Recognizing the threat, New York City Mayor Michael Bloomberg is developing a strategy for mitigating the growing risk from storm surges and flooding along the city's 500 miles of coastline. In such a densely-populated area of so much expensive real estate, surrounded by a complex web of estuaries, tides and ocean, it is a huge challenge. And in the face of global changes, even a city as inventive as New York can only do so much.
The Intergovernmental Panel on Climate Change predicted in 2007 that the global average sea level would rise between seven and 23 inches by the end of this century. More recent projections suggest that the melting of sea ice could mean a rise in excess of 30 inches. The New York State Sea Level Rise Task Force translated that into a local projection of 2 to 5 inches by the 2020s, and with rapid Arctic ice melt the rise could be as much as 5 to 10 inches over the next fifteen years. Combine that with a trend toward more intense storms and New York is "highly vulnerable," professor Michael Oppenheimer of Princeton University told CNN.
"(Superstorm) Sandy is a foretaste of things to come," he predicted, "from the combination of bigger storms and higher sea levels, both of which contribute equally to the growing threat."
New York dodged a bullet by inches last year as the remnants of Hurricane Irene bore down. Ben Orlove, director of the master's program in Climate and Society at Columbia University, wrote on CNN.com: "Irene also arrived at a time of especially high tides, and its storm surge came within inches of flooding the sea wall. Storms and tides are natural, but sea level rise is not. As it continues, New York grows more vulnerable."
Oppenheimer, a professor of geosciences, recently modeled the effect of climate change on storm surges for the New York area. In a paper published by Nature in February, he and three colleagues concluded that the "storm of the century" would become the storm of "every twenty years or less."
New York Governor Andrew Cuomo appears to agree.
"After what happened, what has been happening in the last few years, I don't think anyone can sit back anymore and say 'Well, I'm shocked at that weather pattern,'" he said Tuesday.
The conclusion of Oppenheimer and his colleagues is that storms will become larger and more powerful.
"Climate change will probably increase storm intensity and size simultaneously, resulting in a significant intensification of storm surges," they wrote. Sandy had a diameter of some 900 miles, much larger than most storms.
A study of the New York area in 2010 led by Guy Nordenson, an architect and structural engineer whose offices are in lower Manhattan, concluded: "There is a prevalent risk that the city will be severely paralyzed due to the predicted inundation and wave action associated with storm surge."
In addition, salt-water intrusion could compromise the quality of drinking water and weaken ecosystems, Nordenson and others concluded in the book "On the Water: Palisade Bay."
But the answer, they argue, is not solely in engineering. "Cities fortify their coasts to protect real estate at the expense of nature ... the hard engineering habit has proven costly, unreliable and ineffective."
Nordenson is serving on a task force set up by Mayor Michael Bloomberg to study ways of giving New York a more resilient waterfront. He told CNN that the region needs a combination of strategies that includes more "soft infrastructure." New York is losing tidal marshes at a rapid pace, partly because of the rise in sea level but also because of development.
Among the big ideas in "On the Water: Palisade Bay": create an archipelago of islands and reefs in the New York-New Jersey Upper Bay to dampen powerful storm currents, the islands being "fingered" (with many inlets) and combining tidal marshes and parks.
Nordenson points to the example of the Netherlands and cities like Hamburg that incorporate flood plains into their planning. Similarly Hurricane Katrina showed the importance of preserving Gulf coastal swamps. He hopes a project with the New York Port Authority to begin using dredged material for natural barriers will get underway soon.
At the local level, the Nature Conservancy is working with communities in Long Island to identify the risks from rising sea levels and protect wetlands. Sarene Marshall, who leads the Conservancy's Global Climate Change Team, estimates that every dollar spent in preventive measures saves $5 in disaster recovery, and that long-term investment in natural infrastructure is more effective than hard engineering. She points to the value of the humble oyster reef, nature's version of the sea wall.
Paul Greenburg, writing in The New York Times Tuesday, echoes her point, saying that in previous centuries "a bivalve population that numbered in the trillions ... played a critical role in stabilizing the shoreline from Washington to Boston."
The Nature Conservancy estimates such reefs can reduce the storm risks for 7 million Americans living on the shore.