Three temperature records, the GRIP sequence (red) clearly showing the Younger Dryas event at around 11 kyr BP

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.^[4]

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)^[5] in sharp contrast to the warming of the preceding interstadial deglaciation. The transitions each occurred over a period of a decade or so.^[6] 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^[6] than today. In the UK, coleopteran (fossil beetle) evidence suggests mean annual temperature dropped to approximately 5°C,^[7] and periglacial conditions prevailed in lowland areas, while icefields and glaciers formed in upland areas.^[8] Nothing of the size, extent, or rapidity of this period of abrupt climate change has been experienced since.^[5]

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.^[9] 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).^[10]

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-advance^[11] indicates Younger Dryas cooling occurred in the Pacific Northwest.

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.^[12] 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.^[13]

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;^{[5] [6] [14] [15]} the total warming was 10°±4°.^[16]

The end of the Younger Dryas has been dated to around 9620 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 ^[17]

11530⁺⁴⁰_-60 BP ? Kråkenes Lake, western Norway. ^[18]

11570 BP ? Cariaco Basin core, Venezuela ^[19]

11570 BP ? German oak/pine dendrochronology ^[20]

11640±280 BP ? GISP2 ice core, Greenland ^[14]

The Younger Dryas and the beginning of agriculture

The Younger Dryas is often linked to the adoption of agriculture in the Levant.^[21] 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.^[22] See the Neolithic Revolution, when hunter gatherers turned to farming.

See also

• 1500-year climate cycle
• Timeline of glaciation
• Timeline of environmental events
• Older Dryas
• Oldest Dryas
• Little Ice Age
• Medieval Warm Period

References

External links

• Study Confirms Mechanism for Current Shutdowns, European Cooling, Oregon State University press release (10 April 2007)
• Younger Dryas Caused by ET Impact
• W. S. Broecker, "What If the Conveyor Were to Shut Down? Reflections on a Possible Outcome of the Great Global Experiment"
• Paul A. Mayewski and Michael Bender, "The GISP2 ice core record: The Younger Dryas"
• ARIC Global Climate Change Student Guide 5.3.2.1. The Younger Dryas Event
• William C. Calvin, "The great climate flip-flop" adapted from Atlantic Monthly, 281(1):47-64 (January 1998).
• Lamont-Doherty Earth Observatory, "Two examples of abrupt climate change: 1. The Younger Dryas"
• Lev Tarasov and W.R. Peltier, "Arctic freshwater forcing of the Younger Dryas cold reversal" letter, in ''Nature'' '''435''', 662-665 (2 June 2005)
• "Hugheus radiocarbon and climate shifts during the last deglaciation"
• Friedrich, M., ''et al.'' (1999), Paleo-environment and radiocarbon calibration as derived from Lateglacial/Early Holocene tree-ring chronologies
• Younger Dryas (YD) Impact AGU Press Conference - YouTube Playlist
• Exploding asteroid theory strengthened by new evidence located in Ohio, Indiana (July 2008)
• Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling

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