Why did the Harappan civilisation, which flourished for hundreds of years and once extended across a vast area from northwestern India and across Pakistan, suddenly go into a terminal decline some 4,000 years ago and wither away?

Like their script that has remained indecipherable, the question what caused a sophisticated urban culture, capable of great feats of town planning and which had established a trading network that extended across the Middle East, to suddenly collapse is one that has aroused much scholarly debate and writing.

It has been suggested that reduction in water availability, perhaps as a result of climatic change or because tectonic activity caused rivers to change course, could have played a significant part in the decline of this ancient civilisation.

In a paper being published this week in the Proceedings of the National Academy of Sciences (PNAS), a team of scientists from the U.S., U.K., Pakistan, India and Romania has argued that long-term changes in monsoon rainfall altered river flow, creating conditions that initially allowed the Harappan civilisation to thrive but led later to its demise.

There is evidence that about 10,000 years back the Indian subcontinent went through a period when monsoon rainfall was greater than it is now, according to R. Ramesh of the Physical Research Laboratory in Ahmedabad who works on reconstructing the past climate and is not an author of the PNAS paper.

Then an eastward shift of the monsoon reduced rainfall in the northwestern part of the subcontinent, which became particularly marked some 5,000 years ago.

In their PNAS paper, Liviu Giosan and the other scientists have examined how river dynamics affected the Harappan civilisation. The declining rainfall reduced the cataclysmic floods produced by rivers in the region. This decrease in flood intensity “probably stimulated intensive agriculture initially and encouraged urbanisation around 4,500 years before present.”

The Harappan towns tended to be established on higher elevation, “in close proximity to floodable, agriculturally viable land,” the scientists noted. Lacking canal irrigation, these people relied on floods, which had to be regular and also benign enough to foster intensive agriculture without crippling their towns and cities.

But it was a delicate balance that ultimately tipped against the Harappans. As the monsoon continued to weaken, “rivers gradually dried or became seasonal, affecting habitability along their courses,” the paper pointed out.

“After 500 years of flourishing urbanism, the increasing aridification due to a shifting monsoon led to a crisis in the agriculture of the hinterland that supported the cities,” remarked Ronojoy Adhikari of the Institute of Mathematical Sciences in Chennai, one of the authors of the paper. This led to large-scale migrations towards moister regions to the north and a decline in the urban system of the Harappan civilisation.

The PNAS paper also examined the Ghaggar-Hakra river system that some have identified with the legendary Saraswati, which was described as a mighty glacier-fed river in the Rig Veda. These days, the Ghaggar has a sustained water flow only during a good monsoon.

The paper's findings support those published by V. Rajamani, now retired from the faculty of the Jawaharlal Nehru University, his then doctoral student and two German scientists in the journal Current Science in 2004.

After examining the isotopic characteristics of sediments found in the Ghaggar river in the Thar desert, they reported that these sediments did not appear to have originated in the glaciated regions of the Himalayas.

The Ghaggar-Harappan civilisation was, they concluded, a ‘true river valley civilisation' supported by monsoon rainfall in the sub-Himalayan catchment, the reduction of which was responsible for the extinction of the river and the associated civilisation.

The PNAS paper, however, does not cite the Current Science work.

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