4.2-kiloyear event

"Some of the earliest civilizations are known from the Indus (Harappan) and Yellow River (Qijia and Longshan) valleys, developing along with those in Mesopotamia and Egypt. These cultures collapsed around 4200 y BP at a time of rapid monsoon weakening, owing to direct negative impact on regional agriculture and more indirectly through changes in the river systems."

"M. Berkelhammer led an international team to a cave in Cherrapunji, Meghalaya, ‘among the wettest locations on Earth with an annual average precipitation in excess of 11,000 mm’, and studied the isotopic variations in a stalagmite: Oxygen 18 isotope as an index of precipitation, and the Uranium-Thorium method for absolute dating of the stalagmite, which went back almost 12,000 years for a growth of nearly 2 m. The results highlighted a ‘dramatic event ... ~ 4000 years ago when, over the course of approximately a decade, isotopic values abruptly rose above any seen during the early to mid-Holocene and remained at this anomalous state for almost two centuries.’ This suggested either ‘a shift toward an earlier Indian Summer Monsoon withdrawal or a general decline in the total amount of monsoon precipitation.’ The study’s ‘tight age constraints of the record show with a high degree of certainty that much of the documented deurbanization of the Indus Valley at 3.9 kyr B.P. occurred after multiple decades of a shift in the monsoon’s character....’"

"The 4.2 ka event is coherent with the termination of urban Harappan civilization in the Indus valley."