Then, within the space of a few decades around 4000 years ago their world mysteriously fell apart, and the region reverted to farming, the Indus people replaced by Aryans.
The cause of their sudden demise has long been a cause of speculation—with changing climate and an invasion by the Aryan peoples the two leading contenders, but other possibilities being over-population or rivers changing course. New research into the local climate by a British/Indian team now tips the balance in favour of climate change and a 200 year-long drought being the culprit.
One of the problems of trying to correlate past human history with climate is that climate changes are often very local, so that a drought can be devastating one part of a country whilst another part can be unaffected (much as Australia today). Previously scientists were basing their hypotheses about the fate of the Indus on broader regional changes in the Middle East, but this study has now reconstructed the climate within the Indus Valley itself based on palaeolake sediments in the area.
As a result the authors have been able to say with a high degree of confidence that around 4100 BP the Indian summer monsoon weather abruptly weakened and stayed low for around 200 years, water levels in lakes and rivers dropped dramatically, leading to the collapse of agriculture and hence the cities which depended on them. Not the first such case to be sure, but a well-documented one.
Source : Geology, DOI: 10.1130/G35236.1