Do clouds hold the key?

New Delhi, April 29: It wasn’t raiders from the north but a weakened monsoon that spelled doom for the Indus valley civilisation, suggests a study published this week.

Geologist Anil Gupta at the Indian Institute of Technology, Kharagpur, and fellow Indian and American scientists have analysed monsoon behaviour over thousands of years through geological studies and connected it to archaeological findings.

They say that changes in the Indian monsoon over the past 10,000 years may explain the spread of agriculture in the subcontinent as well as the rise and fall of the civilisation that produced Harrappa.

“We see a clear connection between changes in the monsoon, the growth of agriculture and the movement of people across the subcontinent,” said Gupta, the lead author of the study published in the journal Current Science. “The correlation between the history of the monsoon and archaeology is striking.”

Archaeologists have suspected for decades that an intensified monsoon might have helped the Indus civilisation grow, while a weakening monsoon might have led to its decline. However, in the past, some experts had also suggested invasions by central Asian hordes or a massive earthquake may have snuffed the life out of the Indus cities.

Three years ago, Gupta and his colleagues used the signatures of tiny marine organisms in sediments from the Arabian Sea to determine the history of the monsoon over the past 10,000 years.

These organisms thrive when rainfall is good but their population dwindles during dry periods. “The marine records suggest that 10,000 years ago, the monsoon over the subcontinent was much stronger than it is today,” Gupta said.

Independent studies have shown that 10,000 years ago, the Ganga and Brahmaputra carried double the amount of sediment they do today, Gupta said. This, too, indicates a stronger monsoon.

The earliest settlement in the subcontinent with evidence of agriculture and domestication at Mehrgarh ? now in Pakistan ? is about 9,000 years old. This coincides with the peak intensification of the monsoon, the study said.

Archaeological studies have shown that the Mehrgarh settlers grew wheat and barley and domesticated cattle, sheep and goats. The increased rainfall and the spread of agriculture along the Indus valley over a few centuries would have given rise to the Indus civilisation, the researchers said.

The Arabian Sea sediments and other geological studies show that the monsoon began to weaken about 5,000 years ago. The dry spell, lasting several hundred years, might have led people to abandon the Indus cities and move eastward into the Gangetic plain, which has been an area of higher rainfall than the northwestern part of the subcontinent.

“It’s not high temperatures, but lack of water that drove the people eastward and southward,” Gupta said.

About 1,700 years ago, the monsoon began to improve again, leading to increased farm produce for several centuries and contributing to the relative prosperity in India during the medieval ages, from AD 700 to 1200.

After a weak phase between AD 1400 and 1800, the monsoon has again strengthened over the past 200 years, leading to increasing productivity. Scientists, however, believe that global warming might now be influencing the monsoon.