4000 years ago, the Harappa culture enjoyed the Indus River Valley of what is now modern Pakistan and northern India. They had large, prosperous cities, invented the sewers centuries before the Romans and established extensive trade routes to Mesopotamia. But in less than two centuries their culture fades and their cities fell – and the main guilty may have been climate change.
Indus Valley civilization was a primary urban culture held by surplus of agricultural production and trade. They preferred urban buildings and built at least two major cities: Harappa and Mohenjo Daro, which was remarkably advanced for their time.
But things began to change around 2500 BC. Changing temperatures and weather patterns across the Indus Valley caused a gradual dehydration of the summer consonants, which made farming increasingly difficult.
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Liviu Giosan, a geologist at WHOI and leading author of the new paper, says that this is what ultimately led them to be killed and forced them to slowly retreat to smaller villages in the sky of Himalayan.
"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 to the monsoon floods that Harappans used to see in Indus, there had been relatively little water, but at least it would have been reliable. "
End of an era
This is not a new theory, but evidence of these varying patterns has been difficult to find in soil samples. So instead, Giosan and colleagues took to the sea. They looked at microscopic fossils called foraminifera. Foraminifera are primitive "living fossils", which live from Cambrian (542 million years ago) until today. They have a fossil that is often made of calcium carbonate, which means that they often fossilize, especially in the environment of the Indus area. But instead of looking for fossils, the researchers saw one step deeper: they were looking for DNA fragments.
"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."
"The value of this approach is that it gives you a picture of the past biodiversity that you would lack by relying on skeleton residues or fossil mail. And since we can sequentially bilaterally bilaterally generate DNA molecules, it gives a very high resolution 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.
This is an interesting type of indirect evidence: during winter seasons strong winds lead nutrients from the deeper parts of the ocean to the surface, giving rise to plant and animal life. Proof of this surplus is recorded in DNA sequences in sedimentary layers of the seabed. DNA evidence revealed that winter monsoons seemed to be stronger and summer monsons weaker in recent years by the Harapp civilization, which corresponded to the move from cities to villages. It is not clear exactly when and how fast this process took place, but it was the end of an era.
"We do not know if 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 environment of life, Giosan says.
A lesson for today
This event means a very important warning for today. Unlike us, Harappans did not take this climate change on themselves – the source of this climate change actually came from very far away. A mining age settled and liberated colder air from the Arctic to the Atlantic and then to northern Europe. In turn, this pressed the cold (but relatively warmer air) in northern Europe and created storms in the Mediterranean, which led to an increase in winter tunes over the Indus Valley – it was a remarkable series of domino pieces that fell in place and sparked judgment for the Harappan cities.
A similar process is happening today and climate change is launching a complex mechanism that will affect all parts of the world.
"It is remarkable, and there is a powerful lesson for today," he states. "If you look at Syria and Africa, migration from these areas has had some roots in climate change. This is just the beginning of the sea level due to climate change can lead to huge migrations from low-lying areas like Bangladesh or from hurricane-frequent regions of southern United States. The Harappans handle changes by moving, but today you will run into every possible boundary. Political and social cramps can then follow. "
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