CIRES Fellow Balaji Rajagopalan combined his skills in hydrology with a passion for paleoclimatology to address a question that has long remained a mystery: Why did the Harappan Civilization in the Indus Valley disappear? 

Rajagopalan and a group of international authors provided answers last week in Nature Communications Earth & Environment. They used paleoclimate data and computer models to re-create regional hydrologic conditions from 3,000 to 1,000 years ago. The work explains why the ancient bustling and affluent Harappan civilization located in the Indus River Valley region of modern-day Northwest India and Eastern Pakistan, vanished. 

“Declining rainfall since the mid-Holocene is widely considered the leading cause in the Harappan’s disappearance,” Rajagopalan said. “In this work, we offer richer insights into the variability of the regional hydrology—river flow, soil moisture, and consequently, vegetation—consistent with declining rainfall, that likely catalyzed their decline and eventual disappearance.”

Overall, researchers found increased temperatures, drought, and reduced rainfall dried up bodies of water and reduced flow in the Indus River, forcing the Harappans to relocate to more productive areas with fresh water sources. Lead author Hiren Solanki from the Indian Institute of Technology in Gandhinagar, India, said the Harappan switched crops, diversified trade, and relocated settlements to make their society more resilient to climate change.

The authors identified four severe droughts that affected more than 80 percent of the region, and each drought lasted 85 years or more, and the sequence spanned several centuries. 

They found monsoons occurring 3,000 years ago, the mature and bustling phase of the civilization, brought increased rainfall across the region, creating wetter conditions than we see today. Rajagopalan said a warmer northern hemisphere and a cooler tropical Pacific Ocean during this time period—similar to ‘La Nina’— explains the robust monsoon activity resulting in higher flows in the Indus River and sustained water bodies, enabling settlements to congregate around areas with heavy rain and water resources. Then, the tropical Pacific Ocean began warming, creating drier conditions that decreased rainfall and increased temperatures. 

Results suggest climate shifts coincided with the migration of the Harappan people. Yet, the authors acknowledge climate shifts aren’t the only reason—the full story is more complex. 

“Of course droughts alone wouldn’t be the sole reason for the downfall,” Rajagopalan said. “The civilization lasted a very long time despite the persistent drought conditions with trade and ingenuity, and the rise and decline of Harappan societies along with the hydroclimate variability of that epoch offers useful lessons in our current endeavors of enabling sustainable and resilient societies in a future warmer climate.”