Hotter temperatures and repeated droughts fragmented the Indus Valley Civilisation around 4000 years ago in modern-day Pakistan and India. This ancient urban culture, known for advanced water management, gradually declined as water sources dwindled. The findings highlight potential lessons for today's climate challenges.
The Indus Valley Civilisation, also called the Harappan civilisation, thrived along the Indus River and its tributaries, building cities larger than those in ancient Egypt and Mesopotamia. Harappa, one of its key sites, supported a population of 35,000 people. The society excelled in water management, featuring giant storage cisterns and sewage systems made of terracotta pipes and brick channels. Residents irrigated crops like wheat and barley using annual river floods and revered rivers in their practices.
Research by Vimal Mishra at the Indian Institute of Technology Gandhinagar reveals that four major droughts, each lasting at least 85 years, struck between 4400 and 3400 years ago. "Four different droughts occurred in between pre-Harappan to later Harappan periods," Mishra noted. Temperatures in the region rose by about 0.5°C, causing the Indus River's level to fall. In response, communities migrated toward areas with better water availability, such as the Himalayan foothills and the Ganga River plain. "In response to these events, there has been a continuous migration towards where the water availability was better," Mishra added.
Scientists used three climate models to estimate rainfall, cross-referenced with data from stalactites, stalagmites, and lake sediments. The warming and drying likely began with natural patterns like El Niño and the Atlantic Multidecadal Oscillation, worsened by vegetation loss and dust pollution. Earlier studies pointed to a sudden collapse after a 4200-year-old megadrought, but this work suggests a more gradual fragmentation.
Sebastian Breitenbach at Northumbria University praised the study's blend of modeling and proxy data, though he recommended including evapotranspiration in future analyses. He emphasized adaptation strategies like water storage and groundwater conservation amid rapid modern warming. "These studies can give us a warning shot," Breitenbach said. "They can give us a glimpse of what might happen in the future."
The research appears in Communications Earth & Environment (DOI: 10.1038/s43247-025-02901-1).