There's no question that the problem has worsened and the threat is real. Last year alone, high waters inundated St. Mark's Square, the city's main piazza and lowest point, more than 90 times. That compares to fewer than 60 occasions during the entire decade of the 1920s. Paintings by the Venetian master Canaletto, which show the waterline on buildings, indicate that the tides now lap the walls 80 centimeters, or 31 inches, higher than two centuries ago. Marble steps leading down to the canals, above water at the beginning of the 20th century, are now submerged except at low tide.

That could just be the beginning. The Intergovernmental Panel on Climate Change, a body of experts considered the most authoritative scientific group monitoring the subject, warns that the world's oceans could rise this century by anything from a few centimeters to almost 90 centimeters, or 35 inches.