By 2100 devastating flooding of the sort that Superstorm Sandy unleashed on New York City could happen every two years all along the valuable and densely populated U.S. east coast—anywhere from Boston to Miami.
And unless extreme protection measures are implemented, people could again die.
Hyperbole? Hardly. Even though Sandy’s storm surge was exceptionally high, if sea level rises as much as scientists agree is likely, even routine storms could cause similar destruction. Old, conservative estimates put the increase at two feet (0.6 meter) higher than the 2000 level by 2100. That number did not include any increase in ice melting from Greenland or Antarctica—yet in December new data showed that temperatures in Antarctica are rising three times faster than the rate used in the conservative models. Accelerated melting has also been reported in Greenland. Under what scientists call the rapid ice-melt scenario, global sea level would rise four feet (1.2 meters by the 2080s, according to Klaus Jacob, a research scientist at Columbia University’s Lamont–Doherty Earth Observatory. In New York City by 2100 “it will be five feet, plus or minus one foot,” Jacob says.
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Skeptics doubt that number, but the science is solid. The projection comes in part from the realization that the ocean does not rise equally around the planet. The coast from Cape Cod near Boston to Cape Hatteras in North Carolina is a hot spot—figuratively and literally. In 2012 Asbury Sallenger, a coastal hazards expert at the U.S. Geological Survey (USGS), reported that for the prior 60 years sea level along that section of the Atlantic coast had increased three to four times faster than the global average. Looking ahead to 2100, Sallenger indicated that the region would experience 12 to 24 centimeters—4.7 to 9.4 inches—of sea level rise above and beyond the average global increase.
Sallenger (who died in February) was careful to point out that the surplus was related only to ocean changes—such as expansion of water due to higher temperature as well as adjustments to the Gulf Stream running up along the coastbrought about by melting Arctic ice—not changes to the land.
Unfortunately, that land is also subsiding. Since North American glaciers began retreating 20,000 years ago, the crust from New York City to North Carolina has been sinking, as the larger continent continues to adjust to the unloading. The land will continue to subside by one to 1.5 millimeters (0.04 to 0.06 inch) a year, according to S. Jeffress Williams, a coastal marine geologist with the USGS and the University of Hawaii at Mānoa. The boundary zone where rising crust to the north changes to falling crust to the south runs roughly west to east from central New York State through Massachusetts.
Certain municipalities such as Atlantic City, N.J., are sinking even faster because they are rapidly extracting groundwater. Cities around Chesapeake Bay, such as Norfolk, Va., and Virginia Beach, are subsiding faster still because sediment underneath them continues to slump into the impact crater that formed the bay 35 million years ago.
When all these factors are taken into account, experts say, sea level rise of five feet (1.5 meters) by 2100 is reasonable along the entire east coast. That’s not really a surprise: the ocean was 20 to 26 feet (six to eight meters) higher during the most recent interglacial period.
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.
