How does ocean biology affect atmospheric pCO2? Theory and models

This paper examines the sensitivity of atmospheric pCO₂ to changes in ocean biology that result in drawdown of nutrients at the ocean surface. We show that the global inventory of preformed nutrients is the key determinant of atmospheric pCO₂ and the oceanic carbon storage due to the soft-tissue pump (OCS_soft). We develop a new theory showing that under conditions of perfect equilibrium between atmosphere and ocean, atmospheric pCO₂ can be written as a sum of exponential functions of OCS_soft. The theory also demonstrates how the sensitivity of atmospheric pCO₂ to changes in the soft-tissue pump depends on the preformed nutrient inventory and on surface buffer chemistry. We validate our theory against simulations of nutrient depletion in a suite of realistic general circulation models (GCMs). The decrease in atmospheric pCO₂ following surface nutrient depletion depends on the oceanic circulation in the models. Increasing deep ocean ventilation by increasing vertical mixing or Southern Ocean winds increases the atmospheric pCO₂ sensitivity to surface nutrient forcing. Conversely, stratifying the Southern Ocean decreases the atmospheric CO₂ sensitivity to surface nutrient depletion. Surface CO₂ disequilibrium due to the slow gas exchange with the atmosphere acts to make atmospheric pCO₂ more sensitive to nutrient depletion in high-ventilation models and less sensitive to nutrient depletion in low-ventilation models. Our findings have potentially important implications for both past and future climates.