A new study reveals that a long-lasting drought gradually caused the collapse of the Indus Valley Civilisation1. The dry spell, lasting 900 years, slowly destroyed its irrigation systems, devastated agriculture and eventually forced Indus people at places such as Harappa and Mohenjo-Daro to abandon the thriving urban culture.

Looking for favourable climate and water resources, the drought-hit people migrated towards the Ganga-Yamuna plains and tried to hang on by resorting to village culture.

Clues to such a long drought came from the sediments of Tso Moriri Lake, located in the Ladakh region of Jammu and Kashmir in India. Analysis of the lake’s sediments revealed that the Indian summer monsoon began to weaken around 4,350 years ago. This decreased the moisture transport and the snow deposition in the northwest Himalaya, which, in turn, considerably reduced the water supply in the Indus River and its tributaries.

“This research is of great socio-economic value as it emphasises the role of climate in the evolution of human societies and their livelihoods in South Asia,” said lead researcher Anil Gupta from the Indian Institute of Technology, Kharagpur in India.

The mysterious displacement of the Indus Civilisation is generally attributed to socio-economic and political turmoil as well as climate change in South Asia. A recent study linked its fall to a 200-year-long arid phase that reduced the discharge of the Indus River around 4,200 years ago2.

However, earlier studies from the western Himalayan region could not show any signs of water scarcity that probably affected the Harappan settlements around four millennia ago. To take a fresh look at a possible link between a change in climate and the displacement of the Indus people, Gupta and his co-researchers from the Wadia Institute of Himalayan Geology in Dehradun, India, and East China National University in Shanghai, China, collected sediment samples from the Tso Moriri Lake. This lake is sensitive to fluctuations of the Indian summer monsoon and preserves the records of such fluctuations in its sediments.

They measured how the ratios of elements such as strontium and calcium in the lake’s sediments changed in response to climate change. In warm and humid intervals, water-induced chemical reactions increase the transport of strontium and calcium to the lake. Such reactions decrease in arid and cold intervals, reducing the transport of strontium and calcium to the lake.

Around 4,350 years ago, transport of strontium and calcium to the lake abruptly dropped, suggesting a sudden onset of cold and dry climate in the northwest Himalaya that lingered for 900 years. This weakened Indian summer monsoon reduced the discharge of the Indus River and its tributaries, greatly disrupting the agriculture in the Indus Valley region.

A strong El Niño Southern Oscillation (ENSO), an irregular periodic variation in winds and sea surface temperatures over the Pacific Ocean that affect climate on land in the tropics and subtropics, might have hit South Asia, triggering such a long drought in South Asia including the Himalayan region, Gupta told Nature India .

Cameron Andrew Petrie, an archaeologist from Cambridge University, United Kingdom, who is not involved with the research, is a bit critical about this research. “The new lake record was obtained from the high Himalayas, and is therefore some distance from the areas occupied by Indus people, so there may not be a direct connection between the new lake record evidence and the Indus civilisation,” he said.

However, a 900-year-long drought has the potential to dramatically affect human populations, Petrie added. “Indus people lived in many different environmental zones and it is unlikely that the weakened monsoon affected all of these regions in the same way.” Such a long drought, he believed, didn’t wipe them out.

Instead, they devised ways to cope with a reduction in the rainfall in the region. They built structures for water storage, said Gupta.

Wherever they couldn’t cope, they migrated out of the cities. “Gradually, the Indus cities appear to have declined,” said Petrie. “Indus rural populations emerged and continued possibly for 200 years, suggesting that they were resilient to changes in climate.”