UNSTABLE STADIALS: EVIDENCE FROM THE YOUNGER DRYAS

The conventional view of the most recent stadial of the last glaciation, the Younger Dryas (YD), is that of robust cooling with a gradual onset and a dramatically abrupt termination. The distinct asymmetrical shape has been explained as a consequence of fresh water-triggered switches in North Atlantic Deep Water (NADW) formation. Results of ocean circulation models, changes in atmospheric radiocarbon, and benthic isotopic measurements from the North Atlantic appear to be consistent with that interpretation.

New high resolution oxygen isotope measurements in the GISP2 ice core, however, reveal significant variability within the YD on decadal time scales. Smoothing suggests that the variations are bundled into at least 3 major oscillations with durations of 300 to 500 years. That the oscillations are related to regional temperature changes is suggested by evidence for tripartite divisions of the YD elsewhere, such as in the Salpausselkä moraine system of Finland and in IRD records from the North Atlantic. Broadly similar oscillations are evident in the record of atmospheric radiocarbon during the YD.

Whether the multi-centennial oscillations are linked to changes in NADW production is unclear. It is interesting, though, that they have the same time scale and broadly the same amplitude (in oxygen isotopes) as the multi-centennial cycles in the GISP2 record that punctuate the early Holocene. That would appear to support other suggestions that multi-centennial variability is a pervasive feature of climate operating independently of glacial-interglacial boundary conditions.

The high resolution oxygen isotopic measurements in the GISP2 ice core document a second distinct feature of the YD. In the unsmoothed Bölling to YD record, the amplitude of decadal variability increases markedly with falling temperatures. The same is true for virtually every Dansgaard/Oeschger cycle of the last glaciation. Instrumental records and records from tropical corals suggest that the same large increase in amplitude of high frequency oscillations occurs from about 1,000 years ago (roughly the Medieval Warm Period) to the end of the Little Ice Age. Thus, on short timescales of decades, warmer climates are much more stable than colder climates, regardless of whether climate boundary conditions are glacial or interglacial.