For more than 20 years, Metronome, which includes a 62-foot-wide 15-digit electronic clock that faces Union Square in Manhattan, has been one of the city’s most prominent and baffling public art projects.

Its digital display once told the time in its own unique way, counting the hours, minutes and seconds (and fractions thereof) to and from midnight. But for years observers who did not understand how it worked suggested that it was measuring the acres of rainforest destroyed each year, tracking the world population or even that it had something to do with pi.

On Saturday Metronome adopted a new ecologically sensitive mission. Now, instead of measuring 24-hour cycles, it is measuring what two artists, Gan Golan and Andrew Boyd, present as a critical window for action to prevent the effects of global warming from becoming irreversible. On Saturday at 3:20 p.m., messages including “The Earth has a deadline” began to appear on the display. Then numbers — 7:103:15:40:07 — showed up, representing the years, days, hours, minutes and seconds until that deadline.

As a handful of supporters watched, the number — which the artists said was based on calculations by the Mercator Research Institute on Global Commons and Climate Change in Berlin — began ticking down, second by second. “This is our way to shout that number from the rooftops.” Golan said just before the countdown began. “The world is literally counting on us.” The Climate Clock, as the two artists call their project, will be displayed on the 14th Street building, One Union Square South, through Sept 27, the end of Climate Week. The creators say their aim is to arrange for the clock to be permanently displayed, there or elsewhere. Golan said he came up with the idea to publicly illustrate the urgency of combating climate change about two years ago, shortly after his daughter was born. He asked Boyd, an activist from the Lower East Side, to work with him on the project. The artists said they had previously made a handheld climate clock for Greta Thunberg, the teenage activist from Sweden, before her appearance last year at the United Nations Climate Action Summit.

Their goal of creating a large-scale clock was influenced in part by the Doomsday Clock, maintained online by the Bulletin of the Atomic Scientists, and by the National Debt Clock near Bryant Park in Manhattan. Golan and Boyd decided that the Climate Clock would have the most impact if it were displayed in a conspicuous public space and presented like a statue or an artwork. “This is arguably the most important number in the world,” Boyd said. “And a monument is often how a society shows what’s important, what it elevates, what is at center stage.”

Eventually, Golan and Boyd seized on “Metronome,” a mixed-media work by Andrew Ginzel and Kristin Jones that covers a 10-story-high area on the north wall of One Union Square South, a residential high rise.

The work also includes concentric circles rendered in gold-flecked brick that ripple outward from a round opening. When it was unveiled in 1999, clouds of steam and musical tones issued from the facade.

Over the years the sound and steam have ceased. The numbers, however, kept moving.

The original artists had been thinking about reimagining the work to address the deepening climate crisis when, in February, they got a letter from Golan and Boyd.

“It was kind of magic,” Jones said, calling the timing “beautiful synchronicity.” “The Climate Clock will remind the world every day just how perilously close we are to the brink,” Stephen Ross, chairman of Related Companies, the developer that owns One Union Square South, said in a statement. He added, “This initiative will encourage everybody to join us in fighting for the future of our planet.”