Records of past earth system variability are found in the
glaciers of Greenland [e.g., GRIP Members, 1993], Peru
[ Grootes et al., 1989], Tibet [e.g., Tandong and
Thompson, 1992], and Antarctica [e.g., Mosley-Thompson et
al., 1991]. These records sometimes have high enough resolution
to record ENSO [ Thompson, 1989] and even seasonal events
[e.g., Hammer, 1989]. Ice cores are unique in that they
provide a continuous record of variability in atmospheric
chemistry (up to 220,000 years at Vostok, Antarctica), reflecting
changes in temperature, precipitation, winds, and concentrations
of trace gases such as carbon dioxide and methane [ Jouzel et
al., 1993]. Large changes in climate such as the Younger Dryas
have been dated through analysis of various chemical and isotopic
measures of core ices [e.g., Mayewski et al., 1994].
Smaller-scale events such as volcanic eruptions have been
identified and dated through measurements of dust layers and the
concentration and isotopic signature of gases trapped in the snow
and ice of high altitude and polar regions [ Thompson,
1989]. Abrupt changes of climate have also been identified
through ice core measurements [e.g., Alley et al., 1993;
see also ``Rates of Change'' below]. Measures of the oxygen
isotope 
O from the Quelccaya ice cap, Peru, provide evidence
of the seasonal variability of precipitation over the last 1500
years [ Grootes et al., 1989]. Values of 
O
(change in the 
O isotope concentration), conductivity, and
annual dust layers from the Dunde ice cap, Tibet, also depict
patterns of variability in temperature, atmospheric chemistry,
and precipitation [ Thompson et al., 1989; Tandong and
Thompson, 1992]. For a full review of recent analyses of ice
core records, see Mayewski, this volume.