GOOS Essential Ocean Variables: the backbone of a sustained and evolving international ocean observing system

There is an urgent need for a better understanding of the ocean, and we cannot understand what we do not measure. This contribution describes the origin, evolution and implementation of a set of ocean variables (EOVs) spanning physics, biogeochemistry and biology, which are essential to measure and are managed by the Global Ocean Observing System (GOOS) programme. GOOS EOVs address key information required for understanding and monitoring current and future climate change, changes to a major proportion of global biodiversity, hazards impacting human populations and services provided by the ocean to society. The GOOS EOVs are used to define the needs and design of a sustained, fit for purpose global ocean observing system, aimed at maximising investments in observing infrastructure. Guided by panels of experts of disciplinary expertise, GOOS EOVs can be and are integrated with other observing frameworks that define essential climate or biodiversity variables and indicators. As the GOOS EOVs are increasingly used, it has become important to discuss and refine the process and international governance of this keystone framework, to ensure that the right balance is struck between the essential nature of the GOOS EOVs and the need to expand to new domains and to integrate with key global policies. We discuss some of the challenges in implementing the EOVs and identify a set of recommendations for their continued evolution, contributing to building a useful and responsive global ocean observing system that delivers into societal needs.