“It’s not just global warming,” he said. “But the episodes are more severe and long.”

After about 1,000 years, Venice is imperiled by its sinking foundations and rising waters as well as the hordes of tourists arriving on cruise ships and low-cost flights. They clog the narrow streets and have pushed out residents in favor of Airbnb apartments.

Flooding, though, is the existential danger.

Earlier this week, for only the fifth recorded time in St. Mark’s nine-century history, the water reached the marble floor inside, submerging the area around the altar of the Madonna Nicopeia.

On Wednesday, the floor was dry but a yellow sign reading “Attention: Wet Pavement” stood ready by the entrance. Outside, though, the water still filled St. Mark’s Square. As tourists climbed the steep steps to the basilica’s balcony (“We saw a wedding proposal!” one woman shouted), the Roman Catholic patriarch of Venice, Msgr. Francesco Moraglia, checked in on his church.

Earlier in the week, he had rushed to visit when he heard the water had breached the door.

“I said a prayer and gave a blessing,” Monsignor Moraglia said, showing pictures of himself wearing galoshes. Now, he said, he was hoping for help from the multibillion-dollar Mose project, an unfinished system of floodgates that was initiated more than a decade again to block the rising waters and threats from global warming.