The importance of adding unbiased Argo observations to the ocean carbon observing system

The current coverage of direct, high-quality ship-based observations of surface ocean pCO₂ includes large gaps in time and space, and has been declining since 2017. These ocean observations provide the basis for the data products that reconstruct surface ocean pCO₂ and estimate ocean carbon uptake. Improved data coverage is needed to advance our understanding of the ocean carbon sink and air-sea CO₂ exchange. Targeted sampling from autonomous platforms, such as biogeochemical floats, combined with traditional shipboard measurements represents a promising path forward to improve surface ocean pCO₂ reconstructions. However, floats provide indirect pCO₂ estimates derived from pH, and thus have higher uncertainty and are biased compared to direct shipboard measurements. Here, we use a Large Ensemble Testbed (LET) of Earth System Models and the pCO₂-Residual method to reconstruct surface ocean pCO₂ globally to test the impact of additional float observations, both with and without measurement uncertainties. Through comparison to the 'model truth', the LET allows for robust evaluation of the reconstructions. With only shipboard sampling, surface ocean pCO₂ is overestimated, and the 2000-2016 global ocean carbon sink is underestimated by 0.1 Pg C year^-1. Additional float observations significantly reduce this underestimation, and deviate from the 'model truth' by as little as 0.01 Pg C year^-1, even when floats have random uncertainties of ± 11 μatm. However, systematic bias in the float observations significantly degrades the accuracy of pCO₂ reconstructions, leading to an even stronger underestimation of the global ocean carbon sink of up to 0.32 Pg C year^-1. We conclude that adding float-based observations to the global observing system can significantly improve reconstructions of global surface ocean pCO₂, but only if these data are unbiased.