A REVIEW ON THE RELATIONSHIP BETWEEN THE PACIFIC OCEAN, INDIAN SUMMER MONSOON AND THE DECLINE OF HARAPPA

Abstract

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-Nin˜o events and is positivelycorrelated with temperatures in the western equatorial Pacific. The development, decline and disappearance of Harappan civilization between w9000 and 3000 cal BP occurred against an environmentalbackdrop of decreasing northern hemisphere insolation, decreasing western equatorial Pacific SST’s andincreasing frequency and amplitude of ENSO events. Macro-regional paleo-hydrological records sensitiveto the strength of the Indian Monsoon generally show increasing aridity and variability over this periodconsistent with the changes in insolation and the Pacific Ocean. Evidence also exists for a pronouncedincrease in aridity in the late Holocene and a particularly steep drought at w4200 cal BP. However, theinitiation of transition to the deurbanized Late Harappan Phase may have commenced some 300 yearsprior to that event and the persistence of the Late Harappan Phase until 3000 cal BP suggests a morecomplex story than societal collapse and disappearance induced by one discrete environmental event. Thecontinued long-term trend towards greater aridity and interannual variability during the Late HarappanPhase would have continued to presentwater resource challenges beyond the 4200 cal BP event and couldhave been a contributing factor to Harappan decline and ultimate disappearance.