On Sunday, scientists and policymakers from around the world will gather in Montreal to celebrate an international agreement from 20 years ago that addressed perhaps the first global environmental issue of our age: the hole in the ozone layer.
The hole still exists. Indeed, last year it was the biggest ever, spreading across more than 10 million square miles above Antarctica. Scientists do not expect it to recover for at least a half-century.
But the agreement, the Montreal Protocol, nonetheless has been hailed as an environmental success and a possible template for how global warming can be addressed.
"Twenty years ago, getting rid of ozone-depleting chemicals looked really hard," said David Doniger, climate policy director for the Natural Resources Defense Council.
"But it turned out to be quite easy. When we get serious about global warming, we're going to find that it wasn't as hard as it looked."
The ozone layer, high in the stratosphere, protects life on Earth by absorbing harmful solar radiation.
It forms when sunlight separates an oxygen molecule, which has two atoms of oxygen, and the molecule re-forms as ozone, which has three.
Discovery of the ozone hole triggered a big change.
It became an iconic symbol of environmental degradation, as much a part of the public consciousness as Love Canal and the Exxon Valdez.
It transformed suntan lotion into sunscreen. Sunglasses became not just a fashion statement, but vital protective eyewear.
"It was one of the first symbols of how our mundane daily actions can have global consequences," said Roger Turner, a University of Pennsylvania doctoral candidate who studies the history of science. "For consumers, suddenly your hairspray was tearing a hole in the sky."
As often happens, the substances that caused the depletion of ozone were, at first, extolled as a solution for an entirely different problem.
In the 1920s, the refrigerator began to appear in the American household, but the refrigerants used were flammable and toxic.
In 1928, scientists discovered safer chlorofluorocarbons (CFCs), which were inert, nontoxic, and did not dissolve in water.
By the 1950s, they were widely used not only as refrigerants, but also as propellants in deodorant and other personal care products.
But they presented a new problem. Released into the air, they didn't break down. It took about two years, but they migrated high into the stratosphere.
Concern first began to mount in 1974, with an article in Nature theorizing that CFCs could react with ozone, breaking apart the molecules.
By the end of the decade, personal care products bore labels that proclaimed "No CFCs!"
In 1985, scientists discovered that the ozone layer had a seasonal "hole" - more precisely, a dramatic thinning - over the Antarctic. There, cold temperatures and the isolation of the polar stratosphere accelerated depletion of ozone.
It turned out that ozone was thinning elsewhere on the planet, too, because of CFCs. Across North America, ozone had been depleted by about 7 percent as of the mid-1990s. (It has since recovered somewhat.)
Health-care officials warned of increased skin cancers and cataracts due to increased ultraviolet radiation.
The discovery shocked people, said James R. Fleming, a science and technology historian at Colby College in Maine, who asked: "What does it mean now to be concerned about the clear blue sky?"
He uses a cartoon from the 1980s in his classes. The first frame shows a recognizable character, renamed Little Ozone Annie, singing: "The sun will come out tomorrow." The next frame shows her cooked into a pile of ashes.
But then - as with climate change now - debate raged. Some said the science was unsound. And if there was a hole and it was caused by humans, the fix could cost untold amounts of money and could bring industry, the military, and a lot else to their knees.
Nowhere in American industry, perhaps, did the debate about CFCs and the ozone layer have as much relevance - and urgency - as at the DuPont Co., which had introduced the chemical in the 1930s.
By the '70s, it was one of the biggest manufacturers of CFCs in the world, with much of it produced at the company's Chambers Works in Deepwater, N.J. DuPont's brand name, Freon, was in everyday use.
Once the world's scientists began to get a handle on the issue, the next question was what to do about it. In the 1980s, recalled Mack McFarland, DuPont's chief atmospheric scientist, the United States alone had $135 billion worth of refrigeration and air-cooling equipment in use. It could not simply be junked.
On Sept. 16, 1987, policymakers from around the world agreed to a timetable for phasing out CFCs and other ozone-depleting chemicals. It was called the Montreal Protocol.
"The sheer size, clear signature, and the understanding that something humans did so far away could cause a hole over the Antarctic was a powerful message," said A.R. Ravishankara, a scientist with NOAA's Earth System Research Laboratory, which monitors global ozone levels.
DuPont had become one of the leaders in calling for change, in spite of what it would cost the company economically, McFarland said.
Beyond the company's contribution, he credits policymakers with recognizing "the need to move quickly and decisively, and the need to have global involvement."
It was the first time, he said, "the world had ever approached one of these issues that was global in scope and many decades in time scale."
Now ratified by 191 countries, the protocol continues to address not just CFCs but also other ozone-depleting chemicals, including a less-harmful refrigerant replacement, hydrochlorofluorocarbons, also known as HCFCs.
By 2005, according to a U.N. report, more than 95 percent of the chemicals the protocol covers had been phased out.
The benefits, Doniger and others say, have been huge.
A recent EPA report concluded that 6.3 million skin cancer deaths may have been prevented by 2165 in the United States alone. It cited an expected $4.2 trillion worth of "societal health benefits" between 1990 and 2165.
Ravishankara said that ozone-depleting chemicals in the atmosphere have shown a measurable decline, although the ones still there would remain for as long as 100 years.
At meetings in Montreal next week, the United States will push for moving up the phase-out of HCFCs, the interim replacement for CFCs. DuPont supports the move.
Meanwhile, researchers will be getting their first glimpse of what this year's ozone hole looks like.
The sun, which prompts the chemical reactions in the stratosphere, is just making its annual appearance over Antarctica.