Coastal Cities Are Sinking Faster Than the Ocean Is Rising, Study Finds

Coastal communities around the world face a growing flood threat from two directions at once: the ocean is rising, and in many densely populated regions the ground itself is sinking.

Researchers from the Technical University of Munich (TUM) and Tulane University report in Nature Communications that people living in heavily populated coastal regions experience an average relative sea-level rise of about 6 millimeters per year. That is nearly three times the coastline-weighted global average of 2.1 millimeters per year and almost double the climate-driven rise of roughly 3.15 millimeters per year. More than half a billion people live in low-lying coastal zones.

The difference comes down to land subsidence — the gradual sinking of the ground beneath coastal cities and communities. Among the main drivers are heavy groundwater pumping, oil and gas extraction, the compression of young sediments in river deltas, and the sheer weight of expanding urban areas, alongside natural processes such as tectonic movement and the slow adjustment of the Earth's crust after ancient ice sheets melted.

"If we want to understand sea-level rise along coastlines and respond effectively, we must not only observe the ocean but also the land itself," says study lead author Dr. Julius Oelsmann of DGFI-TUM. "Human activities cause the land to subside more strongly — often due to excessive extraction of water and resources that previously stabilized the subsurface. The sheer weight of cities, along with long-term geological processes, can further intensify this subsidence."

The most exposed countries include Thailand, Bangladesh, Nigeria, Egypt, China and Indonesia, where population-weighted coastal averages range from about 7 to 10 millimeters per year. Jakarta is sinking at an average of 13.7 millimeters per year, Tianjin at 13.5, Bangkok at 8.5, Lagos at 6.7 and Alexandria at 4 — with some spots in Jakarta dropping as much as 42 millimeters per year.

The good news: the problem is partly fixable. Tokyo once subsided by more than 10 centimeters per year in its worst-affected areas, but regulation and alternative water supplies brought those rates down substantially. "Improved groundwater management, stricter regulation of withdrawals, or targeted recharge of aquifers can at least slow subsidence rates and, in some cases, largely halt them," says Florian Seitz, Director of DGFI-TUM.