Ozone depletion involves 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 drop in ozone levels during springtime near Earth’s polar regions. This second event, called the ozone hole, happens in the stratosphere, the layer of the atmosphere that contains the ozone layer. There are also similar ozone decreases in the troposphere, the lower part of the atmosphere, near the poles during springtime.

The main causes of ozone depletion and the ozone hole are human-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 being released into the air, these chemicals rise into the stratosphere through natural mixing processes. Once there, sunlight breaks apart the molecules, releasing halogen atoms. These atoms react with ozone molecules, breaking them down into oxygen molecules. As emissions of these chemicals increased, both types of ozone loss became more severe.

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’s surface. This light can damage human skin, eyes, and 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, modern cars often use R-1234yf refrigerants instead of older ones like R-134a and R-12, which have higher GWP.

The ban on ozone-depleting chemicals took effect in 1989. By the mid-1990s, ozone levels had stabilized, and by the 2000s, they began to recover. Changes in the jet stream over the Southern Hemisphere have slowed the spread of the ozone hole and may even reverse it. Scientists predict the ozone hole will return to pre-1980 levels by around 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 are involved in the ozone-oxygen cycle: oxygen atoms (O), oxygen gas (O₂), and ozone gas (O₃). Ozone is created in the stratosphere when oxygen gas molecules absorb UVC photons and split into two oxygen atoms. These oxygen atoms then combine with other oxygen gas molecules to form two ozone molecules. Ozone absorbs UVB light, causing it to split into an oxygen molecule and an oxygen atom. The oxygen atom then joins with another oxygen molecule to form ozone again. This process continues until an oxygen atom combines with an ozone molecule to form two oxygen gas molecules. Ozone is the only atmospheric gas that absorbs UVB light.

The total amount of ozone in the stratosphere depends on a balance between ozone being created and ozone being destroyed.

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 each has an unpaired electron, making them very reactive. The ability of these elements to destroy ozone depends on how they are used in chemical reactions to regenerate the original radical.

All these radicals come from both natural and human-made sources. As of 2020, most OH· and NO· 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, like chlorofluorocarbons, which can reach the stratosphere without breaking down in the troposphere because they are not reactive. 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 oxygen gas with the help of a catalyst. Chlorine and bromine atoms destroy ozone through chemical reactions. In one simple example, a chlorine atom reacts with an ozone molecule, 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. The chemical shorthand for these reactions is:

• 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, but the rate can be slowed by some chemical processes. More complex reactions have also been found that destroy ozone in the lower stratosphere.

A single chlorine atom can destroy about 100,000 ozone molecules before it is removed from the cycle. This, along with the amount of chlorine released into the atmosphere each year by chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), shows the harm these chemicals cause to the environment.

Observations on ozone layer depletion

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

Between 1985 and now, ozone levels over Antarctica have dropped by up to 70% in spring. In 2010, ozone levels in September and October were still 40–50% lower than they were before the ozone hole formed. By 2016, scientists noticed a slow improvement. In 2017, NASA reported that the ozone hole was the smallest since 1988 because of warm conditions in the stratosphere. Scientists predict the ozone hole will fully recover by around 2070.

Ozone loss happens more unpredictably in the Arctic than in the Antarctic. The biggest drops in the Arctic occur 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 appeared over Antarctica and why it is deeper there. Early models did not consider PSCs and predicted slower ozone loss worldwide, which made the sudden discovery of the Antarctic hole surprising to scientists.

It is more accurate to describe ozone loss in middle latitudes (areas around the middle of Earth) rather than "holes." Between 1980 and 1996, ozone levels in middle latitudes dropped below pre-1980 levels. In the northern middle latitudes, ozone levels began to rise by about 2% from 1996 to 2009 as regulations reduced chlorine in the stratosphere. In the southern middle latitudes, ozone levels stayed the same during that time. In the tropics, there are no major trends 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 been shown to reduce ozone levels, though the effects vary.

Ozone loss also explains much of the cooling in the stratosphere and upper troposphere. Ozone warms the stratosphere by absorbing UV light, so less ozone leads to cooler temperatures. Some cooling is also expected from rising 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, a 1994 assessment by the United Nations Environment Programme overestimated ozone loss during that time.

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 and used in refrigerators, air conditioners, aerosol sprays, and electronics cleaning. 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 from the stratosphere. CFCs stay in the atmosphere for decades. A single CFC molecule takes about 5–7 years to reach the upper atmosphere and can destroy up to 100,000 ozone molecules over a century.

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. This is the only known CFC whose levels are still increasing. Its source is unknown, but some suspect illegal production. CFC-113a has been growing in the atmosphere since 1960, with a 40% increase between 2012 and 2017.

A study published in Nature found that emissions of the banned chemical CFC-11 from northeastern China have been releasing large amounts of the substance into the atmosphere since 2013. Scientists estimate that without action, these emissions could delay the recovery of the ozone hole by 10 years.

Satellites burning up when they re-enter Earth’s atmosphere release 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. These break down in under 6 months. About 90% are naturally produced, like bromine-based chemicals from seaweed and phytoplankton, while 10% are man-made, like dichloromethane.

Scientists have linked ozone loss to the rise in man-made halogen compounds, using data and computer models. These models combine chemical measurements, weather patterns, and reaction rates to track how CFCs and other chemicals interact with ozone. Examples of these models include SLIMCAT and CLaMS (Chemical Lagrangian Model of the Stratosphere).

Ozone hole and its causes

The Antarctic ozone hole is a region in the upper part of Earth's atmosphere above Antarctica where ozone levels have dropped to as low as 33% of their levels before 1975. This hole forms during the Antarctic spring, from September to early December, when strong winds circle the continent and trap air in a large, cold area called the polar vortex. During this time, more than half of the ozone in the lower part of the stratosphere is destroyed.

Ozone loss is mainly caused by chlorine from man-made gases like CFCs and other halocarbons. When sunlight hits these gases, they break apart, releasing chlorine atoms. These atoms help speed up the destruction of ozone. This process happens in the air, but it becomes much faster when it occurs on the surfaces of special clouds called polar stratospheric clouds (PSCs).

Polar stratospheric clouds form during the long, dark Antarctic winter, when there is no sunlight for three months. The extreme cold causes temperatures to drop to about or below -80°C. At these temperatures, cloud particles form. There are three types of these clouds, and they provide surfaces where chemical reactions take place. These reactions prepare the way for chlorine to destroy ozone when sunlight returns in spring.

The chemical processes involved are complicated but well understood. Normally, most chlorine in the stratosphere is stored in "reservoir" compounds, like chlorine nitrate (ClONO₂) and hydrogen chloride (HCl). These compounds keep chlorine from destroying ozone. However, 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. This process also removes nitrogen compounds from the stratosphere, which stops ClO from being converted back into ClONO₂.

Sunlight plays a key role in ozone loss. During winter, even though PSCs are most common, there is no sunlight to drive chemical reactions. In spring, sunlight returns, providing energy for reactions that release large amounts of ClO, which causes the ozone hole. By mid-December, warming temperatures break up the polar vortex, allowing air rich in ozone and nitrogen compounds to flow in from lower latitudes. This stops the ozone destruction process, and the hole closes.

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

Effects

The ozone layer in Earth's atmosphere absorbs UVB radiation from the sun. When the ozone layer becomes thinner, more UVB reaches Earth's surface, which can harm living things. This connection led to the creation of the Montreal Protocol, an agreement to reduce substances that damage the ozone layer. While scientific studies show that decreases in stratospheric ozone are linked to human-made chemicals like CFCs and higher UVB levels on Earth, there is no direct evidence proving that ozone depletion causes more skin cancer or eye damage in people. This is partly because UVA radiation, which also contributes to some types of skin cancer, is not blocked by the ozone layer. It is also hard to separate the effects of ozone depletion from changes in human behavior over time. Ozone depletion may also affect wind patterns.

Ozone is a small part of Earth's atmosphere but absorbs most UVB radiation. The amount of UVB that passes through the ozone layer decreases quickly as the layer becomes thinner or less dense. When ozone levels in the stratosphere drop, more UVB reaches Earth's surface. Scientists have used tree rings to study when ozone depletion began in northern regions, finding evidence dating back to 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 observations over 28 years from 12 international satellites. 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 extremely high and a serious health risk. The report concluded that thinning ozone levels in mid-latitude regions are already harming large populations. A similar study by Peru’s Peruvian Space Agency, CONIDA, found nearly the same results.

The main public concern about the ozone hole is the increase in UV radiation on Earth's surface and its effects on human health. In most places, ozone depletion has been small, around a few percent, and no direct evidence of health harm has been found in most regions. However, if the large ozone holes, like the one over Antarctica, became common worldwide, the effects could be much more severe. These ozone holes have sometimes grown large enough to impact parts of Australia, New Zealand, Chile, Argentina, and South Africa. Excessive UV radiation also harms benthic diatoms, tiny algae that improve water quality and resist pollution, in shallow freshwater areas. Ozone depletion harms plants and trees by damaging their cells, which affects their growth, photosynthesis, water balance, and ability to fight pests and diseases. This can harm soil microbes, insects, wildlife, and entire ecosystems.

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

The most common types of skin cancer in humans, basal and squamous cell carcinomas, are strongly linked to UVB exposure. UVB radiation causes damage to DNA in cells, leading to errors when the DNA is copied. These cancers are usually not fatal, but treating squamous cell carcinoma sometimes requires major surgery. Studies suggest that a 1% decrease in stratospheric ozone over time could increase the risk of these cancers by 2%.

Another type of skin cancer, melanoma, is less common but more deadly, with about 15–20% of cases being fatal. The connection between melanoma and UV exposure is not fully understood, but both UVB and UVA may play a role. 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, a study 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.

Studies suggest a link between UVB exposure and eye problems like cataracts. Research on Chesapeake Bay watermen showed that higher UVB exposure was connected to a greater risk of eye damage. Scientists predict that ozone depletion could cause hundreds of thousands of additional cataract cases by 2050.

Increased UV radiation also raises ground-level ozone, which is harmful to health, especially for children, the elderly, and people with asthma. Ground-level ozone is mainly created when UV radiation reacts with gases from vehicle exhaust.

UVB radiation helps the body produce vitamin D, which is important for health. However, many people have low vitamin D levels, as shown by recent studies. In the U.S., people with the lowest vitamin D levels were found to have a higher risk of death. While very high vitamin D levels can be harmful, the body prevents excessive production from sunlight.

A 2011 study by scientists in London found that whales near California showed signs of severe sun damage, possibly due to thinning ozone. The study found evidence of DNA damage in whale skin similar to human sunburn. These findings suggest that rising UV levels from ozone depletion may be harming whales, like the increase in human skin cancer rates. Other animals, such as dogs, cats, and sheep, and ecosystems on land also face risks from increased UVB radiation.

Higher UV radiation could harm crops. Some important plants, like rice, rely on cyanobacteria on their roots to help them absorb nitrogen. Cyanobacteria are sensitive to UV radiation and could be harmed by increased UVB exposure.

Public policy

The full damage that CFCs caused to the ozone layer is not fully understood and will take many years to know. However, scientists have already noticed large drops in the amount of ozone in the atmosphere. The Montreal and Vienna agreements were created before scientists had complete agreement about the effects of CFCs or before all scientific questions were answered. People who were not scientists understood the ozone issue well because terms like "ozone shield" and "ozone hole" helped explain the problem in simple ways. In the United States, people began using fewer aerosol sprays on their own, which led to a 50% drop in sales before any laws were passed.

In 1976, a report by the United States National Academy of Sciences said that scientific evidence supported the idea that CFCs were harming the ozone layer. Because of this, some countries, including the United States, Canada, Sweden, Denmark, and Norway, started to stop using CFCs in aerosol spray cans. At the time, this was seen as a first step toward more complete rules, but progress slowed later because of political issues (such as opposition from companies that made CFCs and a shift in public opinion during the Reagan administration) and new scientific findings that showed earlier estimates of ozone damage were too high.

A key patent for making Freon, a type of CFC, was set to expire in 1979. In 1978, the United States banned CFCs in aerosol cans. The European Community refused to ban CFCs in aerosol sprays, 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 by 1986, it had nearly returned to its 1976 level. In 1993, DuPont Canada closed its CFC manufacturing plant.

The U.S. government’s position changed again 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, 20 countries, including most major CFC producers, signed the Vienna Convention, which created a plan for global rules on substances that harm the ozone layer. That same year, scientists discovered the Antarctic ozone hole, which made people pay more attention to the problem.

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

At the Montreal meeting, countries agreed to stop increasing CFC production and reduce it by half by 1999. After scientists 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 completely stop using CFCs and halons, except for small amounts needed for essential uses like asthma inhalers, by 2000 in developed countries and by 2010 in less developed countries. In 1992, the phase-out date was moved up to 1996. At that meeting, methyl bromide, a chemical used in farming, was also added to the list of controlled substances. Less developed countries had slower phase-out schedules but received help with technology and money from wealthier countries.

Non-governmental organizations (NGOs) and other groups helped shape the Vienna Conference, the Montreal Protocol, and checked if countries followed the rules. Some companies claimed no safe alternatives to CFCs existed. However, scientists in Germany created 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 2004, companies like Coca-Cola and IKEA supported Greenfreeze. By 2008, over 300 million refrigerators used Greenfreeze. In Latin America, an Argentinian company and Bosch in Brazil started producing Greenfreeze units. By 2013, 700 million refrigerators used Greenfreeze, which was about 40% of the global market.

In the United States, changes happened more slowly. CFCs were sometimes replaced with hydrochlorofluorocarbons (HCFCs), but these also have problems. In some cases, hydrofluorocarbons (HFCs) were used instead. HFCs do not harm the ozone layer but are strong greenhouse gases. HFC-134a, for example, replaced CFC-12 in car air conditioners. In some lab uses, other solvents replaced ozone-depleting substances. Companies like DuPont opposed Greenfreeze and tried to block it in the U.S. until 2011. Companies like Ben & Jerry’s and General Electric, encouraged by Greenpeace, supported Greenfreeze in 2008, which helped the EPA approve it.

In 2009, the European Union updated its Ozone Regulation, which bans substances that harm

Prospects of ozone depletion

The Montreal Protocol has helped reduce the release of CFCs, leading to lower levels of these harmful chemicals in the atmosphere. The most important compounds have been decreasing since their highest levels in 1994. By 2008, the Effective Equivalent Chlorine (EECl) level had dropped about 10 percent. The decline 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). Since the phase-out of CFCs, nitrous oxide (N₂O), which is not regulated by the Montreal Protocol, has become the largest source of ozone-depleting emissions and is expected to stay that way through 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 preindustrial levels. While yearly changes are expected, especially in polar regions where ozone loss is greatest, the ozone layer is projected to recover over the next few decades if ozone-depleting substances continue to decline and the Montreal Protocol is fully followed.

The Antarctic ozone hole is expected to last for many decades. By 2020, ozone levels in the lower stratosphere over Antarctica had increased by 5–10 percent and are predicted to reach pre-1980 levels by 2060–2075. This is 10–25 years later than earlier estimates due to updated predictions about future use of ozone-depleting substances in developing countries. Another factor slowing recovery is the movement of nitrogen oxides from above the stratosphere caused by changing wind patterns. A sign of improvement was reported in 2016, and in 2019, the ozone hole was the smallest in 30 years because of warmer temperatures in the polar stratosphere that weakened the polar vortex. However, 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 report 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 simple processes that create the ozone layer in Earth's stratosphere. Short-wavelength ultraviolet (UV) light splits 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 combine to form two oxygen molecules, written as O + O₃ → 2O₂. In the 1950s, David Bates and Marcel Nicolet showed that certain chemicals, such as hydroxyl (OH) and nitric oxide (NO), could speed up this reaction, reducing ozone levels. These chemicals 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 found that nitrous oxide (N₂O), a gas produced by soil bacteria, could reach the stratosphere. There, it becomes nitric oxide (NO). Crutzen noted that increased use of fertilizers might raise N₂O levels, increasing NO in the stratosphere and affecting ozone. In 1971, Crutzen and Harold Johnston suggested that emissions from supersonic passenger planes could also reduce ozone. However, a 1995 study by David W. Fahey estimated that 500 such planes would cause 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 halogen compounds, like CFCs, could behave like nitrous oxide. James Lovelock had earlier found that most CFCs made since 1930 remained in the atmosphere. Molina and Rowland explained that CFCs reach the stratosphere, where UV light breaks them apart, releasing chlorine atoms. Earlier, scientists had shown that chlorine is more effective than nitric oxide at destroying ozone. However, they had not realized that CFCs were a major source of chlorine, as they had studied smaller HCl emissions from the Space Shuttle instead.

The Rowland–Molina idea was strongly opposed by companies that produced CFCs. A DuPont board member called the theory "science fiction" and "nonsense." Despite this, laboratory and observational studies confirmed their predictions within three years. Measurements showed that CFCs were the main source of chlorine in the stratosphere, and chlorine monoxide (ClO), a form of chlorine that destroys ozone, was found there. Scientists later found that bromine atoms, from compounds like halons used in fire extinguishers, were even more harmful to ozone than chlorine. 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 a 15–18% ozone loss if CFC use continued, but later data reduced this to 5–9%.

Crutzen, Molina, and Rowland won the 1995 Nobel Prize in Chemistry for their work on ozone. In 1985, British scientists discovered the Antarctic "ozone hole," a much larger ozone loss than expected. Satellite data initially dismissed the findings as errors, but reanalysis confirmed the hole existed as early as 1976.

Susan Solomon proposed that chemical reactions on polar stratospheric clouds (PSCs) in Antarctica’s cold stratosphere convert inactive chlorine into reactive forms that destroy ozone. These clouds form at temperatures as low as −80°C and during early spring. The tight polar vortex over Antarctica traps these reactions, leading to the ozone hole. This idea was confirmed by laboratory and field measurements showing high levels of chlorine monoxide (ClO) in the stratosphere.

Alternative theories, such as changes in solar UV or atmospheric circulation, were tested and found to be incorrect. Ground-based Dobson spectrophotometers confirmed that ozone levels were decreasing globally.

Ozone depletion and global warming

Robert Watson played an important role in assessing scientific information and helping to create rules for dealing with ozone depletion and global warming. Before the 1980s, organizations such as the EU, NASA, NAS, UNEP, WMO, and the British government had disagreements in their scientific reports about these issues. Watson helped bring these groups together to create agreed-upon assessments. Using lessons from the ozone depletion case, the IPCC began working on a unified way to share scientific findings to reach a common understanding, which led to the creation of 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 increase the amount of ozone (O₃) loss in polar regions and make ozone holes more common.
• Ozone depletion also affects the climate system. When ozone decreases, the stratosphere absorbs less sunlight, cooling the stratosphere and warming the troposphere. At the same time, a colder stratosphere emits less heat downward, which cools the troposphere. Overall, the cooling effect is stronger. The IPCC states that ozone loss over the past two decades has caused a cooling effect on the surface-troposphere system of about −0.15 ± 0.10 watts per square meter (W/m²).
• A strong 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 depletion because both cause cooling. Scientists use computer models to study this. Research from the National Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory shows that above 20 km (12 mi), greenhouse gases are the main cause of cooling.
• Chemicals that destroy ozone are often also greenhouse gases. Increases in these chemicals have contributed 0.34 ± 0.03 W/m² of radiative forcing, which is about 14 percent of the total radiative forcing from well-mixed greenhouse gases.
• Understanding the process of ozone destruction, measuring it, and developing scientific theories takes many years to confirm and widely accept. Several theories about ozone destruction were proposed in the 1980s, published in the late 1990s, and are now being tested. Drew Schindell and Paul Newman of Goddard Space Flight Center proposed a theory in the late 1990s using computer models to explain ozone loss. Their model accounted for 78 percent of ozone destruction. A later version of the model explained 89 percent of ozone loss but estimated the recovery of the ozone hole to take 150 years instead of 75 years. (The model includes the impact of reduced stratospheric flight due to fossil fuel use.)

Misconceptions

CFC molecules are heavier than air, but they can still reach the stratosphere because wind mixes gases in the atmosphere. Some heavier CFCs are not spread evenly in the atmosphere.

A common misunderstanding is that natural sources of chlorine are much larger than human-made ones. This is true in the lower atmosphere, but it does not affect ozone depletion, which depends on chlorine in the stratosphere. Chlorine from ocean spray dissolves in rain and is removed before it reaches the stratosphere. CFCs do not dissolve and stay in the atmosphere longer, allowing them to reach the stratosphere. In the lower atmosphere, CFCs and related chemicals contain more chlorine than salt spray. In the stratosphere, these chemicals are the main source of chlorine. Only methyl chloride, a type of these chemicals, comes mostly from natural sources and makes up about 20% of stratospheric chlorine. The rest comes from human activities.

Very strong volcanic eruptions can send hydrogen chloride (HCl) into the stratosphere, but this is not a major source compared to CFCs. Some people incorrectly believe that volcanic emissions from Mount Erebus in Antarctica are a major cause of the ozone hole. However, a 2015 study found that emissions from Mount Erebus can reach the Antarctic stratosphere through weather patterns. The amount of HCl added to the stratosphere each year from Mount 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 Antarctica and found they were about 320 DU, which is lower than typical Arctic levels. This was not an ozone hole but a normal seasonal low. Antarctic ozone levels drop sharply in spring, reaching as low as 100–150 DU, while Arctic levels vary smoothly between 300 and 450 DU.

Some people think the ozone hole should appear above areas where CFCs are used. However, CFCs mix evenly in the atmosphere. The ozone hole forms above Antarctica because of very cold temperatures that create special clouds, not because of higher CFC levels. Smaller ozone depletion has been observed in other regions, such as Central Asia.

Public confusion about ozone depletion is common. People often mix up ozone depletion with global warming. Early environmental groups avoided focusing on CFCs because the topic seemed complex. Later, groups like Greenpeace supported efforts to replace CFCs in products like refrigerators.

Terms like "ozone hole" are not exact scientifically. The term describes a large drop in ozone levels, not a complete absence. The ozone layer is not uniformly thin, and the hole is not a physical opening. These terms helped people understand the issue but are not precise. The ozone hole was widely reported in 1985 after scientists confirmed its existence.

The Antarctic ozone hole has a small effect on global ozone levels, but it has drawn public attention because:
– People worry that similar holes might form elsewhere, though only a smaller ozone "dimple" has been observed in the Arctic.
– If stratospheric conditions worsen (colder temperatures, more clouds, more active chlorine), ozone loss could increase. Climate models predict the stratosphere will cool.
– When the Antarctic ozone hole breaks up, ozone-depleted air moves to nearby regions. In New Zealand, ozone levels have dropped by up to 10%, 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 is chosen to remember the signing of the Montreal Protocol on September 16, 1987.