Dansgaard–Oeschger event

Dansgaard–Oeschger events (often abbreviated D–O events), named after palaeoclimatologists

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.^[7]

The course of a D-O event sees a rapid warming, followed by a cool period lasting a few hundred years.^[8] This cold period sees an expansion of the polar front, with ice floating further south across the North Atlantic Ocean.^[8]

D-O events are also believed to cause minor increases in atmospheric carbon dioxide concentrations on the order of around 5 ppm.^[9][10]

During D-O events, positive δ¹⁸O excursions occur in Floresian speleothem records, indicating a weakening of the Indonesian-Australian Monsoon during such events.^[11]

Causes

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".^{[citation needed]}

The events appear to reflect changes in the North Atlantic Ocean circulation, perhaps triggered by an influx of fresh water^[8] or rain.^[12]

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).

These events have been attributed to changes in the size of the ice sheets

Although the effects of the Dansgaard–Oeschger events are largely constrained to ice cores taken from Greenland,[16] there is evidence to suggest that D-O events have been globally synchronous.^[17] A spectral analysis of the American GISP2 isotope record^[18] showed a peak of [¹⁸O:¹⁶O] abundance around 1500 years. This was proposed by Schulz (2002)^[19] to be a regular periodicity of 1470 years. This finding was supported by Rahmstorf (2003);^[20] if only the most recent 50,000 years from the GISP2 core are examined, the variation of the trigger is ±12% (±2% in the 5 most recent events, whose dates are probably most precise).

However, the older parts of the GISP2 core do not show this regularity, nor do the same events in the GRIP core. This may be because the first 50 kyr of the GISP2 core are most accurately dated, by layer counting. The climate system response to the trigger is varying within 8% of the period. Oscillations within the Earth system can be expected to be far more irregular in period. Rahmstorf suggests that the highly regular pattern would point more to an orbital cycle. Such a source has not been identified. The closest orbital cycle, a Lunar cycle of 1,800 years, cannot be reconciled with this pattern.

The ice core's signals now recognised as Dansgaard–Oeschger events are, in retrospect, visible in the original GISP core, as well as the Camp Century Greenland core.^[24] But at the time the ice cores were made, their significance was noted but not widely appreciated. Dansgaard et al. (AGU geophysical monograph 33, 1985) note their existence in the GRIP core as "violent oscillations" in the δ¹⁸O signal, and that they appear to correlate to events in the previous Camp Century core 1,400 km away, thus providing evidence for their corresponding to widespread climatic anomalies (with only the Camp Century core, they could have been local fluctuations). Dansgaard et al. speculate that these may be related to quasi-stationary modes of the atmosphere-ocean system. D-O events tend to be what drives the "Sahara pump" which has had an effect upon human evolution and dispersal.

The cyclicity is also found during the Holocene, where the events are referred to as Bond events.^[25][26]

See also

• Younger Dryas

References