Add them about a million buildings in New York, 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 deforms the ground, like bowling balls on a memory foam mattress, and causes a type of subsidence called subsidence, when the earth slowly compresses.
New research reveals that, on average, subsidence rates in New York are between 1 and 2 millimeters per year, but in some places they can be as high as 4 millimeters. That may not sound like a worrying number, but compounded year after year, it’s a significant sinking that effectively doubles the relative sea level rise in the metropolis. “You have about 1 to 2 millimeters of sea level rising, while on average you have 1 to 2 millimeters falling,” says United States Geological Survey geophysicist Tom Parsons, co-author of a new paper describing the research. “It’s a common problem in cities around the world. There seems to be a definite link between urbanization and subsidence.
Parts of Jakarta, Indonesia, for example, are sinking nearly a foot a year. The San Francisco Bay Area could lose up to 165 square miles of coastline due to a combination of rising seas and subsidence. And just last month, another team of researchers reported finding subsidence along the East Coast of up to 10 millimeters per year in parts of Delaware.
The main way to cause a dramatic sinking is the over-extraction of groundwater, which is the case in Jakarta; drained aquifers collapse like empty water bottles. But in New York, subsidence depends on the composition of the underlying soil. Long ago, glaciers scraped the area, depositing sediment. Lakes also formed, depositing even more sediment. Thus, the metropolis is built on a complex mix of materials like clay, silt and artificial fill, which are more prone to subsidence, as well as sand and gravel, which tend to resist it.
“The softer the ground, the more likely it is to compress 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 really flows pretty well.
Parsons and his colleagues calculated subsidence rates in New York City by first adding up all that urban weight and then combining it with geological data on the composition of the various deposits. They also collected satellite data that measured minute changes in elevation to show which areas sank and which were relatively stable.
Manhattan’s skyscrapers may be the heaviest buildings in the city, but they’re anchored to the underlying bedrock, so they’re not really a problem with subsidence. The problem is more along the coast, where spongy materials like clay and artificial fill are particularly prone to compression and where sea water levels rise.
Subsidence is a hidden vulnerability for coastal cities – models that predict sea level rise in a given area do not yet take this into account. By 2050, the average sea level in the United States will rise one foot, and by then 70% of the world’s population will be urban dwellers, up from 56% today. In coastal cities, this boom will exacerbate the problem as more people will have to extract more groundwater and will need more buildings and roads, which in turn will increase the pressure on sediments.