Stronger monsoon drove ancient Indus civilization into the hills

by Mary Caperton Morton
Wednesday, February 13, 2019

Roughly 4,000 years ago, the Indus River Valley was home to the advanced and thriving Harappa culture. But by 1800 B.C., the civilization’s sophisticated cities along the river, which drains into the Arabian Sea on the coast of what is now Pakistan, were abandoned for smaller villages in the Himalayan foothills. A new study suggests that widespread changes in the Indian winter monsoon may have resulted in flooding that forced people to resettle farther from the Indus.

A team led by Liviu Giosan of Woods Hole Oceanographic Institution collected sediment cores from several sites in the Arabian Sea to study the river’s flooding history. By scanning the samples for plankton DNA and for the abundance of planktonic foraminifera, which are more plentiful in the region in winter, when strong winds stir up nutrients and lead to annual plankton blooms, the team reconstructed the winter monsoon cycle beginning about 4,500 years ago.

The researchers found that winter monsoons became stronger beginning about 2500 B.C., around the time that the Harappan civilization began moving their settlements toward the hills. “We don’t know whether Harappan caravans moved toward the foothills in a matter of months or if this massive migration took place over centuries. What we do know is that when it concluded, their urban way of life ended,” Giosan said in a statement.

The study, published in the journal Climate of the Past, also highlights the long reach of climate change. At about the same time the Indian winter monsoon was strengthening, a prolonged cold spell was sweeping across the Northern Hemisphere, bringing Arctic air masses south into Europe. The new study demonstrates that these cold air masses also affected the Middle East and the Far East, places where climate during this time period has not been as well studied.

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