A new study provides the strongest evidence yet that long, recurring droughts played a decisive role in reshaping — and ultimately weakening — the Indus Valley Civilization, one of the world’s earliest urban societies.
Flourishing between roughly 5,000 and 3,500 years ago, this civilization built sophisticated cities such as Harappa and Mohenjo Daro, managed extensive water systems, and maintained a long-distance trade network across South Asia and into Mesopotamia. For decades, researchers have debated why these vibrant urban centers declined. New findings point toward a prolonged, basin-wide collapse in water availability.
The research, published in Communications Earth & Environment, combines several lines of evidence to recreate ancient climate and river conditions throughout the Indus region. To fill in gaps in the scattered paleoclimate record, the team integrated high-resolution proxies, such as speleothem growth rates from caves and lake sediment changes, with hydrological reconstructions driven by three independent climate simulations. These models span the period from 6,000 years ago through the late preindustrial era and offer a rare, basin-scale view of how rainfall, temperature, and river discharge changed over time.
These records show a drying trend starting from approximately 5,000 years ago, indicated by decreasing rainfall and increasing temperatures that increased evaporation. The researchers identified several major drought periods within this shift, all longer than 85 years. Four are significant between approximately 4,445 and 3,418 years before the present. The most severe, at about 3,757 years ago, may have lasted for a century and a half, with rainfall reduced by roughly 13 percent, affecting almost the entire Indus basin.
When rainfall records were run through a hydrological model, the simulations indicated a steep decline in river discharge during these events. In regions around large urban centers, flow in some waterways fell by over 12 percent. Such declines would have placed significant pressure on farming, urban water supply, and food storage systems — the fundamental infrastructure that supported dense populations. Archaeological evidence matches these patterns: as water levels decreased, communities began to shift away from the traditional urban core toward areas with more reliable water sources, such as the Ganges plains and coastal regions.
The research also puts these droughts into a larger climatic context. Changes in ocean temperatures — such as El Niño-like warming in the Pacific and cooler conditions in the North Atlantic — appear to correspond with many of the intense drought intervals. These patterns are known to weaken the Indian Summer Monsoon, further reducing rainfall over the Indus basin.
The researchers acknowledge that social and economic changes also contributed to the transformation of the Harappan world, but the new analysis confirms that climate-driven water stress was a critical factor. It now seems that, rather than collapsing suddenly, the civilization’s urban decline reflects a long, complex process driven by repeated, widespread droughts that eroded its environmental foundations.
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