The culprit behind the mysterious disappearance of one of the most advanced urban civilizations at the time, contemporaries to Mesopotamians and Egyptians, has finally been identified: a series of severe, long-lasting droughts that dried rivers across the Indus Valley more than 4,000 years ago.
A study published in Communications Earth & Environment reveals that these dry spells likely pushed communities to move, change crops, and fundamentally reorganize their civilization.
“The most surprising finding is that the Harappan decline was driven not by a single catastrophic event, but by repeated, long, and intensifying river droughts lasting centuries,” the lead author of the study, Hiren Solanki, told New Atlas via email.
The Indus Valley Civilization, also known as the Harappan empire, rose to prominence around 5,000 years ago along the fertile plains of northwest India and Pakistan. During its mature period, the society developed well-planned cities with grid systems of roads, advanced water management, sophisticated drainage, and large granaries. The Harappan were also among the first to cultivate and process cotton.
But around 3,900 years ago, things started falling apart and cities emptied out. Historians have presented a number of hypotheses to explain the reasons for the culture’s demise, pointing to floods, shifting rivers, and invasions.
Now, a team of scientists led by Hiren Solanki blended evidence from lake sediments, cave deposits, and other natural archives with high-tech climate models to reconstruct how water moved through the Indus basin around 3,000 years ago. The team found that the decline was not due to a single drought, but a sequence of four major droughts. Researchers dubbed these four events D1, D2, D3 and D4.
Hiren told New Atlas that the most severe droughts were D2 and D3, which lasted between 102 and 164 years, impacting over 90% of the Indus Valley region during the transition from the mature to the late Harappan period.
This drying didn’t lead to an abrupt “collapse” but to a gradual reorganization and migration. The study supports a “push-pull” migration, in which a decline in the Indus river discharge pushed communities to relocate; meanwhile, fertile plains in the foothills of the Himalayas, with a reliable water source, served as a pull factor.
“Smaller communities, diversified crops (mostly in Saurashtra), and locations with stable rainfall (foothills of the Himalaya) proved most resilient during the long climatic downturn,” Hiren Solanki told us.
But what flipped the climatic conditions? Hiren Solanki describes the Pacific and North Atlantic oceans influencing the Indian monsoon through large-scale atmospheric “teleconnections.” When the Pacific warms in an El Niño–like pattern, it weakens the monsoon circulation and reduces summer rainfall over South Asia. At the same time, a cooler North Atlantic (negative AMO phase) suppresses moisture transport into the monsoon system. It shifts atmospheric pressure patterns, further weakening the monsoon.
Initially, the winter rainfall buffered drought impacts and helped the society during the Pre-to-Mature Harappan period. However, during the late Harappan period, the reduction in the winter rainfall hindered the last support for agriculture in the central regions. This fragmented the big urban regions leading to rural relocation, ending the major Harappan period by around 1700 BCE.
The study has been published in Communications Earth & Environment.
