text:
Ozone depletion

Ozone depletion consists of two related events observed since the late 1970s: a steady lowering of about four percent in the total amount of ozone in Earth's atmosphere, and a much larger springtime decrease in stratospheric ozone (the ozone layer) around Earth's polar regions. The latter phenomenon is referred to as the ozone hole. There are also springtime polar tropospheric ozone depletion events in addition to these stratospheric events.

The main causes of ozone depletion and the ozone hole are manufactured chemicals, especially manufactured halocarbon refrigerants, solvents, propellants, and foam-blowing agents (chlorofluorocarbons (CFCs), HCFCs, halons), referred to as ozone-depleting substances (ODS). These compounds are transported into the stratosphere by turbulent mixing after being emitted from the surface, mixing much faster than the molecules can settle. Once in the stratosphere, they release atoms from the halogen group through photodissociation, which catalyze the breakdown of ozone (O3) into oxygen (O2). Both types of ozone depletion were observed to increase as emissions of halocarbons increased.

Ozone depletion and the ozone hole have generated worldwide concern over increased cancer risks and other negative effects. The ozone layer prevents harmful wavelengths of ultraviolet (UVB) light from passing through the Earth's atmosphere. These wavelengths cause skin cancer, sunburn, permanent blindness, and cataracts, which were projected to increase dramatically as a result of thinning ozone, as well as harming plants and animals. These concerns led to the adoption of the Montreal Protocol in 1987, which bans the production of CFCs, halons, and other ozone-depleting chemicals. Currently, scientists plan to develop new refrigerants to replace older ones.

The ban came into effect in 1989. Ozone levels stabilized by the mid-1990s and began to recover in the 2000s, as the shifting of the jet stream in the southern hemisphere towards the south pole has stopped and might even be reversing. Recovery is projected to continue over the next century, and the ozone hole was expected to reach pre-1980 levels by around 2075. In 2019, NASA reported that the ozone hole was the smallest ever since it was first discovered in 1982.

The Montreal Protocol is considered the most successful international environmental agreement to date. Following the bans on ozone-depleting chemicals, the UN projects that under the current regulations the ozone layer will completely regenerate by 2045, thirty years earlier than previously predicted.

Ozone cycle overview
Three forms (or allotropes) of oxygen are involved in the ozone-oxygen cycle: oxygen atoms (O or atomic oxygen), oxygen gas ( or diatomic oxygen), and ozone gas ( or triatomic oxygen). Ozone is formed in the stratosphere when oxygen gas molecules photodissociate after absorbing UVC photons. This converts a single into two atomic oxygen radicals. The atomic oxygen radicals then combine with separate molecules to create two  molecules. These ozone molecules absorb UVB light, following which ozone splits into a molecule of and an oxygen atom. The oxygen atom then joins up with an oxygen molecule to regenerate ozone. This is a continuing process that terminates when an oxygen atom recombines with an ozone molecule to make two molecules. It is worth noting that ozone is the only atmospheric gas that absorbs UVB light.

 * O + → 2

The total amount of ozone in the stratosphere is determined by a balance between photochemical production and recombination.

Ozone can be destroyed by a number of free radical catalysts; the most important are the hydroxyl radical (OH·), nitric oxide radical (NO·), chlorine radical (Cl·) and bromine radical (Br·). The dot is a notation to indicate that each species has an unpaired electron and is thus extremely reactive. All of these have both natural and man-made sources; at present, most of the OH· and NO· in the stratosphere is naturally occurring, but human activity has drastically increased the levels of chlorine and bromine. These elements are found in stable organic compounds, especially chlorofluorocarbons, which can travel to the stratosphere without being destroyed in the troposphere due to their low reactivity. Once in the stratosphere, the Cl and Br atoms are released from the parent compounds by the action of ultraviolet light, e.g.

 * + electromagnetic radiation → Cl· + ·

Ozone is a highly reactive molecule that easily reduces to the more stable oxygen form with the assistance of a catalyst. Cl and Br atoms destroy ozone molecules through a variety of catalytic cycles. In the simplest example of such a cycle, a chlorine atom reacts with an ozone molecule, taking an oxygen atom to form chlorine monoxide (ClO) and leaving an oxygen molecule. The ClO can react with a second molecule of ozone, releasing the chlorine atom and yielding two molecules of oxygen. The chemical shorthand for these gas-phase reactions is: A chlorine atom removes an oxygen atom from an ozone molecule to make a ClO molecule This ClO can also remove an oxygen atom from another ozone molecule; the chlorine is free to repeat this two-step cycle
 * Cl· + → ClO +
 * ClO + → Cl· + 2

The overall effect is a decrease in the amount of ozone, though the rate of these processes can be decreased by the effects of null cycles. More complicated mechanisms have also been discovered that lead to ozone destruction in the lower stratosphere.

A single chlorine atom would continuously destroy ozone (thus a catalyst) for up to two years (the time scale for transport back down to the troposphere) except for reactions that remove it from this cycle by forming reservoir species such as hydrogen chloride (HCl) and chlorine nitrate. Bromine is even more efficient than chlorine at destroying ozone on a per-atom basis, but there is much less bromine in the atmosphere at present. Both chlorine and bromine contribute significantly to overall ozone depletion. Laboratory studies have also shown that fluorine and iodine atoms participate in analogous catalytic cycles. However, fluorine atoms react rapidly with water vapour, methane and hydrogen to form strongly bound hydrogen fluoride (HF) in the Earth's stratosphere, while organic molecules containing iodine react so rapidly in the lower atmosphere that they do not reach the stratosphere in significant quantities.

A single chlorine atom is able to react with an average of 100,000 ozone molecules before it is removed from the catalytic cycle. This fact plus the amount of chlorine released into the atmosphere yearly by chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) demonstrates the danger of CFCs and HCFCs to the environment.

Observations on ozone layer depletion
The ozone hole is usually measured by reduction in the total column ozone above a point on the Earth's surface. This is normally expressed in Dobson units; abbreviated as "DU". The most prominent decrease in ozone has been in the lower stratosphere. Marked decreases in column ozone in the Antarctic spring and early summer compared to the early 1970s and before have been observed using instruments such as the Total Ozone Mapping Spectrometer (TOMS).

Reductions of up to 70 percent in the ozone column observed in the austral (southern hemispheric) spring over Antarctica and first reported in 1985 (Farman et al.) are continuing. Antarctic total column ozone in September and October have continued to be 40–50 percent lower than pre-ozone-hole values since the 1990s. A gradual trend toward "healing" was reported in 2016. In 2017, NASA announced that the ozone hole was the weakest since 1988 because of warm stratospheric conditions. It is expected to recover around 2070.

The amount lost is more variable year-to-year in the Arctic than in the Antarctic. The greatest Arctic declines are in the winter and spring, reaching up to 30 percent when the stratosphere is coldest.

Reactions that take place on polar stratospheric clouds (PSCs) play an important role in enhancing ozone depletion. PSCs form more readily in the extreme cold of the Arctic and Antarctic stratosphere. This is why ozone holes first formed, and are deeper, over Antarctica. Early models failed to take PSCs into account and predicted a gradual global depletion, which is why the sudden Antarctic ozone hole was such a surprise to many scientists.

It is more accurate to speak of ozone depletion in middle latitudes rather than holes. Total column ozone declined below pre-1980 values between 1980 and 1996 for mid-latitudes. In the northern mid-latitudes, it then increased from the minimum value by about two percent from 1996 to 2009 as regulations took effect and the amount of chlorine in the stratosphere decreased. In the Southern Hemisphere's mid-latitudes, total ozone remained constant over that time period. There are no significant trends in the tropics, largely because halogen-containing compounds have not had time to break down and release chlorine and bromine atoms at tropical latitudes.

Large volcanic eruptions have been shown to have substantial albeit uneven ozone-depleting effects, as observed with the 1991 eruption of Mt. Pinatubo in the Philippines.

Ozone depletion also explains much of the observed reduction in stratospheric and upper tropospheric temperatures. The source of the warmth of the stratosphere is the absorption of UV radiation by ozone, hence reduced ozone leads to cooling. Some stratospheric cooling is also predicted from increases in greenhouse gases such as and CFCs themselves; however, the ozone-induced cooling appears to be dominant.

Predictions of ozone levels remain difficult, but the precision of models' predictions of observed values and the agreement among different modeling techniques have increased steadily. The World Meteorological Organization Global Ozone Research and Monitoring Project—Report No. 44 is strongly in favor of the Montreal Protocol, but notes that a UNEP 1994 Assessment overestimated ozone loss for the 1994–1997 period.

CFCs and related compounds in the atmosphere
Chlorofluorocarbons (CFCs) and other halogenated ozone-depleting substances (ODS) are mainly responsible for man-made chemical ozone depletion. The total amount of effective halogens (chlorine and bromine) in the stratosphere can be calculated and are known as the equivalent effective stratospheric chlorine (EESC).

CFCs as refrigerants were invented by Thomas Midgley Jr. in the 1930s. They were used in air conditioning and cooling units, as aerosol spray propellants prior to the 1970s, and in the cleaning processes of delicate electronic equipment. They also occur as by-products of some chemical processes. No significant natural sources have ever been identified for these compounds—their presence in the atmosphere is due almost entirely to human manufacture. As mentioned above, when such ozone-depleting chemicals reach the stratosphere, they are dissociated by ultraviolet light to release chlorine atoms. The chlorine atoms act as a catalyst, and each can break down tens of thousands of ozone molecules before being removed from the stratosphere. Given the longevity of CFC molecules, recovery times are measured in decades. It is calculated that a CFC molecule takes an average of about five to seven years to go from the ground level up to the upper atmosphere, and it can stay there for about a century, destroying up to one hundred thousand ozone molecules during that time.

1,1,1-Trichloro-2,2,2-trifluoroethane, also known as CFC-113a, is one of four man-made chemicals newly discovered in the atmosphere by a team at the University of East Anglia. CFC-113a is the only known CFC whose abundance in the atmosphere is still growing. Its source remains a mystery, but illegal manufacturing is suspected by some. CFC-113a seems to have been accumulating unabated since 1960. Between 2012 and 2017, concentrations of the gas jumped by 40 percent.

A study by an international team of researchers published in Nature found that since 2013 emissions that are predominately from north-eastern China have released large quantities of the banned chemical Chlorofluorocarbon-11 (CFC-11) into the atmosphere. Scientists estimate that without action, these CFC-11 emissions will delay the recovery of the planet's ozone hole by a decade.

Computer modeling
Scientists have attributed ozone depletion to the increase of man-made (anthropogenic) halogen compounds from CFCs by combining observational data with computer models. These complex chemistry transport models (e.g. SLIMCAT, CLaMS—Chemical Lagrangian Model of the Stratosphere) work by combining measurements of chemicals and meteorological fields with chemical reaction rate constants. They identify key chemical reactions and transport processes that bring CFC photolysis products into contact with ozone.

Ozone hole and its causes

The Antarctic ozone hole is an area of the Antarctic stratosphere in which the recent ozone levels have dropped to as low as 33 percent of their pre-1975 values. The ozone hole occurs during the Antarctic spring, from September to early December, as strong westerly winds start to circulate around the continent and create an atmospheric container. Within this polar vortex, over 50 percent of the lower stratospheric ozone is destroyed during the Antarctic spring.

As explained above, the primary cause of ozone depletion is the presence of chlorine-containing source gases (primarily CFCs and related halocarbons). In the presence of UV light, these gases dissociate, releasing chlorine atoms, which then go on to catalyze ozone destruction. The Cl-catalyzed ozone depletion can take place in the gas phase, but it is substantially enhanced in the presence of polar stratospheric clouds (PSCs).

These polar stratospheric clouds form during winter, in the extreme cold. Polar winters are dark, consisting of three months without solar radiation (sunlight). The lack of sunlight contributes to a decrease in temperature and the polar vortex traps and chills the air. Temperatures are around or below −80 °C. These low temperatures form cloud particles. There are three types of PSC clouds—nitric acid trihydrate clouds, slowly cooling water-ice clouds, and rapid cooling water-ice (nacreous) clouds—provide surfaces for chemical reactions whose products will, in the spring lead to ozone destruction.

The photochemical processes involved are complex but well understood. The key observation is that, ordinarily, most of the chlorine in the stratosphere resides in "reservoir" compounds, primarily chlorine nitrate as well as stable end products such as HCl. The formation of end products essentially removes Cl from the ozone depletion process. Reservoir compounds sequester Cl, which can later be made available via absorption of light at wavelengths shorter than 400 nm. During the Antarctic winter and spring, reactions on the surface of the polar stratospheric cloud particles convert these "reservoir" compounds into reactive free radicals (Cl and ClO). Denitrification is the process by which the clouds remove from the stratosphere by converting it to nitric acid in PSC particles, which then are lost by sedimentation. This prevents newly formed ClO from being converted back into.

The role of sunlight in ozone depletion is the reason why the Antarctic ozone depletion is greatest during spring. During winter, even though PSCs are at their most abundant, there is no light over the pole to drive chemical reactions. During the spring, however, sunlight returns and provides energy to drive photochemical reactions and melt the polar stratospheric clouds, releasing considerable ClO, which drives the hole mechanism. Further warming temperatures near the end of spring break up the vortex around mid-December. As warm, ozone and -rich air flows in from lower latitudes, the PSCs are destroyed, the enhanced ozone depletion process shuts down, and the ozone hole closes.

Most of the ozone that is destroyed is in the lower stratosphere, in contrast to the much smaller ozone depletion through homogeneous gas-phase reactions, which occurs primarily in the upper stratosphere.

Effects
Since the ozone layer absorbs UVB ultraviolet light from the sun, ozone layer depletion increases surface UVB levels (all else equal), which could lead to damage, including an increase in skin cancer. This was the reason for the Montreal Protocol. Although decreases in stratospheric ozone are well-tied to CFCs and increases in surface UVB, there is no direct observational evidence linking ozone depletion to higher incidence of skin cancer and eye damage in human beings. This is partly because UVA, which has also been implicated in some forms of skin cancer, is not absorbed by ozone, and because it is nearly impossible to control statistics for lifestyle changes over time. Ozone depletion may also influence wind patterns.

Increased UV
Ozone, while a minority constituent in Earth's atmosphere, is responsible for most of the absorption of UVB radiation. The amount of UVB radiation that penetrates through the ozone layer decreases exponentially with the slant-path thickness and density of the layer. When stratospheric ozone levels decrease, higher levels of UVB reach the Earth's surface. UV-driven phenolic formation in tree rings has dated the start of ozone depletion in northern latitudes to the late 1700s.

In October 2008, the Ecuadorian Space Agency published a report called HIPERION. The study used ground instruments in Ecuador and the last 28 years' data from 12 satellites of several countries, and found that the UV radiation reaching equatorial latitudes was far greater than expected, with the UV Index climbing as high as 24 in Quito; the WHO considers 11 as an extreme index and a great risk to health. The report concluded that depleted ozone levels around the mid-latitudes of the planet are already endangering large populations in these areas. Later, the CONIDA, the Peruvian Space Agency, published its own study, which yielded almost the same findings as the Ecuadorian study.

Biological effects
The main public concern regarding the ozone hole has been the effects of increased surface UV radiation on human health. So far, ozone depletion in most locations has been typically a few percent and, as noted above, no direct evidence of health damage is available in most latitudes. If the high levels of depletion seen in the ozone hole were to be common across the globe, the effects could be substantially more dramatic. As the ozone hole over Antarctica has in some instances grown so large as to affect parts of Australia, New Zealand, Chile, Argentina, and South Africa, environmentalists have been concerned that the increase in surface UV could be significant. Excessive ultraviolet radiation (UVR) has reducing effects on the rates of photosynthesis and growth of benthic diatom communities (microalgae species that increase water quality and are pollution resistant) that are present in shallow freshwater. Ozone depletion not only affects human health but also has a profound impact on biodiversity. It damages plants and trees at the cellular level, affecting their growth, vitality, photosynthesis, water balance, and defense mechanisms against pests and diseases. This sets off a cascade of ecological impacts, harming soil microbes, insects, wildlife, and entire ecosystems.

Ozone depletion would magnify all of the effects of UV on human health, both positive (including production of vitamin D) and negative (including sunburn, skin cancer, and cataracts). In addition, increased surface UV leads to increased tropospheric ozone, which is a health risk to humans.

Basal and squamous cell carcinomas
The most common forms of skin cancer in humans, basal and squamous cell carcinomas, have been strongly linked to UV-B exposure. The mechanism by which UVB induces these cancers is well understood—absorption of UV-B radiation causes the pyrimidine bases in the DNA molecule to form dimers, resulting in transcription errors when the DNA replicates. These cancers are relatively mild and rarely fatal, although the treatment of squamous cell carcinoma sometimes requires extensive reconstructive surgery. By combining epidemiological data with results of animal studies, scientists have estimated that every one percent decrease in long-term stratospheric ozone would increase the incidence of these cancers by 2%.

Malignant melanoma
Another form of skin cancer, malignant melanoma, is much less common but far more dangerous, being lethal in about 15–20 percent of the cases diagnosed. The relationship between malignant melanoma and ultraviolet exposure is not yet fully understood, but it appears that both UV-B and UV-A are involved. Because of this uncertainty, it is difficult to estimate the effect of ozone depletion on melanoma incidence. One study showed that a 10 percent increase in UV-B radiation was associated with a 19 percent increase in melanomas for men and 16 percent for women. A study of people in Punta Arenas, at the southern tip of Chile, showed a 56 percent increase in melanoma and a 46 percent increase in nonmelanoma skin cancer over a period of seven years, along with decreased ozone and increased UVB levels.

Cortical cataracts
Epidemiological studies suggest an association between ocular cortical cataracts and UV-B exposure, using crude approximations of exposure and various cataract assessment techniques. A detailed assessment of ocular exposure to UV-B was carried out in a study on Chesapeake Bay Watermen, where increases in average annual ocular exposure were associated with increasing risk of cortical opacity. In this highly exposed group of predominantly white males, the evidence linking cortical opacities to sunlight exposure was the strongest to date. Based on these results, ozone depletion is predicted to cause hundreds of thousands of additional cataracts by 2050.

Increased tropospheric ozone
Increased surface UV leads to increased tropospheric ozone. Ground-level ozone is generally recognized to be a health risk, as ozone is toxic due to its strong oxidant properties. The risks are particularly high for young children, the elderly, and those with asthma or other respiratory difficulties. At this time, ozone at ground level is produced mainly by the action of UV radiation on combustion gases from vehicle exhausts.

Increased production of vitamin D
Vitamin D is produced in the skin by ultraviolet light. Thus, higher UVB exposure raises human vitamin D in those deficient in it. Recent research (primarily since the Montreal Protocol) shows that many humans have less than optimal vitamin D levels. In particular, in the U.S. population, the lowest quarter of vitamin D (<17.8 ng/ml) were found using information from the National Health and Nutrition Examination Survey to be associated with an increase in all-cause mortality in the general population. While blood level of vitamin D in excess of 100 ng/ml appear to raise blood calcium excessively and to be associated with higher mortality, the body has mechanisms that prevent sunlight from producing vitamin D in excess of the body's requirements.

Effects on animals
A November 2011 report by scientists at the Institute of Zoology in London, England found that whales off the coast of California have shown a sharp rise in sun damage, and these scientists "fear that the thinning ozone layer is to blame". The study photographed and took skin biopsies from over 150 whales in the Gulf of California and found "widespread evidence of epidermal damage commonly associated with acute and severe sunburn", having cells that form when the DNA is damaged by UV radiation. The findings suggest "rising UV levels as a result of ozone depletion are to blame for the observed skin damage, in the same way that human skin cancer rates have been on the increase in recent decades." Apart from whales many other animals such as dogs, cats, sheep and terrestrial ecosystems also suffer the negative effects of increased UV-B radiations.

Effects on crops
An increase of UV radiation would be expected to affect crops. A number of economically important species of plants, such as rice, depend on cyanobacteria residing on their roots for the retention of nitrogen. Cyanobacteria are sensitive to UV radiation and would be affected by its increase. "Despite mechanisms to reduce or repair the effects of increased ultraviolet radiation, plants have a limited ability to adapt to increased levels of UVB, therefore plant growth can be directly affected by UVB radiation."

Effects on plant life
Over the years, the Arctic ozone layer has depleted severely. As a consequence species that live above the snow cover or in areas where snow has melted abundantly, due to hot temperatures, are negatively impacted due to UV radiation that reaches the ground. Depletion of the ozone layer and allowing excess UVB radiation would initially be assumed to increase damage to plant DNA. Reports have found that when plants are exposed to UVB radiation similar to stratospheric ozone depletion, there was no significant change in plant height or leaf mass, but showed a response in shoot biomass and leaf area with a small decrease. However, UVB radiation has been shown to decrease quantum yield of photosystem II. UVB damage only occurs under extreme exposure, and most plants also have UVB absorbing flavonoids which allow them to acclimatize to the radiation present. Plants experience different levels of UV radiation throughout the day. It is known that they are able to shift the levels and types of UV sunscreens (i.e. flavonoids), that they contain, throughout the day. This allows them to increase their protection against UV radiation. Plants that have been affected by radiation throughout development are more affected by the inability to intercept light with a larger leaf area than having photosynthetic systems compromised. Damage from UVB radiation is more likely to be significant on species interactions than on plants themselves.

Another significant impact of ozone depletion on plant life is the stress experienced by plants when exposed to UV radiation. This can cause a decrease in plant growth and an increase in oxidative stress, due to the production of nitric oxide and hydrogen peroxide. In areas where substantial ozone depletion has occurred, increased UV-B radiation reduces terrestrial plant productivity (and likewise carbon sequestration) by about 6%.

Moreover, if plants are exposed to high levels of UV radiation, it can elicit the production of harmful volatile organic compounds, like isoprenes. The emission of isoprenes into the air, by plants, can severely impact the environment by adding to air pollution and increasing the amount of carbon in the atmosphere, ultimately contributing to climate change.

Public policy

The full extent of the damage that CFCs have caused to the ozone layer is not known and will not be known for decades; however, marked decreases in column ozone have already been observed. The Montreal and Vienna conventions were installed long before a scientific consensus was established or important uncertainties in the science field were being resolved. The ozone case was understood comparably well by lay persons as e.g. Ozone shield or ozone hole were useful "easy-to-understand bridging metaphors". Americans voluntarily switched away from aerosol sprays, resulting in a 50 percent sales loss even before legislation was enforced.

After a 1976 report by the United States National Academy of Sciences concluded that credible scientific evidence supported the ozone depletion hypothesis a few countries, including the United States, Canada, Sweden, Denmark, and Norway, moved to eliminate the use of CFCs in aerosol spray cans. At the time this was widely regarded as a first step towards a more comprehensive regulation policy, but progress in this direction slowed in subsequent years, due to a combination of political factors (continued resistance from the halocarbon industry and a general change in attitude towards environmental regulation during the first two years of the Reagan administration) and scientific developments (subsequent National Academy assessments that indicated that the first estimates of the magnitude of ozone depletion had been overly large).

A critical DuPont manufacturing patent for Freon was set to expire in 1979. The United States banned the use of CFCs in aerosol cans in 1978. The European Community rejected proposals to ban CFCs in aerosol sprays, and in the U.S., CFCs continued to be used as refrigerants and for cleaning circuit boards. Worldwide CFC production fell sharply after the U.S. aerosol ban, but by 1986 had returned nearly to its 1976 level. In 1993, DuPont Canada closed its CFC facility.

The U.S. government's attitude began to change again in 1983, when William Ruckelshaus replaced Anne M. Burford as Administrator of the United States Environmental Protection Agency (EPA). Under Ruckelshaus and his successor, Lee Thomas, the EPA pushed for an international approach to halocarbon regulations. In 1985 twenty nations, including most of the major CFC producers, signed the Vienna Convention for the Protection of the Ozone Layer, which established a framework for negotiating international regulations on ozone-depleting substances. That same year, the discovery of the Antarctic ozone hole was announced, causing a revival in public attention to the issue.

In 1987, representatives from 43 nations signed the Montreal Protocol. Meanwhile, the halocarbon industry shifted its position and started supporting a protocol to limit CFC production. However, this shift was uneven with DuPont acting more quickly than its European counterparts. DuPont may have feared court action related to increased skin cancer, especially as the EPA had published a study in 1986 claiming that an additional 40 million cases and 800,000 cancer deaths were to be expected in the U.S. in the next 88 years. The EU shifted its position as well after Germany gave up its defence of the CFC industry and started supporting moves towards regulation. Government and industry in France and the UK tried to defend their CFC producing industries even after the Montreal Protocol had been signed.

At Montreal, the participants agreed to freeze production of CFCs at 1986 levels and to reduce production by 50 percent by 1999. After a series of scientific expeditions to the Antarctic produced convincing evidence that the ozone hole was indeed caused by chlorine and bromine from manmade organohalogens, the Montreal Protocol was strengthened at a 1990 meeting in London. The participants agreed to phase out CFCs and halons entirely (aside from a very small amount marked for certain "essential" uses, such as asthma inhalers) by 2000 in non-Article 5 countries and by 2010 in Article 5 (less developed) signatories. At a 1992 meeting in Copenhagen, Denmark, the phase-out date was moved up to 1996. At the same meeting, methyl bromide (MeBr), a fumigant used primarily in agricultural production, was added to the list of controlled substances. For all substances controlled under the protocol, phaseout schedules were delayed for less developed ('Article 5(1)') countries, and phaseout in these countries was supported by transfers of expertise, technology, and money from non-Article 5(1) Parties to the Protocol. Additionally, exemptions from the agreed schedules could be applied for under the Essential Use Exemption (EUE) process for substances other than methyl bromide and under the Critical Use Exemption (CUE) process for methyl bromide.

Civil society, including especially non-governmental organizations (NGOs), played critical roles at all stages of policy development leading to the Vienna Conference, the Montreal Protocol, and in assessing compliance afterwards. The major companies claimed that no alternatives to HFC existed. An ozone-safe hydrocarbon refrigerant was developed at a technological institute in Hamburg, Germany, consisting of a mixture of the hydrocarbon gases propane and butane, and in 1992 came to the attention of the NGO Greenpeace. Greenpeace called it "Greenfreeze". The NGO then worked successfully first with a small and struggling company to market an appliance beginning in Europe, then Asia and later Latin America, receiving a 1997 UNEP award. By 1995, Germany had made CFC refrigerators illegal. Since 2004, corporations like Coca-Cola, Carlsberg, and IKEA formed a coalition to promote the ozone-safe Greenfreeze units. Production spread to companies like Electrolux, Bosch, and LG, with sales reaching some 300 million refrigerators by 2008. In Latin America, a domestic Argentinian company began Greenfreeze production in 2003, while the giant Bosch in Brazil began a year later. By 2013 it was being used by some 700 million refrigerators, making up about 40 percent of the market.

In the U.S., however, change has been much slower. To some extent, CFCs were being replaced by the less damaging hydrochlorofluorocarbons (HCFCs), although concerns remain regarding HCFCs also. In some applications, hydrofluorocarbons (HFCs) were being used to replace CFCs. HFCs, which contain no chlorine or bromine, do not contribute to ozone depletion although they are potent greenhouse gases. The best known of these compounds is probably HFC-134a (R-134a), which in the United States has largely replaced CFC-12 (R-12) in automobile air conditioners. In laboratory analytics (a former "essential" use) the ozone depleting substances can be replaced with other solvents. Chemical companies like Du Pont, whose representatives disparaged Greenfreeze as "that German technology," maneuvered the EPA to block the technology in the U.S. until 2011. Ben & Jerry's of Unilever and General Electric, spurred by Greenpeace, had expressed formal interest in 2008 which figured in the EPA's final approval.

The EU recast its Ozone Regulation in 2009. The law bans ozone-depleting substances with the goal of protecting the ozone layer. The list of ODS that are subject to the regulation is the same as those under the Montreal Protocol, with some additions.

More recently, policy experts have advocated for efforts to link ozone protection efforts to climate protection efforts. Many ODS are also greenhouse gases, some thousands of times more powerful agents of radiative forcing than carbon dioxide over the short and medium term. Thus policies protecting the ozone layer have had benefits in mitigating climate change. The reduction of the radiative forcing due to ODS probably masked the true level of climate change effects of other greenhouse gases, and was responsible for the "slow down" of global warming from the mid-90s. Policy decisions in one arena affect the costs and effectiveness of environmental improvements in the other.

ODS requirements in the marine industry
The IMO has amended MARPOL Annex VI Regulation 12 regarding ozone depleting substances. As from July 1, 2010, all vessels where MARPOL Annex VI is applicable should have a list of equipment using ozone depleting substances. The list should include the name of ODS, type and location of equipment, quantity in kg and date. All changes since that date should be recorded in an ODS Record book on board recording all intended or unintended releases to the atmosphere. Furthermore, new ODS supply or landing to shore facilities should be recorded as well.

Prospects of ozone depletion

Since the adoption and strengthening of the Montreal Protocol has led to reductions in the emissions of CFCs, atmospheric concentrations of the most-significant compounds have been declining. These substances are being gradually removed from the atmosphere; since peaking in 1994, the Effective Equivalent Chlorine (EECl) level in the atmosphere had dropped about 10 percent by 2008. The decrease in ozone-depleting chemicals has also been significantly affected by a decrease in bromine-containing chemicals. The data suggest that substantial natural sources exist for atmospheric methyl bromide. The phase-out of CFCs means that nitrous oxide, which is not covered by the Montreal Protocol, has become the most highly emitted ozone-depleting substance and is expected to remain so throughout the 21st century.

According to the IPCC Sixth Assessment Report, global stratospheric ozone levels experienced rapid decline in the 1970s and 1980s and have since been increasing, but have not reached preindustrial levels. Although considerable variability is expected from year to year, including in polar regions where depletion is largest, the ozone layer is expected to continue recovering in coming decades due to declining ozone-depleting substance concentrations, assuming full compliance with the Montreal Protocol.

The Antarctic ozone hole is expected to continue for decades. Ozone concentrations in the lower stratosphere over Antarctica increased by 5–10 percent by 2020 and will return to pre-1980 levels by about 2060–2075. This is 10–25 years later than predicted in earlier assessments, because of revised estimates of atmospheric concentrations of ozone-depleting substances, including a larger predicted future usage in developing countries. Another factor that may prolong ozone depletion is the drawdown of nitrogen oxides from above the stratosphere due to changing wind patterns. A gradual trend toward "healing" was reported in 2016. In 2019, the ozone hole was at its smallest in the previous thirty years, due to the warmer polar stratosphere weakening the polar vortex. In September 2023, the Antarctic ozone hole was one of the largest on record, at 26 million square kilometers. The anomalously large ozone loss may have been a result of the 2022 Tonga volcanic eruption.

Research history
The basic physical and chemical processes that lead to the formation of an ozone layer in the Earth's stratosphere were discovered by Sydney Chapman in 1930. Short-wavelength UV radiation splits an oxygen molecule into two oxygen (O) atoms, which then combine with other oxygen molecules to form ozone. Ozone is removed when an oxygen atom and an ozone molecule "recombine" to form two oxygen molecules, i.e. O + → 2. In the 1950s, David Bates and Marcel Nicolet presented evidence that various free radicals, in particular hydroxyl (OH) and nitric oxide (NO), could catalyze this recombination reaction, reducing the overall amount of ozone. These free radicals were known to be present in the stratosphere, and so were regarded as part of the natural balance—it was estimated that in their absence, the ozone layer would be about twice as thick as it currently is.

In 1970 Paul Crutzen pointed out that emissions of nitrous oxide, a stable, long-lived gas produced by soil bacteria, from the Earth's surface could affect the amount of nitric oxide (NO) in the stratosphere. Crutzen showed that nitrous oxide lives long enough to reach the stratosphere, where it is converted into NO. Crutzen then noted that increasing use of fertilizers might have led to an increase in nitrous oxide emissions over the natural background, which would in turn result in an increase in the amount of NO in the stratosphere. Thus human activity could affect the stratospheric ozone layer. In the following year, Crutzen and (independently) Harold Johnston suggested that NO emissions from supersonic passenger aircraft, which would fly in the lower stratosphere, could also deplete the ozone layer. However, more recent analysis in 1995 by David W. Fahey, an atmospheric scientist at the National Oceanic and Atmospheric Administration, found that the drop in ozone would be from 1–2 percent if a fleet of 500 supersonic passenger aircraft were operated. This, Fahey expressed, would not be a showstopper for advanced supersonic passenger aircraft development.

Rowland–Molina hypothesis
In 1974 Frank Sherwood Rowland, Chemistry Professor at the University of California at Irvine, and his postdoctoral associate Mario J. Molina suggested that long-lived organic halogen compounds, such as CFCs, might behave in a similar fashion as Crutzen had proposed for nitrous oxide. James Lovelock had recently discovered, during a cruise in the South Atlantic in 1971, that almost all of the CFC compounds manufactured since their invention in 1930 were still present in the atmosphere. Molina and Rowland concluded that, like, the CFCs would reach the stratosphere where they would be dissociated by UV light, releasing chlorine atoms. A year earlier, Richard Stolarski and Ralph Cicerone at the University of Michigan had shown that Cl is even more efficient than NO at catalyzing the destruction of ozone. Similar conclusions were reached by Michael McElroy and Steven Wofsy at Harvard University. Neither group, however, had realized that CFCs were a potentially large source of stratospheric chlorine—instead, they had been investigating the possible effects of HCl emissions from the Space Shuttle, which are very much smaller.

The Rowland–Molina hypothesis was strongly disputed by representatives of the aerosol and halocarbon industries. The Chair of the Board of DuPont was quoted as saying that ozone depletion theory is "a science fiction tale ... a load of rubbish ... utter nonsense". Robert Abplanalp, the President of Precision Valve Corporation (and inventor of the first practical aerosol spray can valve), wrote to the Chancellor of UC Irvine to complain about Rowland's public statements. Nevertheless, within three years most of the basic assumptions made by Rowland and Molina were confirmed by laboratory measurements and by direct observation in the stratosphere. The concentrations of the source gases (CFCs and related compounds) and the chlorine reservoir species (HCl and ) were measured throughout the stratosphere, and demonstrated that CFCs were indeed the major source of stratospheric chlorine, and that nearly all of the CFCs emitted would eventually reach the stratosphere. Even more convincing was the measurement, by James G. Anderson and collaborators, of chlorine monoxide (ClO) in the stratosphere. ClO is produced by the reaction of Cl with ozone—its observation thus demonstrated that Cl radicals not only were present in the stratosphere but also were actually involved in destroying ozone. McElroy and Wofsy extended the work of Rowland and Molina by showing that bromine atoms were even more effective catalysts for ozone loss than chlorine atoms and argued that the brominated organic compounds known as halons, widely used in fire extinguishers, were a potentially large source of stratospheric bromine. In 1976 the United States National Academy of Sciences released a report concluding that the ozone depletion hypothesis was strongly supported by the scientific evidence. In response the United States, Canada and Norway banned the use of CFCs in aerosol spray cans in 1978. Early estimates were that, if CFC production continued at 1977 levels, the total atmospheric ozone would after a century or so reach a steady state, 15 to 18 percent below normal levels. By 1984, when better evidence on the speed of critical reactions was available, this estimate was changed to 5 to 9 percent steady-state depletion.

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

Antarctic ozone hole
The discovery of the Antarctic "ozone hole" by British Antarctic Survey scientists Farman, Gardiner and Shanklin (first reported in a paper in Nature in May 1985 ) came as a shock to the scientific community, because the observed decline in polar ozone was far larger than had been anticipated. Satellite measurements (TOMS onboard Nimbus 7) showing massive depletion of ozone around the south pole were becoming available at the same time. However, these were initially rejected as unreasonable by data quality control algorithms (they were filtered out as errors since the values were unexpectedly low); the ozone hole was detected only in satellite data when the raw data was reprocessed following evidence of ozone depletion in in situ observations. When the software was rerun without the flags, the ozone hole was seen as far back as 1976.

Susan Solomon, an atmospheric chemist at the National Oceanic and Atmospheric Administration (NOAA), proposed that chemical reactions on polar stratospheric clouds (PSCs) in the cold Antarctic stratosphere caused a massive, though localized and seasonal, increase in the amount of chlorine present in active, ozone-destroying forms. The polar stratospheric clouds in Antarctica are only formed at very low temperatures, as low as −80 °C, and early spring conditions. In such conditions the ice crystals of the cloud provide a suitable surface for conversion of unreactive chlorine compounds into reactive chlorine compounds, which can easily deplete ozone.

Moreover, the polar vortex formed over Antarctica is very tight and the reaction occurring on the surface of the cloud crystals is far different from when it occurs in atmosphere. These conditions have led to ozone hole formation in Antarctica. This hypothesis was decisively confirmed, first by laboratory measurements and subsequently by direct measurements, from the ground and from high-altitude airplanes, of very high concentrations of chlorine monoxide (ClO) in the Antarctic stratosphere.

Alternative hypotheses, which had attributed the ozone hole to variations in solar UV radiation or to changes in atmospheric circulation patterns, were also tested and shown to be untenable.

Meanwhile, analysis of ozone measurements from the worldwide network of ground-based Dobson spectrophotometers led an international panel to conclude that the ozone layer was in fact being depleted, at all latitudes outside of the tropics. These trends were confirmed by satellite measurements. As a consequence, the major halocarbon-producing nations agreed to phase out production of CFCs, halons, and related compounds, a process that was completed in 1996.

Since 1981 the United Nations Environment Programme, under the auspices of the World Meteorological Organization, has sponsored a series of technical reports on the Scientific Assessment of Ozone Depletion, based on satellite measurements. The 2007 report showed that the hole in the ozone layer was recovering and the smallest it had been for about a decade.

A 2010 report found, "Over the past decade, global ozone and ozone in the Arctic and Antarctic regions is no longer decreasing but is not yet increasing. The ozone layer outside the Polar regions is projected to recover to its pre-1980 levels some time before the middle of this century. In contrast, the springtime ozone hole over the Antarctic is expected to recover much later."

In 2012, NOAA and NASA reported "Warmer air temperatures high above the Antarctic led to the second smallest season ozone hole in 20 years averaging 17.9 million square kilometres. The hole reached its maximum size for the season on Sept 22, stretching to 21.2 million square kilometres." A gradual trend toward "healing" was reported in 2016 and then in 2017. It is reported that the recovery signal is evident even in the ozone loss saturation altitudes.

The hole in the Earth's ozone layer over the South Pole has affected atmospheric circulation in the Southern Hemisphere all the way to the equator. The ozone hole has influenced atmospheric circulation all the way to the tropics and increased rainfall at low, subtropical latitudes in the Southern Hemisphere.

Arctic ozone "mini-hole"
On March 3, 2005, the journal Nature published an article linking 2004's unusually large Arctic ozone hole to solar wind activity.

On March 15, 2011, a record ozone layer loss was observed, with about half of the ozone present over the Arctic having been destroyed. The change was attributed to increasingly cold winters in the Arctic stratosphere at an altitude of approximately 20 km, a change associated with global warming in a relationship that is still under investigation. By March 25, the ozone loss had become the largest compared to that observed in all previous winters with the possibility that it would become an ozone hole. This would require that the quantities of ozone to fall below 200 Dobson units, from the 250 recorded over central Siberia. It is predicted that the thinning layer would affect parts of Scandinavia and Eastern Europe on March 30–31.

On October 2, 2011, a study was published in the journal Nature, which said that between December 2010 and March 2011 up to 80 percent of the ozone in the atmosphere at about 20 km above the surface was destroyed. The level of ozone depletion was severe enough that scientists said it could be compared to the ozone hole that forms over Antarctica every winter. According to the study, "for the first time, sufficient loss occurred to reasonably be described as an Arctic ozone hole." The study analyzed data from the Aura and CALIPSO satellites, and determined that the larger-than-normal ozone loss was due to an unusually long period of cold weather in the Arctic, some 30 days more than typical, which allowed for more ozone-destroying chlorine compounds to be created. According to Lamont Poole, a co-author of the study, cloud and aerosol particles on which the chlorine compounds are found "were abundant in the Arctic until mid March 2011—much later than usual—with average amounts at some altitudes similar to those observed in the Antarctic, and dramatically larger than the near-zero values seen in March in most Arctic winters".

In 2013, researchers analyzed the data and found the 2010–2011 Arctic event did not reach the ozone depletion levels to classify as a true hole. A hole in the ozone is generally classified as 220 Dobson units or lower; the Arctic hole did not approach that low level. It has since been classified as a "mini-hole."

Following the ozone depletion in 1997 and 2011, a 90% drop in ozone was measured by weather balloons over the Arctic in March 2020, as they normally recorded 3.5 parts per million of ozone, compared to only around 0.3 parts per million lastly, due to the coldest temperatures ever recorded since 1979, and a strong polar vortex which allowed chemicals, including chlorine and bromine, to reduce ozone.

A rare hole, the result of unusually low temperatures in the atmosphere above the North Pole, was studied in 2020.

Tibet ozone hole
As winters that are colder are more affected, at times there is an ozone hole over Tibet. In 2006, a 2.5 million square kilometer ozone hole was detected over Tibet. Again in 2011, an ozone hole appeared over mountainous regions of Tibet, Xinjiang, Qinghai and the Hindu Kush, along with an unprecedented hole over the Arctic, though the Tibet one was far less intense than the ones over the Arctic or Antarctic.

Potential depletion by storm clouds
Research in 2012 showed that the same process that produces the ozone hole over Antarctica occurs over summer storm clouds in the United States, and thus may be destroying ozone there as well.

Ozone hole over tropics
Physicist Qing-Bin Lu, of the University of Waterloo, claimed to have discovered a large, all-season ozone hole in the lower stratosphere over the tropics in July 2022. However, other researchers in the field refuted this claim, stating that the research was riddled with "serious errors and unsubstantiated assertions." According to Dr Paul Young, a lead author of the 2022 WMO/UNEP Scientific Assessment of Ozone Depletion, "The author's identification of a 'tropical ozone hole' is down to him looking at percentage changes in ozone, rather than absolute changes, with the latter being much more relevant for damaging UV reaching the surface." Specifically, Lu's work defines "ozone hole" as "an area with O3 loss in percent larger than 25%, with respect to the undisturbed O3 value when there were no significant CFCs in the stratosphere (~ in the 1960s)" instead of the general definition of 220 Dobson units or lower. Dr Marta Abalos Alvarez has added "Ozone depletion in the tropics is nothing new and is mainly due to the acceleration of the Brewer-Dobson circulation."

Depletion caused by wildfire smoke
Analyzing the atmospheric impacts of the 2019–2020 Australian bushfire season, scientists led by MIT researcher Susan Solomon found the smoke destroyed 3–5% of ozone in affected areas of the Southern Hemisphere. Smoke particles absorb hydrogen chloride and act as a catalyst to create chlorine radicals that destroy ozone.

Ozone depletion and global warming
Among others, Robert Watson had a role in the science assessment and in the regulation efforts of ozone depletion and global warming. Prior to the 1980s, the EU, NASA, NAS, UNEP, WMO and the British government had dissenting scientific reports and Watson played a role in the process of unified assessments. Based on the experience with the ozone case, the IPCC started to work on a unified reporting and science assessment to reach a consensus to provide the IPCC Summary for Policymakers.

There are various areas of linkage between ozone depletion and global warming science:

 * The same radiative forcing that produces global warming is expected to cool the stratosphere. This cooling, in turn, is expected to produce a relative increase in ozone  depletion in polar areas and the frequency of ozone holes.
 * Conversely, ozone depletion represents a radiative forcing of the climate system. There are two opposing effects: Reduced ozone causes the stratosphere to absorb less solar radiation, thus cooling the stratosphere while warming the troposphere; the resulting colder stratosphere emits less long-wave radiation downward, thus cooling the troposphere. Overall, the cooling dominates; the IPCC concludes "observed stratospheric Ozone losses over the past two decades have caused a negative forcing of the surface-troposphere system" of about −0.15 ± 0.10 watts per square meter (W/m2).
 * One of the strongest predictions of the greenhouse effect is that the stratosphere will cool. Although this cooling has been observed, it is not trivial to separate the effects of changes in the concentration of greenhouse gases and ozone depletion since both will lead to cooling. However, this can be done by numerical stratospheric modeling. Results from the National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory show that above 20 km, the greenhouse gases dominate the cooling.
 * Ozone depleting chemicals are also often greenhouse gases. The increases in concentrations of these chemicals have produced 0.34 ± 0.03 W/m2 of radiative forcing, corresponding to about 14 percent of the total radiative forcing from increases in the concentrations of well-mixed greenhouse gases.
 * The long term modeling of the process, its measurement, study, design of theories and testing take decades to document, gain wide acceptance, and ultimately become the dominant paradigm. Several theories about the destruction of ozone were hypothesized in the 1980s, published in the late 1990s, and are now being investigated. Dr Drew Schindell, and Dr Paul Newman, NASA Goddard, proposed a theory in the late 1990s, using computational modeling methods to model ozone destruction, that accounted for 78 percent of the ozone destroyed. Further refinement of that model accounted for 89 percent of the ozone destroyed, but pushed back the estimated recovery of the ozone hole from 75 years to 150 years. (An important part of that model is the lack of stratospheric flight due to depletion of fossil fuels.)

In 2019, NASA reported that there was no significant relation between size of the ozone hole and climate change.

CFC weight
Since CFC molecules are heavier than air (nitrogen or oxygen), it is commonly believed that the CFC molecules cannot reach the stratosphere in significant amounts. However, atmospheric gases are not sorted by weight at these altitudes; the forces of wind can fully mix the gases in the atmosphere. Some of the heavier CFCs are not evenly distributed.

Percentage of man-made chlorine

Another misconception is that "it is generally accepted that natural sources of tropospheric chlorine are four to five times larger than man-made ones." While this statement is strictly true, tropospheric chlorine is irrelevant; it is stratospheric chlorine that affects ozone depletion. Chlorine from ocean spray is soluble and thus is washed by rainfall before it reaches the stratosphere. CFCs, in contrast, are insoluble and long-lived, allowing them to reach the stratosphere. In the lower atmosphere, there is much more chlorine from CFCs and related haloalkanes than there is in HCl from salt spray, and in the stratosphere halocarbons are dominant. Only methyl chloride, which is one of these halocarbons, has a mainly natural source, and it is responsible for about 20 percent of the chlorine in the stratosphere; the remaining 80 percent comes from manmade sources.

Very violent volcanic eruptions can inject HCl into the stratosphere, but researchers have shown that the contribution is not significant compared to that from CFCs. A similar erroneous assertion is that soluble halogen compounds from the volcanic plume of Mount Erebus on Ross Island, Antarctica are a major contributor to the Antarctic ozone hole.

Nevertheless, a 2015 study showed that the role of Mount Erebus volcano in the Antarctic ozone depletion was probably underestimated. Based on the NCEP/NCAR reanalysis data over the last 35 years and by using the NOAA HYSPLIT trajectory model, researchers showed that Erebus volcano gas emissions (including hydrogen chloride (HCl)) can reach the Antarctic stratosphere via high-latitude cyclones and then the polar vortex. Depending on Erebus volcano activity, the additional annual HCl mass entering the stratosphere from Erebus varies from 1.0 to 14.3 kt.

First observation
G.M.B. Dobson mentioned that when springtime ozone levels in the Antarctic over Halley Bay were first measured in 1956, he was surprised to find that they were ~320 DU, or about 150 DU below spring Arctic levels of ~450 DU. These were at that time the only known Antarctic ozone values available. What Dobson describes is essentially the baseline from which the ozone hole is measured: actual ozone hole values are in the 150–100 DU range.

The discrepancy between the Arctic and Antarctic noted by Dobson was primarily a matter of timing: during the Arctic spring, ozone levels rose smoothly, peaking in April, whereas in the Antarctic they stayed approximately constant during early spring, rising abruptly in November when the polar vortex broke down.

The behavior seen in the Antarctic ozone hole is different. Instead of staying constant, early springtime ozone levels drop from their already low winter values, by as much as 50 percent, and normal values are not reached again until December.

Location of hole
Some people thought that the ozone hole should be above the sources of CFCs. However, CFCs are well mixed globally in the troposphere and stratosphere. The reason for occurrence of the ozone hole above Antarctica is not because there are more CFCs concentrated but because the low temperatures help form polar stratospheric clouds. In fact, there are findings of significant and localized "ozone holes" above other parts of the Earth, such as above Central Asia.

Awareness campaigns
Public misconceptions and misunderstandings of complex issues like ozone depletion are common. The limited scientific knowledge of the public led to confusion about global warming or the perception of global warming as a subset of the "ozone hole". In the beginning, classical green NGOs refrained from using CFC depletion for campaigning, as they assumed the topic was too complicated. They became active much later, e.g. in Greenpeace's support for a CFC-free refrigerator produced by the former East German company VEB dkk Scharfenstein.

The metaphors used in the CFC discussion (ozone shield, ozone hole) are not "exact" in the scientific sense. The "ozone hole" is more of a depression, less "a hole in the windshield". The ozone does not disappear through the layer, nor is there a uniform "thinning" of the ozone layer. However, they resonated better with non-scientists and their concerns. The ozone hole was seen as a "hot issue" and imminent risk as laypeople feared severe personal consequences such as skin cancer, cataracts, damage to plants, and reduction of plankton populations in the ocean's photic zone. Not only on the policy level, ozone regulation compared to climate change fared much better in public opinion. Americans voluntarily switched away from aerosol sprays before legislation was enforced, while climate change failed to achieve comparable concern and public action. The sudden identification in 1985 that there was a substantial "hole" was widely reported in the press. The especially rapid ozone depletion in Antarctica had previously been dismissed as a measurement error. Scientific consensus was established after regulation.

While the Antarctic ozone hole has a relatively small effect on global ozone, the hole has generated a great deal of public interest because:
 * Many have worried that ozone holes might start appearing over other areas of the globe, though to date the only other large-scale depletion is a smaller ozone "dimple" observed during the Arctic spring around the North Pole. Ozone at middle latitudes has declined, but by a much smaller extent (a decrease of about 4–5 percent).
 * If stratospheric conditions become more severe (cooler temperatures, more clouds, more active chlorine), global ozone may decrease at a greater pace. Standard global warming theory predicts that the stratosphere will cool.
 * When the Antarctic ozone hole breaks up each year, the ozone-depleted air drifts into nearby regions. Decreases in the ozone level of up to 10 percent have been reported in New Zealand in the month following the breakup of the Antarctic ozone hole, with ultraviolet-B radiation intensities increasing by more than 15 percent since the 1970s.

World Ozone Day
In 1994, the United Nations General Assembly voted to designate September 16 as the International Day for the Preservation of the Ozone Layer, or "World Ozone Day". The designation commemorates the signing of the Montreal Protocol on that date in 1987.

title: Ozone depletion


text:
Talk:SQL

Wiki Education Foundation-supported course assignment
This article was the subject of a Wiki Education Foundation-supported course assignment, between 6 September 2020 and 6 December 2020. Further details are available on the course page. Student editor(s): ASCXX.

Above undated message substituted from Template:Dashboard.wikiedu.org assignment by PrimeBOT (talk) 08:30, 17 January 2022 (UTC)

External links modified
Hello fellow Wikipedians,

I have just modified 2 external links on SQL. Please take a moment to review my edit. If you have any questions, or need the bot to ignore the links, or the page altogether, please visit this simple FaQ for additional information. I made the following changes:
 * Added archive https://web.archive.org/web/20110628130925/http://iablog.sybase.com/paulley/2008/07/sql2008-now-an-approved-iso-international-standard/ to http://iablog.sybase.com/paulley/2008/07/sql2008-now-an-approved-iso-international-standard/
 * Added archive https://web.archive.org/web/20140706215458/http://www.sqlstream.com/stream-processing-with-sql/ to http://www.sqlstream.com/stream-processing-with-sql/

When you have finished reviewing my changes, you may follow the instructions on the template below to fix any issues with the URLs.

Cheers.— InternetArchiveBot  (Report bug) 16:47, 4 June 2017 (UTC)

DQL, DCL, DDL and DML
The link to data query language redirects to Query_language that is very general and very vague about SQL. The links for DCL, DDL and DML try to be a combination of a relevant language in general and SQL specifically, but perhaps that is relevant to each of those other pages. Sam Tomato (talk) 02:03, 21 March 2019 (UTC)

Turing completeness
The change in "&oldid=947475103" stated that " ... the most recent SQL standard actually *is* Turing complete, but many popular implementations are not ...". This raises the question of what we are speaking about: Reflects the article the SQL standard or the numerous implementations?--Kelti (talk) 08:34, 27 March 2020 (UTC)
 * Don't believe me too much, because I have been away from data bases almost 30 years ago. Being warned, here is what remember.
 * Recursive queries were possible since SQL3, that made possible to compute recursive functions. The other thing that I roughly remember is that recursive queries were explained with fixed point combinators in some data base book that was taken from the lambda calculus. Anyway recursion is necessary for Turing completeness.
 * Remember that relations are sets, and SQL was based in relation operations, i.e. SQL operators are based set operators, union, intersection, cross product, projections, selection and joins. The late can be expressed with cross product, projection and selection. Recursive iteration make sense if you repeat joins until you arrive to a fixed point, otherwise you may be surprised by non-termination. (Really bags or multi-sets, see a comment posted by other person below.)
 * I studied that almost 40 years ago, so it is not a recent feature. — Preceding unsigned comment added by 2806:106E:B:EA86:5571:765A:3CF:AAAF (talk) 03:23, 2 August 2022 (UTC)

Is SQL an open format?
In the second Infobox (file format), SQL is introduced as an Open (Yes value) format.

Although it is not a "free standard" and we should buy it, it costs about CHF 178.00 !!! Do we should change "Open format" to "No" by this criteria?

Thanks, Hooman Mallahzadeh (talk) 10:10, 31 May 2020 (UTC)
 * Seems fair to change this to "no" based on this criteria and from Open format, even SQL is a text-based format. See also List of open formats. + m t  21:51, 1 June 2020 (UTC)
 * Thanks for your reply, Here is the definition of open format:
 * OPEN FORMAT - We will say that a file format is open if the mode of presentation of its data is transparent and/or its specification is publicly available.
 * In here: SQL
 * and here: https://standards.iso.org/ittf/PubliclyAvailableStandards/index.html
 * Explicitly stated that it is "Publicly Available".
 * I changed it to "Yes (but not free)" value.
 * Thanks again, Hooman Mallahzadeh (talk) 05:23, 2 June 2020 (UTC)

HiveQL or HQL be considered as SQL ?
Curious to know whether HiveQL or the language similar to SQL but used in a Hadoop cluster environment and used via Hive interface can be considered as a type of SQL? --TheHiveBeeline (talk) 21:11, 2 September 2020 (UTC)

SQL does stand for Structured Query Language
I've added a source confirming that Donald D. Chamberlin intended SQL to stand for "Structured Query Language." Some people say it doesn't stand for anything; hopefully this will put that claim to rest. Qzekrom (she/her • talk) 16:56, 14 January 2021 (UTC)

Distributed SQL vs SQL
I do not think my description of Distributed SQL database architecture should be merged into this article despite the tag. If I were to create the term I would have called them Distributed Relational. While they do execute SQL in a distributed manner, the means it which they (Spanner, Cockroach, et al) achieve consistency and how they store data is more interesting than that they support SQL the language. In my opinion User:CommanderWaterford's addition of a merge tag seems to be a reaction to the name of the technology rather than the contents of the article. Reboot (talk) 19:58, 26 January 2021 (UTC)
 * , this doesn't appear to be gaining any traction. I've removed the tag from the Distributed SQL article, which I think was added in error before you finished writing the article. I see has since been reviewed as part of the NPP process, so I think we can draw a line under this now. Best Girth Summit  (blether)  18:03, 29 January 2021 (UTC)

Lists or multisets aka bags
The article says:
 * tables and query results are lists of rows; the same row may occur multiple times, and the order of rows can be employed in queries (e.g. in the LIMIT clause)

A relational database with no extensions, contains relations in the mathematical sense, i.e. A relation is a subset of the cross product of sets, thus a set. For efficiency reasons, those relations are indexed by keys. I don't remember the LIMIT clause, but in principle an SQL query may produce a bag as a result. Depending on what is queried, repeated rows may be meaningful or not. Those tables with repeated rows, i.e. bags also called multisets, are not part of the database relations, they are inferred. A relational database should be normalized to model the world it represents. Having bags instead of sets in the database, could make normalization not possible, because the normal forms have smaller relations from which queries infer either relations which may be projected in bags. Those bags in actual queries are ordered for efficiency, not logical, reason. On the other hand, lists are successions, the order is meaningful. $$[1,2,3]\neq [1,3,2]$$ while $$\{1,2,3\}=\{1,3,2\}$$ and $$\{1,2,3\}\cup\{1\}=\{1,2,3\}$$ while $$\{\!\!\{ 1,2,3\}\!\!\} \uplus\{\!\!\{ 1\}\!\!\}=\{\!\!\{ 1,1,2,3\}\!\!\}\neq\{\!\!\{ 1,2,3\}\!\!\}$$. So that part of the article should say: query results are either sets or multisets, where the same row may occur multiple times in multisets. if everybody agree.

Can someone say more about time travelling? How Datalog (by 1984) has influenced SQL (by 1974)?
In the entry "Influenced by" say "Datalog", a posterior query language based in predicate calculus written almost 10 years after SQL. Datalog is a kind of Prolog for relational data bases. It has recursion. On the other side SQL is based in relational calculus, where recursion is achieved via something related with the fixed point combinators in lambda calculus. Recursion was added to SQL3 if my memory doesn't fail me. Because it was needed to make the relational calculus equivalent to predicate calculus, a theoretical requirement. Both, SQL and Datalog, have a solid theoretical foundation, I could say both are discovered languages as opposed to invented languages coming from someone inspiration. It is a case of convergence in their development not of inspiration.

9075-16:2023 Part 16 - SQL/PGQ
Kelti (talk) 07:09, 19 May 2023 (UTC)
 * 1) As of spring 2023 this part is only a working draft, hence not part of the standard.
 * 2) The author says that JSON is introduced as a new data type - in opposition to 'JSON as a character string'. Unfortunatelly I don't have access to the working draft. Can the author publish a link to it?
 * 3) Please explain the stated correlation between 'Property Graph Queries' on the one hand and 'data type JSON' on the other hand.
 * 4) The text 'SQL/MDA' seems to be a copy/past error?

SQL pronunciation clarification tag
What needs cleared up by the tag you placed? - AquilaFasciata (talk | contribs) 16:55, 13 April 2024 (UTC)

 * I don’t understand what “often… for historical reasons” means Zanahary (talk) 17:01, 13 April 2024 (UTC)
 * Because it was historically called sequel? What do you interpret it as? - AquilaFasciata (talk | contribs) 17:11, 13 April 2024 (UTC)
 * I interpreted it to mean that it is presently referred to as sequel only in the context of historical discussion. Does it mean that it used to be called sequel but is no longer? Zanahary (talk) 21:36, 13 April 2024 (UTC)
 * Ahh, gotcha; the latter example is correct but I can see the confusion. - AquilaFasciata (talk | contribs) 14:28, 18 April 2024 (UTC)
 * It was historically named SEQUEL, but that doesn't mean it is true that it is historically pronounced "sequel." It is very common today, especially in the SQL Server community, to pronounce it "sequel." I think both should be given as valid pronunciations without the "historically" qualifier. WanderingDev (talk) 16:24, 11 July 2024 (UTC)

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Talk:SQL


text:
Sound effect

A sound effect (or audio effect) is an artificially created or enhanced sound, or sound process used to emphasize artistic or other content of films, television shows, live performance, animation, video games, music, or other media.

In motion picture and television production, a sound effect is a sound recorded and presented to make a specific storytelling or creative point without the use of dialogue or music. Traditionally, in the twentieth century, they were created with Foley. The term often refers to a process applied to a recording, without necessarily referring to the recording itself. In professional motion picture and television production, dialogue, music, and sound effects recordings are treated as separate elements. Dialogue and music recordings are never referred to as sound effects, even though the processes applied to such as reverberation or flanging effects, often are called sound effects.

This area and sound design have been slowly merged since the late-twentieth century.

History
The term sound effect dates back to the early days of radio. In its Year Book 1931 the BBC published a major article about "The Use of Sound Effects". It considers sound effects deeply linked with broadcasting and states: "It would be a great mistake to think of them as anologous to punctuation marks and accents in print. They should never be inserted into a program already existing. The author of a broadcast play or broadcast construction ought to have used Sound Effects as bricks with which to build, treating them as of equal value with speech and music." It lists six "totally different primary genres of Sound Effect":

 * Realistic, confirmatory effect
 * Realistic, evocative effect
 * Symbolic, evocative effect
 * Conventionalised effect
 * Impressionistic effect
 * Music as an effect

According to the author, "It is axiomatic that every Sound Effect, to whatever category it belongs, must register in the listener's mind instantaneously. If it fails to do so its presence could not be justified."

Film
In the context of motion pictures and television, sound effects refers to an entire hierarchy of sound elements, whose production encompasses many different disciplines, including:
 * Hard sound effects are common sounds that appear on screen, such as door alarms, weapons firing, and cars driving by.
 * Background (or BG) sound effects are sounds that do not explicitly synchronize with the picture, but indicate setting to the audience, such as forest sounds, the buzzing of fluorescent lights, and car interiors. The sound of people talking in the background is also considered a BG, but only if the speaker is unintelligible and the language is unrecognizable (this is known as walla). These background noises are also called ambience or atmos ("atmosphere").
 * Foley sound effects are sounds that synchronize on screen, and require the expertise of a Foley artist to record properly. Footsteps, the movement of hand props (e.g., a tea cup and saucer), and the rustling of cloth are common Foley units.
 * Design sound effects are sounds that do not normally occur in nature, or are impossible to record in nature. These sounds are used to suggest futuristic technology in a science fiction film, or are used in a musical fashion to create an emotional mood.

Each of these sound effect categories is specialized, with sound editors known as specialists in an area of sound effects (e.g. a Car cutter or Guns cutter).

Foley is another method of adding sound effects. Foley is more of a technique for creating sound effects than a type of sound effect, but it is often used for creating the incidental real-world sounds that are very specific to what is going on onscreen, such as footsteps. With this technique, the action onscreen is essentially recreated to try to match it as closely as possible. If done correctly it is very hard for audiences to tell what sounds were added and what sounds were originally recorded (location sound).

In the early days of film and radio, Foley artists would add sounds in real time or pre-recorded sound effects would be played back from analog discs in real time (while watching the picture). Today, with effects held in digital format, it is easy to create any required sequence to be played in any desired timeline.

In the days of silent film, sound effects were added by the operator of a theater organ or photoplayer, both of which also supplied the soundtrack of the film. Theater organ sound effects are usually electric or electro-pneumatic, and activated by a button pressed with the hand or foot.

Photoplayer operators activate sound effects either by flipping switches on the machine or pulling cow-tail pull-strings, which hang above. Sounds like bells and drums are made mechanically, sirens and horns electronically. Due to its smaller size, a photoplayer usually has fewer special effects than a theater organ, or less complex ones.

Notable early uses of sound effects in film

 * 1897: A Canadian filmmaker premiered a silent film in which a blacksmith was placed outside of the audience view and made sounds to coincide with the visuals of a blacksmith in the film.
 * Kelly Gang (1906): An Australian premiere of this film was accompanied by a symphony to add sound effects to the film as viewers watched it.
 * 1914: Britain began producing sound effect devices that people could buy to play different sound effects for films at home.
 * Blackmail (1929): Uses a woman's voice separate from the main actress on screen for screaming scenes.
 * M (1931): Uses several street sounds such as car horns and the sounds of a ball to increase tension within the film.
 * Spellbound (1945) Uses a Theremin to create one of the first instances of electronically produced sound effects.

Video games
The principles involved with modern video game sound effects (since the introduction of sample playback) are essentially the same as those of motion pictures. Typically a game project requires two jobs to be completed: sounds must be recorded or selected from a library and a sound engine must be programmed so that those sounds can be incorporated into the game's interactive environment.

In earlier computers and video game systems, sound effects were typically produced using sound synthesis. In modern systems, the increases in storage capacity and playback quality has allowed sampled sound to be used. The modern systems also frequently utilize positional audio, often with hardware acceleration, and real-time audio post-processing, which can also be tied to the 3D graphics development. Based on the internal state of the game, multiple different calculations can be made. This will allow for, for example, realistic sound dampening, echoes and Doppler effect.

Historically the simplicity of game environments reduced the required number of sounds needed, and thus only one or two people were directly responsible for the sound recording and design. As the video game business has grown and computer sound reproduction quality has increased, however, the team of sound designers dedicated to game projects has likewise grown and the demands placed on them may now approach those of mid-budget motion pictures.

Some pieces of music use sound effects that are made by a musical instrument or by other means. An early example is the 18th century Toy Symphony. Richard Wagner in the opera Das Rheingold (1869) lets a choir of anvils introduce the scene of the dwarfs who have to work in the mines, similar to the introduction of the dwarfs in the 1937 Disney movie Snow White. Klaus Doldingers soundtrack for the 1981 movie Das Boot includes a title score with a sonar sound to reflect the U-boat setting. John Barry integrated into the title song of Moonraker (1979) a sound representing the beep of a Sputnik like satellite.

Sound effects in theater
Gao, Jianliang, Zhao, Yuezhe, and Pan, Lili explained how sound absorption in the stage area influences the acoustics within an opera house auditorium. Their research, using computer models and scale experiments, revealed that sound absorption significantly affects sound clarity and the time it takes for sound to fade, but not the volume. More absorption led to clearer sounds but quicker fades, showing the intricate dance between stage and auditorium acoustics.

In his book "Sound: A Reader in Theatre Practice," Brown effectively connects the dots between theory and practice in the world of theater sound. He presents an engaging look into how sound design in theater has evolved, blending historical insights with current philosophical thoughts. Brown argues that the immersive nature of theater sound goes beyond traditional analysis, providing fresh perspectives on how sound interacts with societal contexts.

Brown offers a fresh look at Ovadija's exploration of sound in theater, questioning the traditional focus on visuals over audio. He points out the significant, yet often overlooked, role of sound in shaping theater's impact and experience. Brown pushes for a broader appreciation of sound's essence in theater, beyond just supporting visuals, to acknowledge its deep influence on storytelling and audience immersion.

Rost explores the criteria for 'good sound' in theater through handbooks and prioritization as guiding principles. These criteria not only dictate the creation and selection of sounds to complement the narrative and mood but also aim to maintain audience focus. Rost's analysis reveals underlying hierarchies in sound selection and emphasizes the need for further research into the historical and practical aspects of theater sound.

Recording

The most realistic sound effects may originate from original sources; the closest sound to machine-gun fire could be an original recording of actual machine guns.

Despite this, real life and actual practice do not always coincide with theory. When recordings of real life do not sound realistic on playback, Foley and effects are used to create more convincing sounds. For example, the realistic sound of bacon frying can be the crumpling of cellophane, while rain may be recorded as salt falling on a piece of tin foil.

Less realistic sound effects are digitally synthesized or sampled and sequenced (the same recording played repeatedly using a sequencer). When the producer or content creator demands high-fidelity sound effects, the sound editor usually must augment his available library with new sound effects recorded in the field.

When the required sound effect is of a small subject, such as scissors cutting, cloth ripping, or footsteps, the sound effect is best recorded in a studio, under controlled conditions in a process known as Foley. Many sound effects cannot be recorded in a studio, such as explosions, gunfire, and automobile or aircraft maneuvers. These effects must be recorded by a professional audio engineer.

When such big sounds are required, the recordist will begin contacting professionals or technicians in the same way a producer may arrange a crew; if the recordist needs an explosion, they may contact a demolition company to see if any buildings are scheduled to be destroyed with explosives in the near future. If the recordist requires a volley of cannon fire, they may contact historical re-enactors or gun enthusiasts.

Depending on the effect, recordists may use several DAT, hard disk, or Nagra recorders and a large number of microphones. During a cannon- and musket-fire recording session for the 2003 film The Alamo, conducted by Jon Johnson and Charles Maynes, two to three DAT machines were used. One machine was stationed near the cannon itself, so it could record the actual firing. Another was stationed several hundred yards away, below the trajectory of the ball, to record the sound of the cannonball passing by. When the crew recorded musket fire, a set of microphones were arrayed close to the target (in this case a swine carcass) to record the musket-ball impacts.

A counter-example is the common technique for recording an automobile. For recording onboard car sounds (which include the car interiors), a three-microphone technique is common. Two microphones record the engine directly: one is taped to the underside of the hood, near the engine block. The second microphone is covered in a wind screen and tightly attached to the rear bumper, within an inch or so of the tailpipe. The third microphone, which is often a stereo microphone, is stationed inside the car to get the car interior.

Having all of these tracks at once gives a sound designer or audio engineer a great deal of control over how they want the car to sound. In order to make the car more ominous or low, they can mix in more of the tailpipe recording; if they want the car to sound like it is running full throttle, they can mix in more of the engine recording and reduce the interior perspective. In cartoons, a pencil being dragged down a washboard may be used to simulate the sound of a sputtering engine.

What is considered today to be the first recorded sound effect was of Big Ben striking 10:30, 10:45, and 11:00. It was recorded on a brown wax cylinder by technicians at Edison House in London on July 16, 1890. This recording is currently in the public domain.

Processing effects
As the car example demonstrates, the ability to make multiple simultaneous recordings of the same subject—through the use of several DAT or multitrack recorders—has made sound recording into a sophisticated craft. The sound effect can be shaped by the sound editor or sound designer, not just for realism, but for emotional effect.

Once the sound effects are recorded or captured, they are usually loaded into a computer integrated with an audio non-linear editing system. This allows a sound editor or sound designer to heavily manipulate a sound to meet his or her needs.

The most common sound design tool is the use of layering to create a new, interesting sound out of two or three old, average sounds. For example, the sound of a bullet impact into a pig carcass may be mixed with the sound of a melon being gouged to add to the stickiness or gore of the effect. If the effect is featured in a close-up, the designer may also add an impact sweetener from his or her library. The sweetener may simply be the sound of a hammer pounding hardwood, equalized so that only the low-end can be heard. The low end gives the three sounds together added weight, so that the audience actually feels the weight of the bullet hit the victim.

If the victim is the villain, and his death is climactic, the sound designer may add reverb to the impact, in order to enhance the dramatic beat. And then, as the victim falls over in slow motion, the sound editor may add the sound of a broom whooshing by a microphone, pitch-shifted down and time-expanded to further emphasize the death. If the film is science-fiction, the designer may phaser the whoosh to give it a more sci-fi feel. (For a list of many sound effects processes available to a sound designer, see the bottom of this article.)

Aesthetics
When creating sound effects for films, sound recordists and editors do not generally concern themselves with the verisimilitude or accuracy of the sounds they present. The sound of a bullet entering a person from a close distance may sound nothing like the sound designed in the above example, but since very few people are aware of how such a thing actually sounds, the job of designing the effect is mainly an issue of creating a conjectural sound which feeds the audience's expectations while still suspending disbelief. Sci-fi and fantasy genres can be more forgiving in terms of audience expectations; the listener will not be caught off guard as much by unusual sound effects. In contrast, when creating sound effects for historical accuracy and realism, the listener likely had a lifetime of exposure to some of these sounds and so there are expectations of what they should sound like.

In the previous example, the phased 'whoosh' of the victim's fall has no analog in real-life experience, but it is emotionally immediate. If a sound editor uses such sounds in the context of emotional climax or a character's subjective experience, they can add to the drama of a situation in a way visuals simply cannot. If a visual effects artist were to do something similar to the 'whooshing fall' example, it would probably look ridiculous or at least excessively melodramatic.

The conjectural sound principle applies even to happenstance sounds, such as tires squealing, doorknobs turning or people walking. If the sound editor wants to communicate that a driver is in a hurry to leave, they will cut the sound of tires squealing when the car accelerates from a stop; even if the car is on a dirt road, the effect will work if the audience is dramatically engaged. If a character is afraid of someone on the other side of a door, the turning of the doorknob can take a second or more, and the mechanism of the knob can possess dozens of clicking parts. A skillful Foley artist can make someone walking calmly across the screen seem terrified simply by giving the actor a different gait.

Techniques

In music and film/television production, some typical effects used in recording and amplified performances are:
 * echo – to simulate the effect of reverberation in a large hall or cavern, one or several delayed signals are added to the original signal. To be perceived as echo, the delay has to be of order 35 milliseconds or above. Short of actually playing a sound in the desired environment, the effect of echo can be implemented using either analog or digital methods.  Analog echo effects are implemented using tape delay or delay lines.
 * flanger – to create an unusual sound, a delayed signal is added to the original signal with a continuously variable delay (usually smaller than 10 ms). This effect is now done electronically using DSP, but originally the effect was created by playing the same recording on two synchronized tape players, and then mixing the signals together.  As long as the machines were synchronized, the mix would sound more-or-less normal, but if the operator placed his finger on the flange of one of the players (hence flanger), that machine would slow down and its signal would fall out-of-phase with its partner, producing a phasing effect.  Once the operator took his finger off, the player would speed up until its tachometer was back in phase with the master, and as this happened, the phasing effect would appear to slide up the frequency spectrum.  This phasing up and down the register can be performed rhythmically.
 * phaser – another way of creating an unusual sound; the signal is split, a portion is filtered with an all-pass filter to produce a phase-shift, and then the unfiltered and filtered signals are mixed. The phaser effect was originally used as a simpler implementation of the flanger effect since delays were difficult to implement with analog equipment. Phasers are often used to give a synthesized or electronic effect to natural sounds, such as human speech.  The voice of C-3PO from Star Wars was created by taking the actor's voice and treating it with a phaser.
 * chorus – a delayed signal is added to the original signal with a constant delay. The delay has to be short in order not to be perceived as echo, but above 5 ms to be audible. If the delay is too short, it will destructively interfere with the un-delayed signal and create a flanging effect.  Often, the delayed signals will be slightly pitch shifted to more realistically convey the effect of multiple voices.
 * Fastoverdrive.oggPitchshift12semitones.oggequalization – different frequency bands are attenuated or boosted to produce desired spectral characteristics. Moderate use of equalization (often abbreviated as EQ) can be used to fine-tune the tone quality of a recording; extreme use of equalization, such as heavily cutting a certain frequency can create more unusual effects.
 * filtering – Equalization is a form of filtering. In the general sense, frequency ranges can be emphasized or attenuated using low-pass, high-pass, band-pass or band-stop filters.  Band-pass filtering of voice can simulate the effect of a telephone because telephones use band-pass filters.
 * Modulation0.oggoverdrive effects such as the use of a fuzz box can be used to produce distorted sounds, such as for imitating robotic voices or to simulate distorted radiotelephone traffic (e.g., the radio chatter between starfighter pilots in the science fiction film Star Wars). The most basic overdrive effect involves clipping the signal when its absolute value exceeds a certain threshold.
 * Compression0.oggpitch shift – similar to pitch correction, this effect shifts a signal up or down in pitch. For example, a signal may be shifted an octave up or down. This is usually applied to the entire signal, and not to each note separately.  One application of pitch shifting is pitch correction.  Here a musical signal is tuned to the correct pitch using digital signal processing techniques.  This effect is ubiquitous in karaoke machines and is often used to assist pop singers who sing out of tune.  It is also used intentionally for aesthetic effect in such pop songs as Cher's "Believe" and Madonna's "Die Another Day".Echoreverse75mil.ogg
 * time stretching – the opposite of pitch shift, that is, the process of changing the speed of an audio signal without affecting its pitch.
 * resonators – emphasize harmonic frequency content on specified frequencies.
 * robotic voice effects are used to make an actor's voice sound like a synthesized human voice.
 * synthesizer – generate artificially almost any sound by either imitating natural sounds or creating completely new sounds.
 * modulation – to change the frequency or amplitude of a carrier signal in relation to a predefined signal. Ring modulation, also known as amplitude modulation, is an effect made famous by Doctor Who's Daleks and commonly used throughout sci-fi.
 * compression – the reduction of the dynamic range of a sound to avoid unintentional fluctuation in the dynamics. Level compression is not to be confused with audio data compression, where the amount of data is reduced without affecting the amplitude of the sound it represents.
 * 3D audio effects – place sounds outside the stereo basis
 * sound on sound – to record over a recording without erasing. Originally accomplished by disabling the tape-erase magnet. This allowed an artist to make parts of the work sound like a duet.
 * reverse echo – a swelling effect created by reversing an audio signal and recording echo and/or delay whilst the signal runs in reverse. When played back forward the last echos are heard before the effected sound creating a rush-like swell preceding and during playback.

title: Sound effect