Now for the flooding: Sandy’s storm surge topped out at about 11 feet (3.4 meters) above the most recent average sea level at the lower tip of Manhattan. But flood maps just updated by the Federal Emergency Management Agency in January indicate that even an eight-foot (2.5-meter) surge would cause widespread, destructive flooding. So if sea level rises by five feet (1.5 meters_, a surge of only three feet is needed to inflict considerable damage.
How frequently could that occur? Municipalities rarely plan for anything greater than the so-called one-in-100-year storm—which means that the chances of such a storm hitting during any given year is one in 100. Sandy was a one-in-500-year storm. If sea level rises by five feet, the chance in any year of a storm bringing a three-foot surge to New York City will increase to as high as one in three or even one in two, according to various projections. The 100-year-height for a storm in the year 2000 would be reached by a two-year storm in 2100.
With hundreds of people still homeless in Sandy’s wake, coastal cities worldwide are watching to see how New York City will fend off rising seas. Scientists and engineers have proposed solutions to pieces of the complex puzzle, and a notable subset of them on the New York City Panel on Climate Change are rushing to present options to Mayor Michael Bloomberg by the end of May. But extensive interviews with those experts leads to several controversial and expensive conclusions: Long-term, the only way to protect east coast cities against storm surges is to build massive flood barriers (pdf). The choices for protecting the long stretches of sandy coastlines between them—New Jersey, Maryland, the Carolinas, Florida—are even more limited.
As for sea level rise, retreat from low-lying shores may be the best option. Despite the gut reaction of “No, we won’t go,” climate forces already in motion may leave few options.