The weight of the buildings in New York City may be causing it to sink faster

The sheer weight of the city's high-rise structures, according to the experts, is one contributing element. The team, lead by US Geological Survey research geophysicist Tom Parsons, examined subsidence models, surface geology, and satellite data from the Interferometric Synthetic Aperture Radar and Global Positioning System.

"Geodetic measurements show a mean subsidence rate of 1-2 mm/year across the city, which is consistent with regional post-glacial deformation," the scientists noted in their report. The problem is exacerbated by urbanization. "Cumulative pressure applied to the ground from large buildings contributes to subsidence not only from initial primary settlement caused by soil compression and reduction of void space, but also through potential secondary settlement caused by creep in clay rich layers that can continue indefinitely," the researchers explain.

The entire mass of the skyscrapers in New York was calculated to be 764,000,000,000 kilograms (1.68 trillion pounds), exerting pressure on the earth below. New York's geology is complicated, with a variety of soils. Buildings atop clay-rich soil and artificial fill "are especially prone to significant building settlement and clay models show the largest potential subsidence, ranging from 75 to 600 mm with a median of 294 mm at a sample point in lower Manhattan," the research says.

Storms Sandy and Ida were also contributors in sinking, and the researchers cautioned that repeated exposure of building foundations to seawater - from storms or increasing sea levels - can cause structural deterioration, corroding the steel and weakening the concrete. The climate catastrophe and rising urbanization will only worsen these issues.

"Major cities around the world are expected to grow disproportionately relative to rural areas, with a projected 70 percent of the world's population living in cities by 2050," concluded the study. Except for Antarctica, major cities are sinking on every continent, and the problem may intensify as people expand. "Increased urbanization will most likely exacerbate subsidence caused by groundwater extraction and/or construction density, implying a growing flood hazard in coastal cities." As these trends continue, it will be critical to consider inundation mitigation methods in increasing coastal communities."

Sea level rise, subsidence, and increased storm strength from natural and manmade factors pose an increasing danger of inundation for New York City. We quantify a hitherto unknown contribution to sinking from the accumulated bulk and downward pressure exerted by the cities built environment. To compute predicted subsidence, we enforce that load distribution in a multiphysics finite element model.

"Increased urbanization will most likely exacerbate subsidence caused by groundwater extraction and/or construction density, implying a growing flood hazard in coastal cities." As these trends continue, it will be critical to consider inundation mitigation methods in increasing coastal communities."

Complex surface geology necessitates the use of different rheological soil models; clay-rich soils and artificial fill are projected to have the most post-construction sinking when compared to more elastic soils. Depending on the soil/rock physical factors and foundation modes, the minimum and maximum predicted building sinking ranges from 0 to 600 mm. We compare observed subsidence rates from satellite data (Interferometric Synthetic Aperture Radar and Global Positioning System) to predicted subsidence and surface geology.

The comparison is complex since urban load has accumulated over a much longer time than reported sinking rates, and subsidence has several causes. Geodetic data suggest that the city has a typical sinking rate of 1-2 mm/year, which is consistent with regional post-glacial deformation, while there are certain regions with substantially higher subsidence rates. Although there are other plausible explanations, some of this deformation is consistent with internal consolidation of artificial fill and other soft silt, which may have been worsened by recent building loads.