Indus megacities possibly met their demise due to extended droughts

New research involving the University of Cambridge has found evidence – locked in an ancient stalagmite from a cave in the Himalayas – of a series of severe and prolonged droughts that may have upended the civilization of Bronze Age Indus.

The beginning of this arid period—which began about 4,200 years ago and lasted for more than two centuries—coincides with the reorganization of the city-building Indus civilization, which extended into present-day Pakistan and India.

The research identified three prolonged droughts—each between 25 and 90 years old—during this arid period. Study co-author Professor Cameron Petrie, from the Department of Archeology at Cambridge, said: “We found clear evidence that this interlude was not a short-term crisis, but a gradual shift in the ecological conditions in which the Indus people lived.” The study has been published in the journal Earth and Environment Communications.

The researchers charted historical rainfall by examining the growth layers in stalagmites collected from a cave near Pithoragarh, India. By measuring a range of environmental tracers — including oxygen, carbon and calcium isotopes — they obtained a reconstruction showing relative precipitation at seasonal resolution. They also used high-resolution uranium-series dating to get a sense of the age and duration of the drought.

“Multiple lines of evidence allow us to piece together the nature of these droughts from different angles – and make sure they fit,” said lead author Alena Giesche, who conducted the research as part of her PhD. in the Department of Earth Sciences, Cambridge.

Giesche and the team identified distinct periods of below-average precipitation in both the summer and winter seasons. “Evidence of drought affecting both growing seasons is very important for understanding the impact of this period of climate change on populations,” Petrie said. He adds that the droughts during this period increased in duration, to the extent that the third period was multi-generational.

The findings support existing evidence that the decline of Sindh’s megacities was related to climate change. “But what has been a mystery until now is the information on the duration of the drought and the season in which it occurred,” Gish said. “These additional details are really important when we think about cultural memory and how people adapt when faced with environmental change.”

According to Petrie, “Archaeological evidence indicates that over a period of 200 years, ancient populations took various steps to adapt and survive in the face of this new normal.” During this shift, the larger urban sites were depopulated in favor of smaller rural settlements towards the eastern extent of the area occupied by the Indus people. At the same time, agriculture shifted towards reliance on summer crops, particularly the drought-tolerant millet, and the population moved to a lifestyle that appears to have been more self-reliant.

Recently major droughts have become a popular cause to explain a number of cultural shifts, including the Indus Valley, explains David Hodel, co-author of the study from the Department of Earth Sciences at Cambridge. “But the links in general are obscure because of the difficulties involved in comparing climatic and archaeological records.” This is now changing, Hodel said, because “paleoclimate records are getting progressively better at refining changes in precipitation on a seasonal and annual basis, which directly affects people’s decision-making.”

The team is now looking to extend the climate reconstruction to the western parts of the Indus River region, where the winter precipitation regime becomes more dominant than the Indian summer monsoon. “What we really need are more records like this, of a west-east oriented cross-section across the region as the summer and winter monsoons interact – and most importantly, capturing the onset of this arid period,” Gish said.

“Currently, we have a huge blind spot on our maps stretching across Afghanistan and Pakistan where the Indian summer monsoon interacts with the west,” said Professor Sebastian Breytenbach, co-author and paleoclimatologist at Northumbria University. “Unfortunately, the political situation is unlikely to allow this type of research in the near future.”

“There is more work to be done by paleoclimatologists and archaeologists,” Hodel said. “We are fortunate at Cambridge to have the two departments next door to each other.”