Climate change likely caused migration, demise of ancient Indus Valley civilization

Beginning in roughly 2500 BCE, a shifting weather and weather pattern over the Indus valley caused by the monsoon rains to gradually dry up, making agriculture difficult or impossible near Harappan cities, says Liviu Giosan, a geologist at WHOI and lead author on the paper published Nov. 13, 2018, in the journal Climate of the Past.

"Although fickle summer monsoons made farming difficult along the Indus, up in the foothills, moisture and rain would come more regularly," Giosan says. "As winter storms from the Mediterranean hit the Himalayas, they created rain on the Pakistan side, and fed up with the floods of monsters that the Harappans were used to see in the Indus, it would have been relatively little water, but at least it would have been reliable. "

Evidence for this shift in seasonal rainfall and the Harapans' switch from relying on Indus floods to rains near the Himalayas in order to water crops-is difficult to find in soil samples. That's why Giosan and his team focused on sediments from the ocean floor Pakistan's coast. In the Arabian Sea, he and his group examined the shells of a single-celled plankton called foraminifera (or "forams") that they found in the sediments, helping them understand which ones in the summer, and which in winter.

Fossil remains, they were able to focus on the clusters to the region's climate: paleo-DNA, fragments of ancient genetic material preserved in the sediments.

"The seafloor near the mouth of the Indus is a very low-oxygen environment, so any grows and dies in the water is very much preserved in the sediment," says Giosan. "You can only get fragments of DNA from anything that's lived there."

During winter monsoons, he notes, strong winds bring nutrients from the ocean to the surface, feeding a surge in plant and animal life. Likewise, less nutrients, slightly less productivity in the offshore waters.

"The value of this approach is that of a fossil record. How does it work? "adds William Orsi, paleontologist and geobiologist at Ludwig Maximilian University of Munich, who collaborates with Giosan on the work.

Sure enough, based on the evidence of the DNA, the rise of the world, and the rise of the world, and the rise of the world, of the Harappan civilization, corresponding to the move from cities to villages.

"We do not know whether Harappan caravans moved to the foothills in a matter of months or this massive migration took place," Giosan says.

The rains in the foothills seem to be more likely to be harvested for the next millennia, but they would eventually dry up, likely contributing to their ultimate demise.

"We can not say that they disappeared entirely from the climate of the same time, the Indo-Aryan culture was born in the region of Iron Age tools and horses and carts.But it is 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 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 the Mediterranean, leading to an upswing in winter monsters over the Indus valley.

"It's remarkable, and there's a powerful lesson for today," he notes. "If you look at Syria and Africa, this is just the beginning-of-the-sea-level change of lead to huge migrations of Bangladesh, or from hurricane -prone regions in the southern US Back then, the Harappans could change the world, but today, you'll be able to go into all sorts of borders.

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Collapse of ancient Indus civilization