Sinking reality: Rising sea levels threaten cities but risks can be mitigated

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·Senior Reporter
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A worker carries a bamboo stick at a crumbled sea wall in an area affected by land subsidence and rising sea levels, in northern coast of Jakarta, Indonesia, December 9, 2019. Picture taken December 9, 2019. REUTERS/Willy Kurniawan
A worker carries a bamboo stick at a crumbled sea wall in an area affected by subsidence on the northern coast of Jakarta in Indonesia. (Reuters file photo)

This article is part of the Yahoo series ‘Sustainability: Fact/Myth’

Is the prospect of rising seas or sinking land a greater threat? The answer is they are equally perilous, especially for those living in populous coastal areas.

Sinking land, or subsidence, leads to higher sea levels and increased flood risks. To make matters worse, people often flock to reside in so-called "delta cities", as well as other areas prone to sinking, further accelerating subsidence.

Coastal residents are experiencing more extreme sea-level rise of about three to four times higher than the global average because they are living in places where land is sinking rapidly, according to a recent study published in Nature Climate Change. Between 1993 and 2015, the sea levels in such places had risen by an average of 7.8 to 9 mm annually.

Among the cities affected, parts of Tokyo had sunk by four metres during the 20th century, while Shanghai, Bangkok, and New Orleans fell by at least two metres over the period. 

Flood-prone Jakarta – at about 40 per cent below sea level – is sinking faster than any other city in the world. If it continues to sink at the current rates of up to 10cm to 20cm per year, one-third of the entire city is expected to be submerged by 2050.

Such consequences can be dire for coastal residents. Subsidence can cause serious damage to buildings, and important infrastructure such as railway lines and roads, and trigger widespread flooding. In the worst-case scenario, it can even lead to the loss of entire coastal areas.

And the problem is expected to worsen as the number of people living in coastal floodplains is expected to rise from 249 million in 2015 to 280 million in 2050.

As the causes behind subsidence are largely driven by human activities like extensive groundwater withdrawal, oil or gas extraction as well as heavy coal mining, they can be mitigated with measures such as groundwater drainage and pumping, and tighter regulations. 

Urban planners can also redesign cities to lessen the impact on the environment, such as creating plenty of green spaces to allow natural groundwater replenishment from rain and building high rises on bedrock. While building higher sea walls can help in the short run, it is not a permanent solution, said experts.

Some authorities are resorting to implementing drastic measures to deal with the dire threat of cities sinking such as relocation. 

In 2019, Indonesia announced that the capital Jakarta would relocate to a site in the sparsely populated East Kalimantan province on the island of Borneo. The plan to move the city's population of some 10 million in the first half of 2024 is expected to cost a staggering 489 trillion rupiah (US$34 billion) and potentially endanger wildlife habitats. 

Many such measures are expected to be introduced by authorities around the world in the years to come as they attempt to avert the sinking reality of irreversible subsidence.

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