The control of atmospheric pCO ₂ by ocean ventilation change: The effect of the oceanic storage of biogenic carbon

A simple analytical framework is developed relating the atmospheric partial pressure of CO ₂ to the globally-averaged concentrations of respired carbon (C _soft) and dissolved carbonate (C _carb) in the ocean. Assuming that the inventory of carbon is conserved in the ocean-atmosphere system (i.e. no seawater-sediment interactions), the resulting formula of ΔpCO ₂/pCO ₂ =-0.0053Δ C _soft + 0.0034Δ C _carb suggests that atmospheric pCO ₂ would decrease by 5.3% and increase by 3.4% when C _soft and C _carb increase by 10 mol kg ^-1, respectively. Using this analytical framework along with a 3-D global ocean biogeochemistry model, we show that the response of atmospheric pCO ₂ to changes in ocean circulation is rather modest because ∼30% of the change in atmospheric pCO ₂ caused by the accumulation of respired carbon is countered by a concomitant accumulation of dissolved carbonate in deep waters. Among the suite of circulation models examined here, the largest reduction in atmospheric pCO ₂ of 44-88 ppm occurs in a model where reduced overturning rates of both southern and northern sourced deep waters result in a four-fold increase in the Southern Ocean deep water ventilation age. On the other hand, when the ventilation rate of the southern-sourced water decreases, but the overturning rate of North Atlantic Deep Water increases, the resulting decrease in atmospheric pCO ₂ is only 14-34 ppm. The large uncertainty ranges in atmospheric pCO ₂ arise from uncertainty in how surface productivity responds to circulation change. Although the uncertainty is large, this study suggests that a synchronously reduced rate for the deep water formation in both hemispheres could lead to the large glacial reduction in atmospheric pCO ₂ of 80-100 ppm.