On Monday afternoon, Jerry Saltz, the longtime art critic at New York Magazine, went to the Metropolitan Museum of Art, where he has gone nearly once a week for three decades. After two hours of taking in “Golden Kingdoms: Luxury and Legacy in the Ancient Americas,” he left and headed toward the fountain on the northern end of the plaza out front, which bears the name of its benefactor, David H. Koch. Mr. Saltz was carrying a long strip of paper that had been printed to blend in with the granite used for the fountain. The words on it matched the typeface of the fountain’s inscription. Mr. Saltz then attached his sign to the fountain, so that it no longer read “David H. Koch Plaza.” Instead it declared, “Climate Change Denier Plaza.”

[David Koch Died on Aug. 23 at the Age of 79]

People cheered and clapped and took out their phones. Mr. Saltz, who is married to the co-chief art critic of The New York Times, Roberta Smith, picked up the scraps of tape he had used that had fallen to the ground — he did not want to litter — and shared a picture of his act of guerrilla resistance with his 502,000 followers on Twitter. The Met removed the sign quickly and offered no comment when I called its spokesman to ask how the gesture was received. Had the museum banned him from going back, Mr. Saltz told me, he would have prostrated himself before its executives and begged to be readmitted. “I can’t live if living is without the Met,’’ he said. “I know the act is riddled with protest, but it was a guttural response. I’d walked by so many times and seeing the name David Koch had bothered me for such a long time.’’

In the fall of 2014, the plaza in front of the museum, which runs along Fifth Avenue for four city blocks, opened to the public after a two-year reconstruction that cost $65 million, the entire sum of which was supplied by Mr. Koch, the famous backer of right-wing causes and work in climate-change skepticism in particular. Among the many recipients of the family’s largess is, for instance, the Competitive Enterprise institute, a nonprofit libertarian think tank that published a blog post several years ago comparing a prominent Penn State climate scientist, Michael Mann, to the former football coach and pedophile Jerry Sandusky, arguing that Mr. Mann had “molested and tortured data in the service of politicized science.”

The Trump presidency has, in many ways, been like a tuning fork applied to the city’s soul, causing it to vibrate again after years in which it had seemed incapable — dulled and flattened by the growing presence of Rite Aid and meal kits and complacency and billionaire dormitories. On a Friday evening last month, a dozen activists stood up in Clyde Frazier’s, a Midtown restaurant, chanting, “No justice, no bread,’’ in protest of Tom Cat Bakery, a major supplier, that has attracted the antagonisms of the group Rise and Resist for its treatment of immigrants. In January, protesters lined up in front of the Museum of Natural History to call for Rebekah Mercer, another influential conservative philanthropist who has supported climate-science denial, to step down from the museum’s board.