The ocean has absorbed approximately one-quarter of anthropogenic carbon dioxide (CO2) emissions since the industrial era, moderating atmospheric warming while driving significant changes in marine carbonate chemistry. Collectively referred to as ocean acidification (OA), these changes include declining seawater pH, reduced carbonate ion availability, and decreasing calcium carbonate saturation states, with consequences for marine ecosystems, biogeochemical cycles, and ocean services. Detecting and quantifying OA is challenging because anthropogenic trends are superimposed on substantial natural variability across a wide range of spatial and temporal scales. Consequently, no single variable, platform, or observing approach can adequately characterize OA.
This chapter reviews the principal components of the contemporary OA observing system and the methodologies used to quantify changes in marine carbonate chemistry. It introduces the marine carbonate system and the analytical methods used to measure key OA variables, including pH, dissolved inorganic carbon (DIC), total alkalinity (TA), and partial pressure of CO2 (pCO2). The chapter examines the role of sustained time-series stations in detecting long-term trends and distinguishing anthropogenic change from natural variability, highlighting evidence for widespread acidification across open-ocean, coastal, polar, and marginal seas. The complementary contributions of surface underway observations, repeat hydrography programs, and autonomous observing platforms are reviewed, including major international initiatives such as SOCONET, SOCAT, GO-SHIP, GLODAP, and Biogeochemical-Argo (BGC-Argo). Particular emphasis is placed on how these observing elements provide complementary information across spatial and temporal scales and together form an integrated OA observing system.
Finally, the chapter discusses uncertainty assessment, trend attribution, data integration, and the role of international coordination mechanisms, including GOOS, GCOS, IOCCP, and GOA-ON. It further examines how OA observations support climate assessments, model evaluation, ecosystem-risk assessments, Sustainable Development Goal (SDG) 14.3.1 reporting, and emerging monitoring, reporting, and verification needs associated with marine carbon dioxide removal.
El Rahman Hassoun A., Sabine C., Krasakopoulou E., Steinhoff T., Tanhua T., Wimart Rousseau C., Álvarez M., Isensee K. & Schoo K. L., 2026. The climate observing system – ocean acidification. Reference Module in Earth Systems and Environmental Sciences. Elsevier. doi: 10.1016/B978-0-443-15748-6.00085-X. Chapter (restricted access)
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