How bad would the ozone hole be if we did nothing?

The “hole” in the ozone layer is sometimes invoked by those who downplay environmental concerns as an example of “sky is falling” warnings that never came to pass. It's an odd example. There's a simple reason ozone problems didn't come to pass: the world came together and agreed to phase out key ozone-depleting chemicals.

It’s a major success story, and one that should be remembered. As we consider the cost of dealing with ongoing environmental problems, it's worth considering: how much better off are we for the action we did take to preserve the ozone layer? Some scientists have now tackled this question.

Ozone gas in the stratosphere is enormously important for life on Earth. Ozone absorbs ultraviolet radiation emitted by the Sun, greatly reducing the amount that reaches the surface. It’s sunscreen for the planet, as UV causes skin cancer and sunburns. In the 1970s and early 1980s, it was discovered that compounds called chlorofluorocarbons (CFCs), commonly used as refrigerants and in aerosol sprays, were breaking down stratospheric ozone.

That chemical reaction requires sunlight and is facilitated by cold temperatures, so it occurred most vigorously over Antarctica in the spring. A hole opened up over Antarctica, where so little ozone survived that most UV radiation passed right through. Ozone was destroyed elsewhere around the world but to a lesser degree.

The havoc-causing chlorine (or, in some cases, bromine) from these compounds can last a long time in the atmosphere and packs a strong pound-for-pound ozone punch. After we cut our emissions, however, atmospheric chlorine peaked in the mid-1990s and has dropped slightly since.

If we had kept emitting these compounds, how bad would things have gotten? A number of studies have looked at this “world avoided” scenario, using climate models to project through the 21^st century. Those simulations are grim, showing an ozone “hole” that grows to encompass the entire planet around mid-century. (Incidentally, CFCs are also very potent greenhouse gases, so the outlook for global warming would be even worse.)

A team led by University of Leeds researcher Martyn Chipperfield took a slightly different approach to address the same question. Instead of simulating the future with climate models, they used a “reanalysis” of the past—a kind of model that fills in the gaps between all the available observations to draw a complete picture of recent conditions around the globe. They used this representation of actual conditions since the 1980s to drive a chemical model calculating the effects of ozone-depleting chemicals. One version of the model reflected the phase-out of CFCs, matching the actual behavior of the ozone layer that we’ve observed over the years. Another version simulated the consequences of CFC emissions continuing to rise at about three percent per year.

In that parallel reality—the world where we never took action—things would already be considerably worse than they are today. The Antarctic hole would be about 40 percent bigger at this point.

And it would have an Arctic twin. In 2011, atmospheric conditions conspired to produce exceptional ozone loss in the Arctic, although it never reached the threshold used to define ozone holes. But in the simulated world where there are continued CFC emissions, those conditions would have produced a legitimate hole extending over most of the Arctic Ocean and south over Scandinavia, Finland, and part of Russia. That’s as big as the Antarctic hole has been.

Beyond that, the Arctic would be seeing a hole one-third to one-half that size every spring.

Moving away from the poles, we would also find a thinner ozone layer. In the tropics, we would have about five percent less ozone today. The mid-latitudes would be seeing around 10 percent additional losses on top of the current four percent decline from pre-1970s levels seen today—after CFCs have been on the decline.

These numbers would have real health impacts. Australia and New Zealand, which have the highest skin cancer mortality rates in the world today, would be bathed in eight to 12 percent more UV radiation. Northern Europe and the UK would be getting about 14 percent more. Unless this triggered a compensating increase in protective behavior, the result would be more cases of skin cancer—not just in the future, but in 2015.

Fortunately, we took responsible actions to avoid that world, which now only exists in computer models.

Open Access at Nature Communications, 2015. DOI: 10.1038/ncomms8233  (About DOIs).