Younger Dryas

The Younger Dryas stadial, named after the alpine / tundra wildflower Dryas octopetala, and also referred to as the Big Freeze ^[1], 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 ^[2], 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.

The Younger Dryas 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].

Abrupt climate change

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)^[3] in sharp contrast to the warming of the preceding interstadial deglaciation. The transitions each occurred over a period of a decade or so ^[4].Thermally fractionated nitrogen and argon isotope data from Greenland ice core GISP2 indicates that the summit of Greenland was ~15 °C colder than today during the Younger Dryas ^[4]. In the UK, coleopteran (fossil beetle) evidence suggests mean annual temperature dropped to approximately -5 °C ^[5], and periglacial conditions prevailed in lowland areas, while icefields and glaciers formed in upland areas ^[6]. Nothing of the size, extent, or rapidity of this period of abrupt climate change has been experienced since ^[3].

Was the Younger Dryas global?

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 LGM conditions with colder temperatures coupled with higher precipitation (high lake stands in the Altiplano). ^[7]

Other features seen include:

• Replacement of forest in Scandinavia with glacial tundra (which is the habitat of the plant Dryas octopetala).
• Glaciation or increased snow in mountain ranges around the world.
• Formation of solifluction layers and loess deposits in Northern Europe.
• More dust in the atmosphere, originating from deserts in Asia.
• Drought in the Levant, perhaps motivating the Natufian culture to invent agriculture.
• The Huelmo/Mascardi Cold Reversal in the Southern Hemisphere began slightly before the Younger Dryas and ended at the same time.

Causes of the Younger Dryas

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.

The end of the Younger Dryas

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 ^{[3] [4] [8] [9] [10]}.

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:

11530±50 BP -- GRIP ice core, Greenland ^[11]

11530⁺⁴⁰_-60 BP -- Kråkenes Lake, western Norway. ^[12]

11570 BP -- Cariaco Basin core, Venezuela ^[13]

11570 BP -- German oak/pine dendrochronology ^[14]

11640±280 BP -- GISP2 ice core, Greenland ^[8]

The Younger Dryas and the beginning of agriculture

The Younger Dryas is often linked to the adoption of agriculture in the Levant ^[15]. 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 ^[16]. See the Neolithic Revolution, when hunter gatherers turned to farming.

References

1. ^ 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
2. ^ How Rapidly did Climate Change in the Distant Past?, Climate Change 2001: Working Group I: The Scientific Basis, Intergovernmental Panel on Climate Change
3. ^ ^{a b c} Alley, R.B.: "The Younger Dryas cold interval as viewed from central Greenland.", page 213-226. Quaternary Science Reviews 19, 2000
4. ^ ^{a b c} Alley et al.: "Abrupt accumulation increase at the Younger Dryas termination in the GISP2 ice core", page 527-529. Nature 362, 1993
5. ^ 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
6. ^ Atkinson, T.C.: "Seasonal temperatures in Britain during the past 22,000 years, reconstructed using beetle remains.", page 587-592. Nature 325, 1987
7. ^ Thompson et al., 2000.
8. ^ ^{a b} Sissons, J.B.: "The Loch Lomond stadial in the British Isles.", page 199-203. Nature 280, 1979
9. ^ 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
10. ^ Dansgaard, W., et al.: "The abrupt termination of the Younger Dryas climate event.", page 532-534. Nature 339, 1989
11. ^ Taylor, K.C., et al.: "The Holocene-Younger Dryas transition recorded at Summit, Greenland.", page 825-827. Science 278, 1997
12. ^ Spurk, M., et al.: "Revisions and extension of the Hohenheim oak and pine chronologies: New evidence about the timing of the Younger Dryas/Preboreal transition", page 1107-1116. Radiocarbon 40, 1998
13. ^ Gulliksen, S., et al.: "A calendar age estimate of the Younger Dryas-Holocene boundary at Krakenes, western Norway", page 3, 249-259. Holocene 8, 1998
14. ^ "Hugheus radiocarbon and climate shifts during the last deglaciation", page 5498, 1951-1954. Science 290
15. ^ 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.
16. ^ 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 also

• Timeline of glaciation
• Timeline of environmental events

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