Observation-based quantification of ocean carbon dioxide (CO2) uptake relies on synthesis data sets such as the Surface Ocean CO2 ATlas (SOCAT). However, the data collection effort has dramatically declined and the number of annual data sets in SOCATv2023 decreased by similar to 35% from 2017 to 2021. This decline has led to a 65% increase (from 0.15 to 0.25 Pg C yr(-1)) in the standard deviation of seven SOCAT-based air-sea CO2 flux estimates. Reducing the availability of the annual data to that in the year 2000 creates substantial bias (50%) in the long-term flux trend. The annual mean CO2 flux is insensitive to the seasonal skew of the SOCAT data and to the addition of the lower accuracy data set available in SOCAT. Our study highlights the need for sustained data collection and synthesis, to inform the Global Carbon Budget assessment, the UN-led climate negotiations, and measurement, reporting, and verification of ocean-based CO2 removal projects.