Hidetaka Kobayashi

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 CO ₂ 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 p CO ₂ disequilibrium exhibits strong scenario dependence. Under the low-emission scenario SSP1–2.6, oceanic CO ₂ undersaturation gradually weakens and approaches near equilibrium by 2100. By contrast, under the high-emission scenario SSP5–8.5, rapid increase in atmospheric p CO ₂ results in increasing undersaturation of CO ₂ in the ocean over the 21st century. Thereafter, surface-ocean CO ₂ supersaturation progressively expand into the equatorial and subtropical oceans between the mid–23rd and late 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. Under SSP5–8.5, oceanic CO ₂ uptake continues to increase until the late 21st century. Thereafter, it gradually weakens as surface-ocean p CO ₂ approaches atmospheric levels, whereas the Southern Ocean remains a persistent carbon sink throughout the simulation. Across all scenarios, early oceanic p CO ₂ 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 CO ₂ buffering capacity, driven by cumulative CO ₂ uptake, increases the sensitivity of surface-ocean p CO ₂ to additional carbon uptake. The buffering capacity approaches its effective minimum levels by the late 22nd century. In parallel, enhanced upper-ocean stratification and a slowdown of the deep overturning circulation reduce the upward transport of high alkalinity waters to the surface. This results in a decline in surface-ocean alkalinity that further enhances the influence of alkalinity on surface-ocean p CO ₂ during this period. Marine biological productivity and export vary significantly as a function of atmospheric CO ₂ trajectory. It increases under low-emission scenarios but declines under sustained high-emission scenarios. The transition between the two behaviors occurs when pCO2atm reaches approximately 3 times the preindustrial level and is closely associated with the weakening of the Atlantic Meridional Overturning Circulation and the resulting nutrient redistribution. 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 CO ₂ 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 p CO ₂ , highlighting the long memory and limited reversibility of oceanic ecosystem stress.

Ice‐core records show that atmospheric carbon dioxide (CO ₂ ) concentrations are approximately 90 ppmv lower during glacial periods than during interglacials. However, climate models fail to fully reproduce this reduction. One source of model uncertainty is the representation of ocean physical conditions that influence ocean carbon storage. We assess the impact of uncertainty in ocean physical fields on glacial atmospheric partial pressure of CO ₂ (pCO ₂ ) using a single offline ocean biogeochemical model with multiple physical fields. These fields are generated from a single Ocean General Circulation Model (OGCM) forced with the surface boundary conditions derived from 12 Paleoclimate Modeling Intercomparison Project (PMIP) models. Keeping the biogeochemical model identical while varying only the physical fields isolates their effect on atmospheric pCO ₂ . The mean glacial atmospheric pCO ₂ reduction over 12 LGM simulations is 40.3 ppmv. This reduction is driven by increased CO ₂ solubility due to sea‐surface temperature (SST) cooling (−30.1 ppmv) and a strengthened organic‐matter pump (−21.6 ppmv). The spread among 12 LGM simulations in atmospheric pCO ₂ (±7.8 ppmv) arises from differences in SST among the models, which in turn affect CO ₂ solubility (±5.6 ppmv), organic‐matter pump efficiency linked to water‐mass age (±6.6 ppmv), and gas‐exchange pump responses (±9.4 ppmv). Proxy constraints indicate that SST cooling contributed approximately 35 ppmv to the glacial CO ₂ drawdown, while the organic‐matter pump may have contributed an additional ∼30 ppmv, albeit with substantial uncertainty. These results show that uncertainty in reproducing glacial atmospheric CO ₂ arises from ocean physical conditions, through SST‐driven solubility and organic‐matter pump efficiency.

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.

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.

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.

During glacial periods, atmospheric CO2 concentrations are known to have been about 90 ppmv lower than during interglacial. However, climate models have not been able to fully reproduce this decrease, partly due to large uncertainties in changes in ocean physical fields. In this study, we evaluate the impact of uncertainties in ocean physical fields on atmospheric pCO2 during the Last Glacial Maximum (LGM) using a single offline ocean biogeochemical model forced by 12 ocean physical states derived from PMIP.The simulated glacial atmospheric pCO2 reduction is 40.3 ± 7.8 ppmv on average, with a large inter-model spread. This reduction mainly comes from the SST-dependent solubility effect (−30.1 ± 5.6 ppmv) and the enhanced efficiency of the organic matter pump (−21.6 ± 6.6 ppmv), cancelled somewhat by the response of the gas-exchange pump (+6.2 ± 9.4 ppmv). Our analysis suggests that the enhanced efficiency of the organic matter pump is associated with the older deep-water age in the glacial ocean and the response of the gas-exchange pump appears controlled by the SST contrast between the North Atlantic and the Southern Ocean.We find that models with older radiocarbon deep-water ages exhibit more efficient sequestration of carbon transported by the organic matter pump into the deep ocean, leading to a larger glacial reduction in atmospheric pCO2. However, all models used in this study underestimate the deep-water radiocarbon ages suggested by Δ14C paleoclimate records. In addition, both the global mean SST and the global mean ocean temperature are tend to be underestimated in the model compared to paleoclimate proxy reconstructions, leading to the smaller contribution of the SST-dependent solubility effect to the pCO2 reduction. If such model biases (i.e. underestimation of deep-water ages and the SST cooling) are corrected, we estimate that the corrected model estimate of the glacial pCO2 reduction becomes up to ~65ppmv which is still not enough for 90 ppmv reduction obtained from ice core record. Our results imply that the improvement in the reproducibility of the glacial ocean physical field alone are insufficient to fully account for the glacial atmospheric CO2 reduction and further improvements in the representation of ocean biogeochemical processes are also required under constraints including carbon isotope records.

The rise in atmospheric CO₂ during the last deglaciation has been attributed to carbon release from the deep ocean, establishing the ocean interior as the dominant reservoir modulating glacial–interglacial CO₂ variability. In contrast, the processes by which carbon was sequestered in the ocean during periods of declining atmospheric CO₂ remain poorly constrained. In particular, three irreversible CO₂ drawdown events since the last interglacial (MIS 5d, MIS 4, and MIS 2) represent critical intervals for understanding how the ocean stored carbon over multi-millennial timescales. Here we reconstruct vertical profiles of carbonate ion concentration ([CO₃²⁻]) from deep-sea sediment records in the South Pacific, located at the junction of the Pacific and Atlantic deep-water pathways. Millennium-scale measurements of carbonate dissolution reveal distinct depth-dependent [CO₃²⁻] changes that correspond to variations in the Atlantic Meridional Overturning Circulation (AMOC) and the Antarctic Circumpolar Current (ACC). The results reveal that during periods of CO₂ drawdown, the South Pacific developed a pronounced two-layer structure, reflecting the contrasting influences of bathypelagic carbon accumulation and abyssal ventilation. By integrating our new records with existing datasets from the equatorial Pacific to Atlantic, we show that each of the three irreversible CO₂ drawdown events was characterized by carbon storage in different oceanic basins and depth ranges. These findings reveal that distinct circulation regimes of the AMOC and ACC governed deep-ocean carbon sequestration during glacial intensification, which advance our understanding of how the deep ocean regulates atmospheric CO₂.

Radiocarbon (Δ14C) and stable carbon isotope (δ13C) proxy records provide important constraints on how carbon is redistributed among Earth’s surface reservoirs during major climate transitions. Previous work by Kobayashi et al. (2024, Climate of the Past) showed that transient simulations with the MIROC 4m climate model reproduce the timing of deglacial atmospheric pCO2 changes but underestimate their magnitude. Here, we extend this analysis by using carbon isotope proxies to better diagnose ocean carbon cycle processes during the last deglaciation (21 to 11 ka BP).We combine three-dimensional transient model output with marine sediment core and ice core records of Δ14C and δ13C to examine how changes in ocean ventilation, biological carbon export efficiency, and alkalinity cycling are reflected in carbon isotope budgets. Particular attention is given to Heinrich Stadial 1 (HS1), the Bølling–Allerød, and the Younger Dryas, periods characterized by abrupt changes in the Atlantic Meridional Overturning Circulation (AMOC) and pronounced interhemispheric climate asymmetry.We analyze three-dimensional transient model output and compare the results with existing marine sediment core and ice core records of Δ14C and δ13C. This comparison is used to examine how changes in ocean circulation and biological carbon export and remineralization are expressed in carbon isotope budgets. We focus on Heinrich Stadial 1 (HS1), the Bolling-Allerod, and the Younger Dryas, periods associated with abrupt changes in the Atlantic Meridional Overturning Circulation (AMOC) and strong interhemispheric climate asymmetry.The model reproduces the sequence of atmospheric pCO2 variations across these events, but comparisons with proxy data reveal a systematic underestimation of enhanced deep-ocean ventilation during HS1, particularly in the Southern Ocean and North Pacific, as indicated by marine Δ14C records. Stable carbon isotope data further suggest that reductions in biological carbon export efficiency during HS1 are weaker in the model than implied by benthic and planktonic δ13C records. During the Younger Dryas, proxy records indicate a continued increase in deep-ocean δ13C, whereas the model simulates an opposite trend, pointing to potential biases in simulated AMOC changes, ecosystem responses, or terrestrial carbon exchange.Overall, radiocarbon and stable carbon isotope comparisons indicate that the redistribution of carbon within the ocean is underestimated in the model. To further investigate these discrepancies, we additionally report sensitivity experiments that revisit the initialization of the Last Glacial Maximum state and assess the respective roles of ocean circulation and the biological pump in shaping deglacial carbon isotope and atmospheric pCO2 evolution.

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.

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.