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. The drought may have initiated southeastward habitat tracking within the Indus Valley Civilization.

The 4.2 kiloyear BP event 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. However, this theory has been criticised by archaeologists, with political causes for the collapse of these polities thought to be more probable.

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.

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