New York is sinking, and its skyscrapers are bringing it down. That's the finding of a new study that modeled the geology beneath the city compared to satellite data showing its footprint is collapsing into Earth.
Technically called subsidence, this gradual settling or sudden sinking of Earth's surface occurs when soft sediments shift, or loads bearing down on the ground push it deeper still. There are many causes, but the weight of cities themselves is rarely studied.
New York is sinking at a rate of 1-2 millimeters per year, the study found, under the weight of its sky-high buildings. A few millimeters might not sound like much, but some parts of the city are subsiding much faster, on par with the fastest observed rates at which tectonic plates rebound when glaciers melt.
The deformation could spell trouble for the low-lying city home to more than 8 million people, so the findings should encourage further efforts to develop mitigation strategies to counter growing flood risk and rising sea levels – though perhaps building gigantic sea walls to fortify the city aren't the right answer.
"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," writes geologist Tom Parsons of the United States Geological Survey and his colleagues at the University of Rhode Island.
It also brings to mind the totality of human construction. As of 2020, scientists estimated that all the stuff humans have ever made was close to or already outweighing the dry weight of every last living thing on Earth. Buildings and roads weigh more than all trees and shrubs combined; plastics double the weight of animals.
In this new study, Parsons and colleagues calculated the cumulative mass of the more than 1 million buildings in New York City, which worked out to be 764,000,000,000 kilograms or 1.68 trillion pounds. Then they divvied up the city into a grid of 100-by-100-meter squares and converted building mass to downward pressure by factoring in gravity's pull.
Their estimates only include the mass of buildings and their contents, not the roads, sidewalks, bridges, railways, and other paved areas of New York City. Even with those limitations, these new calculations refine past observations of subsidence in the city by accounting for the complex surface geology beneath New York that consists of sand, silt, and clay lake deposits, and bedrock outcrops.
Modeling the behavior of these substrates, the researchers showed clay-rich soils and artificial fill are particularly prone to subsidence: the middle value being one ruler's length (294 millimeters) at a sample site in lower Manhattan. More elastic soils bounce back after construction, while bedrock, anchoring many skyscrapers, doesn't budge as much.
Comparing these models with satellite data measuring land surface height, the team mapped its subsidence estimates across the city. Increased urbanization, including the draining and pumping of groundwater, could only add to New York's subsidence problem, the researchers warn.
New York is certainly not alone in its subsidence. A quarter of Indonesia's capital, Jakarta, could be underwater by 2050, with parts of the city sinking by almost 11 centimeters a year due to groundwater extraction. More than 30 million Jakarta residents are now reckoning with the prospect of relocating the city or doubling down on climate action, such as opting for electric buses.
New York, by comparison, ranks third in its exposure to future flooding. Much of lower Manhattan lies only 1 and 2 meters above current sea levels. Hurricanes in 2012 (Sandy) and 2021 (Ida) also showed how quickly the mostly paved city could be flooded by water.
"New York is emblematic of growing coastal cities all over the world that are observed to be subsiding, meaning there is a shared global challenge of mitigation against a growing inundation hazard," the researchers conclude.
A 2022 study of 99 coastal cities around the world found that subsidence might actually pose a bigger, or at least underrated, problem compared to sea-level rise. In most cities surveyed, the land is subsiding faster than sea levels are rising, which means residents will be challenged by flooding sooner than climate models have projected.
While tons of skyscrapers have already been built, our planet's future trajectory is not set in stone – and reducing greenhouse gas emissions is the best bet we have to limit future risks, be they rising seas or hurricanes.
The research has been published in Earth's Future.