{{short description|Severe climatic event starting around 2200 BC}} [[File:Global distribution of the Bond Holocene IRD Event 3 also named 4.2 kiloyear event. The hatched areas were affected by wet conditions or flooding and the dotted areas by drought or dust storms.jpg|thumb|upright=2.2|Global distribution of the 4.2 kiloyear event. The hatched areas were affected by wet conditions or flooding, and the dotted areas by drought or dust storms.<ref>[https://ars.els-cdn.com/content/image/1-s2.0-S027737911731003X-gr1.jpg Another map for reference] in {{cite journal |last1=Railsback |first1=L. Bruce |last2=Liang |first2=Fuyuan |last3=Brook |first3=G. A. |last4=Voarintsoa |first4=Ny Riavo G. |last5=Sletten |first5=Hillary R. |last6=Marais |first6=Eugene |last7=Hardt |first7=Ben |last8=Cheng |first8=Hai |last9=Edwards |first9=R. Lawrence |title=The timing, two-pulsed nature, and variable climatic expression of the 4.2 ka event: A review and new high-resolution stalagmite data from Namibia |journal=[[Quaternary Science Reviews]] |date=15 April 2018 |volume=186 |pages=78–90 |doi=10.1016/j.quascirev.2018.02.015 |issn=0277-3791|bibcode=2018QSRv..186...78R |doi-access=free }} The initial source where this map comes from had the map caption the wrong way around: {{cite journal |last1=Wang |first1=Xinming |last2=Wang |first2=Yuhong |last3=Chen |first3=Liqi |last4=Sun |first4=Liguang |last5=Wang |first5=Jianjun |title=The abrupt climate change near 4,400 yr BP on the cultural transition in Yuchisi, China and its global linkage |journal=[[Scientific Reports]] |date=10 June 2016 |volume=6 |issue=1 |article-number=27723 |doi=10.1038/srep27723 |pmid=27283832 |pmc=4901284 |language=en |issn=2045-2322|bibcode=2016NatSR...627723W}}</ref>]]

The '''4.2-kiloyear event''' (also known as the '''4.2 ka event''') was an [[aridification]] event (long-term drought), 4,200 years ago (4,200 years [[Before_Present|BP]]). It was one of the most severe climatic events of the [[Holocene]] epoch.<ref name="deMenocal2001">{{cite journal |last=deMenocal |first=Peter B. |year=2001 |title=Cultural Responses to Climate Change During the Late Holocene |journal=[[Science (journal)|Science]] |volume=292 |issue=5517 |pages=667–673 |doi=10.1126/science.1059827 |pmid=11303088 |bibcode=2001Sci...292..667D|s2cid=18642937 }}</ref> It defines the beginning of the current [[Meghalayan]] age in the Holocene epoch.

Starting around 2200&nbsp;BC, it most likely lasted the entire [[22nd century BC]]. It has been hypothesised to have caused the collapse of the [[Old Kingdom of Egypt|Old Kingdom]] in [[Ancient Egypt|Egypt]], the [[Akkadian Empire]] in [[Mesopotamia]], and the [[Liangzhu culture]] in the lower [[Yangtze|Yangtze River]] area.<ref>{{cite journal |author=Gibbons, Ann |year=1993 |title=How the Akkadian Empire Was Hung Out to Dry |journal=Science |volume=261 |issue=5124 |page=985 |doi=10.1126/science.261.5124.985 |pmid=17739611 |bibcode=1993Sci...261..985G}}</ref><ref>{{cite journal |last1=Li |first1=Chun-Hai |last2=Li |first2=Yong-Xiang |last3=Zheng |first3=Yun-Fei |last4=Yu |first4=Shi-Yong |last5=Tang |first5=Ling-Yu |last6=Li |first6=Bei-Bei |last7=Cui |first7=Qiao-Yu |date=August 2018 |title=A high-resolution pollen record from East China reveals large climate variability near the Northgrippian-Meghalayan boundary (around 4200 years ago) exerted societal influence |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=512 |pages=156–165 |doi=10.1016/j.palaeo.2018.07.031 |issn=0031-0182 |bibcode=2018PPP...512..156L|s2cid=133896325 }}</ref> The drought may also have initiated the collapse of the [[Indus Valley Civilisation]], with some of its population moving southeast to follow the movement of their desired habitat,<ref name="Staubwasser2003" /> as well as the [[Indo-Aryan migrations|migration of Indo-European-speaking people into India]].<ref name="Kochhar2017"/> Some scientists disagree with that conclusion, citing evidence that the event was not a global drought and did not happen in a clear timeline.<ref name="auto">{{cite journal |author=Voosen |first=Paul |date=August 8, 2018 |title=Massive drought or myth? Scientists spar over an ancient climate event behind our new geological age |url=https://www.science.org/content/article/massive-drought-or-myth-scientists-spar-over-ancient-climate-event-behind-our-new |journal=[[Science (journal)|Science]] |access-date=9 January 2020}}</ref>

==Causes== [[Climate model|Modelling]] evidence suggests that the 4.2 ka event was the result of a significant weakening of the [[Atlantic meridional overturning circulation]] (AMOC), disrupting global ocean currents and generating precipitation and temperature changes in various regions.<ref>{{cite journal |last1=Yan |first1=Mi |last2=Liu |first2=Jian |date=21 February 2019 |title=Physical processes of cooling and mega-drought during the 4.2 ka BP event: results from TraCE-21ka simulations |url=https://cp.copernicus.org/articles/15/265/2019/ |journal=[[Climate of the Past]] |volume=15 |issue=1 |pages=265–277 |doi=10.5194/cp-15-265-2019 |bibcode=2019CliPa..15..265Y |access-date=29 August 2023 |doi-access=free }}</ref><ref>{{cite journal |last1=Ning |first1=Liang |last2=Liu |first2=Jian |last3=Bradley |first3=Raymond S. |last4=Yan |first4=Mi |date=10 January 2019 |title=Comparing the spatial patterns of climate change in the 9th and 5th millennia BP from TRACE-21 model simulations |url=https://cp.copernicus.org/articles/15/41/2019/ |journal=[[Climate of the Past]] |volume=15 |issue=1 |pages=41–52 |doi=10.5194/cp-15-41-2019 |bibcode=2019CliPa..15...41N |access-date=29 August 2023 |doi-access=free }}</ref> The [[Intertropical Convergence Zone]] (ITCZ) abruptly shifted southward.<ref>{{cite journal |last1=Jalali |first1=Bassem |last2=Sicre |first2=Marie-Alexandrine |last3=Azuara |first3=Julien |last4=Pellichero |first4=Violaine |last5=Combourieu-Nebout |first5=Nathalie |date=8 April 2019 |title=Influence of the North Atlantic subpolar gyre circulation on the 4.2 ka BP event |url=https://cp.copernicus.org/articles/15/701/2019/ |journal=[[Climate of the Past]] |volume=15 |issue=2 |pages=701–711 |doi=10.5194/cp-15-701-2019 |bibcode=2019CliPa..15..701J |access-date=29 August 2023 |doi-access=free }}</ref><ref>{{cite journal |last1=Bini |first1=Monica |last2=Zanchetta |first2=Giovanni |last3=Perşoiu |first3=Aurel |last4=Carter |first4=Rosine |last5=Català |first5=Albert |last6=Cacho |first6=Isabel |last7=Dean |first7=Jonathan R. |last8=Di Bine |first8=Federico |last9=Drysdale |first9=Russell N. |last10=Finnè |first10=Martin |last11=Isola |first11=Ilaria |last12=Jalali |first12=Bassem |last13=Lirer |first13=Fabrizio |last14=Magri |first14=Donatella |last15=Massi |first15=Alessia |last16=Marks |first16=Leszek |last17=Mercuri |first17=Anna Maria |last18=Peyron |first18=Odile |last19=Satori |first19=Laura |last20=Sicre |first20=Marie-Alexandrine |last21=Welc |first21=Fabian |last22=Zielhofer |first22=Christoph |last23=Brisset |first23=Elodie |date=27 March 2019 |title=The 4.2 ka BP Event in the Mediterranean region: an overview |url=https://cp.copernicus.org/articles/15/555/2019/ |journal=[[Climate of the Past]] |volume=15 |issue=2 |pages=555–577 |doi=10.5194/cp-15-555-2019 |bibcode=2019CliPa..15..555B |access-date=29 August 2023 |doi-access=free }}</ref> Evidence suggests increased [[El Niño–Southern Oscillation]] (ENSO) variability also played a role in generating the climatic conditions associated with the event.<ref>{{cite journal |last1=Toth |first1=Lauren T. |last2=Aronson |first2=Richard B. |date=16 January 2019 |title=The 4.2 ka event, ENSO, and coral reef development |url=https://cp.copernicus.org/articles/15/105/2019/ |journal=[[Climate of the Past]] |volume=15 |issue=1 |pages=105–119 |doi=10.5194/cp-15-105-2019 |bibcode=2019CliPa..15..105T |access-date=29 August 2023 |doi-access=free }}</ref> Explosive volcanism in [[Iceland]] has also been proposed as a cause,<ref>{{cite journal |last1=Bradley |first1=Raymond S. |last2=Bakke |first2=Jostein |date=2 September 2019 |title=Is there evidence for a 4.2 ka BP event in the northern North Atlantic region? |url=https://cp.copernicus.org/articles/15/1665/2019/ |journal=[[Climate of the Past]] |volume=15 |issue=5 |pages=1665–1676 |doi=10.5194/cp-15-1665-2019 |bibcode=2019CliPa..15.1665B |access-date=29 August 2023 |doi-access=free }}</ref> though the low sulphur content of Icelandic volcanoes has led other studies to suggest it had a negligible impact on global climate.<ref name=":1">{{cite journal |last1=Geirsdóttir |first1=Áslaug |last2=Miller |first2=Gifford H. |last3=Andrews |first3=John T. |last4=Harning |first4=David J. |last5=Anderson |first5=Leif S. |last6=Florian |first6=Christopher |last7=Larsen |first7=Darren J. |last8=Thordarson |first8=Thor |date=8 January 2019 |title=The onset of neoglaciation in Iceland and the 4.2 ka event |url=https://cp.copernicus.org/articles/15/25/2019/ |journal=[[Climate of the Past]] |volume=15 |issue=1 |pages=25–40 |doi=10.5194/cp-15-25-2019 |bibcode=2019CliPa..15...25G |access-date=29 August 2023 |doi-access=free }}</ref>

==Evidence== A phase of intense aridity around 2200 BC is recorded in physical evidence across [[North Africa]],<ref>{{cite journal |last1=Gasse |first1=Françoise |author-link=Françoise Gasse |last2=Van Campo |first2=Elise |year=1994 |title=Abrupt post-glacial climate events in West Asia and North Africa monsoon domains |journal=[[Earth and Planetary Science Letters]] |volume=126 |issue=4 |pages=435–456 |doi=10.1016/0012-821X(94)90123-6 |bibcode=1994E&PSL.126..435G}}</ref> the [[Middle East]],<ref>{{cite journal |last=Bar-Matthews |first=Miryam |author2=Ayalon, Avner |author3=Kaufman, Aaron |year=1997 |title=Late Quaternary Paleoclimate in the Eastern Mediterranean Region from Stable Isotope Analysis of Speleothems at Soreq Cave, Israel |journal=[[Quaternary Research]] |volume=47 |issue=2 |pages=155–168 |doi=10.1006/qres.1997.1883 |bibcode=1997QuRes..47..155B|s2cid=128577967 }}</ref> the [[Red Sea]],<ref>{{cite journal |last=Arz |first=Helge W. |year=2006 |title=A pronounced dry event recorded around 4.2 ka in brine sediments from the northern Red Sea |journal=Quaternary Research |volume=66 |issue=3 |pages=432–441 |doi=10.1016/j.yqres.2006.05.006 |bibcode=2006QuRes..66..432A |s2cid=55910028 |display-authors=etal}}</ref> the [[Arabian Peninsula]],<ref name="Parker2006">{{ cite journal |last=Parker |first=Adrian G. |year=2006 |title=A record of Holocene climate change from lake geochemical analyses in southeastern Arabia |journal=[[Quaternary Research]] |volume=66 |issue=3 |pages=465–476 |doi=10.1016/j.yqres.2006.07.001 |url=http://www.gulfnexus.org/articles/geo/2006a%20Parker%20et%20al.pdf |bibcode=2006QuRes..66..465P |s2cid=140158532 |archive-url=https://web.archive.org/web/20081029174631/http://www.gulfnexus.org/articles/geo/2006a%20Parker%20et%20al.pdf |archive-date=October 29, 2008 |display-authors=etal}}</ref> the [[Indian subcontinent]],<ref name="Staubwasser2003" /> and midcontinental [[North America]].<ref>{{cite journal |author=Booth, Robert K. |s2cid=39419698 |year=2005 |title=A severe centennial-scale drought in midcontinental North America 4200 years ago and apparent global linkages |journal=[[The Holocene]] |volume=15 |issue=3 |pages=321–328 |doi=10.1191/0959683605hl825ft |display-authors=etal |bibcode=2005Holoc..15..321B }}</ref> [[Glacier]]s throughout the mountain ranges of western [[Canada]] advanced around that time.<ref>{{cite journal |last=Menounos |first=B. |year=2008 |title=Western Canadian glaciers advance in concert with climate change c. 4.2 ka |journal=[[Geophysical Research Letters]] |volume=35 |issue= 7|pages=L07501 |doi=10.1029/2008GL033172 |bibcode=2008GeoRL..35.7501M |s2cid=13069875 |display-authors=etal}}</ref> Iceland also experienced glacial advance.<ref name=":1" /> Evidence has also been found in an Italian cave [[flowstone]],<ref name="Drysdale2005">{{cite journal |author=Drysdale, Russell |year=2005 |title=Late Holocene drought responsible for the collapse of Old World civilizations is recorded in an Italian cave flowstone |journal=[[Geology (journal)|Geology]] |volume=34 |issue=2 |pages=101–104 |doi=10.1130/G22103.1 |bibcode=2006Geo....34..101D |display-authors=etal}}</ref> the [[Mount Kilimanjaro|Kilimanjaro]] ice sheet,<ref name="Davis2002">{{cite journal |author=Thompson |first=L. G. |display-authors=etal |year=2002 |title=Kilimanjaro Ice Core Records Evidence of Holocene Climate Change in Tropical Africa |journal=[[Science (journal)|Science]] |volume=298 |issue=5593 |pages=589–93 |bibcode=2002Sci...298..589T |doi=10.1126/science.1073198 |pmid=12386332 |s2cid=32880316}}</ref> and in [[Andes|Andean]] glacier ice.<ref name="Davis2006">{{cite journal |author1=Davis, Mary E. |author2=Thompson, Lonnie G. |year=2006 |title=An Andean ice-core record of a Middle Holocene mega-drought in North Africa and Asia |journal=[[Annals of Glaciology]] |volume=43 |issue=1 |pages=34–41 |doi= 10.3189/172756406781812456 |bibcode=2006AnGla..43...34D |doi-access=free }}</ref> The onset of the [[aridification]] in [[Mesopotamia]] around 2100 BC also coincided with a cooling event in the [[Atlantic Ocean|North Atlantic]], known as [[Bond event]] 3.<ref name="deMenocal2001" /><ref name="Bond1997">{{cite journal |author=Bond, G. |url=http://rivernet.ncsu.edu/courselocker/PaleoClimate/Bond%20et%20al.,%201997%20Millenial%20Scale%20Holocene%20Change.pdf |year=1997 |title=A Pervasive Millennial-Scale Cycle in North Atlantic Holocene and Glacial Climates |journal=[[Science (journal)|Science]] |volume=278 |issue=5341 |pages=1257–1266 |doi=10.1126/science.278.5341.1257 |bibcode=1997Sci...278.1257B |s2cid=28963043 |display-authors=etal |archive-url=https://web.archive.org/web/20080227192411/http://rivernet.ncsu.edu/courselocker/PaleoClimate/Bond%20et%20al.%2C%201997%20Millenial%20Scale%20Holocene%20Change.pdf |archive-date=2008-02-27}}</ref><ref>{{cite web |url=http://www.ldeo.columbia.edu/res/pi/arch/examples.shtml |title=Two examples of abrupt climate change |publisher=Lamont–Doherty Earth Observatory |archive-url=https://web.archive.org/web/20070823214001/http://www.ldeo.columbia.edu/res/pi/arch/examples.shtml |archive-date=2007-08-23}}</ref>

In 2018, the [[International Commission on Stratigraphy]] divided the [[Holocene]] epoch into three periods,<ref>{{Cite news |date=19 July 2018 |title=Meghalaya Age: Newest phase in Earth's history named after Meghalaya rock &#124; – Times of India |website=The Times of India |url=https://timesofindia.indiatimes.com/home/science/newest-phase-in-earths-history-named-after-meghalaya-rock/articleshow/65046837.cms}}</ref> with the late Holocene from around 2250 BC onwards designated as the [[Meghalayan]] stage/age.<ref>{{cite news |url=https://www.bbc.com/news/science-environment-44868527 |title=Welcome to the Meghalayan Age a new phase in history |work=[[BBC News]] |date=18 July 2018 |last1=Amos |first1=Jonathan}}</ref> The boundary [[stratotype]] is a [[speleothem]] in [[Caves of Meghalaya#List of twenty longest caves in Meghalaya|Mawmluh cave]] in India,<ref>{{Cite web|url=http://www.stratigraphy.org/index.php/ics-news-and-meetings/119-collapse-of-civilizations-worldwide-defines-youngest-unit-of-the-geologic-time-scale|title=Collapse of civilizations worldwide defines youngest unit of the Geologic Time Scale}}</ref> and the global auxiliary stratotype is an [[ice core]] from [[Mount Logan]] in Canada.<ref>{{Cite web|url=https://www.qpg.geog.cam.ac.uk/news/formalsubdivisionoftheholoceneseriesgeogr18.pdf|title=Formal subdivision of the Holocene Series/Epoch}}</ref> However, justification for this division is debated as the event was not a global drought and did not happen within a clear timeframe. [[Jessica Tierney]], a paleoclimatologist at the [[University of Arizona]] in [[Tucson]], states that proponents of the new partitioning mistakenly "lumped together evidence of other droughts and wet periods, sometimes centuries away from the event."<ref name="auto"/>

==Effects==

=== Europe ===

==== British Isles ==== In [[Ireland]], there is little definitive record of the 4.2 ka event outside of a brief isotopic excursion in some cave speleothem records. The manner in which this climatic event manifested itself in the region is thus unclear.<ref>{{cite journal |last1=Swindles |first1=Graeme T. |last2=Lawson |first2=Ian T. |last3=Matthews |first3=Ian P. |last4=Blaauw |first4=Maarten |last5=Daley |first5=Timothy J. |last6=Charman |first6=Dan J. |last7=Roland |first7=Thomas P. |last8=Plunkett |first8=Gill |last9=Schettler |first9=Georg |last10=Gearey |first10=Benjamin R. |last11=Turner |first11=T. Edward |last12=Rea |first12=Heidi A. |last13=Roe |first13=Helen M. |last14=Amesbury |first14=Matthew J. |last15=Chambers |first15=Frank M. |last16=Holmes |first16=Jonathan |last17=Mitchell |first17=Fraser J. G. |last18=Blackford |first18=Jeffrey |last19=Blundell |first19=Antony |last20=Branch |first20=Nicholas |last21=Holmes |first21=Jane |last22=Langdon |first22=Peter |last23=McCarroll |first23=Julia |last24=McDermott |first24=Frank |last25=Oksanen |first25=Pirita O. |last26=Pritchard |first26=Oliver |last27=Stastney |first27=Phil |last28=Stefanini |first28=Bettina |last29=Young |first29=Dan |last30=Wheeler |first30=Jane |last31=Becker |first31=Katharina |last32=Armit |first32=Ian |date=November 2013 |title=Centennial-scale climate change in Ireland during the Holocene |url=https://www.sciencedirect.com/science/article/abs/pii/S001282521300144X |journal=[[Earth-Science Reviews]] |volume=126 |pages=300–320 |doi=10.1016/j.earscirev.2013.08.012 |bibcode=2013ESRv..126..300S |s2cid=52248969 |access-date=18 March 2023}}</ref> In Great Britain as in Ireland, the nature of the 4.2 ka event is ambiguous and unclear.<ref name=":0">{{cite journal |last1=Roland |first1=Thomas P. |display-authors=etal |date=2014 |title=Was there a '4.2 ka event' in Great Britain and Ireland? Evidence from the peatland record |url=https://ore.exeter.ac.uk/repository/bitstream/10871/30630/1/4.2%20Symplectic.pdf |journal=[[Quaternary Science Reviews]] |volume=83 |pages=11–27 |bibcode=2014QSRv...83...11R |doi=10.1016/j.quascirev.2013.10.024 |hdl-access=free |hdl=10871/30630}}</ref> The [[yew]] tree's abundance declined in eastern England.<ref>{{Cite journal |last1=Bebchuk |first1=Tatiana |last2=Krusic |first2=Paul J. |last3=Pike |first3=Joshua H. |last4=Piermattei |first4=Alma |last5=Friedrich |first5=Ronny |last6=Wacker |first6=Lukas |last7=Crivellaro |first7=Alan |last8=Arosio |first8=Tito |last9=Kirdyanov |first9=Alexander V. |last10=Gibbard |first10=Philip |last11=Brown |first11=David |last12=Esper |first12=Jan |last13=Reinig |first13=Frederick |last14=Büntgen |first14=Ulf |date=November 2023 |title=Sudden disappearance of yew (Taxus baccata) woodlands from eastern England coincides with a possible climate event around 4.2 ka ago |url=https://www.researchgate.net/publication/375869722 |journal=[[Quaternary Science Reviews]] |language=en |volume=323 |article-number=108414 |doi=10.1016/j.quascirev.2023.108414 |access-date=26 December 2023 |via=ResearchGate}}</ref>

==== Central Europe ==== Analysis of sediments from Lake Spore reveals that in [[Poland]], winters became colder between 4250 and 4000 BP, with this cooling likely responsible for a [[Podzol#Podzolization|podzolisation]] (generation of boreal forest soil type) event around 4200 BP, whereas summer temperatures remained constant. Humidity levels were not affected by the 4.2 ka event.<ref>{{Cite journal |last1=Pleskot |first1=Krzysztof |last2=Apolinarska |first2=Karina |last3=Kołaczek |first3=Piotr |last4=Suchora |first4=Magdalena |last5=Fojutowski |first5=Michał |last6=Joniak |first6=Tomasz |last7=Kotrys |first7=Bartosz |last8=Kramkowski |first8=Mateusz |last9=Słowiński |first9=Michał |last10=Woźniak |first10=Magdalena |last11=Lamentowicz |first11=Mariusz |date=August 2020 |title=Searching for the 4.2 ka climate event at Lake Spore, Poland |url=https://linkinghub.elsevier.com/retrieve/pii/S0341816220301156 |journal=CATENA |language=en |volume=191 |article-number=104565 |doi=10.1016/j.catena.2020.104565|bibcode=2020Caten.19104565P |s2cid=216227365 }}</ref>

==== Iberian Peninsula ==== On the [[Iberian Peninsula]], in general the climate between 2800 and 1100 cal BC is quite stable and relatively humid. A reconstruction of precipitation shows two rapid, pronounced dry phases from 2350 to 2200 cal BC (4.3 - 4.15 ka BP) and from 2100 to 2000 cal BC (4.05 - 3.95 ka BP).<ref name=":3">{{Cite journal |last1=Hinz |first1=Martin |last2=Schirrmacher |first2=Julien |last3=Kneisel |first3=Jutta |last4=Rinne |first4=Christoph |last5=Weinelt |first5=Mara |date=2019-12-12 |title=The Chalcolithic–Bronze Age transition in southern Iberia under the influence of the 4.2 kyr event? A correlation of climatological and demographic proxies |url=https://www.jna.uni-kiel.de/index.php/jna/article/view/170/333 |journal=Journal of Neolithic Archaeology |language=en |pages=1–26 |doi=10.12766/jna.2019.1 |issn=2197-649X}}</ref> The dry phases were followed by a shift towards wetter conditions, suggesting a more complex pattern of climate change than other regions during the 4.2 ka event.<ref>{{cite journal |last1=Schirrmacher |first1=Julien |last2=Weinelt |first2=Mara |last3=Blanz |first3=Thomas |last4=Andersen |first4=Nils |last5=Salgueiro |first5=Emília |last6=Schneider |first6=Ralph R. |date=2 April 2019 |title=Multi-decadal atmospheric and marine climate variability in southern Iberia during the mid- to late-Holocene |url=https://cp.copernicus.org/articles/15/617/2019/ |journal=[[Climate of the Past]] |volume=15 |issue=2 |pages=617–634 |bibcode=2019CliPa..15..617S |doi=10.5194/cp-15-617-2019 |access-date=29 August 2023 |doi-access=free}}</ref>

On the entire Iberian Peninsula, there is a slight decrease in settlement activity from 2500 cal BC, followed by a significant decline between 2300 and 2100 cal BC. In the southwest, and especially in the Evora region, a collapse of settlement activity has been documented, and in the following centuries settlement remained at a low level.<ref name=":3" /> In south-eastern Spain, on the other hand, a few archaeological cultures now started to flourish. One is associated the construction of ''[[motillas]]''-type settlements and the other to [[El Argar]].

The construction of ''[[motillas]]''-type settlements in the period after 2200 BC is believed to be the consequence of the severe [[aridification]] that affected this area. According to M. Mejías Moreno, who reported the first [[historical geology|palaeohydrogeological]] interdisciplinary research in [[La Mancha]], Spain, these ''motillas'' may represent the oldest, most ancient system of groundwater collection in the Iberian Peninsula and their construction might have been directly connected to the prolonged, harsh drought and other climatic perturbations brought by the 4.2 ka event. The authors' analysis verified a relationship between the geological substrate and the spatial distribution of the ''motillas''.<ref>Mejías Moreno, M., Benítez de Lugo Enrich, L., Pozo Tejado, J. del y Moraleda Sierra, J. 2014. [http://www.igme.es/boletin/2014/125_4/5_%20Articulo%203.pdf "Los primeros aprovechamientos de aguas subterráneas en la Península Ibérica. Las motillas de Daimiel en la Edad del Bronce de La Mancha"]. ''Boletín Geológico y Minero'', 125 (4): 455–474 {{ISSN|0366-0176}}</ref>

The other is the El Argar phenomenon, which began to flourish at around the same time, although there was initially a slight collapse, too. However, it soon stabilised and was not negatively affected by the second dry period. It is conceivable, although not perfectly clear as M. Hinz and his colleagues stress, that the two developments of decreasing settlements in the west of the Iberian Peninsula and increasing settlement activities in the east are linked.<ref name=":3" /><ref>{{Cite web |title=Climate-induced migration — Alles bleibt anders |url=https://www.allesbleibtanders.com/en/modules/klimaflucht/ |access-date=2025-02-05 |website=www.allesbleibtanders.com}}</ref>

==== Italian Peninsula ==== In the [[Gulf of Genoa]], mean annual temperature dropped, winters became drier, and summers became wetter and cooler, a phenomenon most likely caused by the southward retreat of the ITCZ in summer that weakened the high pressure and reduced ocean warming over the western Mediterranean, which led to retarded evaporation rates in the autumn and early winter.<ref>{{cite journal |last1=Isola |first1=Ilaria |last2=Zanchetta |first2=Giovanni |last3=Drysdale |first3=Russell N. |last4=Regattieri |first4=Eleonora |last5=Bini |first5=Monica |last6=Bajo |first6=Petra |last7=Hellstrom |first7=John C. |last8=Baneschi |first8=Ilaria |last9=Lionello |first9=Piero |last10=Woodhead |first10=Jon |last11=Greig |first11=Alan |date=22 January 2019 |title=The 4.2 ka event in the central Mediterranean: new data from a Corchia speleothem (Apuan Alps, central Italy) |url=https://cp.copernicus.org/articles/15/135/2019/ |journal=[[Climate of the Past]] |volume=15 |issue=1 |pages=135–151 |doi=10.5194/cp-15-135-2019 |bibcode=2019CliPa..15..135I |access-date=29 August 2023 |doi-access=free }}</ref> The 4.2 ka event appears to have wettened the climate in the [[Alps]].<ref name=":2">{{Cite journal |last1=Zanchetta |first1=Giovanni |last2=Regattieri |first2=Eleonora |last3=Isola |first3=Ilaria |last4=Drysdale |first4=Russell N. |last5=Baneschi |first5=Ilaria |last6=Hellstrom |first6=John C. |date=18 October 2021 |title=THE SO-CALLED "4.2 EVENT" IN THE CENTRAL MEDITERRANEAN AND ITS CLIMATIC TELECONNECTIONS |url=https://amq.aiqua.it/index.php/amq/article/view/91 |journal=Alpine and Mediterranean Quaternary |volume=29 |issue=1 |pages=5–17 |issn=2279-7335 |access-date=3 September 2023}}</ref> Lake Petit saw increased precipitation during the ice-free season, evidenced by an increase in δ<sup>18</sup>O<sub>diatom</sub>.<ref>{{cite journal |last1=Carter |first1=Rosine |last2=Sylvestre |first2=Florence |last3=Paillès |first3=Christine |last4=Sonzogni |first4=Corinne |last5=Couapel |first5=Martine |last6=Alexandre |first6=Anne |last7=Mazur |first7=Jean-Charles |last8=Brisset |first8=Elodie |last9=Miramont |first9=Cécile |last10=Guiter |first10=Frédéric |date=7 February 2019 |title=Diatom-oxygen isotope record from high-altitude Lake Petit (2200 m a.s.l.) in the Mediterranean Alps: shedding light on a climatic pulse at 4.2 ka |url=https://cp.copernicus.org/articles/15/253/2019/ |journal=[[Climate of the Past]] |volume=15 |issue=1 |pages=253–263 |doi=10.5194/cp-15-253-2019 |bibcode=2019CliPa..15..253C |access-date=29 August 2023 |doi-access=free }}</ref> Southern Italy, in contrast, experienced intense aridification.<ref name=":2" /> A major decline in forests occurred in Italy as a result of the climatic perturbation.<ref>{{cite journal |last1=Di Rita |first1=Federico |last2=Magri |first2=Donatella |date=7 February 2019 |title=The 4.2 ka event in the vegetation record of the central Mediterranean |url=https://cp.copernicus.org/articles/15/237/2019/ |journal=[[Climate of the Past]] |volume=15 |issue=1 |pages=237–251 |doi=10.5194/cp-15-237-2019 |bibcode=2019CliPa..15..237D |access-date=29 August 2023 |doi-access=free }}</ref>

===North Africa=== At the site of Sidi Ali in the [[Middle Atlas]], δ<sup>18</sup>O values indicate not a dry spell but a centennial-scale period of cooler and more humid climate.<ref>{{cite journal |last1=Zielhofer |first1=Christoph |last2=Köhler |first2=Anne |last3=Mischke |first3=Steffen |last4=Benkaddour |first4=Abdelfattah |last5=Mikdad |first5=Abdeslam |last6=Fletcher |first6=William J. |date=20 March 2019 |title=Western Mediterranean hydro-climatic consequences of Holocene ice-rafted debris (Bond) events |url=https://cp.copernicus.org/articles/15/463/2019/ |journal=[[Climate of the Past]] |volume=15 |issue=2 |pages=463–475 |doi=10.5194/cp-15-463-2019 |bibcode=2019CliPa..15..463Z |access-date=29 August 2023 |doi-access=free }}</ref> In c. 2150&nbsp;BC, Egypt was hit by a series of exceptionally low [[Flooding of the Nile|Nile floods]] that may have influenced the collapse of the centralised government of the [[Old Kingdom of Egypt|Old Kingdom]] after a famine.<ref>{{cite journal | last = Stanley | first = Jean-Daniel | year = 2003 | title = Nile flow failure at the end of the Old Kingdom, Egypt: Strontium isotopic and petrologic evidence | journal = Geoarchaeology | volume = 18 | issue = 3 | pages = 395–402 | doi = 10.1002/gea.10065 | s2cid = 53571037 | url = https://www.research.manchester.ac.uk/portal/en/publications/short-contribution-nile-flow-failure-at-the-end-of-the-old-kingdom-egypt-strontium-isotopic-and-petrologic-evidence(2a75c9be-5939-467a-b08f-6f08e9bfb86d).html |display-authors=etal}}</ref>

===Middle East=== The south-central [[Levant]] experienced two phases of dry climate punctuated by a wet interval in between and thus the 4.2 ka event in the region has been termed a W-shaped event.<ref>{{cite journal |last1=Kaniewski |first1=David |last2=Marriner |first2=Nick |last3=Cheddadi |first3=Rachid |last4=Guiot |first4=Joël |last5=Van Campo |first5=Elise |date=22 October 2018 |title=The 4.2 ka BP event in the Levant |url=https://cp.copernicus.org/articles/14/1529/2018/ |journal=[[Climate of the Past]] |volume=14 |issue=10 |pages=1529–1542 |doi=10.5194/cp-14-1529-2018 |bibcode=2018CliPa..14.1529K |access-date=29 August 2023 |doi-access=free }}</ref>

Enhanced dust flux coeval with [[δ18O|δ<sup>18</sup>O]] peaks is recorded in [[Mesopotamia]] from 4260 to 3970 BP, reflecting intense aridity.<ref>{{Cite journal |last1=Carolin |first1=Stacy A. |last2=Walker |first2=Richard T. |last3=Day |first3=Christopher C. |last4=Ersek |first4=Vasile |last5=Sloan |first5=R. Alastair |last6=Dee |first6=Michael W. |last7=Talebian |first7=Morteza |last8=Henderson |first8=Gideon M. |date=24 December 2018 |title=Precise timing of abrupt increase in dust activity in the Middle East coincident with 4.2 ka social change |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |language=en |volume=116 |issue=1 |pages=67–72 |doi=10.1073/pnas.1808103115 |issn=0027-8424 |pmc=6320537 |pmid=30584111 |doi-access=free }}</ref> The aridification of Mesopotamia may have been related to the onset of cooler sea-surface temperatures in the [[Atlantic Ocean|North Atlantic]] (Bond event 3), as analysis of the modern instrumental record shows that large (50%) interannual reductions in Mesopotamian water supply result when subpolar northwest Atlantic [[sea surface temperature]]s are anomalously cool.<ref name="Cullen2000">{{cite journal |last1=Cullen |first1=Heidi M. |last2=deMenocal |first2=Peter B. |year=2000 |title=North Atlantic influence on Tigris-Euphrates streamflow |journal=[[International Journal of Climatology]] |volume=20 |issue=8 |pages=853–863 |doi=10.1002/1097-0088(20000630)20:8<853::AID-JOC497>3.0.CO;2-M |bibcode=2000IJCli..20..853C}}</ref> The headwaters of the [[Tigris]] and [[Euphrates]] rivers are fed by elevation-induced capture of winter [[Mediterranean Sea|Mediterranean]] rainfall.

The [[Akkadian Empire]] in 2300 BC was the second civilization to subsume independent societies into a single state (the first being ancient Egypt around 3100 BC). It has been claimed that the collapse of the state was influenced by a wide-ranging, centuries-long drought.<ref>{{cite journal |last1=Kerr |first1=Richard A. |year=1998 |title=Sea-Floor Dust Shows Drought Felled Akkadian Empire |journal=[[Science (journal)|Science]] |volume=279 |issue=5349 |pages=325–326 |doi=10.1126/science.279.5349.325 |bibcode=1998Sci...279..325K|s2cid=140563513 }}</ref><ref>Cullen, H. M. et al., "Climate change and the collapse of the Akkadian empire: Evidence from the deep sea", ''Geology'', vol. 28, iss. 4, pp. 379–382, 2000</ref> Archaeological evidence documents widespread abandonment of the agricultural plains of northern Mesopotamia and dramatic influxes of refugees into southern Mesopotamia, around 2170&nbsp;BC,<ref name="Weiss1993">{{cite journal |last=Weiss |first=H. |year=1993 |title=The Genesis and Collapse of Third Millennium North Mesopotamian Civilization |journal=[[Science (journal)|Science]] |volume=261 |issue=5124 |pages=995–1004 |doi=10.1126/science.261.5124.995 |pmid=17739617 |bibcode=1993Sci...261..995W |s2cid=31745857 |display-authors=etal |url=http://revistas.ucm.es/index.php/ILUR/article/view/61022}}</ref> which may have weakened the Akkadian state.<ref>{{cite book |last1=Danti |first1=Michael |editor-last1=Mainwaring |editor-first1=A. Bruce |editor-last2=Giegengack |editor-first2=Robert |editor-last3=Vita-Finzi |editor-first3=Claudio |date=8 November 2010 |title=Climate Crises in Human History |url=https://books.google.com/books?id=uJikcQAACAAJ |chapter=Late Middle Holocene Climate and Northern Mesopotamia: Varying Cultural Responses to the 5.2 and 4.2 ka Aridification Events |chapter-url=https://www.academia.edu/2391324 |publisher=American Philosophical Society |pages=139–172 |isbn=978-1-60618-921-4 |access-date=3 September 2023}}</ref> A 180-km-long wall, the "Repeller of the [[Amorites]]", was built across central Mesopotamia to stem nomadic incursions to the south. Around 2150 BC, the [[Gutian people]], who originally inhabited the [[Zagros Mountains]], defeated the demoralised Akkadian army, took [[Akkad (city)|Akkad]] and destroyed it around 2115 BC. Widespread agricultural change in the [[Near East]] is visible at the end of the 3rd millennium BC.<ref>{{cite journal |last=Riehl |first=S. |year=2008 |title=Climate and agriculture in the ancient Near East: a synthesis of the archaeobotanical and stable carbon isotope evidence |journal=[[Vegetation History and Archaeobotany]] |volume=17 |issue=1 |pages=43–51 |doi=10.1007/s00334-008-0156-8|bibcode=2008VegHA..17S..43R |s2cid=128622745 }}</ref> Weiss suggests a figure of 300,000 displaced from the zone of uncertainty,<ref name="Weiss1993" /> while Burke suggests no less than 126,400 (99,000 displaced from [[Upper Mesopotamia]]; 17,400 from Middle Euphrates and approximately 10,000 from territories from northeast to southeast of [[Ebla]]).<ref>{{cite book |last1=Burke |first1=Aaron A. |title=The Amorites and the Bronze Age Near East: The Making of a Regional Identity |date=2021 |publisher=Cambridge University Press |isbn=978-1-108-49596-7 |url=https://books.google.com/books?id=qc0HEAAAQBAJ}}</ref> Resettlement of the northern plains by smaller sedentary populations occurred near 1900 BC, three centuries after the collapse.<ref name="Weiss1993" />

In the [[Persian Gulf]] region, there was a sudden change in settlement pattern, style of pottery and tombs. The 22nd century BC drought marks the end of the [[Umm Al Nar culture]] and the change to the [[Wadi Suq culture]].<ref name="Parker2006" /> A study of fossil corals in [[Oman]] provides evidence that prolonged winter [[Shamal (wind)|shamal]] seasons, around 2200 BC, led to the [[Soil salinity|salinization]] of the irrigated field, which made a dramatic decrease in crop production trigger a widespread famine and eventually the collapse of the ancient Akkadian Empire.<ref>{{cite journal |last1=Watanabe |first1=Takaaki K. |last2=Watanabe |first2=Tsuyoshi |last3=Yamazaki |first3=Atsuko |last4=Pfeiffer |first4=Miriam |title=Oman corals suggest that a stronger winter shamal season caused the Akkadian Empire (Mesopotamia) collapse |journal=[[Geology (journal)|Geology]] |volume=47 |issue=12 |pages=1141–1145 |publisher=GeoScienceWorld |year=2019 |doi=10.1130/G46604.1|bibcode=2019Geo....47.1141W|s2cid=204781389 }}</ref><ref>{{cite web |url=https://www.global.hokudai.ac.jp/blog/strong-winter-dust-storms-may-have-caused-the-collapse-of-the-akkadian-empire/ |title=Strong winter dust storms may have caused the collapse of the Akkadian Empire |website=Hokkaido University |date=24 October 2019}}</ref>

===South and Central Asia=== The [[Siberian High]] increased in area and magnitude, which blocked moisture-carrying westerly winds, causing intense aridity in [[Central Asia]].<ref>{{cite journal |last1=Perşoiu |first1=Aurel |last2=Ionita |first2=Monica |last3=Weiss |first3=Harvey |date=11 April 2019 |title=Atmospheric blocking induced by the strengthened Siberian High led to drying in west Asia during the 4.2 ka BP event – a hypothesis |url=https://cp.copernicus.org/articles/15/781/2019/ |journal=[[Climate of the Past]] |volume=15 |issue=2 |pages=781–793 |doi=10.5194/cp-15-781-2019 |bibcode=2019CliPa..15..781P |access-date=29 August 2023 |doi-access=free }}</ref>

The Indian Summer Monsoon (ISM) and Indian Winter Monsoon (IWM) both declined in strength, leading to highly arid conditions in northwestern South Asia.<ref>{{cite journal |last1=Giesche |first1=Alena |last2=Staubwasser |first2=Michael |last3=Petrie |first3=Cameron A. |last4=Hodell |first4=David A. |date=15 January 2019 |title=Indian winter and summer monsoon strength over the 4.2 ka BP event in foraminifer isotope records from the Indus River delta in the Arabian Sea |url=https://cp.copernicus.org/articles/15/73/2019/ |journal=[[Climate of the Past]] |volume=15 |issue=1 |pages=73–90 |doi=10.5194/cp-15-73-2019 |bibcode=2019CliPa..15...73G |access-date=29 August 2023 |doi-access=free }}</ref> The ISM's decline is evident from low Mn/Ti and Mn/Fe values in [[Rara Lake]] from this time.<ref>{{Cite journal |last1=Nakamura |first1=Atsunori |last2=Yokoyama |first2=Yusuke |last3=Maemoku |first3=Hideaki |last4=Yagi |first4=Hiroshi |last5=Okamura |first5=Makoto |last6=Matsuoka |first6=Hiromi |last7=Miyake |first7=Nao |last8=Osada |first8=Toshiki |last9=Adhikari |first9=Danda Pani |last10=Dangol |first10=Vishnu |last11=Ikehara |first11=Minoru |last12=Miyairi |first12=Yosuke |last13=Matsuzaki |first13=Hiroyuki |date=18 March 2016 |title=Weak monsoon event at 4.2 ka recorded in sediment from Lake Rara, Himalayas |url=https://www.sciencedirect.com/science/article/pii/S1040618215005613 |journal=[[Quaternary International]] |series=Japanese Quaternary Studies |volume=397 |pages=349–359 |doi=10.1016/j.quaint.2015.05.053 |bibcode=2016QuInt.397..349N |issn=1040-6182 |access-date=8 September 2023}}</ref> The area around PankangTeng Tso Lake in the [[Tawang district]] of [[Arunachal Pradesh]] had cold and dry conditions and was dominated by subalpine vegetation.<ref>{{Cite journal |last1=Mehrotra |first1=Nivedita |last2=Shah |first2=Santosh K. |last3=Basavaiah |first3=Nathani |last4=Laskar |first4=Amzad H. |last5=Yadava |first5=Madhusudan G. |date=25 February 2019 |title=Resonance of the '4.2ka event' and terminations of global civilizations during the Holocene, in the palaeoclimate records around PT Tso Lake, Eastern Himalaya |url=https://www.sciencedirect.com/science/article/pii/S1040618218300570 |journal=[[Quaternary International]] |series=Holocene Civilization |volume=507 |pages=206–216 |doi=10.1016/j.quaint.2018.09.027 |bibcode=2019QuInt.507..206M |s2cid=135417137 |issn=1040-6182 |access-date=8 September 2023}}</ref> Though some proxy records suggest a prolonged, multicentennial dry period, others indicate that the 4.2 ka event was a series of multidecadal droughts instead.<ref>{{cite journal |last1=Kathayat |first1=Gayatri |last2=Cheng |first2=Hai |last3=Sinha |first3=Ashish |last4=Berkelhammer |first4=Max |last5=Zhang |first5=Haiwei |last6=Duan |first6=Pengzhen |last7=Li |first7=Hanying |last8=Li |first8=Xianglei |last9=Ning |first9=Youfeng |last10=Edwards |first10=Robert Lawrence |date=13 November 2018 |title=Evaluating the timing and structure of the 4.2 ka event in the Indian summer monsoon domain from an annually resolved speleothem record from Northeast India |url=https://cp.copernicus.org/articles/14/1869/2018/ |journal=[[Climate of the Past]] |volume=14 |issue=12 |pages=1869–1879 |doi=10.5194/cp-14-1869-2018 |access-date=29 August 2023 |doi-access=free }}</ref><ref>{{Cite journal |last1=Giesche |first1=Alena |last2=Hodell |first2=David A. |last3=Petrie |first3=Cameron A. |last4=Haug |first4=Gerald H. |last5=Adkins |first5=Jess F. |last6=Plessen |first6=Birgit |last7=Marwan |first7=Norbert |last8=Bradbury |first8=Harold J. |last9=Hartland |first9=Adam |last10=French |first10=Amanda D. |last11=Breitenbach |first11=Sebastian F. M. |date=4 April 2023 |title=Recurring summer and winter droughts from 4.2-3.97 thousand years ago in north India |url=https://www.nature.com/articles/s43247-023-00763-z |journal=[[Communications Earth & Environment]] |language=en |volume=4 |issue=1 |page=103 |doi=10.1038/s43247-023-00763-z |bibcode=2023ComEE...4..103G |s2cid=257915185 |issn=2662-4435 |access-date=8 September 2023}}</ref>

====Effects on the Indus Valley civilization==== {{Main|Indus Valley Civilisation|Indo-Aryan migrations}}

In the 2nd millennium BC, widespread aridification occurred in the [[Eurasian steppes]] and in [[South Asia]].<ref name="Kochhar2017">{{Cite web |last=Kochhar |first=Rajesh |date=2017-07-25 |title=The Aryan chromosome |url=https://indianexpress.com/article/opinion/columns/aryans-dna-genetics-archaeology-4765740/ |access-date=2023-12-19 |website=The Indian Express |language=en}}</ref><ref name="Demkina">{{cite journal |last1=Demkina |first1=T. S. |year=2017 |title=Paleoecological crisis in the steppes of the Lower Volga region in the Middle of the Bronze Age (III–II centuries BC) |journal=Eurasian Soil Science |volume=50 |issue=7 |doi=10.1134/S1064229317070018 |pages=791–804 |bibcode =2017EurSS..50..791D|s2cid=133638705 }}</ref> On the steppes, the vegetation changed, driving "higher mobility and transition to the nomadic cattle breeding."<ref name="Demkina" />{{refn|group=note|Demkina et al. (2017): "In the second millennium&nbsp;BC, humidization of the climate led to the divergence of the soil cover with secondary formation of the complexes of chestnut soils and solonetzes. This paleoecological crisis had a significant effect on the economy of the tribes in the Late [[Catacom culture|Catacomb]] and Post-Catacomb time stipulating their higher mobility and transition to the nomadic cattle breeding."<ref name="Demkina" />}} Water shortage also strongly affected South Asia: {{quote|This time was one of great upheaval for ecological reasons. Prolonged failure of rains caused acute water shortage in large areas, causing the collapse of sedentary urban cultures in south central Asia, Afghanistan, Iran, and India, and triggering large-scale migrations. Inevitably, the new arrivals came to merge with and dominate the post-urban cultures.<ref name="Kochhar2017"/>}}

Urban centers of the [[Indus Valley Civilisation]] were abandoned and replaced by disparate local cultures because of the same [[climate change]] that affected the neighbouring regions to the west.<ref>{{Cite web|url=https://phys.org/news/2014-02-decline-bronze-age-megacities-linked.html|title=Decline of Bronze Age 'megacities' linked to climate change|website=phys.org}}</ref> {{As of|2016}}, many scholars believed that drought and a decline in trade with Egypt and Mesopotamia caused the collapse of the Indus civilization.<ref name="Science">{{cite journal |last1=Lawler |first1=Andrew |date=6 June 2008 |title=Indus Collapse: The End or the Beginning of an Asian Culture? |journal=[[Science (journal)|Science]] |volume=320 |issue=5881 |pages=1282–1283 |doi=10.1126/science.320.5881.1281 |pmid=18535222 |s2cid=206580637}}</ref> The [[Ghaggar-Hakra River|Ghaggar-Hakra system]] was rain-fed,<ref name="Giosan">{{cite journal |last1=Giosan |first1=L. |title=Fluvial landscapes of the Harappan Civilization |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |year=2012 |volume=109 |issue=26 |display-authors=etal |doi=10.1073/pnas.1112743109 |pmid=22645375 |pages=E1688–E1694 |pmc=3387054 |bibcode=2012PNAS..109E1688G|doi-access=free }}</ref><ref name="Clift">Clift et al., 2011, [http://geology.gsapubs.org/content/40/3/211.short "U–Pb zircon dating evidence for a Pleistocene Sarasvati River and capture of the Yamuna River"], ''Geology'', 40, 211–214 (2011).</ref><ref name="Tripathi_2004">{{cite journal |first1=Jayant K. |last1=Tripathi |last2=Tripathi |first2=K. |last3=Bock |first3=Barbara |last4=Rajamani |first4=V. |last5=Eisenhauer |first5=A. |name-list-style=amp |title=Is River Ghaggar, Saraswati? Geochemical Constraints |journal=[[Current Science]] |volume=87 |issue=8 |date=25 October 2004 |url=http://www.ias.ac.in/currsci/oct252004/1141.pdf}}</ref> and water supply depended on the [[monsoon]]s. The [[Indus Valley]] climate grew significantly cooler and drier from around 1800 BC, which is linked to a contemporary general weakening of the monsoon.<ref name="Giosan"/> Aridity increased, with the [[Ghaggar-Hakra River]] retracting its reach towards the foothills of the [[Himalayas]],<ref name="Giosan"/><ref>{{cite web |author=Nuwer |first=Rachel |author-link=Rachel Nuwer |date=28 May 2012 |title=An Ancient Civilization, Upended by Climate Change |url=http://green.blogs.nytimes.com/2012/05/29/an-ancient-civilization-upended-by-climate-change/?_r=0 |access-date=29 May 2012 |publisher=LiveScience}}</ref><ref>{{cite web |author=Choi |first=Charles |date=29 May 2012 |title=Huge Ancient Civilization's Collapse Explained |url=http://www.livescience.com/20614-collapse-mythical-river-civilization.html |access-date=18 May 2016 |work=[[The New York Times]]}}</ref> leading to erratic and less-extensive floods, which made inundation agriculture less sustainable. Aridification reduced the water supply enough to cause the civilization's demise, and to scatter its population eastward.<ref name="Staubwasser2003">{{cite journal |last=Staubwasser |first=M. |s2cid=129178112 |year=2003 |title=Climate change at the 4.2 ka BP termination of the Indus valley civilization and Holocene south Asian monsoon variability |journal=[[Geophysical Research Letters]] |volume=30 |issue=8 |page=1425 |doi=10.1029/2002GL016822 |bibcode=2003GeoRL..30.1425S |display-authors=etal}}</ref><ref name="madella-fuller">{{cite journal |last2=Fuller |first2=Dorian |date=2006 |title=Palaeoecology and the Harappan Civilization of South Asia: a reconsideration|journal=[[Quaternary Science Reviews]] |volume=25 |issue=11–12 |pages=1283–1301 |last1=Madella |first1=Marco |doi=10.1016/j.quascirev.2005.10.012 |bibcode=2006QSRv...25.1283M}}</ref><ref name="macdonald">{{cite journal |last1=MacDonald |first1=Glen |year=2011|title=Potential influence of the Pacific Ocean on the Indian summer monsoon and Harappan decline |journal=[[Quaternary International]] |volume=229 |issue=1–2 |pages=140–148 |doi=10.1016/j.quaint.2009.11.012 |bibcode=2011QuInt.229..140M}}</ref><ref name=brooke-2015>{{citation |last=Brooke |first=John L. |title=Climate Change and the Course of Global History: A Rough Journey |url=https://books.google.com/books?id=O9TSAgAAQBAJ&pg=PA296 |date=2014 |publisher=[[Cambridge University Press]] |isbn=978-0-521-87164-8 |page=296|bibcode=2014cccg.book.....B }}</ref>

===East Asia=== The 4.2 ka event resulted in an enormous reduction in the strength of the East Asian Summer Monsoon (EASM).<ref name="HulunLake" /> This profound weakening of the EASM has been postulated to have resulted from a reduction in the strength of the AMOC;<ref>{{cite journal |last1=Kaboth-Bahr |first1=Stefanie |last2=Bahr |first2=André |last3=Zeeden |first3=Christian A. |last4=Yamoah |first4=Kweku A. |last5=Lone |first5=Mahjoor Ahmad |last6=Chuang |first6=Chih-Kai |last7=Löwemark |first7=Ludvig |last8=Wei |first8=Kuo-Yen |date=25 March 2021 |title=A tale of shifting relations: East Asian summer and winter monsoon variability during the Holocene |journal=[[Scientific Reports]] |volume=11 |issue=1 |page=6938 |doi=10.1038/s41598-021-85444-7 |pmid=33767210 |pmc=7994397 |bibcode=2021NatSR..11.6938K }}</ref> the cooling of North Atlantic waters led to retardation of northward movements of the EASM and diminished rainfall on its northern margin.<ref name="HulunLake">{{cite journal |last1=Xiao |first1=Jule |last2=Zhang |first2=Shengrui |last3=Fan |first3=Jiawei |last4=Wen |first4=Ruilin |last5=Zhai |first5=Dayou |last6=Tian |first6=Zhiping |last7=Jiang |first7=Dabang |date=11 October 2018 |title=The 4.2 ka BP event: multi-proxy records from a closed lake in the northern margin of the East Asian summer monsoon |url=https://cp.copernicus.org/articles/14/1417/2018/ |journal=[[Climate of the Past]] |volume=14 |issue=10 |pages=1417–1425 |doi=10.5194/cp-14-1417-2018 |bibcode=2018CliPa..14.1417X |access-date=29 August 2023 |doi-access=free }}</ref> A stark humidity gradient emerged between northern and southern China because of the EASM's southward move.<ref>{{cite journal |last1=Zhang |first1=Haiwei |last2=Cheng |first2=Hai |last3=Cai |first3=Yanjun |last4=Spötl |first4=Christoph |last5=Kathayat |first5=Gayathri |last6=Sinha |first6=Ashish |last7=Edwards |first7=R. Lawrence |last8=Tan |first8=Liangcheng |date=27 November 2018 |title=Hydroclimatic variations in southeastern China during the 4.2 ka event reflected by stalagmite records |url=https://cp.copernicus.org/articles/14/1805/2018/ |journal=[[Climate of the Past]] |volume=14 |issue=11 |pages=1805–1817 |doi=10.5194/cp-14-1805-2018 |access-date=29 August 2023 |doi-access=free }}</ref> Northeastern China was strongly affected;<ref>{{cite journal |last1=Scuderi |first1=Louis A. |last2=Yang |first2=Xiaoping |last3=Ascoli |first3=Samantha E. |last4=Li |first4=Hongwei |date=21 February 2019 |title=The 4.2 ka BP Event in northeastern China: a geospatial perspective |url=https://cp.copernicus.org/articles/15/367/2019/ |journal=[[Climate of the Past]] |volume=15 |issue=1 |pages=367–375 |doi=10.5194/cp-15-367-2019 |bibcode=2019CliPa..15..367S |access-date=29 August 2023 |doi-access=free }}</ref> proxy records from [[Hulun Lake]] in [[Inner Mongolia]] reveal a major dry event from 4210–3840 BP,<ref name="HulunLake" /> while records from Wudalianchi Crater Lake indicate a sharp decline in evergreen broadleaf forests.<ref>{{Cite journal |last1=Zhou |first1=Xin |last2=Liu |first2=Xiaoyan |last3=Zhan |first3=Tao |last4=Oyebanji |first4=Dorcas B. |last5=Zhang |first5=Jixiao |last6=Tu |first6=Luyao |last7=Jiang |first7=Shiwei |date=April 2024 |title=Low-latitude forcing on 4.2 ka event indicated by records in the Asian monsoon region |url=https://www.sciencedirect.com/science/article/pii/S0921818124000481 |journal=[[Global and Planetary Change]] |language=en |volume=235 |article-number=104401 |doi=10.1016/j.gloplacha.2024.104401 |access-date=11 March 2025 |via=Elsevier Science Direct}}</ref> δ<sup>18</sup>O values from Yonglu Cave in [[Hubei]] confirm that the region became characterised by increased aridity and show that the onset of the event was gradual but that its end was sudden.<ref>{{Cite journal |last1=Yujie |first1=Bai |last2=Jiangying |first2=Wu |last3=Yijia |first3=Liang |last4=Qingfeng |first4=Shao |date=30 July 2020 |title=THE MULTI-PROXY RECORD OF A STALAGMITE FROM YULONG CAVE,HUBEI DURING THE 4.2 KA EVENT |url=http://www.dsjyj.com.cn/en/article/doi/10.11928/j.issn.1001-7410.2020.04.11 |journal=Quaternary Sciences |volume=40 |issue=4 |pages=959–972 |doi=10.11928/j.issn.1001-7410.2020.04.11 |doi-broken-date=11 July 2025 |access-date=3 September 2023}}</ref> In the Luoyang Basin, the 4.2 ka event was much less strongly felt; precipitation was low, but not extremely so.<ref name="LuoyangBasin">{{Cite journal |last1=Chen |first1=Liang |last2=Hui |first2=Zhengchuang |last3=Yong |first3=Zijuan |date=1 October 2025 |title=Precipitation variations around ∼4.2 ka BP in the Luoyang Basin: A pollen-based quantitative reconstruction |url=https://www.sciencedirect.com/science/article/pii/S0031018225003724 |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |language=en |volume=675 |article-number=113087 |doi=10.1016/j.palaeo.2025.113087 |access-date=15 September 2025 |via=Elsevier Science Direct}}</ref> In central China, precipitation increased, meanwhile.<ref>{{Cite journal |last1=Lin |first1=Jie |last2=Jiang |first2=Wenying |last3=Wang |first3=Luo |last4=Zhang |first4=Enlou |last5=Tang |first5=Lingyu |last6=Yang |first6=Xiaoxiao |last7=Chu |first7=Guoqiang |last8=Yang |first8=Shiling |last9=Xiao |first9=Jule |date=15 November 2022 |title=Spatially diverse hydroclimatic response to the 4.2 ka event in the Asian monsoon region |url=https://www.sciencedirect.com/science/article/pii/S0277379122004401 |journal=[[Quaternary Science Reviews]] |language=en |volume=296 |article-number=107809 |doi=10.1016/j.quascirev.2022.107809 |access-date=11 March 2025 |via=Elsevier Science Direct}}</ref>

In the Korean Peninsula, the 4.2 ka event was associated with significant aridification, measured by the large decline in arboreal pollen percentage (AP).<ref>{{cite journal |last1=Park |first1=Jungjae |last2=Park |first2=Jinheum |last3=Yi |first3=Sangheon |last4=Kim |first4=Jin Cheul |last5=Lee |first5=Eunmi |last6=Choi |first6=Jieun |date=25 July 2019 |title=Abrupt Holocene climate shifts in coastal East Asia, including the 8.2 ka, 4.2 ka, and 2.8 ka BP events, and societal responses on the Korean peninsula |journal=[[Scientific Reports]] |volume=9 |issue=1 |page=10806 |doi=10.1038/s41598-019-47264-8 |pmid=31346228 |pmc=6658530 |bibcode=2019NatSR...910806P }}</ref> On [[Jeju Island]], in contrast, the climate was humid, as evidenced by the presence of tychoplanktonic species found in the Sara-oreum and Muljangori-oreum wetlands.<ref>{{Cite journal |last1=Cho |first1=Ara |last2=Lim |first2=Jaesoo |last3=Han |first3=Min |last4=Jun |first4=Chang-Pyo |last5=Ahn |first5=Ung San |date=1 March 2025 |title=Hydroclimatic variation on Jeju Island, Korea during the Holocene and its linkage to the westerly jet: A focus on the 4.2 ka event |url=https://www.sciencedirect.com/science/article/pii/S003101822500015X |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |language=en |volume=661 |article-number=112730 |doi=10.1016/j.palaeo.2025.112730 |access-date=8 April 2025 |via=Elsevier Science Direct}}</ref>

The [[Sannai-Maruyama Site|Sannai-Maruyama site]] in Japan declined during the same period;<ref>[https://sannaimaruyama.pref.aomori.jp/about/iseki/ 三内丸山遺跡について三内丸山遺跡とは]（公式サイト）。</ref> the growing [[:ja:_近代以前の日本の人口統計#地域別推定人口|population]] of the [[Jōmon period|Jomon culture]] gradually turned to decline after that.<ref name="Koyama">(a) Shuzo Koyama, "Jomon Subsistence and Population", ''Senri Ethnological Studies'' no. 2, 1–65 (1978). (b) 小山修三, 『縄文時代』, 中央公論社, 1983. なお『縄文時代』では遺跡数に乗じる係数を、弥生時代57人、縄文時代中期以降24人、縄文時代早期8.5人と紹介しているが、実際の数値計算に合わせ、本文のように修正した。</ref>

[[Rebun Island]] experienced an abrupt, intense cooling around 4,130 BP believed to be associated with the 4.2 ka event.<ref>{{Cite journal |last1=Leipe |first1=Christian |last2=Müller |first2=Stefanie |last3=Hille |first3=Konrad |last4=Kato |first4=Hirofumi |last5=Kobe |first5=Franziska |last6=Schmidt |first6=Mareike |last7=Seyffert |first7=Konrad |last8=Spengler |first8=Robert |last9=Wagner |first9=Mayke |last10=Weber |first10=Andrzej W. |last11=Tarasov |first11=Pavel E. |date=1 August 2018 |title=Vegetation change and human impacts on Rebun Island (Northwest Pacific) over the last 6000 years |url=https://www.sciencedirect.com/science/article/pii/S0277379118303561 |journal=[[Quaternary Science Reviews]] |volume=193 |pages=129–144 |doi=10.1016/j.quascirev.2018.06.011 |bibcode=2018QSRv..193..129L |issn=0277-3791 |access-date=8 September 2023}}</ref>

====Effects on Chinese civilization==== The drought may have caused the collapse of [[List of Neolithic cultures of China|Neolithic cultures around Central China]] in the late 3rd millennium BC,<ref>{{cite journal |last1=Wu |first1=Wenxiang |last2=Liu |first2=Tungsheng |year=2004 |title=Possible role of the "Holocene Event&nbsp;3" on the collapse of Neolithic Cultures around the Central Plain of China |journal=Quaternary International |volume=117 |issue=1 |pages=153–166 |doi=10.1016/S1040-6182(03)00125-3 |bibcode=2004QuInt.117..153W}}</ref><ref>{{cite journal |author1=Chun Chang Huang |year=2011 |title=Extraordinary floods related to the climatic event at 4200&nbsp;a BP on the Qishuihe River, middle reaches of the Yellow River, China |journal=Quaternary Science Reviews |volume=30 |issue=3–4 |pages=460–468 |doi=10.1016/j.quascirev.2010.12.007 |bibcode=2011QSRv...30..460H |display-authors=etal}}</ref> or at least partially contributed to it.<ref>{{Cite journal |last1=Ran |first1=Min |last2=Chen |first2=Liang |date=30 June 2019 |title=The 4.2 ka BP climatic event and its cultural responses |url=https://www.sciencedirect.com/science/article/pii/S1040618219303404 |journal=[[Quaternary International]] |language=en |volume=521 |pages=158–167 |doi=10.1016/j.quaint.2019.05.030 |access-date=11 March 2025 |via=Elsevier Science Direct}}</ref> In the [[:zh:沂沭泗河水系|Yishu River Basin]] (a river basin that consists of the Yi River ([[:zh:沂河|沂河]]) of Shandong and [[Shu River]]), the flourishing [[Longshan culture]] was affected by a cooling that severely reduced rice output and led to a substantial decrease in population and to fewer archaeological sites.<ref>{{cite journal |last1=Gao |first1=Huazhong |last2=Zhu |first2=Cheng |last3=Xu |first3=Weifeng |year=2007 |title=Environmental change and cultural response around 4200&nbsp;cal. yr&nbsp;BP in the Yishu River Basin, Shandong |journal=Journal of Geographical Sciences |volume=17 |issue=3 |pages=285–292 |doi=10.1007/s11442-007-0285-5|s2cid=186227589 |doi-access=free |bibcode=2007JGSci..17..285G }}</ref> Around 2000 BC, Longshan was displaced by the [[Yueshi culture]], which had fewer and less-sophisticated artifacts of ceramic and bronze. The [[Liangzhu culture|Liangzhu civilization]] in the lower reaches of the Yangtze River also declined during the same period.<ref>{{cite web |title=Migration of the Tribe and Integration into the Han Chinese |url=http://museum.shqp.gov.cn/gb/content/2009-02/23/content_237435.htm |archive-url=https://web.archive.org/web/20160304075205/http://museum.shqp.gov.cn/gb/content/2009-02/23/content_237435.htm |archive-date=2016-03-04 |access-date=29 January 2014 |publisher=Qingpu Museum}}</ref> The 4.2 ka event is also believed to have helped collapse the [[Dawenkou culture]].<ref>{{Cite journal |last1=Wang |first1=Jianjun |last2=Sun |first2=Liguang |last3=Chen |first3=Liqi |last4=Xu |first4=Libin |last5=Wang |first5=Yuhong |last6=Wang |first6=Xinming |date=10 June 2016 |title=The abrupt climate change near 4,400 yr BP on the cultural transition in Yuchisi, China and its global linkage |journal=[[Scientific Reports]] |language=en |volume=6 |article-number=27723 |doi=10.1038/srep27723 |issn=2045-2322 |pmc=4901284 |pmid=27283832|bibcode=2016NatSR...627723W }}</ref> The Longshan culture of the less strongly affected Luoyang Basin, however, continued to develop and thrive.<ref name="LuoyangBasin" /> The 4.2 ka event had no discernible impact on the spread of [[millet]] cultivation in the region.<ref>{{Cite journal |last1=Leipe |first1=C. |last2=Long |first2=T. |last3=Sergusheva |first3=E. A. |last4=Wagner |first4=M. |last5=Tarasov |first5=P. E. |date=6 September 2019 |title=Discontinuous spread of millet agriculture in eastern Asia and prehistoric population dynamics |journal=[[Science Advances]] |language=en |volume=5 |issue=9 |article-number=eaax6225 |doi=10.1126/sciadv.aax6225 |issn=2375-2548 |pmc=6760930 |pmid=31579827 |bibcode=2019SciA....5.6225L }}</ref>

=== Southeast Asia === The 4.2 ka event substantially reduced ENSO variability in [[Borneo]], as evidenced by stalagmite [[Δ18O|δ<sup>18</sup>O]] values. The reduction in ENSO variability that occurred was comparable only to the earlier 8.2 ka event.<ref>{{Cite journal |last1=Theaker |first1=C. M. |last2=Carolin |first2=S. A. |last3=Day |first3=C. C. |last4=Cobb |first4=K. M. |last5=Chen |first5=S. |last6=Grothe |first6=P. R. |last7=Couper |first7=H. O. |date=28 March 2024 |title=Borneo Stalagmite Evidence of Significantly Reduced El Niño-Southern Oscillation Variability at 4.1 kyBP |url=https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2023GL107111 |journal=[[Geophysical Research Letters]] |language=en |volume=51 |issue=6 |doi=10.1029/2023GL107111 |issn=0094-8276 |access-date=9 March 2025 |via=Wiley Online Library}}</ref>

=== Southern Africa === [[Stalagmite|Stalagmites]] from northeastern [[Namibia]] demonstrate the region became wetter thanks to the southward shift of the ITCZ.<ref>{{Cite journal |last1=Railsback |first1=L. Bruce |last2=Liang |first2=Fuyuan |last3=Brook |first3=George A. |last4=Cheng |first4=Hai |last5=Edwards |first5=R. Lawrence |date=15 January 2022 |title=Additional multi-proxy stalagmite evidence from northeast Namibia supports recent models of wetter conditions during the 4.2 ka Event in the Southern Hemisphere |url=https://linkinghub.elsevier.com/retrieve/pii/S0031018221005411 |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |language=en |volume=586 |article-number=110756 |doi=10.1016/j.palaeo.2021.110756 |bibcode=2022PPP...58610756R |s2cid=244126683 |access-date=3 September 2023}}</ref> The Namibian humidification event had two pulses.<ref>{{Cite journal |last1=Railsback |first1=L. Bruce |last2=Liang |first2=Fuyuan |last3=Brook |first3=G.A. |last4=Voarintsoa |first4=Ny Riavo G. |last5=Sletten |first5=Hillary R. |last6=Marais |first6=Eugene |last7=Hardt |first7=Ben |last8=Cheng |first8=Hai |last9=Edwards |first9=R. Lawrence |date=15 April 2018 |title=The timing, two-pulsed nature, and variable climatic expression of the 4.2 ka event: A review and new high-resolution stalagmite data from Namibia |url=https://linkinghub.elsevier.com/retrieve/pii/S027737911731003X |journal=[[Quaternary Science Reviews]] |language=en |volume=186 |pages=78–90 |doi=10.1016/j.quascirev.2018.02.015 |bibcode=2018QSRv..186...78R |access-date=3 September 2023}}</ref>

===Mascarenes=== No signal of the 4.2 ka event has been found in [[Rodrigues]].<ref>{{cite journal |last1=Li |first1=Hanying |last2=Cheng |first2=Hai |last3=Sinha |first3=Ashish |last4=Kathayat |first4=Gayatri |last5=Spötl |first5=Christoph |last6=André |first6=Aurèle Anquetil |last7=Meunier |first7=Arnaud |last8=Biswas |first8=Jayant |last9=Duan |first9=Pengzhen |last10=Ning |first10=Youfeng |last11=Edwards |first11=Richard Lawrence |date=7 December 2018 |title=Hydro-climatic variability in the southwestern Indian Ocean between 6000 and 3000 years ago |url=https://cp.copernicus.org/articles/14/1881/2018/ |journal=[[Climate of the Past]] |volume=14 |issue=12 |pages=1881–1891 |doi=10.5194/cp-14-1881-2018 |bibcode=2018CliPa..14.1881L |access-date=29 August 2023 |doi-access=free }}</ref>

==See also== {{Portal|Ancient Egypt|Asia}} *[[Great Flood (China)|2300–2200 BCE Great Flood (China)]] *[[Mid-24th-century BCE climate anomaly|2354–2345 BCE climate anomaly]] *[[8.2-kiloyear event]] *[[African humid period]] *[[Bond event]] *[[Climate variability and change]] *[[Late_Bronze_Age_collapse#Environmental|Late Bronze Age collapse § Environmental]] (c. 1200–1150 BC) *[[Timeline of environmental history]]

== Explanatory notes == {{Reflist|group=note}}

==References== {{reflist|30em}}

==Further reading== *{{cite journal |first1=D. |last1=Kaniewski |display-authors=1 |first2=E. |last2=Paulissen |first3=E. |last3=Van Campo |first4=M. |last4=Al-Maqdissi |first5=J. |last5=Bretschneider |first6=K. |last6=Van Lerberghe |title=Middle East coastal ecosystem response to middle-to-late Holocene abrupt climate changes |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |url=https://www.researchgate.net/publication/23238483 |volume=105 |issue=37 |pages=13941–13946 |year=2008 |doi=10.1073/pnas.0803533105 |pmid=18772385 |pmc=2544558 |bibcode=2008PNAS..10513941K |doi-access=free |access-date=21 June 2023}} *{{cite book |editor-last=Weiss |editor-first=H. |year=2012 |title=Seven Generations Since the Fall of Akkad |location=Wiesbaden |publisher=Harrassowitz |isbn=978-3-447-06823-9}} *{{cite book |last=Weiss |first=H. |chapter=Beyond the Younger Dryas: Collapse as Adaptation to Abrupt Climate Change in Ancient West Asia and the Eastern Mediterranean |title=Environmental Disaster and the Archaeology of Human Response |editor-first=G. |editor-last=Bawden |editor2-first=R. M. |editor2-last=Reycraft |location=Albuquerque, New Mexico |publisher=Maxwell Museum of Anthropology |pages=63–74 |year=2000 |isbn=0-912535-14-8}}

==External links== * [http://www.friesian.com/notes/oldking.htm The Egyptian Old Kingdom, Sumer and Akkad] * [https://web.archive.org/web/20121231174140/http://www.reshafim.org.il/ad/egypt/the_end_of_the_old_kingdom.htm The End of the Old Kingdom] *[https://www.nature.com/articles/d41586-022-00157-9 Michael Marshall (26 January 2022), 'Did a mega drought topple empires 4,200 years ago?', ''Nature''] *[https://www.allesbleibtanders.com/en/modules/klimaflucht/ Digital exhibition of the crisis around 2200 BCE on the Iberian Peninsula.]

{{DEFAULTSORT:4.2 Kiloyear Event}} [[Category:Droughts]] [[Category:23rd century BC]] [[Category:22nd century BC]] [[Category:23rd century BC in Egypt]] [[Category:22nd century BC in Egypt]] [[Category:Ancient Near East]] [[Category:Old Kingdom of Egypt]] [[Category:First Intermediate Period of Egypt]] [[Category:Akkadian Empire]] [[Category:Pepi II Neferkare]]