Scientists finally uncovered why the Indus Valley Civilization collapsed

Prolonged droughts, each lasting over 85 years, likely contributed to the decline of the Indus Valley Civilization, according to a study published in Communications Earth & Environment. The research offers new insight into the gradual reduction of complexity within the ancient society, which flourished near the present-day India-Pakistan border.

Indus Valley Civilization and its Decline

The Indus Valley Civilization (IVC) was one of the earliest known urban cultures, thriving between 5,000 and 3,500 years ago along the Indus River and its tributaries in present-day Pakistan and northwest India. At its peak, from 4,500 to 3,900 years ago, the society was recognized for its well-planned cities, extensive infrastructure, and advanced water management systems. Despite this development, the reasons for its long-term decline have remained unclear to researchers.

Climate Simulations Reveal Temperature Rise and Reduced Rainfall

Vimal Mishra and colleagues reconstructed climate patterns across the region spanning 5,000 to 3,000 years ago to investigate past conditions. Their analysis combined climate modeling with indirect indicators of environmental change, including chemical signatures from stalactites and stalagmites in Indian caves and water level histories from five lakes in northwest India. The data indicates a temperature increase of approximately 0.5 degrees Celsius during this period, alongside a 10 to 20 percent reduction in annual rainfall.

The team identified four extended drought periods occurring between 4,450 and 3,400 years ago. Each drought lasted more than 85 years and impacted between 65 percent and 91 percent of the area associated with the IVC, suggesting widespread and lasting effects on water availability.

Shifts in Settlement Patterns During Prolonged Dry Periods

The authors suggest these droughts likely influenced settlement locations. Between 5,000 and 4,500 years ago, most communities were located in areas with higher rainfall. After 4,500 years ago, settlement patterns shifted, with populations moving closer to the Indus River, potentially reflecting a growing reliance on a more dependable water source as drought conditions worsened.

A particularly long drought, lasting 113 years between 3,531 and 3,418 years ago, coincides with archaeological evidence of widespread deurbanization in the region. The researchers conclude that the Indus Valley Civilization did not experience a sudden collapse due to a single climate event, but rather a prolonged and uneven decline where repeated droughts were a significant contributing factor.