4.2-kiloyear event

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 most likely 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 Civilization, with some of its population moving southeast to follow the movement of their desired habitat, as well as the migration of Indo-European-speaking people into India. Some scientists disagree with that conclusion, citing evidence that the event was not a global drought and did not happen in a clear timeline.