One of the most exciting new developments within the area of global change science has arisen from the growing body of paleoclimatological evidence for rapid changes in the climate system. Until recently it would have been hard to imagine that the global climate, together with other aspects of the Earth system to which it is intimately linked, could have undergone repeated oscillations on millennial timescales as described earlier, let alone that the most dramatic of these "switches" occurred within a matter of decades, with major consequences for temperatures, hydrological regimes, plants, and animals that were at the very least hemispheric in their extent. The evidence now available provides a fascinating insight into the climatic backdrop to the development of human societies in the past. It also highlights one of the major challenges for the future. If the potentially disastrous impacts of climatic "surprises" like the ones that have already been well documented from ice core and sediment records are to be avoided in the future, research devoted to deepening our understanding of abrupt changes in the past is of paramount importance.
Any definition of "abrupt" climate changes is necessarily somewhat subjective, as it depends in large measure on the sample interval used in a particular study and the pattern of longer-term variation within which the sudden shift is embedded. Here, I make no attempt at a general definition but focus attention on two examples of rapid transition found in the paleorecord from the geologically recent past, glacial terminations and the Younger Dryas.
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