An analysis of satellite data reveals that subsidence in the world's major river deltas poses a greater flooding risk to populations than sea-level rise alone. Up to half a billion people, including residents of ten megacities, live in these vulnerable low-lying areas. Groundwater extraction emerges as the primary driver of this sinking land.
The world's most economically and environmentally significant river deltas are subsiding at alarming rates, endangering millions with increased flood risks. Researchers led by Manoochehr Shirzaei at Virginia Tech examined 40 key deltas, such as the Mekong, Mississippi, Amazon, Zambezi, Yangtze, and Nile, using data from the European Space Agency's Sentinel-1 satellite radar collected between 2014 and 2023. This technology measures ground changes with 0.5 mm accuracy, showing that more than a third of each delta's area is sinking, and over half in 38 of the 40 studied regions. Subsidence rates often outpace the global sea-level rise of about 4 mm per year, creating a compounded threat. As Shirzaei notes, “In many, sinking land is a bigger driver of relative sea-level rise than the ocean itself.” Specifically, average subsidence exceeds sea-level rise in 18 deltas, with the effect amplified in areas less than a metre above sea level. The Chao Phraya delta in Thailand, home to Bangkok, faces the most severe impacts, with an average subsidence of 8 mm per year across 94% of its area exceeding 5 mm annually. This results in a combined sea-level rise rate of 12.3 mm per year for the region. Other hotspots include Alexandria in Egypt and Indonesia's Jakarta and Surabaya. Human activities drive this subsidence: groundwater extraction has the strongest overall influence, followed by sediment alteration from upstream dams and levees, and urban expansion that adds weight and water demand to delta surfaces. Shirzaei warns that focusing solely on climate-driven sea-level rise misdirects efforts, as subsidence is locally manageable through groundwater regulation, aquifer recharge, and sediment management. Even data centres, which consume vast water for cooling, could exacerbate risks if reliant on local supplies. In vulnerable spots like the Mekong delta, such demands might undermine flood protections and infrastructure longevity. Shirzaei advises that while data centres can be built on deltas, they must minimize water use and avoid groundwater depletion. The findings, published in Nature (DOI: 10.1038/s41586-025-09928-6), underscore the need for targeted interventions to protect these densely populated ecosystems.
