Takashi Obase

It has been hypothesized that the Earth may have experienced snowball events in the past, during which its surface became completely covered with ice. Previous studies used general circulation models to investigate the onset and climate of such snowball events. Using the MIROC4m coupled atmosphere–ocean climate model, this study examined the changes in the oceanic circulation during the onset of a modern snowball Earth and elucidated their evolution to steady states under the snowball climate. Abruptly changing the solar constant to 94 % of its present-day value caused the modern Earth climate to turn into a snowball state after ∼ 1300 years and initiated rapid increase in sea ice thickness. During onset of the snowball, extensive sea ice formation and melting of sea ice in the mid-latitudes caused substantial freshening of surface waters and salinity stratification. By contrast, such salinity stratification was absent if the duration between the change in the solar flux and the snowball onset was short. After snowball onset, the global sea ice cover and the buildup of salinity stratification caused drastic weakening in the deep ocean circulation. However, the meridional overturning circulation resumed within several hundred years after the snowball onset because the density flux by sea ice production weakens the salinity stratification. While the evolution of the oceanic circulation would depend on the continental distribution and the evolution of continental ice sheets, our results highlight the gradual growth of sea ice and associated brine rejection are essential factors for the transient evolution of the oceanic circulation in the snowball events.

We present our development of the Ice sheet model for Integrated Earth-system Studies (IcIES2) for the Antarctic ice sheet configuration as a model development for CMIP7-ISMIP7. The flow of the ice is calculated with the shallow ice approximation (SIA) and shallow shelf approximation (SSA). To represent the migration of grounding lines, we use the grounding line flux boundary condition of Schoof (2007), following previous implementations (Pollard and DeConto 2012; 2020). The ice velocity fields are calculated using a hybrid approach that combines the SIA and SSA approximations, based on the ratio of basal sliding velocity to the depth-averaged velocity from the SIA solution. We perform sensitivity experiments on ice-sheet model parameters using the present-day bedrock topography and surface mass balance to obtain a reasonable present-day Antarctic ice-sheet configuration. We also perform experiments with an abrupt increase in sub-shelf melting to evaluate the model response to reduced ice shelf buttressing and marine ice sheet instability.

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.

The Quaternary climate is characterized by glacial–interglacial cycles, with the most recent transition from the last glacial maximum to the present interglacial (the last deglaciation) occurring between ∼ 21 and 9 ka. While the deglacial warming at high southern latitudes is mostly in phase with atmospheric CO₂ concentrations, some proxy records indicate that the onset of the warming occurred before the CO₂ increase. In addition, high southern latitudes exhibit a cooling event in the middle of the deglaciation (15–13 ka) known as the “Antarctic Cold Reversal”. In this study, we analyse transient simulations of the last deglaciation performed with six different climate models as part of the 4th phase of the Paleoclimate Modelling Intercomparison Project (PMIP4) to understand the processes driving high-southern-latitude surface temperature changes. As the protocol of the last deglaciation sets the choice of freshwater forcing as flexible, the freshwater forcing is different in each model, thus complicating the multi-model comparison. While proxy records from West Antarctica and the Pacific sector of the Southern Ocean suggest the presence of an early warming before 18 ka, only half the models show a significant warming at this time (∼ 1 °C or ∼ 10 % of the total deglacial warming). All models simulate a major warming during Heinrich Stadial 1 (18–15 ka), concurrent with the CO₂ increase and with a weakening of the Atlantic Meridional Overturning Circulation (AMOC) in some models. However, the simulated Heinrich Stadial 1 warming over Antarctica is smaller than the one suggested from ice core data. During the Antarctic Cold Reversal, simulations with an abrupt AMOC strengthening exhibit a high-southern-latitude cooling of 1 to 2 °C, in relative agreement with proxy records, while simulations with rapid North Atlantic meltwater input exhibit a warming. Using simple models to extract the relative AMOC contribution, we find that all climate models simulate a high-southern-latitude cooling in response to an AMOC increase with a response timescale of several hundred years, suggesting the choice of the North Atlantic meltwater forcing substantially affects high-southern-latitude temperature changes. Thus, further work needs to be carried out to reconcile the deglacial AMOC evolution with the Northern Hemisphere ice sheet disintegration and associated meltwater input. Finally, all simulations exhibit only minimal changes in Southern Hemisphere westerlies and Southern Ocean meridional circulation during the last deglaciation. Improved understanding of the processes impacting Southern Hemisphere atmospheric and oceanic circulation changes accounting for deglacial atmospheric CO₂ increase is needed.

In the past, Earth experienced snowball events, where its surface became completely covered with ice. Previous studies used general circulation models to investigate the onset and climate of such snowball events. Using the MIROC4m coupled atmosphere-ocean climate model, this study examined the changes in the oceanic circulation during the onset of a modern snowball Earth and elucidated their evolution to steady states under the snowball climate. Abruptly changing the solar constant to 94 % of its present-day value caused the modern Earth climate to turn into a snowball state after 1300 years and initiated rapid increase in sea ice thickness. During onset of the snowball event, extensive sea ice formation and melting of sea ice in the mid-latitudes caused substantial freshening of surface waters and salinity stratification. By contrast, such salinity stratification was absent if the duration necessary for snowball onset was short because of stronger solar constant forcing. After snowball onset, the global sea ice cover reduced air–sea fluxes and caused drastic weakening in the deep ocean circulation. However, as the ocean temperature and salinity fields approached near constant states, the meridional overturning circulation resumed in the steady-state snowball climate. Although the evolution of the oceanic circulation would depend on model setting, particularly regarding the treatment of air–sea fluxes and the continental distribution, our results highlight the importance of the oceanic circulation and associated biogeochemical changes in the climate system feedback and sequence of snowball events.

In the past, Earth experienced snowball events, where its surface became completely covered with ice. Previous studies used general circulation models to investigate the onset and climate of such snowball events. Using the MIROC4m coupled atmosphere-ocean climate model, this study examined the changes in the oceanic circulation during the onset of a modern snowball Earth and elucidated their evolution to steady states under the snowball climate. Abruptly changing the solar constant to 94 % of its present-day value caused the modern Earth climate to turn into a snowball state after 1300 years and initiated rapid increase in sea ice thickness. During onset of the snowball event, extensive sea ice formation and melting of sea ice in the mid-latitudes caused substantial freshening of surface waters and salinity stratification. By contrast, such salinity stratification was absent if the duration necessary for snowball onset was short because of stronger solar constant forcing. After snowball onset, the global sea ice cover reduced air–sea fluxes and caused drastic weakening in the deep ocean circulation. However, as the ocean temperature and salinity fields approached near constant states, the meridional overturning circulation resumed in the steady-state snowball climate. Although the evolution of the oceanic circulation would depend on model setting, particularly regarding the treatment of air–sea fluxes and the continental distribution, our results highlight the importance of the oceanic circulation and associated biogeochemical changes in the climate system feedback and sequence of snowball events.

Habitable terrestrial exoplanets around M dwarfs are remarkable targets due to the ease to detect and characterize. These planets are thought to have a different climate from solar system planets because these planets are expected to be in a tidally-locked state due to closer orbits and strong tidal forces. According to the studies of planet formation, terrestrial exoplanets around M dwarfs could sustain a large amount of surface water (tens or hundreds of Earth ocean mass). Such planets with large amount water (deep ocean) would be globally ocean covered, resulting in no continents. Earth’s ocean circulation largely affects the climate (e.g. meridional heat transport, water cycle). Ocean circulation is roughly classified into wind-driven circulation and density-driven circulation. In particular, density-driven circulation, which passes deep ocean, is affected by sea water temperature and salinity, resulting from heat exchange to atmosphere and salinity change by precipitation or sea ice formation. Although salinity has large influences on ocean behavior such as strength of ocean circulation and condition of sea ice formation, this effect on tidally-locked ocean planet has been still unknown. To investigate the influence on climate by salinity, first, we developed an atmospheric and oceanic global climate model (AOGCM) for exoplanets based on MIROC4m. Both atmospheric part and oceanic part solve 3-dimensional hydrodynamics, thermodynamics, and tracer transports (such as water vapor in the atmosphere and salinity in the ocean), and a coupler exchanges the information about the sea surface between both parts for example radiative heating and evaporation. By using the AOGCM, we simulated the TRAPPIST-1e’s climate assuming an ocean covered planet. We set 1 Earth ocean mass (3200 m depth because of its planetary radius) with Earth-like salinity (35.4 psu) or pure water (0 psu). In addition to the AOGCM simulations, we also run AGCM simulation which includes slab ocean instead of ocean GCM part. We integrated over 2000 years for AOGCM simulations and 50 years for AGCM simulation. An eyeball-shape open sea appears on the dayside according to the AGCM simulation, however, AOGCM simulations result in slushball-shape open sea due to the equatorial ocean currents. Comparing the results of AOGCM results, we found that salinity expands open sea distribution due to freezing-point depression and affects intensity of surface ocean current and vertical circulation. In this presentation, we will show the physical mechanisms of these differences by salinity.

In the past, Earth experienced snowball events, where its surface became completely covered with ice. Previous studies used general circulation models to investigate the onset and climate of such snowball events. Using the MIROC4m coupled atmosphere-ocean climate model, this study examined the changes in the oceanic circulation during the onset of a modern snowball Earth and elucidated their evolution to steady states under the snowball climate. Abruptly changing the solar constant to 94 % of its present-day value caused the modern Earth climate to turn into a snowball state after 1300 years and initiated rapid increase in sea ice thickness. During onset of the snowball event, extensive sea ice formation and melting of sea ice in the mid-latitudes caused substantial freshening of surface waters and salinity stratification. By contrast, such salinity stratification was absent if the duration necessary for snowball onset was short because of stronger solar constant forcing. After snowball onset, the global sea ice cover reduced air–sea fluxes and caused drastic weakening in the deep ocean circulation. However, as the ocean temperature and salinity fields approached near constant states, the meridional overturning circulation resumed in the steady-state snowball climate. Although the evolution of the oceanic circulation would depend on model setting, particularly regarding the treatment of air–sea fluxes and the continental distribution, our results highlight the importance of the oceanic circulation and associated biogeochemical changes in the climate system feedback and sequence of snowball events.

In the past, Earth experienced snowball events, where its surface became completely covered with ice. Previous studies used general circulation models to investigate the onset and climate of such snowball events. Using the MIROC4m coupled atmosphere-ocean climate model, this study examined the changes in the oceanic circulation during the onset of a modern snowball Earth and elucidated their evolution to steady states under the snowball climate. Abruptly changing the solar constant to 94 % of its present-day value caused the modern Earth climate to turn into a snowball state after 1300 years and initiated rapid increase in sea ice thickness. During onset of the snowball event, extensive sea ice formation and melting of sea ice in the mid-latitudes caused substantial freshening of surface waters and salinity stratification. By contrast, such salinity stratification was absent if the duration necessary for snowball onset was short because of stronger solar constant forcing. After snowball onset, the global sea ice cover reduced air–sea fluxes and caused drastic weakening in the deep ocean circulation. However, as the ocean temperature and salinity fields approached near constant states, the meridional overturning circulation resumed in the steady-state snowball climate. Although the evolution of the oceanic circulation would depend on model setting, particularly regarding the treatment of air–sea fluxes and the continental distribution, our results highlight the importance of the oceanic circulation and associated biogeochemical changes in the climate system feedback and sequence of snowball events.

Millennial-scale climate variability during Pleistocene Ice Ages, known as Dansgaard-Oeschger (DO) cycles, are characterised by abrupt transitions between Greenland cold stadials and warm interstadials, which coincide with gradual warming and cooling over Antarctica, respectively, via the bipolar seesaw. DO cycles are associated with reorganisations of the Atlantic Meridional Overturning Circulation (AMOC), but the mechanisms driving them remain unclear. In this study, from nudging experiments based on intrinsic millennial-scale AMOC variability in a complex climate model, we show that gradual changes in sea ice over the Southern Ocean induced by the bipolar seesaw act as a negative feedback to maintain the millennial-scale AMOC variability. Southern Ocean surface cooling during the interstadial phases enhances regional sea ice-related salt and freshwater fluxes, which eventually weakens the AMOC by strengthening the oceanic stratification over the North Atlantic by increasing and decreasing the salinity of Antarctic bottom water and Antarctic intermediate water, respectively. The Southern Ocean feedback becomes particularly important for DO cycles with long periodicities, such as those occurring during Marine Isotope Stages 5, 4, 2 and those appearing after major Heinrich events. Our results suggest that the Southern Ocean feedback helps drive the DO cycles, demonstrating the globally connected nature of these events.