Editing Younger Dryas (section)

== Cause ==
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" />{{Rp|page=1148}} 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>{{cite journal |last1=Keigwin |first1=L. D. |last2=Schlegel |first2=M. A. |date=22 June 2002 |title=Ocean ventilation and sedimentation since the glacial maximum at 3 km in the western North Atlantic |journal=[[Geochemistry, Geophysics, Geosystems]] |volume=3 |issue=6 |page=1034 |doi=10.1029/2001GC000283 |bibcode=2002GGG.....3.1034K |s2cid=129340391 |doi-access=free }}</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>{{Cite journal |last1=Grimm |first1=Eric C. |last2=Watts |first2=William A. |last3=Jacobson |first3=George L. Jr. |last4=Hansen |first4=Barbara C.S. |last5=Almquist |first5=Heather R. |last6=Dieffenbacher-Krall |first6=Ann C. |date=September 2006 |title=Evidence for warm wet Heinrich events in Florida|journal=[[Quaternary Science Reviews]] |volume=25 |issue=17–18 |pages=2197–2211 |doi=10.1016/j.quascirev.2006.04.008 |bibcode=2006QSRv...25.2197G}}</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">{{cite journal |last=Broecker |first=Wallace S. |year=2006 |title=Was the Younger Dryas triggered by a flood? |journal=Science |volume=312 |issue=5777 |pages=1146–1148 |doi=10.1126/science.1123253 |pmid=16728622 |s2cid=39544213}}</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>{{cite journal |last1=Murton |first1=Julian B. |last2=Bateman |first2=Mark D. |last3=Dallimore |first3=Scott R. |last4=Teller |first4=James T. |last5=Yang |first5=Zhirong |date=2010 |title=Identification of Younger Dryas outburst flood path from Lake Agassiz to the Arctic Ocean |journal=Nature |language=en |volume=464 |issue=7289 |pages=740–743 |bibcode=2010Natur.464..740M |doi=10.1038/nature08954 |issn=0028-0836 |pmid=20360738 |s2cid=4425933}}</ref><ref>{{Cite journal |last1=Keigwin |first1=L. D. |last2=Klotsko |first2=S. |last3=Zhao |first3=N. |last4=Reilly |first4=B. |last5=Giosan |first5=L. |last6=Driscoll |first6=N. W. |date=August 2018 |title=Deglacial floods in the Beaufort Sea preceded Younger Dryas cooling |journal=Nature Geoscience |language=en |volume=11 |issue=8 |pages=599–604 |doi=10.1038/s41561-018-0169-6 |bibcode=2018NatGe..11..599K |hdl=1912/10543 |s2cid=133852610 |issn=1752-0894}}</ref> and sediment cores show that the strongest outburst had occurred right before the onset of Younger Dryas.<ref name="Süfke2022">{{Cite journal |last1=Süfke |first1=Finn |last2=Gutjahr |first2=Marcus |last3=Keigwin |first3=Lloyd D. |last4=Reilly |first4=Brendan |last5=Giosan |first5=Liviu |last6=Lippold |first6=Jörg |date=25 April 2022 |title=Arctic drainage of Laurentide Ice Sheet meltwater throughout the past 14,700 years |journal=[[Communications Earth & Environment]] |language=en |volume=3 |issue=1 |page=98 |doi=10.1038/s43247-022-00428-3 |bibcode=2022ComEE...3...98S |issn=2662-4435 }}</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>{{Cite journal |last1=Muschitiello |first1=Francesco |last2=Pausata |first2=Francesco S. R. |last3=Watson |first3=Jenny E. |last4=Smittenberg |first4=Rienk H. |last5=Salih |first5=Abubakr A. M. |last6=Brooks |first6=Stephen J. |last7=Whitehouse |first7=Nicola J. |last8=Karlatou-Charalampopoulou |first8=Artemis |last9=Wohlfarth |first9=Barbara |date=17 November 2015 |title=Fennoscandian freshwater control on Greenland hydroclimate shifts at the onset of the Younger Dryas |journal=Nature Communications |language=en |volume=6 |issue=1 |page=8939 |doi=10.1038/ncomms9939 |issn=2041-1723 |pmc=4660357 |pmid=26573386|bibcode=2015NatCo...6.8939M }}</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">{{cite journal |last1=Eisenman |first1=I. |last2=Bitz |first2=C.M. |author2-link=Cecilia Bitz |last3=Tziperman |first3=E. |year=2009 |title=Rain driven by receding ice sheets as a cause of past climate change |journal=Paleoceanography |volume=24 |issue=4 |page=PA4209 |bibcode=2009PalOc..24.4209E |doi=10.1029/2009PA001778 |s2cid=6896108 |doi-access=free}}</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">{{Cite journal |last1=Wang |first1=L. |last2=Jiang |first2=W. Y. |last3=Jiang |first3=D. B. |last4=Zou |first4=Y. F. |last5=Liu |first5=Y. Y. |last6=Zhang |first6=E. L. |last7=Hao |first7=Q. Z. |last8=Zhang |first8=D. G. |last9=Zhang |first9=D. T. |last10=Peng |first10=Z. Y. |last11=Xu |first11=B. |last12=Yang |first12=X. D. |last13=Lu |first13=H. Y. |date=27 December 2018 |title=Prolonged Heavy Snowfall During the Younger Dryas |journal=Journal of Geophysical Research: Atmospheres |language=en |volume=123 |issue=24 |doi=10.1029/2018JD029271 |bibcode=2018JGRD..12313748W |issn=2169-897X}}</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>{{Cite journal |last1=Condron |first1=Alan |last2=Joyce |first2=Anthony J. |last3=Bradley |first3=Raymond S. |date=31 January 2020 |title=Arctic sea ice export as a driver of deglacial climate |journal=Geology |language=en |volume=48 |issue=4 |pages=395–399 |doi=10.1130/G47016.1 |bibcode=2020Geo....48..395C |issn=0091-7613}}</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">{{Cite journal |last1=Abdul |first1=N. A. |last2=Mortlock |first2=R. A. |last3=Wright |first3=J. D. |last4=Fairbanks |first4=R. G. |date=February 2016 |title=Younger Dryas sea level and meltwater pulse 1B recorded in Barbados reef crest coral Acropora palmata |url= |journal=Paleoceanography |language=en |volume=31 |issue=2 |pages=330–344 |doi=10.1002/2015PA002847 |bibcode=2016PalOc..31..330A |issn=0883-8305}}</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 [[aerosol]]s, 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>{{cite web |date=18 February 2021 |title=Aerosol pollution has caused decades of global dimming |url=https://news.agu.org/press-release/aerosol-pollution-caused-decades-of-global-dimming/ |website=[[American Geophysical Union]] |access-date=18 December 2023 |archive-url=https://web.archive.org/web/20230327143716/https://news.agu.org/press-release/aerosol-pollution-caused-decades-of-global-dimming/ |archive-date=27 March 2023 }}</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>{{Cite journal |last1=Brauer |first1=Achim |last2=Endres |first2=Christoph |last3=Günter |first3=Christina |last4=Litt |first4=Thomas |last5=Stebich |first5=Martina |last6=Negendank |first6=Jörg F.W. |date=March 1999 |title=High resolution sediment and vegetation responses to Younger Dryas climate change in varved lake sediments from Meerfelder Maar, Germany |journal=Quaternary Science Reviews |language=en |volume=18 |issue=3 |pages=321–329 |doi=10.1016/S0277-3791(98)00084-5|bibcode=1999QSRv...18..321B }}</ref><ref>{{Cite journal |last=van den Bogaard |first=Paul |date=June 1995 |title=40Ar/39Ar ages of sanidine phenocrysts from Laacher See Tephra (12,900 yr BP): Chronostratigraphic and petrological significance |journal=Earth and Planetary Science Letters |language=en |volume=133 |issue=1–2 |pages=163–174 |doi=10.1016/0012-821X(95)00066-L}}</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>{{Cite journal |last1=Reinig |first1=Frederick |last2=Wacker |first2=Lukas |last3=Jöris |first3=Olaf |last4=Oppenheimer |first4=Clive |last5=Guidobaldi |first5=Giulia |last6=Nievergelt |first6=Daniel |last7=Adolphi |first7=Florian |last8=Cherubini |first8=Paolo |last9=Engels |first9=Stefan |last10=Esper |first10=Jan |last11=Land |first11=Alexander |last12=Lane |first12=Christine |author-link12=Christine Lane |last13=Pfanz |first13=Hardy |last14=Remmele |first14=Sabine |last15=Sigl |first15=Michael |date=2021-07-01 |title=Precise date for the Laacher See eruption synchronizes the Younger Dryas |journal=Nature |language=en |volume=595 |issue=7865 |pages=66–69 |bibcode=2021Natur.595...66R |doi=10.1038/s41586-021-03608-x |issn=0028-0836 |pmid=34194020 |s2cid=235696831}}</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">{{Cite journal |last1=Baldini |first1=James U. L. |last2=Brown |first2=Richard J. |last3=Wadsworth |first3=Fabian B. |last4=Paine |first4=Alice R. |last5=Campbell |first5=Jack W. |last6=Green |first6=Charlotte E. |last7=Mawdsley |first7=Natasha |last8=Baldini |first8=Lisa M. |date=2023-07-05 |title=Possible magmatic CO2 influence on the Laacher See eruption date |journal=Nature |volume=619 |issue=7968 |pages=E1–E2 |doi=10.1038/s41586-023-05965-1 |pmid=37407686 |s2cid=259336241 |issn=0028-0836|url=https://durham-repository.worktribe.com/output/1709112 }}</ref> For now, the debate continues without a conclusive proof or rejection of the volcanic hypothesis.<ref name="Abbott2021" />

=== Younger Dryas impact hypothesis ===
The [[Younger Dryas impact hypothesis]] (YDIH) attributes the cooling to the impact of a disintegrating comet or asteroid.<ref name="Powell2022">{{Cite journal |last=Powell |first=James Lawrence |date=January 2022 |title=Premature rejection in science: The case of the Younger Dryas Impact Hypothesis |journal=Science Progress |language=en |volume=105 |issue=1 |pages=003685042110642 |doi=10.1177/00368504211064272 |issn=0036-8504 |pmc=10450282 |pmid=34986034}}</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">{{Cite journal |last1=Holliday |first1=Vance T. |last2=Daulton |first2=Tyrone L. |last3=Bartlein |first3=Patrick J. |last4=Boslough |first4=Mark B. |last5=Breslawski |first5=Ryan P. |last6=Fisher |first6=Abigail E. |last7=Jorgeson |first7=Ian A. |last8=Scott |first8=Andrew C. |last9=Koeberl |first9=Christian |last10=Marlon |first10=Jennifer R. |last11=Severinghaus |first11=Jeffrey |last12=Petaev |first12=Michail I. |last13=Claeys |first13=Philippe |date=December 2023 |title=Comprehensive refutation of the Younger Dryas Impact Hypothesis (YDIH) |journal=Earth-Science Reviews |language=en |volume=247 |pages=104502 |doi=10.1016/j.earscirev.2023.104502|bibcode=2023ESRv..24704502H }}</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">{{Cite news |last=Gramling |first=Carolyn |name-list-style=vanc |date=2018-06-26 |title=Why won't this debate about an ancient cold snap die? |url=https://www.sciencenews.org/article/younger-dryas-comet-impact-cold-snap |url-status=live |archive-url=https://web.archive.org/web/20210805112551/https://www.sciencenews.org/article/younger-dryas-comet-impact-cold-snap |archive-date=2021-08-05 |access-date=2023-02-23 |work=[[Science News]] |language=en-US}}</ref> [[Mineralogy|mineralogical]] and [[Geochemistry|geochemical]] evidence<ref>{{Cite journal |last=Jaret |first=Steven J. |last2=Scott Harris |first2=R. |date=2022-03-25 |title=No mineralogic or geochemical evidence of impact at Tall el-Hammam, a Middle Bronze Age city in the Jordan Valley near the Dead Sea |url=https://www.nature.com/articles/s41598-022-08216-x |journal=Scientific Reports |language=en |volume=12 |issue=1 |pages=5189 |doi=10.1038/s41598-022-08216-x |issn=2045-2322}}</ref> or for simultaneous human population declines and mass animal extinctions which would have been required by this hypothesis.<ref name="Holliday2023" />

=== Similar events ===
[[File:Ice-core-isotope.png|thumb|upright=1.4|right|Temperature proxy from four ice cores for the last 140,000&nbsp;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 26 [[Dansgaard–Oeschger event]]s (D–O events) over the past 120,000 years.<ref name="Nye2014">{{Cite journal |last1=Nye |first1=Henry |last2=Condron |first2=Alan |date=30 June 2021 |title=Assessing the statistical uniqueness of the Younger Dryas: a robust multivariate analysis |journal=Climate of the Past |language=en |volume=17 |issue=3 |pages=1409–1421 |doi=10.5194/cp-17-1409-2021 |issn=1814-9332 |doi-access=free |bibcode=2021CliPa..17.1409N }}</ref> These episodes are characterized by abrupt changes in the AMOC on timescales of decades or centuries.<ref>{{cite journal |last1=Dansgaard |first1=W |last2=Clausen |first2=H. B. |last3=Gundestrup |first3=N. |last4=Hammer |first4=C. U. |last5=Johnsen |first5=S. F. |last6=Kristinsdottir |first6=P. M. |last7=Reeh |first7=N. |title=A new Greenland deep ice core |journal=Science |date=1982 |volume=218 |issue=4579 |pages=1273–1277 |doi=10.1126/science.218.4579.1273 |pmid=17770148 |bibcode=1982Sci...218.1273D |s2cid=35224174 }}</ref><ref>{{cite journal |last1=Lynch-Stieglitz |first1=J |date=2017 |title=The Atlantic meridional overturning circulation and abrupt climate change. |journal=Annual Review of Marine Science |volume=9 |pages=83–104 |bibcode=2017ARMS....9...83L |doi=10.1146/annurev-marine-010816-060415 |pmid=27814029}}</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>{{Cite journal |last1=Baldini |first1=James U.L. |last2=Brown |first2=Richard J. |last3=McElwaine |first3=Jim N. |date=30 November 2015 |title=Was millennial scale climate change during the Last Glacial triggered by explosive volcanism? |journal=Scientific Reports |volume=5 |issue=1 |page=17442 |doi=10.1038/srep17442 |pmid=26616338 |pmc=4663491 |bibcode=2015NatSR...517442B |issn=2045-2322}}</ref><ref>{{Cite journal |last1=Lohmann |first1=Johannes |last2=Svensson |first2=Anders |date=2022-09-02 |title=Ice core evidence for major volcanic eruptions at the onset of Dansgaard–Oeschger warming events |journal=Climate of the Past |language=en |volume=18 |issue=9 |pages=2021–2043 |doi=10.5194/cp-18-2021-2022 |issn=1814-9332 |doi-access=free |bibcode=2022CliPa..18.2021L }}</ref> 

Events similar to the Younger Dryas appear to have occurred during the other [[termination (geomorphology)|terminations]] - a term used to describe a comparatively rapid transition from cold glacial conditions to warm interglacials.<ref name="Eglinton1992">[[Geoffrey Eglinton|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&nbsp;I, II and IV.'' Nature. 356:423–426.</ref><ref name="Bardley2015">{{cite book |author=Bradley, R. |year=2015 |title=Paleoclimatology: Reconstructing climates of the Quaternary |edition=3rd |publisher=Academic Press |place=Kidlington, Oxford, UK |isbn=978-0-12-386913-5}}</ref>{{Page needed|date=July 2021}} The analysis of lake and marine sediments can reconstruct past temperatures from the presence or absence of certain [[lipid]]s and long chain [[alkenone]]s, as these molecules are very sensitive to temperature.<ref name="Eglinton1992" /><ref name="Bardley2015" /> This analysis provides evidence for YD-like events during Termination&nbsp;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">{{cite journal |author1=Chen, S. |author2=Wang, Y. |author3=Kong, X. |author4=Liu, D. |author5=Cheng, H. |author6=Edwards, R.L. |year=2006 |title=A possible Younger Dryas-type event during Asian monsoonal Termination&nbsp;3 |journal=Science China Earth Sciences |volume=49 |issue=9 |pages=982–990|doi=10.1007/s11430-006-0982-4 |bibcode=2006ScChD..49..982C |s2cid=129007340 }}</ref> and Termination&nbsp;IV (the end of Marine Isotope Stage 10, ~337,000 years BP.<ref>{{cite journal |author1=Schulz, K.G. |author2=Zeebe, R.E. |year=2006 |title=Pleistocene glacial terminations triggered by synchronous changes in Southern and Northern Hemisphere insolation: The insolation canon hypothesis |journal=[[Earth and Planetary Science Letters]] |volume=249 |issue=3–4 |pages=326–336 |doi=10.1016/j.epsl.2006.07.004 |bibcode=2006E&PSL.249..326S |url=http://www.soest.hawaii.edu/oceanography/faculty/zeebe_files/Publications/SchulzZeebeEPSL06.pdf |via=[[University of Hawaii|U. Hawaii]] }}</ref><ref>{{cite journal|doi=10.1029/2004PA001071|url=http://www.lorraine-lisiecki.com/LR04_MISboundaries.txt|title=A Pliocene-Pleistocene stack of 57 globally distributed benthic δ18O records|journal=Paleoceanography|volume=20|author1= Lisiecki, Lorraine E.| author2= Raymo, Maureen E.|authorlink = Lorraine Lisiecki|authorlink2 = Maureen Raymo|year=2005|issue=1|pages=n/a|bibcode=2005PalOc..20.1003L|hdl=2027.42/149224|s2cid=12788441 |hdl-access=free}}</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>{{cite journal |author1=Sima, A. |author2=Paul, A. |author3=Schulz, M. |year=2004 |title=The Younger Dryas — an intrinsic feature of late Pleistocene climate change at millennial timescales |journal=[[Earth and Planetary Science Letters]] |volume=222 |issue=3–4 |pages=741–750|doi=10.1016/j.epsl.2004.03.026 |bibcode=2004E&PSL.222..741S }}</ref><ref>{{cite journal |last1 = Xiaodong |first1 = D. |last2 = Liwei |first2 = Z. |last3 = Shuji |first3 = K. |year = 2014 |title = A review on the Younger Dryas event |journal = Advances in Earth Science |volume = 29 |issue = 10 |pages = 1095–1109 }}</ref>