4.2-kiloyear event

The 4.2-kiloyear (thousand years) BP aridification event (long-term drought) 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 as well as 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. Some scientists disagree with that conclusion, citing evidence that the event was not a global drought and did not happen in a clear timeline. 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 about that 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 in about 4100 BP also coincided with a cooling event in the North Atlantic, known as Bond event 3. Despite the geographic diversity of these examples, evidence for the 4.2 kyr event in Northern Europe is ambiguous, which suggests that the origins and effects of the event are spatially complex. In 2018, the International Commission on Stratigraphy divided the Holocene epoch into three periods, with the late Holocene from approximately 2250 BC onwards designated as the Meghalayan stage/age. 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. However, justification for this division is debated as the event was not a global drought and did not happen within a clear timeframe.