Glacial-Interglacial Variations of Atmospheric CO2 Concentration: the Effect of Ocean Physical Field on the Carbon Cycle

Ancient air trapped in Antarctic ice cores shows that atmospheric CO2 concentration, pCO2, was lower during glacial periods than during interglacial periods. At the Last Glacial Maximum (LGM), pCO2 was 180-200 ppm which is 80-100 lower than the preanthropogenic value. This observation suggests that variations in pCO2 has played an important role in global climate change during the late Quaternary. Although many hypotheses have been proposed to explain the pCO2 variations, the reason for the low glacial pCO2 is still unclear. Concerning this problem, we investigate the effect of physical changes in ocean (the circulation field, temperature, salinity) on pCO2 in this study. We consider two physical ocean fields which are based on reproduction by an atmosphere-ocean coupled general circulation model (MIROC3.2). One corresponds to the preindustrial state, and the other to the LGM state. In order to consider the carbon cycle in the ocean, we use an ocean general circulation model (COCO3.4), which is an ocean component of MIROC3.2, coupled with a simple biogeochemical model. First, we simulate distribution of chemical tracers, and obtain pCO2 of 280 ppm when the preindustrial physical ocean field is assumed. On the other hand, when the LGM ocean field is assumed (any other conditions are the same), pCO2 is lowered by ~ 30 ppm. Next, in order to evaluate the effect of solubility change by lower sea surface temperature (SST) on pCO2, we calculate the solubility using SST at the LGM, although we assume that the circulation field is identical to the preindustrial state. As a result, pCO2 also is lowered by ~ 30 ppm. This result suggest that the effect of solubility change could be dominant in the effects of physical changes in ocean.