The quiet crossing of ocean tipping points

Abstract

Anthropogenic climate change profoundly alters the ocean's environmental conditions, which, in turn, impact marine ecosystems. Some of these changes are happening fast and may be difficult to reverse. The identification and monitoring of such changes, which also includes tipping points, is an ongoing and emerging research effort. Prevention of negative impacts requires mitigation efforts based on feasible research-based pathways. Climate-induced tipping points are traditionally associated with singular catastrophic events (relative to natural variations) of dramatic negative impact. High-probability high-impact ocean tipping points due to warming, ocean acidification, and deoxygenation may be more fragmented both regionally and in time but add up to global dimensions. These tipping points in combination with gradual changes need to be addressed as seriously as singular catastrophic events in order to prevent the cumulative and often compounding negative societal and Earth system impacts.

Keywords: biogeochemistry; climate change; ocean; regime shifts; tipping points.

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).

Fig. 2.

Candidates for high-probability high-impact marine tipping elements that concern warming, deoxygenation, and ocean acidification as well as their impacts. Further details concerning drivers, variables affected, and potential hazards are listed in SI Appendix, Table S1. The areas indicated in the map are approximate only. References/examples: ①, refs. and ; ②, refs. and ; ③, refs. , , and ; ④, refs. and ; ⑤, ref. ; ⑥, refs. and ; ⑦, refs. and ; ⑧, refs. –; ⑨, refs. , , and ; ⑩, refs. , , and ; ⑪, refs. , , and .

Fig. 3.

Time scale of the large-scale ocean mixing and circulation. Image credit: V. Byfield, National Oceanography Centre (NOC), licensed under CC BY 3.0. The original figure was modified with mean age indications of water masses since their last contact with atmosphere; the age indications are based on ideal age tracers following refs. and 136).