On Sept. 23, the city of Kolkata in eastern India came to a standstill: The capital of West Bengal state received more than 12% of the city’s average annual rainfall in just 24 hours, some 247.5 millimeters (9.7 inches). The subsequent flooding claimed lives and caused extensive property damage.
Scientists say climate change has made such extreme weather events in Kolkata more frequent, but the city’s stressed and aging drainage system exacerbated the impact, reports contributor Snigdhendu Bhattacharya for Mongabay India.
Research shows that the Indian Ocean, including the Bay of Bengal, which borders the coastline of West Bengal, is warming faster than the global average. Under such conditions, a low-pressure area over the bay draws in more moisture from the oceans, leading to more intense weather events, said Mahesh Palawat of Skymet Weather, a private weather forecast service.
Kolkata itself has the highest recorded urban warming among global megacities over the past seven decades: about 2.6° Celsius (4.7° Fahrenheit) between 1950 and 2018. Warm air can hold more moisture, so “every degree rise leads to a rise in moisture content by 7%,” said Kartiki Negi of Climate Trends, a New Delhi-based research and advocacy group. More atmospheric moisture can also lead to more intense weather activities, he added.
Kolkata’s climate action report notes the city is facing increasing threats from heat waves, flooding, sea level rise and more intense cyclones.
Urbanization, too, has taken a toll. New townships and roadways have been built on natural drainage paths of rivers and swamps. What were once mighty rivers and wetlands have shrunk considerably, affecting the city’s drainage and flood control.
Bhattacharya writes that the Sept. 23 flooding in Kolkata showed that the city’s aging underground drains and canals, clogged with silt and plastic waste, couldn’t handle the combined strain of urbanization and extreme weather.
Tarak Singh, from the municipal sewage and drainage department, said the agency has been working on maintaining the aging system. They removed more than 200,000 metric tons of silt from the city’s underground sewer in 2024-2025. “Pumping stations, the number of pumps, mechanised desiltation … every capacity has been significantly enhanced. The September 23 rain would have left any megacity waterlogged,” he added.
Pankaj Kumar Roy, a professor at Jadavpur University in Kolkata, said recent upgrades to the drainage system by the municipality likely lessened the impacts from the September rain. However, such intense rainfall events are likely to happen again, he said, so the city needs to focus on restoring the drainage system and increasing its capacity, particularly the outlets where pumped water will be discharged during future cloudburst-like situations.
Read the full story by Snigdhendu Bhattacharya here.
Banner image: Kolkata was waterlogged after a sudden intense downpour on Sept. 23. Image by Dipanwita Saha.
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