Fig. 1.

Abrupt system changes in the physical, biogeochemical, and ecosystem compartments of the ocean (big colored boxes) can be induced by external drivers (rounded black and white boxes) or by couplings between the compartments. Tipping behavior can be potentially induced by any type of boundary conditions and can cascade from a change of the system state in one compartment to the boundary conditions for another compartment. “System state” on the y axes of the small diagrams denotes all relevant state variables such as temperature, O₂ concentration, pH etc. “Boundary conditions” on the x axes includes all relevant forcings either from external sources or from other compartments of the ocean system. N_r stands for reactive nitrogen (such as nutrient inputs from land). Both boundary conditions and system states are time dependent. Abrupt changes can potentially be reversible, so that one system state occurs for a unique type of conditions directly following the forcing. Particularly critical are abrupt system changes that show hysteresis. In this case, a certain system state is not coupled to one unique forcing. Strong negative forcing may be needed to enable a return to the initial system state (or such a return may be completely impossible in case of irreversibility).