Climate change: Major droughts linked to ancient Indus Valley Civilization’s collapse

Successive major droughts, each lasting longer than 85 years, were likely a key factor in the eventual fall of the Indus Valley Civilization, according to a paper in Communications Earth & Environment. The findings may help explain why this major ancient civilization — a contemporary of ancient Egypt located around the modern India-Pakistan border — slowly declined, and highlights how environmental factors could shape ancient societies.

The Indus Valley Civilization (IVC) was one of the earliest urban cultures, existing between 5,000 and 3,500 years ago around the Indus River and its tributaries in modern-day Pakistan and northwest India. At its peak, between 4,500 and 3,900 years ago, its culture was characterised by advanced cities and sophisticated water management systems. However, the reasons for the civilization’s prolonged decline after this peak are not yet fully understood.

Vimal Mishra and colleagues simulated climate conditions across the IVC region between 5,000 and 3,000 years ago. They combined these results with data from several indirect measures of past climate conditions, including the geochemistry of stalactites and stalagmites in two Indian caves, and the water level records of five lakes in northwest India. From this, the authors identified a temperature increase over the period of approximately 0.5 degrees Celsius, and a decrease in average annual rainfall across the region of between 10% and 20%. They were also able to identify four long-term droughts between 4,450 and 3,400 years ago, each lasting more than 85 years and affecting between 65% and 91% of the IVC region.

The authors suggest that these droughts influenced settlement location choice in the IVC. Between 5,000 and 4,500 years ago, they report that settlements were concentrated in areas with more rainfall. However, from 4,500 years ago onwards, settlements shifted closer to the Indus River, probably as the droughts started to affect water availability. The final 113-year-long drought that they identify, between 3,531 and 3,418 years ago, coincides with archaeological evidence of major deurbanization in the IVC. The authors conclude that the IVC likely did not collapse suddenly as a result of any one climate event, but instead declined slowly, with the prolonged droughts a major contributory factor.