The ocean is being reshaped by the combined effects of multiple climate stressors in a warming climate, including the concurrent change in warming, deoxygenation, acidification, and salinity shifts. However, most of the previous studies, as well as many observing and modelling frameworks developed for Digital Twins of the Ocean (DTOs), still tend to treat these variables separately or focus on single stressors. A synthesis assessment framework for compound ocean state changes, indicated by multiple climate stressors (double/triple stressors) remains insufficiently developed.

Drawing on a series of studies by our team, this presentation will first introduce a definition and an assessment framework for ocean compound state change. It then examines how the ocean’s climate state is evolving on a long-term scale, presenting observational evidence that the ocean has experienced large-scale and deep-reaching compound state changes over the past 65 years. The regions that are most exposed to the ocean state change are identified. This indicates that the ocean climate system has already exhibited threshold-crossing (tipping point) behavior, implying the system has entered into a new ocean state. Then, this presentation will focus on the South Atlantic as a regional study to explore the physical mechanisms behind these compound state changes. In particular, the water-mass variability in terms of ‘spice’ and ‘heave’ components (i.e., local signal), together with the ongoing South Atlantic intermediate layer “Indianification” (i.e., remote signal), are key drivers of compound state change in this region.

Finally, a new ocean state will exacerbate compound climate risks, making marine governance increasingly urgent. The presentation will take the BBNJ Agreement (UN High Seas Treaty), which was recently entered into force, as a policy framework, to explore how the compound ocean state change informs the spatial designation of climate-ready Marine Protected Areas (MPAs) in areas beyond national jurisdiction (ABNJ). More broadly, we expect such actionable knowledge for international ocean governance to be embedded in the decision-support and governance application layer of DTOs in the near future.