Home to 8.8 million people as of 2020, New York City is by far the most populous city in the U.S. And the mass of the buildings needed to support all those residents—and the work they do—really adds up. New research published on May 8 in Earth’s Future suggests that the weight of the city itself is pressing down on the land it occupies and contributing to local sea-level rise that increases flood risks.
“In terms of worrying about sea-level rise globally, generally the notion that most people have is that ice is melting, and that changes sea level,” says Jacky Austermann, a Columbia University geophysicist, who was not involved in the new research. But “it’s only part of the contribution to sea-level rise at any given location.” The sinking of land, which can happen for a range of reasons, is another important factor to consider, she says. “If you’re standing at the shoreline, whether the land’s going down or sea level’s going up, both cause the same amount of flooding,” Austermann adds.
The scientists behind the new research wanted to try to understand how the weight of a city itself might contribute to local sea-level rise—and New York City boasts quite a lot of weight to study. “We’ve just imported all this mass,” says lead author Tom Parsons, a geophysicist at the U.S. Geological Survey. “In lower Manhattan, it almost looks like a mountain range that we’ve kind of built up there, so all that weight is also pressing down.”
The researchers’ first step was to analyze the city’s weight, which, for the purposes of the paper, meant the city’s buildings: all 1,084,954 of them across all five boroughs. Parsons and his colleagues mapped the city on a grid and then consulted a database that included the footprint and total height of every building in the city. They used building codes to estimate the weight in each grid square—and came up with a total of 764 billion kilograms (1.68 trillion pounds) for all of New York City’s buildings. “It’s not a perfectly exact weight, but it gives us a rough idea of what the concentration of buildings is,” Parsons says. (For simplicity’s sake, the team didn’t factor in the weight of roads and sidewalks.)
Next the researchers mapped New York City’s geology. In some neighborhoods, such as midtown Manhattan, bedrock lies close to the surface, and there’s relatively little soil to compress. This makes it less susceptible to weight-induced subsidence. In other areas, such as along the southern coast of Brooklyn, the city has artificially expanded its footprint using fill. Artificial fill can consist of a variety of materials, but it is particularly vulnerable to pressure from any mass above it because it is not as compact as the natural landscape. Elsewhere, the geology lies somewhere between these extremes.
The researchers fed their maps of building distribution and ground type into a set of models designed to predict how different geologies respond to pressure. The results allowed them to spot areas where subsidence from the city’s own weight might be particularly prevalent.
Finally, they looked at satellite data to see how much sinking had actually happened across the city over the past decade and found an average rate of one to two millimeters per year. Importantly, that overall subsidence includes factors besides urban weight—so comparing the models with the satellite observations only shows where the city’s weight might account for a larger portion of this sinking. The researchers concluded that in areas where building weight is concentrated on looser soils, it is likely contributing substantially to subsidence. The study is meant not as a definitive analysis but as a first step in understanding how cities around the world may be contributing to the sea-level rise that threatens them. “As [the study authors] highlight..., the comparison between the data and the models is complicated. There’s a lot of things we don’t understand,” Austermann says, adding that the study essentially gives a rough estimate of subsidence from urban weight instead of an exact calculation.
Another limitation of the study is that the scientists weren’t able to mimic the 400-year development of the city to fully capture and project how its weight—and therefore any related sinking—could play out over the coming years. “They model the overall subsidence from an initial load of all the buildings as though they were somehow constructed at the same time—magically appearing on uncompressed soil or rock simultaneously,” says Cathleen Jones, a physicist at NASA’s Jet Propulsion Laboratory (JPL). Jones wasn’t involved in the new research, although she specializes in using satellite data to study subsidence and other types of land deformation. “Of course, the buildings were built at different times, so that part of the model is unrealistic,” she says, adding that this is her own opinion, not that of either NASA or JPL. Jones does note that the team’s long-term subsidence estimates are more realistic and match satellite observations.*
While the new research examined only New York City, the study is an important reminder of issues that are playing out in coastal cities around the world. About 40 percent of people already live within 100 kilometers of the world’s coasts, and by 2050 nearly 70 percent are expected to live in a city. That combination means cities already vulnerable to sea-level rise may also struggle under their own weight—and while much of New York City benefits from strong bedrock, other cities are on more perilous footing. “Most of the coastal cities around the world are expanding in significant ways,” Parsons says. “It’s likely to be a growing concern, given that kind of change in the distribution of people.”
*Editor’s Note (5/30/23): This paragraph was updated after posting to reflect the full extent of Cathleen Jones’s comments on the accuracy of the study’s results.