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Three temperature records, the GRIP sequence (red) clearly showing the Younger Dryas event at around 11 kyr BP
The Younger Dryas stadial, named after the alpine / tundra wildflower Dryas octopetala, and also referred to as the Big Freeze, Berger, W.H.: "The Younger Dryas cold spell – a quest for causes.", page 219-237. Palaeogeography, Palaeoclimatology, Palaeoecology (Global and Planetary Change Section) 89, 1990 was a brief (approximately 1300 ± 70 years [1]) cold climate period following the Bölling/Allerød interstadial at the end of the Pleistocene between approximately 12,700 to 11,500 years Before Present, How Rapidly did Climate Change in the Distant Past?, Climate Change 2001: Working Group I: The Scientific Basis, Intergovernmental Panel on Climate Change and preceding the Preboreal of the early Holocene. In Ireland, the period has been known as the Nahanagan Stadial, while in the UK it has been called the Loch Lomond Stadial and most recently Greenland Stadial 1 (GS1). See INTIMATE Project (Integration of Ice, Marine and Terrestrial records), an INQUA Palaeoclimate subcommittee.
The Younger Dryas (GS1) is also a Blytt-Sernander climate period detected from layers in north European bog peat. It is dated approximately 12,900-11,500 BP calibrated, or 11,000-10,000 BP uncalibrated. An Older Dryas stadial had preceded the Allerød, approximately 1000 years before the Younger Dryas; it lasted 300 years.[2]
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The Younger Dryas saw a rapid return to glacial conditions in the higher latitudes of the Northern Hemisphere between 12,900 – 11,500 years before present (BP)Alley, R.B.: "The Younger Dryas cold interval as viewed from central Greenland.", page 213-226. Quaternary Science Reviews 19, 2000 in sharp contrast to the warming of the preceding interstadial deglaciation. The transitions each occurred over a period of a decade or so.Alley et al.: "Abrupt accumulation increase at the Younger Dryas termination in the GISP2 ice core", page 527-529. Nature 362, 1993 Thermally fractionated nitrogen and argon isotope data from Greenland ice core GISP2 indicates that the summit of Greenland was ~15 °C colder during the Younger Dryas than today. In the UK, coleopteran (fossil beetle) evidence suggests mean annual temperature dropped to approximately 5 °C,Severinghaus, J.P.: "Timing of abrupt climate change at the end of the Younger Dryas interval from thermally fractionated gases in polar ice.", page 141-146. Nature 391, 1998 and periglacial conditions prevailed in lowland areas, while icefields and glaciers formed in upland areas.Atkinson, T.C.: "Seasonal temperatures in Britain during the past 22,000 years, reconstructed using beetle remains.", page 587-592. Nature 325, 1987 Nothing of the size, extent, or rapidity of this period of abrupt climate change has been experienced since.
Answering this question is hampered by the lack of a precise definition of "Younger Dryas" in all the records. In western Europe and Greenland, the Younger Dryas is a well-defined synchronous cool period.[3] But cooling in the tropical North Atlantic may have preceded this by a few hundred years; South America shows a less well defined initiation but a sharp termination. The Antarctic Cold Reversal appears to have started a thousand years before the Younger Dryas, and has no clearly defined start or end; Huybers has argued that there is fair confidence in the absence of the Younger Dryas in Antarctica, New Zealand and parts of Oceania. Similarly the Southern Hemisphere cooling known as the Deglaciation Climate Reversal (DCR) began approximately 1kyr before the YD, between 14kya and 11.5 kya as noted in the Sajama ice core. The Andean climate returned to Last Glacial Maximum conditions with colder temperatures coupled with higher precipitation (high lake stands in the Altiplano).L G Thompson, E Mosley-Thompson and K A Henderson (2000). "Ice-core palaeoclimate records in tropical South America since the Last Glacial Maximum". Journal of Quaternary Science 15 (4): 377-394. doi:<377::AID-JQS542>3.0.CO;2-L 10.1002/1099-1417(200005)15:4%3C377::AID-JQS542%3E3.0.CO;2-L.
In western North America it is likely that the effects of the Younger Dryas were less intense than in Europe; however, evidence of glacial re-advanceFriele, P.A., Clague, J.J.: "Younger Dryas readvance in Squamish river valley, southern Coast mountains, British Columbia.", page 1925-1933. Quaternary Science Reviews 21, 2002 indicates Younger Dryas cooling occurred in the Pacific Northwest.
Other features seen include:
The prevailing theory holds that the Younger Dryas was caused by a significant reduction or shutdown of the North Atlantic thermohaline circulation in response to a sudden influx of fresh water from Lake Agassiz and deglaciation in North America.[4] The global climate would then have become locked into the new state until freezing removed the fresh water "lid" from the north Atlantic Ocean. This theory does not explain why South America cooled first.
Previous glacial terminations probably did not have Younger Dryas-like events, suggesting that whatever the mechanism is, it has a random component.
However there is evidence that termination II had a post glacial cooling period similar to the younger Dryas but lasting longer and being more severe.
There is evidence that the so-called Younger Dryas impact event, 12,900 years ago in North America could have initiated the Younger Dryas cooling.[5]
Measurements of oxygen isotopes from the GISP2 ice core suggest the ending of the Younger Dryas took place over just 40 - 50 years in three discrete steps, each lasting five years. Other proxy data, such as dust concentration, and snow accumulation, suggest an even more rapid transition, requiring a ~7 °C warming in just a few years; Sissons, J.B.: "The Loch Lomond stadial in the British Isles.", page 199-203. Nature 280, 1979 Alley, R.B., et al.: "Abrupt increase in Greenland snow accumulation at the end of the Younger Dryas event.", page 527-529. Nature 362, 1993 Dansgaard, W., et al.: "The abrupt termination of the Younger Dryas climate event.", page 532-534. Nature 339, 1989 the total warming was 10°±4°.Takuro Kobashia, Jeffrey P. Severinghausa and Jean-Marc Barnolab (2008 (in press)). "4 ± 1.5 °C abrupt warming 11,270 years ago identified from trapped air in Greenland ice". doi:10.1016/j.epsl.2008.01.032.
The end of the Younger Dryas has been dated to around 9600 BC (11550 calendar years BP, occurring at 10000 radiocarbon years BP, a "radiocarbon plateau") by a variety of methods, with mostly consistent results:
The Younger Dryas is often linked to the adoption of agriculture in the Levant.Bar-Yosef, O. and A. Belfer-Cohen: "Facing environmental crisis. Societal and cultural changes at the transition from the Younger Dryas to the Holocene in the Levant." In: The Dawn of Farming in the Near East. Edited by R.T.J. Cappers and S. Bottema, pp. 55-66. Studies in Early Near Eastern Production, Subsistence and Environment 6. Berlin: Ex oriente. It is argued that the cold and dry Younger Dryas lowered the carrying capacity of the area and forced the sedentary Early Natufian population into a more mobile subsistence pattern. Further climatic deterioration is thought to have brought about cereal cultivation. While there exists relative consensus regarding the role of the Younger Dryas in the changing subsistence patterns during the Natufian, its connection to the beginning of agriculture at the end of the period is still being debated.Munro, N. D.: "Small game, the younger dryas, and the transition to agriculture in the southern levant", page 47–64. Mitteilungen der Gesellschaft für Urgeschichte 12, 2003 See the Neolithic Revolution, when hunter gatherers turned to farming.
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