Science Briefs

Rising Sea Levels, Frequent Floods: Effects on New York City Neighborhoods

Melting of glaciers and ice sheets, which now accounts for nearly two thirds of the present 3 mm per year global sea level rise, is likely to increase in the future due to warming temperatures associated with mounting atmospheric greenhouse gases. These trends, coupled with potential ice sheet instabilities, raise prospects of higher-than-anticipated sea levels. As sea levels rise ever upward, more frequent coastal flooding endangers people and structures in low-lying areas, worldwide. Plans for the design and construction of long-lived urban infrastructure and transportation networks should therefore include high-consequence, although low-likelihood events, as well as more likely outcomes.

Photo of flooded street in Broad Channel, Queems

Fig. 1: Street flooding in Broad Channel, Queens. (Credit: New York City Department of City Planning)

Much of the West Antarctic Ice Sheet (WAIS) is potentially unstable because it sits on bedrock that lies largely below sea level. As warmer ocean water encroaches and thaws the undersides of floating ice shelves, their ability to buttress grounded ice is weakened. This could eventually initiate a Marine Ice Sheet Instability (MISI) on tidewater glaciers whose beds tilt landward. Once an MISI is initiated, the grounding line would continue its landward retreat until the glacier bed slope levels off. Atmospheric warming and hydrofracturing would further hasten ice shelf breakup, leading to collapse of high ice cliffs (≥100 m above water). Shelf thinning and grounding line retreats already occur at a number of WAIS glaciers.

Such considerations motivated the New York City Panel on Climate Change (NPCC), an advisory team of academic and private-sector experts, to introduce a physically plausible late 21st century upper-end, low-probability sea level rise scenario for New York City in its 2019 report. This scenario, the Antarctic Rapid Ice Melt (ARIM) scenario, includes the possibility of future WAIS destabilization, particularly under high carbon-emissions futures.

The ARIM scenario projections lie within the prior NPCC (2015) 10th-90th percentile range up to the 2050s (Table 1). By the 2080s, under ARIM, sea level climbs to 2.1 m and up to 2.9 m by 2100. In contrast, NPCC (2015) reported a sea level rise of 1.5 m by the 2080s and 1.9 m by 2100 (90th percentile; Table 1). [It should be noted that the NPCC (2015) projections still remain the scientific basis of New York City resiliency planning].

Time interval NPCC 2015 Sea Level Rise Projections, m ARIM Scenario, m
Low estimate
(10th percentile)
Middle range
(25th to 75th percentile)
High estimate
(90th percentile)
ARIM scenario
2020s 0.05 0.10-0.20 0.25
2050s 0.20 0.28-0.53 0.76
2080s 0.33 0.46-0.99 1.47 2.06
2100 0.38 0.56-1.27 1.91 2.90

Higher sea levels and increasing coastal flood exposure pose growing challenges for the large population and major economic assets along New York City's shoreline. Historically, a number of severe coastal floods (both hurricanes and nor'easters) have struck the city, causing great harm. Superstorm Sandy in 2012 generated the highest water levels in at least 300 years and caused an estimated $19 billion in damages and 43 fatalities. But major coastal storms are not the only hazard associated with rising seas.

An early manifestation of sea level rise is the increasing incidence of clear-weather “nuisance flooding”, or tidally related flooding of streets, basements, and low-elevation highways. The number of such events has increased substantially in the United States since the 1950s. Parts of several low-lying New York City neighborhoods surrounding Jamaica Bay and Rockaway Peninsula already experience frequent flooding not associated with storms (Fig. 1).

A worst-case sea level rise scenario such as ARIM would not only increase the frequency and severity of coastal flooding, but would also lead to a progressive landward expansion of the floodplain over time (see map below). Under ARIM, monthly tidal flooding would affect a much broader area than under the 90th percentile NPCC 2015 projections, during the same time periods. Moreover, neighborhoods around Jamaica Bay, previously exposed to frequent tidal flooding could become permanently submerged by late century, without strengthened coastal protection. Several other city waterfront communities face a similar risk.

Guided by NPCC findings, New York City undertakes programs to strengthen local coastal defenses tailored to specific neighborhood needs. Actions include installation of temporary or permanent barriers (such as floodwalls or berms), raising bulkheads and seawalls, construction of neighborhood-scale levees and storm surge barriers. Streets subject to frequent nuisance flooding can be raised, as already proposed. However, long-term planning should also include land-use zoning to minimize or avoid high-density development in flood-prone areas and preserve natural vegetation, such as salt marshes, as buffer zones against high waves.

Will these measures suffice in light of a potential high-end sea level future? Will managed relocation of the most vulnerable neighborhoods become an eventual necessity? The potential consequences of economic and societal disruptions from low probability, yet high- consequence flood events in major coastal urban centers such as New York City highlight the urgency of investigating upper-end sea level scenarios for long-term coastal risk management.

Map of monthly tidal flooding around Jamaica Bay, current and projected

Landward extension of the area affected by monthly tidal flooding around Jamaica Bay, Coney Island, and the Rockaways, New York City, for NPCC (2015) 90th percentile and ARIM SLR scenarios. Results assume no future shoreline changes due either to coastal erosion or coastal flood protection, and may therefore over- or underestimate flood area. Because of remaining uncertainties associated with ARIM, this map is not intended for planning purposes.

Reference

NPC, 2015: Building the Knowledge Base for Climate Resiliency: New York City Panel on Climate Change 2015 Report [Special Issue]. Ann. New York Acad. Sci., 1336.

NPC, 2019: Advancing Tools and Methods for Flexible Adaptation Pathways and Science Policy Integration: New York City Panel on Climate Change 2019 Report [Special Issue]. Ann. New York Acad. Sci., 1439.

Gornitz, V., M. Oppenheimer, R. Kopp, P. Orton, M. Buchanan, N. Lin, R. Horton, and D. Bader, 2019: New York City Panel on Climate Change 2019 Report Chapter 3: Sea level rise. Ann. New York Acad. Sci., 1439, 71-94, doi:10.1111/nyas.14006.

Horton, R., C. Little, V. Gornitz, D.A. Bader, and M. Oppenheimer, 2015: New York City Panel on Climate Change 2015 Report: Sea level rise and coastal storms. Ann. New York Acad. Sci., 1336, 36-44, doi:10.1111/nyas.12593.

Orton, P., N. Lin, V. Gornitz, B. Colle, J. Booth, K. Feng, M. Buchanan, M. Oppenheimer, and L. Patrick, 2019: New York City Panel on Climate Change 2019 Report Chapter 4: Coastal flooding. Ann. New York Acad. Sci., 1439, 95-114, doi:10.1111/nyas.14011.

Please address all inquiries about this research to Dr. Vivien Gornitz.

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