A series of severe and prolonged droughts may have caused the decline of Indus Civilization cities, according to a study that examined ancient rock formations from a cave in Uttarakhand.
The beginning of this dry period, which began about 4,200 years ago and lasted for more than two centuries, coincides with the reorganization of the metropolis-building Indus civilization that spread over the territory of present-day Pakistan and India.
Research published in the journal Communications Earth & Environment identified three protracted droughts lasting 25 to 90 years during this dry period.
“We find clear evidence that this interval was not a short-term crisis, but a progressive transformation of the environmental conditions under which the Indus people lived,” said study co-author Cameron Petrie, a professor at the University of Cambridge in the UK.
The researchers mapped historical precipitation by examining growth layers in a stalagmite – a type of rock formation that rises from the cave floor – collected from a cave near Pithoragarh, Uttarakhand.
The decline of Indus Civilization cities in Uttarakhand
By measuring a range of environmental indicators including isotopes of oxygen, carbon and calcium, they obtained a reconstruction showing relative precipitation at seasonal resolution.
The team also used high-precision dating of the Uranus series to gain insight into the age and duration of the drought.
“Several lines of evidence allow us to piece together the nature of these droughts from different angles and confirm that they match,” said lead study author Alena Giesche, who conducted the research as part of her Ph.D. in Cambridge’s Department of Earth Sciences.
Giesche and team identified distinct periods of below-average precipitation in both the summer and winter seasons.
“Evidence that drought affects both harvest seasons is extremely important for understanding the impact of this period of climate change on human populations,” Petrie said.
Droughts lasted longer during this period, to the point where a third would last for multiple generations, the researchers said.
The findings support existing evidence that the decline of the Indus cities was linked to climate change.
“But what has been a mystery until now is information about the duration of the drought and the season in which it occurred,” Giesche said.
“These other details are really important when we consider cultural memory and how people adapt when faced with environmental changes,” the researcher added.
Archaeological evidence over 200 years
According to Petrie, archaeological evidence suggests that over a period of 200 years, ancient inhabitants took various steps to adapt and remain sustainable in the face of this new normal.
During this transformation, larger urban sites were depopulated in favor of smaller rural settlements towards the eastern extent of the area occupied by the Indus population.
At the same time, agriculture has shifted towards a reliance on summer crops, especially the drought-resistant millet, and the population has shifted to a lifestyle that appears to be more self-sufficient.
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