The ground beneath New York City is sinking, in part due to the mass of all its buildings – and it’s not the only coastal city to be impacted by this problem.
As sea levels rise in these concrete jungle regions, can they still be saved?
On September 27, 1889, workers put the finishing touches on the Tower Building. It was an 11-story building that, with its steel structure, is considered New York City’s first skyscraper.
The construction marked the beginning of a wave that still hasn’t stopped.
In the 777 km² that comprise the city of New York, there are 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 number includes some generalizations about building materials, this tonnage does not include the light fixtures, fixtures, and furniture that are in these buildings.
Nor does it include the transport infrastructure that connects them, nor the 8.5 million people who inhabit them.
All that weight is having an extraordinary effect on the ground it was built on. That soil, according to a study published in May, is sinking by 1 to 2 mm per year, partly due to the pressure exerted on it by city buildings. And this is worrying for experts.
That might not seem like a lot, but over a few years it adds up to significant problems for a coastal city.
New York has already been suffering from sinking since the end of the last ice age.
Relieved of the weight of ice sheets, some land on the East Coast is expanding, while other parts of the coastal zone, including the chunk where New York City sits, appear to be settling down.
“This relaxation causes subsidence,” says Tom Parsons, a research geophysicist at the USGS’s Pacific Coastal and Marine Science Center in Moffett Field, Calif., and one of the study’s four authors.
But the sheer weight of the city’s built environment makes that sinking worse, says Parsons.
And this is a global phenomenon. New York City, says Parsons, “can be seen as an example for other coastal cities in the United States and the world, which have growing populations of people migrating to them, which have associated urbanization and which face rising levels of sea”.
There are a wide range of reasons why coastal cities are sinking, but the mass of human infrastructure pressing down on land is playing an important role.
The scale of this infrastructure is vast: in 2020, the mass of human-made objects surpassed that of all living biomass.
Can anything be done to stop these cities – some with hundreds of millions of people – from sinking into the sea?
Some cities around the world – like 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 in Narragansett.
At this rate, the city is sinking much faster than sea levels are rising.
“There would have to be a significant increase in melting ice 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 subsidence rates in 99 coastal counties around the world.
“If subsidence continues at recent rates, they will be challenged by severe flooding much sooner than projected,” wrote D’Hondt and colleagues Pei-Chin Wu and Matt Wei, both of the University of Rhode Island.
Southeast Asia featured heavily on the list of cities that experienced the fastest subsidence. Parts of Jakarta are sinking between 2 and 5 cm per year.
Alongside Jakarta, which is being replaced as the capital by a city built 1,996 km away, were Manila (Philippines), Chittagong (Bangladesh), Karachi (Pakistan) and Tianjin (China). These cities are already suffering infrastructure damage and frequent flooding.
Meanwhile, although it’s not on the coast, Mexico City is sinking at a staggering 50 centimeters a year, thanks to the Spanish draining underground aquifers when they occupied it as a colony.
Research suggests it could take another 150 years for the sinking to stop.
But it is coastal cities that are the focus of study by D’Hondt and his colleagues. A large part of Indonesia’s Semarang, for example, is sinking by 2 to 3 cm per year, while a significant area north of Tampa Bay, Florida is sinking by 6 mm annually.
Some level of this subsidence happens naturally, says Wei. However, it can be greatly accelerated by humans – not just by loading our buildings, but also by our groundwater extraction and our deep-sea oil and gas production.
The relative contribution of each of these phenomena, says Wei, “varies from place to place, making understanding and addressing coastal subsidence a challenging task.”
But we must deal with it. The rising water wreaks havoc long before it starts hitting the flood barriers: it’s a high tide that sinks all boats.
The first effects of a relative sea level rise, says D’Hondt, occur below the surface.
“You have buried utility lines, buried infrastructure, foundations for buildings, and then seawater starts working with these things long before you see them above ground.” As this happens, storms drive water deeper and deeper into cities.
Solutions vary according to the local causes of subsidence.
An obvious approach, albeit with its own problems, is to stop building. As Parsons explains, laying the ground under buildings “is usually completed within a year or two of construction.” Although much of New York City has bedrock of shale, marble, and gneiss, these rocks have a degree of elasticity and fracture that account for some of the subsidence. But the clay-rich soil and artificial fill materials that are particularly prevalent in lower Manhattan may cause some of the greatest amounts of subsidence, say Parsons and his colleagues. Therefore, ensuring that the largest buildings are positioned on the most solid bedrock can help reduce the downward trend.
Another solution, at least for some places, is to delay groundwater withdrawal and aquifer extraction. Parsons and his colleagues caution that increased urbanization is likely to increase the amount of groundwater being drawn, as well as being combined with even more construction to cope with population growth. Finding more sustainable ways to meet the city’s water needs and maintain groundwater levels can help.
However, the most common approach is a messy and imperfect program of building and maintaining flood defenses such as sea walls. Tokyo’s adaptation to land subsidence is twofold. The city built physical structures such as concrete dikes, sea walls, pumping stations and locks. These are combined with social measures such as evacuation rehearsals and an early warning system.
Some level of this subsidence occurs naturally, but it can be greatly accelerated by humans – not just by loading our buildings, but also by extracting groundwater and our deep-sea oil and gas production.
Sometimes, it is the residents themselves who intervene. A 2021 study documented how residents of Jakarta, Manila and Ho Chi Minh City took their own informal measures. This includes raising floors, moving appliances and, in Manila, building makeshift bridges between homes in swampy areas.
Other useful tools include attenuation tanks: large reservoirs that sit underground and release stormwater at a controlled, slow rate. Martin Lambley, a drainage expert at pipe maker Wavin, says that attenuation tanks should be combined with natural elements like ponds, sinkholes (gravel pits from which water drains slowly) and ditches (swamp basins).
“The challenges we face today differ drastically from when urban sewers 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 cities on water, which can take the form of floating structures.
Finally, halting climate change by eliminating greenhouse gas emissions would prevent or slow down at least some of the melting of the polar ice caps, slowing sea level rise.
“I think governments need to be concerned,” says D’Hondt. “If they don’t want to have a massive loss of infrastructure and economic capacity in a few decades, they need to start planning now.”