Glacial CO₂ cycle as a succession of key physical and biogeochemical processes

Abstract. During glacial-interglacial cycles, atmospheric CO₂ concentration varied by about 100 ppmv in amplitude. While testing mechanisms that have led to the low glacial CO₂ level could be done in equilibrium model experiments, an ultimate goal is to explain CO₂ changes in transient simulations through the complete glacial-interglacial cycle. The computationally efficient Earth System model of intermediate complexity CLIMBER-2 is used to simulate global biogeochemistry over the last glacial cycle (126 kyr). The physical core of the model (atmosphere, ocean, land and ice sheets) is driven by orbital changes and reconstructed radiative forcing from greenhouses gases, ice, and aeolian dust. The carbon cycle model is able to reproduce the main features of the CO₂ changes: a 50 ppmv CO₂ drop during glacial inception, a minimum concentration at the last glacial maximum 80 ppmv lower than the Holocene value, and an abrupt 60 ppmv CO₂ rise during the deglaciation. The model deep ocean δ¹³C also resembles reconstructions from deep-sea cores. The main drivers of atmospheric CO₂ evolve in time: changes in sea surface temperatures and in the volume of bottom water of southern origin control atmospheric CO₂ during the glacial inception and deglaciation; changes in carbonate chemistry and marine biology are dominant during the first and second parts of the glacial cycle, respectively. These feedback mechanisms could also significantly impact the ultimate climate response to the anthropogenic perturbation.