Toward an integrated observing system for ocean carbon and biogeochemistry at a time of change

Ocean biogeochemical cycles are currently undergoing fundamental changes – largely as a consequence of the addition of greenhouse gases to the atmosphere. The oceans are getting warmer, and their pH and oxygen levels are decreasing. Further changes may arise as a consequence of the perturbation of the global nitrogen cycle, leading to higher inputs of fixed nitrogen to the ocean by rivers and enhanced deposition of nitrogen to the surface ocean. These biogeochemical changes plus the concomitant changes in ocean circulation will have profound effects on some of the ocean’s key services, i.e. its capability to mitigate climate change by removing anthropogenic CO2 from the atmosphere, and its provision of important ecosystem services such as food and biodiversity. Documenting, understanding, and predicting these biogeochemical changes require a concerted and sustained observational effort that includes both the continuation of well-tested approaches and the development and implementation of novel systems. Of particular importance for the first set of approaches are the sustaining and extension of (i) a surface ocean volunteer ocean ship-based observing system primarily focusing on the determination of the air-sea exchange of CO2 and upper ocean changes in carbonate chemistry, of (ii) an interior ocean research-ship based system focusing on large-scale interior changes of the ocean’s biogeochemistry (carbon, oxygen, nutrients, etc), and of (iii) ship-based and moored time-series observations at key sites, including the coastal ocean. Of particular importance for the second set of approaches are (i) the accelerated improvement, development and implementation of new observational elements on the Argo array (especially oxygen, but also nutrient and bio-optical sensors), and (ii) the development, testing, and deployment of novel sensors for the ocean’s carbon system. Concerted synthesis efforts involving also novel approaches for merging observations with biogeochemical models will ensure that these observational elements realize their synergistic potential.