jan 1, 3500 BC - End of the African humid period, possibly linked to the Piora Oscillation: a rapid and intense aridification event.

Description:

...which probably started the current Sahara Desert dry phase and a population increase in the Nile Valley due to migrations from nearby regions. It is also believed this event contributed to the end of the Ubaid period in Mesopotamia.

https://en.wikipedia.org/wiki/African_humid_period

https://en.wikipedia.org/wiki/Piora_Oscillation

https://en.wikipedia.org/wiki/Nile

https://en.wikipedia.org/wiki/Ubaid_period

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