Archaeological site of the Harappan civilization in Punjab Province, Pakistan

The double climate catastrophe may have ended the ancient Harappan civilization

By Richard Kemeny

An archaeological site of the Harappan civilization in Punjab Province, Pakistan

Suzuki Kaku / Alamy

Even for a civilization as advanced as the Harappan, perhaps a second drought was one too many. A two-pronged climate catastrophe could have caused ancient society to disperse and eventually disappear.

The Harappan originated about 5200 years ago in the Indus Valley between northeast Afghanistan and northwest India and reached its peak around 2600 BC. Much is unknown about her as her written script has not yet been deciphered. Archaeological remains, however, tell the story of a highly developed people who know their way around metallurgy, trade, and town planning, and are particularly skilled at controlling water. Their huge cities with complicated sewer systems, reservoirs and public baths were long before the Roman Empire.

But around 1900 BC Society seemed to be in decline and around 1300 B.C. The Harappan civilization collapsed.

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Various theories have been put forward to explain the extinction, including invasion and climate change. A more recent hypothesis leads to a major drought in the northern hemisphere around 4,200 years ago. This event was recently declared to mark the beginning of the Holocene Meghalayan Period. It is believed to have disrupted climate systems around the world, including the summer monsoon rains that the Harappan depended on.

Nick Scroxton of University College Dublin is now questioning this idea after analyzing ten recently reported paleoclimate records. These come mainly from stalagmites from cave areas around the Indian Ocean, including one from Madagascar and a sediment core from the Arabian Sea. Together they provide a region-specific overview of the developing climate during the rise and fall of Harappan.

Scroxton and his team found evidence of a relatively sudden drought that began about 4,260 years ago. Rather than affecting the summer monsoons, the analysis suggests that the Harappan has experienced a sharp drop in northern winter rains.

“Civilization has suffered, that’s for sure,” says Scroxton. But that was not the end of Harappan. Archaeological finds suggest that they left their great cities in the Indus Valley and continued to live in the south around what is now Gujarat. Botanical evidence suggests that the Harappan has also switched from winter crops like barley and wheat to ones that prefer summer rainy conditions like millet. “Their policies could change, the crops could change, the location of their cities could change, but they adapt,” he says.

About 300 years later, as the winter rains began to recover, a tropical drought set in. This was a gradual, longer lasting decrease in summer monsoon rainfall over several centuries. Scroxton and colleagues say this second drought turned the Harappan into a rural agrarian society that eventually faded.

The conclusions are entirely plausible, says Peter Clift of Louisiana State University, and fit with other records from Rajasthan and the Indus Delta. He’s a little concerned that the study is largely based on stalagmites, but points out some in China that have recently been shown to be unreliable.

Julien Emile-Geay of the University of Southern California says the study provides a well-constructed argument and adds a more refined view of the changing climate at the time.

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