Impact of Surface Ocean CO2 Atlas (SOCAT) observing network extensions on the quantification of global air-sea CO2 fluxes

Observing seawater CO2 concentrations is fundamental to understanding the dynamics of CO2 exchanges at the air-sea interface. Over the last decades, sustained efforts in global data collection have established the Surface Ocean CO2 Atlas (SOCAT) as the leading database for quantifying CO2 fluxes and the ocean carbon budget. SOCAT continuously integrates new data while performing annual quality reassessments, and such adjustments could significantly influence the global flux estimation. This study provides a pioneering analysis of the evolution of data updates across the five most recent SOCAT releases (SOCATv2020 to SOCATv2024) and consequences on flux estimates by mapping in situ CO2 fugacity with reanalysis data of environmental predictor variables. Five neural network ensembles are trained on SOCAT datasets to reproduce global maps of air-sea CO2 fluxes for the period 2000-2023. SOCATv2024 incorporates approximately 20% more data than SOCATv2020, revising the global ocean net CO2 sink estimate to 1.77±0.17 PgC yr-1, approximately 4.5% to 9.6% lower than earlier versions. Global uncertainty (1σ) improves by 0.3% to 7.6% compared to SOCATv2023 and SOCATv2020. However, a decline in data collection since 2017 causes a significant divergence in flux estimates, with annual net sink estimates from SOCATv2024 being 0.2 PgC yr-1 to 1 PgC yr-1 lower than previous estimates. Recent updates primarily fine-tune data coverage, with pronounced impacts on flux estimates in the Southern Ocean, upwelling zones, and subpolar Pacific.