The Baltic Sea as a Natural Experiment of Climate Change: From Physical, Chemical and Biological Changes to Ecosystem Responses—Implications for Sustainability

The Baltic Sea is one of the best-studied marine areas in the world, while at the same time being among the most sensitive to human pressure and related environmental change. This review manuscript synthesizes current knowledge on the impacts of climate change on the Baltic Sea using the observed state and functioning of the ecosystem as a starting point to identify the physical, chemical, and biological mechanisms driving these changes. At the same time, climate change is treated as a dominant modifying factor of these mechanisms rather than as the single primary cause of the observed processes. The manuscript discusses changes in the structure and functioning of the Baltic Sea ecosystem, including shifts in plankton communities, the expansion of hypoxic zones, changes in benthic functioning, responses of fish populations, and the increasing role of pollution and anthropogenic pressures originating from both marine activities and processes within the Baltic Sea catchment. By treating the Baltic Sea as a model system, this review manuscript shows that the processes and ecosystem responses currently observed may serve as early signals of changes that in other seas may only become evident in the future. The insights gained from this natural experiment highlight the need to integrate climatic, physical, chemical, and biological knowledge into marine management strategies.