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The discovery of the Antarctic ozone hole

The unexpected discovery of a hole in the atmospheric ozone layer over the Antarctic revolutionized science — and helped to establish one of the most successful global environmental policies of the twentieth century.
Susan Solomon is in the Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
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In 1985, Joe Farman, Brian Gardiner and Jonathan Shanklin reported1 unanticipated and large decreases in stratospheric ozone levels over the Antarctic stations of Halley and Faraday. Their data showed that, after about 20 years of fairly steady values, ozone levels began dropping in the austral spring months around the late 1970s (Fig. 1). By 1984, the stratospheric ozone layer over Halley in October was only about two-thirds as thick as that seen in earlier decades — a phenomenon that became known as the Antarctic ozone hole. Farman et al. boldly suggested a link to human use of compounds called chlorofluorocarbons (CFCs), often used in aerosol cans and cooling devices such as fridges. Their findings transformed the fields of atmospheric science and chemical kinetics, and led to global changes in environmental policy.

Figure 1 | Ozone over Antarctica. a, In 1985, Farman et al.1 reported that stratospheric ozone levels over the Halley and Faraday stations in Antarctica during the austral spring had declined greatly from previously steady values. The graph shows the Halley times series, extended to 2016. b, Subsequent satellite monitoring revealed that the area of ozone depletion — the ozone hole — extended over a vast region. This map shows a satellite ozone map for 10 September 2000, when ozone depletion was close to its maximum: blue indicates low ozone levels; red, high levels. The position of the Halley station is indicated.

The stability of the stratospheric ozone layer has attracted the interest of scientists, the public and policymakers for more than 50 years because this layer protects life on Earth’s surface from biologically damaging ultraviolet radiation. The potential for pollutants known as nitrogen oxides to deplete global ozone prompted much research2 on the influence of aviation on the ozone layer3. A study4 in 1974 suggested that chlorine monoxide (ClO) produced from CFCs might similarly deplete ozone. By the early 1980s, the best projections from stratospheric models indicated that continuing production of CFCs at then-current amounts risked the destruction of only about 2–4% of the ozone layer by the end of the twenty-first century3. There was no suggestion that ozone at polar latitudes would be especially sensitive.

The expected depletion was relatively small and far in the future, but posed serious threats, including increased incidence of skin cancers and ecological damage. International policymakers therefore concluded that a cautious ozone-protection strategy was needed, and, in March 1985, the United Nations Vienna Convention for the Protection of the Ozone Layer was signed. It called for more ozone research, but contained no legally binding goals for CFC reductions5.

Farman and colleagues’ report of a loss of one-third of the springtime ozone layer over Antarctica was published a few months later. The paper’s strengths were the authors’ careful analysis of the seasonal character of the change, and the fact that changes were detected using two different instruments. The authors suggested that Antarctica’s extremely cold temperatures during winter and spring made the region “uniquely sensitive to growth of inorganic chlorine” produced in the atmosphere from CFCs, although the chemical mechanism they proposed was incorrect. The careers of hundreds of scientists and dozens of diplomats worldwide were abruptly transformed by this single paper.

At that time, the atmospheric chemistry of the Antarctic was terra incognita. Measurements needed to be made both at ground level and from aircraft to understand whether CFCs had a role in producing the ozone hole. Scientists were energized and excited to attack the challenge.

I was fortunate to be among a group of scientists who went to the US station at McMurdo in 1986, where the first Antarctic measurements of ClO6 and of another CFC-derived ozone-depleting compound, chlorine dioxide (OClO)7, were obtained. These compounds were roughly 100-fold more abundant than elsewhere. The ‘smoking gun’ for the role of CFCs in ozone depletion came from aircraft measurements taken in 1987. They revealed8 a dramatic enhancement in ClO levels (comparable to those at McMurdo) and a co-located decrease in ozone concentrations as the plane flew south from Chile into the Antarctic.

Nature PastCast: A gaping hole

These independently obtained data sets indicated that the Antarctic was indeed uniquely sensitive to chlorine compounds9, as Farman et al. had suggested. Unusual changes in atmospheric abundances of related chemicals were also measured10. Moreover, satellite monitoring confirmed that depletion extended over a vast region (typically up to about 20 million square kilometres; see ref. 10, for example).

The response of policymakers to Farman and colleagues’ paper was initially cool. In my view, this was because they did not want to upset the apple cart of the delicate diplomacy embarked on with the Vienna convention until it was clear that the science was correct. Nevertheless, they argued that precautionary principles were part of the convention, and — even as the research planes were flying from Chile — signed the 1987 Montreal Protocol on Substances that Deplete the Ozone Layer. This was an agreement to freeze production and consumption of ozone-depleting substances at then-current rates, and to meet over time to consider whether to decrease production.

But the signing of global environmental agreements is only a ceremonial first step; they must subsequently be ratified and strengthened over time5. I believe that Farman and colleagues’ paper led to the remarkably fast ratification of the protocol in 1989, and to later amendments (beginning with the London Amendment in 1990) that included ever-tightening restrictions on the global production and consumption of ozone-depleting substances.

So why was the ozone hole not seen in computational simulations of the stratosphere? It turned out that the models lacked a key ingredient: by considering only gas-phase atmospheric chemistry, they overlooked the activation of ozone-destroying chlorine species that occurs on and within polar stratospheric cloud particles at extremely low temperatures11,12. The discovery of the missing ingredient drew physical chemists in increasing numbers to study the surface chemistry involved13. Previously unknown gas-phase reactions associated with ozone depletion were also identified, particularly those involving a ClO dimer (see ref. 10, for example). Laboratory and field studies were carried out, and microphysical models were developed (see ref. 14, for example), to determine what polar stratospheric clouds are made of: ice, nitric acid hydrates or supercooled liquids. The answer was that they could be all three, depending on temperature and the histories of the sampled air parcels.

Ground-based and airborne missions to understand Arctic ozone chemistry15 were also inspired by Farman and colleagues’ paper and related studies. It emerged that ozone loss in the Arctic is generally much less severe than in the Antarctic, broadly because temperatures in the region are warmer as a result of meteorological differences between the two regions. The coupling of chlorine-containing species with bromine-containing ones was found to be a key ingredient in polar ozone depletion, especially in the Arctic16.

Atmospheric modelling also progressed to simulate the newly discovered processes, evolving from two dimensions (latitude–altitude) to three (latitude–altitude–longitude), to better represent global stratospheric temperatures, winds and circulation17. Dynamical studies have shown that the ozone hole influences Antarctic winds and temperatures not just in the stratosphere, but also in the underlying troposphere, and there is evidence for climate connections at other latitudes18. Modern global climate models therefore include increasingly detailed representations of stratospheric chemistry and dynamics. The ozone hole has thus inspired a new generation of scientists to probe climate–chemistry interactions, forging connections between previously separate disciplines.

The Montreal Protocol led to global CFC production and consumption phase-outs by 2010, and now the Antarctic ozone hole is slowly healing10. The protocol thus prevented the ozone layer from collapsing19 and is a signature success story for global environmental policy. Because CFCs have atmospheric lifetimes of 50 years or more, the atmosphere will not fully recover until after 2050, even in the absence of further emissions.

However, recent work20 provides strong evidence of the continuing production and release of one type of CFC (trichlorofluoromethane). The source is not large enough to reverse the healing of the ozone hole, but it is slowing recovery and shows that there is still a need for scrutiny in this field. Research into, and policy to protect, the stratosphere will thus continue to be inspired by Farman and colleagues’ research — and will probably do so until the ozone hole finally closes.

Nature 575, 46-47 (2019)

doi: 10.1038/d41586-019-02837-5

References

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    Farman, J. C., Gardner, B. G. & Shanklin, J. D. Nature 315, 207–210 (1985).

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    National Research Council. Causes and Effects of Changes in Stratospheric Ozone: Update 1983 (Natl Acad. Press, 1984).

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    Molina, M. J. & Rowland, F. S. Nature 249, 810–812 (1974).

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    Benedick, R. A. Ozone Diplomacy: New Directions in Safeguarding the Planet (Harvard Univ. Press, 1998).

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    de Zafra, R. L. et al. Nature 328, 408–411 (1987).

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    Solomon, S., Mount, G. H., Sanders, R. W. & Schmeltekopf, A. L. J. Geophys. Res. Atmos. 92, 8329–8338 (1987).

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    Anderson, J. G. et al. J. Geophys. Res. Atmos. 94, 11480–11520 (1989).

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    Solomon, S. Nature 347, 347–354 (1990).

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    World Meteorological Organization. Scientic Assessment of Ozone Depletion: 2018 – Report No. 58 (WMO, 2018).

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    Solomon, S., Garcia, R. R., Rowland, F. S. & Wuebbles, D. J. Nature 321, 755–758 (1986).

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    Tolbert, M. A., Rossi, M. J., Malhotra, R. & Golden, D. M. Science 238, 1258–1260 (1987).

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    Ravishankara, A. R. & Hanson, D. R. J. Geophys. Res. Atmos. 101, 3885–3890 (1996).

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    Peter, T. & Groos, J.-U. in Stratospheric Ozone Depletion and Climate Change (ed. Muller, R.) Ch. 4, 108–144 (R. Soc. Chem., 2011).

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    Pyle, J. A. et al. Geophys. Res. Lett. 21, 1191–1194 (1994).

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    Frieler, K. et al. Geophys. Res. Lett. 33, L10812 (2006).

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    Eyring, V. et al. Atmos. Chem. Phys. 10, 9451–9472 (2010).

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    Thompson, D. W. J. et al. Nature Geosci. 4, 741–749 (2011).

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    Montzka, S. A. et al. Nature 557, 413–417 (2018).

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Transcript

Nature PastCast: A gaping hole

Kerri Smith

This is the Nature PastCast, each month raiding Nature’s archive and looking at key moments in science. In this show, we’re exploring a paper published in the 1980s.

Music

News report

And now, ozone in the news. Recently, scientists discovered a weak spot in the ozone layer over Antarctica. Satellite observations have confirmed a progressive deterioration in the Earth’s protective ozone layer above Antarctica, according to scientists…

Voice of Nature: John Howe

Nature, international weekly journal of science. 16th May 1985. Letters to Nature, p207. Large losses of total ozone in Antarctica.

Jonathan Shanklin

The paper really changed the way people look at the environment.

Erik Conway

It provided an image of nearly global environmental damage that people could see.

Richard Stolarski

All of a sudden, you look at it differently – wow, we really can affect the planet as a whole.

Music

Voice of Nature: John Howe

J. C. Farman, B. G. Gardiner and J. D. Shanklin, British Antarctic Survey, Cambridge, UK.

Music

Jonathan Shanklin

What we discovered at our Antarctic station was quite curious. It seemed that each Antarctic spring – which for the Antarctic is September, October – ozone levels were dropping. I’m Jonathan Shanklin and I was one of the team of scientists that discovered the Antarctic ozone hole.

Music

Jonathan Shanklin

Concerns were raised, really, in the 1960s and 70s that substances that we were manufacturing, in particular chlorofluorocarbons – the CFCs – might put chlorine high into the atmosphere where it could then photocatalytically interact with the ozone and destroy ozone.

News report

Ozone is an invisible upper atmospheric gas that protects all forms of life on Earth from most of the Sun’s damaging radiation, radiation that can cause skin cancer, eye damage and suppression of the immune system. The harvesting of fish and plant life are also affected. A vast amount of aquatic life has its beginnings…

Jonathan Shanklin

When we were working up to publishing our paper, concerns had been expressed that these CFCs might destroy the ozone layer, that exhaust gases from Concorde might destroy the ozone layer, and the general thoughts were that any destruction would probably take place at high altitude in the tropics.

Richard Stolarski

My name is Richard Stolarksi, and I’m a research scientist who’s worked on ozone for many years. I’m now at the Johns Hopkins University in Baltimore, Maryland. We were looking for, at most, a few percent change in ozone and we weren’t necessarily looking in Antarctica. What they found in that paper was, during the springtime, a 40% or more decline and that that 40% only ten years earlier was not there, so this happened very rapidly. So, basically, it was more than an order of magnitude greater than we expected.

Erik Conway

My name is Erik Conway. I am the historian at NASA's Jet Propulsion Laboratory in Pasadena, California. So, when the British Antarctic Survey paper is published in 1985, a lot of people are rather shocked because during the early 1980s, they weren’t sure whether there was evidence of depletion and that’s because it’s a difficult question to answer. There is a great deal of variation in the density of stratospheric ozone – it varies by season, it varies by latitude – so there were claims of depletion but they weren’t widely accepted yet.

Richard Stolarski

The fluorocarbon-ozone theory was a theory. It’s just a theory. We can’t see it happening. Boom – look at it. There it is.

Voice of Nature: John Howe

Total ozone has been measured at the British Antarctic Survey stations, Argentine islands and Halley Bay since 1957. Whatever the absolute error of the recent values may be, the annual variation of total ozone at Halley Bay has undergone a dramatic change.

Richard Stolarski

Boom – look at it. There it is.

Jonathan Shanklin

One of the tasks that I had when I first joined the Survey was to process the missing data. We had a period of about ten years where the data hadn’t been reduced and doing it manually was quite time consuming. Computers were just coming in. I was given the task of writing the programmes, supervising the digitisation of the data and processing it. I started at the present and was going to work back, and my colleagues really didn’t believe that this was significant. They thought, well, just because you’ve got it one year, it doesn’t count. So, I eventually worked up all the missing years and was able to show it was systematic and if you’ve got something systematic, there has to be something real going on. I think I was probably the little thing in the background that made the difference and it was really a little voice from me being quite persistent that look, we’ve got to do something about this, that finally sort of drove them to right, well, we better write this up and better go to Nature because I think they had a better grasp of what a fundamental discovery this was than I did.

Music

Erik Conway

Many scientists told me that, in fact, they trusted the British Antarctic Survey people. In other words, they knew them to be good and rigorous scientists, so the initial questioning of those scientists was, well, how could they get such results because what they had found was ozone concentrations are far lower than anyone had ever seen on Earth.

Richard Stolarski

I was with NASA for 35 years, and I had just come off of a stint as a low-level manager and was just getting back into doing some real science, and we had people who had been taking satellite data of ozone. At the time, the amount of data was so massive it was overwhelming and very difficult to deal with. You had to learn how to find the right 9 track tape in a room full of tapes, and it’s really ironic to look at it today because the satellite was taking about 5 GB of data per year and at that time, when you said that, people were overwhelmingly impressed and now I think a lot of people probably run that through their smartphone every month. But at the time, it was an overwhelming amount of data.

Erik Conway

The Goddard team was scooped and they knew it. They went back to their own data and tried to figure out why this ozone hole wasn’t apparent, and it turned out that it was. Well, the Goddard folks had set quality control software to look at the data automatically as it came back from the satellite and mark as potentially bad data anything that showed levels below a certain amount, and these levels of the prediction of the Antarctic Surveys were below that amount. So, when the Goddard folks mapped all these flags of potentially bad data, they were all over the Antarctic, exactly where the British Antarctic Survey said it should be. So, they ultimately wound up corroborating the British Antarctic Survey’s story and getting a bit of egg on their faces too.

News report

It’s being called an unprecedented display of international cooperation to protect the world’s environment. The Montreal Protocol signed today aims at stopping the deterioration of the ozone layer in the atmosphere – that’s the layer which shields us from damaging ultraviolet radiation from the Sun. But there’s a lot of work ahead before…

Erik Conway

So, after the discovery of the 1985 ozone hole, there was already scheduled a set of negotiations over chlorofluorocarbons to reduce, but not ban, the production of chlorofluorocarbons, and what the discovery of the ozone hole does is demonstrate that there really is a problem and probably helped accelerate the negotiations that were already ongoing. The Montreal Protocol, as it’s initially written, was a 50% reduction in production of chlorofluorocarbons – it was not a ban. The ban actually took a number of more years to negotiate and became possible as more evidence that chlorofluorocarbons were actually responsible for the ozone hole was developed by the scientific research that was going on.

News reports

They have discovered a trend. Each spring, over Antarctica a hole in the ozone develops… Satellites photo show that a hole opens for a few months during Antarctica’s springtime…

Jonathan Shanklin

It was very clear when we first looked at the data that we’ve got an ozone decline and I think that’s probably the term we used – a decline in ozone above the Antarctic – and it wasn’t really until you got the satellite images that you could see oh, there’s a hole there. And yeah, with nice coloured graphics it hits you in the face.

Richard Stolarski

It is not clear to me to this day who coined the term ozone hole and when and where it was done. Some complained we shouldn’t use that term, but once you let that term out of the bag, there was no going back. It became the ozone hole and it seems to me that it certainly made it easier to reach a greater part of the public by having a simple key word that you could describe it by.

Erik Conway

The ozone hole was mentioned in newspapers and on TV quite commonly during the 1980s because the interest of environmental organisations kept it on the public radar, I think. The ozone hole actually still appears every year, and it has completely disappeared from public conversation. We have judged it a solved problem even though scientists don’t actually expect it to go away for another 80-100 years.

Richard Stolarski

I really think that the whole episode of the ozone hole discovery and all the subsequent things that happened, in terms of explaining it, mapping it etc., are a huge success story for the science of understanding the stratosphere. We were able, in a very short time, to put together a lot of pieces and to draw a much clearer picture of how this part of the Earth’s system works and what might affect it and what might not affect it.

Erik Conway

So, the legacy of the ozone hole story was that effective environmental action can be taken at the international level to resolve global environmental problems. A great many people compare the ozone depletion problem and the climate change problem.

Richard Stolarski

You have to remember that the ozone hole story was sort of simple and straightforward, and that CFCs had not been part of our economy for that long and they were not the same as carbon dioxide which is coming from our basic energy generation, so that the regulations that limited the CFCs could be handled by the economy quite easily. So, while there are lessons, I think with carbon dioxide we are dealing with a much more difficult problem that goes way beyond the parameters that were involved in the ozone hole.

Jonathan Shanklin

It’s been quite fascinating going back, particularly along the Antarctic Peninsula, on various trips. When I first went to Rothera, we used to get drinking water in the summer from a melt pool that formed on a patch of ice nearby. The last time I was at Rothera, we couldn’t do that because the ice level had dropped so much that the water just spilled out into the sea and I’ve seen the glaciers retreating, slowly, admittedly, but I can see that they’re going back and yes, change is very obvious along the Antarctic Peninsula. The ozone hole story clearly shows that if we have the will to do things, we can succeed. Climate change – the will isn’t there yet, and I fear it will take a disaster or two before that will appears. But once it does, we have the knowledge and the technology to do something about it.

Music

Voice of Nature: John Howe

Nature. Published by Macmillan Journals Limited, 4 Little Essex Street, London, WC2R 3LF. 16th May 1985.

Music

Kerri Smith

You’ve been listening to the Nature PastCast, produced by Charlotte Stoddart and with contributions from Jonathan Shanklin, Richard Stolarski and historian Erik Conway. Next month, it’s back to the 1870s and the starring role of the gorilla in the quest to understand man’s place in nature.