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    Woods Hole scientists say this ancient civilization declined for a worrisome reason: climate change

    Peter Clift/Louisiana State University
    Researchers used a piston corer to take soil samples from the floor of the Arabian Sea.

    Thousands of years ago in what is now modern Pakistan and northwestern India, people lived in cities with populations perhaps as high as 35,000. They invented sewage systems before the Romans and engaged in long-distance trade.

    But the Harappa culture of the Indus River Valley went into decline, and nothing is left but ruins. Researchers believe the reason may be climate change, and they say that what happened to the Harappans may be a cautionary tale for people today.

    A new Woods Hole Oceanographic Institution study has found additional evidence that climate change was behind the move of the Harappans from the floodplains of the river, where they practiced inundation agriculture, to smaller villages in the Himalayan foothills that eventually dwindled away.

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    The Harappan culture was thriving over 4,600 years ago, but beginning around 2500 BC, summer monsoon rains began to dry up, said Liviu Giosan, a Woods Hole geologist who was lead author of a paper published in the journal Climate of the Past. By 1800 BC, the Harappan cities had been abandoned.

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    Changes in the summer monsoon have been previously studied by researchers, including Giosan, and cited as a reason for the Harappan decline. In his new study, Giosan looked at another angle: whether winter rains were increasing in the area the Harappans moved to. Giosan’s team found that winter monsoons seemed to become stronger as the civilization declined, the statement said.

    “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,” Giosan said in the statement.

    “They left there to go to a region that became more attractive because it had winter rain,” Giosan said in a telephone interview.

    A stone seal depicting a mythological animal and pictographic symbols from Mohenjo-Daro, one of the Indus Valley cities. The Harappan language has only been partly and tentatively deciphered.
    Harappan National Museum of Karachi/Bridgeman Art Library
    A stone seal depicting a mythological animal and pictographic symbols from Mohenjo-Daro, one of the Indus Valley cities. The Harappan language has only been partly and tentatively deciphered.

    The researchers made the case for stronger winter monsoons by collecting and examining core samples of the sediment on the ocean floor in the Arabian Sea off the coast of Pakistan. Giosan said the researchers went back through the millennia by examining layers of dead microscopic sea life deposited on the seafloor. They looked at the shells of foraminifera, tiny single-celled plankton, and at DNA fragments of other plankton.

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    Different organisms thrive during different seasons, including a particular species of foraminifera, G. falconensis, Giosan said. By looking at the makeup of what was deposited in the mud, the researchers could determine years when winters were stronger.

    The study concluded: “Changes in the planktonic community structure indicative of cool, productive waters highlight strong winter monsoon conditions. . . . We propose that the combined changes in summer and winter monsoon hydroclimate triggered the metamorphosis of the urban Harappan civilization into a rural society.

    “We don’t know whether Harappan caravans moved toward the foothills in a matter of months or this massive migration took place over centuries,” Giosan said. “What we do know is that when it concluded, their urban way of life ended.

    “It’s remarkable, and there’s a powerful lesson for today,” he 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 US. Back then, the Harappans could cope with change by moving; but today, you’ll run into all sorts of borders. Political and social convulsions can then follow.

    “History tells us things,” Giosan said. “We just need to notice.”

    Martin Finuane can be reached at martin.finucane@globe.com