Sensitivity of Ocean Carbon Sink Estimates to Rare Observations

Abstract We evaluate the impact of new data in the annual release of SOCATv2024 on air‐sea carbon dioxide (CO 2 ) flux reconstructions. Using the p CO 2 ‐Residual and LDEO‐HPD methods, we reconstruct surface ocean f CO 2 based on SOCATv2023 and SOCATv2024. For both products we find strong agreement in global mean air‐sea CO 2 fluxes for 1990–2017, but a notable divergence beginning in 2018. Reconstructions based on SOCATv2024 estimate substantially less ocean carbon uptake compared to those based on SOCATv2023, with a global mean difference of 0.25–0.27 PgCyr −1 over 2018–2022. We find that flux differences are due primarily to large changes in the Southern Ocean where data from 63 cruises was added for SOCATv2024. Analysis with a model‐testbed indicates that these data, if unbiased, should substantially increase reconstruction fidelity. We conclude that there is high sensitivity of air‐sea CO 2 flux reconstructions to the addition of data in poorly‐sampled regions.