How the Weight of Buildings Contributes to New York City’s Sinking and Flood Risk

(CNN) — New York City sinks under the collective weight of all its buildings, according to a new study.

This gradual process could pose a problem for a city around which the sea level has been rising at twice the global rate, and is anticipated increase between 20 and 76 centimeters between now and 2050.

In addition, scientists are forecasting more frequent and extreme precipitation, such as nor’easters, or northeastern systems, and hurricanes, due to the human-caused climate crisis.

“The ocean is still a long way from moving inland,” says Tom Parsons, lead author of the study and a research geophysicist with the US Geological Survey. “But we’ve had a couple of big hurricanes with Sandy and Ida in New York, where heavy rains caused flooding in the city, and some of the effects of development have allowed water to come in.”

The research, published in the academic journal Earth’s Futureaims to demonstrate how high-rise buildings in coastal, riverine or lake areas could contribute to future flood risk, and that steps should be taken to mitigate potential hazardous effects.

Risks of sinking cities and a mystery

The researchers calculated the mass of the 1,084,954 buildings that existed in the five boroughs of New York City at the time, concluding that they weighed about 1.68 trillion pounds (762 billion kilograms), which which is equivalent to approximately 1.9 million Boeing 747-400 fully charged.

The study team then used simulations to calculate the effects of that weight on the ground and compared them with satellite data showing the actual geology of the surface. That analysis revealed the rate at which the city is sinking: “The average is 1 to 2 millimeters a year, with some areas of greater subsidence reaching 4.5 millimeters a year,” explains Parsons.

Subsidence is the technical term that designates the subsidence or settlement of the earth’s surface due to natural or artificial causes. According to a study As of September 2022, 44 of the 48 most populous coastal cities have areas that are sinking faster than the sea level is rising. The novel approach of this latest study is to specifically consider the weight of New York City buildings and how they contribute to the subsidence of the underlying land.

However, not all subsidence is due to buildings. “We might see some correspondences where there is construction on very soft soil and artificial fill,” Parsons said. “In other places we see subsidence that is difficult to explain. And it has many different causes, such as postglacial relaxation after the last ice age, or groundwater pumping.”

Some areas of lower Manhattan, Brooklyn and Queens are among those sinking at an above-average rate, according to the study.

“Part of that seems to correspond to the construction projects that are taking place,” Parsons said.

“But we also see subsidence off the northern tip of Staten Island that I can’t explain, and I’ve researched all sorts of things, so they’re still a mystery.”

Mitigating risk around sinking cities

Subsidence may pose a flood threat even earlier than sea level rise, the research suggests, and not just in New York City. “It’s a global problem. My coauthors at the University of Rhode Island analyzed 99 cities all over the world, not only coastal but also inland, and the vast majority of them have sinking problems,” Parsons explains, citing the case of Jakarta, which is sinking so fast that the Indonesian government plans to build a new capital in another place.

“We know that global sea levels are rising and coastlines are changing, and that is critical to understanding the impact of human activities, such as greenhouse gas emissions, on our warming world,” said the geophysicist Sophie Coulson, a postdoctoral fellow at Los Alamos National Laboratory who was not involved in the study. “This research examines an important human factor that has only recently come to light: the effect of urban building loads on coastal land subsidence.”

The authors, he added, use a clever combination of computer models, satellite measurements and GPS data to estimate short-term and long-term subsidence rates for different parts of the city and identify the areas most at risk.

“New York City is one of the most densely populated coastal areas in the world, and much of its critical infrastructure is built in low-lying coastal areas,” he explained.

“Understanding how and why the landscape is changing, and identifying the areas most vulnerable to flooding is essential to making the right preparations to mitigate future sea level rise.”