Ozone depletion includes two related events first noticed in the late 1970s: a decrease in the total amount of ozone in Earth's upper atmosphere and a much larger drop in ozone levels during spring near Earth's polar regions. This second event, called the ozone hole, happens in the stratosphere, which is the layer of the atmosphere that contains the ozone layer. There are also similar decreases in ozone levels in the troposphere, the lower part of the atmosphere, during spring near the poles.

The main causes of ozone depletion and the ozone hole are man-made chemicals, such as refrigerants, solvents, propellants, and foam-blowing agents. These chemicals include chlorofluorocarbons (CFCs), HCFCs, and halons, which are known as ozone-depleting substances (ODS). After these chemicals are released into the air, they rise into the stratosphere through natural air movement. Once there, sunlight breaks them apart, releasing atoms that help break down ozone into oxygen. Scientists observed that both types of ozone loss increased as more of these chemicals were released into the atmosphere.

Ozone depletion and the ozone hole have raised global concerns because they increase the risk of health problems, such as skin cancer, sunburn, blindness, and cataracts. The ozone layer normally blocks harmful ultraviolet (UVB) light from reaching Earth. This light can harm humans, plants, and animals. These dangers led to the creation of the Montreal Protocol in 1987, an international agreement that banned the production of CFCs, halons, and other ozone-depleting chemicals. Scientists later developed new refrigerants with lower global warming potential (GWP) to replace older ones. For example, many new cars now use R-1234yf refrigerants instead of older ones like R-134a and R-12, which have higher GWP.

The ban on ozone-depleting substances began in 1989. By the mid-1990s, ozone levels had stabilized, and by the 2000s, they started to recover. Changes in wind patterns in the Southern Hemisphere have slowed or reversed the movement of air that once worsened ozone loss. Scientists predict that the ozone hole will return to its 1980 levels by about 2075. In 2019, NASA reported that the ozone hole was the smallest since it was first discovered in 1982. The United Nations now estimates that the ozone layer will fully recover by 2045. The Montreal Protocol is widely regarded as the most successful international agreement for protecting the environment.

Ozone cycle overview

Three forms of oxygen—oxygen atoms (O), oxygen gas (O₂), and ozone gas (O₃)—are involved in the ozone-oxygen cycle. Ozone is created in the stratosphere when oxygen gas molecules absorb ultraviolet light with very short wavelengths (UVC). This causes oxygen gas (O₂) to split into two oxygen atoms (O). These oxygen atoms then combine with other oxygen gas molecules to form two ozone molecules (O₃). Ozone molecules absorb ultraviolet light with medium wavelengths (UVB), causing them to split into one oxygen gas molecule (O₂) and one oxygen atom (O). The oxygen atom then joins with 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 gas in the atmosphere that absorbs UVB light.

The total amount of ozone in the stratosphere depends on a balance between the chemical creation of ozone and its recombination.

Ozone can be destroyed by reactive molecules called free radicals. The most important ones are the hydroxyl radical (OH·), nitric oxide radical (NO·), chlorine radical (Cl·), and bromine radical (Br·). The dot in these names shows that each molecule has an unpaired electron, making them very reactive. The ability of these molecules to destroy ozone depends on how they react and how they are reused after interacting with ozone or oxygen gas.

All these radicals come from both natural and human-made sources. As of 2020, most hydroxyl and nitric oxide radicals in the stratosphere are naturally produced. However, human activities have greatly increased the levels of chlorine and bromine. These elements are found in stable chemical compounds, such as chlorofluorocarbons (CFCs), which do not break down in the lower atmosphere and can reach the stratosphere. Once in the stratosphere, ultraviolet light causes chlorine and bromine atoms to be released from these compounds.

Ozone is a reactive molecule that can change into a more stable form of oxygen with the help of a catalyst. Chlorine and bromine atoms destroy ozone through repeated chemical reactions. In one simple example, a chlorine atom combines 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 forming two oxygen gas molecules. These reactions can be written as:

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

These reactions reduce the amount of ozone in the atmosphere. However, some chemical processes can slow these reactions. More complex mechanisms also contribute to ozone loss in the lower stratosphere.

A single chlorine atom can destroy up to 100,000 ozone molecules before it stops reacting. This ability, combined with the large amounts of chlorine released into the atmosphere by CFCs and HCFCs, shows the harm these chemicals cause to the environment. While bromine is more efficient than chlorine at destroying ozone, there is much less bromine in the atmosphere today. Both chlorine and bromine contribute to ozone loss. Studies also show that fluorine and iodine atoms can participate in similar reactions, but fluorine reacts quickly with other substances in the stratosphere, and iodine-containing compounds break down before reaching the stratosphere in significant amounts.

Observations on ozone layer depletion

The ozone hole is measured by how much ozone is present above a location on Earth's surface. This is usually shown in Dobson units, written as "DU." The largest drop in ozone has been in the lower part of the stratosphere. Scientists have noticed a big decrease in ozone levels in Antarctica during spring and early summer compared to the 1970s and earlier, using tools like the Total Ozone Mapping Spectrometer (TOMS).

Between 1985 and now, ozone levels over Antarctica have dropped by up to 70% during the Southern Hemisphere's spring. In 2010, ozone levels in September and October were still 40–50% lower than 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 warmer. Scientists expect full recovery by around 2070.

Ozone loss changes more each year in the Arctic than in Antarctica. The biggest drops in the Arctic happen in 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 very cold stratosphere of the Arctic and Antarctica. This is why ozone holes first appeared and are deeper in Antarctica. Early models did not consider PSCs and predicted slow global ozone loss, which made the sudden discovery of the Antarctic ozone hole surprising to scientists.

It is more accurate to describe ozone loss in middle latitudes instead of "holes." Between 1980 and 1996, ozone levels in mid-latitudes dropped below pre-1980 levels. In the northern mid-latitudes, ozone levels rose by about 2% from 1996 to 2009 as regulations reduced chlorine in the stratosphere. In the Southern Hemisphere's mid-latitudes, ozone levels stayed the same during that time. Ozone levels in the tropics have not changed much because chemicals that release chlorine and bromine have not had time to break down there.

Large volcanic eruptions, like the 1991 eruption of Mount Pinatubo in the Philippines, have caused uneven but significant ozone loss.

Ozone loss explains much of the drop in temperatures in the stratosphere and upper troposphere. Ozone warms the stratosphere by absorbing UV light, so less ozone means cooling. Some cooling is also predicted from greenhouse gases like CO₂ and CFCs, but ozone-related cooling is more important.

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 but notes that an earlier assessment overestimated ozone loss between 1994 and 1997.

Chlorofluorocarbons (CFCs) and other man-made chemicals that destroy ozone are mainly responsible for human-caused ozone loss. Scientists can calculate the total amount of chlorine and bromine in the stratosphere, called equivalent effective stratospheric chlorine (EESC).

CFCs were invented by Thomas Midgley Jr. in the 1930s and used in refrigeration, air conditioning, aerosol sprays, and cleaning processes. They are not naturally produced and are almost entirely from human activity. When CFCs reach the stratosphere, UV light breaks them apart, releasing chlorine atoms. These atoms act as catalysts, destroying tens of thousands of ozone molecules before being removed. CFCs stay in the atmosphere for decades, with one molecule taking 5–7 years to reach the stratosphere and up to 100 years to destroy up to 100,000 ozone molecules.

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. It is the only known CFC whose levels are still rising. Its source is unknown, but some believe it may be from illegal production. CFC-113a has been increasing since 1960, with a 40% rise between 2012 and 2017.

A study published in Nature found that emissions from northeastern China have released large amounts of the banned chemical CFC-11 into the atmosphere since 2013. Scientists estimate that without action, these emissions could delay ozone recovery by a decade.

Satellites burning up in Earth's atmosphere produce aluminum oxide (Al₂O₃) nanoparticles that stay in the atmosphere for decades. In 2022 alone, about 17 metric tons of these particles were released from satellites. More satellites could eventually cause significant ozone loss.

"Very short-lived substances" are ozone-depleting chemicals allowed by the Montreal Protocol. They break down in under 6 months. 90% are naturally made, like bromine from seaweed and phytoplankton, while 10% are man-made, such as dichloromethane.

Scientists have linked ozone loss to man-made halogen compounds from CFCs by combining data and computer models. These models, like SLIMCAT and CLaMS, track chemical reactions and how CFCs interact with ozone in the atmosphere.

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 circle around the continent, creating a closed area of air. Inside this area, more than 50% of the ozone in the lower stratosphere is destroyed during the spring season.

The main reason for ozone loss is the presence of chlorine-containing gases, such as CFCs and other halocarbons. When these gases are exposed to sunlight, they break apart, releasing chlorine atoms. These chlorine atoms then help speed up the destruction of ozone. This process can happen in the air, but it becomes much more active when polar stratospheric clouds (PSCs) are present.

Polar stratospheric clouds form during the long, dark Antarctic winter, when there are three months without sunlight. The lack of sunlight causes temperatures to drop to around or below −80 °C. At these extreme cold temperatures, clouds form. There are three types of PSCs: clouds made of nitric acid, clouds formed slowly by freezing water, and clouds formed quickly by freezing water. These clouds provide surfaces where chemical reactions occur, and the results of these reactions lead to ozone loss during the spring.

The chemical reactions 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 the ozone destruction process. However, when sunlight with wavelengths shorter than 400 nm is present, these compounds can release chlorine. During the Antarctic winter and spring, reactions on the surfaces of PSCs change these "reservoir" compounds into reactive chlorine forms, such as Cl and ClO. Denitrification is the process where PSCs remove nitrogen dioxide (NO₂) from the stratosphere by turning it into nitric acid, which then falls out of the atmosphere. This stops ClO from being converted back into ClONO₂.

Sunlight plays a key role in ozone loss, 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. In spring, sunlight returns, providing energy for reactions that break apart PSCs and release large amounts of ClO, which causes the ozone hole. As temperatures warm by mid-December, the wind pattern around Antarctica breaks apart, allowing air rich in ozone and nitrogen dioxide to flow in from lower latitudes. This stops the ozone loss process, and the ozone hole closes.

Most of the ozone destroyed is in the lower stratosphere, while much smaller ozone loss from chemical reactions in the gas phase happens mainly in the upper stratosphere.

Effects

The ozone layer in Earth’s atmosphere helps protect life by absorbing harmful UVB radiation from the sun. When the ozone layer becomes thinner, more UVB radiation reaches Earth’s surface, which can increase the risk of health problems, such as skin cancer. This connection led to the creation of the Montreal Protocol, an international agreement aimed at reducing substances that damage the ozone layer. While scientific studies show that decreases in ozone are linked to human-made chemicals like CFCs and higher UVB levels on Earth, there is no clear evidence yet that ozone depletion directly causes more skin cancer or eye damage in people. This is partly because UVA radiation, which can also contribute to skin cancer, is not blocked by the ozone layer, and it is hard to separate the effects of ozone depletion from changes in human behavior over time. Ozone depletion may also change wind patterns.

Ozone is a small part of Earth’s atmosphere but plays a major role in absorbing UVB radiation. The amount of UVB that reaches Earth’s surface depends on the thickness and density of the ozone layer. When ozone levels drop, more UVB radiation passes through. Scientists have used tree rings to study when ozone depletion began in northern areas, tracing it back to the late 1700s.

In 2008, the Ecuadorian Space Agency released a report called HIPERION. The study combined data from ground instruments in Ecuador and satellite observations from 12 countries over 28 years. It found that UV radiation in equatorial regions was much higher than expected, with the UV index reaching as high as 24 in Quito, Ecuador. The World Health Organization considers a UV index of 11 to be extremely dangerous. The report concluded that thinning ozone in mid-latitude regions is already putting many people at risk. A similar study by Peru’s space agency, CONIDA, reached nearly the same conclusions.

The most common concern about ozone depletion is its impact on human health, especially the increased risk of skin cancer. In most areas, ozone levels have decreased by only a few percent, and there is no clear evidence of widespread health damage. However, if ozone depletion were as severe globally as it is in the Antarctic ozone hole, the effects could be much worse. The Antarctic ozone hole has grown large enough to affect parts of Australia, New Zealand, Chile, Argentina, and South Africa. Scientists have also found that increased UV radiation harms freshwater ecosystems by reducing the growth of microalgae that improve water quality. Ozone depletion damages plants and trees at the cellular level, affecting their growth, photosynthesis, and ability to resist pests and diseases. This can harm entire ecosystems, including soil microbes, insects, and wildlife.

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 levels of ground-level ozone, which is harmful to human health, especially for children, the elderly, and people with respiratory conditions. Ground-level ozone is mainly created when sunlight interacts with pollutants from vehicle exhaust.

Two common types of skin cancer, basal and squamous cell carcinomas, are strongly linked to UVB exposure. UVB radiation causes chemical changes in DNA, leading to errors that can result in cancer. These cancers are usually not fatal, though some may require surgery. Studies suggest that a 1% drop in ozone levels could increase the risk of these cancers by about 2%.

Another type of skin cancer, melanoma, is less common but more dangerous. It is linked to both UVB and UVA radiation, though the exact relationship is not fully understood. Studies have shown that increased UVB exposure may raise melanoma rates in men and women. In Punta Arenas, Chile, researchers found a 56% increase in melanoma and a 46% increase in non-melanoma skin cancer over seven years, along with lower ozone levels and higher UVB exposure.

Research also suggests a link between UVB exposure and eye problems like cataracts. A study of workers on Chesapeake Bay showed that higher UV exposure was associated with a greater risk of eye damage. Scientists predict that ozone depletion could cause hundreds of thousands of additional cataracts by 2050.

Increased UV radiation also raises ground-level ozone, which is harmful to human health. People with asthma or respiratory issues are especially at risk. Ground-level ozone is mainly created when sunlight reacts with pollutants from vehicles.

Vitamin D is produced in the skin when exposed to UVB radiation. Higher UVB levels can help people with low vitamin D levels. However, recent studies show that many people have suboptimal vitamin D levels. Very high vitamin D levels can be harmful, but the body naturally limits how much it produces.

In 2011, scientists in London found that whales near California showed signs of severe sun damage, which they believe is linked to thinning ozone. The study found evidence of DNA damage in whale skin, similar to human skin cancer. Other animals, like dogs, cats, and sheep, are also affected by increased UVB radiation.

Increased UV radiation could harm crops. Some important plants, like rice, rely on tiny organisms called cyanobacteria on their roots to help them grow. These organisms are sensitive to UV radiation and would be harmed by higher UV levels.

Public policy

The full effects of damage caused by CFCs to the ozone layer are not fully known and will take many years to understand. However, scientists have already noticed a clear decrease in the amount of ozone in the atmosphere. The Montreal and Vienna conventions were created before scientists agreed on the facts or resolved many scientific questions. People who are not scientists understood the ozone issue well because terms like "ozone shield" and "ozone hole" helped explain the problem. In the United States, people stopped using aerosol sprays voluntarily, leading to a 50% drop in sales 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 damage the ozone layer, several countries, including the United States, Canada, Sweden, Denmark, and Norway, began banning CFCs in aerosol spray cans. This was seen as a first step toward stronger rules, but progress slowed later because of political issues, such as resistance from companies that make CFCs and changes in public attitudes during the early years of the Reagan administration. Scientific studies later found that early 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 aerosol sprays, and in the United States, CFCs were still used for refrigeration and cleaning circuit boards. Global CFC production dropped sharply after the U.S. ban but returned to nearly 1976 levels by 1986. In 1993, DuPont Canada closed its CFC production facility.

The U.S. government changed its position in 1983 when William Ruckelshaus became the head of the Environmental Protection Agency (EPA). Under Ruckelshaus and his successor, Lee Thomas, the EPA pushed for international rules on CFCs. In 1985, twenty countries, including most major CFC producers, signed the Vienna Convention for the Protection of the Ozone Layer, which created a plan for global rules on ozone-depleting substances. That same year, scientists discovered the Antarctic ozone hole, which renewed public interest in the issue.

In 1987, 43 countries signed the Montreal Protocol. At the same time, companies that make CFCs began supporting limits on CFC production. However, this change was not equal, as DuPont acted faster than European companies. DuPont may have been worried about legal action related to increased skin cancer risks, especially after the EPA reported that 40 million more skin cancer cases and 800,000 cancer deaths could occur in the U.S. over the next 88 years. The European Union also changed its stance after Germany stopped defending CFC-producing industries. In France and the UK, government and industry leaders tried to protect their CFC industries even after the Montreal Protocol was signed.

At the Montreal meeting, countries agreed to stop increasing CFC production and reduce it by 50% by 1999. After scientific studies in Antarctica proved that the ozone hole was caused by chlorine and bromine from human-made chemicals, the Montreal Protocol was strengthened in 1990. Countries agreed to stop using CFCs and halons completely by 2000 in most countries and by 2010 in less developed countries. At a 1992 meeting in Copenhagen, the phase-out date was moved up to 1996. Methyl bromide, a chemical used in farming, was added to the list of controlled substances. Less developed countries had longer timeframes to phase out CFCs and received help from wealthier countries in the form of technology, money, and expertise. Special exceptions could be requested for certain uses.

Groups outside of government, such as non-profit organizations, played important roles in creating the Vienna Convention, the Montreal Protocol, and in checking if countries followed the rules. Some companies claimed no alternatives to CFCs existed. However, scientists in Germany developed a refrigerant made of propane and butane, called "Greenfreeze," which was safe for the ozone layer. Greenpeace helped promote this technology, and by 1995, Germany banned CFC refrigerators. By 2008, over 300 million refrigerators used Greenfreeze. Companies like Coca-Cola, Carlsberg, and IKEA later supported the technology.

In the United States, changes were slower. CFCs were partly replaced by HCFCs, which are less harmful but still not perfect. HFCs, which do not damage the ozone layer but are strong greenhouse gases, were used in some areas. HFC-134a, for example, replaced CFC-12 in car air conditioners. In laboratories, ozone-depleting substances were replaced with other solvents. Companies like DuPont opposed Greenfreeze and blocked it in the U.S. until 2011. Companies like Ben & Jerry's and General Electric supported Greenfreeze, which helped the EPA approve it.

The European Union updated its Ozone Regulation in 2009. The law bans ozone-depleting substances to protect the ozone layer. The list of banned substances matches the Montreal Protocol, with some additions.

Recently, experts have suggested linking ozone protection efforts with climate change efforts. Many ozone-depleting substances are also strong greenhouse gases, some thousands of times more powerful than carbon dioxide. Policies to protect the ozone layer have also helped reduce climate change effects. The reduction of these gases may have hidden the true impact of other greenhouse gases, which contributed to a slowdown in global warming from the mid-1990s. Decisions in one area can affect the cost and success of

Prospects of ozone depletion

Since the Montreal Protocol was adopted and improved, the release of CFCs has decreased, leading to lower levels of the most important ozone-depleting compounds in the atmosphere. These substances are slowly being removed from the air. After reaching their highest levels in 1994, the Effective Equivalent Chlorine (EECl) in the atmosphere had decreased by about 10% by 2008. The drop in ozone-depleting chemicals has also been influenced by reduced use of bromine-containing substances. Studies show that natural sources contribute significantly to atmospheric methyl bromide (CH₃Br). The phase-out of CFCs has made nitrous oxide (N₂O), which is not controlled by the Montreal Protocol, the largest source of ozone-depleting emissions. It is expected to remain the largest source throughout the 21st century.

According to the IPCC Sixth Assessment Report, global stratospheric ozone levels dropped quickly during the 1970s and 1980s but have since started to rise. However, they have not yet returned to levels seen before industrial activity began. While yearly changes are expected, especially in polar regions where ozone loss is greatest, the ozone layer is projected 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 predicted to return to levels seen before 1980 by about 2060–2075. This is 10–25 years later than earlier predictions, due to updated estimates of ozone-depleting substance concentrations, including higher expected use in developing countries. Another factor that may slow recovery is the reduction of nitrogen oxides from above the stratosphere caused by changes in wind patterns. A sign of recovery was reported in 2016. In 2019, the ozone hole was the smallest in the previous 30 years because warmer temperatures in the polar stratosphere weakened the polar vortex. In September 2023, the Antarctic ozone hole was among the largest ever recorded, 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 globally, 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 apart an oxygen (O₂) molecule into two oxygen (O) atoms. These oxygen 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 free radicals, such as hydroxyl (OH) and nitric oxide (NO), can speed up this reaction, reducing the amount of ozone. These radicals naturally exist in the stratosphere, and without them, the ozone layer would be about twice as thick as it is now.

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

In 1974, Frank Sherwood Rowland, a chemistry professor at the University of California, and his colleague Mario J. Molina proposed that long-lasting organic halogen compounds, like CFCs, might act like nitrous oxide. James Lovelock had earlier discovered that nearly all CFCs made since 1930 remained in the atmosphere. Molina and Rowland 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 (Cl) is more effective than NO at destroying ozone. Similar findings were made by Michael McElroy and Steven Wofsy. These groups did not realize that CFCs were a major source of stratospheric chlorine, as they had studied smaller HCl emissions from the Space Shuttle instead.

The Rowland–Molina hypothesis faced strong opposition from the aerosol and halocarbon industries. A DuPont board member called the ozone depletion theory "science fiction" and "nonsense." Despite this, within three years, laboratory tests and stratospheric observations confirmed most of Rowland and Molina’s ideas. Scientists measured CFCs and chlorine compounds in the stratosphere, proving that CFCs were the main source of chlorine there. James G. Anderson’s team also found chlorine monoxide (ClO) in the stratosphere, showing that chlorine atoms were destroying ozone. McElroy and Wofsy later found that bromine atoms were even better at destroying ozone than chlorine and warned that brominated compounds like halons, used in fire extinguishers, could add more bromine to the stratosphere. In 1976, the U.S. National Academy of Sciences confirmed the ozone depletion theory. In response, the U.S., Canada, and Norway banned CFCs in aerosol cans in 1978. Early estimates predicted that continued CFC use would reduce ozone by 15–18% over a century, but later studies revised this to 5–9%.

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

In 1985, British scientists Farman, Gardiner, and Shanklin discovered the Antarctic "ozone hole" in a paper published in Nature. This finding shocked scientists because the ozone loss was much larger than expected. Satellite data from the Nimbus 7 satellite showed large ozone depletion near the South Pole, but early data was rejected as errors due to unusually low values. The ozone hole was only confirmed when raw data was reprocessed after ground observations showed ozone depletion.

Susan Solomon, an atmospheric chemist, proposed that chemical reactions on polar stratospheric clouds (PSCs) in Antarctica’s cold stratosphere increase the amount of chlorine in active, ozone-destroying forms. These clouds form only at very low temperatures (as low as −80°C) and during early spring. The ice crystals in PSCs provide surfaces where unreactive chlorine compounds become reactive, destroying ozone. The tight polar vortex over Antarctica also helps trap these reactions, leading to the ozone hole. This idea was confirmed by laboratory tests and direct measurements of high chlorine monoxide (ClO) levels in the Antarctic stratosphere.

Other theories, such as changes in solar UV radiation or atmospheric circulation, were tested and found to be incorrect. Analysis of ozone data from ground-based Dobson spectrophotometers worldwide led scientists to conclude that the ozone layer is indeed depleting at all latitudes outside the tropics.

Ozone depletion and global warming

Robert Watson played an important role in studying the science of 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 agreed-upon assessments. Inspired by the way scientists worked together on the ozone issue, the IPCC began creating unified reports to help leaders make informed decisions.

There are several ways the science of ozone depletion and global warming are connected:

• The same carbon dioxide (CO₂) that causes global warming also cools the stratosphere. This cooling may increase ozone loss in polar regions and make ozone holes more common.
• Ozone depletion also affects the climate. Less ozone means the stratosphere absorbs less sunlight, cooling the stratosphere and warming the troposphere. The cooler stratosphere emits less heat downward, further cooling the troposphere. Overall, the cooling effect is stronger. The IPCC reports that ozone loss over the past 20 years has caused a cooling effect of about −0.15 ± 0.10 watts per square meter (W/m²).
• A major prediction of the greenhouse effect is that the stratosphere will cool. While this cooling has been observed, it is difficult to tell if it is caused by greenhouse gases or ozone depletion. Scientists use computer models to separate these effects. Studies by the National Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory show that above 20 km (12 miles), greenhouse gases are the main cause of cooling.
• Many chemicals that destroy ozone are also greenhouse gases. Increases in these chemicals have added 0.34 ± 0.03 W/m² of cooling to the climate, which is about 14% of the total cooling caused by all greenhouse gases.
• Understanding the process of ozone destruction, measuring it, and developing scientific theories takes many years. Some theories about ozone loss were proposed in the 1980s, published in the 1990s, and are still being studied today. Drew Schindell and Paul Newman of Goddard Space Flight Center created a model in the 1990s that explained 78% of ozone loss. A more detailed version of the model explained 89% of ozone loss but suggested that the ozone hole may take 150 years to recover instead of 75 years. (The model includes the impact of reduced stratospheric flights due to fossil fuel use.)

Misconceptions

CFC molecules are heavier than air (like nitrogen or oxygen), but they can still reach the stratosphere. Wind mixes gases in the atmosphere, so heavier gases like CFCs are not always evenly spread. Some heavier CFCs are not distributed equally in the atmosphere.

A common misunderstanding is that natural sources of chlorine are much larger than human-made ones. This is true for chlorine in the lower atmosphere, but it does not affect ozone depletion. Natural chlorine from ocean spray dissolves in rain and is washed out before reaching the stratosphere. In contrast, CFCs do not dissolve and stay in the atmosphere for a long time, allowing them to reach the stratosphere. In the lower atmosphere, CFCs and similar chemicals contain more chlorine than salt spray. In the stratosphere, these chemicals (called halocarbons) are the main source of chlorine. Only one of these halocarbons, methyl chloride, comes mostly from natural sources. It contributes about 20% of the chlorine in the stratosphere, while the remaining 80% comes from human-made sources.

Very strong volcanic eruptions can send hydrogen chloride (HCl) into the stratosphere, but scientists say this is not a major cause of ozone depletion compared to CFCs. A similar incorrect idea is that HCl from Mount Erebus volcano in Antarctica contributes greatly to the ozone hole.

A 2015 study found that Mount Erebus may have a bigger role in Antarctic ozone depletion than previously thought. Using data from the past 35 years and a model called HYSPLIT, researchers showed that HCl from the volcano can reach the Antarctic stratosphere through strong winds called high-latitude cyclones and the polar vortex. The amount of HCl added to the stratosphere each year from Erebus 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 and found they were about 320 DU, which was lower than typical Arctic levels of around 450 DU. However, Dobson’s observation was not an ozone hole but a normal high level of ozone for that time of year. Actual ozone hole values are much lower, between 100–150 DU. In the Arctic, ozone levels change gradually throughout the year, peaking in the spring. In the Antarctic, ozone levels drop sharply in the spring and do not rise again until December.

Some people believe the ozone hole should be above places where CFCs are used. However, CFCs are spread evenly around the world in the lower and upper atmosphere. The ozone hole forms over Antarctica because of very cold temperatures that create special clouds in the stratosphere, not because of high CFC levels. Ozone holes have also been found in other areas, such as over Central Asia.

Public confusion about ozone depletion is common. Many people misunderstand complex scientific issues, such as confusing global warming with the ozone hole. Early environmental groups avoided using the ozone hole as a campaign topic because they thought it was too complicated. Later, groups like Greenpeace supported efforts to stop CFC use, such as promoting refrigerators without CFCs.

Terms like “ozone shield” and “ozone hole” are not scientifically precise. The “ozone hole” is more like a dip in ozone levels, not a complete hole. Ozone does not disappear entirely, and the layer is not uniformly thin. These terms helped people understand the issue better. The ozone hole was seen as an urgent problem because people worried about health risks like skin cancer and harm to plants and ocean life. Public support for ozone regulations was stronger than for climate change policies. For example, Americans stopped using aerosol sprays before laws were passed, while climate change faced less public action. The discovery of the ozone hole in 1985 was widely reported, and scientists later agreed on the need for action.

The Antarctic ozone hole has a small effect on global ozone levels, but it has caused public interest because:

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

World Ozone Day

In 1994, the United Nations General Assembly chose 16 September as the International Day for the Preservation of the Ozone Layer, also known as "World Ozone Day." This date honors the signing of the Montreal Protocol on 16 September 1987.