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Younger Dryas
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The Younger Dryas stadial, named after the alpine/tundra wildflower Dryas octopetala, and also referred to as the Big Freeze, was a brief (approximately 1,300 ± 70 years) cold climate period following the Bölling/Allerød interstadial at the end of the Pleistocene between approximately 12,800 to 11,500 years Before Present, and preceding the Preboreal of the early Holocene.
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The Younger Dryas stadial, named after the alpine/tundra wildflower Dryas octopetala, and also referred to as the Big Freeze, was a brief (approximately 1,300 ± 70 years) cold climate period following the Bölling/Allerød interstadial at the end of the Pleistocene between approximately 12,800 to 11,500 years Before Present, 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).
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 1,000 years before the Younger Dryas; it lasted 300 years.
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) in sharp contrast to the warming of the preceding interstadial deglaciation. The transitions each occurred over a period of a decade or so. 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, and periglacial conditions prevailed in lowland areas, while icefields and glaciers formed in upland areas. Nothing of the size, extent, or rapidity of this period of abrupt climate change has been experienced since.
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. 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. Timing of the tropical counterpart to the Younger Dryas – the Deglaciation Climate Reversal (DCR) – is difficult to establish as low latitude ice core records generally lack independent dating over this interval. An example of this is the Sajama ice core (Bolivia), for which the timing of the DCR has been pinned to that of the GISP2 ice core record (central Greenland). Climatic change in the central Andes during the DCR, however, was significant and characterized by a shift to much wetter, and likely colder, conditions. The magnitude and abruptness of these changes would suggest that low latitude climate did not respond passively during the YD/DCR.
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 indicates Younger Dryas cooling occurred in the Pacific Northwest.
Other features seen include:
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. 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 its cause has a random component. Nevertheless, 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 and population bottleneck or near extinction of the Clovis people.
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; the total warming was 10°±4°.
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
- 11530+40-60 BP — Kråkenes Lake, western Norway.
- 11570 BP — Cariaco Basin core, Venezuela
- 11570 BP — German oak/pine dendrochronology
- 11640±280 BP — GISP2 ice core, Greenland
The Younger Dryas and the beginning of agriculture
The Younger Dryas is often linked to the adoption of agriculture in the Levant. 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. See the Neolithic Revolution, when hunter gatherers turned to farming.
The Younger Dryas in contemporary culture and writing
The failure of North Atlantic thermohaline circulation is used to explain rapid climate change in some of Kim Stanley Robinson's novels, particularly Fifty Degrees Below. It also underpinned the 1999 book, The Coming Global Superstorm. Likewise, the idea of rapid climate change caused by disruption of North Atlantic ocean currents creates the setting for 2004 apocalyptic science-fiction film The Day After Tomorrow. Similar sudden cooling events have featured in other novels, such as John Christopher's The World in Winter, though not always with the same explicit links to the Younger Dryas event as is the case of Robinson's work.
See also
External links
- , Oregon State University press release (10 April 2007)
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- adapted from Atlantic Monthly, 281(1):47-64 (January 1998).
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- (July 2008)
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