‘Sheer weight’ of cities is pushing famous locations under water, study warns

It leaves a sinking feeling.

Rising seas aren’t the only flood risk afoot on Earth. Scientists have found that the planet’s most populated metropolises are sinking under their own weight, leaving millions of people at risk of going into the drink.

This worldwide urban subsidence has the potential to amplify the effect of “sea-level rise,” according to the study’s lead author Dr. Julius Oelsmann, of Technical University of Munich.

Per a study in the journal Nature Communications, this double threat of elevating oceans and plunging land has caused sea levels to rise by 6 millimeters annually along urbanized areas of the coast — around three times the global average.

What’s to blame for this metropolitan plunge? Oelsmann pointed to the “sheer weight of cities,” explaining that the push to build taller, heavier buildings puts a strain on the ground, causing the hub to plummet relative to its surroundings.

This effect is compounded by the excessive groundwater and oil extraction, which causes the terrain to become increasingly tenuous.

“In many large coastal cities, groundwater extraction is a major driver of land subsidence,” said study co–author Professor Florian Seitz, of the Technical University of Munich.

The countries with the most rapid rate of sea-level rise are Thailand, Bangladesh, Nigeria, Egypt, China, and Indonesia, where the ocean is creeping up 10 millimeters higher every year.

The downward trend isn’t limited to Africa and Asia: the US, the Netherlands and Italy are seeing a similar surge, with sea levels inching up by around 4 millimeters to 5 millimeters annually.

Naturally, the sinkage is concentrated in urban areas with the world’s most populous metropolis, Jakarta — home to 42 million people — descending at a rate of 13.7 millimeters per year.

The subsidence isn’t evenly distributed as some parts of the city are sinking at a staggering 42 millimeters annually, while other areas are experiencing an uplift.

At this rate, Jakarta, 40% of which is already below sea level, could see half its area swamped and uninhabitable by 2050.

Following behind the Indonesian Capital is Tianjin, China, which is sinking by 13.5 millimeters per year.

Meanwhile, Bangkok, Lagos and Egypt’s Alexandria are also suffering from subsidence rates of 8.5 millimeters, 6.7 millimeters and 4 millimeters per year, respectively.

And while these might not seem like giant amounts, every millimeter of subsidence causes the risk of flooding to go up exponentially amid severe storms.

Reversing the trend is not easy, although the researchers point out that water management decisions can help slow the decline.

In Tokyo, for example, the rates of subsidence once surpassed 10 millimeters annually — and 24 millimeters in the worst areas — but Japanese government policies to introduce new water resources helped curb the damage.

“Improved groundwater management, stricter regulation of withdrawals, or targeted recharge of aquifers can at least slow subsidence rates and, in some cases, largely halt them,” said Seitz.

Ultimately, the study demonstrates that the sea isn’t the only indicator of coastal flood dangers.

“If we want to understand sea–level rise along coastlines and respond effectively, we must not only observe the ocean but also the land itself,” said Oelsmann.