FIGURE 2.5 History of temperature in central Greenland over the last 100,000 years, as calculated by Cuffey and Clow (1997) from the data of Grootes and Stuiver (1997). The large Younger Dryas temperature oscillation (labeled YD), and the smaller temperature change of the event about 8,200 years ago (labeled 8ka) are just the most recent in a long sequence of such abrupt temperature jumps. Changes in materials from beyond Greenland trapped in the ice cores, including dust and methane, demonstrate that just as for the YD and 8ka events, the earlier events affected large areas of the earth nearly simultaneously.

North Atlantic deep water formation (e.g., Lehman and Keigwin, 1992; Oppo and Lehman, 1995; Bond et al., 1993; Bond and Lotti, 1995). The geographic pattern of climate anomalies associated with the cold phases of the Dansgaard/Oeschger oscillations is thus quite similar to that of the Younger Dryas event.

The millennial Dansgaard/Oeschger oscillations are bundled into multimillennial Bond cycles, although with variable spacing (Bond et al., 1993). Each Dansgaard/Oeschger oscillation is slightly colder than the previous one through a few oscillations; then there is an especially long, cold interval, followed by an especially large, abrupt warming. The latter parts of the especially cold intervals are marked by the enigmatic Heinrich layers in the North Atlantic (Heinrich, 1988).