More than 4,000 years ago, Harappa culture developed rapidly in the Indus River Valley where today it is modern and northwestern India, where they built sophisticated cities, created sewage systems that preceded ancient Rome, and were involved in long-distance trade with settlements in Mesopotamia. . But in 1800 BC, these advanced cultures had left their cities, moved to smaller villages in the foothills of the Himalayas. A new study from Woods Hole Oceanographic Institution (WHOI) found evidence that climate change is likely to encourage Harappans to resettle far from the Indus floodplain.
Starting in around 2500 BC, shifts in temperature and weather patterns over the Indus valley caused the summer rainy season to gradually dry up, making agriculture difficult or impossible near the cities of Harappan, said Liviu Giosan, a geologist at WHOI and lead author in the newspaper. published November 13, 2018, in the journal Climate of the Past.
"Although the volatile summer rainy season makes agriculture difficult along Indus, at the foot of the hill, humidity and rain will come more regularly," Giosan said. "When winter storms from the Mediterranean hit the Himalayas, they created rain on the side of Pakistan, and fed small rivers there. Compared to the floods of monsoons that Harappans used to see on the Indus, it would be relatively small water, but at least that reliable. "
Evidence for this seasonal rainfall shift – and the transition from & # 39; The Mustans & # 39; from relying on Indus floods to rain near the Himalayas for aquatic plants – hard to find in soil samples. That's why Giosan and his team focus on sediments from the seabed off the coast of Pakistan. After taking core samples at several sites in the Arabian Sea, he and his group examined the single-celled plankton shells called foraminifera (or "forams") they found in sediments, helping them understand which ones developed in the summer, and which in winter.
After he and the team identified the season based on fossil remains of forams & # 39; they could then focus on deeper clues to the region's climate: paleo-DNA, fragments of ancient genetic material preserved in sediments.
"The seabed near the mouth of the Indus River is a very low oxygen environment, so anything that grows and dies in water is very well preserved in sediments," Giosan said. "You can basically get DNA fragments from almost all who live there."
During the winter, he noted, strong winds bring nutrients from the sea deeper into the surface, feeding spikes in plant and animal life. Likewise, weaker winds at other times of the year provide less nutrition, causing productivity to decrease slightly in offshore waters.
"The value of this approach is to give you an idea of the past biodiversity that you miss by relying on skeletal remains or fossil records. And because we can arrange billions of DNA molecules in parallel, they give very high values." the resolution picture of how ecosystems change over time, "added William Orsi, paleontologist and geobiologist at Ludwig Maximilian University of Munich, who worked with Giosan on his work.
Sure enough, based on evidence from DNA, the couple found that winter seems to be stronger – and summer is weaker – towards the later years of the Harappan civilization, according to the movement from city to village.
"We do not know whether the Harappan caravan moved to the foothills in a matter of months or this massive migration lasted for centuries. What we know is that when concluded, their urban way of life ended," Giosan said.
Rain at the foot of the hill seems to be enough to hold back the remaining countryside for the next millennium, but even eventually it will dry up, possibly contributing to their final destruction.
"We cannot say that they disappeared entirely because of the climate – at the same time, Indo-Aryan culture arrived in the region with Iron Age tools and horses and carts. But it is very possible that the winter monsoon played a role," said Giosan.
The big surprise of this study, Giosan noted, is how far the root of climate change might be. At that time, the "new ice age" was being settled, forcing cold air down from the Arctic to the Atlantic and Northern Europe. Which in turn drives the storm down into the Mediterranean, which causes the winter wind to rise above the Indus valley.
"This is amazing, and there are strong lessons for today," he said. "If you look at Syria and Africa, migration from these areas has some roots in climate change. This is just the beginning – rising sea levels due to climate change can cause massive migration from low-lying areas such as Bangladesh, or from hurricanes. regions in the southern US At that time, Harappan was able to deal with change by moving, but today, you will meet all kinds of borders. Politics and social seizures can then follow. "
The researchers concluded that climate change caused the collapse of the ancient Indus civilization