India’s monsoon is expanding westward under global warming, portending an enhanced risk of extreme rainfall and floods across the country’s semi-arid northwestern region, including the Thar desert, but promising greener landscapes there.

Climate scientists, who analysed monsoon rainfall across the country over 115 years, have detected a broad 25 per cent increase in the average rainfall in the west and northwest alongside a 10 per cent decrease in the average rainfall in the northeast.

Their study has also predicted that the average monsoon rainfall over the semi-arid northwest could double within decades, rising 50 per cent to 100 per cent by the turn of the century.

The findings also challenge the “wet gets wetter, dry gets drier” paradigm assumed under global warming scenarios.

“Our findings throw up a question — will the monsoon’s westward expansion reverse the present-day aridity in northwest India? This region was not always as arid as it is now,” said P.V. Rajesh, a scientist at the Indian Institute of Tropical Meteorology (IITM), Pune.

Several earlier studies probing climate history have suggested that the monsoon was far more active in the northwest than it is at present. But an eastward shift centuries ago led to arid conditions and likely contributed to the collapse of the Indus Valley civilisation.

“The mechanisms that led to the eastward shift centuries ago remains unclear — but the westward expansion we’re seeing now appears driven by global warming,” said Bhupendra Nath Goswami, a senior climate physicist and former IITM director now at Cotton University, Guwahati.

Rajesh and Goswami analysed monsoon rainfall data from across the country between 1900 and 2015 and noted a 100 per cent increase — a doubling — in the average rainfall over the northwest region with every degree rise in the average global temperature. Rainfall over the east is decreasing by 8 per cent per degree.

They also used computer simulations to project future climate changes which indicated a 50 per cent to 100 per cent increase in rainfall by the end of the century. “We used 34 climate simulation models — and all pointed to similar results,” Rajesh told TheTelegraph.

The researchers said a westward shift in the warm Indian Ocean waters — also observed in the Pacific Ocean waters — under the present-day global warming conditions appeared to be driving the westward expansion of the monsoon over the Indian subcontinent.

While earlier studies have pointed to a stronger monsoon under global warming, there has not been enough research on how the monsoon's spatial distribution will change in the future, said Madhavan Rajeevan, a distinguished scientist at the National Centre for Earth Science Studies, Thiruvananthapuram, who was not associated with the IITM study.

"That makes these findings significant — that monsoon rainfall will increase over northwest India and adjoining Pakistan," Rajeevan said. "Under the warm Indian Ocean water shifts, low-pressure systems over the Bay of Bengal are also likely to move more westward, thus increasing the rainfall."

The expected increase in the average rainfall during the monsoon season, Goswami said, is likely to occur through spurts or short-lasting, extreme rainfall events. Weather observations over the past two decades have pointed to an increase in the frequency of such extreme rainfall events.

The floods in Gujarat and Rajasthan in recent years and in Pakistan in 2022 are signals of the process leading to increased rainfall in the northwest region “is underway in earnest,” the scientists have said in their study published in the journal Earth’s Future.

While the anticipated changes would require policymakers and the public to adopt strategies to cope with the intensified rainfall, Goswami said, the enhanced rainfall could also be “harnessed” to improve local crop productivity and to add to green landscapes.

“Intense rainfall usually leads to rapid runoff — but efforts to capture and store the surplus water could prevent such loss of water. Harvesting the additional rainwater could help replenish the groundwater levels in the region,” Goswami said.