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STEVE INSKEEP, host:

Climate Connections, our series with National Geographic, goes underwater this month. As the Earth continues to warm, rising sea levels will threaten islands and coastlines around the world. Many places hope they're going to be able to engineer their way out of trouble when the time comes.

RENEE MONTAGNE, host:

Some, like Venice, have already started. Over the last century, this 1,300-year-old island city has seen a growing number of high-water tides. The increased flooding is due to the land shrinking and also rising sea levels brought about by climate change. A huge and controversial engineering project is under way, aimed at parting the waters to protect the Pearl of the Adriatic from disappearing under the sea.

NPR's Sylvia Poggioli reports.

SYLVIA POGGIOLI: A boat ride in the Venice lagoon is a discovery of how man and nature have created one of the world's most extraordinary experiments. Left alone, lagoons tend to either dry up and become land or are overwhelmed by the sea and turn into bays.

Starting in the 16th century, the Venetians diverted major rivers outside of the lagoon to prevent silt from filling it up. It covers 212 square miles. And along with the city of Venice in the center, there are some 50 smaller islands, as well as dozens of mudflats and sandbanks — havens for thousands of aquatic birds that flock here even in winter.

The delicate and fragile ecosystem is the largest wetland in the Mediterranean. Today, it's being threatened by rising seas, and it must be protected if Venice is to be saved.

Ms. FRANCESCA de POL (Consorzio Venezia Nuova): You see these windows, which are almost at the sea level? Those windows have been closed as they are too much exposed to the waters.

POGGIOLI: Francesca de Pol works for Consorzio Venezia Nuova, the consortium entrusted with the task of safeguarding Venice. She points to windows of a building on the Grand Canal filled with cement. No Venetian lives on the ground floor anymore.

In the last century, the city sank 11 inches, mostly due to pumping of groundwater and methane gas for local industries. But it's also being affected by rising sea levels.

Ms. de POL: What does it means? That the same tides that were not flooding the city 100 years ago today are high-tide events.

POGGIOLI: Here, it's called acqua alta. High water afflicts Venice mostly in the winter. A century ago, it happened seven times a year, now it's more like a hundred.

The visionaries who first began building Venice 1,300 years ago used materials for the foundations that could withstand water. But with the seabed sinking, brick walls of the ground floors are being corroded, and waterlogged buildings are crumbling.

Sophisticated technology is now being used to rescue the lagoon. The ambitious engineering project, the biggest public works in Italian history, is called MOSE, the acronym in Italian for experimental electromechanic module. It also happens to mean the Italian word for Moses, recalling the biblical parting of the Red Sea.

It consists of building 78 floodgates at the three inlets that link the lagoon to the Adriatic. Francesca de Pol says one of the gates' characteristics is their flexibility.

Ms. de POL: Depending on the different kind of tides, you have a different management of the gates. You are not obliged to close the whole lagoon. You can choose to close one part, one inlet and not the other, in case of wind coming from a certain direction. You can choose of not closing the whole system, but only parts of the gates for certain types of tides. So you continue to have this exchange of water, which is diminished but not totally blocked.

POGGIOLI: This is the worksite at the Malamocco inlet. The walls at each end are being built just like Venice was. Instead of wooden pilings, teams of workers are driving 125-foot-long steel and concrete pilings into the lagoon bed.

Ms. de POL: The doors will be in between this area and in front of us where you see that boat. And it will be from one side to the other. When at rest, they will be under, lying on the bottom of the inlet channel. When a tide - a dangerous tide is forecast, compressed air will be inflated inside the gates. It will empty the gates from the water. And they will rise and block the entrance of the tide.

POGGIOLI: Submerged gates will each be up to 92 feet long, 65 feet wide, and weigh 300 tons. When not in use, they'll be invisible. In another effort not to alter the landscape, the worksite is on a specially built artificial island that will be demolished once the entire project is completed.

The debate over the floodgates has been under way for nearly four decades. The design was finally approved by the Italian government in 2003. Costs now stand at $7 billion.

Claudio Mantovan is the supervisor at the Malamocco worksite.

Mr. CLAUDIO MANTOVAN (Supervisor, MOSE, Malamocco ): (Speaking in Italian)

POGGIOLI: We have finished the navigation lock, he says, that will allow large ships to enter the lagoon when the gates are up. We're on schedule, 37 percent of the entire project is completed, and he says it should open as planned in 2012.

Mantovan acknowledges a few days of work were lost due to peaceful protests, and it's not just environmentalists who opposed MOSEs. Journalist Alberto Vitucci has been covering the project for years. He sums up the opponents' counter arguments.

Mr. ALBERTO VITUCCI (Journalist): (Through translator) In order to build trenches for the MOSE gates, they are going to dig up millions of cubic meters of seabed and replace them with cement, which seriously could alter the ecosystem. The entire mechanism will be underwater, making maintenance extremely difficult and costly. And the authorities never took any alternative project into serious consideration.

POGGIOLI: Other proposals included narrowing the inlet channels to reduce the water flow from the sea into the lagoon, and banning tankers and large ships from entering. Some criticize the project as irreversible and outdated. They say it was designed without taking into account predictions of rising sea levels over the next century.

Engineers at the MOSE project respond that the mobile gates are designed to last at least a century and to protect Venice from a difference in water level between the sea and lagoon of up to six and a half feet.

The latest prediction of the Intergovernmental Panel on Climate Change is for a one or two-foot increase by the end of this century. Despite opposition, the MOSEs project is moving ahead, and it's being closely watched not only by Venetians.

Coastal cities all over the world, from New Orleans to Singapore to Bombay, know that due to rising sea levels, Venice's seasonal flooding could soon become a shared, global phenomenon.

Sylvia Poggioli, NPR News.

MONTAGNE: Later today on ALL THINGS CONSIDERED, you can hear how Venetians are dealing with the high water and nipping at their knees. And at npr.org/climateconnections, you can get videos of climate science in action. That's from public television's Wild Chronicles.

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