A global three-dimensional ocean carbon cycle model was used to investigate the use of macronutrient fertilization of the ocean to increase the oceanic uptake of CO₂. To simulate macronutrient fertilization, phosphate was added to the 18°–50°S surface ocean. The carbon sequestration efficiency of fertilization was determined from the ratio of increased ocean uptake of anthropogenic CO₂ to the rate of phosphate addition to the upper ocean (converted to carbon units using the C/P ratio of organic matter, 106). The model simulation produced a maximum efficiency of 78%. However, the simulations demonstrated that changes in calcium carbon production with macronutrient fertilization could significantly reduce carbon sequestration efficiency. When calcium carbonate production increases at the same rate as export production, the carbon sequestration efficiency is reduced by 25% when compared to a simulation where calcium carbonate production is held constant. The study also discusses several other potential process that could impact the efficiency phosphate fertilization to sequester carbon in the ocean and the potential consequences of large-scale macronutrient fertilization of the ocean.