Dansgaard-Oeschger events
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Dansgaard-Oeschger events are rapid climate fluctuations during and at the end of the last ice age. There were 23 such events between 110,000 and 23,000 years BP.
In the Northern Hemisphere, they take the form of rapid warming episodes, each followed by gradual cooling. The pattern in the Southern Hemisphere is different, with slow warming and much smaller temperature fluctuations. Indeed, the Vostok core was done first, and the existence of D-O events was not widely recognised until the Greenland (GRIP/GISP2) cores were done later; after which there was some reexamination of the Vostok core to see if these events had somehow been "missed". The processes behind the timing and amplitude of these events (as recorded in ice cores) are still unclear.
Dansgaard-Oeschger events are closely related to Heinrich events. Heinrich events disrupt North Atlantic Thermohaline Circulation, causing cooling in the Northern Hemisphere. Cooler climate increases ice cover, consequently increasing solar insolation and promoting further ice growth. There is evidence to suggest Dansgaard-Oeschger events were globally synchronous [Bond et al. (1999)].
Rahmstorf (2003) proposes that the events are paced by a regular cycle of 1,470 years, and are not themselves cycles. 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. 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 (Rahmstorf, 2003).
History
The ice cores signals now recognised as D-O events are, in retrospect, visible in the original GISP core, as well as the Camp Century core. But at the time, 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 delta-O-18 signal, and that they appear to correlate to events in the previous Camp Century core 1400 km away, thus providing evidence for their corresponding to widespread climatic anomalies (with only the 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.
References
Links
- http://www.agu.org/revgeophys/mayews01/node7.html
- http://content.aip.org/products/esva/Dansgaard_Willi.html - Photo of Dansgaard and Oerschger in a sub ice trench at Dye-3.
- Bond et al (1999). Bond, G.C., Showers, W., Elliot, M., Evans, M., Lotti, R., Hajdas, I., Bonani, G., Johnson, S., 1999. The North Atlantic's 1–2 kyr climate rhythm: relation to Heinrich events, Dansgaard/Oeschger cycles and the little ice age. In: Clark, P.U., Webb, R.S., Keigwin, L.D. (Eds.), Mechanisms of Global Change at Millennial Time Scales. Geophysical Monograph 112, American Geophysical Union, Washington DC, pp. 59–76.fi:D-O-tapahtuma