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Listen to an interview with Alain Robert

Updated, 6:30 p.m. | Alain Robert, a French stuntman known for climbing tall buildings, scaled the north face of the New York Times building on Thursday, ascending 52 stories to the roof and clutching a bright green banner, before police officers arrested him around 12:22 p.m. (Update: Hours later, a second man was arrested after climbing to the roof on another part of the building. For a complete account of the day’s events, see the article prepared for Friday print editions.)

Police and security officials cordoned off the sidewalk below, on West 41st Street, as a crowd assembled. The words on the banner were illegible from the sidewalk, but from office windows inside the tower the slogan on the banner could be clearly read: “Global warming kills more people than 9/11 every week.”

The man later confirmed, moments after being arrested on the roof of the tower, that he was Alain Robert, a 46-year-old stuntman famous for scaling structures like the National Bank of Abu Dhabi, the Sydney Opera House in Australia and the Eiffel Tower and Montparnasse Tower in Paris.

He wore a T-shirt with his name and the address of a Web site (thesolutionissimple.org), exercise pants and climbing shoes. He had long blond hair. He used no rope, harness or parachute.

Police officers blocked off the sidewalk at the base of the building and asked members of the crowd to move along. Construction workers on a building directly across West 41st Street, facing the northern face of the building, looked on with expressions of astonishment and amusement. A crowd gathered on the sidewalk, pointing, gawking and capturing pictures and images with cellphones, digital cameras and video cameras.

“This is a publicity stunt, it looks like,” Janet L. Robinson, the chief executive of The New York Times Company, said as she entered the building. “There is definitely going to be an arrest.”

Wearing a backpack slung over one shoulder, Arthur Ochs Sulzberger Jr., the chairman of The Times Company and publisher of The Times, who is himself an avid climber, ducked under the police tape and examined the spectacle. He declined to comment.