add The million or so buildings in New York City, and you get something on the order of 1.7 trillion pounds of weight pressing down on the Earth – and that’s not even counting all the other infrastructure like roads and sidewalks. All that weight is deforming the ground, like a bowling ball on a memory foam mattress, and causing a type of sinking, known as subsidence, when the ground slowly shrinks.
New research shows that, on average, the subsidence rate in NYC is between 1 and 2 millimeters per year, but in some places it is up to 4 millimeters. This may not sound like a worrying statistic, but compounded year-on-year, this is significant sinking that is effectively doubling the relative sea-level rise in the metropolis. “You have about 1 to 2 millimeters of sea level going up, whereas you have, on average, 1 to 2 millimeters going down,” says United States Geological Survey geophysicist Tom Parsons, co-author of a new study. paper Describing the research. “This is a common issue with cities around the world. It appears that there is a definite relationship between urbanization and subsidence.”
For example, parts of Jakarta, Indonesia are sinking about a foot per year. The San Francisco Bay Area could lose 165 square miles of shoreline due to a combination of rising seas and subsidence. And just last month, another team of researchers reported subsidence up and down the East Coast in parts of Delaware as high as 10 millimeters per year.
The primary method of the dramatic sinking is over-extraction of groundwater, which is the case in Jakarta; Empty waterholes collapse like empty water bottles. But in NYC, subsidence depends on the composition of the underlying soil. Long ago, glaciers strewn across the region, depositing sediments. Lakes were also formed, and more sediments were deposited. The metropolis is therefore built on a complex mix of materials such as clay, silt and artificial fill, which are more prone to subsidence, as well as sand and gravel, which resist it.
“The softer the soil, the more likely it is to sag under load,” says Parsons. “Even if you don’t build on it, it will sink under its own weight. But if you build on it, it definitely sinks in pretty well.
Parsons and his colleagues calculated subsidence rates by first adding up the urban load in NYC, then combining it with geological data on the composition of the various deposits. They also collected satellite data that shows the subtle changes in elevation that show which areas are sinking and which are relatively stable.
Manhattan skyscrapers may be the heaviest of the city’s buildings, but they’re anchored to the underlying bedrock, so they’re not much of a problem. The problem is greater along the coast, where spongy materials such as soil and artificial fill are particularly prone to compression – and where sea levels are rising.
Subsidence is a hidden vulnerability for coastal cities—models that project how much sea level rise will occur in a given area don’t yet take it into account. By 2050, the average sea level in the US will rise by a foot, and by that time, 70 percent The world’s people will be urban, up from 56 percent today. In coastal cities, that boom will exacerbate the issue because more people will need to extract more groundwater and more buildings and roads will be needed, which in turn will increase pressure on sediment.
