Younger Dryas

Three temperature records, the GRIP sequence (red) showing the Younger Dryas event at around 11.0 ka BP. The vertical axis shows δ¹⁸O, which is a temperature proxy showing the water molecule isotopic composition of ¹⁸O in an ice core.

The Younger Dryas stadial, also referred to as the Big Freeze,^[1] was a geologically brief (1,300 ± 70 years) period of cold climatic conditions and drought between approximately 12.8 and 11.5 ka BP, or 12,800 and 11,500 years before present.^[2] The Younger Dryas stadial is thought to have been caused by the collapse of the North American ice sheets, although rival theories have been proposed.

The period followed the Bølling-Allerød interstadial at the end of the Pleistocene and preceded the Preboreal of the early Holocene. It is named after an indicator genus, the alpine-tundra wildflower Dryas octopetala. In Ireland, the period has been known as the Nahanagan Stadial, while in the United Kingdom it has been called the Loch Lomond Stadial and most recently Greenland Stadial 1 (GS1).^[3][4]

The Younger Dryas (GS1) is also a Blytt-Sernander climate period detected from layers in north European bog peat. It is dated approximately 12.9–11.5 ka BP calibrated, or 11–10 ka BP uncalibrated (radiocarbon dating). An Older Dryas stadial had preceded the Allerød, approximately 1,000 years before the Younger Dryas; it lasted 300 years.^[5]

Abrupt climate change

The Younger Dryas saw a rapid return to glacial conditions in the higher latitudes of the Northern Hemisphere between 12.9–11.5 ka BP^[6] in sharp contrast to the warming of the preceding interstadial deglaciation. It has been believed that the transitions each occurred over a period of a decade or so,^[7] but the onset may have been faster.^[8] Thermally fractionated nitrogen and argon isotope data from Greenland ice core GISP2 indicate that the summit of Greenland was approximately 15 °C (27.0 °F) colder during the Younger Dryas^[7] than today. In the UK, coleopteran fossil evidence (from beetles) suggests that mean annual temperature dropped to approximately 5 °C (41 °F),^[9] and periglacial conditions prevailed in lowland areas, while icefields and glaciers formed in upland areas.^[10] Nothing of the size, extent, or rapidity of this period of abrupt climate change has been experienced since.^[6]

Global effects

In western Europe and Greenland, the Younger Dryas is a well-defined synchronous cool period.^[11] 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.^[12] 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.^[13] 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^[14] 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 develop agriculture.
• The Huelmo/Mascardi Cold Reversal in the Southern Hemisphere ended at the same time.
• Decline of the Clovis Culture and extinction of animal species in North America.

Causes

The prevailing theory holds that the Younger Dryas was caused by a significant reduction or shutdown of the North Atlantic "Conveyor", which circulates warm tropical waters northward, in response to a sudden influx of fresh water from Lake Agassiz and deglaciation in North America; however, geological evidence for such an event is thus far lacking.^[15] The global climate would then have become locked into the new state until freezing removed the fresh water "lid" from the north Atlantic Ocean. A recent alternative theory suggests instead that the jet stream shifted northward in response to the changing topographic forcing of the melting North American ice sheet, bringing more rain to the North Atlantic which freshened the ocean surface enough to slow the thermohaline circulation.^[16]

Previous glacial terminations probably did not have Younger Dryas-like events, suggesting that its cause has a random component. Nevertheless, there is evidence that some previous glacial terminations had post glacial cooling periods somewhat similar to the Younger Dryas.^[17]

A hypothesized Younger Dryas impact event, presumed to have occurred in North America around 12.9 ka BP, has been proposed as the mechanism to have initiated the Younger Dryas cooling, but reviews of the evidence for an impact event has shown that none of the original impact signatures that were attributed to the YD have been corroborated by independent tests.^[18]

End of the climate period

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 about a 7 °C (12.60 °F) warming in just a few years.^{[6][7][19][20]} Total warming was 10 ± 4 °C (18 ± 7 °F).^[21]

The end of the Younger Dryas has been dated to around 11.55 ka BP, occurring at 10 ka BP (radiocarbon year), a "radiocarbon plateau") by a variety of methods, with mostly consistent results:

11.50 ± 0.05	ka BP	— GRIP ice core, Greenland [22]
11.53 + 0.04 − 0.06	ka BP	— Kråkenes Lake, western Norway.[23]
11.57	ka BP	— Cariaco Basin core, Venezuela [24]
11.57	ka BP	— German oak/pine dendrochronology [25]
11.64 ± 0.28	ka BP	— GISP2 ice core, Greenland [19]

Effect on agriculture

The Younger Dryas is often linked to the adoption of agriculture in the Levant.^[26][27] 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.^[28][29] See the Neolithic Revolution, when hunter gatherers turned to farming.

Cultural references

The failure of North Atlantic thermohaline circulation is used to explain rapid climate change in some science fiction writings as early as Stanley G. Weinbaum's 1937 short story "Shifting Seas" where the author described the freezing of Europe after the Gulf Stream was disrupted, and more recently in 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

• Shutdown of thermohaline circulation
• Heinrich event
• 1500-year climate cycle
• Timeline of glaciation
• Timeline of environmental events
• Neoglaciation
• Older Dryas
• Oldest Dryas
• 8.2 kiloyear climate event
• Little Ice Age
• Medieval Warm Period

References

1. ^ Berger, W. H. (1990). "The Younger Dryas cold spell — a quest for causes". Global and Planetary Change 3 (3): 219–237. Bibcode 1990GPC.....3..219B. doi:10.1016/0921-8181(90)90018-8. 
2. ^ Muscheler, Raimund et al. (2008). "Tree rings and ice cores reveal ¹⁴C calibration uncertainties during the Younger Dryas". Nature Geoscience 1 (4): 263–267. Bibcode 2008NatGe...1..263M. doi:10.1038/ngeo128. 
3. ^ Seppä, H.; Birks, H. H.; Birks, H. J. B. (2002). "Rapid climatic changes during the Greenland stadial 1 (Younger Dryas) to early Holocene transition on the Norwegian Barents Sea coast". Boreas 31 (3): 215–225. doi:10.1111/j.1502-3885.2002.tb01068.x.  edit
4. ^ Walker, M. J. C. (2004). "A Lateglacial pollen record from Hallsenna Moor, near Seascale, Cumbria, NW England, with evidence for arid conditions during the Loch Lomond (Younger Dryas) Stadial and early Holocene". Proceedings of the Yorkshire Geological Society 55: 33–42. doi:10.1144/pygs.55.1.33. 
5. ^ Mangerud, J.; Andersen, S. T.; Berglund, B. E.; Donner, J. J. (2008). "Quaternary stratigraphy of Norden, a proposal for terminology and classification". Boreas 3 (3): 109–126. doi:10.1111/j.1502-3885.1974.tb00669.x.  edit
6. ^ ^{a b c} Alley, Richard B. (2000). "The Younger Dryas cold interval as viewed from central Greenland". Quaternary Science Reviews 19 (1): 213–226. Bibcode 2000QSRv...19..213A. doi:10.1016/S0277-3791(99)00062-1. 
7. ^ ^{a b c} Alley, Richard B. et al. (1993). "Abrupt accumulation increase at the Younger Dryas termination in the GISP2 ice core". Nature 362 (6420): 527–529. Bibcode 1993Natur.362..527A. doi:10.1038/362527a0. 
8. ^ Choi, Charles Q. (2 December 2009). Big Freeze: Earth Could Plunge into Sudden Ice Age. http://www.livescience.com/environment/091202-fast-ice-age.html. Retrieved December 2, 2009. 
9. ^ Severinghaus, Jeffrey P. et al. (1998). "Timing of abrupt climate change at the end of the Younger Dryas interval from thermally fractionated gases in polar ice". Nature 391 (6663): 141–146. Bibcode 1998Natur.391..141S. doi:10.1038/34346. 
10. ^ Atkinson, T. C. et al. (1987). "Seasonal temperatures in Britain during the past 22,000 years, reconstructed using beetle remains". Nature 325 (6105): 587–592. Bibcode 1987Natur.325..587A. doi:10.1038/325587a0. 
11. ^ How Stable was the Holocene Climate?
12. ^ http://www.sciencedaily.com/releases/2010/09/100908132214.htm
13. ^ Thompson, L. G. et al. (2000). "Ice-core palaeoclimate records in tropical South America since the Last Glacial Maximum". Journal of Quaternary Science 15 (4): 377–394. Bibcode 2000JQS....15..377T. doi:10.1002/1099-1417(200005)15:4<377::AID-JQS542>3.0.CO;2-L. 
14. ^ Friele, P. A.; Clague, J. J. (2002). "Younger Dryas readvance in Squamish river valley, southern Coast mountains, British Columbia". Quaternary Science Reviews 21 (18–19): 1925–1933. Bibcode 2002QSRv...21.1925F. doi:10.1016/S0277-3791(02)00081-1. 
15. ^ Broecker, Wallace S. (2006). "Was the Younger Dryas Triggered by a Flood?". Science 312 (5777): 1146–1148. doi:10.1126/science.1123253. PMID 16728622. 
16. ^ Eisenman, I.; Bitz, C. M.; Tziperman, E. (2009). "Rain driven by receding ice sheets as a cause of past climate change". Paleoceanography 24 (4): PA4209. Bibcode 2009PalOc..24.4209E. doi:10.1029/2009PA001778.  edit
17. ^ Carlson, A. (2008). "Why there was not a Younger Dryas-like event during the Penultimate Deglaciation". Quaternary Science Reviews 27 (9–10): 882–887. Bibcode 2008QSRv...27..882C. doi:10.1016/j.quascirev.2008.02.004.  edit
18. ^ Pinter, N.; Scott, A. C.; Daulton, T. L.; Podoll, A.; Koeberl, C.; Anderson, R. S.; Ishman, S. E. (2011). "The Younger Dryas impact hypothesis: A requiem". Earth-Science Reviews. doi:10.1016/j.earscirev.2011.02.005.  edit
19. ^ ^{a b} Sissons, J. B. (1979). "The Loch Lomond stadial in the British Isles". Nature 280 (5719): 199–203. Bibcode 1979Natur.280..199S. doi:10.1038/280199a0. 
20. ^ Dansgaard, W. et al. (1989). "The abrupt termination of the Younger Dryas climate event". Nature 339 (6225): 532–534. Bibcode 1989Natur.339..532D. doi:10.1038/339532a0. 
21. ^ Kobashia, Takuro et al. (2008). "4 ± 1.5 °C abrupt warming 11,270 years ago identified from trapped air in Greenland ice". Earth and Planetary Science Letters 268 (3–4): 397–407. Bibcode 2008E&PSL.268..397K. doi:10.1016/j.epsl.2008.01.032. 
22. ^ Taylor, K. C. (1997). "The Holocene-Younger Dryas transition recorded at Summit, Greenland". Science 278 (5339): 825–827. Bibcode 1997Sci...278..825T. doi:10.1126/science.278.5339.825. 
23. ^ Spurk, M. (1998). "Revisions and extension of the Hohenheim oak and pine chronologies: New evidence about the timing of the Younger Dryas/Preboreal transition". Radiocarbon 40 (3): 1107–1116. http://www.radiocarbon.org/Journal/v40n3/Abstracts/4.html. 
24. ^ Gulliksen, Steinar et al. (1998). "A calendar age estimate of the Younger Dryas-Holocene boundary at Krakenes, western Norway". Holocene 8 (3): 249–259. doi:10.1191/095968398672301347. 
25. ^ Hughen, Konrad A. et al. (2000). "Synchronous Radiocarbon and Climate Shifts During the Last Deglaciation". Science 290 (5498): 1951–1954. Bibcode 2000Sci...290.1951H. doi:10.1126/science.290.5498.1951. PMID 11110659. 
26. ^ 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.
27. ^ Mithen, Steven J.: After The Ice: A Global Human History, 20,000–5000 BC, pages 46–55. Harvard University Press paperback edition, 2003.
28. ^ Munro, N. D. (2003). "Small game, the younger dryas, and the transition to agriculture in the southern levant". Mitteilungen der Gesellschaft für Urgeschichte 12: 47–64. http://www.anth.uconn.edu/faculty/munro/assets/Mitteilungen.pdf. 
29. ^ Balter, Michael (2010). "Archaeology: The Tangled Roots of Agriculture". Science 327 (5964): 404–406. doi:10.1126/science.327.5964.404. PMID 20093449. http://scienceonline.org/cgi/content/summary/sci;327/5964/404. Retrieved 4 February 2010. 

External links

• "Study Confirms Mechanism for Current Shutdowns, European Cooling". Oregon State University. 2007. http://oregonstate.edu/dept/ncs/newsarch/2007/Apr07/currents.html. Retrieved 2011-04-11. 
• Broecker WS (1999). "What If the Conveyor Were to Shut Down?". GSA Today 9 (1): 1–7. http://faculty.washington.edu/wcalvin/teaching/Broecker99.html. Retrieved 2011-04-11. 
• Calvin WH (January 1998). "The Great Climate Flip-flop". The Atlantic Monthly 281: 47–64. http://williamcalvin.com/1990s/1998AtlanticClimate.htm. Retrieved 2011-04-11. 
• Tarasov L, Peltier WR (June 2005). "Arctic freshwater forcing of the Younger Dryas cold reversal". Nature 435 (7042): 662–665. Bibcode 2005Natur.435..662T. doi:10.1038/nature03617. PMID 15931219.