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 Kilimanjaroice sheet, and in Andeanglacier ice. The onset of the aridification in Mesopotamia in 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 effect of this event is spatially complex. In 2018, the International Commission on Stratigraphy divided the Holocene epoch into three, with the late Holocene being called the Meghalayan stage/age starting around 2250 BC. The boundary stratotype is a speleothem in Mawmluh cave in India, and the global auxiliary stratotype is an ice core from Mount Logan in Canada. Some scientists disagree with this conclusion and point out that the event was neither a global drought nor did it happen in a clear timeline. Jessica Tierney, a paleoclimatologist at the University of Arizona in Tucson, assumes that scientists mistakenly "lumped together evidence of other droughts and wet periods, sometimes centuries away from the event".
Effects
Iberian peninsula
On the Iberian Peninsula, the construction of motillas-type settlements in the period after 2200 BC is believed to be the consequence of the severe aridification that affected this area. According to Mejías et al., who reported the first palaeohydrogeologicalinterdisciplinary research in La Mancha, Spain, The authors' analysis verified a relationship between the geological substrate and the spatial distribution of the motillas.
Ancient Egypt
In c. 2150 BC, the Old Kingdom, then ruled by the Ninth Dynasty, was hit by a series of exceptionally low Nilefloods. This may have influenced the collapse of centralised government in ancient Egypt after a famine.
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 culture and the change to the Wadi Suq culture.
Mesopotamia
The aridification of Mesopotamia may have been related to the onset of cooler sea-surface temperatures in the North Atlantic, as analysis of the modern instrumental record shows that large 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 civilisation to subsume independent societies into a single state. 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 4,200 years ago, led to the salinization of the irrigated fields; hence, a dramatic decrease in crop production triggered a widespread famine and eventually the collapse of the ancient Akkadian Empire.
South and Central Asia
In the 2nd millennium BC, widespread aridification occurred in the Eurasian steppes and South Asia. On the steppes, the vegetation changed, driving "higher mobility and transition to the nomadic cattle breeding". Water shortage also strongly affected South Asia: Urban centers of the Indus Valley Civilisation were abandoned and replaced by disparate local cultures, due to the same climate change that affected the neighbouring regions to the west., 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, linked to a general weakening of the monsoon at that time. The Indian monsoon declined and aridity increased, with the Ghaggar-Hakra River retracting its reach towards the foothills of the Himalayas, 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.
