Biology-mediated temperature control on atmospheric pCO₂ and ocean biogeochemistry

The remarkable correspondence between glacial-interglacial changes in atmospheric CO₂ levels and global climate over much of the Pleistocene suggests that CO₂ is also a key climate change driver. However, there is as yet no widely accepted explanation of the low glacial CO₂ levels. Here I use an intermediate-complexity climate model to show that glacial cooling, acting on the rates of organic carbon production and decay in the ocean, can explain a significant portion of the glacial CO₂ lowering. New model results show that cooling strengthens the vertical transport of organic carbon from the surface ocean to the deep ocean, reduces atmospheric pCO₂, and shifts nutrients from the Atlantic basin to the Indo-Pacific basins. The overall vertical transport is increased because the cooling effect on reducing the degradation rate of sinking particulate organic carbon is greater than on reducing the export production. This net temperature effect on atmospheric pCO₂ mediated by biology is comparable to the temperature effect on atmospheric pCO₂ driven by solubility, which is almost always mentioned as a large factor in the glacial CO₂ levels. An implication for the future is that higher ocean temperatures will act as a positive feedback on atmospheric CO₂ by reducing the vertical transport of carbon to the deep ocean and thereby increasing CO₂ degassing from the ocean.