page_title: Dansgaard–Oeschger event
section_title: Causes
hierarchical_section_title: Dansgaard–Oeschger event / Causes
caption_attribution_description: GRIP and NGRIP ice core delta-O-18 data; closeup between 30kyr and 50kyr BP. Note that the GRIP data is plotted on its native timescale; the NGRIP timescale has been shifted linearly so that the jumps at 35.5 and 44.5 kyr coincide between the cores. The offset in delta-O-18 between the cores is real.
context_page_description: Dansgaard–Oeschger 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.
context_section_description: 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 or rain.
  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).
  More recently, these events have been attributed to changes in the size of the ice sheets and atmospheric carbon dioxide. The former determines the strength of the Atlantic Ocean circulation via altering the northern hemisphere westerly winds, gulf stream, and sea-ice systems. The latter modulates atmospheric inter-basin freshwater transport across Central America, which changes the freshwater budget in the North Atlantic and thus the circulation. They further suggest the existence of a window of AMOC bistability ('sweet spot' for abrupt climate changes) associated with ice volume and atmospheric CO2, accounting for the occurrences of D-O type events under intermediate glacial conditions in the late Pleistocene.
caption: A closeup near 40 kyr BP, showing reproducibility between cores
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