How the weight of skyscrapers contributes to the subsidence of New York City

New York City is sinking 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, USA) 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, the researchers calculate the load that is added above the crust, by studying the downward pressure exerted by the buildings of the city.

A subsidence which adds to the climatic risks

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

The US team observed from a combination of InSAR (a radar imaging technique used for ground monitoring and deformation) and GPS data that the average rate of subsidence in New York City is order of 1 to 2 mm/year. This change is consistent with the tectonic subsidence that the city is already undergoing: “During the last ice age, a large ice cap in the center of the continent caused this part of the earth’s crust to sink. There was what is known as a “foreedge” around the edges of the removed area, which coincides with the east coast of the United States. When the ice melted, that foreedge began to sink and continues to sink today.” says Tom Parsons, co-author of the study. But in certain very dense areas, the researchers show that the weight of the buildings is added to this subsidence and contributes to it all the more so as the ground is loose.

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

Cities threatened by rising sea levels and extreme weather events

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

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By 2050, the sea level is expected to rise by 20 to 60 cm and many coastal towns are threatened by this rising sea level. Early 2022, a study from the American National Oceanic and Atmospheric Agency (NOAA) estimated that the 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 death 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 concrete infrastructure. New York City is therefore particularly vulnerable to these changes, especially since the buildings located in areas at risk of flooding are insufficiently adapted to them.

The case of New York is not exceptional

But New York is not an isolated case and coastal cities are increasingly threatened by rising waters. Those that sag could also suffer the effects of rising waters four times faster than the 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 decrease as population increases. increase. In some coastal areas, land is subsiding 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 by a study published in 2022 in the journal Geophysical Research Letters. In Indonesia, some areas of the city of Jakarta are sinking up to 50mm/year. By way of 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 larger coastal cities, there will likely be an increase in subsidence due to groundwater extraction, diversion of waterways, and the weight of buildings, among other things. As many climate projections predict an acceleration in sea level rise, we could see more flooding in the future.”warns Tom Parsons.