Impact of Ocean Physical Conditions on Oceanic Carbon Pumps and Atmospheric Carbon Dioxide at the Last Glacial Maximum

Abstract

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.