Location map of study site. Credit: USGS
Superstorm Sandy has provided the most convincing evidence yet that powerful storms and
what are called “black swan” storms, combined with sea level rise
are pushing barrier islands out from under beach cities.
Earth scientists studying the
changes to islands of the mid-Atlantic coast and the Gulf of Mexico
announced on Monday that the islands are not disappearing, but moving
to stay in the same depth of water, which means they are shifting
landward as sea levels go up.
Island towns facing this losing battle against the sea
are in danger of popping a real estate market bubble,
inflated by state and federal subsidies that make beach protection
and replenishment projects possible.
“This is how barrier
islands respond to sea level rise,” said Hilary Stockdon, an
oceanographer with the U.S. Geological Survey. Stockdon and her
colleagues have been building a survey of predictions of specific
changes that are likely to occur on different barrier islands, which
proved accurate for islands pummeled by Sandy.
Using before and after
images and measurements of islands like Fire Island, NY,
Stockdon and her colleagues showed that the ocean facing beaches had
been eroded away one to three meters in elevation while inland sides
of the island had gained a meter. Not only does that shift confirm
that the island is moving, but it makes the island communities more
vulnerable to even smaller storms in the future, Stockdon said at a
press conference at the meeting of the American Geophysical Union in
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For decades many barrier
island beach communities managed to survive by dredging sand from
offshore and “nourishing beaches.” But political pressures have
been building to cut off government funding of such projects.
“The subsidy is slowly
being removed, so we wanted to model the effects of changes with
that,” said Dylan McNamara, an oceanographer at the University of
North Carolina Wilmington. Using his home town Ocean City Maryland as
a case study, he found that when you remove the government subsidy,
property values drop a whopping 40 percent. That bubble bursting
could be enough to cause the abandonment of some cities altogether.
The effects of the loss of the subsidy was equaled only by sea level
rise itself, said McNamara.
As for whether more such
island-shifting storms are more likely, they aren't necessarily, but
at the same time, Sandy wasn't the worst that the nature can dish
out, said Ning Lin, a civil engineer at Princeton University. She
conducted a study to see what sorts of very powerful, low probability
storms, black swan storms, are possible in New York; Darwin,
Australia and the Persian Gulf.
Lin concluded that the
2.75-meter storm surge of Sandy in New York was not the worst that a
storm could do in that area. Rather, a full 5 meters is possible in a
true black swan storm. Though it may be little consolation to storm
Sandy survivors, “Sandy was not a black swan,” said Lin.