4.2-kiloyear event
It has been hypothesised to have caused the collapse of the Old Kingdom in Egypt, the Akkadian Empire in Mesopotamia, and the Liangzhu culture in the lower Yangtze River area.
[4][5] 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,[6] as well as the migration of Indo-European-speaking people into India.
[8] 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.
[11][12] Evidence suggests increased El Niño–Southern Oscillation (ENSO) variability also played a role in generating the climatic conditions associated with the event.
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.
[34] 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.
According to M. Mejías Moreno, who reported the first 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.
[41] At the site of Sidi Ali in the Middle Atlas, δ18O values indicate not a dry spell but a centennial-scale period of cooler and more humid climate.
[42] In c. 2150 BC, Egypt was hit by a series of exceptionally low Nile floods that may have influenced the collapse of the centralised government of the Old Kingdom after a famine.
[19] A study of fossil corals in Oman provides evidence that prolonged winter shamal seasons, around 2200 BC, led to the 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.
[53][54] The Siberian High increased in area and magnitude, which blocked moisture-carrying westerly winds, causing intense aridity in Central Asia.
[57] The area around PankangTeng Tso Lake in the Tawang district of Arunachal Pradesh had cold and dry conditions and was dominated by subalpine vegetation.
[7]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.
[64] Aridity increased, with the Ghaggar-Hakra River retracting its reach towards the foothills of the Himalayas,[64][67][68] leading to erratic and less-extensive floods, which made inundation agriculture less sustainable.
[74] Northeastern China was strongly affected;[75] proxy records from Hulun Lake in Inner Mongolia reveal a major dry event from 4210–3840 BP.
[72] δ18O 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.
[76] In the Korean Peninsula, the 4.2 ka event was associated with significant aridification, measured by the large decline in arboreal pollen percentage (AP).
[83] Around 2000 BC, Longshan was displaced by the Yueshi culture, which had fewer and less-sophisticated artifacts of ceramic and bronze.The Liangzhu civilization in the lower reaches of the Yangtze River also declined during the same period.
