Box 3.1 Thresholds and Hysteresis
Many simple physical systems exhibit abrupt change, as demonstrated in the simple diagram presented in Box 1.1. The more complex figure (Figure 3.1 ) provides a schematic view of hysteresis in the thermohaline circulation. The upper branch denotes climate states in which the THC is strong and North Atlantic temperatures are relatively high (similar to present conditions). The lower branch represents a much-reduced or collapsed THC, in which the Atlantic meridional heat flux by the ocean is small. A given perturbation (indicated by the horizontal arrows) in the freshwater balance of the North Atlantic (precipitation plus runoff minus evaporation) first causes transitions from an initial state 1 to state 2. The reverse perturbation then causes a transition back to state 1, or to state 3. Three structurally different responses are possible for the same pair of perturbations, depending on whether threshold values (dashed line) are crossed. This, in turn, depends on where state 1 is, relative to the threshold: a) small, reversible response; b) large, reversible response; c) large, irreversible response (Stocker and Marchal, 2000).