What led to the fall of the Indus Valley Civilisation (IVC), one of the greatest and most ancient civilisations in the world? That was a question plaguing scientists and historians for a long time. There were numerous conjectures; it could have been caused by adverse weather conditions, earthquakes, or the arrival of armed, hostile “invaders”. Now, we finally have an answer. 

Scientists from Rochester Institute of Technology (RIT) in the US, using mathematical methods to study ancient climate patterns in North India, have surmised that shifting monsoon patterns linked to climate change likely caused the rise and fall of the civilisation. Indian-origin Nishant Malik, who headed the study, analysed data from North India covering the past 5,700 years.

What is the Indus Valley Civilisation?

Spread over 2600 and 1900 BCE, the mature IVC—or Harappa—was considered one of the largest urban societies of its time, with Dholavira in Gujarat, Mohenjo Daro in Pakistan’s Sindh and Rakhigarhi in Haryana as some of its major centres. Its historical importance cannot be overstated.

British explorers first conducted large-scale excavations of the civilisation in the 1920s. In 2016, researchers claimed the Indus Valley Civilisation began nearly 2,500 years earlier than thought. This meant it began nearly 8,000 years ago, making it older than the Mesopotamian and Egyptian civilisations, bringing into the question whether these Middle Eastern settlements can be considered as the ‘cradle of civilisation’.

The area was populated by Harappans, considered a mix of Iranian farmers who arrived from the Zagros region and the earliest migrants out of Africa. Harappans built for themselves a highly evolved urban ecosystem—in infrastructure like town planning and drainage systems, the IVC was second to none. It was largely peaceful, cosmopolitan, with distant trade ties, and no outward, overarching influence of religion.

It is widely presumed in academia that they spoke a language resembling the now-defunct Iranian Elamite, closely related to the Dravidian languages now spoken mainly in South India.

The IVC decline is believed to have commenced in the turn of the second millennium (2000 BCE).

What caused its decline?

Some in academia point to traditional literature which claim an influx of central Asian Steppe pastoralists (commonly referred to as the ‘Aryans’), who brought with them Vedic culture and Indo-European languages including the first iterations of Sanskrit. There are a lot of holes in that narrative, some of which claim a hostile, military invasion by the pastoralists.

There are yet others, like the Rochester study, which claim climate change and shifting weather patterns as a reason for the decline of the civilisation. Malik analysed the presence of a particular isotope in stalagmites—calcium carbonate deposits that trap surface climate signatures in different layers as they continually grow—in a North Indian cave, which should reveal the amount of water that fell as rain over time; scientists have previously been able to estimate monsoon rainfall in the region over the past 5,700 years. In the new research, Malik was able to identify patterns in this data showing a major shift in monsoon patterns as the civilisation began to rise, and then a reverse shift that matched its decline.

This is not the only study to make similar claims. Researchers at IIT-Kharagpur had claimed that the Indus Valley Civilisation died out as a result of a 900-year-long drought around 4,350 years ago. The researchers had came to the conclusion of the drought after studying variations in the monsoons over the past 5,000 years. They argued that rainfall levels were erratic for 900 years in the north-west Himalayas. This dried up water supply to the Indus. This could have also have led to the disappearance of a Himalayan snow-fed river (quite like the mythological Saraswati river) which once flowed in the Rann of Kutch. The long duration of the drought meant that residents were forced to abandon settlements for areas in the east and south where rainfall was better: modern-day Uttar Pradesh, West Bengal, Gujarat and Madhya Pradesh.

According to Malik’s analysis, there was a major shift in monsoon patterns just before the dawn of this civilisation and that the pattern reversed course right before it declined, indicating it was in fact climate change that caused the fall. The RIT scientists believe the new method will allow academics to develop more automated methods of finding transitions in ancient climate data, leading to additional important historical discoveries.

“Usually the data we get when analysing paleoclimate is a short time series with noise and uncertainty in it. As far as mathematics and climate is concerned, the tool we use very often in understanding climate and weather is dynamical systems,” Malik stated. “But dynamical systems theory is harder to apply to paleoclimate data. This new method can find transitions in the most challenging time series, including paleoclimate, which are short, have some amount of uncertainty and have noise in them,” he explained.

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