in the atmosphere-ocean system is not a prerequisite for such behavior, whose time scale is dictated essentially by that of the forcing.1
Slow changes in forcing can induce the crossing of a threshold and result in the transition to a second equilibrium of the system. The evolution of such a change would be governed by the system dynamics rather than by the external time scale of the slow change. In considering the whole earth system rather than just the oceans and atmosphere, massive discharges of freshwater from disintegrating ice sheets would be a result of threshold-crossing. Slow melting at the end of the last ice age produced ice-marginal lakes. When the ice margin reached a particular location, such as the path of a former river that the ice had dammed, a threshold was crossed, the ice dam broke, and the water was released rapidly (Broecker et al., 1988).
Regime transitions can occur spontaneously in a chaotic system. In this case, external triggers for transitions are not required, so a series of regime changes could continue indefinitely or until slow changes in external forcing or system dynamics removed the chaotic behavior.
Changes in ocean circulation, and especially THC in the North Atlantic, have been implicated in abrupt climate change of the past, such as the Younger Dryas and the Dansgaard/Oeschger and Heinrich/Bond oscillations (Broecker et al., 1988; Alley and Clark, 1999; Stocker, 2000). Today, relatively warm waters reach high latitudes only in the North Atlantic. The high salinity of the Atlantic waters allows them to sink into the deep ocean when they cool, and warmer waters flowing along the surface then replace them. This yields a net heat transport into the high northern latitudes of the Atlantic and northward heat transport throughout the South Atlantic, carrying heat into the North Atlantic (Ganachaud and Wunsch, 2000; see also Plate 4b.)
Outburst floods, which would have freshened the North Atlantic and reduced the ability of its waters to sink, immediately preceded the coolings of the Younger Dryas and the short cold event about 8,200 years ago (Broecker et al., 1988; Barber et al., 1999); this suggests causation. Evidence of reduction or elimination of northern sinking of waters during cold times (Sarnthein et al., 1994; Boyle, 2000) provides further support, as does