A new study identifies long-duration droughts—four or more periods lasting around 85 years between 4,450 and 3,400 years ago—as a major factor driving the gradual decline of the Indus Valley Civilization (IVC).

In particular, the final 113-year drought from roughly 3,531 to 3,418 years ago aligns with evidence of deurbanization, suggesting a slow, protracted decline rather than a single catastrophic collapse.

A hydrological model translated rainfall and temperature data into river and water-source changes, which were then compared with settlement patterns that moved closer to water over time.

Drought evidence comes from multiple proxies—stalagmite/stalactite geochemistry and historical lake levels in northwest India—integrated with climate simulations.

The study underscores policy implications for today, advocating adaptation measures like water storage and groundwater conservation to mitigate climate risks on civilizations past and present.

While acknowledging site biases, resolution limits, and proxy-to-hydrology translation challenges, the researchers report broad agreement between model outputs and speleothem records, supporting climate-driven hydrological stress as a driver of IVC trajectories.

Researchers argue that a series of decades-long droughts reduced water availability and spurred population shifts away from major urban centers, culminating in the IVC’s decline.

Experts note the study’s novelty for combining climate modelling with proxy records, while indicating future work should incorporate evapotranspiration to better capture regional hydrology.

Independent experts praised the study’s sophisticated modelling and its contribution to understanding hydroclimate’s role in urban and agricultural changes within ancient civilizations.

The droughts show distinct SST teleconnections: D1/D2 link to warmer central/east Pacific and cooler North Atlantic SSTs, while D3/D4 align with warmer eastern Pacific and neutral North Atlantic SSTs, consistent with ENSO-related ISM variability.

Three independent global climate simulations consistently show rainfall declines from 5,000 to 3,000 years ago, signaling multi-century droughts, monsoon weakening, and shifts in winter rainfall as persistent signals.

Potential natural drivers include El Niño and the Atlantic Multidecadal Oscillation, with feedbacks like vegetation loss and dust amplification worsening drought conditions.