The 4.2-kiloyear (thousand years) BP aridification event (long-term
drought), also known as the 4.2 ka event , was one of the most severe
climatic events of the Holocene epoch. It defines the beginning of the current
Meghalayan age in the Holocene epoch.
Starting around 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, 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 Civilisation, with some of its population moving southeastward to
follow the movement of their desired habitat, as well as the migration of
Indo-European-speaking people into India.