How does ocean biology affect atmospheric p CO 2 ? Theory and models

This paper examines the sensitivity of atmospheric p CO 2 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 p CO 2 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 p CO 2 can be written as a sum of exponential functions of OCS soft . The theory also demonstrates how the sensitivity of atmospheric p CO 2 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 p CO 2 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 p CO 2 sensitivity to surface nutrient forcing. Conversely, stratifying the Southern Ocean decreases the atmospheric CO 2 sensitivity to surface nutrient depletion. Surface CO 2 disequilibrium due to the slow gas exchange with the atmosphere acts to make atmospheric p CO 2 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.