4.2 kiloyear event
The 4.2 kiloyear BP aridification event was one of the most severe climatic events of the Holocene period. Starting in about 2200 BC, it probably lasted the entire 22nd century BC. It has been hypothesised to have caused the collapse of the Old Kingdom in Egypt as well as the Akkadian Empire in Mesopotamia, and the Liangzhu culture in the lower Yangtze River area. The drought may also have initiated the collapse of the Indus Valley Civilization, and southeastward habitat tracking of its population, as well as the migration of Indo-European speaking people into India.
A phase of intense aridity about 4.2 ka BP is recorded across North Africa, the Middle East, the Red Sea, the Arabian peninsula, the Indian subcontinent, and midcontinental North America. Glaciers throughout the mountain ranges of western Canada advanced at about this time. Evidence has also been found in an Italian cave flowstone, the Kilimanjaro Ice sheet, and in Andean glacier ice. The onset of the aridification in Mesopotamia about 4100 BP also coincided with a cooling event in the North Atlantic, known as Bond event 3. Despite this, evidence for the 4.2 kyr event in northern Europe is ambiguous, suggesting the origin and impact of this event is spatially complex.
According to Moreno et al., who reported the first palaeohydrogeological interdisciplinary research in La Mancha, Spain,
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, the Old Kingdom was hit by a series of exceptionally low Nile floods. It has been suggested that may have impacted the collapse of the centralised government in ancient Egypt. Contemporary texts claim that famines, social disorder and fragmentation subsequently occurred. There may however be a strong element of political bias to these writings since the Egyptian elite believed the stability of Egypt was dependent on a unified state, and they would have been motivated to present decentralisation as disastrous. After a phase of rehabilitation and restoration of order in various provinces, Egypt was eventually reunified within a new paradigm of kingship. The process of recovery depended on capable provincial administrators, a more formalised justice system, irrigation projects, and an administrative reform.
In the Persian Gulf region, there is a sudden change in settlement pattern, style of pottery and tombs at this time. The 22nd century BC drought marks the end of the Umm an-Nar Culture and the change to the Wadi Suq period.
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 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. 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.
South central Asia and IndiaEdit
In the second millennium BCE widespread aridization lead to water shortages and ecological changes in both the Eurasian steppes and south Asia. At the steppes, humidization lead a change of vegetation, triggering "higher mobility and transition to the nomadic cattle breeding."[note 1][note 2] Water shortage also had a strong impact in south Asia:
This time was one of great upheaval for ecological reasons. Prolonged failure of rains caused acute water shortage in a large area, 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.
The drought may have initiated southeastward habitat tracking within the Indus Valley Civilization. The Indus Valley Civilisation was localised, that is, urban centers disappeared and were replaced by local cultures, due to a climate change that is also signalled for the neighbouring areas of the Middle East. As of 2016[update] many scholars believe 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 BCE, linked to a general weakening of the monsoon at that time. The Indian monsoon declined and aridity increased, with the Ghaggar-Hakra retracting its reach towards the foothills of the Himalaya, leading to erratic and less extensive floods that made inundation agriculture less sustainable. Aridification reduced the water supply enough to cause the civilisation's demise, and to scatter its population eastward.
The drought may have caused the collapse of Neolithic Cultures around Central China during the late third millennium BC. At the same time, the middle reaches of the Yellow River saw a series of extraordinary floods. In the Yishu River Basin, the flourishing Longshan culture was hit by a cooling that made the paddies shortfall in output or even no seeds were gathered. The scarcity in natural resource led to substantial decrease in population and subsequent drop in archaeological sites. In about 2000 BC, Longshan was displaced by the Yueshi culture, which was relatively underdeveloped.
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