Centuries of repeated droughts linked to the Indus Valley Civilisation’s decline

The Indus Valley Civilisation (IVC) did not collapse abruptly but declined gradually under repeated and prolonged droughts, according to a study¹ that integrates palaeoclimate evidence with archaeological records.

Reduced rainfall, rising temperatures and riverine droughts disrupted agriculture and water availability over several centuries, triggering migration, urban fragmentation and cultural adaptation in one of the world’s earliest urban societies, which flourished between about 5,000 and 3,500 years ago along the Indus River and its tributaries in present-day Pakistan and northwest India.

To disentangle the sequence of events, researchers reconstructed climate conditions in the Indus Valley between roughly 5,000 and 3,000 years ago using transient climate model simulations combined with palaeoclimate proxies. These included geochemical records from stalactites and stalagmites in two Indian caves and water-level data from five lakes in northwest India.The analysis revealed a regional temperature rise of around 0.5 °C and a 10–20% decline in average annual rainfall. Four major droughts, each lasting more than 85 years, struck the region between 4,450 and 3,400 years ago, each affecting 65–91% of the Indus basin. These prolonged dry phases, the authors say, played a central role in shaping settlement patterns.

“It has long been recognised that shifts in monsoon rainfall and river flow influenced the civilisation, but earlier work relied largely on site-specific, qualitative proxy records,” says Vimal Mishra, the study’s corresponding author at the Indian Institute of Technology (IIT) Gandhinagar.

“By combining transient climate simulations with hydrological models, we show that multiple droughts drove the decline over nearly a millennium — it was not a single event.”

The team identified four regional clusters that experienced distinct river-flow responses during major droughts: the Upper Indus (Harappa region), Middle Indus (including Kot Diji, Ganweriwala, Kalibangan and Banawali), Lower Indus (Mohenjo-Daro and Chanhudaro) and the Saurashtra region (Dholavira, Surkotada and Lothal).

Water stress varied across these regions during the same droughts, creating spatial push–pull factors that shaped where people could farm and settle. During droughts between 3,826–3,663 years ago and 3,531–3,418 years ago, for instance, the central Indus was more severely affected than peripheral regions such as Saurashtra.

These spatial patterns mirror archaeological evidence of distinct and consistent cropping strategies across river basins, suggesting that early agricultural practices were closely tuned to local water availability.

Settlement shifts and deurbanisation

The reconstructed climate record shows clear links to changes in settlement patterns. Between 5,000 and 4,500 years ago, populations clustered in regions receiving relatively higher rainfall. After about 4,500 years ago, as aridity intensified, communities increasingly gravitated toward the Indus River in search of more reliable water supplies.

A particularly severe drought lasting 113 years between 3,531 and 3,418 years ago coincided with widespread deurbanisation across the civilisation’s core.

Large urban centres declined as populations dispersed into smaller settlements. Rather than disappearing, communities reorganised — shifting from water-intensive wheat and barley to more drought-tolerant millets and migrating toward wetter regions. Progressive drying in the central Indus appears to have driven eastward and southward movements into the Ganga plains and Saurashtra.

This interval overlaps with the 4.2-thousand-year (4.2 ka) event, a globally recognised phase of aridification that marks the onset of the Meghalayan age. The study quantifies the drop in rainfall during this period, which has previously been linked to cultural transitions in South Asia.

“The findings highlight the need to better understand the degree to which early societies adapted to climate stress,” says palaeoclimate scientist Yama Dixit of IIT Delhi.

“These changes are intricately linked to variations in the Indian summer monsoon over northwest India.”

After this phase, a persistent weakening of winter rainfall — known as Neoglacial drying — further reduced both summer and winter precipitation, potentially compounding stresses on the civilisation.

Climate among multiple drivers

Hiren Solanki, lead author of the study at IIT Gandhinagar, emphasises that the civilisation’s decline was driven by multiple interacting factors. “Prolonged aridity was important, but evidence for drought-tolerant crops, long-distance agricultural trade and changes in material culture point to a complex process of fragmentation and transformation — one of social adjustment rather than abrupt disappearance,” he says.

The study revisits earlier hypotheses that attributed the decline to invasions or epidemics, noting that archaeological evidence for such explanations remains sparse compared with indicators of long-term hydroclimatic change.

Co-author Thirumalai Kaustubh of the University of Arizona, USA, cautions against climate-only explanations. “No single factor can account for the decline. Our work helps place climate and monsoon variability in a broader context alongside other social and environmental drivers,” he says.

Mahesh Thakkar, director of the Birbal Sahni Institute of Palaeosciences in Lucknow, adds that tectonic processes also played a role by altering river courses in western India, suggesting that “the combined effects of climate and tectonics may have been critical.”

“Indus Valley societies offer valuable lessons as we confront a warming world,” says Balaji Rajagopalan, a co-author at the University of Colorado Boulder, USA. “Efficient water management, adaptive agriculture and interconnected trade networks underpinned resilience then — and remain central to sustainability today.”