New York City is sinking under the weight of its skyscrapers, new research shows, which could put its population of more than 8 million people at an increased risk of coastal flooding.

A new study by the United States Geological Survey found the city is sinking at a rate of about 1–2mm per year, although certain parts of lower Manhattan, Brooklyn, Queens, and northern Staten Island are sinking at a faster rate of 2.75mm each year. 

This could worsen the city’s already high risk of coastal flooding as a result of climate-amplified sea level rise. The New York City Panel on Climate Change estimates that, while the world’s sea levels have risen by about 0.5 inches per decade, New York City has had a much faster rate of about 1.2 inches per decade. By 2050, sea levels are expected to rise by eight to 30 inches, depending on how much the world limits greenhouse gas emissions.

The new study now warns that new buildings in the city could be at increased threat of floods, highlighting the need for a strategy to adapt around this. “Every additional high-rise building constructed at coastal, river, or lakefront settings could contribute to future flood risk,” the study published in the journal Earth’s Future says.

The researchers estimated the weight of all of New York City’s buildings to be around 842 million tons. But to find the areas more vulnerable to sinking — or, as they call it in more scientific terms, “subsidence”— a key factor to consider was the type of soil beneath the buildings. The paper found a higher sinking rate in areas with clay-rich soils due to their “material softness and ability to flow under pressure.”

While the study focuses on New York City, this problem goes well beyond just the Big Apple. “Major cities on every continent except Antarctica are observed to be subsiding,” according to the paper, but coastal cities in particular are facing an increased risk of flooding as the global climate warms. “As coastal cities grow globally, the combination of construction densification and sea level rise imply increasing inundation hazard,” the study reads.

At a global scale, around 800 million people are expected to live in coastal cities where sea levels will rise by more than a foot, according to a report by the C40 group of major cities taking action on climate change. The cost from these impacts, including rising seas and inland flooding, could reach up to $1 trillion, the report notes.

A separate 2022 study by the University of Rhode Island analyzed the sinking rate of 99 coastal cities across the world and found that, in most cases, urban areas are sinking faster than sea levels are rising, posing subsidence as a major long-term risk. The fastest rates were found in Asia. For example, in Jakarta, Indonesia, some parts of the city sank at a rate of 20mm per year. On the East Coast of the US, subsidence is typically attributed to deglaciation, but as the new research shows, urban constructions will also determine how the region adapts to rising seas.