India’s coasts sinking up to 44 mm/year, 8.5 million people in high-subsidence zones

DEHRADUN: Large stretches of India's 7,500-km coastline are sinking, often rapidly, and in many places at rates that rival or exceed global sea-level rise, as per a new study by researchers from the University of Rhode Island, US. The study, the first comprehensive satellite-based assessment of coastal land subsidence along India's shoreline, recorded a maximum subsidence rate of 44mm per year in localised hotspots, while rates elsewhere ranged from a few millimetres to over 20mm annually.Titled ‘Vertical Land Motion and Human Exposure Across India's Coastal Regions', the study analysed eight years (2016–2024) of Sentinel-1 satellite radar data to measure vertical land motion along India's coastline, focusing on areas within a 100-km coastal buffer zone. It was authored by Quantao Zhu, Pei-Chin Wu, and Meng (Matt) Wei of the Graduate School of Oceanography, University of Rhode Island, along with lead data scientist Estelle Chaussard of FM Global's research division, US. More than 200 million people live within 100 km of India's coast. Of the 207.4 million residents in this coastal belt, 27.1 million (13.1%) live in districts subsiding faster than 2 mm per year. About 8.5 million (4.1%) live in areas sinking more than 5 mm per year, while 3.4 million (1.6%) face severe subsidence exceeding 10 mm per year.Using nearly 4,900 radar interferograms and over 39 million measurement points, the researchers identified major subsidence hotspots in cities including Ahmedabad, Chennai, Amaravati, Kochi, Kakinada, and Kolkata. In Ahmedabad, more than half the metropolitan area is subsiding, with peak rates of up to 44mm per year. In Kolkata, widespread areas are sinking at less than 5mm per year, but localised hotspots reach 13mm per year. Chennai shows mostly localised subsidence under 5mm per year, with peaks up to 11mm per year. Kochi and Puducherry exhibit widespread sinking exceeding 6mm per year across much of their urban areas, with maximum rates of 21mm and 13mm per year, respectively. Kakinada shows rates between 5mm and 19mm per year, with localised maxima reaching 30mm per year.The five major deltas along India's east coast — the Ganges-Brahmaputra, Mahanadi, Godavari, Krishna, and Cauvery systems — also show extensive sinking, with rates locally reaching 20mm per year or more.Tide-gauge data indicates that sea levels near Kochi have risen much faster in recent years. Between 1940 and 2000, sea level rose at about 1.45mm per year. This increased to around 2.47mm per year between 2000 and 2016, but surged to more than 17mm per year between 2016 and 2025, pointing to a sharp acceleration in relative sea-level rise.Subsidence was mainly concentrated near population centres, found the study. While only 1.7% of coastal land was sinking faster than 5mm per year, 4.1% of the population was exposed to those rates and more than half of severely subsiding areas lay at elevations below 25m above sea level, the study found.On the causes, the study noted that subsidence can result from natural processes such as tectonic activity, glacial isostatic adjustment, and sediment compaction, as well as human-induced factors including surface water drainage, groundwater extraction, mining, and structural loading. However, the findings suggest groundwater extraction is likely the primary driver across much of India, while in delta regions, it reflects a combination of sediment compaction and groundwater withdrawal.The authors warned that conventional coastal risk assessments — which consider only climate-driven sea-level rise — may substantially underestimate future flood exposure. Accounting for land subsidence, they said, is no longer optional but essential for long-term resilience planning along India's coasts.