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As he stood on the Queens shoreline on Earth Day, Mayor Bill de Blasio issued a stern warning that the familiar Manhattan skyline behind him was about to change.

“We are going to introduce legislation to ban the glass and steel skyscrapers that have contributed so much to global warming,” he said on Monday. “They have no place in our city or on our Earth anymore.”

But as far as an actual prohibition against glass and steel skyscrapers? Not exactly.

Mr. de Blasio’s plan involves legislation that would institute new energy code requirements as a prerequisite for a building permit. “The kind of the glass and steel buildings of the past, and some bluntly were being built very recently, are just not going to be allowed any more,” the mayor said.

City officials said that they did not know the specifics of Mr. de Blasio’s proposal, and that any such change would require City Council approval.