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.
The 4.2-kiloyear (thousand years) BParidification event (long-term drought) was one of the most severe climatic events of the Holocene epoch. It defines the beginning of the current Meghalayan age in the Holocene epoch.
Recent studies show that the "motilla" sites from the Bronze Age in La Mancha may be the most ancient system of groundwater collection in the Iberian Peninsula. ... These were built during the Climatic Event 4.2 ka cal BP in a time of environmental stress due to a period of severe, prolonged drought.
The authors' analysis verified a relationship between the geological substrate and the spatial distribution of the motillas.
In c. 2150 BC, Egypt was hit by a series of exceptionally low Nilefloods that may have influenced the collapse of the centralised government of the Old Kingdom after a famine.
The aridification of Mesopotamia may have been related to the onset of cooler sea-surface temperatures in the 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 temperatures are anomalously cool. The headwaters of the Tigris and Euphrates rivers are fed by elevation-induced capture of winter Merranean rainfall.
The Akkadian Empire in 2300 BC was the second civilisation to subsume independent societies into a single state (the first being ancient Egypt in around 3100 BC). It has been claimed that the collapse of the state was influenced by a wide-ranging, centuries-long drought. Archaeological evidence documents widespread abandonment of the agricultural plains of northern Mesopotamia and dramatic influxes of refugees into southern Mesopotamia, around 2170 BC. 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 and destroyed it around 2115 BC. Widespread agricultural change in the Near East is visible at the end of the 3rd millennium BC.
Resettlement of the northern plains by smaller sedentary populations occurred near 1900 BC, three centuries after the collapse.
A study of fossil corals in Oman provides evidence that prolonged winter shamal seasons, around 4200 years ago, 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.
In the 2nd millennium BC, widespread aridification occurred in the Eurasian steppes and in South Asia. On the steppes, the vegetation changed, driving "higher mobility and transition to the nomadic cattle breeding."[note 1][note 2] Water shortage also strongly affected South Asia:
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.
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. As of 2016[update], many scholars believed that drought and a decline in trade with Egypt and Mesopotamia caused the collapse of the Indus civilisation. The Ghaggar-Hakra system was rain-fed, and water supply depended on the monsoons. The Indus Valley climate grew significantly cooler and drier from about 1800 BC, which is linked to a contemporary general weakening of the monsoon. Aridity increased, with the Ghaggar-Hakra River retracting its reach towards the foothills of the Himalayas, leading to erratic and less-extensive floods, which made inundation agriculture less sustainable. Aridification reduced the water supply enough to cause the civilisation's demise, and to scatter its population eastward.
^Demkina et al. (2017): "In the second millennium 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 Catacomb and Post-Catacomb time stipulating their higher mobility and transition to the nomadic cattle breeding."
^Riehl, S. (2008). "Climate and agriculture in the ancient Near East: a synthesis of the archaeobotanical and stable carbon isotope evidence". Vegetation History and Archaeobotany. 17 (1): 43–51. doi:10.1007/s00334-008-0156-8. S2CID128622745.
^Watanabe, Takaaki K.; Watanabe, Tsuyoshi; Yamazaki, Atsuko; Pfeiffer, Miriam (2019). "Oman corals suggest that a stronger winter shamal season caused the Akkadian Empire (Mesopotamia) collapse". Geology. GeoScienceWorld. 47 (12): 1141–1145. Bibcode:2019Geo....47.1141W. doi:10.1130/G46604.1. S2CID204781389.
^Gao, Huazhong; Zhu, Cheng; Xu, Weifeng (2007). "Environmental change and cultural response around 4200 cal. yr BP in the Yishu River Basin, Shandong". Journal of Geographical Sciences. 17 (3): 285–292. doi:10.1007/s11442-007-0285-5. S2CID186227589.
Weiss, H., ed. (2012). Seven Generations Since the Fall of Akkad. Wiesbaden: Harrassowitz. ISBN9783447068239.
Weiss, H. (2000). "Beyond the Younger Dryas: Collapse as Adaptation to Abrupt Climate Change in Ancient West Asia and the Eastern Merranean". In Bawden, G.; Reycraft, R. M. (eds.). Environmental Disaster and the Archaeology of Human Response. Albuquerque, NM: Maxwell Museum of Anthropology. pp. 63–74. ISBN0-912535-14-8.