The climate system contains, even in its simplest possible representation, nonlinearities that can give rise to multiple equilibria. The role of the ocean in this context is discussed, and recent progress is reviewed. The paleoclimatic record indicates that such different equilibria are relevant to our understanding of past changes and likely are fundamental for a correct assessment of future changes.

9.1 Introduction

During the past two decades, paleoclimatic research has been the key to a quantitative understanding and appreciation of the full dynamics of the climate system. It has long been thought that the major climatic shifts have been caused by changes in the Earth's orbit, evidenced most dramatically by the sequence of ice ages during the Pleistocene (Imbric et aK, 1992). The advent of high-resolution archives such as ice cores from the polar regions modified this view considerably. Measurements of the stable isotopes of polar ice from Greenland indicated a succession of abrupt events during the last glacial (Dansgaard et al., 1984), The late Hans Oesehger, one of the pioneers in ice core research, demonstrated that the changes seen d uring the last deglaciation about 15,000-11,000 years ago are coeval with those registered in Swiss Gerzensce (Oesehger et al., 1984). This historical and bold hypothesis is reproduced here (Figure 9.1), Oesehger and his colleagues showed a strong correlation between these two records for the Younger Dryas (YD), the last of the series of abrupt events found by Dansgaard et al. {1984). This was surprising because these two paleoclimatic archives have very different characteristics and are located far apart. In honor of this finding, these events have become known as Dansgaard/Oeschger events, of which 24 have been identified during approximately the last 100,000 years (Dansgaard et al., 1993).

Earlier paleoceanographic reconstructions by Ruddiman and Mclntyre (1981) showed that the polar front in the Atlantic moved farther south during the YD. This suggested that the common cause of these events was the ocean, in particular changes in the sea surface temperature distribution and hence circulation patterns. The hypothesis of an oceanic flip-flop was formulated (Oesehger et al., 1984; Broecker et al, 1985).

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