Plain Language Summary: The ocean has absorbed roughly 40% of fossil fuel carbon dioxide (CO₂) emissions since the beginning of the industrial era. This so-called “ocean carbon sink,” which primarily sequesters emissions in the form of dissolved inorganic carbon (DIC), plays a key role in regulating climate and mitigating global warming. However, we still lack a mechanistic understanding of how physical, chemical, and biological processes impact the ocean DIC reservoir in both space and time, and hence how the storage rates of emissions may change in the future.

Here we use a global-ocean biogeochemistry model Estimating the Circulation and Climate of the Ocean-Darwin, which ingests both physical and biogeochemical observations to improve its accuracy, to map how ocean circulation, air-sea CO₂ exchange, and marine ecosystems have modulated the combined natural and anthropogenic ocean DIC budget for 1995–2018. We find that in the upper ocean, circulation provides the largest supply of DIC while biological processes drive the largest loss. Year-to-year changes in the ocean carbon sink are dominated by El Niño-Southern Oscillation events in the equatorial Pacific Ocean, which then affect DIC globally.

In summary, our data-constrained, global-ocean DIC budget constitutes a significant step forward toward understanding climate-related changes to the ocean DIC reservoir.