Excerpt:
The weight of New York City’s 1.1 million buildings is making the city slowly sink.
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…
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