Figure 2.22: Variations of temperature, methane, and atmospheric carbon dioxide concentrations derived from air trapped within ice cores from Antarctica (adapted from Sowers and Bender, 1995; Blunier et al., 1997; Fischer et al., 1999; Petit et al., 1999).
Only during the 1980s was the possibility of rapid climatic changes occurring at the time-scale of human life more or less fully recognised, largely due to the Greenland ice core drilled at Dye 3 in Southern Greenland (Dansgaard et al., 1982, 1989). A possible link between such events and the mode of operation of the ocean was then subsequently suggested (Oeschger et al., 1984; Broecker et al., 1985; see Broecker, 1997, for a recent review). The SAR reviewed the evidence of such changes since the peak of the last inter-glacial period about 120 ky BP (thousands of years Before Present). It concluded that: (1) large and rapid climatic changes occurred during the last Ice Age and during the transition towards the present Holocene; (2) temperatures were far less variable during this latter period; and (3) suggestions that rapid changes may have also occurred during the last inter-glacial required confirmation.
These changes are now best documented from ice core, deep-sea sediment and continental records. Complementary and generally discontinuous information comes from coral and lake level data. The time-scale for the Pleistocene deep-sea core record is based on the orbitally tuned oxygen isotope record from marine sediments (Martinson et al., 1987), constrained by two radiometrically dated horizons, the peak of the last inter-glacial (about 124 ky BP) and the Brunhes/Matuyama reversal of the Earth's magnetic field at about 780 ky BP. 14C-dating is also used in the upper 50 ky BP; the result is a deep-sea core chronology believed to be accurate to within a few per cent for the last million years. 14C-dating is also used for dating continental records as well as the counting of annual layers in tree rings and varved lake records, whereas ice-core chron-ologies are obtained by combining layer counting, glaciological models and comparison with other dated records. The use of globally representative records, such as changes in continental ice volume recorded in the isotopic composition of deep-sea sediments, or changes in atmospheric composition recorded in air bubbles trapped in ice cores, now allow such local records to be put into a global perspective. Studies still largely focus on the more recent glacial-interglacial cycle (the last 120 to 130 ky). Table 2.4 is a guide to terminology.
|Table 2.4: Guide to terminology used in palaeoclimate studies of the last 150,000 years.|
|"Event", Stage||Estimatet age (calendar years)|
||~10 ky BP to present|
|Holocene maximum warming (also referred to as "climatic optimum")||Variable?|
|~4.5 t 6 ky BP (Europe) 10 to 6 ky BP (SH)|
|Last deglaciation||~18 to 10 ky BP|
|Termination 1||~14 ky BP|
|Younger Dryas||~12.7 to 11.5 ky BP|
|Antarctic cold reversal||14 to 13 ky BP|
|Bölling-Alleröd warm period||14.5 to 13 ky BP (Europe)|
|Last glacial||~74 to 14 ky BP|
|LGM (last glacial maximum)||~25 to 18 ky BP|
|Last interglacial peak||~124 ky BP|
|Termination 2||~130 ky BP|
|Eemian/MIS stage 5e||~128 ky BP|
|Heinrich events||Peaks of ice-rafted detritus in marine sediments, ~7 to 10 ky time-scale.|
|Dasgaard-Oeschger events||Warm-cold oscillations determend from ice cores with duration ~2 to 3 ky.|
|Bond Cycles||A quasi-cycle during the last Ice Age whose period is equal to the time between successive Heinrich events.|
|Periods of rapid deglaciation.|
Figure 2.23: Time-series illustrating temperature variability over the last about 400 ky (updated from Rostek et al., 1993; Schneider et al., 1996; MacManus et al., 1999; Reille et al., 2000). The uppermost time-series describes the percentage of tree pollen that excludes pollen from pine tree species. The higher this percentage, the warmer was the climate.
Before reviewing important recent information about rapid changes, we briefly mention progress made on two aspects of the palaeoclimate record of relevance for future climate. The first deals with the relationship between modern and past terrestrial data and SSTs around the time of the Last Glacial Maximum (about 20 ky BP); this is important because of the use of glacial data to validate climate models. New results obtained since the SAR both from marine and terrestrial sources (reviewed in Chapter 8), agree on a tropical cooling of about 3°C. The second concerns the greenhouse gas record (CO2 and CH4) which has now been considerably extended due to the recent completion of drilling of the Vostok ice core in central East Antarctica. The strong relationship between CO2 and CH4 and Antarctic climate documented over the last climatic cycle has been remarkably confirmed over four climatic cycles, spanning about 420 ky (Figure 2.22). Present day levels of these two important greenhouse gases appear unprecedented during this entire interval (Petit et al., 1999; and Figure 2.22). From a detailed study of the last three glacial terminations in the Vostok ice core, Fischer et al. (1999) conclude that CO2 increases started 600 ± 400 years after the Antarctic warming. However, considering the large uncertainty in the ages of the CO2 and ice (1,000 years or more if we consider the ice accumulation rate uncertainty), Petit et al. (1999) felt it premature to ascertain the sign of the phase relationship between CO2 and Antarctic temperature at the initiation of the terminations. In any event, CO2 changes parallel Antarctic temperature changes during deglaciations (Sowers and Bender, 1995; Blunier et al., 1997; Petit et al., 1999). This is consistent with a significant contribution of these greenhouse gases to the glacial-interglacial changes by amplifying the initial orbital forcing (Petit et al., 1999).We also now have a better knowledge of climate variability over the last few climatic cycles as illustrated by selected palaeo-temperature records back to about 400 ky (Figure 2.23). The amplitude of the glacial-interglacial temperature change was lower in tropical and equatorial regions (e.g., curve c) than in mid- and high latitudes (other curves). During glacial periods, the climate of the North Atlantic and adjacent regions (curves a and b) was more variable than in the Southern Hemisphere (curve d). Also (not shown), full glacial periods were characterised by very high fluxes of dust (seen in ice-core records and in continental and marine records). A combination of increased dust source area, stronger atmospheric transport and a weaker hydrological cycle (Yung et al., 1996; Mahowald et al., 1999; Petit et al., 1999) probably generated these changes.
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