The River Ganga.
Credit: Vimal Mishra
New Delhi: The mighty Ganga is undergoing its worst drying phase in the last 1,300 years, Indian researchers reported on Monday, noting that the worrying situation can’t be explained by natural climatic variability.
Scientists at Indian Institute of Technology, Gandhinagar and University of Arizona found the ongoing trend being 76 per cent more severe than its closest analogue, a drought in the 16th century CE.
No extreme wet years in the Ganga river basin were observed between 1991 and 2010, and only two extreme wet years occurred between 2011 and 2020.
“Between 1991 and 2020, there have been 15 drought years, leading to massive decline in monsoon rainfall in the Ganga river basin. This is the worst period in the last 1,300 years as we found after reconstructing the streamflow,” team leader Vimal Mishra, a scientist at IIT Gandhinagar told DH.
Over 2,500 km long Ganga – the lifeline for north India – travels from the Himalayas to Bangladesh and holds enormous religious, cultural and economic significance for the people of India, Nepal and Bangladesh besides supporting the agriculture and water-needs of 600 million people.
The scientists recreated the historical streamflow from 700 to 2012 CE using instrumental data, paleo-hydrological records, and hydrological modelling.
The data matches with past records like major famines caused by repeated monsoon failures, such as those between 1344 and 1355, the Bengal famine from 1769 to 1771, and the famine of 1802–1804 establishing the reliability of the reconstructed data.
They found that in general, wet years outnumbered dry years, but the current drying trend, since the 1990s, was the most severe in the past 1,300 years and 76 per cent more severe than a 16th century drought (between 1501 and 1530), the next worst one.
The 2014–2018 drought years stand out as the most intense spell of five year drought in the Ganga basin in the last millennium.
Even more striking are two unprecedented 7-year drought events that occurred between 1991 and 2020. Among these, the 2004–2010 drought ranks as the most severe drought of the past 1,300 years whereas 1991–1997 was the second-worst 7-years drought period. Both rank among the ten longest droughts in the Ganga basin’s 1,300-y history.
The severity of the drying trend cannot be explained by natural variability and is not captured by global climate models, which predict an increase in streamflow with global warming.
“Climate models mostly tell us about the response to greenhouse gas emissions and can’t capture such a complex trend responsible for declining rainfall,” Mishra said, the recent drying trend was due to declining monsoon rainfall
According to the scientists, the factors influencing such a trend may include interactions between the forces driving summer monsoon precipitation, trends in the large-scale climate, anthropogenic warming, and aerosol emissions.
The study has appeared in the Proceedings of the National Academy of Sciences.