Role of the Ocean in Changes of Atmospheric CO2 on Glacial-Interglacial and Millennial Timescales

Measurements of air trapped in ice cores show changes in atmospheric CO₂ contentrations of ~80 ppmv on glacial-interglacial timescales and intra-glacial variability of ~20 ppmv on millennial timescales. Using highly idealized (box or zonally averaged) models, it has been suggested that the ocean accounts both for glacial to interglacial changes of atmospheric CO₂ as well as for the intra-glacial variability. Here I reassess both problems with a global coupled climate model (UVic ESCM) containing state-of-the-art 3D ocean circulation, ecosystem and carbon cycle modules. Simulated variations of atmospheric CO₂ in response to changes in North Atlantic Deep Water (NADW) formation are much smaller (~2 ppmv) than previously found. This suggests a much larger role of the terrestrial biosphere in millennial CO₂ variations than previously assumed. Much larger variations (~20 ppmv) are simulated for glacial-interglacial changes, although still largely underestimated compared to the observations. The processes leading to these modelled changes, particularly deep ocean ventilation, air-sea gas exchange and productivity, are analysed. The potential role of missing processes such as increased productivity through iron fertilisation and calcium carbonate compensation within the sediments are discussed. I conclude that we are still a far way from understanding the natural variability of the carbon cycle and that the simulation of both glacial-interglacial and millennial variability remains a key challenge for Earth System Models.