Large coastal cities like New York face a double risk from climate change and land subsidence.
New York is the most populous city in the United States. Due to climate change, it also faces an increasing risk of coastal flooding and typhoon surges. An estimated 1. 3 million New Yorkers now live in or near the city’s floodplain, and that number could reach 2. 2 million by the end of the century.
Flood trends have led many low-lying coastal cities to invest in protective infrastructure: building levees, raising roads, creating synthetic barriers, levees, and reefs. These “hard” structures are the default option for cities that want to preserve themselves against flooding. In 2017, 14% of the total U. S. coastline was shielded in this way, and that percentage is expected to rise. But the truth is that the dikes are a brutal instrument; it is not a fine-tuned solution. They’re expensive to build and require constant maintenance: The U. S. Army Corps of Engineers’ most recent plan is a major one. The U. S. budget to protect New York from coastal storms is valued at $52. 6 billion. Creating an area for a seawall also plays a role in the destruction or displacement of vital coastal habitats: a 2016 study found that “seawalls were home to 23% less biodiversity and 45% fewer organisms than natural coastlines. “This is rarely a permanent solution, as existing walls require “additions” to the construction up to their height.
And as an organization of researchers from the United States Geological Survey (USGS) and the University of Rhode Island (URI) has shown, rising waters are the only threat to New York. The relentless structure is also causing the city to sink, exacerbating the threat of long-term flooding.
Subsidence is a general term used to describe the sinking of the Earth’s surface as a result of herbal processes or human activities. Earthquakes, erosion, groundwater extraction, and mining can cause vertical downward movement of soil and rocks. In their (open access) paper, published in the journal Earth’s Future, the USGS/URI team focused on the contribution that “cumulative mass and downward stress exerted through the built environment” make to the city’s overall subsidence. . In other words, they wanted to calculate the collective weight of New York’s buildings*. Combined with knowledge of the underlying geology, they can estimate the degree of subsidence (in millimeters) resulting from those buildings.
They accessed the surface area and height of 1,084,954 buildings in the five boroughs of New York City through a public database developed through Microsoft. To get the total square footage of each building, the researchers estimated the number of floors (based on a constant ceiling height) and multiplied it by the footprint. The mass calculation also required some mandatory simplifications: for example, they assumed that the design itself exerted 2. 0 kN/m2 (called “dead load” in engineering). This price is representative of reinforced concrete; In fact, a ubiquitous building material. The “live load”, which represents the content of the building, was calculated on average at 4. 79 kN/m2 to take into account the wide variety of construction uses in the city. They found that New York’s buildings together weigh 7. 64 x 1,011 kg (1. 68 trillion pounds), spread over a domain of 778. 2 km2.
The surface geology of New York City has been widely studied, but is fairly complex; a patchwork that includes “silt, sand, and clay lake deposits, glacial moraines, outwash and till, beach deposits, and bedrock outcrops,” as well as artificial fill along the water line. Because of this, it’s not possible to know precisely what sits beneath every building. And with more than a million buildings involved in the calculation, information on the specific foundation style of each one was fairly limited. To address this, the researchers to create a series of regional soil and bedrock models, and applied them to the city.
Their modeling effects were very diverse and had a high degree of uncertainty due to the simplifications involved. However, everyone agreed that some areas of the town had been weakened more temporarily than others. There is very little construction-related subsidence in spaces like midtown Manhattan because foundations there tend to anchor directly into hard bedrock. However, lower Manhattan and southern Brooklyn are largely built on synthetic fill, a combination of materials much less compact than any natural geological terrain. This makes those spaces vulnerable to collapsing under the pressure of construction.
The researchers then compared their effects with those of previous studies of satellite techniques (i. e. , interferometry and GPS measurements) to map floor deformation. Although it was not a direct comparison, it allowed them to infer the possible contribution of the urban structure’s load to the overall subsidence of the city.
They discovered that the average rate of subsidence in the city is 1 to 2 mm/year, however, in spaces like Queens and Brooklyn it is particularly higher; “. . . up to about 4½ millimeters per year,” Parsons told CNN. This may not seem like much, but considering that global sea level has risen 98. 5 mm in the last 30 years (an average of 3. 28 mm/year), the effect becomes clearer. Speaking to Time, senior writer Tom Parsons described the likelihood that parts of New York could end up being permanently underwater as “…inevitable. The ground goes down and the water goes up. At some point those two degrees will meet.
The Big Apple is not alone in this. A global study published in 2021 concluded that declining cities will experience rising sea temperatures at a rate up to four times faster than solid regions. A separate 2022 study looked at the world’s 48 largest coastal cities. , which combined represent approximately one-fifth of the world’s urban population. It found that 44 of those cities had spaces that were sinking faster than sea levels were rising. It has already led some governments to take drastic measures. Indonesia is moving its capital from Jakarta to Nusantara, a purpose-built city on the island of Borneo (which has its own significant environmental problems).
The authors of this Earth’s Future study aren’t calling for the urgent evacuation of New York City. Rather, they write that their goal “… is to raise awareness that every additional high-rise building constructed at coastal, river, or lakefront settings could contribute to future flood risk, and that mitigation strategies may need to be included.” Let’s see if their message sinks in.
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*They excluded “…. roads and other paved areas, sidewalks, parks, bridges, railways, etc. From your calculation, your load is probably underestimated.
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