Younger Dryas
Template:Short description Template:Use dmy dates Template:Infobox geologic timespan The Younger Dryas (YD, Greenland Stadial GS-1)<ref>Template:Cite journalTemplate:Rp</ref> was a period in Earth's geologic history that occurred circa 12,900 to 11,700 years Before Present (BP).<ref>Template:Cite journal</ref> 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 ~Template:Convert over North America, Template:Convert in Europe and up to Template:Convert in Greenland, in a few decades.<ref name="Carlson2013" /> Cooling in Greenland was particularly rapid, taking place over just 3 years or less.<ref name="Partin2015" /><ref>Template:Cite web</ref> At the same time, the Southern Hemisphere experienced warming.<ref name="Carlson2013">Template:Cite encyclopedia</ref><ref>Template:Cite journal</ref> This period ended as rapidly as it began, with dramatic warming over ~50 years, the transition from the glacial Pleistocene epoch into the current Holocene.<ref name="Partin2015" />
The Younger Dryas onset was not fully synchronized; in the tropics, the cooling was spread out over several centuries, and the same was true of the early-Holocene warming.<ref name="Partin2015" /> 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.<ref name="Buizert2014" /><ref name="Schenk2018">Template:Cite journal</ref> Substantial changes in precipitation also took place, with cooler areas experiencing substantially lower rainfall, while warmer areas received more of it.<ref name="Carlson2013" /> In the Northern Hemisphere, the length of the growing season declined.<ref name="Schenk2018" /> Land ice cover experienced little net change,<ref name="Shakun2012" /> but sea ice extent had increased, contributing to ice–albedo feedback.<ref name="Carlson2013" /> This increase in albedo was the main reason for net global cooling of Template:Convert.<ref name="Carlson2013" />
During the preceding period, the Bølling–Allerød Interstadial, rapid warming in the Northern Hemisphere<ref name="IPCC AR6 WG1 CH5">Template:Cite report</ref>Template:Rp was offset by the equivalent cooling in the Southern Hemisphere.<ref name="Obase2021">Template:Cite journal</ref><ref name="Shakun2012" /> 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 is weak.<ref name="Obase2021" /> The scientific consensus is that severe AMOC weakening explains the climatic effects of the Younger Dryas.<ref name="IPCC AR6 WG1 Ch.8">Template:Cite journal</ref>Template:Rp 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.<ref name="Shakun2012" />
AMOC weakening causing polar seesaw effects is also consistent with the accepted explanation for Dansgaard–Oeschger events, with YD likely to have been the last and the strongest of these events.<ref name="Nye2014" /> 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.<ref>Template:Cite journal</ref> While there is evidence of meltwater travelling via the Mackenzie River,<ref name="Süfke2022" /> this hypothesis may not be consistent with the lack of sea level rise during this period,<ref name="Abdul2016" /> so other theories have also emerged.<ref name="Broecker2010">Template:Cite journal</ref> Another proposed explanation is an extraterrestrial impact, but this is rejected by most experts. A volcanic eruption as an initial trigger for cooling and sea ice growth has been proposed more recently,<ref name="Baldini2018">Template:Cite journal</ref> and the presence of anomalously high levels of volcanism immediately preceding the onset of the Younger Dryas has been confirmed in both ice cores<ref name="Abbott2021">Template:Cite journal</ref> and cave deposits.<ref name="Sun2020">Template:Cite journal</ref>
Etymology
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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 was abundant in Europe are the Oldest Dryas (approx. 18,500-14,000 BP) and Older Dryas (~14,050–13,900 BP), respectively.<ref>Template:Cite journal</ref><ref name="Shakun2012">Template:Cite journal</ref> 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.<ref name="EGL2022">Template:Cite book</ref>
In Ireland, the Younger Dryas has also been known as the Nahanagan Stadial, and in Great Britain it has been called the Loch Lomond Stadial.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> 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).<ref>Template:Cite journal</ref>
Climate
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As with the other geologic periods, paleoclimate during the Younger Dryas is reconstructed through proxy data such as traces of pollen, ice cores and layers of marine and lake sediments.<ref name="Yu1998">Template:Cite journal</ref> Collectively, this evidence shows that significant cooling across the Northern Hemisphere began around 12,870 ± 30 years BP.<ref name="Cheng2020">Template:Cite journal</ref> It was particularly severe in Greenland, where temperatures declined by Template:Convert,<ref name="Buizert2014">Template:Cite journal</ref> in an abrupt fashion.<ref name="Alley2000" /> Temperatures at the Greenland summit were up to Template:Convert colder than at the start of the 21st century.<ref name="Alley2000">Template:Cite journal</ref><ref name="Severinghaus1998">Template:Cite journal</ref>
Strong cooling of around Template:Convert had also taken place in Europe.<ref name="Carlson2013" /> Icefields and glaciers formed in upland areas of Great Britain, while many lowland areas developed permafrost,<ref name="Sissons1979">Template:Cite journal</ref> implying a cooling of Template:Convert and a mean annual temperature no higher than Template:Convert.<ref name="Severinghaus1998" /><ref>Template:Cite journal</ref> North America also became colder, particularly in the eastern and central areas.<ref name="Yu1998" /> While the Pacific Northwest region cooled by Template:Convert, cooling in western North America was generally less intense.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref name="Elias2013">Template:Cite book</ref> While the Orca Basin in the Gulf of Mexico still experienced a drop in sea surface temperature of 2.4 ± 0.6°C,<ref>Template:Cite journal</ref> land areas closer to it, such as Texas, the Grand Canyon area<ref>Template:Cite journal</ref> and New Mexico, ultimately did not cool as much as the continental interior.<ref name="Meltzer2010">Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> The Southeastern United States became warmer and wetter than before.<ref name="Griggs2017" /><ref name="Meltzer2010" /><ref name="Elias2013" /> There was warming in and around the Caribbean Sea, and in West Africa.<ref name="Carlson2013" />
It was once believed that the Younger Dryas cooling started at around the same time across the Northern Hemisphere.<ref>Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref> Evidence from Lake Suigetsu cores in Japan and 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.<ref name="Nakagawa2003">Template:Cite journal</ref><ref name="Partin2015" /> 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 the summer.<ref name="Buizert2014" /><ref name="Schenk2018" /> An exception appears to have taken place in what is now Maine, where winter temperatures remained stable, yet summer temperatures decreased by up to Template:Convert.<ref name="Dieffenbacher-Krall2016">Template:Cite journal</ref>
While the Northern Hemisphere cooled, considerable warming occurred in the Southern Hemisphere.<ref name="Partin2015" /> Sea surface temperatures were warmer by Template:Convert, and Antarctica, South America (south of Venezuela) and New Zealand all experienced warming.<ref name="Carlson2013" /> The net temperature change was a relatively modest<ref>Template:Cite journal</ref> cooling of Template:Convert.<ref name="Carlson2013" /> Temperature changes of the Younger Dryas lasted 1,150–1,300 years.<ref name="Bjorck2007a" /><ref name="Bjorck1996" /> According to the International Commission on Stratigraphy, the Younger Dryas ended around 11,700 years ago,<ref>Template:Cite journal</ref> although some research places it closer to 11,550 years ago.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
The end of Younger Dryas was also abrupt: in previously cooled areas, warming to previous levels took place over 50–60 years.<ref name="Alley1993">Template:Cite journal</ref><ref name="Partin2015" /> The tropics experienced more gradual temperature recovery over several centuries;<ref name="Partin2015" /> the exception was in tropical Atlantic areas such as Costa Rica, where temperature change was similar to Greenland's.<ref>Template:Cite journal</ref> The Holocene warming then proceeded across the globe, following an increase in carbon dioxide levels during the YD period<ref name="Shakun2012" /> (from ~210 ppm to ~275 ppm<ref>Template:Cite journal</ref>).
Ice cover
[edit]Younger Dryas cooling was often accompanied by glacier advance and lowering of the regional snow line, with evidence found in areas such as Scandinavia,<ref name="Bjorck2007a">Björck, S. (2007) Younger Dryas oscillation, global evidence. In S. A. Elias, (Ed.): Encyclopedia of Quaternary Science, Volume 3, pp. 1987–1994. Elsevier B.V., Oxford.</ref> the Swiss Alps<ref name="Carlson2013" /> and the Dinaric Alps in the Balkans,<ref>Template:Cite journal</ref> northern ranges of North America's Rocky Mountains,<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> Two Creeks Buried Forest in Wisconsin and western parts of the New York State,<ref>Template:Cite journal</ref> and in the Pacific Northwest,<ref>Template:Cite journal</ref> including the Cascade Range.<ref>Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref> Southeastern Alaska appears to have escaped glaciation; speleothem calcite deposition continued in the region despite being retarded, indicating the absence of permafrost and glaciation.<ref>Template:Cite journal</ref>
On the other hand, the warming of the Southern Hemisphere led to ice loss in Antarctica, South America and New Zealand.<ref>Template:Cite web</ref><ref name="Carlson2013" /> Moreover, while Greenland as a whole had cooled, glaciers had only grown in the north of the island,<ref>Template:Cite journal</ref> and they had retreated from the rest of Greenland's coasts. This was likely driven by the strengthened Irminger Current.<ref name="Rainsley2018">Template:Cite journal</ref> The Jabllanica mountain range in 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.<ref>Template:Cite journal</ref> Unlike now, the glaciers were still present in northern Scotland, but they had thinned during the Younger Dryas.<ref>Template:Cite book</ref>
The amount of water contained within glaciers directly influences global sea levels - sea level rise occurs if the glaciers retreat, and it drops if glaciers grow. Altogether, there appears to have been little change in sea level throughout the Younger Dryas.<ref name="Shakun2012" /> This is in contrast to rapid increases before and after, such as the Meltwater Pulse 1A.<ref name="Shakun2012" /> On the coasts, glacier advance and retreat also affects relative sea level. Western Norway experienced a relative sea level rise of Template:Cvt as the Scandinavian ice sheet advanced.<ref>Template:Cite journal</ref><ref name="Lohne2007" /> Notably, ice sheet advance in this area appears to have begun about 600 years before the global onset of the Younger Dryas.<ref name="Lohne2007">Template:Cite journal</ref> Underwater, the deposits of methane clathrate - methane frozen into ice - remained stable throughout the Younger Dryas, including during the rapid warming as it ended.<ref>Template:Cite journal</ref>
Weather systems
[edit]As the Northern Hemisphere cooled and the Southern Hemisphere warmed, 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.<ref>Template:Cite journal</ref> Shifts in atmospheric patterns are believed to be the main reason why Northern Hemisphere summers generally did not cool during the Younger Dryas.<ref name="Schenk2018" />
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,<ref>Template:Cite journal</ref> the North American Midwest, <ref>Template:Cite journal</ref> Anatolia and southern China.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref name="Hong2014">Template:Cite journal</ref> As North Africa, including the Sahara Desert, became drier, the amount of dust blown by wind had also increased.<ref name="Carlson2013" /> Other areas became wetter including northern China<ref name="Hong2014"/> (possibly excepting the Shanxi region)<ref>Template:Cite journal</ref>
Biosphere
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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.<ref name="Mangerud2020" /><ref name="Bjorck1996">Template:Cite journal</ref><ref>Template:Cite book</ref><ref>Template:Cite journal</ref> The analysis of fossilized pollen had consistently shown how Dryas octopetala, a plant which only thrives in glacial conditions, began to dominate where forests were able to grow during the preceding B-A Interstadial.<ref name="Mangerud2020">Template:Cite journal</ref> This makes the Younger Dryas a key example of how biota responded to abrupt climate change.<ref>Template:Cite journal</ref>
For instance, in what is now New England,<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> cool summers, combined with cold winters and low precipitation, resulted in a treeless tundra up to the onset of the Holocene, when the boreal forests shifted north.<ref name="Dieffenbacher-Krall2016" /> 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.<ref>Template:Cite journal</ref> 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 the Holocene.<ref name="Liu2012">Template:Cite journal</ref> Conversely, pollen and macrofossil evidence from near Lake Ontario indicates that cool, boreal forests persisted into the early Holocene.<ref name="Griggs2017">Template:Cite journal</ref>
An increase of pine pollen indicates cooler winters within the central Cascades.<ref>Template:Cite journal</ref> Speleothems from the Oregon Caves National Monument and Preserve in southern Oregon's Klamath Mountains yield evidence of climatic cooling contemporaneous to the Younger Dryas.<ref name="Vacco2005">Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref> Speleothem records indicate an increase in precipitation in southern Oregon,<ref name="Vacco2005" /><ref>Template:Cite journal</ref> the timing of which coincides with increased sizes of pluvial lakes in the northern Great Basin.<ref>Template:Cite journal</ref> Pollen record from the Siskiyou Mountains suggests a lag in timing of the Younger Dryas, indicating a greater influence of warmer Pacific conditions on that range.<ref>Template:Cite journal</ref>
Effects in the Rocky Mountain region were varied.<ref>Template:Cite book</ref><ref>Template:Cite journal</ref> Several sites show little to no changes in vegetation.<ref name="Brunelle2003" /> In the northern Rockies, a significant increase in pines and firs suggests warmer conditions than before and a shift to subalpine parkland in places.<ref name="Mumma2012">Template:Cite journal</ref><ref name="Brunelle2003">Template:Cite journal</ref><ref>Template:Cite web</ref><ref>Template:Cite journal</ref> That is hypothesized to be the result of a northward shift in the jet stream, combined with an increase in summer insolation<ref name="Mumma2012" /><ref>Template:Cite journal</ref> as well as a winter snow pack that was higher than today, with prolonged and wetter spring seasons.<ref>Template:Cite journal</ref>
Human societies
[edit]Northwestern Europe faced a significant population reduction during the first half of the Younger Dryas.<ref>Template:Cite journal</ref>
The Younger Dryas is often linked to the Neolithic Revolution, with the adoption of agriculture in the Levant.<ref name="Bar-Yosef">Template:Cite book Template:Isbn.</ref><ref>Template:Cite book</ref> The cold and dry Younger Dryas arguably lowered the carrying capacity of the area and forced the sedentary early Natufian population into a more mobile subsistence pattern.<ref>Template:Cite book</ref> 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.<ref name="Munro">Template:Cite journal</ref><ref>Template:Cite journal</ref>
Cause
[edit]The scientific consensus links 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).<ref name="Carlson2013" /><ref name="IPCC AR6 WG1 Ch.8" />Template:Rp This is consistent with climate model simulations,<ref name="Partin2015" /> 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 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.<ref>Template:Cite journal</ref> 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 the coastal waters.<ref>Template:Cite journal</ref>
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.<ref name="Broecker2006">Template:Cite journal</ref> For instance, the salinity in the Saint Lawrence Seaway did not decline, as would have been expected from massive quantities of meltwater.<ref name="Eisenman2009" /> More recent research instead shows that floodwaters followed a pathway along the Mackenzie River in present-day Canada,<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> and sediment cores show that the strongest outburst had occurred right before the onset of Younger Dryas.<ref name="Süfke2022">Template:Cite journal</ref>
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 the most abrupt climatic changes during the YD.<ref>Template:Cite journal</ref> 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.<ref name="Wang2018" /> 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 the AMOC.<ref name="Eisenman2009">Template:Cite journal</ref> 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.<ref name="Wang2018">Template:Cite journal</ref> Other modelling shows that sea ice in the Arctic Ocean could have been tens of meters thick by 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.<ref>Template:Cite journal</ref> 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.<ref name="Abdul2016">Template:Cite journal</ref>
Some scientists also explain the lack of sea level rise during the Younger Dryas onset by connecting it with a volcanic eruption.<ref name="Baldini2018" /> 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.<ref>Template:Cite web</ref> Cooling from a high latitude volcanic eruption could have accelerated North Atlantic sea ice growth, finally tipping the AMOC sufficiently to cause the Younger Dryas.<ref name="Baldini2018" /> 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,<ref name="Sun2020" /><ref name="Abbott2021" /> perhaps even completely matching the stalagmite-derived date for the onset of the Younger Dryas event.<ref name="Cheng2020" /> 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.<ref name="Abbott2021" />
According to 1990s research, the Laacher See eruption (present-day volcanic lake in Rhineland-Palatinate, Germany) would have matched the criteria,<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> but radiocarbon dating done in 2021 pushes the date of the eruption back to 13,006 years BP, or over a century before the Younger Dryas began.<ref>Template:Cite journal</ref> This analysis was also challenged in 2023, with some researchers suggesting that the radiocarbon analysis was tainted by magmatic carbon dioxide.<ref name="Baldini2023">Template:Cite journal</ref> For now, the debate continues without a conclusive proof or rejection of the volcanic hypothesis.<ref name="Abbott2021" />
Younger Dryas impact hypothesis
[edit]The Younger Dryas impact hypothesis (YDIH) attributes the cooling to the impact of a disintegrating comet or asteroid.<ref name="Powell2022">Template:Cite journal</ref> 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,<ref name="Gramling2021" /> or that it was an airburst, which would only leave micro- and nanoparticles behind as evidence.<ref name="Powell2022" /> Most experts reject the hypothesis, and argue that all of the microparticles are adequately explained by the terrestrial processes.<ref name="Holliday2023">Template:Cite journal</ref> 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.<ref name="Sun2020" /> Opponents argue that there is no evidence for massive wildfires which would have been caused by an airburst of sufficient size to affect the thermohaline circulation,<ref name="Gramling2021">Template:Cite news</ref> mineralogical and geochemical evidence<ref>Template:Cite journal</ref> or for simultaneous human population declines and mass animal extinctions which would have been required by this hypothesis.<ref name="Holliday2023" />
Similar events
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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.<ref name="Nye2014">Template:Cite journal</ref> These episodes are characterized by abrupt changes in the AMOC on timescales of decades or centuries.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> 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.<ref name="Nye2014" /> On the other hand, some research links volcanism with D–O events, potentially supporting the volcanic hypothesis.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
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.<ref name="Eglinton1992">Eglinton, G., A.B. Stuart, A. Rosell, M. Sarnthein, U. Pflaumann, and R. Tiedeman (1992) Molecular record of secular sea surface temperature changes on 100-year timescales for glacial terminations I, II and IV. Nature. 356:423–426.</ref><ref name="Bardley2015">Template:Cite book</ref>Template:Page needed 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.<ref name="Eglinton1992" /><ref name="Bardley2015" /> This analysis provides evidence for YD-like events during Termination II (the end of the Marine Isotope Stage 6, ~130,000 years BP), III (the end of Marine Isotope Stage 8, ~243,000 years BP)<ref name="Chen2006">Template:Cite journal</ref> and Termination IV (the end of Marine Isotope Stage 10, ~337,000 years BP.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> When combined with additional evidence from ice cores and paleobotanical data, some have argued that YD-like events inevitably occur during every deglaciation.<ref name="Chen2006" /><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
In popular culture
[edit]The 2004 film, The Day After Tomorrow depicts catastrophic climatic effects following 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. <ref>Template:Cite web</ref>
See also
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- Preboreal oscillation – Cooling episode within the preboreal
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References
[edit]External links
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