The Younger Dryas is clearly observable in paleoclimate records from many parts of the world. In the Cariaco Basin north of Venezuela, for example, temperatures decreased about 3°C (5.5°F), although some of this cooling might have been due to greater upwelling of colder subsurface water (Lea et al., 2003). In many parts of the Northern Hemisphere tropics, conditions also became drier (Hughen et al., 2000; Wang et al., 2001). The story in Antarctica is somewhat different, however. The ice core record at Dome C (Figure 6) shows that climate changes in Antarctica were out-of-phase with those in the Northern Hemisphere (EPICA, 2004). At Dome C, the amount of the hydrogen isotope called deuterium, expressed here as δD, is proportional to temperature. The deuterium record indicates that, contrary to the Northern Hemisphere records, temperatures were relatively low prior to the Younger Dryas (a period called the Antarctic Cold Reversal) and rose during the Younger Dryas. This pattern provides an important clue about what caused the Younger Dryas, as will be discussed next.
What caused the Younger Dryas?
The Younger Dryas occurred during the transition from the last glacial period into the present interglacial (the Holocene). During this time, the continental ice sheets were rapidly melting and adding freshwater to the North Atlantic. Figure 6 shows the reconstructed freshwater flux from the melting Laurentide ice sheet through the St. Lawrence River. Just prior to the Younger Dryas, meltwater fluxes into the North Atlantic increased dramatically. In addition, there was probably a short-lived period of particularly high freshwater flux about 13,000 years ago that is not shown in this figure, resulting from a large discharge of freshwater from a glacial lake in North America. Scientists have hypothesized that meltwater floods reduced the salinity and density of the surface ocean in the North Atlantic, causing a reduction in the ocean's thermohaline circulation and climate changes around the world. Eventually, as the meltwater flux abated, the thermohaline circulation strengthened again and climate recovered.
The record from Dome C in Antarctica supports this explanation. If the thermohaline circulation were to slow, less heat would be transported from the South Atlantic to the North Atlantic (Crowley, 1992; Broecker, 1998). This would cause the South Atlantic to warm and the North Atlantic to cool. This pattern, sometimes called the "bipolar see-saw", is observable when comparing the GISP2 and Dome C records for the Younger Dryas.
Notice the second period of large freshwater discharge following the Younger Dryas in Figure 6. Interestingly, this discharge did not cause a second major climate change similar to the Younger Dryas. One possible explanation for this is that, after the Younger Dryas, the thermohaline circulation had become more vigorous as the climate finally entered the interglacial. A vigorous thermohaline circulation might be less susceptible to freshwater discharges.
Some important datasets related to the Younger Dryas:
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