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Assessment of present day climate variability benefits from comparison with conditions during inter-glacial periods that are broadly comparable with the Holocene. The most recent such inter-glacial began about 130 ky BP, lasting until about 71 ky BP when final deterioration into the last glacial began. However, only the Eemian interval, from about 130 to 120 ky BP corresponds to a climate as warm as, or warmer than, today e.g., Figure 2.22.
The study of atmospheric composition changes has revealed that rapid changes of properties observed for the lowest part of the Greenland cores (GRIP Project Members, 1993; Grootes et al., 1993) do not correspond to climatic instabilities during the last inter-glacial (Chappellaz et al., 1997). The extent to which climate was more or less stable during this last inter-glacial than during the Holocene is unclear. Early evidence from marine cores (CLIMAP, 1984; McManus et al., 1994) and other ice cores (Jouzel et al., 1993) indicated that the Eemian climate was rather stable. A high resolution North Atlantic record shows a lack of substantial fluctuations during the last inter-glacial but also indicates that the Eemian began and ended with abrupt changes in deep-water flow, with transitions occurring in less than 400 years (Adkins et al., 1997). In New Zealand, there were at least three periods of milder climate than typical of the Holocene during the last inter-glacial (Salinger and McGlone, 1989). Study of an Indonesian fossil coral indicates that ENSO was robust during the last glacial period (Hughen et al., 1999).
A rapid and significant cooling event within the Eemian period has been detected from European continental pollen records (Cheddadi et al., 1998). High winter temperatures prevailed for 3.5 to 4 ky after the deglaciation, but then dropped by as much as 6 to 10°C in mid-Eemian times, accompanied by a decrease in precipitation. In Antarctica, the last inter-glacial is also marked by a short (about 5 ky) period of warm temperatures followed by a slightly cooler interval (Petit et al., 1999). Further evidence for Eemian climate variability is found in marine records. An invasion of cold, low salinity water in the Norwegian Sea (Cortijo et al., 1994) was probably associated with a reduction in warm water transport by the North Atlantic Drift and the thermohaline circulation. Overall, the last inter-glacial appears, at least during its first part, warmer than present day climates by at least 2°C in many sites, i.e., comparable to anthropogenic warming expected by the year 2100. However, the geographical coverage of reliable and well-dated temperature time-series is too sparse to provide a global estimate.
Current evidence indicates that very rapid and large temperature changes, generally associated with changes in oceanic and atmospheric circulation, occurred during the last glacial period and during the last deglaciation, particularly in higher latitudes of the Northern Hemisphere. During the warming phases, and the Younger Dryas pause, there is evidence of almost worldwide, nearly synchronous events. However, as with the Holocene maximum warming and the Last Glacial Maximum, these changes appear to have occurred asynchronously between the Northern Hemisphere and at least part of the Southern Hemisphere. During the Holocene smaller but locally quite large climate changes occurred sporadically; similar changes may have occurred in the last inter-glacial. Evidence is increasing, therefore, that a rapid reorganisation of atmospheric and ocean circulation (time-scales of several decades or more) can occur during inter-glacial periods without human interference.
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