Dansgaard-Oeschger event

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.

Evidence

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.

Effect

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 [http://www.pnas.org/cgi/content/full/97/4/1331 Alley (2000)] , [http://oceanworld.tamu.edu/resources/ocng_textbook/chapter13/chapter13_01.htm 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).

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".Verify source|date=March 2008

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).

Timing

Although the effects of the D-O events are largely constrained to ice cores taken from Greenland (Bond "et al." 1999), there is evidence to suggest Dansgaard-Oeschger events have been globally synchronous (Voelker 2003). Rahmstorf [http://www.pik-potsdam.de/~stefan/Publications/Journals/rahmstorf_grl_2003.pdf (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).

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).

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 [http://www.ncdc.noaa.gov/paleo/icecore/greenland/gisp/campcentury/campc.html] . 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 (Bond "et al". 1997, 2001)

ee also

* 1500-year climate cycle
* Younger Dryas

References

*

*cite journal | author = Bond, G.C. | coauthors = Lotti, R. | date = 1995-02-17 | title = Iceberg Discharges into the North Atlantic on Millennial Time Scales During the Last Glaciation | journal = Science | volume = 267 | issue = 5200 | pages = 1005 | doi = 10.1126/science.267.5200.1005 | url = | accessdate = 2007-06-28 | pmid = 17811441

*cite journal| author = Bond, G. | coauthors = "et al." | url=http://rivernet.ncsu.edu/courselocker/PaleoClimate/Bond%20et%20al.,%201997%20Millenial%20Scale%20Holocene%20Change.pdf | year = 1997 | title = A Pervasive Millennial-Scale Cycle in North Atlantic Holocene and Glacial Climates | journal = Science | volume = 278 | issue = 5341 | pages = 1257–1266 | doi = 10.1126/science.278.5341.1257
* Bond et al (1999). cite book|author=Bond, G.C., Showers, W., Elliot, M., Evans, M., Lotti, R., Hajdas, I., Bonani, G., Johnson, S., |year=1999|chapter= The North Atlantic's 1–2 kyr climate rhythm: relation to Heinrich events, Dansgaard/Oeschger cycles and the little ice age|editor=Clark, P.U., Webb, R.S., Keigwin, L.D.|title=Mechanisms of Global Change at Millennial Time Scales|series=Geophysical Monograph|number=112|publisher=American Geophysical Union, Washington DC|pages=59–76|isbn=0-87590-033-X | chapterurl=http://rivernet.ncsu.edu/courselocker/PaleoClimate/Bond%20et%20al%201999%20%20N.%20Atlantic%201-2.PDF

*cite journal |last=Bond |first=G. |authorlink= |coauthors="et al." |year=2001 |month= |title=Persistent Solar Influence on North Atlantic Climate During the Holocene |journal=Science |volume=294 |issue=5549 |pages=2130–2136 |doi=10.1126/science.1065680 |url= |accessdate= |quote= |pmid=11739949
*cite journal
author = Maslin, M.
coauthors = Seidov, D.; Lowe, J.
year = 2001
title = Synthesis of the nature and causes of rapid climate transitions during the Quaternary
journal = Geophysical monograph
volume = 126
pages = 9–52
url = http://www.essc.psu.edu/~dseidov/pdf_copies/maslin_seidov_levi_agu_book_2001.pdf
accessdate = 2008-03-06

*

*cite journal |last=Braun |first=Holger |authorlink= |coauthors="et al." |year=2005 |month= |title=Possible solar origin of the 1,470-year glacial climate cycle demonstrated in a coupled model |journal=Nature |volume=438 |issue= |pages=208–211 |doi=10.1038/nature04121 |url= |accessdate= |quote=

*cite journal |last=Schulz |first=Michael |authorlink= |coauthors= |year=2002 |title=On the 1470-year pacing of Dansgaard-Oeschger warm events |journal=Paleoceanography |volume=17 |issue=2 |pages=1014 |doi=10.1029/2000PA000571 |url=

*

External links

* [http://oceanworld.tamu.edu/resources/ocng_textbook/contents.html Introduction to Physical Oceanography] by Robert H. Stewart
* http://www.agu.org/revgeophys/mayews01/node7.html
* http://content.aip.org/products/esva/Dansgaard_Willi.html (subcription required) - Photo of Dansgaard and Oeschger in a sub ice trench at Dye-3