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Today's tourists often need wading boots to explore the
architectural wonders of Venice. Will they one day need scuba gear?
NOVA covers the battle to keep the world's most unusual city from
drowning beneath the rising tides of the Adriatic Sea. The lessons
we learn about how to stop rising sea levels will prove essential
for other coastal cities around the world, from New York to
Shanghai.
For centuries, Venetians have been fighting the forces of nature
that threaten to alter their city's precarious relationship with the
encircling lagoon that has long served as protection from invading
armies. In the 15th century, the trouble was silt, which was filling
up the lagoon from nearby rivers. Environmental engineers of the day
solved this problem at enormous expense by diverting the rivers to
more distant outlets.
More recently the peril has been the subsidence of the ground on
which Venice is built, combined with ever higher tides caused by
rising sea levels. When storm winds blow from the south, the
Adriatic floods the lagoon, causing acqua alta, or high
water. Under certain weather conditions, the flooding can be
catastrophic. Venetians have kept up with the rising water by
raising the level of floors and pavement. This has deformed elegant
buildings, created awkward doorways, and left very little room to
keep building up in advance of acqua alta.
Another alarming sign is the green ring of algae at the high-water
mark. Ominously, this line has crept above the impermeable
foundation stones that have long kept saltwater from seeping into
walls, slowly destroying buildings from within. Infrared images show
that the saltwater is indeed inside doing its damage.
Venetians, however, are not without a plan to save their city.
Engineers have proposed a multibillion-dollar series of gates that
will rise off the seafloor at the entrances of the lagoon whenever
acqua alta is forecast, holding the sea back until the high tides
subside (Watch a
video about the proposed
gates).
Not everyone supports the gate project. American archeologist Albert
Ammerman recently made headlines when he announced his conclusion
that Venice is sinking faster than previously believed. Ammerman and
others also criticize the planners of the gates for not taking into
account the worst-case scenarios for sea-level rise caused by global
warming. If the sea rises faster than the engineers predict,
frequent gate closings could severely inhibit water circulation in
the lagoon, turning the lagoon into a cesspool of industrial waste,
agricultural runoff, and raw sewage, all of which now flow out to
sea with the tides.
If Ammerman is right, the working life of the gates may be short.
"The people who are claiming that this is going to be a successful
solution for 100 years, 200 years have no basis for that whatsoever,
in terms of the best guesses about global warming," worries
Ammerman. He believes more studies should be done on the impact of
the gates and that it may be necessary to go back to the drawing
board to find a more viable long-term solution.
But supporters of the gates counter that rapidly rising sea levels
mean the need for the gates is more urgent than ever. "I think you
have to act and learn simultaneously," says environmental studies
expert Pier Vellinga. "We think this solution is very good for the
short term. And if you postpone and study and study and study, we
may study until the sea has risen 50 centimeters [20 inches]. Then
you're not safe. If you have the barrier in place, you have
insurance for the future."
In December 2001, the Italian government announced that it would
indeed build the gates, though as John Keahey points out in
Weighing the Solutions, Venetian leaders remain ambivalent about the gates, and the
current pro-gates national government might be replaced by one not
so supportive of the idea. So whether the gates will actually be
built remains to be seen. In the meantime, the seas continue to
rise.
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