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Dansgaard–Oeschger events (often abbreviated D–O events) are rapid climate fluctuations that occurred 25 times during the last glacial period. Some scientists say that the events occur quasi-periodically with a recurrence time being a multiple of 1,470 years, but this is debated. The comparable climate cyclicity during the Holocene is referred to as Bond events.
The best evidence for Dansgaard–Oeschger events remains in the Greenland ice cores, which only go back to the end of the last interglacial, the Eemian interglacial. Ice core evidence from Antarctic cores suggests that the Dansgaard–Oeschger events are related to the so-called Antarctic Isotope Maxima by means of a coupling of the climate of the two hemispheres, the Bi-polar Seesaw. If this relationship holds also for the previous glacials, Antarctic data suggest that D-O events were present in previous glacial periods as well. Unfortunately, current ice core records from Greenland extend only through the last most recent glacial period so direct evidence of D-O events in earlier glacial periods from Greenland ice is unavailable. However, work by Stephen Barker and colleagues has shown that the existing Greenland record can be reconstructed by deriving the Antarctic ice core record. This allows for the reconstruction of an older Greenland record through the derivation of the nearly million-year-long Antarctic ice core record.
In the Northern Hemisphere, they take the form of rapid warming episodes, typically in a matter of decades, each followed by gradual cooling over a longer period. For example, about 11,500 years ago, averaged annual temperatures on the Greenland ice sheet warmed by around 8 °C over 40 years, in three steps of five years (see,Stewart, chapter 13), where a 5 °C change over 30–40 years is more common.
Heinrich events only occur in the cold spells immediately preceding D-O warmings, leading some to suggest that D-O cycles may cause the events, or at least constrain their timing.
The course of a D-O event sees a rapid warming of temperature, followed by a cool period lasting a few hundred years. This cold period sees an expansion of the polar front, with ice floating further south across the North Atlantic Ocean.
Dansgaard and Oeschger in a sub ice trench at Dye-3, Greenland
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