New Delhi: Climate change was the primary factor that drove people of the Indus Valley Civilisation also known as Harappan Civilisation away from the floodplains of the Indus, a new study has found.
The study, conducted by the Woods Hole Oceanographic Institution (WHOI) used under-sea fossil evidence, marine DNA which allowed researchers to pinpoint that climate change, in the form of an increase in winter monsoon resulted in the migration of people – leading to the decline of the ancient civilization.
Liviu Giosan, a geologist at WHOI, the lead author of the paper published in the journal Climate of the Past said that beginning roughly around 2,500 BCE – a shift in temperature and weather patterns over the Indus Valley caused summer monsoon rains to gradually dry up, which in turn made agriculture difficult or impossible near Harappan cities.
Describing it as a “powerful lesson,” Giosan said, “If you look at Syria and Africa, the migration out of those areas has some roots in climate change. This is just the beginning—sea level rise due to climate change can lead to huge migrations from low lying regions like Bangladesh, or from hurricane-prone regions in the southern U.S.”
He added that this migration took place at a time when the world wasn’t divided by borders and said, “Back then, the Harappans could cope with change by moving, but today, you will run into all sorts of borders. Political and social convulsions can then follow.”
Shift in seasonal rainfall
While fickle summer monsoons made agriculture difficult along the Indus, in the foothills, moisture and rain would come more routinely, said Giosan. He added, “As winter storms from the Mediterranean hit the Himalayas, they created rain on the Pakistan side, and fed little streams there. Compared to the floods from monsoons that the Harappans were used to seeing in the Indus, it would have been relatively little water, but at least it would have been reliable.”
The evidence of the shift in seasonal rainfall – and the resultant switch away from the Indus floods to rains near the foothills to irrigate crops – is hard to ascertain through soil samples. Which is why, Giosan and his team focused their efforts on the sediments from the ocean floor of Pakistan’s coast.
The team took core samples at different sites in the Arabian sea and then examined the shells of single cell plankton called foraminifera (or “forams”) that they found in the sediments, helping them understand which ones thrived in the summer and which in winter.
Clues left behind in ancient DNA
Upon identifying the season, based on these fossil remains, the team was then able to focus on the clues on the region’s climate: paleo-DNA or fragments of ancient genetic material preserved in the sediments.
“The seafloor near the mouth of the Indus is a very low-oxygen environment, so whatever grows and dies in the water is very well preserved in the sediment. You can basically get fragments of DNA of nearly anything that’s lived there.” says Giosan.
So for instance, during winter monsoons, strong winds would bring nutrients from the deeper ocean to the surface that fed a surge in plant and animal life. On the other hand, weaker winds other times of year provide fewer nutrients, causing slightly less productivity in the waters offshore.
“The value of this approach is that it gives you a picture of the past biodiversity that you would miss by relying on skeletal remains or a fossil record. And because we can sequence billions of DNA molecules in parallel, it gives a very high-resolution picture of how the ecosystem changed over time,” adds William Orsi, paleontologist and geobiologist at Ludwig Maximilian University of Munich, who collaborated with Giosan.
The end of urban life
Based on this evidence, the team found that as winter monsoons became stronger and summer monsoons became weaker. The later year of the Harappan civilization resulted in the move away from cities to villages.
“We don’t know whether Harappan caravans moved toward the foothills in a matter of months or this massive migration took place over centuries. What we do know is that when it concluded, their urban way of life ended,” Giosan says.
While Giosan adds that they can’t say the civilization disappeared entirely due to climate, at the same time, “it's very likely that the winter monsoon played a role,” Giosan says.
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