资源环境科技发展态势分析平台

By Richard Kemeny

Suzuki Kaku/Alamy

Even for a civilisation as advanced as the Harappan, a second drought was perhaps one too many. A two-pronged climate catastrophe may be what drove the ancient society to disperse and eventually disappear.

The Harappan arose in the Indus valley between north-east Afghanistan and north-west India around 5200 years ago, peaking around 2600 BC. Much about them is unknown, as their written script is still undeciphered. Yet archaeological remains tell the story of a sophisticated people, skilled in metallurgy, trade and urban planning, and particularly adept at controlling water. Their huge cities, complete with intricate sewer systems, reservoirs and public baths, long predated the Roman Empire.

But by 1900 BC, their society seemed to be in decline, and by 1300 BC, the Harappan civilisation had collapsed.

Several ideas have been put forward to explain the downfall, including invasion and climate change. One recent hypothesis pins it to a major drought in the northern hemisphere around 4200 years ago. This event was recently declared as the start of the Meghalayan period of the Holocene geological epoch. It is thought to have disrupted climate systems around the world, including the summer monsoon rainfall the Harappan depended on.

Nick Scroxton at University College Dublin, Ireland, and his colleagues are now challenging this idea after analysing 10 recently reported palaeoclimate records. These come mostly from stalagmites from cave sites around the Indian Ocean, including one from Madagascar and a sediment core from the Arabian Sea. Together, they provide a region-specific view of the evolving climate during the rise and fall of the Harappan.

Scroxton and his team found some evidence of a relatively sudden drought starting around 4260 years ago. Rather than affecting summer monsoons though, the analysis suggests the Harappan faced a sharp decrease in winter rain.

“The civilisation suffered, that’s for sure,” says Scroxton. But that wasn’t the end of the Harappan. Archaeological findings suggest they abandoned their grand cities in the Indus valley and continued living in the south around present-day Indian state of Gujarat. Botanical evidence suggests the Harappan also switched from winter crops like barley and wheat to those like millet that favour summer rain conditions. “Their politics might change, the crops might change, the location of their cities changes, but they adapt,” says Scroxton.

Some 300 years later, however, just as the winter rains were starting to recover, a tropical drought kicked off. This was a gradual reduction in the summer monsoon rains over several centuries. Scroxton and his colleagues say this second drought transformed the Harappan into a rural, agrarian society that eventually faded away.

The conclusions are quite plausible, says Peter Clift at Louisiana State University and fit with other records from Rajasthan in western India and the Indus river delta. He is a little concerned about the study being based largely on stalagmites, however, pointing to some in China that were recently shown to be unreliable.

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

Journal reference: Climate of the Past, DOI: 10.5194/cp-2020-138