It doesn't get rehashed often these days, but in the 1980s, the world narrowly avoided a massive environmental debacle. As early as 1974, Paul Crutzen, Sherwood Rowland, and Mario Molina had discovered that the Earth's ozone layer, which protects the planet from harmful ultraviolet rays, could be depleted by manmade chemicals—chlorofluorocarbons, to be precise, which had been used since the 1920s for everything from refrigeration to making Styrofoam. As the science advanced, and the link between CFCs and the dramatic ozone "holes" over the Earth's poles—especially Antarctica—became more certain, the countries of the world finally got together and, in 1987, drafted the Montreal Protocol, agreeing to ban a variety of hydrocarbons. It was a stunning success: CFCs were phased out, and the ozone hole is slowly (if unevenly) on its way to recovery.

What's always stomach-clenching about this story is how close a call this actually was. If, instead of CFCs, companies had been using bromine-based fluorocarbons since the 1920s—a comparable substance that would've just chewed through ozone molecules much faster than chlorine-based fluorocarbons did—then much of the ozone layer would've been depleted by the time Crutzen and his colleagues made their discovery. What's more, according to a new NASA report released today, had the countries of the world not joined together and phased out CFCs under the Montreal Protocol, two-thirds of the earth's ozone layer would have vanished by 2065—not just at the poles, but all over the planet.

What would that have meant for humans? For a few clues, look at the recovering ozone hole near the South Pole, which still lets an abnormally high percentage of UV rays through. People living south of the 40 latitude mark have seen a spectacular rise in skin cancer risk—in Punta Arenas, Chile (sitting at 53S), skin cancer rates rose some 66 percent. If ozone depletion had spread, then in the future you'd be able to get sunburn here in Washington, D.C. after standing outside for just five minutes. Sunscreen would be a must just to grab the mail. UV rays have also been linked to eye damage and an increased frequency of cataracts, while the extra ultraviolet would've wreak havoc on the one-celled organisms that undergird the ocean's food chain. Agriculture would've been screwed, too: Pea and bean yields drop roughly one percent for every extra percent of UV radiation received.

Anyway, this always strikes me as a good precedent for negotiating a greenhouse-gas treaty—here we had scientists telling us that the world was in real danger if we continued emitting certain substances, so the world's governments got together and acted swiftly. The aerosol and halocarbon industries bitched and moaned, sure—even sending out paid flacks to cast doubt on the science (the CEO of Dupont, the inventor and largest manufacturer of CFCs at the time, scoffed at the ozone discoveries as "a load of rubbish"). But once the treaty passed, companies got to work finding substitutes for CFCs, and did so in remarkably short order. The economy didn't skip a beat. So if it worked with CFCs, why can't it work with carbon-dioxide and manmade global warming?

Alas, it's not a perfect analogy. I asked Riley Dunlap, an environmental sociologist at Oklahoma State University, about the parallels, and he added some smart caveats. The first is that DuPont, for all its early stubbornness, actually led the way in phasing out CFCs when pressed by environmentalists and Congress. The company had been researching alternatives over the past decade as a way of preparing for the expiration of its patents, and had easy substitutes to CFCs close at hand. In the current situation, it will likely prove trickier to shift away from fossil fuels—and oil and coal companies are a lot more resistant to regulation than Dupont ever was in the 1980s. Plus, of course, phasing out CFCs didn't force anyone to make any lifestyle changes, the way curbing carbon-dioxide emissions might.

The other point, Dunlap notes, is that ozone depletion was a fairly easy issue for scientists and environmentalists to convey in a concrete way to the public. "Although we technically couldn't see the hole, it was easy to portray with computer mapping. And then there was the looming threat of skin cancer," Dunlap says. "So you could easily pinpoint the problem and highlight the dramatic effects." Global warming's not quite as simple or dramatic as "hole in sky = horrible sunburn." Manmade climate change is a gradual process that will unfold over many years; it's hard to tie this or that hurricane or drought explicitly to rises in CO2 levels. Climate scientists talk in terms of probabilities and risks and percentages—they're correct to do so, but it's not nearly as eye-catching. So while the ozone layer tale is heartening, there's no question that climate change is a much bigger—and more daunting—challenge.

--Bradford Plumer