Forty years ago, a group of scientists from the British Antarctic Survey announced the discovery of the ozone hole during Antarctic springtime, and 28 countries signed the Vienna Convention for the Protection of the Ozone Layer — the first global treaty of its kind. What has happened since?
Recent studies give us a glimpse into the status of the ozone hole.
In the late ‘80s, the Antarctic ozone hole was a hot topic. Then a geology student, I remember reading John Gribbin’s “The Hole in the Sky ; Man’s Threat to the Ozone Layer” – a good book on the discovery of the ozone hole, the 1985 Vienna Convention and its successor, the 1987 Montreal Protocol. The Montreal Protocol called on all countries to phase out production of chlorofluorocarbons responsible for depletion of the ozone layer. By 2000, more than 90 percent of CFC production was terminated and replaced with alternative chemicals.
A recent MIT study, as well as reports from the World Meteorological Organization and NASA, indicate signs of recovery in the ozone layer, although the Antarctic ozone hole still strongly forms every September–October.
Fingerprinting the Antarctic Ozone Changes
In 1995, the Nobel Prize for chemistry was given to Sherwood Rowland and Mario Molina, who in the early 1970s showed that man-made CFCs – then widely used in refrigerators, air conditioners, fire extinguishers, solvents and aerosol sprayers – can deplete the ozone layer as chlorine atoms released from the CFCs by the sun’s ultraviolet radiation split ozone molecules into oxygen and chlorine monoxide. Each chlorine atom can destroy thousands of ozone molecules before it is removed from the atmosphere. A depleted ozone layer cannot protect humans, animals, crops and phytoplankton from harmful ultraviolet radiation.
But the question remained: Why did the ozone hole appear mainly in Antarctica?
This problem was resolved in 1986–87 when Susan Solomon, then at the National Oceanic and Atmospheric Administration, led expeditions to Antarctica and showed that stratospheric clouds form in the especially cold (-78° Celsius) and polar-vortex isolated Antarctica and provide surfaces for chemical reaction of CFCs to destroy the ozone layer.
In 2016, Solomon led another study of the Antarctic ozone layer and showed that the ozone hole was slowly “healing.” This study also suggested that “volcanic eruptions have episodically interfered with healing.”
In March, Peidong Wang, an MIT graduate student supervised by Solomon, published an article in Nature that differentiates the effects of CFCs from climate factors in the recovery of the Antarctic ozone hole. The MIT researchers applied the climate change fingerprinting method pioneered by Klaus Hasselmann, the 2021 winner of the Nobel Prize for physics. They ran simulations of Earth’s atmosphere between 66- and 82-degrees south for the period from 2005 to 2023 under multiple atmospheric scenarios, each with different starting conditions. Then they compared the simulation results with the Antarctic ozone data, analyzing how the ozone hole changed with season and altitudes in response to different starting conditions. Their study demonstrated for the first time that “data and simulations show compelling agreement in the fingerprint pattern of the ozone response to decreasing man-made ozone-depleting substances.”
A View from 2024
The September issue of WMO Ozone and UV Bulletin reports that in 2024, “high values were observed for mean total ozone columns” compared to the previous 23 years. The report acknowledges that natural climate factors such as intense solar activity and strong El Niño conditions in 2024 also influenced the overall ozone recovery.
The WMO report states that the 2024 Antarctic ozone hole formed relatively slowly, recovered rapidly and was notably below the 1990–2020 average depth. According to NASA’s Earth Observatory, during the peak of ozone depletion season from Sept. 7 through Oct. 13, the 2024 area of the Antarctic ozone hole “ranked the seventh smallest” since 1992. The 2024 Antarctic ozone concentration reached its lowest point at 109 dobson units, still far below the 1979 level of 225 dobson units.
Looking Ahead
The ozone layer is dynamic. In 2020 and 2022, the Antarctica ozone hole was at its maximum on record. Why?
Solomon commented: “The 2020 Australian wildfires and the 2022 Hunga Tonga–Hunga Ha’apai volcanic eruption may have played some role, but it is also quite possible that this just reflects natural internal variability. It’s not a good idea to overreact to one or two years of big or small holes. With a treaty, we’re in it for the longer game of a decade or more.”
In other words, the long-term trend around how the ozone hole changes is more important than a single odd year.
Solomon estimates the residence time of CFCs in the atmosphere to be 50 to 100 years. The 2022 report on Scientific Assessment of Ozone Depletion sponsored by WMO, UNEP, NASA, NOAA and the European Commission forecasts that the ozone column will “return to 1980 values around 2066 in the Antarctic, around 2045 in the Arctic and around 2040 for the near-global average (60°N–60°S).”