More Floods Ahead: Adapting to Sea Level Rise in New York City
Our planet is heating up, largely due to atmospheric build-up of greenhouse gases since the onset of the Industrial Revolution. As mountain glaciers melt, ice sheets thin, and oceans warm, sea level rise is accelerating. As sea level rises, urban areas near the coast like New York City will undergo more frequent and intense episodic flooding following major storms, as well as permanent inundation of some low-lying areas.
Accelerated sea level rise and exacerbated coastal flooding are issues of critical concern for New York City and its broader metropolitan region. With over 600 miles of densely populated coastline, this urban environment is prone to losses from weather-related natural disasters. NYC ranks in the top ten worldwide in population vulnerable to coastal flooding, and is second only to Miami in port cities with assets exposed to coastal flooding.
New York City currently experiences a number of coastal climate hazards. In spite of its northerly location, the city is not immune to tropical cyclones. In 1821, a hurricane that struck the city produced a surge of 13 feet (about 4 m) in one hour and flooded lower Manhattan as far north as Canal Street. In 1893, another hurricane flooded southern Brooklyn and Queens, wiping out a small barrier island off the Rockaways. During the 20th century, the "Long Island Express" (1938) (see Fig. 1), Hurricane Donna (1960), and to a lesser extent, Hurricane Gloria (1985) inflicted considerable damage on nearby Long Island and New Jersey. Even winter storms, such as the nor'easter of December 1992, can cause extensive flooding in low-lying neighborhoods and seriously disrupt ground and air transportation.
Recognizing that New York City faces potential impacts of climate change, Mayor Michael Bloomberg in 2008 convened the New York City Panel on Climate Change (NPCC), consisting of experts from the NASA Goddard Institute for Space Studies; Columbia University, the City University of New York, and other regional universities; and the private sector to advise the government of New York City on issues related to climate change and adaptation (defined as steps taken to reduce the impacts of climate variability and change) of critical infrastructure (note 1). As part of the city's overall development of policies for climate change adaptation, the NPCC provided information on future risks stemming from changes in temperature, precipitation, sea level rise, and extreme events.
In the NPCC's 2010 Foundation Report, Climate Change Adaptation in New York City: Building a Risk Management Response, future sea level rise was projected for New York City for a combination of seven global circulation models (GCMs) and three emissions scenarios (IPCC SRES A2, A1B, and B2). One set of projections, based on the methods used by the Intergovernmental Panel on Climate Change (IPCC) for the 2007 Fourth Assessment Report, include global contributions from thermal expansion of the oceans and meltwater (glaciers, ice caps, and ice sheets). Local terms include land subsidence and changes in local water height (relative to the oceans as a whole) caused by changes in ocean currents, temperature, salinity, and other factors.
Recognizing that GCM-based sea level projections may underestimate the potential for accelerated ice sheet melting, the NPCC created additional "rapid ice-melt" scenario projections. These projections differ in the meltwater term and assume that glaciers and ice sheets will melt at rates comparable to those at the end of the last Ice Age — the 10,000-12,000-year period of rapid sea level rise starting roughly 20,000 years ago — when sea level climbed at an average rate of approximately 3-4 feet (0.9 to 1.2 m) per century (note 2).
The GCM-based projections show a sea level rise of 7 to 12 inches (18 to 30 cm) by the 2050s and 12 to 23 inches (30 to 58 cm) by the 2080s. In the rapid ice-melt scenario, sea level jumps to 41 to 55 inches (104 to 140 cm) by the 2080s (Fig. 2).
Rising sea level alone is expected to increase the frequency, intensity and duration of coastal flooding. The one-in-100 year event — a flood having a recurrence probability of once per century — is a common measure of flood risk. The GCM-based sea level rise shrinks the return period for the one-in-100 year flood to once in 15 to 35 years by the 2080s. The return interval for the one-in-10 year flood event is reduced to once in 1 to 3 years. Possible changes in storms themselves are not included in this analysis, due to uncertainty in how storm characteristics may alter in the future.
For New York City, a higher average sea level would exacerbate street, basement, and sewer flooding and create more frequent transportation disruptions. It would increase rates of beach erosion, necessitating additional beach nourishment programs. Saltwater would encroach further on freshwater sources, potentially causing structural damage to infrastructure and compromising some drinking water sources on Long Island.
Given that temperatures and sea level are already rising, the NPCC recommended that New York City should begin adapting to climate change today. Because changes in the amount of future sea level rise and other climate variables are uncertain, an effective response requires "flexible adaptation pathways" that can be adjusted periodically in light of new information. Other important types of information to monitor include changes in coastal population and infrastructure, emerging evidence of climate impacts, changes in available technologies, and costs of adaptation strategies.
The NPCC prepared three workbooks, each addressing different aspects of the challenge:
- Climate Risk Information, which includes projections for mean and extreme events, sea level rise and storm surge. (Horton et al., 2010)
- Adaptation Assessment Guidebook, which is designed to help city agencies plan for future climate change, including sea level rise (Major et al., 2010)
- Climate Protection Levels, which outlines a general process for developing a set of climate change-related design and performance standards (Solecki et al., 2010).
Recommended steps included preparing an inventory of infrastructure and assets at risk and linking adaptation strategies to capital and rehabilitation cycles. As part of the risk and hazard management strategy, current one-in-100 year FEMA flood zone maps and Coastal Zone Boundaries for waterfront revitalization should be updated to cope with a rising sea level (Fig. 3). Design standards for bridges can also be revised to encompass higher water levels.
By monitoring current climate impacts, periodically updating climate projections, and developing and testing adaptation strategies, New York City can build and maintain climate resilience in the 21st century.
Horton, R., et al., 2010: Climate Risk Information, New York City Panel on Climate Change. Ann. New York Acad. Sci., 1196, 147-228, doi:10.1111/j.1749-6632.2010.05323.x.
Major, D.C., and M. O'Grady, 2010: Adaptation assessment guidebook, New York City Panel on Climate Change. Ann. New York Acad. Sci., 1196, 229-292, doi:10.1111/j.1749-6632.2010.05324.x.
Nicholls, R.J. et al., 2008: Ranking Port Cities with High Exposure and Vulnerability to Climate Extremes: Exposure Estimates, OECD Environment Working Papers, No. 1, OECD Publishing, doi:10.1787/011766488208.
Rosenzweig, C., et al., 2011: Developing coastal adaptation to climate change in the New York City infrastructure-shed: process, approach, tools, and strategies. Climatic Change, 106, 93-127, doi:10.1007/s10584-010-0002-8.
Rosenzweig, C., and Solecki, W. eds., 2010. Climate Change Adaptation in New York City: Building a Risk Management Response. New York City Panel on Climate Change 2010 Report. Annals of the New York Academy of Sciences, v. 1196.
Solecki, W., Patrick, L., and Brady, M, 2010: Climate Protection Levels, New York City Panel on Climate Change. Ann. New York Acad. Sci., 1196, 293-352, doi:10.1111/j.1749-6632.2010.05325.x.
Please address all inquiries about this research to Dr. Cynthia Rosenzweig.
1 "Critical infrastructure" is defined as systems and assets (excluding residential and commercial buildings, which are addressed by other efforts) that support vital city activities and for which the diminished functioning or destruction of such systems and assets would have a debilitating impact on public safety and/or economic security. (Back to text)
2 In this scenario, meltwater is assumed to rise exponentially from the present mean ice melt rate of 0.43 in/decade (1.1 cm/decade) between 2000 and 2004, reaching about 3 feet (1 meter) by the end of the century. The meltwater term is added to the other three sea level terms (i.e., thermal expansion, local land subsidence and local changes in water height) which remain unchanged. (Back to text)