Dansgaard-Oeschger events are rapid climate fluctuations occurring every ≈1470 (± 532) years throughout the last glacial period. Twenty-three such events have been identified between 110,000 and 23,000 years BP; 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. Somewhat less direct evidence from Antarctic cores (the pattern of warmings; and the methane record) suggests that they were present in previous glacial periods as well.
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 icepack warmed by around 8°C over 40 years, in three steps of five years (see Alley (2000), Stewart, chapter 13) - 5°C change over 30-40 yrs 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 (Bond & Lotti 1995).
The course of a D-O event sees a rapid warming of temperature, followed by a cool period lasting a few hundred years (Bond et al. 1999). This cold period sees an expansion of the polar front, with ice floating further south across the North Atlantic ocean (Bond et al. 1999).
The processes behind the timing and amplitude of these events (as recorded in ice cores) are still unclear. The pattern in the Southern Hemisphere is different, with slow warming and much smaller temperature fluctuations. Indeed, the Vostok ice core was drilled before the Greenland cores, and the existence of Dansgaard-Oeschger events was not widely recognised until the Greenland (GRIP/GISP2) cores were done; after which there was some reexamination of the Vostok core to see if these events had somehow been "missed".
The events appear to reflect changes in the North Atlantic ocean circulation, perhaps triggered by an influx of fresh water (Bond et al. 1999).
The events may be caused by an amplification of solar forcings, or by a cause internal to the earth system - either a "binge-purge" cycle of ice sheets accumulating so much mass they become unstable, as postulated for Heinrich events, or an oscillation in deep ocean currents (Maslin et al. 2001, p25).
D-O cycles may set their own timescale. Maslin et al. (2001) suggested that each ice sheet had its own conditions of stability, but that on melting, the influx of freshwater was enough to reconfigure ocean currents - causing melting elsewhere. More specifically, D-O cold events, and their associated influx of meltwater, reduce the strength of the North Atlantic Deep Water current (NADW), weakening the northern hemisphere circulation and therefore resulting in an increased transfer of heat polewards in the southern hemisphere. This warmer water results in melting of Antarctic ice, thereby reducing density stratification and the strength of the Antarctic Bottom Water current (AABW). This allows the NADW to return to its previous strength, driving northern hemisphere melting - and another D-O cold event. This theory may also explain Heinrich events' apparent connection to the D-O cycle; when the accumulation of meltwater in the oceans reaches a threshold, it may have raises sea level enough to undercut the Laurentide ice sheet - causing a Heinrich event and resetting the cycle.
The little ice age of ~400 to 200 years ago has been interpreted as the cold part of a D-O cycle, putting us (even without the effects anthropogenic global warming) in a period of warming climate (Bond et al. 1999).
The cyclicity is also found during the Holocene, where the events are referred to as Bond events (Bond et al. 1997, 2001)