New York City’s land subsidence and the challenge of sea level rise

By Vivek Saini 

Scientists have discovered that various parts of the New York City metropolitan area are experiencing differential sinking and rising rates, attributed to land-use practices and historical glacier effects. Although the elevation changes may appear minimal, measured in fractions of inches per year, they play a crucial role in amplifying or mitigating local flood risks associated with rising sea levels.

Sinking City, rising concerns 

New York City, known as the city that never sleeps, is undergoing a paradoxical transformation as it steadily sinks at an average rate of 1-2 mm per year, with specific areas experiencing twice this rate, according to recent research published in the Earth’s Future journal. This subsidence is exacerbating the impact of accelerated sea level rise, which is double the global average, driven by the melting of glaciers and the expansion of seawater due to global heating. Since 1950, the water surrounding the city has risen by approximately 9 inches (22cm).

A study published in Science Advances by a team of researchers from NASA’s Jet Propulsion Laboratory (JPL) and Rutgers University sheds light on this phenomenon. The researchers employed interferometric Synthetic Aperture Radar (InSAR) to analyze vertical land motion, including uplift and subsidence, across the metropolitan area from 2016 to 2023. This remote sensing technique revealed that much of the observed motion occurred in areas where past modifications to the Earth’s surface, such as land reclamation and the construction of landfills, had rendered the ground looser and more compressible beneath subsequent buildings.

This sinking trend, coupled with the intensification of hurricanes due to climate change, could result in significant flooding events being up to four times more frequent by the end of the century, presenting a substantial challenge to the city’s future resilience.

Human Handprints and Glacial Echoes Driving Land Movement in NYC 

As the climate crisis deepens, the risks faced by New York City are not unique but rather shared by coastal cities globally. The intertwining factors of tectonic and human-induced subsidence, sea level rise, and the escalating intensity of hurricanes signify an accelerating problem for coastal and riverfront areas worldwide.

The sheer mass of New York City’s built infrastructure amplifies this trend. The researchers highlighted that the total weight of the city’s structures, including iconic landmarks like the Empire State Building and Chrysler Building, amounts to a staggering 1.68 trillion pounds – equivalent to approximately 140 million elephants. This substantial weight exerts pressure on a mix of materials in the city’s ground, with solid bedrock supporting many large buildings. At the same time, other areas consist of sands and clays built over time. This sinking effect, a natural occurrence along much of the U.S. East Coast as the land adjusts to the retreat of massive glaciers since the last ice age, contributes to the ongoing risk of inundation from flooding. Tom Parsons, a geophysicist leading the research at the US Geological Survey, told CFC, “The most likely cause of uplift in New York City is groundwater recharge. This is when drainage patterns and pumping can cause pressured water to fill the pore space of near-surface sediments, causing the land to rise slightly. I’m not sure this would be a perfect mitigation strategy against subsidence. Pumping water into the ground would likely have unpredictable outcomes due to the complex subsurface pathways fluids move and seasonal variations.”

The observed motion in New York City is not solely a consequence of recent human activities. Still, it is also influenced by natural processes dating back thousands of years to the last ice age. Approximately 24,000 years ago, a massive ice sheet extended across much of New England, with a towering ice wall over a mile high covering what is now Albany in upstate New York. Since then, Earth’s mantle, akin to a flexed mattress, has been gradually readjusting. New York City, situated on land uplifted just beyond the periphery of the ice sheet, is now experiencing a subsidence as part of this ongoing geological process.

Vulnerable Hotspots

The research team identified significant subsidence hotspots, including LaGuardia Airport’s runway 13/31 sinking at a rate of about 0.15 inches per year, prompting an $8 billion renovation to address flooding concerns. Arthur Ashe Stadium, subsiding at 0.18 inches per year, required a lightweight roof during renovation to counteract its heaviness. Other subsidence areas include the southern portion of Governors Island, Coney Island, Arverne by the Sea in Queens, Route 440, Interstate 78 in suburban New Jersey, and Rikers Island – all associated with historical fill locations.

In addition to subsidence, the scientists uncovered previously unnoticed uplift in East Williamsburg and Woodside. East Williamsburg rises by about 0.06 inches annually, potentially influenced by groundwater pumping and injection wells treating polluted water. Woodside experienced a rise of 0.27 inches per year between 2016 and 2019, stabilizing after that. Co-author Robert Kopp of Rutgers University highlighted the potential of using high-resolution InSAR for measuring short-term environmental modifications associated with uplift. 

Cities like New York, actively investing in coastal defences and infrastructure to combat sea level rise, can derive significant benefits from high-resolution estimates of land motion. The ongoing JPL-led OPERA (Observational Products for End-Users from Remote Sensing Analysis) project aims to deliver detailed surface displacement data across North America in a forthcoming data product. Leveraging InSAR data from ESA’s Sentinel-1 and the upcoming NISAR (NASA-Indian Space Research Organization Synthetic Aperture Radar) mission, set to launch in 2024, OPERA will play a pivotal role in providing valuable insights for monitoring vertical land motion and other changes linked to natural hazards.

This collaborative effort underscores the importance of utilizing advanced remote sensing technologies to inform coastal resilience strategies. By harnessing the capabilities of OPERA, scientists can enhance their ability to track and understand shifts in land motion, facilitating more informed decision-making for coastal cities facing the challenges of sea level rise and associated environmental changes.

Beyond New York: Implications of the Study for Coastal Cities Facing Sea Level Rise

Looking beyond New York City, the study’s implications resonate globally for coastal cities facing rising sea levels. The intricate dance of sinking and rising areas in New York is not an isolated occurrence but mirrors a broader phenomenon faced by coastal metropolises. The insights and lessons gleaned from NASA’s study on New York serve as a valuable guide and a cautionary narrative for these vulnerable cities, prompting a reassessment of their land management strategies and the adoption of proactive measures to navigate the uncertainties that lie ahead.

Land subsidence, often exacerbated by human activities such as groundwater extraction and land reclamation, poses a mounting threat to coastal cities worldwide. Cities like Venice, Jakarta, Bangkok, and Shanghai grapple with the sinking ground, witnessing increased stress on their infrastructure and cultural heritage due to the advancing sea. The findings underscore the global nature of this challenge, emphasizing the need for a collective and vigilant response to safeguard the future of coastal urban areas.

References 

1. https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2022EF003465
2. https://sealevelrise.org/states/new-york/
3. https://www.jpl.nasa.gov/news/nasa-led-study-pinpoints-areas-of-new-york-city-sinking-rising
4. https://cpo.noaa.gov/Divisions-Programs/Earth-System-Science-and-Modeling/MAPP/ArtMID/6170/ArticleID/719/NYC-could-experience-a-twenty-fold-increase-in-seasonal-coastal-flooding-episodes-by-the-end-of-the-21st-century
5. https://amp.theguardian.com/us-news/2023/may/19/new-york-city-sinking-skyscrapers-climate-crisis
6. https://climate.nasa.gov/news/3285/nasa-led-study-pinpoints-areas-of-new-york-city-sinking-rising/
7. https://www.jpl.nasa.gov/go/opera
8. https://nisar.jpl.nasa.gov/
9. https://www.theguardian.com/environment/2019/dec/10/venice-floods-sea-level-rise-mose-project
10. https://www.nytimes.com/interactive/2017/12/21/world/asia/jakarta-sinking-climate.html
11. https://www.bangkokpost.com/thailand/general/2499786/bangkok-is-still-sinking-and-fast-
12. https://science.time.com/2012/05/21/soaring-to-sinking-how-building-up-is-bringing-shanghai-down/

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