The land beneath New York City is sinking in part due to the sheer mass of all its buildings, and it's not the only coastal city facing this fate. As sea levels rise and meet these concrete jungles, is there a chance to save 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 having been occupied in 1914, but its construction ushered in a real estate rush that continues unabated.
Throughout the 777 km² that make up the city of New York there are approximately 762 million tons of concrete, glass and steel, according to estimates by researchers from the United States Geological Survey (USGS, for its acronym in English).
While this figure involves some generalizations about building materials, that prodigious load does not include the fixtures, furnishings, and equipment for those millions of buildings. Nor does it include the transportation infrastructure that connects them, nor the 8.5 million people who inhabit them.
All that weight is having an extraordinary effect on the ground on which it is built.
According to a study published in May, that floor is sinking between 1 and 2 mm per year, partly due to the pressure exerted by the city's buildings.
And that worries the experts: by adding the subsidence of the land with the rise in sea levels, the relative rise in sea level is 3 to 4 mm per year .
It may not sound like much, but over the course of a few years, this translates into significant problems for a coastal city.
New York has been experiencing subsidence since the end of the last ice age.
Having shed the weight of the ice sheets, some areas on the East Coast have been expanding, while other parts of the coastline, including the area around New York City, appear to be settling.
“That relaxation causes the subsidence,” says Tom Parsons, a research geophysicist at the USGS Pacific Coastal and Marine Science Center in California, and one of the four authors of the study.
But the sheer 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 considered representative of other coastal cities in the US and around the world that have growing populations due to migration, experience associated urbanization, and are dealing with rising sea levels.”
There are a number of reasons why coastal cities are sinking, but the weight of human infrastructure on the ground plays a role .
The scale of this infrastructure is vast: in 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 people, from sinking into the sea?
Some cities around the world, like Jakarta, the Indonesian capital, are sinking much faster than others.
“In some cities, we're seeing subsidence of a few inches a year,” says Steven D'Hondt, a professor of oceanography at the University of Rhode Island in Narragansett.
At this rate, the city is sinking much faster than sea levels are rising to meet it. “We would have to increase ice melting 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 sink rates in 99 coastal cities around the world.
“If subsidence continues at recent rates, these cities will face severe flood events much sooner than projected,” wrote D 'Hondt' and colleagues Pei-Chin Wu and Matt Wei, both from the University of Rhode Island.
Southeast Asia frequently appeared on the list of cities suffering the fastest sinking. 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, are Manila (Philippines), Chittagong (Bangladesh), Karachi (Pakistan) and Tianjin (China).
These cities are already suffering infrastructure damage and frequent flooding .
Meanwhile, though not on the coast, Mexico City is sinking at an astonishing rate of 20 inches (50 cm) a year as the Spanish drained the underlying aquifers when they occupied it as a colony.
Research has suggested that it could be another 150 years (that is, around 30 meters of additional sinking) before that subsidence stops.
But it is the coastal cities that are the focus of the study by D'Hondt and his colleagues.
A large part of Semarang, Indonesia, for example, is sinking at 2-3 cm per year, while a significant area in northern Tampa Bay, Florida, is sinking at 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 loading our buildings, but also from groundwater extraction and oil and gas production at depth .
The relative contribution of each of these phenomena, Wei explains, “varies from one place to another, making it difficult to understand and address coastal subsidence.”
But we must address it. Rising waters do damage long before the flood barriers begin to break: it's a rising tide that sinks all ships.
The seas that rise
The first effects of a relative rise in sea level, according to D 'Hondt', occur below the surface.
“You have buried utility lines, buried infrastructure, buried building foundations, and then seawater starts interacting with those elements long before you see them on the surface.”
As this happens, storms carry more and more water into cities.
Solutions vary depending on the local causes of the subsidence .
An obvious approach, though with its own problems, is to stop construction.
As Parsons explains, the settlement of the soil under buildings “is usually complete a year or two after construction.”
Although much of New York City has a rocky base of shale, 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 can cause some of the greatest amounts of subsidence, Parsons and his colleagues note.
This is why ensuring that the largest buildings sit on the strongest bedrock could help reduce downward trend.
Another solution, at least for some places, is to reduce the extraction of groundwater and the exploitation of underground aquifers .
Parsons and his colleagues caution that increased urbanization will likely increase the amount of groundwater abstracted and will be combined with increased construction to cope with population growth.
Finding more sustainable ways to meet the city's water needs and maintain groundwater levels could help.
However, the most common method is a cumbersome and imperfect program to build and maintain flood defenses, such as seawalls.
Adapting to change
Tokyo's adaptation to land subsidence is two-pronged.
The city has built physical structures such as concrete levees, boardwalks, water pumping stations, and flood gates.
They are combined with social measures such as evacuation exercises and an early warning system .
Sometimes it is the residents themselves who take action.
A 2021 study documented how residents of 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 marshy 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, drainage expert at pipe manufacturer Wavin, says attenuation tanks should be combined with natural features such as ponds, seepage pits (rocky reservoirs through which water slowly drains) and ditches (swampy basins).
“The challenges we face today differ drastically from when urban sewerage and drainage systems were first introduced,” he says.
We may see more innovative solutions as the 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 need to worry,” says D'Hondt. “If you don't want to have huge losses of infrastructure and economic capacity in a few decades, you need to start planning right now.”