Ozone depletion

• 1. Introduction

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  Ozone depletion is the story of a shield we nearly lost. High in the stratosphere, a thin layer of ozone gas absorbs the ultraviolet light that would otherwise damage DNA, trigger skin cancer, and blind animals and people. Starting in the late 1970s, scientists began observing that this shield was shrinking. Not gradually. Not uniformly. In some places, especially over Antarctica each southern spring, it was collapsing by staggering amounts. What caused it? Who sounded the alarm? And how did the world respond? Those are the questions this documentary will answer.

• 2. The Chemistry of Destruction

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  Chlorofluorocarbons, invented by Thomas Midgley Jr. in the 1930s, were once celebrated as marvels of industrial chemistry. They were used in refrigerators, air conditioners, aerosol cans, and the cleaning of delicate electronics. They seemed harmless. In the troposphere, the lower atmosphere, they are almost completely inert. That inertness is precisely the problem.

  Because CFCs do not break down in the lower atmosphere, they drift upward over years. It takes an average of about five to seven years for a CFC molecule to travel from ground level into the upper atmosphere. Once there, ultraviolet light shatters the molecule, releasing chlorine atoms. Each chlorine atom then acts as a catalyst in a repeating chemical cycle: it strips an oxygen atom from an ozone molecule, forming chlorine monoxide, and then strips another oxygen from a second ozone molecule, freeing the chlorine atom to start again. A single chlorine atom can destroy an average of 100,000 ozone molecules before it is finally removed from the stratosphere. And a CFC molecule can persist in the upper atmosphere for roughly a century.

  Bromine, released by halons used in fire extinguishers, is even more efficient than chlorine at this destruction on a per-atom basis. In the lower atmosphere, by contrast, ocean spray does produce chlorine, but that chlorine dissolves in water and is washed out by rain before reaching the stratosphere. CFCs are insoluble, which is why they make it through. About 80 percent of the chlorine in the stratosphere comes from human-made sources, with only roughly 20 percent traceable to the naturally occurring compound methyl chloride.

• 3. The Antarctic Hole

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  British Antarctic Survey scientists Farman, Gardiner, and Shanklin first reported the Antarctic ozone hole in a paper published in Nature in May 1985, and the scientific community was stunned. The decline in polar ozone was far larger than anyone had predicted. Satellite data from the Total Ozone Mapping Spectrometer aboard Nimbus 7 had actually captured the phenomenon earlier, but the values were so far below expectation that quality-control algorithms filtered them out as errors. When the raw data was reprocessed, the hole was visible as far back as 1976.

  The reason Antarctica is the epicenter of ozone loss comes down to cold and chemistry. Each polar winter, three months of darkness drive temperatures in the stratosphere down to around or below negative 80 degrees Celsius. At those temperatures, three types of polar stratospheric clouds form on ice and nitric acid particles. Chlorine that normally sits in stable reservoir compounds such as hydrogen chloride gets converted on the surface of those cloud crystals into reactive free radicals. Then, when sunlight returns in the southern spring, those radicals unleash a cascade of destruction. Over 50 percent of the lower stratospheric ozone is destroyed within the Antarctic polar vortex during spring. Reductions of up to 70 percent in the ozone column above Antarctica have been measured. Since the 1990s, Antarctic total column ozone in September and October has continued to be 40-50 percent lower than pre-ozone-hole values.

  Susan Solomon, an atmospheric chemist at the National Oceanic and Atmospheric Administration, proposed the polar stratospheric cloud mechanism that explains these observations. It was confirmed first in laboratory measurements and then by direct measurement from aircraft of extremely high concentrations of chlorine monoxide in the Antarctic stratosphere. By the early 1990s, the ozone hole had grown large enough in some seasons to affect parts of Australia, New Zealand, Chile, Argentina, and South Africa.

• 4. The Science That Led There

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  The basic chemistry of the ozone layer was first worked out by Sydney Chapman in 1930. Chapman described how short-wavelength UV radiation splits oxygen molecules into reactive oxygen atoms, which then combine with other oxygen molecules to form ozone. He also described how ozone is naturally destroyed when an oxygen atom and an ozone molecule recombine. In the 1950s, David Bates and Marcel Nicolet showed that free radicals, particularly hydroxyl and nitric oxide, could catalyze this recombination and reduce overall ozone levels. Without those natural catalysts, the ozone layer would be roughly twice as thick as it is.

  In 1970, Paul Crutzen identified a new threat. He showed that nitrous oxide, a stable gas emitted by soil bacteria, lives long enough to reach the stratosphere, where it converts into nitric oxide. Crutzen noted that increasing fertilizer use could raise nitrous oxide emissions above natural background levels, pushing more ozone-destroying nitric oxide into the stratosphere. The following year, he and Harold Johnston independently raised the alarm about supersonic passenger aircraft flying in the lower stratosphere.

  In 1974, Frank Sherwood Rowland, a chemistry professor at the University of California at Irvine, and his postdoctoral associate Mario J. Molina made the critical link to CFCs. James Lovelock had recently found, during a cruise in the South Atlantic in 1971, that nearly all CFC compounds manufactured since their invention in 1930 were still present in the atmosphere. Molina and Rowland concluded that those molecules would eventually reach the stratosphere and release chlorine. Richard Stolarski and Ralph Cicerone at the University of Michigan had already shown that chlorine is more efficient than nitric oxide at catalyzing ozone destruction. McElroy and Wofsy at Harvard University reached similar conclusions and extended the work to show that bromine from halons was also a significant threat.

  Industry pushed back hard. The chair of the board of DuPont was quoted calling ozone depletion theory a science fiction tale, a load of rubbish, and utter nonsense. Robert Abplanalp, the inventor of the first practical aerosol spray can valve and president of Precision Valve Corporation, wrote directly to the chancellor of UC Irvine to complain about Rowland's public statements. Within three years, though, most of the assumptions behind the Rowland-Molina hypothesis had been confirmed by laboratory measurements and direct stratospheric observations. James G. Anderson and collaborators measured chlorine monoxide in the stratosphere, proving that chlorine radicals were not only present but actively destroying ozone. In 1976, the United States National Academy of Sciences concluded that the evidence strongly supported the ozone depletion hypothesis. Crutzen, Molina, and Rowland were awarded the 1995 Nobel Prize in Chemistry for this body of work.

• 5. Global Response and the Montreal Protocol

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  Before any legislation existed, Americans voluntarily reduced their use of aerosol sprays, producing a 50 percent sales decline. That shift in public behavior preceded the bans that followed. In 1978, the United States banned CFCs in aerosol cans. Canada, Sweden, Denmark, and Norway moved in a similar direction. But progress then slowed, partly because of continued resistance from the halocarbon industry and partly because of a change in attitude toward environmental regulation during the early years of the Reagan administration.

  The political momentum shifted again in 1983, when William Ruckelshaus replaced Anne M. Burford as administrator of the United States Environmental Protection Agency. Under Ruckelshaus and his successor Lee Thomas, the EPA pushed for international coordination. In 1985, twenty nations signed the Vienna Convention for the Protection of the Ozone Layer, establishing a framework for international regulation. That same year, the discovery of the Antarctic ozone hole reignited public concern.

  In 1987, representatives from 43 nations signed the Montreal Protocol. The initial agreement froze CFC production at 1986 levels and called for a 50 percent reduction by 1999. The halocarbon industry shifted position and began supporting limits, though unevenly. DuPont moved faster than its European counterparts, partly because the EPA had published a 1986 study projecting an additional 40 million skin cancer cases and 800,000 cancer deaths in the United States over the following 88 years. The EU shifted after Germany abandoned its defence of the CFC industry. France and the United Kingdom initially resisted even after the Protocol was signed.

  After scientific expeditions to Antarctica produced conclusive evidence that chlorine and bromine from manufactured compounds were responsible for the hole, the Protocol was strengthened at a 1990 meeting in London, where participants agreed to phase out CFCs and halons entirely by 2000 in developed countries and by 2010 in less developed signatories. At a 1992 meeting in Copenhagen, the phase-out date for developed nations was moved up to 1996, and methyl bromide, a fumigant used in agriculture, was added to the controlled list.

  One alternative technology took an unexpected path to market. An ozone-safe hydrocarbon refrigerant made from propane and butane was developed at a technological institute in Hamburg and came to the attention of the NGO Greenpeace in 1992. Greenpeace named it Greenfreeze and helped bring it to market, first in Europe, then Asia and Latin America. The effort received a 1997 UNEP award. By 1995, Germany had made CFC refrigerators illegal. By 2008, sales of Greenfreeze units had reached some 300 million refrigerators, and by 2013 the technology was in use in roughly 700 million refrigerators, accounting for about 40 percent of the global market. Chemical companies including DuPont, whose representatives dismissed Greenfreeze as that German technology, maneuvered the EPA to block its use in the United States until 2011.

• 6. Health and Ecological Consequences

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  Skin cancer is the effect most people associate with ozone loss, and the link to UV-B radiation is well established. UV-B causes pyrimidine bases in DNA to form dimers, which produce errors when DNA replicates. Basal and squamous cell carcinomas, the most common forms of skin cancer, are strongly tied to UV-B exposure. Scientists have estimated that every one percent decrease in long-term stratospheric ozone would raise the incidence of these cancers by 2 percent. Melanoma, though far less common, is lethal in roughly 15-20 percent of diagnosed cases. A study in Punta Arenas at the southern tip of Chile found a 56 percent increase in melanoma and a 46 percent increase in non-melanoma skin cancer over seven years, during a period of decreased ozone and increased UV-B levels.

  Eye damage is another documented risk. A study of watermen working on Chesapeake Bay found a clear association between annual ocular UV-B exposure and the risk of cortical cataracts. Based on those findings, ozone depletion is predicted to cause hundreds of thousands of additional cataracts by 2050.

  In October 2008, the Ecuadorian Space Agency published a report called HIPERION, which drew on ground instruments in Ecuador and 28 years of data from 12 satellites. The study found that UV radiation reaching equatorial latitudes was far greater than expected, with the ultraviolet index climbing as high as 24 in Quito. The World Health Organization considers an index of 11 to be extreme and a significant health risk. The Peruvian Space Agency's CONIDA subsequently published its own study reaching nearly the same conclusions.

  A November 2011 report by scientists at the Institute of Zoology in London found that whales off the coast of California showed a sharp rise in sun damage, with skin biopsies taken from over 150 whales in the Gulf of California revealing widespread epidermal damage consistent with acute and severe sunburn caused by UV radiation.

  For plants, the effects are more complex. While some studies found little change in plant height or leaf mass under UV-B exposure similar to levels from ozone depletion, UV-B has been shown to reduce the quantum yield of photosystem II. In areas of substantial ozone depletion, increased UV-B radiation has been found to reduce terrestrial plant productivity by about 6 percent. Rice and other crops that depend on cyanobacteria on their roots for nitrogen retention are at particular risk, since cyanobacteria are sensitive to UV radiation. Plants exposed to high UV levels can also produce harmful volatile organic compounds including isoprenes, which contribute to air pollution and add carbon to the atmosphere.

• 7. Recovery and Remaining Risks

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  Since the adoption of the Montreal Protocol, atmospheric concentrations of the most significant ozone-depleting compounds have been declining. After peaking in 1994, the Effective Equivalent Chlorine level in the atmosphere had dropped about 10 percent by 2008. Ozone levels stabilized by the mid-1990s and began recovering in the 2000s. In 2019, the ozone hole was the smallest it had been in the previous thirty years.

  According to a 2023 United Nations assessment, the ozone layer is on track to recover to 1980 levels by around 2066 over Antarctica, by 2045 over the Arctic, and by 2040 for the rest of the world, assuming current regulations remain in place. The Montreal Protocol is widely regarded as the most successful international environmental agreement ever concluded.

  Several threats complicate that trajectory. A team at the University of East Anglia discovered four previously undetected man-made chemicals in the atmosphere, including CFC-113a, the only known CFC still growing in atmospheric abundance. Its source is unknown, though illegal manufacturing is suspected. Between 2012 and 2017, concentrations of CFC-113a jumped by 40 percent. A separate study published in Nature found that since 2013, emissions predominantly from north-eastern China have released large quantities of the banned compound CFC-11. Scientists estimate that without corrective action, those emissions would delay ozone hole recovery by a decade.

  Satellites burning up on re-entry produce aluminum oxide nanoparticles that persist in the atmosphere for decades. Estimates for 2022 alone were roughly 17 metric tons. As satellite constellations grow larger, that could become a meaningful source of ozone depletion. And in September 2023, the Antarctic ozone hole was one of the largest on record, reaching 26 million square kilometers, a size possibly linked to the 2022 Tonga volcanic eruption. Nitrous oxide, which is not covered by the Montreal Protocol, has meanwhile become the most highly emitted ozone-depleting substance and is expected to hold that position throughout the 21st century.