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About 4,000 years ago, Harappa culture flourished in the Indus Valley, in present-day Pakistan and northern India. They had large prosperous cities, had invented sewage centuries before the Romans and had established far-reaching trade routes to Mesopotamia. But in less than two centuries, their culture has faded and their cities have fallen – and the main culprit may have been climate change.
The civilization of the Indus Valley was an essentially urban culture supported by a surplus of agricultural production and trade. They prefer urban settlements and build at least two big cities: Harappa and Mohenjo Daro, which are remarkably advanced for their time.
But things started to change around 2500 BCE. The changes in temperature and weather conditions in the Indus Valley have caused the gradual drying up of summer monsoons, which has made farming more and more difficult.
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Liviu Giosan, a WHOI geologist and lead author of the new article, explains that this is what ultimately caused their disappearance and forced them to slowly retreat to smaller villages at the foot of the Himalayas.
"Although unstable summer monsoons have made agriculture difficult along the Indus, in the foothills, moisture and rain would come more regularly," says Giosan. "When the winter storms of the Mediterranean hit the Himalayas, they created rain on the Pakistan side and fed small streams. Compared to the monsoon floods that the Harappans used to see in the Indus, it would have taken relatively little water, but at least it would have been reliable. "
The end of an era
This is not a new theory, but it has been difficult to find evidence of these changing patterns in soil samples. So Giosan and his colleagues took the ocean. They examined microscopic fossils called foraminifera. Foraminifers are primitive living fossils living in the Cambrian (542 million years ago). They have a fossil usually made of calcium carbonate, which means that they often fossilize, especially in the environment of the Indus region. However, instead of searching for fossils, the researchers looked for one more step: they searched for fragments of DNA.
"The seabed located near the mouth of the Indus is a very low oxygen environment. Anything that grows and dies in the water is therefore very well preserved in the sediment, "explains Giosan. "You can get DNA fragments from almost everything that has lived there."
"The value of this approach is that it gives you a picture of the past biodiversity that you lacked by relying on skeletal remains or fossil records. And since we can sequence billions of DNA molecules in parallel, this gives a very high resolution image of the evolution of the ecosystem over time, "adds William Orsi, paleontologist and geobiologist at the Ludwig Maximilian University of Munich, who collaborated with Giosan at work. .
This is an interesting type of indirect evidence: during the winter monsoon, high winds bring nutrients from the deepest parts of the ocean to the surface, fueling a surge in animal life and plant – evidence is given in DNA sequences located at the bottom of the ocean. DNA evidence revealed that winter monsoons appeared to be stronger and summer monsoons weaker in the later years of the Harappa civilization, which corresponds to the movement of towns to villages. It is not known exactly when and how quickly this process took place, but it was the end of an era.
"We do not know if Harappan caravans have moved to the foot of the mountains in a few months or if this massive migration has taken place over the centuries. What we do know is that when it ended, their urban lifestyle ended, "said Giosan.
A lesson for today
This event is a very important warning for today. Unlike ours, the Harappai have not been at the origin of this climate change: the source of this climate change actually comes from very far. A mini ice age has settled, releasing colder air from the Arctic into the Atlantic and, subsequently, into northern Europe. In turn, this pushed the cold (but relatively warmer air) of northern Europe around, creating storms in the Mediterranean, which caused a rise in winter monsoons on the valley of the Indus – a remarkable series of domino pieces setting up and producing sparks Damage to the Harappa towns.
A similar process is happening today and climate change is putting in place a complex mechanism that will affect all regions of the world.
"It's remarkable and the lesson is powerful for today," he notes. "If you look at Syria and Africa, migration out of these regions has roots in climate change. It is only the beginning – the rise in sea level due to climate change can result in large migrations from lowland areas like Bangladesh or from hurricane-prone southern regions of the United States. At the time, the Harappans could cope with change by moving, but today you will encounter all kinds of borders. Political and social convulsions may then ensue.
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