8.7.3 Unforced Abrupt Climate Change
Formally, as noted above, the changes discussed here do not fall into the definition of abrupt climate change. In the literature, unforced abrupt climate change falls into two general categories. One is just a red noise time series, where there is power at decadal and longer time scales. A second category is a bimodal or multi-modal distribution. In practice, it can be very difficult to distinguish between the two categories unless the time series are very long – long enough to eliminate sampling as an issue – and the forcings are fairly constant in time. In observations, neither of these conditions is normally met.
Models, both AOGCMs and less complex models, have produced examples of large abrupt climate change (e.g., Hall and Stouffer 2001; Goosse et al., 2002) without any changes in forcing. Typically, these events are associated with changes in the ocean circulation, mainly in the North Atlantic. An abrupt event can last for several years to a few centuries. They bear some similarities with the conditions observed during a relatively cold period in the recent past in the Arctic (Goosse et al., 2003)
Unfortunately, the probability of such an event is difficult to estimate as it requires a very long experiment and is certainly dependent on the mean state simulated by the model. Furthermore, comparison with observations is nearly impossible since it would require a very long period with constant forcing which does not exist in nature. Nevertheless, if an event such as the one of those mentioned above were to occur in the future, it would make the detection and attribution of climate changes very difficult.