Some coastal cities are sinking faster than the ocean is rising

Coastal cities worldwide are experiencing a dual threat: rising ocean levels combined with the sinking of the land beneath them. This heightened risk is particularly dire for densely populated regions, where over half a billion people reside. Researchers from Technical University of Munich and Tulane University have found that the downward movement of land significantly amplifies the perceived rate of sea level rise for millions of coastal residents.

On average, densely populated coastal areas are experiencing a relative sea level rise of about 6 millimeters per year, nearly three times the global coastline average of 2.1 millimeters per year and nearly twice the climate-driven absolute sea level rise of 3.15 millimeters per year. The primary driver of this accelerated rise is land subsidence, the gradual sinking of the ground beneath coastal areas.

Several factors contribute to subsidence, including heavy groundwater pumping, oil and gas extraction, the compression of young sediments in river deltas, and the weight of burgeoning urban areas. Natural processes like tectonic movement and adjustments in the Earth's crust following the loss of ancient ice sheets also play a role.

To effectively address the issue, it's crucial to monitor both sea-level rise and land subsidence. In heavily populated coastal regions, human activities often exacerbate subsidence, mainly due to excessive water and resource extraction. The weight of cities and long-term geological processes further intensify this sinking.

Countries like Thailand, Bangladesh, Nigeria, Egypt, China, and Indonesia face the highest relative sea level rises, ranging from 7 to 10 millimeters per year. The United States, the Netherlands, and Italy also have elevated rates, averaging 4 to 5 millimeters per year. Cities like Jakarta, Tianjin, Bangkok, Lagos, and Alexandria are notable hotspots, with some sinking at rates exceeding 10 millimeters per year.

While subsidence is not uniform across all coastlines, some regions, such as Sweden and Finland, are experiencing the opposite effect due to postglacial rebound following the last Ice Age. This uplift occurs faster than sea level rise in these areas.

Groundwater use, a direct human influence, is a significant cause of land subsidence in many large coastal cities. Local policies and water management decisions can significantly impact subsidence rates. Improved groundwater management, stricter regulation of withdrawals, or aquifer recharge can help mitigate subsidence, as seen in Tokyo and the Harris-Galveston region of Texas.