FIGURE 4.1 Evolution of the maximum overturning in the Atlantic (strength of the thermohaline circulation [THC]) for a coupled model of reduced complexity for 100 realizations of the model. Radiative forcing is increased from years 1000 to 1140, equivalent to a doubling of CO2, and then held constant. The warming pushes the model closer to the bifurcation point, and transitions usually occur when the overturning is weakened. Two individual realizations are highlighted by the black lines (Knutti and Stocker, 2001).

THC (the Faroe Bank overflow from the Nordic Seas; Hansen et al., 2001b) has experienced both a reduction in total flow and a warming and freshening over the last 50 years, consistent with many model projections. However, the entire THC is not monitored. This highlights the importance of maintaining and increasing measurements and data-model comparisons that capture the behavior of the entire system.


Possible instabilities of the THC also have important implications for the predictability of future climate change. Model simulations show that as an instability is approached, small deviations in initial or boundary conditions can determine whether a transition to a different equilibrium will occur, which inherently limits predictability. This behavior has been investigated with a climate model of reduced complexity (Knutti and Stocker, 2001). The threshold is approached by a prescribed global warming over about 140 years, equivalent to a doubling of carbon dioxide. Small random fluctuations, as produced by atmospheric disturbances at the ocean surface, can excite large changes in the THC when the system is close to a threshold (Figure 4.1). Many experiments with the same model but slightly different initial conditions (Monte Carlo simulations) indicate that the North Atlan

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