific value of the study.
Specific Comments
(1) Introduction
The Introduction requires substantial reorganization. As it stands, it interweaves results and concepts related to the evolution of the ocean carbon sink, primary productivity, and ecosystem stressors (e.g., acidification), without clearly distinguishing these components or explaining why they should be considered together, nor how they potentially interact or feedback on one another. Some statements appear overly general and insufficiently supported by the literature; for instance, the claim that “low-latitude productivity is crucial for the global carbon cycle” is too simplistic and would benefit from a more explicit and better substantiated discussion.In addition, the paragraph starting at line 59 (“Analyses using CMIP models have also advanced understanding of present-day oceanic uptake of anthropogenic CO₂”) transitions abruptly from present-day understanding to long-term projections without a clear logical bridge. Given the relatively limited number of studies addressing multi-centennial ocean biogeochemistry, the motivation for focusing on long-term responses needs to be strengthened. In particular, the authors should better justify why centennial-to-millennial timescales are relevant, explicitly discussing ocean adjustment timescales, (ir)reversibility of carbon-cycle changes, and the relevance for net-zero or overshoot scenarios.
(2) Results structure and metrics
The first results section is relatively short and focuses primarily on dissolved inorganic carbon (DIC) and net primary production (NPP)/export production. This choice is not sufficiently justified. In particular, it is unclear why the evolution of ocean carbon uptake itself is not treated as a central diagnostic, given that it is arguably one of the key variables of the study.(3) NPP response under contrasting scenarios
The evolution of NPP differs markedly between scenarios, with a decrease under high-emission conditions and an increase under low-emission conditions. This is a striking and potentially highly significant result. However, it is not discussed or explained in any depth, which represents a missed opportunity given its implications.(4) Carbon uptake drivers and biological pump
The section describing the drivers of carbon uptake is generally well developed and clearly presented. However, it would be particularly interesting to further explore the role of the biological pump across these long timescales, especially in comparison with the physical pump. In particular, the manuscript could attempt to assess whether biological processes become increasingly important in controlling carbon uptake over centennial to millennial scales.(5) Ecosystem focus
The analysis of specific ecosystems (e.g., kelp forests, coral reefs) appears underdeveloped and only marginally integrated into the overall narrative. It is not entirely clear that this level of ecosystem-specific detail is essential for the scope of the manuscript, and reconsideration of this section may help improve focus and coherence.Citation: https://doi.org/10.5194/egusphere-2026-120-RC2
Data sets
Code and scripts for Kobayashi et al. entitled "Multi-centennial ocean biogeochemical responses to extended Shared Socioeconomic Pathways" Hidetaka Kobayashi https://doi.org/10.5281/zenodo.18193548
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- 1
This manuscript explores the ocean biogeochemical consequence of extended emissions scenarios up to 2500 using the MRI Earth system model. The analysis presented is robust, the manuscript is well-written, well referenced and adds an interesting long-term perspective to past publications which have overwhelmingly concentrated on 21st century perturbations. My comments are minor, typically focusing on text which could be clarified and additional references which could enhance the discussion in certain areas. Subject to these minor recommendations, I wholeheartedly support publication.
L15 Would be nice to mention the driver(s) of alkalinity declines.
L65 I would rephrase this to make it clear the stratification and not the reduced buffering capacity that reduces alkalinity.
L68 Maybe explain why this is the case (computing costs) and that EMICs often do run this far but with potentially less biogeochemical realism.
L102-104 Perhaps compliment this with the model TCRE if known.
L122-124 I think a citation or reference to a figure is needed to support this statement.
L140-149 I think this excessive detail given this is quite fundamental knowledge.
L179-180 The logic here is confusing. Wouldn’t declining nutrients in deep water formation regions mean greater upper ocean nutrient availability (less loss to depth).
L202-203 Is this confirmed by simulated fluxes? Presumably so if changes in winds are minimal.
L210 Does the Southern Ocean therefore remain a sink? Interestingly under concentration-driven overshoot scenarios the Southern Ocean tends to switch form sink to source in multi-model assessments (Koven et al., 2022). This difference in model behavior should probably be discussed.
L230 Should the p in pCO2 be capitalized here to represent potential pCO2?
L237 Rephrase “neutralizing increases in DIC”. Should DIC be “acidity”?
L242 Although closely related, I don’t think this is exactly the inverse as it has units (umol kg-1/uatm) unlike 1/R.
L246 Here and elsewhere be explicit when you are referring to the surface ocean.
L246-255 One wonders what are the respective roles of DIC vs. alkalinity in the differences in buffering capacity recovery. Presumably this is nearly all DIC driven?
L255 Maybe mention that there is still some regional variability in buffering capacity in SSP126 but this disappears in SSP585.
L270 I think “in contrast” should be “similarly”.
L268-277 Does this surface ocean alkalinity decline occur despite reductions in PIC export? i.e. is physics dominating biotic effects? It is worth mentioning here or later in the discussion on model limitations whether sediment carbonate dissolution is possible (presumably not). On these timescales, one might expect a non-negligible benthic alkalinity flux associated with such dissolution.
L290-291 Does export production decline though, as seen in most CMIP6 models and therefore is it remineralization associated with increasing water mass age which is dominating ? (e.g. Wilson et al., 2022).
L322 Are undersaturation metrics with respect to the surface ocean only? Clarify this.
L379 Maybe the relative pCO2atm decline with respect to increase should be mentioned here. This relative decline is much greater in SSP126 than SSP585
L380-381 Would be useful to compare with Koven et al. (2022) here.
L383 This wording “as expressed as a decline...” doesn’t work in light of the previous sentence.
L389 and 391 Can you say “reductions” instead of “changes”?
L398-399 It might be worth reelecting on biogeochemical drivers of alkalinity anomalies here.
L415 Not sure it makes sense to call the carbonate system CO2 dominated as CO2 will still be <<10% of DIC with nearly everything presumably bicarbonate.
L424 Could also mention the absence of sediment feedbacks here.
L444-445 This is a little confusing. Please clarify what is meant by this decoupling.
L447 “Ice sheet derived” freshwater forcing? What follows this is also a bit confusing. Is this a consequence of enhanced remineralization in these older waters?
L452-455 I’m not sure I have seen robust increases in export even under high mitigation scenarios in multi-model comparisons (e.g. Wilson et al., 2022). The novelty in these projections should be highlighted.
L468 “they” typo
L544 Or an overestimation. Greenland hosing simulations have been shown to reduce ocean carbon uptake (e.g. Swingedouw et al., 2007).
L563-565 This is a feature of carbonate chemistry projections at depth but not in the global surface ocean where there is generally high model agreement for acidification projections with the exception of regions dominated by sea ice dynamics or riverine fluxes. I therefore suspect the impact on ocean carbon uptake to be minimal. That being said, a general overestimation of Revelle factor has been shown to low bias CMIP6 ocean carbon uptake simulations (Terhaar et al., 2022).
References
Koven, C. D., Arora, V. K., Cadule, P., Fisher, R. A., Jones, C. D., Lawrence, D. M., et al. (2022). Multi-century dynamics of the climate and carbon cycle under both high and net negative emissions scenarios. Earth System Dynamics, 13(2), 885–909. https://doi.org/10.5194/esd-13-885-2022
Swingedouw, D., Bopp, L., Matras, A., & Braconnot, P. (2007). Effect of land-ice melting and associated changes in the AMOC result in little overall impact on oceanic CO2 uptake. Geophysical Research Letters, 34(23). https://doi.org/10.1029/2007GL031990
Terhaar, J., Frölicher, T. L., & Joos, F. (2022). Observation-constrained estimates of the global ocean carbon sink from Earth system models. Biogeosciences, 19(18), 4431–4457. https://doi.org/10.5194/bg-19-4431-2022
Wilson, J. D., Andrews, O., Katavouta, A., de Melo Viríssimo, F., Death, R. M., Adloff, M., et al. (2022). The biological carbon pump in CMIP6 models: 21st century trends and uncertainties. Proceedings of the National Academy of Sciences, 119(29), e2204369119. https://doi.org/10.1073/pnas.2204369119