Climate change was almost certainly one of the key factors that forced the Harappans people to shift away from the floodplains of the Indus River, a new study by researchers from the Woods Hole Oceanographic Institution (WHOI) has claimed.
The Indus Valley civilisation thrived more than 4,000 years ago across the plains of the Indus River in modern day India and Pakistan.
The civilisation is believed to have spanned an area of more than one million square kilometers, from the Arabian Sea to the Ganges, over what is now Pakistan, northwestern India, and even into eastern Afghanistan. This great but poorly known urban culture also included parts of Mesopotamia and Egypt.
Extent of the Indus Valley civilisation, 2600-1900BCE. Image by Sue McIntosh via Wikipedia
The Harappa culture was named after Harappa - one of the largest cities in the Indus civilisation on the Ravi River. Inhabitants of this culture lived near rivers and used the fertile plains to make their livelihood. However, this advanced culture slowly disappeared between 3,900 and 3,000 years ago.
The new study by the researchers at the WHOI claims to have found evidence that inhabitants of Harappan culture had left their cities by 1800 BCE and shifted to smaller villages in the Himalayan foothills.
The migration started roughly around 2500 BCE, when drastic changes in weather patterns over the Indus valley made agriculture increasingly difficult across the region. With changes in climate, summer monsoon rains over the Indus valley gradually dried up. However, winter rains became more regular in the foothills, feeding little streams there.
Although the winter rains in the foothills brought less water compared to the floods from summer monsoons in Indus valley region, they were at least "reliable", according to Liviu Giosan, a geologist at WHOI.
It was difficult for the team to find evidence in soil samples for the shift in seasonal rainfall. Instead, they analysed samples of sediments collected from the ocean floor at different sites in the Arabian Sea. They examined the shells of foraminifera (or "forams") - the single-celled plankton - in sediments to understand which ones thrived in winters, and which in the summer.
After that, researchers examined ancient genetic material (paleo-DNA) preserved in the sediments. The analysis revealed that towards the end of the Harappan civilisation, summer monsoons had weakened, while winter monsoons became stronger, forcing people to migrate from big cities to smaller villages.
However, researchers are unsure whether this migration happened quickly or whether it took hundreds of years to complete.
"We can't say that they disappeared entirely due to climate - at the same time, the Indo-Aryan culture was arriving in the region with Iron Age tools and horses and carts. But it's very likely that the winter monsoon played a role," Giosan says.
The findings of the study are published in the journal Climate of the Past.
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