A shift in temperatures and weather patterns over the Indus valley starting about 2500 BC may have driven the Harappans to resettle far away from the floodplains of the Indus, a study has found.
More than 4,000 years ago, the Harappa culture thrived in the Indus River Valley of what is now modern Pakistan and northwestern India, said researchers from the Woods Hole Oceanographic Institution (WHOI) in the US.
The Harappans built sophisticated cities, invented sewage systems that predated ancient Rome's, and engaged in long-distance trade with settlements in Mesopotamia, they said.
Yet by 1800 BC, this advanced culture had abandoned their cities, moving instead to smaller villages in the Himalayan foothills.
Beginning in roughly 2500 BC, a shift in temperatures and weather patterns over the Indus valley caused summer monsoon rains to gradually dry up, making agriculture difficult or impossible near Harappan cities, said Liviu Giosan, a geologist at WHOI.
"Although fickle summer monsoons made agriculture difficult along the Indus, up in the foothills, moisture and rain would come more regularly," said Giosan, lead author of the study published in the journal Climate of the Past.
"As winter storms from the Mediterranean hit the Himalayas, they created rain on the Pakistan side, and fed little streams there," said Glosan.
"Compared to the floods from monsoons that the Harappans were used to seeing in the Indus, it would have been relatively little water, but at least it would have been reliable," he said.
Evidence for this shift in seasonal rainfall -- and the Harapans' switch from relying on Indus floods to rains near the Himalaya in order to water crops -- is difficult to find in soil samples.
Giosan and his team focused on sediments from the ocean floor off Pakistan's coast.
After taking core samples at several sites in the Arabian Sea, he and his group examined the shells of single-celled plankton called foraminifera (or "forams") that they found in the sediments, helping them understand which ones thrived in the summer, and which in winter.
Once he and the team identified the season based on the forams' fossil remains, they were able to then focus on deeper clues to the region's climate: paleo-DNA, fragments of ancient genetic material preserved in the sediments.
Based on evidence from the DNA, the researchers found that winter monsoons seemed to become stronger -- and summer monsoons weaker -- towards the later years of the Harappan civilisation, corresponding with the move from cities to villages.
"We don't know whether Harappan caravans moved toward the foothills in a matter of months or this massive migration took place over centuries. What we do know is that when it concluded, their urban way of life ended," Giosan said.
The rains in the foothills seem to have been enough to hold the rural Harappans over for the next millennium, but even those would eventually dry up, likely contributing to their ultimate demise.
"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 said.
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