climate change is essential for assessing the importance of their role in our climate future.

Ice Core Evidence of the Younger Dryas

The Younger Dryas cold reversal is especially prominent in ice-core records from Greenland, but it is also observed in ice cores from other locations. The ice-core records provide a unique perspective that demonstrates the synchronous nature of the large, widespread changes observed.

Annual-layer counting in Greenland ice cores allows determination of the age, duration, and rapidity of change of the Younger Dryas event with dating errors of about one percent (Alley et al., 1993; Meese et al., 1997). Annual-layer thicknesses corrected for the effects of ice flow give the history of snow accumulation rate in Greenland (Alley et al., 1993). Concentrations of wind-blown materials—such as dust (which in central Greenland has characteristics showing its origin in central Asia [Biscaye et al., 1997]) and sea salt—reveal changes in atmospheric concentrations of these particles (Mayewski et al., 1997) after correction for variations in dilution caused by changing snow accumulation rate (Alley et al., 1995a). Gases trapped in bubbles reveal past atmospheric composition. Methane is of special interest because it probably records the global area of wetlands. Furthermore, differences between methane concentrations observed in Greenland ice cores and those from Antarctica allow inference of changes in the wetland areas in the tropics and high latitudes (Chappellaz et al., 1997; Brook et al., 1999).

The combination of the isotopic record of water making up the Greenland ice (see Plate 2; Figure 1.2) (Johnsen et al., 1997; Grootes and Stuiver, 1997) and the physical temperature of the ice (Cuffey et al., 1994, 1995; Johnsen et al., 1995) yields estimates of past temperatures in central Greenland, which can be checked by using two additional thermometers based on the thermal fractionation of gas isotopes after abrupt temperature changes (Severinghaus et al., 1998). Ice-core records from Greenland thus provide high-resolution reconstructions of local environmental conditions in Greenland (temperature and snow accumulation rate), conditions well beyond Greenland (wind-blown materials including sea salt and Asian dust), and even some global conditions (wetland area inferred from methane), all on a common time scale (Figures 2.1, 2.2, and 2.3).

A review of available Greenland ice-core data is given by Alley (2000). The data were collected by two international teams of investigators from multiple laboratories. The duplication shows the high reliability of the

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