44 of the 48 most populous coastal cities have areas that are sinking faster than the sea is rising, driven by groundwater pumping and compacted soil from heavy buildings


September 19, 2022

Ho Chi Minh City in Vietnam is sinking at 16.2 millimeters per year

Shutterstock/Nguyen Quang Ngoc Tonkin

Almost all of the 48 most populous coastal cities have areas that are sinking faster than seas are rising.

Globally, sea levels are rising by an average of about 3.7 millimeters per year, much of which is due to melting ice caused by climate change. In many places, the land is also sinking due to groundwater pumping, oil and gas extraction, and sediment compacted by heavy buildings—a process called land subsidence.

Cheryl Tay at Nanyang Technological University in Singapore and colleagues used radar from satellites to measure subsidence rates in the world’s 48 most populous coastal cities between 2014 and 2020.

In 44 cities, the fastest-sinking areas are sinking faster than sea levels are rising. Cities in South and Southeast Asia are some of the fastest-subsiding cities, including Tianjin in China and Ahmedabad in India, both of which are sinking faster than 20 millimeters a year.

The average rate of subsidence varies from 16.2 millimeters per year in Ho Chi Minh City, Vietnam to a rise of 1.1 millimeters per year in Nanjing, China. These levels are not adjusted for other factors that can raise or lower the land level, such as a phenomenon where ice-pressed land rises over thousands of years after the ice melts. The researchers measured all areas within a city’s borders, even if it was miles from the coast.

In a case study of Ho Chi Minh City, researchers found that an additional 20 square kilometers will be below sea level and could be flooded if current rates of subsidence continue until 2030. About 880 square kilometers will be below sea level without subsidence. Rio de Janeiro could see an additional 2 square kilometers flooded, representing a 16% increase in flooding without subsidence.

Manuchehr Shirzaei at Virginia Tech says that the elevation maps used in case studies to estimate the extent of flooding are not reliable for many parts of the world, and that focusing on the fastest sinking inland areas of cities rather than areas directly on the coast, may “exaggerate the impact” of land subsidence. A recent IPCC report looking at global coastal land level changes over a century, we found a narrower range – the fastest sinking was 5.2 millimeters per year.

But if the rapid sinking reported here persists, it could be a “very significant driver” of sea-level hazards, says Robert Kopp at Rutgers University in New Jersey, who worked on the IPCC report.

“Land subsidence has always been treated as an afterthought,” says Shirzaei. “It puts him in the spotlight.”

Journal reference: Resilience of nature, DOI: 10.1038/s41893-022-00947-z

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