Differing methods of accounting ocean carbon sequestration efficiency

Presently, much of CO₂ fossil-fuel emissions are removed from the atmosphere through natural ocean uptake of CO₂. Many schemes have now been proposed by which the accumulation of anthropogenic CO₂ in the atmosphere could be slowed with intentional further storage of CO₂ in the ocean. Our review of the literature indicates inconsistency in whether ambient ocean carbon uptake is included when accounting for the effectiveness of such schemes. This inconsistency is a consequence of differing choices of atmospheric boundary condition. In the case of one particular form of ocean sequestration, namely direct injection of liquefied CO₂ emissions into the ocean interior, this choice is the determination of whether the atmospheric CO₂ concentration responsively increases due to leakage of injected carbon from the ocean or retains a specified value. We first show how results of simulations using these two different boundary conditions can be related with the convolution of an atmosphere pulse release. We then use a numerical model to present a more complete analysis of the role of these boundary conditions. Finally, we suggest that a responsive atmospheric CO₂ boundary condition is appropriate for predicting future carbon concentrations, but a specified atmospheric CO₂ boundary condition is appropriate for evaluating how much CO₂ storage should be attributed to an ocean storage project.