The ozone hole over Antarctica in 2006. Credit:NASA In 1974 came the Nobel prize winning discovery that ozone-depleting chemicals called chlorofluorocarbons (CFCs) - carried in refrigerants, spray cans, foams and other substances - could damage the stratospheric layer that protects us from ultraviolet solar radiation (and thus, skin cancer). But it wasn't until the sudden discovery of a vast seasonal ozone "hole" over Antarctica in 1985 that the world was shocked into action. The so-called "hole" represents a region of the stratosphere over Antarctica, between about 10 and 25 kilometres in altitude, where "the ozone gets destroyed completely", Solomon said. However, some ozone remains above and below this region, amounting to a 40 or 50 per cent loss of atmospheric ozone overall in a very large area of air.

Ozone has been depleted in the stratosphere all across the globe. But Antarctica in the southern hemisphere spring presents uniquely conducive conditions for it to happen, as extremely cold polar stratospheric clouds provide a surface that enables the chemical reactions in which destructive forms of chlorine to be created. Discovery of the "hole" galvanised action and, in 1987, the Montreal Protocol, which is still today hailed as the epitome of a successful environmental agreement, led to a phase-out of the use of ozone depleting chemicals. Here was a case that now appears so very different from the story of climate change, because everything basically functioned as it was supposed to - scientists identified a problem, the public grew concerned, and politicians acted to solve it. "You have to put yourself back in the time when the ozone hole was discovered," Solomon, who has been studying the issue for more than three decades, said. "We thought we were going to see a few per cent change in the ozone layer in a century. And then all of a sudden, boom, we've got half as much ozone in a part of the world where nobody ever expected it, already happening in 1986. It became a tremendous hot environmental crisis as a result of that."

Ever since the Montreal Protocol was adopted, it has been a process of waiting for ozone depletion in the atmosphere to slow down, then for decline to cease entirely, and then, finally, to see the ozone layer turn the corner and begin to grow back. And it is this last observation that is finally here for the Antarctic ozone hole in particular, the new study asserts. In the research, the researchers used satellite and balloon data to examine the seasonal Antarctic ozone hole for a 15-year period between 2000 and 2015. And they found that, in September, the size of the hole had generally declined by more than 3.9 million square kilometres, and that this was a trend that could be statistically separated from the "noise" of natural variations. "The September size of the ozone hole shows this very systematic trend of getting smaller, and the September [measurements] also show that the ozone has begun to recover just exactly in the height range where the polar stratospheric clouds are," she said. The study also found that "roughly half" of the improvement seen in September was "chemical" in nature, or, in other words, the result of fewer ozone-depleting chemicals in the stratosphere. October, the peak month for Antarctic ozone depletion, is another matter. October 2015 actually showed a quite large ozone hole over the ice continent. At its peak size, it was 28 million square kilometres.

But another part of the new study is to explain why October remains a highly variable month for ozone depletion over Antarctica - one in which it is hard to detect a healing signal - and why last October was so bad, even as the ozone hole is clearly healing. And the answer is volcanoes - specifically, the eruption of a volcano named Calbuco in Chile. Large volcanic eruptions fill the stratosphere with sulfur dioxide and, depending on where they occur, this can circulate around much of the globe. Southern hemisphere eruptions spread sulfate across that hemisphere, Solomon said, and the sulfur dioxide aids in the formation of polar stratospheric clouds, once again enhancing ozone depletion. The study used a climate model to simulate how volcanic contributions enhanced the ozone hole in October. Therefore, the paper was able to conclude that one bad October did not detract from an overall, if slow, early ozone recovery.

Granted, there is a long way to go. The study suggests that the October ozone hole over Antarctica will still be with us until about 2050 and, depending on volcano behaviour, there could be more setbacks and large swings along the way. Still, it's hard to interpret the current paper as anything other than a piece of (rare) good news when it comes to the interactions between humans and their planet. And it tempts one to think bigger about its broader significance. The ozone issue and the climate issue have long been conjoined, not merely because they are both problems involving the global atmospheric commons, or because chlorofluorocarbons are also greenhouse gases, but also because they emerged into public consciousness about the same time. This can be seen in a historic 1986 hearing before Congress in which scientists not only raised early climate alarms, but paired those with presentations about ozone depletion and the Antarctic hole. However, the world went on to solve one of the problems before too much damage set in - but has failed to do nearly as well with the other. The precise lesson to draw here, though, is less than clear. On the pessimistic side, one could say that the contrast proves that a modest-sized global environmental problem can be solved by the world, while a mega-sized one is another matter entirely.

"On the scale of things this was a different type of problem," Solomon said. "It was an industry measured in the billions, not the trillions." Yet she said the evidence that the Antarctic ozone hole was finally getting better made her optimistic about our capacity ultimately to come to grips with climate change as well. "Technology and innovation can do miraculous things," she said. "We still have airconditioners, we still have refrigerators, we still have hair spray. We didn't have to give up much, and yet we got to a state in which the atmosphere is much better off, and we're better off." Washington Post