19 March 2014 Withering monsoon may have doomed past Asian society. NewScientist.

The Indian summer monsoon abruptly weakened 4200 years ago. The ensuing drought may have led to the collapse of the Indus Valley Civilization, which flourished around the Indus river, in what is now Pakistan and north-west India. It was at its height from 2600 BC to 1900 BC, but after that its cities were mysteriously abandoned.

Shifts in the monsoon have also been linked to the fall of China’s Tang dynasty, and of the Mayan civilization in South America, both around AD 900.

Yama Dixit and her colleagues at the University of Cambridge dug up snail shells from Kotla Dahar, a lake near one of the cities. The shells record changes in the lake’s water level in their composition. The team found that the lake was deep from 4500 to 3800 BC. Although it shallowed a little up to 2200 BC, after this time there was a sharp drop in the water level, suggesting the summer monsoon abruptly weakened for 200 years, meaning less rainfall. The Indus valley people relied on the monsoon for crops, says Dixit. “It is inevitable that they were affected.” The dates of the drought don’t match perfectly with the collapse, but Dixit says both are uncertain. The idea is credible because the results agree with data elsewhere, says Supriyo Chakraborty of the Indian Institute of Tropical Meteorology in Pune.

The journal article:

Dixit, Y., et al. February 24, 2014. Abrupt weakening of the summer monsoon in northwest India ~4100 years ago. Geology.