At Battery Park, near the South Ferry station, the storm surge from Sandy rose 2.75 meters above the mean high-water level — the highest since gauges were installed there in 1923. In a study published last week in Risk Analysis, researchers working with data from simulated storms concluded that a surge of that magnitude would be expected to hit Battery Park about once every 500 years in the current climate (J. C. J. H. Aerts et al. Risk Anal. http://dx.doi.org/10.1111/risa.12008; 2013).
But the study authors and other scientists say that the real risks may be higher. The study used flooding at Battery Park as a measure of hurricane severity, yet it also showed that some storms could cause less damage there and still hammer the city elsewhere. Factoring in those storms could drive up the probability estimates of major hurricane damage to New York.
The 1-in-500 estimate also does not take into account the unusual nature of Sandy. Dubbed a Frankenstorm, Sandy was a marriage of a tropical cyclone and a powerful winter snowstorm, and it veered into the New Jersey coast along with the high tide of a full Moon. “It was a hybrid storm,” says Kerry Emanuel, a hurricane researcher at the Massachusetts Institute of Technology (MIT) in Cambridge and one of the study's co-authors. “We need to understand how to assess the risks from hybrid events, and I'm not convinced that we do.”
The risks will only increase as the world warms. The New York City Panel on Climate Change's 2010 assessment suggests that local sea level could rise by 0.3–1.4 meters by 2080. Last year, Emanuel and his colleagues found that floods that occur once every 100 years in the current climate could happen every 3–20 years by the end of this century if sea level rises by 1 meter. What is classified as a '500-year' event today could come every 25–240 years (N. Lin et al. Nature Clim. Change 2, 462–467; 2012).
For city planners, the challenge is to rebuild and protect the city in the face of scientific uncertainty. A few scientists have said for more than a decade that the city should armor New York's harbor with a storm-surge barrier similar to the Thames barrier in London. In Sandy's wake, that idea has gained renewed interest, and a New York state panel last month called for a formal assessment of it.
Bridges and barriers
Malcolm Bowman, who heads the storm-surge modeling laboratory at the State University of New York at Stony Brook, has spearheaded the drive for barriers. He imagines a structure roughly 8 kilometers wide and 6 meters high at the entrance to the harbor, and a second barrier where the East River drains into the Long Island Sound. The state panel's cost estimates for such a system range from $7 billion to $29 billion, depending on the design. The harbor barrier could also serve as a bridge for trains and vehicles to the city's airports, suggests Bowman. “My viewpoint is not that we should start pouring concrete next week, but I do think we need to do the studies,” he says. But whether Sandy will push the city to build major defenses, Bowman says, “I don't know.”
Disasters have spurred costly action in the past. The 1888 blizzard helped to drive New York to put its elevated commuter trains underground. And in 2012, the US Army Corps of Engineers completed a $1.1-billion surge barrier in New Orleans, Louisiana, as part of a $14.6-billion effort to protect the city after it was battered by hurricanes Katrina and Rita in 2005. But the New York metropolitan area is bigger and more complex than New Orleans, and protecting it will require a multi-pronged approach. Several hundred thousand city residents live along more than 800 kilometers of coastline, and a barrier would not protect much of coastal Long Island, where Sandy wrought considerable damage. Moreover, the barrier would work only against occasional storm surges. It would not hold back the slowly rising sea or protect against flooding caused by rain.