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60-765: The Younger Dryas (YD, Greenland Stadial GS-1) was a period in Earth's geologic history that occurred circa 12,900 to 11,700 years Before Present (BP). It is primarily known for the sudden or "abrupt" cooling in the Northern Hemisphere, when the North Atlantic Ocean cooled and annual air temperatures decreased by ~3 °C (5.4 °F) over North America , 2–6 °C (3.6–10.8 °F) in Europe and up to 10 °C (18 °F) in Greenland , in

120-537: A few decades. Cooling in Greenland was particularly rapid, taking place over just 3 years or less. At the same time, the Southern Hemisphere experienced warming. This period ended as rapidly as it began, with dramatic warming over ~50 years, which transitioned the Earth from the glacial Pleistocene epoch into the current Holocene . The Younger Dryas onset was not fully synchronized; in the tropics,

180-523: A high latitude volcanic eruption could have accelerated North Atlantic sea ice growth, finally tipping the AMOC sufficiently to cause the Younger Dryas. Cave deposits and glacial ice cores both contain evidence of at least one major volcanic eruption taking place in the northern hemisphere at a time close to Younger Dryas onset, perhaps even completely matching the stalagmite-derived date for the onset of

240-463: A lag in timing of the Younger Dryas, indicating a greater influence of warmer Pacific conditions on that range. Effects in the Rocky Mountain region were varied. Several sites show little to no changes in vegetation. In the northern Rockies, a significant increase in pines and firs suggests warmer conditions than before and a shift to subalpine parkland in places. That is hypothesized to be

300-505: A pathway along the Mackenzie River in present-day Canada, and sediment cores show that the strongest outburst had occurred right before the onset of Younger Dryas. Other factors are also likely to have played a major role in the Younger Dryas climate. For instance, some research suggests climate in Greenland was primarily affected by the melting of then-present Fennoscandian ice sheet , which could explain why Greenland experienced

360-468: A plant which only thrives in glacial conditions, began to dominate where forests were able to grow during the preceding B-A Interstadial. This makes the Younger Dryas a key example of how biota responded to abrupt climate change . For instance, in what is now New England , cool summers, combined with cold winters and low precipitation, resulted in a treeless tundra up to the onset of the Holocene, when

420-461: Is reconstructed through proxy data such as traces of pollen , ice cores and layers of marine and lake sediments . Collectively, this evidence shows that significant cooling across the Northern Hemisphere began around 12,870 ± 30 years BP. It was particularly severe in Greenland , where temperatures declined by 4–10 °C (7.2–18.0 °F), in an abrupt fashion. Temperatures at the Greenland summit were up to 15 °C (27 °F) colder than at

480-489: Is sometimes used for dates established by means other than radiocarbon dating, such as stratigraphy . This usage differs from the recommendation by van der Plicht & Hogg, followed by the Quaternary Science Reviews , both of which requested that publications should use the unit "a" (for "annum", Latin for "year") and reserve the term "BP" for radiocarbon estimations. Some archaeologists use

540-430: Is weak. The scientific consensus is that severe AMOC weakening explains the climatic effects of the Younger Dryas. It also explains why the Holocene warming had proceeded so rapidly once the AMOC change was no longer counteracting the increase in carbon dioxide levels. AMOC weakening causing polar seesaw effects is also consistent with the accepted explanation for Dansgaard–Oeschger events , with YD likely to have been

600-677: The Nahanagan Stadial , and in Great Britain it has been called the Loch Lomond Stadial . In the Greenland Summit ice core chronology, the Younger Dryas corresponds to Greenland Stadial 1 (GS-1). The preceding Allerød warm period (interstadial) is subdivided into three events: Greenland Interstadial-1c to 1a (GI-1c to GI-1a). As with the other geologic periods, paleoclimate during the Younger Dryas

660-700: The Puerto Princesa cave complex in the Philippines shows that the onset of the Younger Dryas in East Asia was delayed by several hundred years relative to the North Atlantic. Further, the cooling was uniform throughout the year, but had a distinct seasonal pattern. In most places in the Northern Hemisphere, winters became much colder than before, but springs cooled by less, while there was either no temperature change or even slight warming during

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720-464: The Scandinavian ice sheet advanced. Notably, ice sheet advance in this area appears to have begun about 600 years before the global onset of the Younger Dryas. Underwater, the deposits of methane clathrate - methane frozen into ice - remained stable throughout the Younger Dryas, including during the rapid warming as it ended. As the Northern Hemisphere cooled and the Southern Hemisphere warmed,

780-950: The Swiss Alps and the Dinaric Alps in the Balkans , northern ranges of North America's Rocky Mountains , Two Creeks Buried Forest in Wisconsin and western parts of the New York State , and in the Pacific Northwest, including the Cascade Range . The entire Laurentide ice sheet had advanced between west Lake Superior and southeast Quebec , leaving behind a layer of rock debris ( moraine ) dated to this period. Southeastern Alaska appears to have escaped glaciation; speleothem calcite deposition continued in

840-485: The boreal forests shifted north. Along the southern margins of the Great Lakes, spruce dropped rapidly, while pine increased, and herbaceous prairie vegetation decreased in abundance, but increased west of the region. The central Appalachian Mountains remained forested during the Younger Dryas, but they were covered in spruce and tamarack boreal forests, switching to temperate broadleaf and mixed forests during

900-641: The thermal equator would have shifted to the south. Because trade winds from either hemisphere cancel each other out above the thermal equator in a calm, heavily clouded area known as the Intertropical Convergence Zone (ITCZ), a change in its position affects wind patterns elsewhere. For instance, in East Africa , the sediments of Lake Tanganyika were mixed less strongly during this period, indicating weaker wind systems in this area. Shifts in atmospheric patterns are believed to be

960-488: The AMOC on timescales of decades or centuries. The Younger Dryas is the best known and best understood because it is the most recent, but it is fundamentally similar to the previous cold phases over the past 120,000 years. This similarity makes the impact hypothesis very unlikely, and it may also contradict the Lake Agassiz hypothesis. On the other hand, some research links volcanism with D–O events, potentially supporting

1020-475: The AMOC. Once the Younger Dryas began, lowered temperatures would have elevated snowfall across the Northern Hemisphere, increasing the ice-albedo feedback . Further, melting snow would be more likely to flood back into the North Atlantic than rainfall would, as less water would be absorbed into the frozen ground. Other modelling shows that sea ice in the Arctic Ocean could have been tens of meters thick by

1080-474: The Balkans also experienced ice loss and glacial retreat: this was likely caused by the drop in annual precipitation, which would have otherwise frozen and helped to maintain the glaciers. Unlike now, the glaciers were still present in northern Scotland , but they had thinned during the Younger Dryas. The amount of water contained within glaciers directly influences global sea levels - sea level rise occurs if

1140-490: The Holocene. Conversely, pollen and macrofossil evidence from near Lake Ontario indicates that cool, boreal forests persisted into the early Holocene. An increase of pine pollen indicates cooler winters within the central Cascades. Speleothems from the Oregon Caves National Monument and Preserve in southern Oregon 's Klamath Mountains yield evidence of climatic cooling contemporaneous to

1200-510: The Marine Isotope Stage 6, ~130,000 years BP), III (the end of Marine Isotope Stage 8, ~243,000 years BP) and Termination IV (the end of Marine Isotope Stage 10, ~337,000 years BP. When combined with additional evidence from ice cores and paleobotanical data, some have argued that YD-like events inevitably occur during every deglaciation. The 2004 film, The Day After Tomorrow depicts catastrophic climatic effects following

1260-480: The Northern Hemisphere, the length of the growing season declined. Land ice cover experienced little net change, but sea ice extent had increased, contributing to ice–albedo feedback . This increase in albedo was the main reason for net global cooling of 0.6 °C (1.1 °F). During the preceding period, the Bølling–Allerød Interstadial , rapid warming in the Northern Hemisphere was offset by

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1320-592: The Younger Dryas event. It has been suggested that this eruption would have been stronger than any during the Common Era , some of which have been able to cause several decades of cooling. According to 1990s research, the Laacher See eruption (present-day volcanic lake in Rhineland-Palatinate , Germany ) would have matched the criteria, but radiocarbon dating done in 2021 pushes the date of

1380-482: The Younger Dryas with a significant reduction or shutdown of the thermohaline circulation , which circulates warm tropical waters northward through the Atlantic meridional overturning circulation (AMOC). This is consistent with climate model simulations, as well as a range of proxy evidence, such as the decreased ventilation (exposure to oxygen from the surface) of the lowest layers of North Atlantic water. Cores from

1440-568: The Younger Dryas. On the Olympic Peninsula, a mid-elevation site recorded a decrease in fire, but forest persisted and erosion increased during the Younger Dryas, which suggests cool and wet conditions. Speleothem records indicate an increase in precipitation in southern Oregon, the timing of which coincides with increased sizes of pluvial lakes in the northern Great Basin. Pollen record from the Siskiyou Mountains suggests

1500-621: The amount of dust blown by wind had also increased. Other areas became wetter including northern China (possibly excepting the Shanxi region) The Younger Dryas was initially discovered around the start of the 20th century, through paleobotanical and lithostratigraphic studies of Swedish and Danish bog and lake sites, particularly the Allerød clay pit in Denmark. The analysis of fossilized pollen had consistently shown how Dryas octopetala ,

1560-572: The coastal waters. It was originally hypothesized that the massive outburst from paleohistorical Lake Agassiz had flooded the North Atlantic via the Saint Lawrence Seaway , but little geological evidence had been found. For instance, the salinity in the Saint Lawrence Seaway did not decline, as would have been expected from massive quantities of meltwater. More recent research instead shows that floodwaters followed

1620-744: The continental interior. The Southeastern United States became warmer and wetter than before. There was warming in and around the Caribbean Sea , and in West Africa . It was once believed that the Younger Dryas cooling started at around the same time across the Northern Hemisphere. However, varve (sedimentary rock) analysis carried out in 2015 suggested that the cooling proceeded in two stages: first along latitude 56–54°N, 12,900–13,100 years ago, and then further north, 12,600–12,750 years ago. Evidence from Lake Suigetsu cores in Japan and

1680-434: The cooling was spread out over several centuries, and the same was true of the early-Holocene warming. Even in the Northern Hemisphere, temperature change was highly seasonal, with much colder winters, cooler springs, yet no change or even slight warming during the summer. Substantial changes in precipitation also took place, with cooler areas experiencing substantially lower rainfall, while warmer areas received more of it. In

1740-522: The disruption of the North Atlantic Ocean circulation that results in a series of extreme weather events that create an abrupt climate change that leads to a new ice age . Before Present In a convention that is not always observed, many sources restrict the use of BP dates to those produced with radiocarbon dating; the alternative notation "RCYBP" stands for the explicit "radio carbon years before present". The BP scale

1800-641: The eastern and central areas. While the Pacific Northwest region cooled by 2–3 °C (3.6–5.4 °F), cooling in western North America was generally less intense. While the Orca Basin in the Gulf of Mexico still experienced a drop in sea surface temperature of 2.4 ± 0.6°C, land areas closer to it, such as Texas , the Grand Canyon area and New Mexico , ultimately did not cool as much as

1860-476: The equivalent cooling in the Southern Hemisphere. This "polar seesaw" pattern is consistent with changes in thermohaline circulation (particularly the Atlantic meridional overturning circulation or AMOC), which greatly affects how much heat is able to go from the Southern Hemisphere to the North. The Southern Hemisphere cools and the Northern Hemisphere warms when the AMOC is strong, and the opposite happens when it

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1920-404: The eruption back to 13,006 years BP, or over a century before the Younger Dryas began. This analysis was also challenged in 2023, with some researchers suggesting that the radiocarbon analysis was tainted by magmatic carbon dioxide. For now, the debate continues without a conclusive proof or rejection of the volcanic hypothesis. The Younger Dryas impact hypothesis (YDIH) attributes the cooling to

1980-482: The exception was in tropical Atlantic areas such as Costa Rica , where temperature change was similar to Greenland's. The Holocene warming then proceeded across the globe, following an increase in carbon dioxide levels during the YD period (from ~210 ppm to ~275 ppm). Younger Dryas cooling was often accompanied by glacier advance and lowering of the regional snow line , with evidence found in areas such as Scandinavia,

2040-476: The exponential decay relation and the "Libby half-life" 5568 a. The ages are expressed in years before present (BP) where "present" is defined as AD 1950. The year 1950 was chosen because it was the standard astronomical epoch at that time. It also marked the publication of the first radiocarbon dates in December 1949, and 1950 also antedates large-scale atmospheric testing of nuclear weapons , which altered

2100-484: The glaciers retreat, and it drops if glaciers grow. Altogether, there appears to have been little change in sea level throughout the Younger Dryas. This is in contrast to rapid increases before and after, such as the Meltwater Pulse 1A . On the coasts, glacier advance and retreat also affects relative sea level . Western Norway experienced a relative sea level rise of 10 m ( 32 + 2 ⁄ 3  ft) as

2160-480: The global ratio of carbon-14 to carbon-12 . Dates determined using radiocarbon dating come as two kinds: uncalibrated (also called Libby or raw ) and calibrated (also called Cambridge ) dates. Uncalibrated radiocarbon dates should be clearly noted as such by "uncalibrated years BP", because they are not identical to calendar dates. This has to do with the fact that the level of atmospheric radiocarbon ( carbon-14 or C) has not been strictly constant during

2220-531: The hypothesis, and argue that all of the microparticles are adequately explained by the terrestrial processes. For instance, mineral inclusions from YD-period sediments in Hall's Cave, Texas, have been interpreted by YDIH proponents as extraterrestrial in origin, but a paper published in 2020 argues that they are more likely to be volcanic. Opponents argue that there is no evidence for massive wildfires which would have been caused by an airburst of sufficient size to affect

2280-465: The impact of a disintegrating comet or asteroid. Because there is no impact crater dating to the Younger Dryas period, the proponents usually suggest the impact had struck the Laurentide ice sheet , so that the crater would have disappeared when the ice sheet melted during the Holocene, or that it was an airburst, which would only leave micro- and nanoparticles behind as evidence. Most experts reject

2340-412: The lack of sea level rise during this period, so other theories have also emerged. An extraterrestrial impact into the Laurentide ice sheet (where it would have left no impact crater) was proposed as an explanation, but this hypothesis has numerous issues and no support from mainstream science. A volcanic eruption as an initial trigger for cooling and sea ice growth has been proposed more recently, and

2400-408: The lack of sea level rise during the Younger Dryas onset by connecting it with a volcanic eruption. Eruptions often deposit large quantities of sulfur dioxide particles in the atmosphere, where they are known as aerosols , and can have a large cooling effect by reflecting sunlight. This phenomenon can also be caused by anthropogenic sulfur pollution, where it is known as global dimming . Cooling from

2460-493: The last and the strongest of these events. However, there is some debate over what caused the AMOC to become so weak in the first place. The hypothesis historically most supported by scientists was an interruption from an influx of fresh, cold water from North America's Lake Agassiz into the Atlantic Ocean. While there is evidence of meltwater travelling via the Mackenzie River , this hypothesis may not be consistent with

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2520-801: The lowercase letters bp , bc and ad as terminology for uncalibrated dates for these eras. The Centre for Ice and Climate at the University of Copenhagen instead uses the unambiguous "b2k", for "years before 2000 AD", often in combination with the Greenland Ice Core Chronology 2005 (GICC05) time scale. Some authors who use the YBP dating format also use "YAP" ("years after present") to denote years after 1950. SI prefix multipliers may be used to express larger periods of time, e.g. ka BP (thousand years BP), Ma BP (million years BP) and many others . Radiocarbon dating

2580-557: The main reason why Northern Hemisphere summers generally did not cool during the Younger Dryas. Since winds carry moisture in the form of clouds, these changes also affect precipitation . Thus, evidence from the pollen record shows that some areas have become very arid, including Scotland, the North American Midwest , Anatolia and southern China . As North Africa, including the Sahara Desert , became drier,

2640-453: The most abrupt climatic changes during the YD. Climate models also indicate that a single freshwater outburst, no matter how large, would not have been able to weaken the AMOC for over 1,000 years, as required by the Younger Dryas timeline, unless other factors were also involved. Some modelling explains this by showing that the melting of Laurentide Ice Sheet led to greater rainfall over the Atlantic Ocean, freshening it and so helping to weaken

2700-516: The name (standard codes are used) of the laboratory concerned, and other information such as confidence levels, because of differences between the methods used by different laboratories and changes in calibrating methods. Conversion from Gregorian calendar years to Before Present years is by starting with the 1950-01-01 epoch of the Gregorian calendar and increasing the BP year count with each year into

2760-406: The onset of the Younger Dryas, so that it would have been able to shed icebergs into the North Atlantic, which would have been able to weaken the circulation consistently. Notably, changes in sea ice cover would have had no impact on sea levels, which is consistent with the absence of significant sea level rise during the Younger Dryas, and particularly during its onset. Some scientists also explain

2820-484: The presence of anomalously high levels of volcanism immediately preceding the onset of the Younger Dryas has been confirmed in both ice cores and cave deposits. The Younger Dryas is named after the alpine – tundra wildflower Dryas octopetala , because its fossils are abundant in the European (particularly Scandinavian ) sediments dating to this timeframe. The two earlier geologic time intervals where this flower

2880-528: The region despite being retarded, indicating the absence of permafrost and glaciation. On the other hand, the warming of the Southern Hemisphere led to ice loss in Antarctica, South America and New Zealand. Moreover, while Greenland as a whole had cooled, glaciers had only grown in the north of the island, and they had retreated from the rest of Greenland's coasts. This was likely driven by the strengthened Irminger Current . The Jabllanica mountain range in

2940-535: The result of a northward shift in the jet stream, combined with an increase in summer insolation as well as a winter snow pack that was higher than today, with prolonged and wetter spring seasons. The Younger Dryas is often linked to the Neolithic Revolution , with the adoption of agriculture in the Levant . The cold and dry Younger Dryas arguably lowered the carrying capacity of the area and forced

3000-482: The sedentary early Natufian population into a more mobile subsistence pattern. Further climatic deterioration is thought to have brought about cereal cultivation. While relative consensus exists 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 scientific consensus links

3060-596: The span of time that can be radiocarbon-dated. Uncalibrated radiocarbon ages can be converted to calendar dates by calibration curves based on comparison of raw radiocarbon dates of samples independently dated by other methods, such as dendrochronology (dating based on tree growth-rings) and stratigraphy (dating based on sediment layers in mud or sedimentary rock). Such calibrated dates are expressed as cal BP, where "cal" indicates "calibrated years", or "calendar years", before 1950. Many scholarly and scientific journals require that published calibrated results be accompanied by

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3120-406: The start of the 21st century. Strong cooling of around 2–6 °C (3.6–10.8 °F) had also taken place in Europe. Icefields and glaciers formed in upland areas of Great Britain , while many lowland areas developed permafrost , implying a cooling of −5 °C (23 °F) and a mean annual temperature no higher than −1 °C (30 °F). North America also became colder, particularly in

3180-493: The summer. An exception appears to have taken place in what is now Maine , where winter temperatures remained stable, yet summer temperatures decreased by up to 7.5 °C (13.5 °F). While the Northern Hemisphere cooled, considerable warming occurred in the Southern Hemisphere. Sea surface temperatures were warmer by 0.3–1.9 °C (0.54–3.42 °F), and Antarctica , South America (south of Venezuela ) and New Zealand all experienced warming. The net temperature change

3240-404: The thermohaline circulation, mineralogical and geochemical evidence or for simultaneous human population declines and mass animal extinctions which would have been required by this hypothesis. Statistical analysis shows that the Younger Dryas is merely the last of 25 or 26 Dansgaard–Oeschger events (D–O events) over the past 120,000 years. These episodes are characterized by abrupt changes in

3300-527: The volcanic hypothesis. Events similar to the Younger Dryas appear to have occurred during the other terminations - a term used to describe a comparatively rapid transition from cold glacial conditions to warm interglacials. The analysis of lake and marine sediments can reconstruct past temperatures from the presence or absence of certain lipids and long chain alkenones , as these molecules are very sensitive to temperature. This analysis provides evidence for YD-like events during Termination II (the end of

3360-560: The western subtropical North Atlantic show that the "bottom water" lingered there for 1,000 years, twice the age of Late Holocene bottom waters from the same site around 1,500 BP. Further, the otherwise anomalous warming of the southeastern United States matches the hypothesis that as the AMOC weakened and transported less heat from the Caribbean towards Europe through the North Atlantic Gyre , more of it would stay trapped in

3420-601: Was a relatively modest cooling of 0.6 °C (1.1 °F). Temperature changes of the Younger Dryas lasted 1,150–1,300 years. According to the International Commission on Stratigraphy , the Younger Dryas ended around 11,700 years ago, although some research places it closer to 11,550 years ago. The end of Younger Dryas was also abrupt: in previously cooled areas, warming to previous levels took place over 50–60 years. The tropics experienced more gradual temperature recovery over several centuries;

3480-635: Was abundant in Europe are the Oldest Dryas (approx. 18,500-14,000 BP) and Older Dryas (~14,050–13,900 BP), respectively. On the contrary, Dryas octopetala was rare during the Bølling–Allerød Interstadial . Instead, European temperatures were warm enough to support trees in Scandinavia, as seen at the Bølling and Allerød sites in Denmark . In Ireland , the Younger Dryas has also been known as

3540-446: Was first used in 1949. Beginning in 1954, metrologists established 1950 as the origin year for the BP scale for use with radiocarbon dating, using a 1950-based reference sample of oxalic acid . According to scientist A. Currie Lloyd: The problem was tackled by the international radiocarbon community in the late 1950s, in cooperation with the U.S. National Bureau of Standards . A large quantity of contemporary oxalic acid dihydrate

3600-525: Was prepared as NBS Standard Reference Material (SRM) 4990B. Its C concentration was about 5% above what was believed to be the natural level, so the standard for radiocarbon dating was defined as 0.95 times the C concentration of this material, adjusted to a C reference value of −19 per mil (PDB). This value is defined as "modern carbon" referenced to AD 1950. Radiocarbon measurements are compared to this modern carbon value, and expressed as "fraction of modern" (fM). "Radiocarbon ages" are calculated from fM using

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