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 (Imbrie et al., 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 Oeschger, one of the pioneers in ice core research, demonstrated that the changes seen during the last deglaciation about 15, 000-11, 000 years ago are coeval with those registered in Swiss Gerzensee (Oeschger et al., 1984). This historical and bold hypothesis is reproduced here (Figure 9.1). Oeschger 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.