Warming and droughts led to collapse of the Indus Valley Civilisation

Climate warming and severe droughts dealt a fatal blow to the Indus Valley Civilisation, an enigmatic urban culture that thrived around 4000 years ago in present-day Pakistan and India.

This civilisation established settlements along the Indus river and its tributaries, surpassing even the other major ancient civilisations in Egypt and Mesopotamia. They constructed several cities and are commonly referred to as the Harappan civilisation, named after Harappa, a city with a population of 35,000, a significant number for that era.

Although the script they used remains incomprehensible to us, the Harappans are renowned for their advanced water management techniques such as large storage cisterns and sophisticated sewage systems made of terracotta pipes and brick channels. However, these innovations proved inadequate in the face of a thousand years of scorching, arid conditions.

“Four distinct droughts occurred between the pre-Harappan and later Harappan periods,” explains Vimal Mishra from the Indian Institute of Technology Gandhinagar. “In response to these events, there was a continuous migration towards areas with better water availability.”

Previous studies had indicated that monsoon rains weakened in the Indus valley following a global megadrought 4200 years ago, leading to the collapse of the civilisation. Mishra and his team argue that it was a more gradual disintegration.

The researchers utilized three climate models to estimate rainfall across the region and compared these findings with rainfall estimates obtained from stalactites, stalagmites, and lake sediments.

The results revealed that the Indus Valley Civilisation experienced four droughts lasting at least 85 years each between 4400 and 3400 years ago, accompanied by a temperature increase of about 0.5°C.

Further modeling indicated a decline in the level of the Indus river. The Harappans, who revered rivers and irrigated crops like wheat and barley using annual floods, congregated nearer to water sources. As successive droughts ensued, they abandoned their urban centers and migrated towards the Himalayan foothills and the Ganga river plain.

The warming and drying trend may have been initiated by natural climate phenomena like El Niño and the Atlantic Multidecadal Oscillation, exacerbated by feedback mechanisms such as vegetation depletion and dust pollution, as proposed by the study.

While the study’s integration of modeling and proxy measurements is groundbreaking, future research should strive to incorporate evapotranspiration – the process of water movement from the ground to the atmosphere, which can be significant in such a hot region, suggests Sebastian Breitenbach from Northumbria University in the UK.

Given that our climate is warming at a much faster pace than during the time of the Harappans, policymakers should contemplate adaptation strategies like water storage systems and groundwater conservation, Breitenbach emphasizes.

“These studies serve as a cautionary tale,” adds Breitenbach. “They offer us a glimpse into what the future may hold.”

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