Climate Change Likely Caused Migration, Demise of Ancient Indus Valley Civilization

During winter monsoons, he notes, strong winds bring nutrients from the deeper ocean to the surface, feeding a surge in plant and animal life. Likewise, weaker winds other times of year provide fewer nutrients, causing slightly less productivity in the waters offshore.

“The value of this approach is that it gives you a picture of the past biodiversity that you’d miss by relying on skeletal remains or a fossil record. And because we can sequence billions of DNA molecules in parallel, it gives a very high-resolution picture of how the ecosystem changed over time,” adds William Orsi, paleontologist and geobiologist at Ludwig Maximilian University of Munich, who collaborated with Giosan on the work.

Sure enough, based on evidence from the DNA, the pair found that winter monsoons seemed to become stronger–and summer monsoons weaker–towards the later years of the Harappan civilization, 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 says.

The rains in the foothills seem to have been enough to hold the rural Harapans 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 says.

The big surprise of the research, Giosan notes, is how far-flung the roots of that climate change may have been. At the time, a “new ice age” was settling in, forcing colder air down from the Arctic into the Atlantic and northern Europe. That in turn pushed storms down into the Mediterranean, leading to an upswing in winter monsoons over the Indus valley.

“It’s remarkable, and there’s a powerful lesson for today,” he notes. “If you look at Syria and Africa, the migration out of those areas has some roots in climate change. This is just the beginning–sea level rise due to climate change can lead to huge migrations from low lying regions like Bangladesh, or from hurricane-prone regions in the southern U.S. Back then, the Harappans could cope with change by moving, but today, you’ll run into all sorts of borders. Political and social convulsions can then follow.”

Also collaborating on the study was Ann G. Dunlea, Samuel E. Munoz, Jeffrey. P. Donnelly, and Valier Galy of WHOI; William D. Orsi of Ludwig-Maximilians-Universität MuÌ�nchen; Marco Coolen and Cornelia Wuchter of Curtin University in Australia; Kaustubh Thirumalai of Brown University; Peter D. Clift of Louisiana State University; and Dorian Q. Fuller of University College, London.

The work was supported by the National Science Foundation’s Division of Ocean Sciences and internal WHOI funds.

The Woods Hole Oceanographic Institution is a private, non-profit organization on Cape Cod, Mass., dedicated to marine research, engineering, and higher education. Established in 1930 on a recommendation from the National Academy of Sciences, its primary mission is to understand the oceans and their interaction with the Earth as a whole, and to communicate a basic understanding of the oceans’ role in the changing global environment. For more information, please visit http://www.whoi.edu.

Contacts and sources:
The Woods Hole Oceanographic Institution