“If you use the medical analogy, first the patient was getting worse and worse, and then the patient is stabilized, and now, the really encouraging thing, is that the patient is really starting to get better,” said MIT atmospheric scientist Susan Solomon, lead author of the study, and former co-chair of the United Nations’ Intergovernmental Panel on Climate Change.

And moreover, that patient — the Earth’s vital ozone layer — is getting better directly because of our choices and policies.

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The initial, 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) came in 1974. 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” refers to a region of the stratosphere over Antarctica, between about 10 and 25 kilometers in altitude, where “the ozone gets destroyed completely,” explains Solomon, who conducted the new research with scientists from the National Center for Atmospheric Research and the University of Leeds in the UK. However, some ozone remains above and below this region, amounting to a 40 or 50 percent loss of atmospheric ozone overall in a very large area of air.

Ozone has been depleted in the stratosphere all across the globe, to be sure. But Antarctica in the spring (which is autumn in the northern hemisphere) 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 are created.

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Discovery of the “hole” galvanized 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. It’s a case that now appears so very different from the story of climate change, because everything basically functioned like 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,” remembers Solomon, who has been studying the issue for over three decades. “We thought we were going to see a few percent 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’s adoption, then, it has been a lengthy process of waiting for ozone depletion in the atmosphere to slow down, then for decline to cease entirely, and then finally, seeing 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.

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In the research, the scientists 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 the month of September, the size of the hole has generally declined by over 1.5 million square miles, and that this is a trend that can 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,” Solomon said. The study also found that “roughly half” of the improvement seen in September is “chemical” in nature, or in other words, the result of less ozone depleting chemicals in the stratosphere.

October, the peak month for Antarctic ozone depletion, is another matter. October of 2015 actually showed a quite large ozone hole over the ice continent. At its peak size, it was 10.9 million square miles in size.

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But another insight of the new study turns on explaining why October remains a highly variable month for ozone depletion over Antarctica — one where it is hard to detect a healing signal — and why last October was so bad, even as the ozone hole is shrinking overall. 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 upon where they occur, this can circulate around much of the globe. Southern hemisphere eruptions spread sulfate across that hemisphere, said Solomon, and the sulfur dioxide aids in the formation of polar stratospheric clouds, thus once again enhancing ozone depletion.

The study used a climate model to simulate how volcanic contributions enhanced the ozone hole in October of last year. Therefore, the paper was able to conclude that one bad October doesn’t detract from an overall, if slow, ozone recovery.

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Granted, there is a long way to go. The study suggests that the October ozone hole over Antarctica will still be with us until around 2050, and depending on volcano behavior, 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 at around the same time. You can see as much 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 ozone hole.

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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, you could say that the contrast basically 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,” said Solomon. “It was an industry measured in the billions, not the trillions.”

Yet Solomon says the evidence that the Antarctic ozone hole is finally getting better makes her optimistic about our capacity to ultimately come to grips with climate change as well.

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“Technology and innovation can do miraculous things,” she said. “We still have air conditioners, we still have refrigerators, we still have hair spray, for crying out loud. 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.”

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