tic THC can undergo many oscillations before it settles in an active or a collapsed state. In some cases, a rapid collapse of the THC occurs many thousands of years after the perturbation. Obviously, beyond the problem of approaching an instability point and the increased vulnerability of the THC to further perturbation, such an evolution results in a much more unpredictable climate system (Figure 4.1).
These simulations are performed with a simplified model that includes only a limited set of processes. Experiments with comprehensive models are necessary to determine whether more realistic models exhibit similar behavior close to bifurcation points. In such models it would be crucial to identify whether there are regions or components in the climate system that could serve as early warning systems for a potential THC reduction or shutdown.
Data suggest that there has been an increase in the frequency of occurrence of El Niño conditions and a possible change in characteristic time scales of this phenomenon (e.g., Urban et al., 2000; Tudhope et al., 2001; see Chapter 2). However, the time series is too short to establish whether such rapid mode shifts are part of the natural, long-term variability inherent in this phenomenon or whether this is already a manifestation of a perturbed system. It has been suggested that this change in mode may be a response of the tropical Pacific to anthropogenic warming (Trenberth and Hoar, 1996).
Comprehensive climate models have just started to include credible representations of ENSO, which makes available tools to investigate the possible responses of the tropical Pacific atmosphere-ocean system to climate change. One model suggests that warming will cause El Niño events to become more frequent and stronger (Timmermann et al., 1999). Apart from regional implications, such a development could have far-reaching impact on other components of the climate system. For example, ENSO causes worldwide teleconnections, and it changes the freshwater balance of other ocean provinces (such as the tropical Atlantic). This provides a mechanism to modify the sea-surface salinity and hence the THC (Latif et al., 2000). Changes in the frequency and amplitudes of natural modes might not only evolve rapidly and thus manifest themselves as abrupt climate change, but they may also trigger other processes that lead to abrupt climate change.
As discussed in Chapter 2, strong trends have been occurring in the