The Indian summer monsoon abruptly weakened 4200 years ago. The ensuing drought may have led to the collapse of the advanced Indus Valley Civilisation.
A complex society flourished on the banks of the Indus river, located in what is now Pakistan and north-west India, and was at its height between 2600 BC and 1900 BC. Cities like Harappa and Mohenjo-daro were well planned, and the society even developed its own script. But after 1900 BC, the cities of the Indus Valley Civilisation were gradually abandoned. Nobody knows why.
Yama Dixit and her colleagues at the University of Cambridge excavated in Kotla Dahar, a lake close to one of the civilisation's greatest cities, Rakhigarhi in Haryana. They unearthed the shells of snails called red-rim melanias, and used them to reconstruct changes in climate over the last 5000 years.
The snails use oxygen from the lake water to make their shells. But when water evaporates from the lake, the lighter form of oxygen, oxygen-16, is lost to the air and heavier oxygen-18 builds up. So shells with more oxygen-18 reflect periods of drought.
Dixit found that Kotla Dahar was a deep freshwater lake between 4500 and 3800 BC. It then started getting shallower, until about 2200 BC, when the summer monsoon suddenly weakened for 200 years.
That would have been bad news for the people living there. The Indus Valley Civilisation depended on the monsoons for their crops, says Dixit. "It is inevitable that they were affected by a pronounced drought of this kind."
The dates of the drought do not match perfectly with the date of the collapse, but Dixit says both figures are quite uncertain.
By itself, the lake is not representative of the entire civilisation, says Supriyo Chakraborty of the Indian Institute of Tropical Meteorology in Pune. "But the authors have compared their results with various other observations and found agreement, giving credence to their claim," he says.
It is not the first time shifts in the monsoon have been linked to the collapse of civilisations.
Around AD 900 one of China's biggest empires, the Tang dynasty, collapsed. At the same time, halfway across the world, the Mayan civilisation in South America all but disappeared. Records from a lake in China show that stronger winds made the summer monsoon fail, causing widespread drought.
Dixit says the drying events at AD 900 and 2200 BC were both linked to shifts in the position of the Intertropical Convergence Zone, a band of cloud that runs east to west in the tropics and has a big influence on rainfall. "These climate phenomena were not regional but global in nature," she says.
Journal reference: Geology, DOI: 10.1130/G35236.1
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