Significant cooling in the Northern Hemisphere took place during the Younger Dryas, but there was also warming in the Southern Hemisphere.Precipitation had substantially decreased (brown) or increased (green) in many areas across the globe. Altogether, this indicates large changes inthermohaline circulation as the cause[1]
TheYounger Dryas (YD, Greenland Stadial GS-1)[2] was a period in Earth's geologic history that occurred circa 12,900 to 11,700 yearsBefore Present (BP).[3] It is primarily known for the sudden or "abrupt" cooling in the Northern Hemisphere, when the NorthAtlantic Ocean cooled and annual air temperatures decreased by ~3 °C (5 °F) overNorth America, 2–6 °C (4–11 °F) in Europe and up to 10 °C (18 °F) inGreenland, in a few decades.[4] Cooling in Greenland was particularly rapid, taking place over just 3 years or less.[1][5] At the same time, the Southern Hemisphere experienced warming.[4][6] This period ended as rapidly as it began, with dramatic warming over ~50 years, the transition from the glacialPleistocene epoch into the currentHolocene.[1]
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.[1] 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.[7][8] Substantial changes inprecipitation also took place, with cooler areas experiencing substantially lower rainfall, while warmer areas received more of it.[4] In the Northern Hemisphere, the length of thegrowing season declined.[8] Land ice cover experienced little net change,[9] butsea ice extent had increased, contributing toice–albedo feedback.[4] This increase in albedo was the main reason for net global cooling of 0.6 °C (1.1 °F).[4]
During the preceding period, theBølling–Allerød Interstadial, rapid warming in the Northern Hemisphere[10]: 677 was offset by the equivalent cooling in the Southern Hemisphere.[11][9] This "polar seesaw" pattern is consistent with changes inthermohaline circulation (particularly theAtlantic 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.[11] Thescientific consensus is that severe AMOC weakening explains the climatic effects of the Younger Dryas.[12]: 1148 It also explains why the Holocene warming had proceeded so rapidly once the AMOC change was no longer counteracting the increase incarbon dioxide levels.[9]
AMOC weakening causing polar seesaw effects is also consistent with the accepted explanation forDansgaard–Oeschger events, with YD likely to have been the last and the strongest of these events.[13] 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'sLake Agassiz into the Atlantic Ocean.[14] While there is evidence of meltwater travelling via theMackenzie River,[15] this hypothesis may not be consistent with the lack ofsea level rise during this period,[16] so other theories have also emerged.[17] Another proposed explanation is anextraterrestrial 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,[18] and the presence of anomalously high levels of volcanism immediately preceding the onset of the Younger Dryas has been confirmed in both ice cores[19] and cave deposits.[20]
The Younger Dryas is named after thealpine–tundra wildflowerDryas octopetala, because itsfossils are abundant in the European (particularlyScandinavian) sediments dating to this timeframe. The two earlier geologic time intervals where this flower was abundant in Europe are theOldest Dryas (approx. 18,500-14,000 BP) andOlder Dryas (~14,050–13,900 BP), respectively.[21][9] On the contrary,Dryas octopetala was rare during theBølling–Allerød Interstadial. Instead, European temperatures were warm enough to support trees in Scandinavia, as seen at the Bølling and Allerød sites inDenmark.[22]
InIreland, the Younger Dryas has also been known as theNahanagan Stadial, and in Great Britain it has been called theLoch Lomond Stadial.[23][24] In theGreenland Summitice 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).[25]
Greenland ice cores since theLast Glacial Maximum show very low temperatures for the most part of the Younger Dryas, which then rise rapidly during theHolocene transition[26]Temperature changes, determined asproxy temperatures, taken from the central region of Greenland's ice sheet during the Late Pleistocene and beginning of the Holocene.Temperatures in Antarctica, derived from EPICA Dome C Ice Core
As with the other geologic periods,paleoclimate during the Younger Dryas is reconstructed throughproxy data such as traces ofpollen,ice cores and layers of marine and lakesediments.[27] Collectively, this evidence shows that significant cooling across the Northern Hemisphere began around 12,870 ± 30 years BP.[28] It was particularly severe inGreenland, where temperatures declined by 4–10 °C (7.2–18.0 °F),[7] in an abrupt fashion.[29] Temperatures at the Greenland summit were up to 15 °C (27 °F) colder than at the start of the 21st century.[29][30]
It was once believed that the Younger Dryas cooling started at around the same time across the Northern Hemisphere.[45] 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.[46] Evidence fromLake Suigetsu cores inJapan and thePuerto Princesa cave complex in thePhilippines shows that the onset of the Younger Dryas in East Asia was delayed by several hundred years relative to the North Atlantic.[47][1] 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.[7][8] An exception appears to have taken place in what is nowMaine, where winter temperatures remained stable, yet summer temperatures decreased by up to 7.5 °C (13.5 °F).[48]
While the Northern Hemisphere cooled, considerable warming occurred in the Southern Hemisphere.[1] Sea surface temperatures were warmer by 0.3–1.9 °C (0.5–3.4 °F), andAntarctica,South America (south ofVenezuela) andNew Zealand all experienced warming.[4] The net temperature change was a relatively modest[49] cooling of 0.6 °C (1.1 °F).[4] Temperature changes of the Younger Dryas lasted 1,150–1,300 years.[50][51] According to theInternational Commission on Stratigraphy, the Younger Dryas ended around 11,700 years ago,[52] although some research places it closer to 11,550 years ago.[53][54][55][56][57]
The end of Younger Dryas was also abrupt: in previously cooled areas, warming to previous levels took place over 50–60 years.[58][1] The tropics experienced more gradual temperature recovery over several centuries;[1] the exception was in tropical Atlantic areas such asCosta Rica, where temperature change was similar to Greenland's.[59] TheHolocene warming then proceeded across the globe, following an increase incarbon dioxide levels during the YD period[9] (from ~210 ppm to ~275 ppm[60]).
On the other hand, the warming of the Southern Hemisphere led to ice loss in Antarctica, South America and New Zealand.[70][4] Moreover, while Greenland as a whole had cooled, glaciers had only grown in the north of the island,[71] and they had retreated from the rest of Greenland's coasts. This was likely driven by the strengthenedIrminger Current.[72] TheJabllanica 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.[73] Unlike now, the glaciers were still present in northernScotland, but they had thinned during the Younger Dryas.[74]
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.[9] This is in contrast to rapid increases before and after, such as theMeltwater Pulse 1A.[9] On the coasts, glacier advance and retreat also affectsrelative sea level. WesternNorway experienced a relative sea level rise of 10 m (32+2⁄3 ft) as theScandinavian ice sheet advanced.[75][76] Notably, ice sheet advance in this area appears to have begun about 600 years before the global onset of the Younger Dryas.[76] Underwater, the deposits ofmethane clathrate - methane frozen into ice - remained stable throughout the Younger Dryas, including during the rapid warming as it ended.[77]
As the Northern Hemisphere cooled and the Southern Hemisphere warmed, thethermal equator would have shifted to the south. Becausetrade winds from either hemisphere cancel each other out above the thermal equator in a calm, heavily clouded area known as theIntertropical Convergence Zone (ITCZ), a change in its position affects wind patterns elsewhere. For instance, inEast Africa, the sediments ofLake Tanganyika were mixed less strongly during this period, indicating weaker wind systems in this area.[78] Shifts in atmospheric patterns are believed to be the main reason why Northern Hemisphere summers generally did not cool during the Younger Dryas.[8]
Since winds carry moisture in the form of clouds, these changes also affectprecipitation. Thus, evidence from the pollen record shows that some areas have become very arid, including Scotland,[79] the North AmericanMidwest,[80]Anatolia and southernChina.[81][82][83] As North Africa, including theSahara Desert, became drier, the amount of dust blown by wind had also increased.[4] Other areas became wetter including northern China[83] (possibly excepting theShanxi region)[84]
The Younger Dryas was initially discovered around the start of the 20th century, through paleobotanical and lithostratigraphic studies ofSwedish andDanish bog and lake sites, particularly theAllerød clay pit in Denmark.[85][51][86][87] The analysis of fossilizedpollen had consistently shown howDryas octopetala, a plant which only thrives in glacial conditions, began to dominate where forests were able to grow during the preceding B-A Interstadial.[85] This makes the Younger Dryas a key example of howbiota responded to abruptclimate change.[88]
For instance, in what is nowNew England,[89][90][91] cool summers, combined with cold winters and low precipitation, resulted in a treeless tundra up to the onset of the Holocene, when theboreal forests shifted north.[48] 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.[92] The centralAppalachian Mountains remained forested during the Younger Dryas, but they were covered inspruce andtamarack boreal forests, switching totemperate broadleaf and mixed forests during the Holocene.[93] Conversely, pollen andmacrofossil evidence from nearLake Ontario indicates that cool, boreal forests persisted into the early Holocene.[44]
Depiction of different land cover during the Younger Dryas event
An increase of pine pollen indicates cooler winters within the central Cascades.[94]Speleothems from theOregon Caves National Monument and Preserve in southernOregon'sKlamath Mountains yield evidence of climatic cooling contemporaneous to the Younger Dryas.[95] 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.[96] Speleothem records indicate an increase in precipitation in southern Oregon,[95][97] the timing of which coincides with increased sizes ofpluvial lakes in the northern Great Basin.[98] Pollen record from theSiskiyou Mountains suggests a lag in timing of the Younger Dryas, indicating a greater influence of warmer Pacific conditions on that range.[99]
Effects in theRocky Mountain region were varied.[100][101] Several sites show little to no changes in vegetation.[102] In the northern Rockies, a significant increase in pines and firs suggests warmer conditions than before and a shift tosubalpine parkland in places.[103][102][104][105] That is hypothesized to be the result of a northward shift in the jet stream, combined with an increase in summerinsolation[103][106] as well as a winter snow pack that was higher than today, with prolonged and wetter spring seasons.[107]
Northwestern Europe faced a significantpopulation reduction during the first half of the Younger Dryas.[108]
The Younger Dryas is often linked to theNeolithic Revolution, with the adoption of agriculture in theLevant.[109][110] The cold and dry Younger Dryas arguably lowered thecarrying capacity of the area and forced the sedentary earlyNatufian population into a more mobile subsistence pattern.[111] Further climatic deterioration is thought to have brought aboutcereal 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.[112][113]
The scientific consensus links the Younger Dryas with a significant reduction or shutdown of thethermohaline circulation, which circulates warm tropical waters northward through theAtlantic meridional overturning circulation (AMOC).[4][12]: 1148 This is consistent withclimate model simulations,[1] 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.[114] 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 theNorth Atlantic Gyre, more of it would stay trapped in the coastal waters.[115]
It was originally hypothesized that the massive outburst from paleohistoricalLake Agassiz had flooded the North Atlantic via theSaint Lawrence Seaway, but little geological evidence for this hypothesis has been found.[116] For instance, thesalinity in the Saint Lawrence Seaway did not decline, as would have been expected from massive quantities of meltwater.[117] More recent research instead shows that floodwaters followed a pathway along theMackenzie River in present-day Canada,[118][119] and sediment cores show that the strongest outburst had occurred right before the onset of Younger Dryas.[15]
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-presentFennoscandian ice sheet, which could explain why Greenland experienced the most abrupt climatic changes during the YD.[120] 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.[121] Some modelling explains this by showing that the melting ofLaurentide Ice Sheet led to greater rainfall over the Atlantic Ocean, freshening it and so helping to weaken the AMOC.[117] Once the Younger Dryas began, lowered temperatures would have elevated snowfall across the Northern Hemisphere, increasing theice-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.[121] Other modelling shows thatsea 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.[122] 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.[16]
Some scientists also explain the lack of sea level rise during the Younger Dryas onset by connecting it with a volcanic eruption.[18] Eruptions often deposit large quantities ofsulfur dioxide particles in the atmosphere, where they are known asaerosols, and can have a large cooling effect by reflecting sunlight. This phenomenon can also be caused by anthropogenic sulfur pollution, where it is known asglobal dimming.[123] 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.[18] 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,[20][19] perhaps even completely matching the stalagmite-derived date for the onset of the Younger Dryas event.[28] It has been suggested that this eruption would have been stronger than any during theCommon Era, some of which have been able to cause several decades of cooling.[19]
According to 1990s research, theLaacher See eruption (present-day volcanic lake inRhineland-Palatinate,Germany) would have matched the criteria,[124][125] butradiocarbon 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.[126] This analysis was also challenged in 2023, with some researchers suggesting that the radiocarbon analysis was tainted by magmatic carbon dioxide.[127] For now, the debate continues without a conclusive proof or rejection of the volcanic hypothesis.[19]
TheYounger Dryas impact hypothesis (YDIH) attributes the cooling to the impact of a disintegrating comet or asteroid.[128] Because there is noimpact crater dating to the Younger Dryas period, the proponents usually suggest the impact had struck theLaurentide ice sheet, so that the crater would have disappeared when the ice sheet melted during the Holocene,[129] or that it was an airburst, which would only leave micro- and nanoparticles behind as evidence.[128] Most experts reject the hypothesis, and argue that all of the microparticles are adequately explained by the terrestrial processes.[130] For instance, mineral inclusions from YD-period sediments in Hall's Cave, Texas, have been interpreted by YDIH proponents as extraterrestrial in origin, while a paper published in 2020 argues that they are more likely to be volcanic.[20] Opponents argue that there is no evidence for the massive wildfires which they assert would have been caused by an airburst of sufficient size to affect the thermohaline circulation,[129] nor anymineralogical orgeochemical evidence[131] for the simultaneous human population declines and mass animal extinctions that would ensue from such an impact.[130]
Temperature proxy from four ice cores for the last 140,000 years. They show the distinct "sawtooth" pattern of the D-O events in the Northern Hemisphere, compared to the more muted changes in the Southern Hemisphere
Statistical analysis shows that the Younger Dryas is merely the last of 25 or 26Dansgaard–Oeschger events (D–O events) over the past 120,000 years.[13] These episodes are characterized by abrupt changes in the AMOC on timescales of decades or centuries.[132][133] 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.[13] On the other hand, some research links volcanism with D–O events, potentially supporting the volcanic hypothesis.[134][135]
Events similar to the Younger Dryas appear to have occurred during the otherterminations - a term used to describe a comparatively rapid transition from cold glacial conditions to warm interglacials.[136][137][page needed] The analysis of lake and marine sediments can reconstruct past temperatures from the presence or absence of certainlipids and long chainalkenones, as these molecules are very sensitive to temperature.[136][137] 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)[138] and Termination IV (the end of Marine Isotope Stage 10, ~337,000 years BP.[139][140] When combined with additional evidence from ice cores and paleobotanical data, some have argued that YD-like events inevitably occur during every deglaciation.[138][141][142]
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