The City of New York is sinking from the weight of its buildings, slowly, into the sea.
That was the conclusion reached by a geological research team at the University of Rhode Island led by geologist Tom Parsons of the US Geological Survey in a study whose findings were published last week in the journal Earth’s Future.
“New York City faces accelerating inundation risk from sea level rise, subsidence, and increasing storm intensity from natural and anthropogenic causes,” the research team wrote.
The study finds that New York is sinking at a rate of 1-2 millimeters each year; but some parts of the city are sinking even faster, at a rate like that of tectonic plates rebounding when glaciers melt.
The phenomenon is called “subsidence,” a gradual settling or sudden sinking of the Earth’s surface, according to Science Alert. It occurs when soft sediments shift, or when loads bearing down on the ground push it deeper.
To reach their conclusion, the research team calculated the cumulative mass of the more than one million buildings in New York City, weighing 1.68 trillion pounds (764 billion kilos). The weight of paved surfaces such as roads, sidewalks, bridges, railways and other structures was not included.
They divided the city into a grid of 100×100 meter squares, and converted the building mass to downward pressure, including the effect of the pull of gravity.
The team then compared the models with satellite data measuring land surface height and mapped its subsidence estimates across the city.
The study is not the first to examine the subsidence in the city. A 2022 study of 99 coastal cities around the world found that subsidence could pose even a bigger problem than sea-level rise. “New York is emblematic of growing coastal cities all over the world that are observed to be subsiding,” the research team wrote.
But it builds on prior research by refining those observations by also considering the complex surface geology beneath the city – comprised mostly of sand, silt, clay lake deposits and bedrock outcrops.
Artificial landfill and clay-rich soils are particularly prone to subsidence, the researchers note, adding that more than eight million people live in New York City, which is sinking at one to two millimeters per year, “while the sea level rises.”
Parson warns the cumulative weight will eventually sink the city into the surrounding sea, adding that the problem is not exclusive to New York.
“The point of the paper is to raise awareness that every additional high-rise building constructed at coastal, river, or lakefront settings could contribute to future flood risk, and mitigation strategies may need to be included,” the researchers wrote.