Dansgaard-Oeschger event

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Temperature proxy from four ice cores for the last 140 kyr, showing D-O events in the NH but not the SH

Dansgaard-Oeschger events are rapid climate fluctuations during and at the end of the last ice age. Twenty-three such events have been identified between 110,000 and 23,000 years BP.

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 BP, 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 chap 13) - 5°C change over 30-40yrs more common.

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 done 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".

A closeup near 40 kyr BP, showing reproducibility between cores

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 surface albedo — more of the incoming solar energy is reflected back to space — and promoting further ice growth. There is evidence to suggest Dansgaard-Oeschger events have been 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. 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 (Rahmstorf, 2003).

The best evidence for Dansgaard-Oeschger events remains in the Greenland cores, which only go back to the end of the interglacial previous to the present one, 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.

[edit] History

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 [1]. 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 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 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.

[edit] References

Richard B. Alley (2000). "Ice-core evidence of abrupt climate changes". PNAS 97 (4): 1331-1334.
Stefan Rahmstorf (2003). "Timing of abrupt climate change: A precise clock". Geophys. Res. Lett. 30 (10): 1510. DOI:10.1029/2003GL017115.
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. ISBN 0-87590-033-X.