Harappa archaeological site, cradle of the Harappan civilization, in the Indus Valley
It did not disappear suddenly: Harappan society would have adapted to the drought and fragmented into smaller communities, according to a new study.
For decades, the decline of one of the world’s first great urban civilizations was often described as a sudden and mysterious collapse, but new research points to a different scenario.
Instead of an abrupt disappearance, the Indus Valley Civilizationalso known as Harapean, will have undergone a slow transformation, driven by persistent droughts in the rivers that supported cities and agriculture.
The study, in Communications Earth & Environment in late November, argues that the history of the Indus civilization, which flourished more than 5,000 years ago in the territory of present-day Pakistan and northwestern India, is deeply linked to a gradual degradation of water security.
According to the authors, cited by , prolonged episodes of river drought, lasting from decades to centuries, have cumulatively weakened the capacity to maintain large urban centers, leading populations to adapt, move and reorganize themselves socially.
The Indus Valley Civilization is considered one of the most extensive and advanced urban societies of the Bronze Age. At its peak, it may have gathered millions of people and built cities comparable to, and in some respects superior to, those of contemporary civilizations in Mesopotamia and Egypt.
Centers like Mohenjo-daro e Harappa presented a grid urban layout, buildings constructed with standardized bricks fired in kilns, and a level of uniformity that suggests technical coordination and, possibly, sophisticated forms of governance.
One of the most striking features was the water management. Cities had elaborate drainage systems under streets, covered sewers, and, in many homes, bathing areas connected to sanitation networks. Reservoirs, wells and canals guaranteed supply in a region dependent on seasonal rivers and monsoon rains.
Economic activity included specialized crafts (metallurgy, bead production, ceramics) and commercial networks that linked the Indus world to Central Asia and Mesopotamia.
Despite this material sophistication, great mysteries about the place: the language remains unknown, the writing remains undeciphered and the political organization is the subject of enormous debate among scholars.
What is most consensual is that, around 3,900 years ago, large urban centers began to fragment. The causes have been attributed, over time, to climate change, social instability, changes in the course of rivers and economic transformations.
The new investigation puts the focus on the water factor, not just precipitation, but the actual river runoff.
To reach this conclusion, the team combined high-resolution paleoclimate records (such as cave deposits and lake sediments) with advanced hydrological modeling fed by long-term climate simulations. This method made it possible to reconstruct, on a river basin scale and over thousands of years, how the flow of the rivers that supported the Harappan cities varied.
The results point to “severe and persistent droughts” which affected the Indus basin approximately between 4,400 and 3,400 years before present. More than just occasional episodes of scarcity, researchers identified four major phases of drought, each lasting more than 85 years, during the transition from the “mature” Harappan period to the “late” period.
One of these droughts lasted approximately 160 years and covered more than 90% of the geographical area associated with civilization, according to the analysis. During these intervals, precipitation decreased significantly, flows decreased and temperatures gradually increased, a potentially devastating combination for agriculture, commerce and urban water supply systems.
And these droughts were not isolated or localized phenomena. Climate simulations indicate that periods of lower river runoff coincided with widespread precipitation deficits in the region, intensifying large-scale water stress. In practical terms, this would mean a “double pressure”: less rain and less water in rivers at the same time, compromising the safety margins of cities dependent on seasonal predictability.
Still, the authors avoid the narrative of a collapse considered simple. What emerges is a gradual metamorphosis. As the droughts repeated and deepened, populations appear to have dispersed from large urban centers to areas with greater water reliability, such as the foothills of the Himalayasthe plains of Ganges and coastal areas of western India. Modeling suggests that while the central Indus region faced the largest drops in flow, other areas maintained relatively more stable water availability, making them more attractive destinations for resettlement.
The study also contributes to reassessing the debate surrounding the so-called “”, a global climate anomaly often associated with crises in ancient civilizations, from the Middle East to North Africa. The analysis proposes that, although there was a relevant episode of drought during this period, it was not a single catastrophic moment: it would have been part of a longer sequence of dry phases that, over centuries, repeatedly put pressure on the Harappan social and economic systems.
The study refers to archaeological evidence compatible with changes in agricultural strategiesincluding greater reliance on more drought-tolerant cereals such as millet. Commercial networks, including maritime links, may have acted as buffers in some regions. Rather than disappearing, Harappan society fragmented into smaller, more localized communities, potentially better suited to unpredictable water conditions.
