The most dramatic recent contributions to our understanding of paleoclimate during the last glacial cycle have come in the millennial-scale range of climate variability. Unprecedented swings in the Earth's climate have now been recorded in two ice cores from central Greenland, instigating new higher-resolution investigations of land and marine paleoclimate records.
In 1993 the Greenland Ice Sheet Project Two (GISP2) successfully completed drilling to the base of the Greenland ice sheet in central Greenland. In so doing, GISP2, along with its European companion project GRIP (Greenland Ice Core Program), developed the longest high-resolution continuous paleoenvironmental record (>250,000 years) available from the northern hemisphere. Based on the comparison of electrical conductivity and oxygen isotope series between the two cores,4 at least the upper 90 percent displays extremely similar if not absolutely equivalent records.
The central Greenland ice cores provide a framework for other paleoclimate records because of their relatively precise dating. The current best estimate of the age at ~2,800 m is ~110,000 years, based on a combination of multiparameter annual layer counting combined with measurements of the d18O of atmospheric O2 calibrated with the Vostok ice core in Antarctica.5 Error estimates in the dating are quite remarkable, from 2 percent for 0 to 11,640 years ago to 10 percent for over 40,000 years.6 Agreement between the GISP2 and GRIP ice cores (separated by 30 km or ~10 ice thicknesses) over the record period of the past ~110,000 years provides strong support for the climatic origin of even the minor features of these records and implies that investigations of subtle environmental signals can be rigorously pursued. The climatic significance of the deeper part of these ice cores (>110,000 years in age) is a matter of considerable controversy. Without additional records, the evidence for rapid climate change in Greenland during the last interglacial remains equivocal.
The millennial-scale events recorded in the upper 110,000 years of the two central Greenland ice cores are, however, unequivocally climate events. They represent large climate deviations (massive reorganizations of the ocean-atmosphere system) that occur over decades or less and during which ice-age temperatures in central Greenland may have been as much as 20°C colder than today (see Figure 6.2).7 These events have their greatest magnitude during the glacial portion of the record, prior to ~14,500 years ago), when large northern hemisphere ice sheets provided positive climate feedbacks.8
Examination of one of these events, the Younger Dryas (a near return to glacial conditions during the last deglaciation, previously identified in a variety of paleoclimate records), demonstrates the importance of conducting multiparameter high-resolution paleoclimate investigations on well-dated records. During this event lowered temperatures were accompanied by up to twofold and greater changes in snow accumulation, order-of-magnitude changes in the amount