What are the effects of ozone depletion?

Answer 1

Even minor problems of ozone depletion can have major effects. Every time even a small amount of the ozone layer is lost, more ultraviolet light from the sun can reach the Earth.

Every time 1% of the ozone layer is depleted, 2% more UV-B is able to reach the surface of the planet. UV-B increase is one of the most harmful consequences of ozone depletion because it can cause skin cancer.

The increased cancer levels caused by exposure to this ultraviolet light could be enormous. The EPA estimates that 60 million Americans born by the year 2075 will get skin cancer because of ozone depletion. About one million of these people will die.

In addition to cancer, some research shows that a decreased ozone layer will increase rates of malaria and other Infectious Diseases. According to the EPA, 17 million more cases of cataracts can also be expected.

The environment will also be negatively affected by ozone depletion. The life cycles of plants will change, disrupting the food chain. Effects on animals will also be severe, and are very difficult to foresee.

Oceans will be hit hard as well. The most basic microscopic organisms such as plankton may not be able to survive. If that happened, it would mean that all of the other animals that are above plankton in the food chain would also die out. Other ecosystems such as forests and deserts will also be harmed.

The planet's climate could also be affected by depletion of the ozone layer. Wind patterns could change, resulting in climatic changes throughout the world.

Environmental Effects of Ozone Depletion

Effects of increased ultraviolet radiation on biological systems had been investigated even before the ozone-depletion issue came to prominence. Effects such as alterations in tropospheric chemistry and potential global warming due to chlorofluorocarbons (CFCs) did not present themselves, however, until depletion and the rise in CFC levels was thought to be possible. Moan (1991) offers a brief overview of such environmental ramifications of ozone depletion in "Ozone Holes and Biological Consequences."

Several possible Ultraviolet-B Effects on Terrestrial Plants have been investigated, including reduction in yield, alteration in species competition, decrease in photosynthetic activity, susceptibility to disease, and changes in plant structure and pigmentation. Studies carried out on loblolly pine indicate retardation of growth and photosynthesis resulting from enhanced levels of ultraviolet-B (UV-B). Similar effects, including yield reduction, were found in certain rice cultivars. In field study experiments, soybean harvests showed decreases under a simulated 25 percent ozone reduction. Existing microclimatic conditions, such as drought and mineral deficiency, can reduce sensitivity to UV-B, however.

Most field studies of Ultraviolet-B Effects on Aquatic Ecosystems have taken place in the Antarctic region, due to the presence of the ozone hole during the polar springtime, and have focused on the effects on phytoplankton, the primary producers at the base of the Antarctic food web. Phytoplankton are sensitive to increased UV-B doses, resulting in decreased mobility and orientation, and changes in photosynthetic and enzymatic reactions. These effects may lead to reduction in primary productivity, which indirectly affects higher trophic levels. Because humans and other consumers are dependent on higher species such as fish and shrimp, populations outside the local ecosystem are potentially at risk. Prokaryotic microorganisms responsible for nitrogen fixation are also susceptible to UV-B, which could result in changes in the biogeochemical cycling of nitrogen, potentially leading to detrimental effects on plant growth. Other possible indirect effects of higher UV-B stress are decreased planktonic production of dimethylsulfide (DMS), an important source of sulfur and cloud condensation nuclei to the atmosphere, and reduced uptake of CO2 by the oceans.

Global climate may also be influenced by Changes in Tropospheric Chemistry. Studies have suggested that the recent slowdown in the rate of increase of methane levels in the atmosphere may be due, in part, to increased UV-B irradiance in the lower atmosphere. Photochemical smog production in urban areas would also increase under enhanced UV-B levels, reducing air quality and leading to possible effects on human health and agriculture.

Chlorofluorocarbons and potential replacement substances also enter into the global climate picture because of their radiative characteristics. Some of these compounds absorb longwave infrared radiation from the Earth's surface that no other substances absorb, thus adding to the greenhouse effect. The Global Warming Potential of Chlorofluorocarbons and Their Replacements has been evaluated relative to carbon dioxide warming potential. This factor is significant when evaluating whether alternatives to CFCs are suitable for distribution in widespread applications on a worldwide basis.

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The ozone hole and its causes

Ozone hole in North America during 1984 (abnormally warm reducing ozone depletion) and 1997 (abnormally cold resulting in increased seasonal depletion). Source: NASAhttp://www.answers.com/topic/ozone-depletion#cite_note-14

The Antarctic ozone hole is an area of the Antarctic stratosphere in which the recent ozone levels have dropped to as low as 33% 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 http://www.answers.com/topic/polar-vortex, over 50% of the lower stratospheric ozone is destroyed during the Antarctic spring.http://www.answers.com/topic/ozone-depletion#cite_note-15

As explained above, the primary cause of ozone depletion was 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 dramatically enhanced in the presence of http://www.answers.com/topic/nacreous-cloud (PSCs).http://www.answers.com/topic/ozone-depletion#cite_note-16 By 2008, the primary cause of ozone depletion was taken over by nitrous oxide (N2O), and was expected to remain the largest throughout the 21st century. http://www.answers.com/topic/ozone-depletion#cite_note-17

These polar stratospheric clouds form during winter, in the extreme cold. Polar winters are dark, consisting of 3 months without solar radiation (sunlight). Not only lack of sunlight contributes to a decrease in temperature but also the http://www.answers.com/topic/polar-vortex traps and chills air. Temperatures hover around or below -80 °C. These low temperatures form cloud particles and are composed of either nitric acid (Type I PSC) or ice (Type II PSC). Both types provide surfaces for chemical reactions that lead to ozone destruction.[citation needed]

The http://www.answers.com/topic/photochemistry processes involved are complex but well understood. The key observation is that, ordinarily, most of the chlorine in the stratosphere resides in stable "reservoir" compounds, primarily hydrochloric acid (HCl) and chlorine nitrate (ClONO2). During the Antarctic winter and spring, however, reactions on the surface of the polar stratospheric cloud particles convert these "reservoir" compounds into reactive free radicals (Cl and ClO). The clouds can also remove NO2 from the atmosphere by converting it to nitric acid, which prevents the newly formed ClO from being converted back into ClONO2.

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 the chemical reactions. During the spring, however, the sun comes out, providing energy to drive photochemical reactions, and melt the polar stratospheric clouds, releasing the trapped compounds.[citation needed]

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.[citation needed]

Warming temperatures near the end of spring break up the vortex around mid-December. As warm, ozone-rich air flows in from lower latitudes, the PSCs are destroyed, the ozone depletion process shuts down, and the ozone hole closes.[citation needed]

Interest in ozone layer depletion

While the effect of the Antarctic ozone hole in decreasing the global ozone is relatively small, estimated at about 4% per decade, the hole has generated a great deal of interest because:

• The decrease in the ozone layer was predicted in the early 1980s to be roughly 7% over a 60 year period.[citation needed]
• The sudden recognition 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.[citation needed]
• Many[citation needed] were worried that ozone holes might start to appear over other areas of the globe but to date the only other large-scale depletion is a smaller ozone "dimple" observed during the Arctic spring over the North Pole. Ozone at middle latitudes has declined, but by a much smaller extent (about 4-5% decrease).
• If the conditions became more severe (cooler stratospheric temperatures, more stratospheric clouds, more active chlorine), then global ozone may decrease at a much greater pace. Standard http://www.answers.com/topic/global-warming theory predicts that the stratosphere will cool.http://www.answers.com/topic/ozone-depletion#cite_note-18
• When the Antarctic ozone hole breaks up, the ozone-depleted air drifts out into nearby areas. Decreases in the ozone level of up to 10% have been reported in New Zealand in the month following the break-up of the Antarctic ozone hole.

Consequences of ozone layer depletion

Since the ozone layer absorbs http://www.answers.com/topic/ultraviolet ultraviolet light from the Sun, ozone layer depletion is expected to increase surface UVB levels, which could lead to damage, including increases in http://www.answers.com/topic/skin-cancer. This was the reason for the Montreal Protocol. Although decreases in stratospheric ozone are well-tied to CFCs and there are good theoretical reasons to believe that decreases in ozone will lead to increases in surface UVB, there is no direct observational evidence linking ozone depletion to higher incidence of skin cancer in human beings. This is partly due to the fact that http://www.answers.com/topic/ultraviolet, which has also been implicated in some forms of skin cancer, is not absorbed by ozone, and it is nearly impossible to control statistics for lifestyle changes in the populace.

Increased UV

Ozone, while a minority constituent in the Earth's atmosphere, is responsible for most of the absorption of UVB radiation. The amount of UVB radiation that penetrates through the ozone layer http://www.answers.com/topic/exponential-decay with the slant-path thickness/density of the layer. Correspondingly, a decrease in atmospheric ozone is expected to give rise to significantly increased levels of UVB near the surface.

Increases in surface http://www.answers.com/topic/ultraviolet due to the ozone hole can be partially inferred by http://www.answers.com/topic/radiative-transfer model calculations, but cannot be calculated from direct measurements because of the lack of reliable historical (pre-ozone-hole) surface UV data, although more recent surface UV observation measurement programmes exist (e.g. at Lauder, http://www.answers.com/topic/new-zealand).http://www.answers.com/topic/ozone-depletion#cite_note-19

Because it is this same UV radiation that creates ozone in the ozone layer from O2 (regular oxygen) in the first place, a reduction in stratospheric ozone would actually tend to increase photochemical production of ozone at lower levels (in the http://www.answers.com/topic/troposphere), although the overall observed trends in total column ozone still show a decrease, largely because ozone produced lower down has a naturally shorter photochemical lifetime, so it is destroyed before the concentrations could reach a level which would compensate for the ozone reduction higher up.[citation needed]

Biological effects of increased UV and microwave radiation from a depleted ozone layer

The main public concern regarding the ozone hole has been the effects of surface UV 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. Were the high levels of depletion seen in the ozone hole ever 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 reach southern parts of http://www.answers.com/topic/australia and http://www.answers.com/topic/new-zealand, environmentalists have been concerned that the increase in surface UV could be significant.[citation needed]

Effects of ozone layer depletion on humans

http://www.answers.com/topic/ultraviolet (the higher energy UV radiation absorbed by ozone) is generally accepted to be a contributory factor to http://www.answers.com/topic/skin-cancer. In addition, increased surface UV leads to increased tropospheric ozone, which is a health risk to humans.[citation needed] The increased surface UV also represents an increase in the http://www.answers.com/topic/vitamin-d synthetic capacity of the sunlight.http://www.answers.com/topic/ozone-depletion#cite_note-20

The cancer preventive effects of vitamin D represent a possible beneficial effect of ozone depletion.[8][9] In terms of health costs, the possible benefits of increased UV irradiance may outweigh the burden. [10]

1. Basal and Squamous Cell Carcinomas -- The most common forms of skin cancer in humans, http://www.answers.com/topic/basal-cell-carcinoma and http://www.answers.com/topic/squamous-cell-carcinoma cell carcinomas, have been strongly linked to UVB exposure. The mechanism by which UVB induces these cancers is well understood - absorption of UVB radiation causes the pyrimidine bases in the DNA molecule to form http://www.answers.com/topic/dimer, 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 a one percent decrease in stratospheric ozone would increase the incidence of these cancers by 2%.http://www.answers.com/topic/ozone-depletion#cite_note-gcrio.org-consequnces-21

2. Malignant Melanoma -- Another form of skin cancer, malignant melanoma, is much less common but far more dangerous, being lethal in about 15% - 20% of the cases diagnosed. The relationship between malignant melanoma and ultraviolet exposure is not yet well understood, but it appears that both UVB and UVA are involved. Experiments on fish suggest that 90 to 95% of malignant melanomas may be due to UVA and visible radiationhttp://www.answers.com/topic/ozone-depletion#cite_note-22 whereas experiments on opossums suggest a larger role for UVB.http://www.answers.com/topic/ozone-depletion#cite_note-gcrio.org-consequnces-21 Because of this uncertainty, it is difficult to estimate the impact of ozone depletion on melanoma incidence. One study showed that a 10% increase in UVB radiation was associated with a 19% increase in melanomas for men and 16% for women.http://www.answers.com/topic/ozone-depletion#cite_note-23 A study of people in http://www.answers.com/topic/punta-arenas-chile-1, at the southern tip of http://www.answers.com/topic/chile, showed a 56% increase in melanoma and a 46% increase in nonmelanoma skin cancer over a period of seven years, along with decreased ozone and increased UVB levels.http://www.answers.com/topic/ozone-depletion#cite_note-24

3. Cortical Cataracts -- Studies are suggestive of an association between ocular cortical http://www.answers.com/topic/cataract 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 http://www.answers.com/topic/ozone-depletion#cite_note-25. In this highly exposed group of predominantly white males, the evidence linking cortical opacities to sunlight exposure was the strongest to date. However, subsequent data from a population-based study in Beaver Dam, WI suggested the risk may be confined to men. In the Beaver Dam study, the exposures among women were lower than exposures among men, and no association was seen.http://www.answers.com/topic/ozone-depletion#cite_note-26 Moreover, there were no data linking sunlight exposure to risk of cataract in African Americans, although other eye diseases have different prevalences among the different racial groups, and cortical opacity appears to be higher in African Americans compared with whites.http://www.answers.com/topic/ozone-depletion#cite_note-27http://www.answers.com/topic/ozone-depletion#cite_note-28

4. Increased Tropospheric Ozone -- Increased surface UV leads to increased http://www.answers.com/topic/troposphere ozone. Ground-level ozone is generally recognized to be a health risk, as ozone is toxic due to its strong http://www.answers.com/topic/redox properties. At this time, ozone at ground level is produced mainly by the action of UV radiation on http://www.answers.com/topic/combustion gases from vehicle exhausts.[citation needed]

Effects on crops

An increase of UV radiation would be expected to affect crops. A number of economically important species of plants, such as http://www.answers.com/topic/rice, depend on http://www.answers.com/topic/cyanobacteria residing on their roots for the retention of http://www.answers.com/topic/nitrogen. Cyanobacteria are sensitive to UV light and they would be affected by its increase.http://www.answers.com/topic/ozone-depletion#cite_note-29

Public policy in response to the ozone hole

NASA projections of stratospheric ozone concentrations if chlorofluorocarbons had not been banned.

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 (as explained above).

After a 1976 report by the http://www.answers.com/topic/national-academy-of-sciences concluded that credible scientific evidence supported the ozone depletion hypothesis, a few countries, including the United States, Canada, Sweden, 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 which indicated that the first estimates of the magnitude of ozone depletion had been overly large). The United States banned the use of CFCs in aerosol cans in 1978. The European Community rejected proposals to ban CFCs in aerosol sprays while even 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 1980, http://www.answers.com/topic/dupont-2 closed down its research program into halocarbon alternatives.

The US Government's attitude began to change again in 1983, when http://www.answers.com/topic/william-ruckelshaus replaced http://www.answers.com/topic/anne-gorsuch-burford as Administrator of the http://www.answers.com/topic/environmental-protection-agency. Under Ruckelshaus and his successor, Lee Thomas, the EPA pushed for an international approach to halocarbon regulations. In 1985 20 nations, including most of the major CFC producers, signed the http://www.answers.com/topic/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 http://www.answers.com/topic/montreal-protocol. Meanwhile, the halocarbon industry shifted its position and started supporting a protocol to limit CFC production. The reasons for this were in part explained by "Dr. Mostafa Tolba, former head of the UN Environment Programme, who was quoted in the 30 June 1990 edition of The http://www.answers.com/topic/new-scientist, '...the chemical industry supported the Montreal Protocol in 1987 because it set up a worldwide schedule for phasing out CFCs, which [were] no longer protected by patents. This provided companies with an equal opportunity to market new, more profitable compounds.'"http://www.answers.com/topic/ozone-depletion#cite_note-greenpeace-ozone-30

At Montreal, the participants agreed to freeze production of CFCs at 1986 levels and to reduce production by 50% 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 http://www.answers.com/topic/metered-dose-inhaler) by 2000. At a 1992 meeting in Copenhagen, the phase out date was moved up to 1996.

To some extent, CFCs have been replaced by the less damaging hydro-chloro-fluoro-carbons (http://www.answers.com/topic/haloalkane-1), although concerns remain regarding HCFCs also. In some applications, hydro-fluoro-carbons (http://www.answers.com/topic/hfc-1) have been used to replace CFCs. HFCs, which contain no chlorine or bromine, do not contribute at all to ozone depletion although they are potent greenhouse gases. The best known of these compounds is probably HFC-134a (http://www.answers.com/topic/1-1-1-2-tetrafluoroethane), which in the United States has largely replaced CFC-12 (http://www.answers.com/topic/dichlorodifluoromethane) in automobile air conditioners. In laboratory analytics (a former "essential" use) the ozone depleting substances can be replaced with various other solvents.http://www.answers.com/topic/ozone-depletion#cite_note-31

Ozone Diplomacy, by Richard Benedick (Harvard University Press, 1991) gives a detailed account of the negotiation process that led to the Montreal Protocol. Pielke and Betsill provide an extensive review of early US government responses to the emerging science of ozone depletion by CFCs.

Past and current events and future prospects of ozone depletion

Ozone-depleting gas trends.

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. By 2015, the Antarctic ozone hole would have reduced by only 1 million km² out of 25 (Newman et al., 2004); complete recovery of the Antarctic ozone layer will not occur until the year 2050 or later. Work has suggested that a detectable (and statistically significant) recovery will not occur until around 2024, with ozone levels recovering to 1980 levels by around 2068.http://www.answers.com/topic/ozone-depletion#cite_note-32

The decrease in ozone-depleting chemicals has also been significantly affected by a decrease in http://www.answers.com/topic/bromine-containing chemicals. The data suggest that substantial natural sources exist for atmospheric http://www.answers.com/topic/bromomethane (CH3Br).http://www.answers.com/topic/ozone-depletion#cite_note-33

The 2004 ozone hole ended in November 2004, daily minimum stratospheric temperatures in the Antarctic lower stratosphere increased to levels that are too warm for the formation of polar stratospheric clouds (PSCs) about 2 to 3 weeks earlier than in most recent years.http://www.answers.com/topic/ozone-depletion#cite_note-34

The Arctic winter of 2005 was extremely cold in the stratosphere; PSCs were abundant over many high-latitude areas until dissipated by a big warming event, which started in the upper stratosphere during February and spread throughout the Arctic stratosphere in March. The size of the Arctic area of anomalously low total ozone in 2004-2005 was larger than in any year since 1997. The predominance of anomalously low total ozone values in the Arctic region in the winter of 2004-2005 is attributed to the very low stratospheric temperatures and meteorological conditions favorable for ozone destruction along with the continued presence of ozone destroying chemicals in the stratosphere.http://www.answers.com/topic/ozone-depletion#cite_note-35

A 2005 http://www.answers.com/topic/intergovernmental-panel-on-climate-change summary of ozone issues observed that observations and model calculations suggest that the global average amount of ozone depletion has now approximately stabilized. Although considerable variability in ozone is expected from year to year, including in polar regions where depletion is largest, the ozone layer is expected to begin to recover in coming decades due to declining ozone-depleting substance concentrations, assuming full compliance with the Montreal Protocol.http://www.answers.com/topic/ozone-depletion#cite_note-36

Temperatures during the Arctic winter of 2006 stayed fairly close to the long-term average until late January, with minimum readings frequently cold enough to produce PSCs. During the last week of January, however, a major warming event sent temperatures well above normal - much too warm to support PSCs. By the time temperatures dropped back to near normal in March, the seasonal norm was well above the PSC threshold.http://www.answers.com/topic/ozone-depletion#cite_note-37 Preliminary satellite instrument-generated ozone maps show seasonal ozone buildup slightly below the long-term means for the Northern Hemisphere as a whole, although some high ozone events have occurred.http://www.answers.com/topic/ozone-depletion#cite_note-38 During March 2006, the Arctic stratosphere poleward of 60 degrees North Latitude was free of anomalously low ozone areas except during the three-day period from 17 March to 19 when the total ozone cover fell below 300 DU over part of the North Atlantic region from Greenland to Scandinavia.http://www.answers.com/topic/ozone-depletion#cite_note-39

The area where total column ozone is less than 220 DU (the accepted definition of the boundary of the ozone hole) was relatively small until around 20 August 2006. Since then the ozone hole area increased rapidly, peaking at 29 million km² 24 September. In October 2006, http://www.answers.com/topic/nasa reported that the year's ozone hole set a new area record with a daily average of 26 million km² between 7 September and 13 October 2006; total ozone thicknesses fell as low as 85 DU on 8 October. The two factors combined, 2006 sees the worst level of depletion in recorded ozone history. The depletion is attributed to the temperatures above the Antarctic reaching the lowest recording since comprehensive records began in 1979.http://www.answers.com/topic/ozone-depletion#cite_note-40http://www.answers.com/topic/ozone-depletion#cite_note-41

On October 2008 the http://www.answers.com/topic/ecuadorian-civilian-space-agency published a report called HIPERION, a study of the last 28 years data from 10 satellites and dozens of ground instruments around the world among them their own, and found that the UV radiation reaching equatorial latitudes was far greater than expected, climbing in some very populated cities up to 24 UVI, the http://www.answers.com/topic/world-health-organization http://www.answers.com/topic/ultraviolet-index standard considers 11 as an extreme index and a great risk to health. The report concluded that the ozone depletion around mid latitudes on the planet is already endangering large populations in this areas. Later, the CONIDA, the Peruvian Space Agency, made its own study, which found almost the same facts as the Ecuadorian study.

The Antarctic ozone hole is expected to continue for decades. Ozone concentrations in the lower stratosphere over Antarctica will increase by 5%-10% by 2020 and return to pre-1980 levels by about 2060-2075, 10-25 years later than predicted in earlier assessments. This is because of revised estimates of atmospheric concentrations of Ozone Depleting Substances - and a larger predicted future usage in developing countries. Another factor which may aggravate ozone depletion is the draw-down of nitrogen oxides from above the stratosphere due to changing wind patterns.http://www.answers.com/topic/ozone-depletion#cite_note-42

History of the research

The basic physical and chemical processes that lead to the formation of an ozone layer in the Earth's stratosphere were discovered by http://www.answers.com/topic/sydney-chapman-1 in 1930. These are discussed in the article http://www.answers.com/topic/ozone-oxygen-cycle - briefly, short-wavelength UV radiation splits an oxygen (O2) 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 + O3 → 2O2. 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 Prof. http://www.answers.com/topic/paul-j-crutzen pointed out that emissions of nitrous oxide (N2O), 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 http://www.answers.com/topic/fertilizer 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 have an impact on the stratospheric ozone layer. In the following year, Crutzen and (independently) Harold Johnston suggested that NO emissions from http://www.answers.com/topic/supersonic http://www.answers.com/topic/aircraft, which fly in the lower stratosphere, could also deplete the ozone layer.

The Rowland-Molina hypothesis

In 1974 http://www.answers.com/topic/frank-sherwood-rowland, Chemistry Professor at the University of California at Irvine, and his postdoctoral associate http://www.answers.com/topic/mario-j-molina suggested that long-lived organic halogen compounds, such as http://www.answers.com/topic/cfc-1, might behave in a similar fashion as Crutzen had proposed for nitrous oxide. http://www.answers.com/topic/james-lovelock (most popularly known as the creator of the http://www.answers.com/topic/gaia-hypothesis-1) 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 N2O, the CFCs would reach the stratosphere where they would be dissociated by UV light, releasing Cl atoms. (A year earlier, Richard Stolarski and http://www.answers.com/topic/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 CFC's 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 http://www.answers.com/topic/halocarbon industries. The Chair of the Board of http://www.answers.com/topic/dupont-2 was quoted as saying that ozone depletion theory is "a science fiction tale...a load of rubbish...utter nonsense".http://www.answers.com/topic/ozone-depletion#cite_note-greenpeace-ozone-30 http://www.answers.com/topic/robert-abplanalp, the President of Precision Valve Corporation (and inventor of the first practical aerosol spray can valve), wrote to the Chancellor of http://www.answers.com/topic/university-of-california-irvine to complain about Rowland's public statements (Roan, p 56.) 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 ClONO2) 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 http://www.answers.com/topic/halon, widely used in fire extinguishers, were a potentially large source of stratospheric bromine. In 1976 the U.S. National Academy of Sciences released a report which concluded that the ozone depletion hypothesis was strongly supported by the scientific evidence. Scientists calculated that if CFC production continued to increase at the going rate of 10% per year until 1990 and then remain steady, CFCs would cause a global ozone loss of 5 to 7% by 1995, and a 30 to 50% loss by 2050. In response the United States, Canada and Norway banned the use of CFCs in aerosol spray cans in 1978. However, subsequent research, summarized by the National Academy in reports issued between 1979 and 1984, appeared to show that the earlier estimates of global ozone loss had been too large.[citation needed]

Crutzen, Molina, and Rowland were awarded the 1995 http://www.answers.com/topic/nobel-prize-in-chemistry for their work on stratospheric ozone.

The ozone hole

The discovery of the Antarctic "ozone hole" by http://www.answers.com/topic/british-antarctic-survey scientists Farman, Gardiner and Shanklin (announced in a paper in nature-journal in May 1985) came as a shock to the scientific community, because the observed decline in polar ozone was far larger than anyone had anticipated.[citation needed] Satellite measurements showing massive depletion of ozone around the http://www.answers.com/topic/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 situobservations. When the software was rerun without the flags, the ozone hole was seen as far back as 1976.http://www.answers.com/topic/ozone-depletion#cite_note-43

http://www.answers.com/topic/susan-solomon, an atmospheric chemist at the National Oceanic and Atmospheric Administration (NOAA), proposed that chemical reactions on http://www.answers.com/topic/nacreous-cloud (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 when there are very low temperatures, as low as -80 degrees http://www.answers.com/topic/celsius, 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 deplete ozone easily.

Moreover the polar vortex formed over Antarctica is very tight and the reaction which occurs 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.[citation needed]

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.[citation needed]

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.[citation needed] 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 http://www.answers.com/topic/united-nations-environment-programme has sponsored a series of reports on http://www.answers.com/topic/scientific-assessment-of-ozone-depletion. The most recent is from 2007 where satellite measurements have shown the hole in the ozone layer is recovering and is now the smallest it has been for about a decade[11].

Ozone depletion and global warming

Although they are often interlinked in the http://www.answers.com/topic/mass-media, the connection between global warming and ozone depletion is not strong. There are five areas of linkage:

http://www.answers.com/topic/radiative-forcing from various http://www.answers.com/topic/greenhouse-gas and other sources.

• The same CO2 radiative forcing that produces near-surface global warming is expected to cool the http://www.answers.com/topic/stratosphere.http://www.answers.com/topic/ozone-depletion#cite_note-ipcc2007-44 This cooling, in turn, is expected to produce a relative increase in polar http://www.answers.com/topic/ozone (O3) depletion 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 http://www.answers.com/topic/troposphere; the resulting colder stratosphere emits less long-wave radiation downward, thus cooling the troposphere. Overall, the cooling dominates; the IPCC concludes that "observed stratospheric http://www.answers.com/topic/ozone losses over the past two decades have caused a negative forcing of the surface-troposphere system"http://www.answers.com/topic/ozone-depletion#cite_note-45 of about −0.15 ± 0.10 http://www.answers.com/topic/watt per square meter (W/m²).http://www.answers.com/topic/ozone-depletion#cite_note-spm_ozone-46
• One of the strongest predictions of the greenhouse effect is that the stratosphere will cool.http://www.answers.com/topic/ozone-depletion#cite_note-ipcc2007-44 Although this cooling has been observed, it is not trivial to separate the effects of changes in the concentration of http://www.answers.com/topic/greenhouse-gas and ozone depletion since both will lead to cooling. However, this can be done by numerical stratospheric modeling. Results from the http://www.answers.com/topic/weather-bureau's http://www.answers.com/topic/geophysical-fluid-dynamics-laboratory show that above 20 km (12.4 miles), the greenhouse gases dominate the cooling.http://www.answers.com/topic/ozone-depletion#cite_note-47
• Ozone depleting chemicals are also greenhouse gases. The increases in concentrations of these chemicals have produced 0.34 ± 0.03 W/m² of radiative forcing, corresponding to about 14% of the total radiative forcing from increases in the concentrations of well-mixed greenhouse gases.http://www.answers.com/topic/ozone-depletion#cite_note-spm_ozone-46
• The long term modeling of the process, its measurement, study, design of theories and testing take decades to both 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 currently being proven. Dr Drew Schindell, and Dr Paul Newman, NASA Goddard, proposed a theory in the late 1990s, using a http://www.answers.com/topic/sgi-origin-2000 supercomputer, that modeled ozone destruction, accounted for 78% of the ozone destroyed. Further refinement of that model, accounted for 89% 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.)

Misconceptions about ozone depletion

A few of the more common misunderstandings about ozone depletion are addressed briefly here; more detailed discussions can be found in the ozone-depletion FAQ.

CFCs are "too heavy" to reach the stratosphere

It is sometimes stated that since CFC molecules are much heavier than nitrogen or oxygen, they cannot reach the stratosphere in significant quantities.http://www.answers.com/topic/ozone-depletion#cite_note-48 But atmospheric gases are not sorted by weight; the forces of wind (turbulence) are strong enough to fully intermix gases in the atmosphere. CFCs are heavier than air, but just like http://www.answers.com/topic/argon, http://www.answers.com/topic/krypton and other heavy gases with a long lifetime, they are uniformly distributed throughout the http://www.answers.com/topic/turbopause-1 and reach the upper atmosphere.http://www.answers.com/topic/ozone-depletion#cite_note-49

Man-made chlorine is insignificant compared to natural sources

Another objection occasionally voiced is that It is generally agreed that natural sources of tropospheric chlorine (volcanoes, ocean spray, etc.) are four to five orders of magnitude larger than man-made sources. While strictly true, troposphericchlorine is irrelevant; it is stratospheric chlorine that affects ozone depletion. Chlorine from http://www.answers.com/topic/sea-spray-2 is soluble and thus is washed out by rainfall before it reaches the stratosphere. CFCs, in contrast, are insoluble and long-lived, which allows them to reach the stratosphere. Even in the lower atmosphere there is more chlorine present in the form of CFCs and related http://www.answers.com/topic/haloalkane-1 than there is in HCl from salt spray, and in the stratosphere halocarbons dominate overwhelmingly.http://www.answers.com/topic/ozone-depletion#cite_note-50 Only one of these halocarbons, methyl chloride, has a predominantly natural sourcehttp://www.answers.com/topic/ozone-depletion#cite_note-51, and it is responsible for about 20 percent of the chlorine in the stratosphere; the remaining 80% comes from manmade compounds.

Very large volcanic eruptions can inject HCl directly into the stratosphere, but direct measurementshttp://www.answers.com/topic/ozone-depletion#cite_note-52 have shown that their contribution is small compared to that of chlorine from CFCs. A similar erroneous assertion is that soluble halogen compounds from the volcanic plume of http://www.answers.com/topic/mount-erebus on Ross Island, Antarctica are a major contributor to the Antarctic ozone hole.[citation needed]

An ozone hole was first observed in 1956

http://www.answers.com/topic/g-m-b-dobson-1 (Exploring the Atmosphere, 2nd Edition, Oxford, 1968) mentioned that when springtime ozone levels over http://www.answers.com/topic/halley-research-station were first measured, he was surprised to find that they were ~320 DU, about 150 DU below spring levels, ~450 DU, in the Arctic. These, however, were the pre-ozone hole normal climatological values. 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 distinctly different. Instead of staying constant, early springtime ozone levels suddenly drop from their already low winter values, by as much as 50%, and normal values are not reached again until December.http://www.answers.com/topic/ozone-depletion#cite_note-53

If the theory were correct, the ozone hole should be above the sources of CFCs

CFCs are well mixed in the http://www.answers.com/topic/troposphere and the http://www.answers.com/topic/stratosphere. The reason the ozone hole occurs above Antarctica is not because there are more CFCs there but because the low temperatures allow polar stratospheric clouds to form.http://www.answers.com/topic/ozone-depletion#cite_note-54 There have been anomalous discoveries of significant, serious, localized "holes" above other parts of the globe.http://www.answers.com/topic/ozone-depletion#cite_note-autogenerated1-55

The "ozone hole" is a hole in the ozone layer

When the "ozone hole" forms, essentially all of the ozone in the lower stratosphere is destroyed. The upper stratosphere is much less affected, however, so that the overall amount of ozone over the continent declines by 50 percent or more. The ozone hole does not go all the way through the layer; on the other hand, it is not a uniform 'thinning' of the layer either. It is a "hole" in the sense of "a hole in the ground", that is, a depression; not in the sense of "a hole in the windshield."