Four protracted droughts, each spanning more than 85 years, underpinned a slow, centuries-long decline of the Indus Valley Civilisation, an India-US study has found, challenging earlier speculation about a rapid collapse tied to shorter dry spells.
The study by researchers at the Indian Institute of Technology, Gandhinagar, and two US universities has suggested rainfall declined by 10 to 20 per cent across the Indus Valley region during those four droughts that occurred between 4,450 and 3,400 years ago.
Each drought affected between 65 to 91 per cent of the Indus Valley region, the longest lasting 163 years, the scientists said, describing their findings in the journal Communications Earth and Environment on Thursday. (See chart)
“There was no abrupt collapse — instead, the evidence points to a slow decline that lasted more than a thousand years,” Vimal Mishra, a hydrologist and climate science faculty member at IIT Gandhinagar, who led the study, told The Telegraph.
Each of the prolonged droughts, Mishra said, would have likely spurred responses from the inhabitants ranging from local migrations from drier zones to relatively wetter zones, as lakes or tributaries of rivers dried up under the deficient rainfall.
“Prolonged droughts lasting decades also point to a resilience of the civilisation — they adapted or they moved to survive,” Mishra said.
An atmospheric physicist and meteorologist who was not associated with the paper cautioned that the
results need to be interpreted with caution. The India Meteorological Department defines drought as seasonal rainfall that is at least 25 per cent below the long-term normal for a region.
Prolonged periods of drought may have occurred, but this does not necessarily mean the rains were deficient 10 to 20 per cent every year-after-year for decades, said Madhavan Rajeevan, a monsoon specialist and former secretary of earth sciences.
The Indus Valley Civilisation, a contemporary of ancient Egypt, had featured well-planned cities — including Dholavira, Kalibangan, Lothal in India, and Chanhudaro, Harappa, and Mohenjodaro in Pakistan — with advanced water management systems, including channels, wells and reservoirs during its mature phase between 4,500 and 3,900 years ago.
Earlier studies over the past two decades had already established that the Indus Valley settlements expanded as the monsoon rains grew stronger and declined as those rains weakened.
The strongest earlier evidence for such a pattern had emerged in 2012 when scientists at the Woods Hole Oceanographic Institution in the US combined multiple sets of data to suggest that a weakening monsoon and reduced river flows initially stimulated intensive agriculture and urbanisation, but precipitated the eventual decline of the subcontinent’s earliest cities.
Liviu Giosan at Woods Hole and his colleagues had found evidence for a proliferation of smaller village-type settlements between 3,900 and 3,000 years ago along the western part of the Ganges basin, the Yamuna river, the Himalayan foothills, and the Yamuna-Ganga zone.
Giosan had also argued for a “protracted” decline rather than a rapid collapse, but the pattern of the dry spells remained unclear.
Now, Mishra and his colleagues used geochemical readings from stalactites and stalagmites in two Indian caves — Sahiya in Uttarakhand and Mawmluh in Meghalaya — and water-level records of five lakes in Gujarat and Rajasthan and ran computer simulations of climate to reconstruct past climate conditions that point to four prolonged droughts.
The final 113-year drought between 3,531 years and 3,418 years ago coincided with archaeological evidence for a large-scale de-urbanisation across the Indus Valley sites.
The new results also suggest that the prolonged droughts coincided with eastward and southward migration of the Indus Valley populations into the Ganga plains and Saurashtra.
Mishra said independent earlier archaeological evidence has pointed to “adaption strategies” by the Indus Valley people. One such strategy was a shift from wheat and barley to drought-tolerant millets, which likely sustained agricultural production in Saurashtra.
An independent study in 2021 by Mishra’s collaborator Balaji Rajagopalan at the University of Colorado and colleagues had suggested that maritime trade with Mesopotamia “likely provided another buffer” that allowed coastal and Indus societies to make up for local shortfalls through imports of staples.
“The decline was not a sudden collapse but a complex process of fragmentation and cultural transformation, indicating a societal adaptation rather than a complete disappearance,” Mishra and colleagues wrote in their study.
The study collaborators were Rajagopalan, Kaustubh Thirumalai at the University of Arizona, and research scholars Hiren Solanki and Vikrant Jain.
The precise reasons for the prolonged droughts remain unclear but, Mishra said, distant weather processes — called teleconnections — may have played a role. Even in the present day, sea surface temperatures in the Pacific Ocean and the Indian Ocean, among other factors, influence the annual behaviour of the Indian monsoon.