With 8.8 million people as of 2020, New York City is by far the most populous city in the United States. And the amount of buildings needed to support all these inhabitants and their jobs would be truly staggering. A new study was published on May 8. the future of the earth It suggests that the weight of the city itself squeezes the land it occupies, contributing to localized sea level rise and increasing the risk of flooding.
“In terms of worrying about global sea level rise, the general concept that most people have is that the ice is melting and that’s causing sea levels to change,” he said, referring to the new study. Not so, says geophysicist Jackie Austerman of Columbia University. But “it’s only part of the contribution to sea-level rise in a given location.” Land subsidence can occur for a variety of reasons and is another important factor to consider, she said. says. “If you’re standing on the coastline, both lower land and rising sea levels will cause the same amount of flooding,” Austermann added.
The scientists behind the new study wanted to understand how the weight of the city itself contributes to regional sea level rise. New York City boasts a great deal of weight to study. “We just imported this mass,” says lead author Tom Parsons, a geophysicist with the US Geological Survey. “In Lower Manhattan it looks like the mountain range we’ve built up there, so it’s pushing all that weight down as well.”
The researchers’ first step was to analyze the city weights. For the purposes of this paper, this means all 1,084,954 buildings in the city’s buildings, in all five administrative districts. Parsons and his colleagues mapped the city on a grid and looked at a database containing the footprint and total height of every building in the city. They used building codes to estimate the weight of each grid square and derived that the sum of all buildings in New York City is 764 billion kilograms (1.68 trillion pounds). “It’s not a perfectly accurate weight, but it gives you a rough idea of how concentrated the building is,” Parsons says. (For simplicity, the team did not consider road and sidewalk weights.)
The researchers then created a geological map of New York City. In some areas, such as Midtown Manhattan, bedrock is close to the surface and there is relatively little soil to compress. This makes it less prone to sinking due to weight. In other areas, such as along Brooklyn’s south coast, the city used embankments to artificially expand the area. Artificial embankments are constructed from a variety of materials, but they are not as dense as natural terrain, making them particularly vulnerable to pressure from the overlying mass. Elsewhere the geology lies somewhere between these extremes.
The researchers fed maps of building distribution and soil types into a series of models designed to predict how different geological features would respond to pressure. As a result, we were able to identify areas where subsidence due to the city’s own weight may be particularly prevalent.
Finally, we looked at satellite data to see how much subsidence has actually occurred across the city over the past decade, and found that the average subsidence is one to two millimeters per year. Importantly, that overall subsidence includes factors other than the city’s weight. Therefore, comparing models and satellite observations only shows that the weight of the city may account for most of this subsidence. The researchers concluded that in areas where the weight of buildings is concentrated on loose ground, it is likely a significant contributor to land subsidence. The study is not intended as a definitive analysis, but as a first step toward understanding how cities around the world contribute to threatening sea-level rise. “as [the study authors] Highlight…, comparing data and models is complex. There are a lot of things that we don’t understand,” Austermann said, adding that the study is essentially a rough estimate of land subsidence from the weight of the city rather than an exact calculation.
Another limitation of the study is that scientists can mimic the city’s 400-year development to fully capture and predict how its weight, and the associated subsidence, will affect it in the years to come. was not possible. Physicist Kathleen Jones of NASA’s Jet Propulsion Laboratory (JPL) said, “These buildings model the overall settlement due to the preload of all the buildings as if they were somehow constructed at the same time. It magically appears on uncompacted soil and rock at the same time.” Jones, who wasn’t involved in the new study, 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 said, adding that this was her own opinion, not that of NASA or her JPL. rice field.
Although the new study only looked at New York City, the study is an important reminder of the problems occurring in coastal cities around the world. About 40 percent of the world’s population already lives within 100 kilometers of the world’s coasts, and nearly 70 percent are expected to live in cities by 2050. This combination means that cities already vulnerable to rising sea levels could also suffer under their own weight, with much of New York City benefiting from solid bedrock, while others are more vulnerable. on dangerous footing. “Most coastal cities around the world are expanding significantly,” Parsons says. “Considering such changes in the distribution of people, concerns may be further heightened.”