Warming and droughts led to collapse of the Indus Valley Civilisation

A warming climate and severe droughts have dealt a mortal blow to the Indus Valley Civilization, a mysterious urban culture that flourished some 4,000 years ago in what is now Pakistan and India.

This culture established settlements along the Indus River and its tributaries, spreading even further than the other major ancient civilizations of Egypt and Mesopotamia. He built several cities and is also known as the Harappan civilization after Harappa, a city of 35,000 people, huge for its time.

Although we don't fully understand the script they used, the Harappans are known for their water management, such as giant storage cisterns and sewer systems made of terracotta pipes and brick channels. But these methods could not withstand millennia of hot and dry conditions.

“Four different droughts occurred between the pre-Harappan and late Harappan periods,” says Vimal Mishra from Indian Institute of Technology Gandhinagar. “In response to these events, there was a continuous migration to where water availability was better.”

Previous research has shown that monsoon rains in the Indus Valley weakened after a global megadrought 4,200 years ago, hastening the collapse of civilization. Mishra and his colleagues argue that this was a more gradual fragmentation.

The scientists estimated rainfall in the region using three climate models and compared them with rainfall estimates derived from stalactites and stalagmites, as well as lake sediments.

The results showed that four droughts lasting at least 85 years struck the Indus Valley Civilization between 4,400 and 3,400 years ago. The air temperature also increased by about 0.5°C.

Additional modeling has shown that the level of the Indus River has fallen. The Harappans, who are believed to have prayed to rivers and irrigated crops such as wheat and barley during annual floods, clustered closer to waterways. When another drought followed, they abandoned their cities and migrated to the foothills of the Himalayas and the plain of the Ganges River.

The study suggests the warming and drying may have been driven by natural climate conditions such as El Niño and the Atlantic Multidecadal Oscillation, and exacerbated by feedback effects such as vegetation loss and dust pollution.

While the study is groundbreaking in its combination of modeling and indirect measurements, future work should try to include evapotranspiration – the movement of water from underground into the atmosphere – which can be large in such a hot region, he says. Sebastian Breitenbach at Northumbria University in the UK.

Since our climate is warming much faster than the Harappans, policymakers should consider adaptation measures such as water storage systems and groundwater conservation, he adds.

“These studies could be a warning shot for us,” says Breitenbach. “They can give us an idea of ​​what might happen in the future.”

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