New research suggests the demise of the Bronze age Harappan Indus River civilisation is directly linked to changes in the monsoon.
Results from a team of researchers, including Dr Thomas Stevens from the Department of Geography| at Royal Holloway, University of London, provide the first good explanation for why the Indus valley flourished for two millennia, with large complex cities, strong trade systems and power that rivalled ancient Egypt, then dwindled away to small villages and isolated farms.
Dr Stevens explains: “The demise of this advanced civilisation has been a mystery for some time but our results now reveal how the Harappans took advantage of a monsoon driven reduction in devastating flooding of the mighty rivers around their settlements and used the more stable rivers to develop agriculture and urbanisation.
"This heyday of the civilization lasted for around 1,500 years, but the monsoon continued to decline and the balance tipped to drastically reduced river flow that forced an eastward population shift and a change towards many more small farming communities and the decline of cities.”
The Indus civilisation was the largest, but least known, of the first great urban cultures more than 4,000 years ago that also included Egypt and Mesopotamia. At its height, the culture spread across about 600,000 square miles in what is now Pakistan, northwest India and eastern Afghanistan.
The researchers worked in Pakistan and used photographs taken by shuttle astronauts and images from the Shuttle Radar Topography Mission to prepare maps of fluvial land forms in the region, then verified them on the ground using drilling, coring and manually dug trenches.
These landforms were also dated in detail to understand the timing of change in these systems. This new information on the area’s geological history enabled them to re-examine what they knew about settlements, what river systems were doing in response to changes in the monsoon, and how both agriculture and settlement patterns changed. Their findings are published in Proceedings of the National Academy of Sciences| (PNAS).
Dr Stevens adds: “Ultimately both the development and demise of the civilisation is intimately tied to monsoon system that fed the Harappan rivers. There was a kind of happy medium when the rivers were strong enough to support an agricultural surplus, but not so prone to flooding that agriculture was restricted. However, when the monsoon weakened further the rivers dried and people were forced to move elsewhere.
"The Indus today is still at the mercy of changes in the monsoon. Predicted monsoon shifts under global warming are likely to have a devastating effect on the current Indus irrigation system, the largest such in the world."
Posted on Tuesday 3rd July 2012