Younger Dryas

The Younger Dryas marks the final stage of the last glacial termination, the transition to the Holocene. The transition out of the Younger Dryas cold reversal into the Holocene is one of the most well-studied rapid climate transitions, both in terms of regional climate expression, primarily in the North Atlantic and Europe but also elsewhere, but also its influence on ecosystems (Amman and Oldfield 2000). Some sense of the suddenness of this transition may be obtained from the record of ice accumulation and temperature in central Greenland shown in O Figure 12.6 (Dansgaard et al. 1993). At this location, most of the transition, a near doubling of snow accumulation rate and a temperature shift of around 10° C, was accomplished within a decade or less. By any standards, this is a truly dramatic, abrupt shift in climate, yet the generally accepted external forcing, an increase in solar irradiance, was relatively modest and certainly gradual. Again, we see external forcing pushing the Earth system over some crucial threshold beyond which internal dynamics can generate extremely rapid

O Figure 12.5

Millennial climate variability during the Last Glacial period (60-15 ka). The timing of the large Heinrich events is indicated by the labels H2 to H5, while the vertical gray lines mark subsequent warming in the Northern North Atlantic. (a) Air temperature over Greenland as inferred from ice core oxygen isotope measurements. (b) North Atlantic Sea Surface Temperature inferred from planktonic foraminifera based transfer functions. (c) North-South shifts in the polar front in the midlatitude Atlantic inferred from foraminifera isotope measurements, (d,e) Deep ocean ventilation and overturning inferred from carbon and oxygen isotope measurements in benthic foraminifera. (f) Regional temperature and moisture balance changes in Eastern China inferred from oxygen isotope measurements in cave deposits. (g) Air temperature over West Antarctica from oxygen isotope measurements in the Byrd ice core (Labeyrie et al. 2003)

Figure 12.5 Continued

Figure 12.5 Continued

O Figure 12.6

Accumulation and isotopically inferred temperature over the past 50,000 years as measured in an ice core from central Greenland. The termination of the last glacial period was an abrupt climatic change. The termination of the Younger Dryas cold event some 11,600 years ago is, in this record, manifested as a warming of around 10°C and a doubling in annual precipitation volume, occurring within about a decade. Figure generated from data of (Dansgaard et al. 1993) stored at the World Data Center for Paleoclimatology http:// (Alverson and Edwards 2003)

responses through a range of, as yet poorly understood or quantified, feedback mechanisms.

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