How drought may have helped bring down the Indus Valley civilization

As rainfall declined and temperatures rose, droughts began to bite. (Representative image)

At its height, the Indus Valley civilization was one of the most advanced societies on Earth. In cities such as Harappa, people lived in multistorey brick houses laid out along gridded streets. They used early versions of flush toilets, built drainage systems and ran busy markets.

Traders moved gold, precious stones and crafted goods along rivers. Artisans carved intricate figurines and shaped clay toys. Farmers cultivated wheat, barley and cotton, using tools to bring water from nearby rivers to their fields, the Washington Post.

This civilization flourished across what is now Pakistan and northwest India at roughly the same time as ancient Egypt and Mesopotamia. Then, without clear signs of war, invasion or internal power struggles, the urban centres declined and dispersed — a disappearance that has puzzled scholars for decades.

Reconstructing an ancient climate

A new study published in the journal Communications Earth & Environment uses paleoclimate data and computer models to reconstruct the environmental backdrop to that decline between 3000 and 1000 B.C. An international team drew on multiple sources: cave formations in India, lake level records and climate simulations. Their goal was to understand how rainfall, river flow and soil moisture evolved over two millennia.

The researchers found that the Indus world did not collapse because of a single catastrophic event. Instead, they identified four prolonged droughts, each lasting more than 85 years, that gradually reshaped the landscape and strained society. The most intense of these, around 1733 B.C., lasted roughly 164 years, reduced annual rainfall by about 13 percent and affected nearly the entire region.

Overall, the team estimated that temperatures rose by about 0.5 degrees Celsius and rainfall fell by 10 to 20 percent over the course of the civilization’s history. Those changes dried rivers, shrank lakes and parched soils, making both trade and agriculture more difficult.

From strong monsoons to centuries of drought

Early in the Indus period, between about 3000 and 2475 B.C., the region experienced strong monsoons and much wetter conditions than today. The models suggest this wetter phase was linked to a cooler tropical Pacific Ocean, similar to persistent La Niña conditions that boost South Asian monsoon rains. It was during this time that settlements expanded around monsoon-fed tributaries and river systems.

Over the following centuries, the tropical Pacific warmed and the monsoon weakened. As rainfall declined and temperatures rose, droughts began to bite. According to the study, river flows diminished, lakes and shallow waterbodies contracted and soils dried out. For a civilization dependent on river transport and irrigation, those shifts created mounting challenges.

Boats and barges could no longer travel easily through shallower rivers, complicating long-distance trade. Moving goods became seasonal and more constrained. Farming in areas further from the main waterways became harder as rainfall and soil moisture declined.

Adaptation, movement and slow decline

The study suggests that the Harappans did not simply succumb to these stresses. Instead, they adapted in ways that prolonged the civilization’s life. Archaeological and climate records together show that settlements gradually shifted toward more reliable water sources, concentrating first along tributaries and then further west along the Indus River itself.

Researchers say the Harappans appear to have altered cropping patterns, diversified trade routes, and relocated communities to cope with recurring drought. Those strategies may help explain why the civilization endured for centuries despite repeated climate shocks. But they also came at a cost. As people moved and consolidated, urban centres shrank or transformed, contributing to the long-term decline of society as a coherent urban network.

Lessons for a warming world

Outside experts have called the work an important advance in linking hydroclimate change to the evolution of ancient civilizations. Previous studies often relied on a single type of evidence, such as cave deposits, to infer rainfall. By combining multiple datasets and climate modelling, the new study sketches a fuller picture of how drying rivers and shifting monsoons may have influenced settlement patterns and social resilience.

The findings also carry contemporary relevance. Modern South Asia is again experiencing rising temperatures and shifting rainfall, though on a planet altered by human-driven greenhouse gas emissions. The study’s authors argue that the Harappans’ long survival under repeated climate stress highlights the value of proactive adaptation: diversified water sources, resilient agricultural systems and flexible settlement patterns.

At the same time, they warn that prolonged climatic pressure can weaken societies if governance, food systems and infrastructure cannot keep pace. How the tropical Pacific behaves in a warmer world — and how that shapes monsoon variability — remains one of the major open questions in climate science. For researchers, that question is central to understanding whether regions dependent on monsoon rains might face stresses echoing, in a very different context, those that helped reshape the Indus Valley thousands of years ago.