The Younger Dryas stadial, also referred to as the Big Freeze, was a 1,300 (± 70) year period of cold climatic conditions and drought which occurred between approximately 12,900 and 11,500 years BP (between 10,900 and 9500 BC) in calendar years. The cause of the Younger Dryas stadial is an issue of ongoing debate. Possible scenarios include the collapse of the North American ice sheets, bringing a significant influx of freshwater to disrupt the thermohaline circulation. An alternative scenario is offered in Younger Dryas impact hypothesis, whereby a bolide (meteor) collision could have caused widespread cooling through dust and aerosols entering the stratosphere.
It followed the Bølling-Allerød interstadial (warm period) 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). The Younger Dryas (GS1) is also a Blytt-Sernander climate period detected from layers in north European bog peat.
The Dryas stadials were cold periods which interrupted the warming trend since the Last Glacial Maximum 20,000 years ago. The Older Dryas occurred approximately 1,000 years before the Younger Dryas and lasted about 300 years. The Oldest Dryas is dated between approximately 18,000 and 15,000 BP (16000 to 13000 BC).
Based upon solid geological evidence, consisting largely of the analysis of numerous deep cores from coral reefs, variations in rates of sea level rise have been reconstucted for the early Holocene. During this period of deglacial sea level rise, 3 major periods of accelerated sea level rise, called meltwater pulses, occurred. They are Meltwater pulse 1A between circa 14,600 and 14,300 calendar years ago; Meltwater pulse 1B between circa 11,400 and 11,100 calendar years ago; and Meltwater pulse 1C between 8,200 and 7,600 calendar years ago. The Younger Dryas occurred after Meltwater pulse 1A, which was a 13.5 m rise over about 290 years centered at about 14,200 calendar years ago and before Meltwater pulse 1B, which was a 7.5 m rise over about 160 years centered at about 11,000 calendar years ago. Between Meltwater pulse 1A and Meltwater pulse 1B, the Younger Dryas was a interval of a significantly reduced rate of sea level rise relative to the periods of time before and after it. For example, the analysis of cores from Tahiti coral reefs found that sea level rose at a rate of about 7.5 ± 1.1 mm/yr during the Younger Dryas. Just after the end of the Younger Dryas, the rate of sea level rise accelerated to 17.4 ± 0.4 mm/yr and just before its start, it was 12.1 ± 0.6 mm/yr. This reduction in the rate of sea level rise directly reflected a substantial reduction of the global inflow of meltwater into the world's oceans during the younger Dryas.
Possible evidence of short term sea level changes have been reported for the beginning of the Younger Dryas. First, the plotting of data by Bard and others suggested a small step, less than 6 m, in sea level near the onset of the Younger Dryas. There is also possibly a corresponding change in the rate of change of sea level rise that they saw in the data from both Barbados and Tahiti. Given that this change is …within the overall uncertainty of the approach…, its existence remains speculative and unproven and significance is uncertain. Finally, research by Lohe and others in western Norway reported a sea-level low-stand at 13,640 calender years ago and a subsequent Younger Dryas transgression starting at 13,080 calendar years ago. They concluded that the timing of the Allerød low-stand and the subsequent transgression were the result of increased regional loading of the crust and geoid changes caused by an expanding ice sheet that started growing and advancing in early Allerød about 13,600 calender years ago and well before the start of the Younger Dryas.
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 BP in calendar years, 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, but the onset may have been faster. Thermally fractionated nitrogen and argon isotope data from Greenland ice core GISP2 indicate that the summit of Greenland was approximately 15 °C (27 °F) colder during the Younger Dryas than today. In the UK, coleopteran (beetle) fossil evidence suggests that mean annual temperature dropped to −5 °C (23 °F), 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.
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; Peter 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:
- 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
The prevailing theory is that the Younger Dryas was caused by 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. Geological evidence for such an event is thus far lacking. The global climate would then have become locked into the new state until freezing removed the fresh water "lid" from the north Atlantic Ocean. An 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. There is also some evidence that a solar flare may have been responsible for the megafaunal extinction, though it cannot explain the apparent variability in the extinction across all continents.
There is evidence that some previous glacial terminations had post glacial cooling periods similar to the Younger Dryas.
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. Amongst other things findings of melt-glass material in sediments in Pennsylvania, South Carolina, and Syria have been reported. These researchers argue that this material, which dates back nearly 13,000 years, was formed at temperatures of 1,700 to 2,200 °C (3,100 to 4,000 °F) as the result of a bolide impact. They argue that these findings support the controversial Younger Dryas Boundary (YDB) hypothesis, that the bolide impact occurred at the onset of the Younger Dryas. The hypothesis has been questioned by research that stated that most of the conclusions cannot be repeated by other scientists, misinterpretation of data, and the lack of confirmatory evidence. After a review of the sediments that are found at the sites, new research found that sediments claimed, by the hypothesis proponents, to be deposits resulting from a bolide impact were, in fact, dated from much later or much earlier time periods than the proposed date of the cosmic impact. The researchers examined 29 sites that are commonly referenced to support the impact theory to determine if they can be geologically dated to around 13,000 years ago. Crucially, only 3 of the sites actually date from that time.
In a study in the Journal of Geology (August 2014), Prof.Kennett (et al.) looked at the distribution of nanodiamonds produced during extraterrestrial collisions; 50 million square kilometers of Northern Hemisphere at YDB was found to have these nanodiamonds. Only two layers exist showing such nanodiamonds: the YDB 12,800YA and the Cretaceous-Tertiary boundary 65 million YA, which is also marked by the mass extinctions 
“The evidence we present settles the debate about the existence of abundant YDB nanodiamonds,” Kennett said. “Our hypothesis challenges some existing paradigms within several disciplines, including impact dynamics, archaeology, paleontology and paleoceanography/paleoclimatology, all affected by this relatively recent cosmic impact.”
Although there may be several causes of the Younger Dryas, volcanic activity is considered one possibility. The Laacher See volcano in Germany was of sufficient size, VEI 6, with over 10 km3 (2.4 cu mi) tephra ejected, to have caused significant temperature changes in the northern hemisphere. Laacher See tephra is found throughout the Younger Dryas boundary layer. This possibility has been disputed by 14
C analysis. In the view of Cambridge University volcanologist, Clive Oppenheimer, the magnitude of Laacher See was similar to the 1991 Mount Pinatubo eruption, and the effects were a year or two of northern hemisphere summer cooling and winter warming, and up to two decades of environmental disruption in Germany.
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 (13 °F) warming in just a few years. Total warming in Greenland was 10 ± 4 °C (18 ± 7 °F).
The end of the Younger Dryas has been dated to around 11.55 ka BP, occurring at 10 ka bp (uncalibrated radiocarbon year), a "radiocarbon plateau" by a variety of methods, with mostly consistent results:
11.50 ± 0.05 ka BP: GRIP ice core, Greenland 11.53 + 0.04
ka BP: Krakenes Lake, western Norway 11.57 ka BP: Cariaco Basin core, Venezuela 11.57 ka BP: German oak/pine dendrochronology 11.64 ± 0.28 ka BP: GISP2 ice core, Greenland
Effect on 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.
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.
- Shutdown of thermohaline circulation
- Heinrich event
- 1500-year climate cycle
- Timeline of glaciation
- Timeline of environmental events
- Older Dryas
- Oldest Dryas
- 8.2 kiloyear climate event
- Little Ice Age
- Medieval Warm Period
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