The combined weight of its skyscrapers, transportation systems and 8.5 million people are causing it to shrink by 1 to 2 mm per year.

The land beneath New York City is sinking in part due to the sheer mass of all its buildings, and it is not the only coastal city facing this fate. As sea levels rise and meet these concrete jungles, is there any chance of saving them?

On September 27, 1889, workers put the finishing touches on the Tower Building. It was an 11-story building that, thanks to its steel skeleton structure, is considered the first skyscraper in New York City.

The Tower Building no longer exists, its prime spot on Broadway was taken in 1914, but its construction marked the beginning of a real estate rush that continues unabated.

Across the 777 square kilometers that make up New York City lie approximately 762 million tons of concrete, glass and steel, according to estimates by researchers at the U.S. Geological Survey (USGS).

While this figure involves some generalizations about building materials, that prodigious load does not include the fixtures, furnishings and equipment in those millions of buildings. Nor does it include the transportation infrastructure that connects them, or the 8.5 million people who inhabit them.

All that weight is having an extraordinary effect on the land on which they are built.

According to a study published in May, that ground is sinking by 1 to 2 mm per year, partly due to the pressure exerted by the city’s buildings.

And that worries experts: when adding soil subsidence to rising sea levels, the relative sea level rise is 3 to 4 mm per year.

It may not sound like much, but over a few years, this translates into significant problems for a coastal city.

The subsidence

New York has been suffering from subsidence since the end of the last ice age.

Having been freed from the weight of the ice sheets, some areas on the east coast have been expanding, while other parts of the coastline, including the area where New York City is located, appear to be settling.

“That easing causes subsidence,” says Tom Parsons, a research geophysicist at the USGS Pacific Coastal and Marine Science Center in California, and one of the study’s four authors.

But the enormous weight of the city’s built environment makes this subsidence worse, Parsons says.

And this is a global phenomenon.

New York City, according to Parsons, “can be seen as representative of other coastal cities in the U.S. and around the world that have growing populations due to migration, experiencing associated urbanization, and facing sea level rise.”

There are a variety of reasons why coastal cities are sinking, but the weight of human infrastructure on the land plays a major role.

The scale of this infrastructure is vast: by 2020, the mass of human-made objects exceeded living biomass.

Can anything be done to prevent these cities, which together are home to hundreds of millions of inhabitants, from sinking into the sea?

Other cases

Some cities around the world, such as Jakarta, the capital of Indonesia, are sinking much faster than others.

“In some cities, we’re seeing subsidence of a few centimeters a year,” says Steven D’Hondt, professor of oceanography at the University of Rhode Island at Narragansett.

At this rate, the city is sinking much faster than sea levels are rising to meet it. “We would have to increase ice melt by an order of magnitude to match that.”

In addition to co-authoring the New York study, D’Hondt is one of three authors of a 2022 study that used satellite imagery to measure sinking rates in 99 coastal cities around the world.

“If subsidence continues at recent rates, these cities will face severe flood events much earlier than projected,” wrote D’Hondt and colleagues Pei-Chin Wu and Matt Wei, both of the University of Rhode Island.

Southeast Asia frequently appeared on the list of cities suffering the fastest subsidence. Parts of Jakarta are sinking between 2 cm and 5 cm per year.

In addition to Jakarta, which is being replaced as Indonesia’s capital by a city being built 1,996 km away, Manila (Philippines), Chittagong (Bangladesh), Karachi (Pakistan) and Tianjin (China) are also on the list.

These cities are already suffering from infrastructure damage and frequent flooding.

Meanwhile, although not on the coast, Mexico City is sinking at an astonishing rate of 50 cm (20 inches) a year because the Spanish drained the underlying aquifers when they occupied it as a colony.

Research has suggested that it could be another 150 years (i.e., about 30 meters of additional subsidence) before that sinking stops.

But it is the coastal cities that are the focus of the study by D’Hondt and colleagues.

A large part of Semarang, Indonesia, for example, is sinking by 2 to 3 cm per year, while a significant area in northern Tampa Bay, Florida, is sinking at the rate of 6 mm per year.

Some level of this subsidence occurs naturally, Wei notes.

However, it can be greatly accelerated by human activity, not only from the loading of our buildings, but also from groundwater extraction and deep oil and gas production.

The relative contribution of each of these phenomena, Wei explains, “varies from place to place, making it difficult to understand and address coastal subsidence.”

But we must address it. Rising waters cause damage long before flood barriers begin to break down: it is a rising tide that sinks all ships.

Rising seas

The first effects of a relative sea level rise, according to D’Hondt, occur below the surface.

“You have buried utility lines, buried infrastructure, buried foundations of buildings, and then, seawater starts interacting with those elements long before you see them above the surface.”

As this happens, storms bring water further and further into cities.

Solutions vary depending on the local causes of subsidence.

One obvious approach, albeit with its own problems, is to stop construction.

As Parsons explains, settlement of the ground beneath buildings “is usually complete a year or two after construction.”

Although much of New York City has a bedrock base of schist, marble and gneiss, these rocks have a degree of elasticity and fractures that contribute to some of the subsidence.

However, the clay-rich soil and artificial fill materials that are particularly prevalent in lower Manhattan may cause some of the greatest amounts of subsidence, Parsons and colleagues note.

That’s why making sure that larger buildings are placed on the more solid bedrock could help reduce the downward trend.

Another solution, at least for some locations, is to reduce groundwater extraction and groundwater aquifer mining.

Parsons and colleagues caution that increased urbanization will likely increase the amount of groundwater withdrawn and combine with more construction to meet population growth.

Finding more sustainable ways to meet city water needs and maintain groundwater levels could help.

However, the most common method is a cumbersome and imperfect program of construction and maintenance of flood defenses, such as seawalls.

Adapting to change

Tokyo’s adaptation to land subsidence is a two-pronged approach.

The city has built physical structures such as concrete levees, seawalls, water pumping stations and flood gates.

These are combined with social measures such as evacuation drills and an early warning system.

Sometimes it is the residents themselves who take action.

A 2021 study documented how residents in Jakarta, Manila and Ho Chi Minh City have taken measures of their own, such as raising floors, moving appliances and building makeshift bridges between houses in swampy areas, as is done in Manila.

Other useful tools include attenuation tanks: large tanks that sit underground and release rainwater at a slow, controlled rate.

Martin Lambley, a drainage expert with pipe manufacturer Wavin, says attenuation tanks should be combined with natural features such as ponds, infiltration pits (rocky deposits through which water slowly drains) and ditches (swampy basins).

“The challenges we face today differ dramatically from when urban sewer and drainage systems were first introduced,” he says.

We may see more innovative solutions as waters rise. In 2019, the UN held a roundtable on floating cities, which could take the form of pontoon structures.

Finally, halting climate change by eliminating greenhouse gas emissions would prevent or delay at least some of the melting of the ice caps, thus slowing sea level rise.

“I think governments should be concerned,” says D’Hondt. “If they don’t want to have huge losses of infrastructure and economic capacity in a few decades, they need to start planning now.”


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