FACING HIGHER SEA LEVELS AND INCREASED COASTAL FLOODING IN NEW YORK CITY

The 837 km of New York City shoreline is lined by significant economic assets and population vulnerable to coastal flooding. The population within the 100-year flood zone exceeds that of other U.S. coastal cities, including Miami, New Orleans, and Houston. After Hurricane Sandy in 2012, New York City developed a comprehensive plan to reduce present and future climate risks, drawing upon the scientific expertise of the New York Panel on Climate Change (NPCC), a special advisory group, as well as city and regional agencies, universities, and the private sector. This case study highlights the latest NPCC findings regarding accelerated sea level rise (SLR) and coastal flooding, with some of the City’s ongoing and planned responses.

The average 20^th century New York City metropolitan area sea level rise of 3.0 cm per decade exceeds the global average rise of 1.7 cm per decade, underscoring a potentially enhanced regional risk to coastal hazards. The NPCC (2015) projects future sea level change at the Battery (lower Manhattan) based on CMIP5 model output, expert judgment, and literature surveys. Mid-range (25^th -75^th percentile) sea level climbs 28-53 cm by the 2050s and 46-99 cm by the 2080s, relative to 2000-2004. High-end SLR estimates (90^th percentile) reach 76 cm by the 2050s, up to 1.9 m by 2100. Given these projections, updated Federal Emergency Management Agency (FEMA, 2013) flood return curves, and assuming unchanged storm behavior, flood heights for the 100-year storm (excluding waves) would rise from 3.4 m in the 2000s to 3.9-4.5 m by the 2080s (mid-range), to a high end of 4.9 m. The annual likelihood of such a flood would increase from 1 percent to 2.0-5.4 percent by the 2080s, with an upper bound of 12.7 percent. The flood zone would expand significantly.

The NPCC will continue to collaborate with New York City to strengthen the City’s resiliency to coastal hazards. The NPCC plans to update down-scaled local climate projections, improve coastal flood mapping on the neighborhood level, and work closely with stakeholders and community groups. New work will study combined impacts of coastal storm and sea level rise, develop a monitoring and indicator framework for coastal hazards, focusing on critical infrastructure, such as energy and transportation. Results will be presented using advanced mapping techniques.