Decline of IVC and El Nino effect


4.2 kilo-year event
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This article of Wikipedia describes the change in El Nino, which affected civilizations all over the world in that process we should include decline of IVC also because the period coincides with the decline of other civilizations. The tectonic plate theory  on IVC decline may not be correct , most plausible reason is the extensive drought occurred during that period, any how such  a drought would not have wiped out IVC because Indus river is a perennial river fed by mountain glaciers, the effect of of drought would not have been as catastrophic as it was in Egypt and Mesopotamia.

The 4.2 kiloyear BP aridification event was one of the most severe climatic events of the Holocene period in terms of impact on cultural upheaval.[1] Starting in ≈2200 BC, it probably lasted the entire 22nd century BC. It is very likely to have caused the collapse of the Old Kingdom inEgypt as well as the Akkadian Empire in Mesopotamia.[2] Also, the drought may have initiated southeastward habitat tracking within the Harappan cultural domain.[3]

Evidence

A phase of intense aridity in ≈4.2 ka BP is well recorded across North Africa,[4] the Middle East,[5] the Red Sea,[6] the Arabian peninsula,[7] theIndian subcontinent,[3] and even midcontinental North America.[8] Glaciers throughout the mountain ranges of western Canada advanced at about this time.[9] Evidence has also been found in an Italian cave flowstone,[10] and in Andean glacier ice.[11]

The onset of the aridification in Mesopotamia near 4100 B.P also coincided with a cooling event in the North Atlantic, known as Bond event3.[1][12][13]

Aftermath

In ca. 2150 BC the Old Kingdom was hit by a series of exceptionally low Nile floods, which was instrumental in the sudden collapse of centralized government in ancient Egypt.[14] Famines, social disorder, and fragmentation during a period of approximately 40 years were followed by 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, the deployment of the idea of justice, irrigation projects, and an administrative reform.
Mesopotamia

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.[15] The headwaters of the Tigris and Euphrates Rivers are fed by elevation-induced capture of winter Mediterranean rainfall.

The Akkadian Empire—which in 2300 B.C. was the first to subsume independent societies into a single state—was brought low by a wide-ranging, centuries-long drought.[16] Archaeological evidence documents widespread abandonment of the agricultural plains of northern Mesopotamia and dramatic influxes of refugees into southern Mesopotamia around 2170 BC.[17] 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 Guti, who originally inhabited the Zagros Mountains, defeated the demoralized Akkadian army, took Akkad, and destroyed it around 2115 BC. Widespread agricultural change in the Near East is visible at the end of the third millennium BC.[18]

Resettlement of the northern plains by smaller, sedentary populations occurred near 1900 BC, three centuries after the collapse.[17]
Arabian peninsula

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 al-Nar period and the change to the Wadi Suq period.[7]
[edit]China

The drought may have caused the collapse of Neolithic Cultures around Central China during the late third millennium BC.[19] 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.[20] About 4000 cal. yr BP Longshan culture was displaced by Yueshi culture which was relatively underdeveloped, simple and unsophisticated.
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