The World's Fastest-Sinking Coastal Cities

Rising sea levels pose a threat to the future of coastal cities around the world, but another major concern is the sinking of the cities themselves. Known as "Relative Local Land Subsidence" (RLLS), this phenomenon occurs when underground materials compact or collapse, causing the surface above to sink.

This subsidence can exacerbate the effects of rising sea levels, which are currently averaging 3.7 mm/year, making it a crucial metric to track for coastal communities.

Using research that monitored changes in land subsidence in 48 high-population coastal cities located within 50 kilometers of the coastline between 2014 and 2020, Planet Anomaly mapped the fastest-sinking coastal cities across the globe.

Key Findings:

• Out of the 44 sinking coastal cities observed to be sinking faster than sea levels were rising, 30 are located in Asia.
• Tianjin, China, and Ho Chi Minh City, Vietnam, lead the list of the fastest-sinking coastal cities overall, experiencing peak Relative Local Land Subsidence (RLLS) velocities of 43 mm per year during the period under examination.
• Among the 10 fastest-sinking coastal cities, the only non-Asian areas include Houston, US; Lagos, Nigeria; and technically, Istanbul, Turkey, situated across both Asia and Europe.
• The study's authors highlight that many of the identified sinking coastal areas are "rapidly expanding megacities," with "high demands for groundwater extraction and loading from densely constructed building structures" cited as contributing factors to local land subsidence.

One of the leading cities on the list is Tianjin, China, with a population exceeding 14 million people. Certain areas of the city experienced peak Relative Local Land Subsidence (RLLS) velocities of 43 mm per year between 2014 and 2020. However, the median velocity is significantly lower, at 6 mm/year, indicating that some areas are sinking much faster than the overall metropolitan area.

Tianjin is situated adjacent to Beijing municipality in the northwest and the Bohai Gulf in the east. In June 2023, significant cracks appeared on Tianjin’s streets due to underground land collapses, a consequence of extensive geothermal drilling, according to the local government.

Ho Chi Minh City in Vietnam, with a population of 9 million, also contends with comparable RLLS rates as Tianjin, although its median velocity is notably higher at 16 mm/year.

Chittagong, Bangladesh; Yangon, Myanmar; and Jakarta, Indonesia, complete the top five fastest-sinking coastal cities by relative land subsidence. All face similar contributing factors, as noted by the authors of the paper:

“Many of these fast-subsiding coastal cities are rapidly expanding megacities, where anthropogenic factors, such as high demands for groundwater extraction and loading from densely constructed building structures, contribute to local land subsidence.” — TAY, C., LINDSEY, E.O., CHIN, S.T. ET AL.

Indonesia has ambitious plans to relocate its sinking capital, Jakarta, to another island, a move estimated to cost the Indonesian government more than $120 billion. This initiative follows forecasts suggesting that one-third of Jakarta could be submerged as early as 2050. Beyond the regular flooding, Jakarta is also highly susceptible to earthquakes.