Thermohaline Catastrophe

A thermohaline catastrophe is a rapid, large-scale shutdown or reorganization of the ocean's density-driven circulation due to a perturbation in temperature or salinity. The term gained currency after the discovery that freshwater pulses from melting ice sheets — such as during the \u200bYounger Dryas or the 8.2 kiloyear event — could weaken or halt North Atlantic Deep Water formation, triggering hemispheric cooling within decades.

The mechanism is a \u200bbistable switch: the circulation can exist in either a strong or weak mode, and a sufficiently large freshwater forcing can push it from one basin of attraction to the other. Once in the weak mode, the system exhibits \u200bhysteresis — the original strong mode is not restored simply by removing the perturbation. This nonlinearity makes thermohaline catastrophe a paradigmatic example of a \u200bcritical transition in the Earth system. See Atlantic Thermohaline Circulation and Abrupt Climate Change.