Tropical Sources Dominate the Ocean Carbonyl Sulfide Budget

Carbonyl sulfide (OCS) regulates stratospheric aerosol–climate interactions and helps to track terrestrial photosynthesis. Using OCS to infer drivers of stratospheric aerosol loading or biospheric productivity requires understanding its largest and most uncertain natural source—ocean emissions. Top-down ocean OCS budget estimates inferred from atmospheric observations range from 230 to 1,083 Gg S yr⁻¹ (median: 677 Gg S yr⁻¹), with most estimates far beyond bottom-up estimates informed by process understanding and ship-based observations, which range from 182 to 313 Gg S yr⁻¹ (median: 280 Gg S yr⁻¹). While ship-based observations suggest that direct OCS emissions are unlikely to account for this gap, limited understanding of the marine cycling of carbon disulfide (CS₂), a key OCS precursor, hampers the assessment of CS₂-derived OCS emissions. Here, we construct a data-driven model to predict OCS and CS₂ fluxes from ship-based observations and reanalysis and remotely sensed environmental covariates. We estimate an ocean OCS budget of 270 ± 28 Gg S yr⁻¹, half of which results from CS₂ emissions. Of the portion of OCS emissions resulting from direct and CS₂-derived emissions (217 Gg S yr⁻¹), more than half come from tropical (23.5°S–23.5°N) oceans, contrasting with process-based models that show stronger high-latitude contributions. Our new data-driven ocean OCS budget estimate remains substantially lower than most top-down estimates and implies a weaker land OCS sink. Our findings suggest that the OCS budget imbalance could have resulted from an overestimated land sink rather than the commonly presumed missing ocean source, with major implications for tracking terrestrial photosynthesis.