Marine carbon dioxide removal (mCDR) is gaining interest as a tool to meet global climate goals. However, because the response of the coupled ocean-atmosphere system to mCDR takes years to centuries, modeling is required to assess the impact of mCDR on atmospheric CO2 reduction. Here, we use a model with an interactive atmosphere coupled to a data-assimilated ocean circulation model to quantify the magnitude and timescales of atmospheric CO2 reduction in response to a CDR perturbation. We define two metrics to characterize the atmospheric CO2 reduction in response to both instantaneous ocean alkalinity enhancement (OAE) and direct air capture (DAC); the cumulative additionality (α), which measures the reduction in atmospheric CO2 relative to the initial CDR perturbation, and the relative efficiency (e), which quantifies the cumulative additionality relative to DAC. For DAC, α is 100% immediately following CDR deployment, and declines to roughly 50% by 100 years post-deployment. For instantaneous OAE, α is zero initially and reaches a maximum of 40-90% several years to decades later, depending on regional CO2 equilibration rates and ocean circulation processes. The global mean e approaches 100% after 40 years, showing that instantaneous OAE is nearly as effective as DAC after several decades. However, there are significant spatial variations, with e approaching 100% most rapidly in the low latitudes while e stays well under 100% for decades to centuries near deep and intermediate water formation sites. The model presented provides a quantitative framework for evaluating sequestration timescales and carbon market valuation that can be applied to any mCDR strategy.
