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India should fear great droughts, warns a study on Indus Valley civilisation

January 07, 2018 11:15 AM

Punjab News Express/Y.S.Rana
CHANDIGARH: It is time to rewrite history text books as a new research study has revealed that weak summer monsoon had played a key role in the collapse of Indus Valley civilization and warned now modern India should fear the great droughts. The study may force researchers to go deep to find out more about so-called ‘cradles of civilization.’ and ended up unearthing something bigger.    

Newly studied summer monsoon rainfall record over 5,700 years reveals periods of normality and severe droughts that have ended the Indus Valley civilization and the Vedic eras that followed. Ashish Sinha, professor of earth sciences at the California State University, USA, studying calcite deposits in Sahiya cave in Uttrakhand, gleaned 5700 years rainfall data using these deposits, stated prolonged draughts that ended two civilizations. New data obtained from deposits made of calcium carbonate dissolved from the surrounding limestone by ground water in an Uttrakhand cave now offer rainfall patterns from 5700 years ago.         

The study shows long, deadly droughts ended the Indus Valley and the Vedic era that followed. Two of the nine authors of the study—scientists of India-origin working in China and the USA—tell us why modern India should be concerned about the likely coming of a new great dry period. It states starting 3400 years BP when a strong Indian summer monsoon had attracted the influx of Indo-Aryans into the north Indus Valley marking the beginning of the Vedic civilization. Later on with weakening of Indian summer monsoon event around 3100 years BP caused them to shift to other places.

The new study is not the first study to identify climate change as the reason for the fall of the Harappan civilization but it is the first study to reconstruct Indi8an monsoon precipatyion patterns over the last 5700 years by studying calcite deposits—usually composed of calcium carbonate dissolved from the surrounding limestone by ground water in a cave in Uttrakhand.

The records reveal a correlation between weak summer monsoons and the periods over which the Indus Valley and Vedic eras are known to have disintegrated. Both civilizations prospered during climatically stable intervals and disintegrated during years of drought. With the earliest period, between 4,550 years before present (BP) and 3,850 years BP, a fairly wet and warm, climatically stable interval facilitated the expansion of the agrarian communities of the early Indus Valley civilisation into large urban centres marking the civilisation’s mature phase. During an ensuing prolonged dry spell, these sites were abandoned as communities relocated to the eastern precincts of the Indo-Gangetic plain, a region enjoying a heavier monsoon, records the study.

“There is a good chance that monsoons which are keys to India prosperity will see anomalies,” also warns the latest assessment report from the Inter-governmental Panel on climate change ((IPCC). The report further states that it is likely the area encompassed by monsoon system will increase over this century means weakening of monsoon. Monsoon precipitation is likely to intensify due to increase in atmospheric moisture.  

But many thought that there might not have been one single that drove the demise of Harappan Civilization. Prof A.D. Ahluwalia, former Head, Department of Geology, Panjan University, Chandigarh told HTW that weakening of summer monsoon might be one of the reasons. The destruction of Indus Valley Civilization  cannot linked to weakening of summer monsoon,  but may most likely be linked to  major earthquake which tilted the Saraswati river bed leading to dissipation of water and destruction of the towns. Floods and droughts are normal cycles of nature and  these cannot be the cause of destruction of a huge civilization, he added.

The variability in the monsoon is expressed most strongly at its western limit, meaning that any weakening of the system will impact the fringe areas most. The speleothems, or mineral deposits within the cave, are formed from groundwater, so in a broad sense, yes, the variability of rainfall would influence their growth. However, more deeply, we studied the changes in the chemistry of speleothems (technically, the ratio of heavy to light isotopes of oxygen) to determine how their growth has been impacted by variations in the monsoon rainfall. This is what makes us confident of the Indian summer monsoon precipitation record of the last 5,700 years based on the speleothems of the Sahiya Cave.

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