Potential influence of the Pacific Ocean on the Indian summer monsoon and Harappan decline

Harappan agriculture and urban water usage were likely sensitive to variations in Indian Summer Monsoon (ISM) rainfall. The strength of the ISM displays negative correlations with sea surface temperatures (SST's) in the eastern and central equatorial Pacific Ocean and with El Niño events and is positively correlated with temperatures in the western equatorial Pacific. The development, decline and disappearance of Harappan civilization between ∼9000 and 3000 cal BP occurred against an environmental backdrop of decreasing northern hemisphere insolation, decreasing western equatorial Pacific SST's and increasing frequency and amplitude of ENSO events. Macro-regional paleohydrological records sensitive to the strength of the Indian Monsoon generally show increasing aridity and variability over this period consistent with the changes in insolation and the Pacific Ocean. Evidence also exists for a pronounced increase in aridity in the late Holocene and a particularly steep drought at ∼4200 cal BP. However, the initiation of transition to the deurbanized Late Harappan Phase may have commenced some 300 years prior to that event and the persistence of the Late Harappan Phase until 3000 cal BP suggests a more complex story than societal collapse and disappearance induced by one discrete environmental event. The continued long-term trend towards greater aridity and interannual variability during the Late Harappan Phase would have continued to present water resource challenges beyond the 4200 cal BP event and could have been a contributing factor to Harappan decline and ultimate disappearance.