The Hole We Made and the Treaty That Closed It

A sliver of ozone — no thicker than a few millimeters if brought down to the surface — is all that stands between life on land and the Sun’s most destructive rays. For most of history we did not know it was there, and then we nearly destroyed it by accident.

High in the stratosphere, between roughly fifteen and thirty-five kilometers up, sits a thin layer of ozone, a form of oxygen with three atoms instead of two. There is not much of it; if you gathered all the ozone in the atmosphere and brought it to the surface it would form a layer only a few millimeters thick. Yet that sliver does something life on land cannot do without. It absorbs the most damaging ultraviolet radiation from the Sun, the high-energy wavelengths that break apart DNA and cause skin cancer, cataracts, and damage to crops and plankton. Without the ozone layer, the land surface would be sterilized of much of its life. For most of human history we did not know it was there, and then, almost by accident, we started destroying it.

The culprits were a class of chemicals called chlorofluorocarbons, or CFCs, and the bitter irony is that they were adopted precisely because they seemed so harmless. Invented in the 1920s and 1930s, CFCs are wonderfully stable and unreactive, non-toxic, non-flammable, which made them ideal as refrigerants, as propellants in spray cans, and as foaming agents. They do not react with anything at ground level. That same stability is what doomed the ozone layer, because a molecule that reacts with nothing at the surface is a molecule that survives long enough to drift, intact, all the way up into the stratosphere.

In 1974 two chemists, Mario Molina and Sherwood Rowland, worked out what happens when it gets there. High in the stratosphere, the intense ultraviolet light that the ozone layer absorbs is strong enough to break a CFC molecule apart, releasing a single chlorine atom. And a chlorine atom is a catalyst for destroying ozone. It reacts with an ozone molecule, breaks it, and then is freed again to attack another, and another. A single chlorine atom can destroy on the order of a hundred thousand ozone molecules before it is finally removed. Molina and Rowland’s calculation was alarming: the stable, useful, apparently benign chemicals in millions of refrigerators and spray cans were a slow-acting poison for the shield that makes land life possible. Their warning was not universally welcomed, and industry pushed back hard, but the chemistry was sound, and they shared a Nobel Prize for it in 1995.

The abstract warning became undeniable in 1985, when scientists from the British Antarctic Survey reported something nobody had predicted: over Antarctica, each spring, the ozone was not merely thinning but collapsing, dropping to a fraction of its normal level across a vast area. The press called it the ozone hole. The Antarctic conditions, with their extreme cold and isolated swirling winds, turned out to create the perfect setting for chlorine to do its worst. The hole was a shock precisely because it was so much larger and more sudden than the gradual thinning the models had forecast.

 Antarctic springtime ozone, schematic

 ozone
 level |---....                                    .....----
       |       ''..                            ..''
       |           '.        the hole         .'
       |             '.                      .'
       |               '...            ...''
       |                   ''..______..''
       +----+----+----+----+----+----+----+----+----  year
          1960   1975   1985   1995   2010   2025  (recovery underway)

What happened next is the part worth holding onto, because environmental news so rarely ends this way. The world acted, and acted fast. In 1987, just two years after the hole was confirmed, nations signed the Montreal Protocol, a treaty to phase out the production of CFCs and related ozone-destroying chemicals. It was strengthened repeatedly as the science firmed up, it has been ratified by every country in the world, and crucially it included help for developing nations to switch to safer alternatives. Production of the worst chemicals fell sharply through the 1990s and beyond.

The atmosphere responds slowly, because the CFCs already up there persist for decades, but it is responding. Atmospheric concentrations of the banned chemicals peaked around the late 1990s and have been declining since. Scientific assessments now project that the ozone layer is on track to recover to its 1980 levels around the middle of this century for most of the world, and over Antarctica by roughly the 2060s. The hole still opens each spring, but the trend is toward healing, and researchers have detected the first clear signs of it. The Montreal Protocol is regularly called the most successful environmental treaty ever enacted, and the title is deserved.

I think the ozone story is worth telling often, and not only as history, because it is the rare case where the whole arc completed. Scientists identified an invisible, slow-moving, global threat from first principles. The threat was confirmed dramatically and unexpectedly. The nations of the world, against significant commercial resistance, agreed to give up a profitable and useful technology. And the planet, decades later, is measurably recovering. It is proof that a diffuse global problem caused by a convenient chemical is not automatically unsolvable. We made the hole, we understood it, and then we did the difficult, coordinated, expensive thing, and it worked.

Scientists named an invisible threat from first principles, the world gave up a profitable technology against commercial resistance, and the planet is measurably recovering. A diffuse global problem turned out not to be unsolvable after all.