An abrupt weakening of the summer monsoon in north-west India may have led to the collapse of the mighty Indus Valley civilisation over 4,000 years ago, according to a new study.
A research team from the University of Cambridge has been studying the impact of climate change on the ancient civilisation and found that the resulting drought from a weak monsoon falls within the radiocarbon age range for the beginning of Indus de-urbanisation.
Their findings are published by the Geological Society of America this week as part of a larger research project funded by the British Council UK-India Education and Research Initiative (UKIERI).
"We think that we now have a really strong indication that a major climate event occurred in the area where a large number of Indus settlements were situated," said David Hodell, from Cambridge University's Department of Earth Sciences.
"Taken together with other evidence from Meghalaya in northeast India, Oman and the Arabian Sea, our results provide strong evidence for a widespread weakening of the Indian summer monsoon across large parts of India 4,100 years ago," he added.
The research team, which included Cambridge University archaeologist Cameron Petrie and Gates scholar Yama Dixit, collected snail shells preserved in the sediments of the ancient lake bed of Kotla Dahar in Haryana, which is adjacent to Indus settlements.
They found an increase of an oxygen isotope, which suggests that the lake was drying up due to a weakening of the source of water, which was the monsoon.
"We observed there was an abrupt change, when the amount of evaporation from the lake exceeded the rainfall - indicative of a drought," said Dixit.
"We know that there was a clear shift away from large populations living in megacities. But precisely what happened to the Indus Civilization has remained a mystery. It is unlikely that there was a single cause, but a climate change event would have induced a whole host of knock-on effects," added Petrie.
The Indus Valley civilisation, the largest of the ancient civilisations, has perplexed experts over its collapse for at least a century but the climate change angle has been suspected all along.
The UKIERI project, alongside Benaras Hindu University, hopes to provide new understanding of the relationships between humans and their environment and also involves researchers at Imperial College London, the University of Oxford, Indian Institute of Technology, Kanpur, and Uttar Pradesh State Archaeology Department.