More than 4000 years ago, the Harappa culture in the Indus River Valley flourished from what is now modern Pakistan and North-West India, where they built sophisticated cities, invented sewage systems that preceded ancient Romans and engaged in long-distance trade with settlements in Mesopotamia. But at 1800 BC had this advanced culture abandoned its cities and instead moved to smaller villages in the sky of Himalayan. A new study from the Woods Hole Oceanographic Institution (WHOI) found evidence that climate change was likely to drive Harappans to resettle far from Indus floods.
Beginning about 2500 BC caused a change in temperatures and weather patterns across the Indus Valley that summer rainforest rain would dry up, making farming difficult or impossible near the Harappan cities, "said Liviu Giosan, a geologist at WHOI, and author of the paper as published November 13, 2018, in the newspaper Former climate.
"Although weak Mormons made farming difficult along Indus, up at the foot, moisture and rain would come more regularly," says Giosan. "When the Mediterranean storms hit the Himalayas, they created rain on the Pakistani side and fed small streams there. Compared with the floods of monsoons that Harappans used to see in Indus, there would have been relatively small water, but at least it would have been reliable. "
Proof of this shift in the season – and Harapan's switch from relying on Indus floods to rain near the Himalayas to water crops – is difficult to find in soil samples. Therefore, Giosan and his team focused on sediment from the seabed off the Pakistani coast. After taking nuclear tests in several places in the Arabian Sea, he and his group examined the single-plankton skeletons called foraminifera (or "forams") found in the sediments, which helped them understand who enjoyed the summer and as in the winter .
When he and the team identified the season based on the fossil remains of the fora, they could then focus on deeper clues to the region's climate: paleo DNA, fragments of old genetic material preserved in the sediments.
"The seabed near the Indus mouth is a very low oxygen environment, so everything that grows and dies in the water is very well preserved in the sediment," says Giosan. "You can basically get DNA fragments of almost all that live there."
During the winter's monsoons, he notes that strong winds lead to nutrients from deeper sea to surface and give rise to plant and wildlife. In the same way, weaker winds produce fewer nutrients other times of the year, which gives a little less productivity in the water offshore.
"The value of this approach is that it gives you a picture of the past biodiversity that you would miss by relying on skeleton residues or fossil mail. And because we can sequentially bill billions of DNA molecules, it gives a very high resolution image of how the ecosystem changed over time, "adds William Orsi, palaeontologist and geobiologist at the Ludwig Maximilian University of Munich, who worked with Giosan at work.
Certainly, based on evidence from DNA, the couple saw that winter monsoons seemed to be stronger – and summer monsons weaker – towards recent years of the Harapp civilization, which corresponds to the move from cities to villages.
"We do not know whether the Harapp caravans moved to the foot in a few months or this massive migration took place over the centuries. What we know is that when that conclusion ended its urban lifestyle," says Giosan.
The rain in the foot seems to have been enough to keep Harapans in the countryside for the next millennium, but they also eventually wiped up, which probably contributes to their final decline.
"We can not say that they completely disappeared because of the climate. At the same time, the Indo-Arya culture arrived in the region with Iron Age tools and horses and wagons. But it is very likely that the winter sunshine played a role, says Giosan.
The great surprise of the research, Giosan notes, is how to remove the roots that climate change may have been. At that time, a "new ice age" lived and forced colder air from the Arctic to the Atlantic and northern Europe. It, in turn, stormed into the Mediterranean, which led to a boom in winter seasons over the Indus Valley.
"It is remarkable, and there is a powerful lesson for today," he states. "Looking at Syria and Africa, migration from these areas has had some roots in climate change. This is just the beginning of the sea level because climate change can lead to huge migrations from lowlanders such as Bangladesh or from hurricane-rained regions in southern United States. Harappans handle changes by moving, but today you will run into all possible boundaries. Political and social cramps can then follow. "
Researchers conclude that climate change led to the collapse of the old indus civilization