10:45 AM - 11:00 AM
[U03-06] Mechanisms behind the transient changes in the ocean carbon cycle during the last deglaciation: insights from MIROC4m climate model simulations and proxy comparisons
★Invited Papers
Keywords:Atlantic meridional overturning circulation, ocean carbon cycle, glacial-interglacial cycle
The Atlantic Meridional Overturning Circulation (AMOC) plays a critical role in the interhemispheric heat transport and is vital to the Earth's climate system. Climate model studies suggest that global warming-induced changes in the hydrological cycle and ocean stratification could weaken the AMOC. In addition, several studies have suggested the possibility of an abrupt weakening or even a collapse of the AMOC if certain thresholds are exceeded. Changes in the AMOC could lead not only to cooling in the North Atlantic region but also to variations in tropical monsoons and changes in global climate patterns. Therefore, expanding the ocean observing network and improving climate models are essential for more accurately predicting future changes in the AMOC.
During the transition from the Last Glacial Maximum (LGM) to the Holocene, there was a significant change in the AMOC amidst global warming. Studying this last deglaciation period is important for understanding future climate change analogues. During the last deglaciation, the atmospheric carbon dioxide concentration (pCO2) increased by about 80ppm. The main cause was the release of CO2 from the oceans, which is thought to be related to the abrupt changes in the AMOC and the associated interhemispheric climate change. However, the factors causing changes in the ocean carbon cycle during the deglaciation period and their impact on atmospheric pCO2 are still not fully understood.
In this study, we investigated the temporal changes in the ocean carbon cycle during the last deglaciation (21–11ka BP) using the MIROC4m climate model, which shows rapid AMOC changes consistent with proxies. By comparing calculated carbon isotope ratios and sediment core data, we investigated the model's reproducibility, potential biases, and underestimated processes. The calculated changes in atmospheric pCO2 are in qualitative agreement with the ice core record. Atmospheric pCO2 increased by 10.2ppm during the Heinrich Stadial 1 (HS1) period, then decreased by 7.0ppm during the Bølling-Allerød (BA) period, and increased by 6.8ppm during the Younger Dryas (YD) period. However, the model underestimates the changes in atmospheric pCO2 compared to the values derived from ice core data.
The increase in atmospheric pCO2 during the HS1 period was primarily due to a decrease in solubility caused by rising water temperatures. However, analyses of radiocarbon (Δ14C) and stable carbon isotope ratios (δ13C) suggest that the model underestimates the activation of deep-sea ventilation, the decrease in biological carbon export efficiency in the Southern Ocean, and the activation of ventilation in the mid-depth North Pacific during the HS1 period. This underestimation could be related to the relatively small changes in atmospheric pCO2 observed during the HS1 period.
The strengthening and weakening of the AMOC during the BA and YD periods are generally consistent with changes in Δ14C values obtained from sediment core records. However, in contrast to the data, which show a continuous increase in δ13C in the deep sea during the YD period, the model shows an opposite trend. This discrepancy suggests that the model may overestimate the weakening of the AMOC during the YD period or that the representation of biogeochemical processes, including the response of marine ecosystems and terrestrial carbon storage, is limited.
The analysis revealed that changes in temperature and alkalinity have the most significant impact on changes in ocean pCO2, and thus atmospheric pCO2. Specifically, increases (or decreases) in temperature decrease (or increase) the solubility of pCO2 in the ocean, thereby increasing (or decreasing) pCO2, while changes in alkalinity decrease (or increase) pCO2. The interplay of these effects suggests that the changes in the AMOC and associated bipolar climate changes contribute to the slight decrease (during the BA period) and increase (during the YD period) in atmospheric pCO2.
During the transition from the Last Glacial Maximum (LGM) to the Holocene, there was a significant change in the AMOC amidst global warming. Studying this last deglaciation period is important for understanding future climate change analogues. During the last deglaciation, the atmospheric carbon dioxide concentration (pCO2) increased by about 80ppm. The main cause was the release of CO2 from the oceans, which is thought to be related to the abrupt changes in the AMOC and the associated interhemispheric climate change. However, the factors causing changes in the ocean carbon cycle during the deglaciation period and their impact on atmospheric pCO2 are still not fully understood.
In this study, we investigated the temporal changes in the ocean carbon cycle during the last deglaciation (21–11ka BP) using the MIROC4m climate model, which shows rapid AMOC changes consistent with proxies. By comparing calculated carbon isotope ratios and sediment core data, we investigated the model's reproducibility, potential biases, and underestimated processes. The calculated changes in atmospheric pCO2 are in qualitative agreement with the ice core record. Atmospheric pCO2 increased by 10.2ppm during the Heinrich Stadial 1 (HS1) period, then decreased by 7.0ppm during the Bølling-Allerød (BA) period, and increased by 6.8ppm during the Younger Dryas (YD) period. However, the model underestimates the changes in atmospheric pCO2 compared to the values derived from ice core data.
The increase in atmospheric pCO2 during the HS1 period was primarily due to a decrease in solubility caused by rising water temperatures. However, analyses of radiocarbon (Δ14C) and stable carbon isotope ratios (δ13C) suggest that the model underestimates the activation of deep-sea ventilation, the decrease in biological carbon export efficiency in the Southern Ocean, and the activation of ventilation in the mid-depth North Pacific during the HS1 period. This underestimation could be related to the relatively small changes in atmospheric pCO2 observed during the HS1 period.
The strengthening and weakening of the AMOC during the BA and YD periods are generally consistent with changes in Δ14C values obtained from sediment core records. However, in contrast to the data, which show a continuous increase in δ13C in the deep sea during the YD period, the model shows an opposite trend. This discrepancy suggests that the model may overestimate the weakening of the AMOC during the YD period or that the representation of biogeochemical processes, including the response of marine ecosystems and terrestrial carbon storage, is limited.
The analysis revealed that changes in temperature and alkalinity have the most significant impact on changes in ocean pCO2, and thus atmospheric pCO2. Specifically, increases (or decreases) in temperature decrease (or increase) the solubility of pCO2 in the ocean, thereby increasing (or decreasing) pCO2, while changes in alkalinity decrease (or increase) pCO2. The interplay of these effects suggests that the changes in the AMOC and associated bipolar climate changes contribute to the slight decrease (during the BA period) and increase (during the YD period) in atmospheric pCO2.