New York City is sinking under the weight of its skyscrapers

New York City is sinking by 1 to 2 mm per year, and the weight of its 1,084,954 buildings could contribute to this. In a study published in the journal Earth’s Future, researchers from the University of Rhode Island (Kingston, United States) show how the weight of skyscrapers contributes to the subsidence of the city. This phenomenon, qualified in geology by the term “subsidence”, designates the sinking of the earth’s crust under the effect of a load which is added above, below or inside it. Here, researchers calculate the added load above the crust, by studying the downward pressure exerted by the city’s buildings.

A subsidence which adds to climatic risks

764 million tonnes. It’s hard to imagine what such a figure represents; this is an estimate of the total weight of New York City’s skyscrapers, calculated by researchers at the University of Rhode Island. And again, this figure does not take into account the weight of the city’s 8.4 million inhabitants, or that of various infrastructures such as roads, bridges or railways. Could the weight of these constructions then contribute to the subsidence of the city? This is what the researchers tried to understand.

The American team observed from the combination of InSAR (a radar imaging technique used for ground monitoring and deformation) and GPS data that the average subsidence rate of New York City is order of 1 to 2 mm/year. This change is consistent with the tectonic subsidence that the city is already experiencing: “During the last ice age, a large ice sheet in the center of the continent pushed down this part of the Earth’s crust. There was what is called a “foreshore” around the edges of the suppressed area, which coincides with the eastern seaboard of the United States. When the ice melted, this foreboard began to sink and continues to sink today. specifies Tom Parsons, co-author of the study. But in certain very dense areas, researchers show that the weight of buildings adds to this subsidence and contributes even more when the soil is soft.

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The neighborhoods of Brooklyn, Queens and southern Manhattan, whose soil is made up of a mixture of clay, sand and silt, are particularly affected and could thus subside up to twice as fast as the average. “Many areas of high subsidence have been observed where construction loads are minor, such as in northern Staten Island”nevertheless nuance the researchers in their study.

Cities threatened by rising water levels and extreme weather events

Added to this anthropogenic and natural subsidence is the threat of rising sea levels, soil drainage and an increase in the frequency and intensity of extreme climatic events. So, although 1 to 2 mm/year may not seem like much, it nevertheless adds to the balance of risks already weighing on the city.

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By 2050, sea levels are expected to rise by 20 to 60 cm and many coastal cities are threatened by rising sea levels. At the beginning of 2022, a study of the American National Oceanic and Atmospheric Agency (NOAA) estimated that sea level should rise 30 cm over the period 2020-2050, and could rise 60 cm by 2100.

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New York’s 8.4 million residents face a significant risk of flooding in the coming years. Already in 2012, Hurricane Sandy brought seawater into the city, and Imani (Ida) in 2021 overwhelmed drainage systems, caused billions of dollars in damage and directly caused the deaths of 55 people. (and 32 indirectly). With climate change, these events are likely to be more and more frequent and violent, especially as seawater corrodes steel and weakens the concrete of infrastructure. New York City is therefore particularly vulnerable to these changes, especially since buildings located in areas at risk of flooding are insufficiently adapted.

The case of New York is not exceptional

But New York is not an isolated case and coastal cities are increasingly threatened by rising seas. Those that are sinking could also suffer the effects of rising water levels four times faster than others.

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According to the United Nations, 70% of the population is expected to live in cities by 2050. Major cities on every continent (except Antarctica) are experiencing subsidence, and the problem is not expected to diminish as population increases. increase. In some coastal areas, land is sinking faster than sea level rises, due to human activities (infrastructure, groundwater extraction). This is particularly the case in South, Southeast and East Asia, where the rate of subsidence is greater than 20mm/year, as shown in a study published in 2022 in the journal Geophysical Research Letters. In Indonesia, certain areas of the city of Jakarta are sinking up to 50mm/year. For comparison, the city of Venice is sinking by 1 to 2mm/year.

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“New York’s problems are emblematic of many cities around the world. As the population migrates to large coastal cities, we will likely see an increase in subsidence due to groundwater extraction, watercourse diversion, and the weight of buildings, among other things. As many climate projections predict accelerating sea level rise, we may see more flooding in the future.”warns Tom Parsons.