Ozone depletion refers to two related events first noticed in the late 1970s: a general decrease in the total amount of ozone in Earth’s upper atmosphere, and a much larger loss of ozone during springtime in Earth’s polar regions. The second event, called the ozone hole, happens when ozone in the stratosphere (the ozone layer) is greatly reduced during spring near the poles. In addition, there are similar ozone losses in the troposphere (lower atmosphere) during spring near the poles.

The main cause of ozone depletion and the ozone hole is man-made chemicals, especially halocarbons used in refrigerants, solvents, propellants, and foam-blowing agents (such as chlorofluorocarbons (CFCs), HCFCs, and halons). These chemicals are called ozone-depleting substances (ODS). After being released into the air, they rise into the stratosphere through mixing in the atmosphere. Once there, sunlight breaks apart the molecules, releasing halogen atoms that speed up the breakdown of ozone (O₃) into oxygen (O₂). Both types of ozone loss increased as more halocarbons were released into the atmosphere.

Ozone depletion and the ozone hole have caused global concern because they increase the risk of health problems, such as skin cancer, sunburn, blindness, and cataracts. The ozone layer blocks harmful ultraviolet (UVB) light from reaching Earth’s surface. UVB light can damage human skin and eyes, harm plants and animals, and increase the risk of these health issues. These dangers led to the Montreal Protocol in 1987, which banned the production of CFCs, halons, and other ozone-depleting chemicals. Scientists have since created new refrigerants with lower global warming potential (GWP) to replace older ones. For example, R-1234yf is now commonly used in new cars instead of older refrigerants like R-134a and R-12, which have higher GWP.

The ban began in 1989. By the mid-1990s, ozone levels stopped decreasing and started to recover in the 2000s. Changes in the jet stream in the Southern Hemisphere, which had moved toward the South Pole, may now be reversing. Scientists predict the ozone layer will fully recover by 2045. The ozone hole is expected to return to its 1980 levels by about 2075. In 2019, NASA reported the ozone hole was the smallest since its discovery in 1982. The Montreal Protocol is widely seen as the most successful international agreement to protect the environment.

Ozone cycle overview

Three forms of oxygen are involved in the ozone-oxygen cycle: oxygen atoms (O or atomic oxygen), oxygen gas (O₂ or diatomic oxygen), and ozone gas (O₃ or triatomic oxygen). Ozone is created in the stratosphere when oxygen gas molecules absorb ultraviolet (UVC) light and break apart, forming two oxygen atoms. These oxygen atoms then combine with other oxygen gas molecules to form two ozone molecules. Ozone molecules absorb ultraviolet (UVB) light, causing them to split into an oxygen gas molecule and an oxygen atom. The oxygen atom then joins another oxygen gas molecule to form ozone again. This process continues until an oxygen atom combines with an ozone molecule to create two oxygen gas molecules. Ozone is the only atmospheric gas that absorbs UVB light.

The total amount of ozone in the stratosphere depends on the balance between the chemical creation of ozone and its recombination. Ozone can be destroyed by free radical catalysts, including the hydroxyl radical (OH·), nitric oxide radical (NO·), chlorine radical (Cl·), and bromine radical (Br·). The dot in these names shows that each has an unpaired electron, making them highly reactive. The ability of different elements to destroy ozone depends on how they react and how they are reused in chemical processes.

All of these radicals come from both natural and human sources. As of 2020, most hydroxyl and nitric oxide radicals in the stratosphere are naturally produced, but human activity has greatly increased the levels of chlorine and bromine. These elements are found in stable compounds, such as chlorofluorocarbons (CFCs), which do not break down in the lower atmosphere and can reach the stratosphere. In the stratosphere, ultraviolet light breaks apart these compounds, releasing chlorine and bromine atoms.

Ozone is a reactive molecule that can be converted into more stable oxygen gas with the help of a catalyst. Chlorine and bromine atoms destroy ozone through repeated chemical reactions. In one example, a chlorine atom reacts with an ozone molecule (O₃), taking an oxygen atom to form chlorine monoxide (ClO) and leaving an oxygen gas molecule (O₂). The ClO can then react with another ozone molecule, releasing the chlorine atom and creating two oxygen gas molecules. These reactions are written as:

• Cl· + O₃ → ClO + O₂ (A chlorine atom removes an oxygen atom from an ozone molecule to form ClO and O₂)
• ClO + O₃ → Cl· + 2 O₂ (ClO removes an oxygen atom from another ozone molecule, freeing the chlorine atom to repeat the cycle)

These processes reduce the amount of ozone, but some reactions can slow them down. More complex chemical mechanisms also contribute to ozone destruction in the lower stratosphere.

A single chlorine atom can destroy up to 100,000 ozone molecules before it is removed from the cycle by forming other compounds, such as hydrogen chloride (HCl) or chlorine nitrate (ClONO₂). Bromine is even more effective at destroying ozone per atom, but there is less bromine in the atmosphere. Both chlorine and bromine contribute significantly to ozone loss. Laboratory studies also show that fluorine and iodine atoms can destroy ozone in similar ways. However, fluorine reacts quickly with water vapor, methane, and hydrogen in the stratosphere to form hydrogen fluoride (HF), while iodine-containing compounds break down too fast in the lower atmosphere to reach the stratosphere in large amounts.

Each year, chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) release large amounts of chlorine into the atmosphere. A single chlorine atom can destroy many ozone molecules before it is removed from the cycle, showing how harmful CFCs and HCFCs are to the environment.

Observations on ozone layer depletion

The ozone hole is measured by the amount of ozone above a point on Earth's surface. This is often shown in Dobson units (DU). The biggest drop in ozone has been in the lower part of the stratosphere. Scientists have noticed large decreases in ozone levels over Antarctica during the spring and early summer compared to the 1970s. Instruments like the Total Ozone Mapping Spectrometer (TOMS) have been used to observe these changes.

From 1985, scientists found that ozone levels over Antarctica dropped by up to 70% during the austral spring. These decreases have continued. By 2010, ozone levels in Antarctica during September and October were still 40–50% lower than levels before the ozone hole formed in the 1990s. In 2016, scientists reported a slow improvement in ozone levels. In 2017, NASA said the ozone hole was the smallest since 1988 because the stratosphere was unusually warm. Scientists predict the ozone hole will fully recover by 2070.

Ozone loss varies more each year in the Arctic than in the Antarctic. In the Arctic, the biggest drops in ozone happen during winter and spring, reaching up to 30% when the stratosphere is coldest.

Chemical reactions on polar stratospheric clouds (PSCs) help speed up ozone loss. These clouds form more easily in the extremely cold stratosphere of the Arctic and Antarctic. This is why the first large ozone hole formed over Antarctica, and why it is deeper there. Early models did not account for PSCs and predicted a slow, global loss of ozone. The sudden discovery of the Antarctic ozone hole surprised many scientists.

It is more accurate to describe ozone loss in middle latitudes (areas not near the poles) rather than "holes." Between 1980 and 1996, ozone levels in middle latitudes dropped below 1980 levels. In the northern mid-latitudes, ozone levels began to rise by about 2% from 1996 to 2009 as rules were followed and chlorine levels in the stratosphere decreased. In the southern mid-latitudes, ozone levels stayed the same during that time. In the tropics, there are no clear trends because chemicals that release chlorine and bromine have not had time to break down in those areas.

Large volcanic eruptions can cause uneven ozone loss. For example, the 1991 eruption of Mount Pinatubo in the Philippines had a major impact on ozone levels.

Ozone loss also explains much of the drop in temperatures in the stratosphere and upper troposphere. Ozone warms the stratosphere by absorbing ultraviolet light, so less ozone causes cooling. Some cooling is also expected from greenhouse gases like CO₂ and CFCs, but ozone-related cooling is the main cause.

Predicting ozone levels is still difficult, but models have become more accurate over time. A report by the World Meteorological Organization supports the Montreal Protocol, which helps protect the ozone layer. However, an earlier report from 1994 overestimated ozone loss between 1994 and 1997.

Chlorofluorocarbons (CFCs) and other man-made chemicals called halogenated ozone-depleting substances (ODS) are the main causes of human-made ozone loss. Scientists can calculate the total amount of chlorine and bromine in the stratosphere, which is called equivalent effective stratospheric chlorine (EESC).

CFCs were invented by Thomas Midgley Jr. in the 1930s. They were used in refrigerators, air conditioners, aerosol sprays, and cleaning processes for electronics. They are also by-products of some chemical reactions. No natural sources of CFCs have been found, so their presence in the atmosphere is almost entirely due to human activity. When CFCs reach the stratosphere, ultraviolet light breaks them apart, releasing chlorine atoms. These atoms act as catalysts, breaking down tens of thousands of ozone molecules before being removed from the stratosphere. Because CFCs last a long time, their effects take decades to reverse. A CFC molecule can take 5–7 years to reach the upper atmosphere and may stay there for about 100 years, destroying up to 100,000 ozone molecules during that time.

A chemical called 1,1,1-Trichloro-2,2,2-trifluoroethane (CFC-113a) was recently discovered in the atmosphere by scientists at the University of East Anglia. CFC-113a is the only known CFC whose levels in the atmosphere are still increasing. Its source is unknown, but some scientists believe it may be from illegal production. CFC-113a has been growing in the atmosphere since 1960. Between 2012 and 2017, its levels increased by 40%.

A study published in Nature found that emissions of CFC-11 from northeastern China have been releasing large amounts of this banned chemical into the atmosphere since 2013. Scientists estimate that these emissions could delay the recovery of the ozone hole by about 10 years if no action is taken.

Satellites burning up in Earth's atmosphere produce tiny particles called aluminum oxide (Al₂O₃) nanoparticles. These particles can stay in the atmosphere for decades. In 2022 alone, about 17 metric tons of these particles were released from satellites. As more satellites are launched, this could lead to significant ozone loss.

"Very short-lived substances" are a group of ozone-depleting chemicals allowed by the Montreal Protocol. These chemicals break down in less than 6 months. About 90% are naturally made, such as bromine-based chemicals from seaweed and phytoplankton. The remaining 10% are man-made, like dichloromethane.

Scientists have linked ozone loss to the increase in man-made halogen compounds, such as CFCs. They use observational data and computer models to study this. These models combine measurements of chemicals, weather patterns, and chemical reaction rates. They help identify key reactions and processes that bring CFCs into contact with ozone. Examples of these models include SLIMCAT and CLaMS (Chemical

Ozone hole and its causes

The Antarctic ozone hole is a region in the stratosphere above Antarctica where ozone levels have dropped to as low as 33% of their levels before 1975. This hole appears during the Antarctic spring, from September to early December, when strong winds form a circular pattern around the continent, creating a contained area of air. Within this area, more than 50% of the ozone in the lower stratosphere is destroyed during the spring season.

Ozone depletion is mainly caused by chlorine-containing gases, such as CFCs and similar compounds. When these gases are exposed to ultraviolet light, they break apart, releasing chlorine atoms. These chlorine atoms then help destroy ozone in the atmosphere. This process happens in the gas phase, but it becomes much more active when polar stratospheric clouds (PSCs) are present.

Polar stratospheric clouds form during the cold Antarctic winter, when there is no sunlight for three months. The lack of sunlight lowers temperatures to around or below −80 °C, allowing cloud particles to form. There are three types of PSCs: nitric acid trihydrate clouds, slowly cooling water-ice clouds, and rapidly cooling water-ice (nacreous) clouds. These clouds provide surfaces where chemical reactions occur, leading to ozone destruction when sunlight returns in spring.

The chemical processes involved are complex but well understood. Normally, most chlorine in the stratosphere is stored in "reservoir" compounds, such as chlorine nitrate (ClONO₂) and hydrogen chloride (HCl). These compounds keep chlorine away from ozone-depletion reactions. However, during the Antarctic winter and spring, reactions on the surfaces of PSCs convert these reservoir compounds into reactive chlorine forms, like Cl and ClO. Denitrification is the process by which PSCs remove nitrogen dioxide (NO₂) from the stratosphere by turning it into nitric acid, which is then removed from the atmosphere. This prevents ClO from being converted back into ClONO₂.

Sunlight plays a key role in ozone depletion, which is why the Antarctic ozone hole is most severe during spring. In winter, even though PSCs are most common, there is no sunlight to drive chemical reactions. When sunlight returns in spring, it provides energy for reactions that break down PSCs and release ClO, which accelerates ozone destruction. By mid-December, warming temperatures disrupt the polar vortex, allowing ozone and nitrogen dioxide-rich air from lower latitudes to enter. This stops the ozone-depletion process, and the ozone hole begins to close.

Most ozone loss occurs in the lower stratosphere, unlike the smaller ozone depletion caused by gas-phase reactions, which happens mainly in the upper stratosphere.

Effects

The ozone layer helps protect Earth by absorbing UVB radiation from the sun. When the ozone layer becomes thinner, more UVB radiation reaches Earth's surface, which can cause harm, such as an increased risk of skin cancer. This understanding led to the creation of the Montreal Protocol, an agreement to reduce harmful substances that damage the ozone layer. While scientists have linked lower ozone levels in the stratosphere to increased UVB at Earth's surface, there is no direct proof that ozone depletion causes more skin cancer or eye damage in humans. This is partly because UVA radiation, which also contributes to some skin cancers, is not blocked by the ozone layer, and it is hard to separate the effects of lifestyle changes over time. Ozone depletion may also change wind patterns.

Although ozone makes up only a small part of Earth's atmosphere, it absorbs most UVB radiation. The amount of UVB that reaches Earth's surface depends on the thickness and density of the ozone layer. When ozone levels in the stratosphere decrease, more UVB radiation reaches Earth. Scientists have used patterns in tree rings to determine that ozone depletion began in northern latitudes around the late 1700s.

In 2008, the Ecuadorian Space Agency released a report called HIPERION. The study used data from ground instruments in Ecuador and satellite information from 12 countries over 28 years. It found that UV radiation levels in equatorial regions were much higher than expected, with the UV index reaching as high as 24 in Quito. The World Health Organization considers a UV index of 11 to be extremely high and a major health risk. The report stated that thinning ozone in mid-latitude regions could harm large populations. A similar study by Peru's space agency, CONIDA, reached nearly the same conclusions.

Most public concern about the ozone hole focuses on the health effects of increased UV radiation. In most areas, ozone depletion has been small—only a few percent—and there is no clear evidence of health damage in most regions. However, if the large ozone depletion seen over Antarctica were common worldwide, the effects could be much more severe. The ozone hole has grown large enough to affect parts of Australia, New Zealand, Chile, Argentina, and South Africa. Excessive UV radiation harms benthic diatom communities in shallow freshwater, which improve water quality and resist pollution. Ozone depletion also harms plants and trees by damaging cells, reducing growth, photosynthesis, and defenses against pests. This can harm soil microbes, insects, wildlife, and ecosystems.

Ozone depletion increases the effects of UV radiation on human health, both positive (like vitamin D production) and negative (like sunburn, skin cancer, and cataracts). Increased UV radiation also raises ground-level ozone, which is harmful to human health.

The most common skin cancers, basal and squamous cell carcinomas, are strongly linked to UVB exposure. UVB radiation causes DNA damage by forming dimers in DNA, leading to errors during replication. These cancers are usually not fatal, though treatment for squamous cell carcinoma may require surgery. Studies suggest that a 1% decrease in stratospheric ozone could increase these cancers by 2%.

Melanoma is a less common but more dangerous type of skin cancer, with a 15–20% fatality rate. Its link to UV exposure is not fully understood, but both UVB and UVA may be involved. A study found that a 10% increase in UVB radiation was linked to a 19% rise in melanoma cases in men and 16% in women. In Punta Arenas, Chile, melanoma and non-melanoma skin cancer rates increased by 56% and 46%, respectively, over seven years, alongside lower ozone levels and higher UVB exposure.

Epidemiological studies suggest a connection between UVB exposure and ocular cortical cataracts. A study on Chesapeake Bay watermen found that higher UVB exposure was linked to increased cataract risk. Based on these findings, ozone depletion is predicted to cause hundreds of thousands of additional cataracts by 2050.

Increased UV radiation raises ground-level ozone, which is harmful to health, especially for children, the elderly, and those with respiratory conditions. Ground-level ozone is mainly produced by UV radiation reacting with gases from vehicle exhaust.

Vitamin D is produced in the skin when exposed to UVB radiation. Higher UVB exposure can increase vitamin D levels in people who are deficient. However, many people have low vitamin D levels, and very high levels can be harmful. The body has ways to prevent overproduction of vitamin D from sunlight.

A 2011 study by scientists in London found that whales near California showed signs of sun damage, possibly due to thinning ozone. The study found evidence of DNA damage in whale skin, similar to human skin cancer. Other animals, including dogs, cats, sheep, and ecosystems, also face risks from increased UVB radiation.

Higher UV radiation could harm crops. Some plants, like rice, rely on cyanobacteria on their roots for nitrogen. Cyanobacteria are sensitive to UV radiation and could be affected by increased UVB levels.

Public policy

Scientists do not fully understand the long-term damage that CFCs have caused to the ozone layer, but they have seen a clear decrease in ozone levels. The Montreal and Vienna conventions were created before scientists agreed on the full effects of CFCs or before all scientific questions were answered. People without scientific training understood the ozone issue well, especially because terms like "ozone shield" and "ozone hole" helped explain the problem in simple ways. Americans stopped using aerosol sprays voluntarily, reducing sales by 50% before laws were passed.

After a 1976 report by the United States National Academy of Sciences showed that scientific evidence supported the idea that CFCs harm the ozone layer, several countries, including the United States, Canada, Sweden, Denmark, and Norway, began banning CFCs in aerosol sprays. This was seen as a first step toward stronger rules, but progress slowed later because of political issues, such as resistance from companies that made CFCs, and changes in how the public viewed environmental laws. Scientific studies later showed that earlier estimates of ozone damage were too high.

A key patent for Freon, a type of CFC, was set to expire in 1979. The United States banned CFCs in aerosol cans in 1978. The European Community refused to ban CFCs in aerosols, and in the United States, CFCs were still used for refrigeration and cleaning electronics. Global CFC production dropped sharply after the U.S. ban but returned to 1976 levels by 1986. In 1993, DuPont Canada closed its CFC production facility.

The U.S. government changed its stance in 1983 when William Ruckelshaus became the head of the Environmental Protection Agency (EPA). Under Ruckelshaus and his successor, the EPA pushed for international rules on CFCs. In 1985, 20 countries, including major CFC producers, signed the Vienna Convention to protect the ozone layer. That same year, scientists discovered the Antarctic ozone hole, which renewed public interest in the issue.

In 1987, 43 countries signed the Montreal Protocol, an agreement to limit CFC production. The halocarbon industry, which made CFCs, began supporting the protocol, though some companies, like DuPont, acted faster than others. DuPont may have worried about legal action over health risks, especially after the EPA estimated that CFCs could cause millions of additional skin cancer cases in the U.S. The European Union changed its position after Germany stopped defending CFC producers.

At the Montreal meeting, countries agreed to stop increasing CFC production and reduce it by half by 1999. After scientific studies confirmed that the ozone hole was caused by manmade chemicals, the Montreal Protocol was strengthened in 1990. Countries agreed to stop using CFCs and halons completely by 2000 in developed nations and by 2010 in less developed countries. In 1992, the phase-out date was moved to 1996, and methyl bromide, a chemical used in farming, was added to the list of banned substances. Less developed countries received help, such as funding and technology, to meet the rules.

Groups outside of government, like non-profit organizations, played a key role in creating the Vienna Convention and Montreal Protocol and in checking if countries followed the rules. Some companies claimed no alternatives to CFCs existed, but a safe refrigerant made of propane and butane was developed in Germany. Greenpeace promoted this technology, called "Greenfreeze," and helped it spread globally. By 1995, Germany banned CFC refrigerators. Companies like Coca-Cola, Carlsberg, and IKEA later supported Greenfreeze, and by 2008, over 300 million refrigerators used it.

In the United States, changes were slower. CFCs were partly replaced by HCFCs, which are less harmful but still have some issues. HFCs, which do not harm the ozone layer but are strong greenhouse gases, were used in some applications. HFC-134a replaced CFC-12 in car air conditioners. Companies like DuPont opposed Greenfreeze in the U.S. until 2011. Greenpeace helped companies like Ben & Jerry’s and General Electric push for its approval.

The European Union updated its ozone rules in 2009, banning harmful substances to protect the ozone layer. These rules match those in the Montreal Protocol, with some additions.

Recently, experts have suggested linking ozone protection to climate efforts. Many ozone-depleting substances are also strong greenhouse gases, which may have hidden the true effects of climate change. Policies to protect the ozone layer have also helped reduce global warming. Decisions in one area, like ozone rules, can affect how well climate policies work.

Prospects of ozone depletion

Since the use and improvement of the Montreal Protocol have reduced emissions of CFCs, the amounts of the most important ozone-depleting compounds in the atmosphere have decreased. These chemicals are slowly leaving the atmosphere. After reaching their highest levels in 1994, the Effective Equivalent Chlorine (EECl) in the atmosphere had dropped by about 10% by 2008. The decline in ozone-depleting chemicals has also been influenced by a reduction in bromine-containing substances. Studies show that natural sources produce significant amounts of atmospheric methyl bromide (CH₃Br). Because CFCs are being phased out, nitrous oxide (N₂O), which is not regulated by the Montreal Protocol, has become the largest source of ozone-depleting emissions and is expected to remain so throughout the 21st century.

According to the IPCC Sixth Assessment Report, global ozone levels in the upper atmosphere fell quickly during the 1970s and 1980s but have since started to rise. However, these levels have not yet returned to those before industrial activity began. Although yearly changes are expected, including in polar regions where ozone loss is greatest, the ozone layer is predicted to continue recovering in the coming decades as ozone-depleting substances decrease, assuming full compliance with the Montreal Protocol.

The Antarctic ozone hole is expected to persist for many years. By 2020, ozone levels in the lower stratosphere over Antarctica had increased by 5–10% and are projected to return to pre-1980 levels by about 2060–2075. This is 10–25 years later than earlier predictions, due to updated estimates of ozone-depleting substance levels, including higher expected future use in developing countries. Another factor that may slow recovery is the movement of nitrogen oxides from above the stratosphere, caused by changes in wind patterns. A sign of gradual improvement was reported in 2016. In 2019, the ozone hole was the smallest in the previous 30 years, likely because warmer temperatures in the polar stratosphere weakened the polar vortex. In September 2023, the Antarctic ozone hole was among the largest on record, covering 26 million square kilometers. This unusually large loss may have been caused by the 2022 Tonga volcanic eruption. A 2023 United Nations assessment states that the ozone layer is expected 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 unchanged.

Research history

In 1930, Sydney Chapman discovered the basic physical and chemical processes that create the ozone layer in Earth’s stratosphere. Short-wavelength ultraviolet (UV) light breaks an oxygen molecule (O₂) into two oxygen atoms (O). These atoms then combine with other oxygen molecules to form ozone (O₃). Ozone is removed when an oxygen atom and an ozone molecule recombine to form two oxygen molecules, as shown in the reaction: O + O₃ → 2 O₂. In the 1950s, David Bates and Marcel Nicolet found that certain chemical particles, such as hydroxyl (OH) and nitric oxide (NO), can speed up this recombination process, reducing ozone levels. These particles naturally exist in the stratosphere and were thought to help maintain a balance, as their absence would likely make the ozone layer about twice as thick as it is today.

In 1970, Paul Crutzen showed that nitrous oxide (N₂O), a gas produced by soil bacteria, can reach the stratosphere. There, it changes into nitric oxide (NO). Crutzen suggested that increased use of fertilizers might raise N₂O levels, leading to more NO in the stratosphere and affecting the ozone layer. The following year, Crutzen and Harold Johnston noted that NO emissions from supersonic passenger aircraft flying in the lower stratosphere could also reduce ozone. However, a 1995 study by David W. Fahey found that 500 such aircraft would likely cause only a 1–2% drop in ozone, which he said would not stop the development of these planes.

In 1974, Frank Sherwood Rowland and Mario J. Molina proposed that long-lasting organic compounds containing halogens, such as chlorofluorocarbons (CFCs), might behave like nitrous oxide. James Lovelock had earlier found that nearly all CFCs produced since 1930 remained in the atmosphere. Rowland and Molina concluded that CFCs would reach the stratosphere, where UV light would break them apart, releasing chlorine atoms. Earlier, Richard Stolarski and Ralph Cicerone showed that chlorine is more effective than NO at destroying ozone. Michael McElroy and Steven Wofsy reached similar conclusions but were studying smaller HCl emissions from the Space Shuttle instead of CFCs.

The Rowland–Molina hypothesis faced strong opposition from industry groups, including DuPont, which called ozone depletion theory "science fiction." However, within three years, laboratory and observational studies confirmed their claims. Measurements showed that CFCs were the main source of stratospheric chlorine and that nearly all CFCs released would eventually reach the stratosphere. James G. Anderson’s detection of chlorine monoxide (ClO) in the stratosphere proved that chlorine atoms were actively destroying ozone. McElroy and Wofsy later showed that bromine atoms, from compounds like halons used in fire extinguishers, were even more damaging to ozone. In 1976, the U.S. National Academy of Sciences supported the ozone depletion theory, leading to bans on CFCs in aerosol cans in 1978. Early estimates suggested that continued CFC use could reduce global ozone by 15–18% over a century, but later studies revised this to 5–9%.

Crutzen, Molina, and Rowland were awarded the 1995 Nobel Prize in Chemistry for their work on stratospheric ozone.

In 1985, scientists from the British Antarctic Survey, Farman, Gardiner, and Shanklin, discovered the Antarctic "ozone hole," a much larger ozone loss than expected. Satellite data from Nimbus 7 initially missed the hole because it was filtered out as an error, but reprocessing of raw data confirmed its existence as early as 1976.

Susan Solomon proposed that chemical reactions on polar stratospheric clouds (PSCs) in Antarctica’s cold stratosphere increase the amount of active chlorine, which destroys ozone. These clouds form only at temperatures as low as −80°C during early spring. Ice crystals in the clouds provide surfaces where inactive chlorine compounds become reactive, enabling ozone destruction. The tight polar vortex over Antarctica traps these reactions, leading to the ozone hole. This theory was confirmed by laboratory and direct measurements of high chlorine monoxide (ClO) levels in the Antarctic stratosphere.

Alternative theories, such as changes in solar UV radiation or atmospheric circulation, were tested and found unsupported. Ground-based Dobson spectrophotometers worldwide confirmed that the ozone layer is depleting at all latitudes outside the tropics.

Ozone depletion and global warming

Robert Watson played a role in evaluating scientific research and creating rules to address ozone depletion and global warming. Before the 1980s, organizations such as the EU, NASA, NAS, UNEP, WMO, and the British government had different scientific reports about these issues. Watson helped bring these groups together to create unified assessments. After learning from the way scientists worked together on the ozone issue, the IPCC began creating unified reports to reach agreement and produce the IPCC Summary for Policymakers.

There are several connections between the science of ozone depletion and global warming:

• The same carbon dioxide (CO₂) that causes global warming is expected to cool the stratosphere. This cooling may lead to more ozone (O₃) loss in polar regions and more frequent ozone holes.
• Ozone loss affects the climate system. Two effects occur: less ozone means the stratosphere absorbs less sunlight, cooling the stratosphere and warming the troposphere. A colder stratosphere emits less heat downward, cooling the troposphere. Overall, cooling is the main effect. The IPCC reports that ozone loss over the past two decades has caused a cooling effect of about −0.15 ± 0.10 watts per square meter (W/m²) on the surface-troposphere system.
• A major prediction of the greenhouse effect is that the stratosphere will cool. This cooling has been observed, but it is difficult to separate the effects of greenhouse gases and ozone loss because both cause cooling. Scientists use computer models to distinguish these effects. Models from the National Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory show that above 20 km (12 mi), greenhouse gases are the main cause of cooling.
• Chemicals that destroy ozone are also greenhouse gases. Increases in these chemicals have caused a radiative forcing of 0.34 ± 0.03 W/m², which is about 14 percent of the total warming effect from greenhouse gases.
• Studying and testing the process of ozone loss takes many years to understand and accept. Theories about ozone destruction were suggested in the 1980s, published in the late 1990s, and are still being tested. Drew Schindell and Paul Newman from Goddard Space Flight Center proposed a theory in the late 1990s using computer models. This model explained 78 percent of ozone loss. After improving the model, it explained 89 percent of ozone loss, but it delayed the predicted recovery of the ozone hole from 75 years to 150 years. (The model includes the impact of reduced stratospheric flight due to fossil fuel use.)

Misconceptions

CFC molecules are heavier than air (nitrogen or oxygen), so some people think they cannot reach the stratosphere in large amounts. However, in the atmosphere, gases are not separated by weight. Wind can mix gases evenly at high altitudes. Some heavier CFCs are not spread uniformly in the atmosphere.

A common misunderstanding is that natural sources of chlorine are much larger than human-made ones. While this is true for chlorine in the lower atmosphere, it does not affect ozone depletion, which depends only on chlorine in the stratosphere. Chlorine from ocean spray dissolves in water and is removed by rain before reaching the stratosphere. CFCs, however, do not dissolve and stay in the atmosphere for a long time, allowing them to reach the stratosphere. In the lower atmosphere, chlorine from CFCs and similar chemicals is much greater than chlorine from salt spray. In the stratosphere, halocarbons (chemicals containing chlorine) are the main source of chlorine. Only methyl chloride, a type of halocarbon, comes mostly from natural sources. It contributes about 20% of the chlorine in the stratosphere, while the remaining 80% comes from human activities.

Very strong volcanic eruptions can release hydrogen chloride (HCl) into the stratosphere. However, studies show that this contribution is much smaller compared to that from CFCs. Another incorrect idea is that soluble halogen compounds from Mount Erebus volcano in Antarctica are a major cause of the Antarctic ozone hole.

A 2015 study found that Mount Erebus may have a greater role in Antarctic ozone depletion than previously thought. Using data from the past 35 years and a model called HYSPLIT, scientists showed that gas emissions from the volcano, including HCl, can reach the Antarctic stratosphere through high-latitude storms and the polar vortex. The amount of HCl added to the stratosphere from Erebus each year depends on the volcano’s activity and ranges from 1.0 to 14.3 kilotons.

In 1956, G.M.B. Dobson measured ozone levels in the Antarctic over Halley Bay and found they were about 320 DU, which was 150 DU lower than typical Arctic spring levels of around 450 DU. Dobson’s observation was not an ozone hole but a normal maximum ozone level for Antarctica. Actual ozone hole values are much lower, between 100–150 DU. In the Arctic, ozone levels vary smoothly throughout the year, peaking in the northern hemisphere spring. In Antarctica, ozone levels drop sharply in the southern hemisphere spring, reaching much lower values, and then rise again by December.

Some people believe the ozone hole should appear above areas where CFCs are released. However, CFCs are evenly mixed in the atmosphere globally. The ozone hole forms over Antarctica not because there are more CFCs there but because cold temperatures create polar stratospheric clouds. Ozone holes have also been observed in other regions, such as Central Asia.

Public confusion about complex issues like ozone depletion is common. Limited scientific knowledge has led to misunderstandings, such as viewing global warming as part of the "ozone hole" problem. Early environmental groups avoided using CFC depletion in campaigns, thinking the topic was too complicated. Later, organizations like Greenpeace supported efforts to eliminate CFCs, such as promoting CFC-free refrigerators.

Terms like "ozone shield" and "ozone hole" are not scientifically precise. The "ozone hole" is more like a depression in ozone levels, not a complete absence of ozone. These terms helped raise public awareness because they highlighted risks like skin cancer, cataracts, and harm to plants and ocean life. Ozone regulations gained more public support than climate change policies. For example, Americans stopped using aerosol sprays before laws were passed. The discovery of the Antarctic ozone hole in 1985 received widespread media attention, even though earlier rapid ozone loss in Antarctica was dismissed as a measurement error. Scientific agreement was reached after regulations were implemented.

Although the Antarctic ozone hole has a small effect on global ozone levels, it has drawn significant public attention because:

• People worry that ozone holes might appear elsewhere, though only a smaller "dimple" has been observed in the Arctic. Ozone levels at mid-latitudes have decreased slightly (about 4–5%).
• If stratospheric conditions worsen (colder temperatures, more clouds, more active chlorine), global ozone loss could increase. Global warming is expected to cool the stratosphere.
• When the Antarctic ozone hole breaks up each year, ozone-depleted air moves into nearby regions. In New Zealand, ozone levels have dropped by up to 10% in the month after the hole breaks up, and ultraviolet-B radiation has increased by more than 15% since the 1970s.

World Ozone Day

In 1994, the United Nations General Assembly decided to name September 16 as the International Day for the Preservation of the Ozone Layer, also called "World Ozone Day." This date was chosen to remember the signing of the Montreal Protocol on September 16, 1987.