a stronger poleward atmospheric transport of moisture, leading to more precipitation in the North Atlantic region. Those two effects, in concert, lead to an increase in buoyancy of the North Atlantic surface waters, which reduces the THC. Although the relative strength of the two mechanisms is debated and uncertain (Dixon et al., 1999; Mikolajewicz and Voss, 2000), most climate models seem to show a general reduction in the Atlantic THC in response to global warming.
The exceptions to this behavior remind us of the inherent uncertainties present in the simulations. It is not clear whether all relevant feedback mechanisms are considered properly in the current generation of climate models and whether their strength is simulated realistically. A simulation by Latif et al. (2000) suggested that changes in the El Niño-Southern Oscillation (ENSO) frequency and amplitude might change the freshwater balance of the tropical Atlantic in such a way that increases in buoyancy in the high latitudes are compensated for by drier (and hence more saline) conditions in the tropics. Gent (2001) reported on a simulation in which evaporation from a warmer sea surface in the North Atlantic is not compensated for by enhanced precipitation, and this simulation results in a stabilization of the THC. While it is not currently possible to decide which simulations are more realistic—those of Plate 7 showing a THC decrease or those that do not—the two simulations by Latif et al. (2000) and Gent (2001) illustrate that the quantitatively correct simulation of heat and freshwater flux changes is essential for the projection of the evolution of the THC under global warming.
However, there are other uncertainties regarding the fate of the THC. Research indicates that the realized warming and the associated changes in the hydrological cycle constitute a threshold for the THC (Manabe and Stouffer, 1993). Also, the rate of warming appears to influence the stability of the circulation (Stocker and Schmittner, 1997; Ahmad et al., 1997), because the ocean heat uptake is limited by mixing; faster warming in the atmosphere produces stronger vertical density gradients in the ocean, which tend to reduce the sinking. Faster warming makes the THC less stable to perturbations. Furthermore, both theoretical arguments (Marotzke, 1996) and model simulations (Tziperman, 1997) suggest that the THC becomes less stable when it is weaker (i.e., once reduced, the THC is more susceptible to perturbations). In the extreme case very close to a threshold, the evolution of the THC loses predictability altogether (Knutti and Stocker, 2001) (Figure 4.1) as discussed in the next section. It is intriguing that recent measurements show that an important part of the North Atlantic