Many coastal cities around the world are sinking

Istanbul, Shanghai, Houston, Bombay, Lagos, Taipei, Auckland, Manila… These major coastal cities are all sinking. This is revealed a study published in Geophysical Research Letters for which the rates of subsidence (slow subsidence of the surface of the earth’s crust) of 99 cities were measured using satellite data acquired over six years. Its authors conclude that:

“Land subsidence is faster than sea level rise in most cities that are at risk of flooding sooner than expected.”

Quartz reports that this is the case for at least 33 cities that “sink more than a centimeter per year, five times faster than the rate of sea level rise” as estimated in a 2018 study. The site notes that “the fastest sinking cities, concentrated in South and Southeast Asia, are being forced to adapt”.

This is the case of Jakarta, Indonesia. hakai signal than at the current rate “a third of the city will be under water by the middle of the century”, which partly explains why the Indonesian authorities decided to move their capital to Kalimantan, on the island of Borneo.

“Many cities are predicting sea level rise, but they are unaware of the cumulative effect of coastal subsidence,” explains to the magazine the American oceanographer Meng Wei, who participated in the study.

A problem: groundwater extraction

Groundwater pumping is believed to be one of the main causes of subsidence, note Scientific American, which specifies:

“The burgeoning urban population is putting increased pressure on underground aquifers to wash, cook and clean. Oil and gas production and new construction also contribute to the problem.”

Although the sagging phenomenon cannot be reversed, it can nevertheless be mitigated. The magazine explains that some cities like Jakarta, Houston and Shanghai have taken steps to improve groundwater management. Tokyo has even succeeded in halting subsidence by regulating groundwater extraction, claims hakai Roderik van de Wal, a sea level change expert from the University of Utrecht.

But cities are already experiencing the effects of coastal flooding from a combination of subsidence and sea level rise. Quartz points out that subsidence has been cited as part of the explanation for the collapse of a building in Surfside, Florida, which killed 98 people in 2021. The site warns:

“Many of the factors that lead to worsening flooding in cities are irreversible. At best, local governments will only be able to adapt through regulations and, at times, backsliding.”

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