Ozone depletion 44183 225835326 2008-07-15T16:52:52Z Evolauxia 100838 introduce wikilink to O3 layer in lead {{Pollution}} [[Image:TOMS Global Ozone 65N-65S.png|thumb|350px|Global monthly average total ozone amount]] '''Ozone depletion''' describes two distinct, but related observations: a slow, steady decline of about 4 percent per decade in the total amount of [[ozone]] in [[Earth|Earth's]] [[stratosphere]] since the late 1970s; and a much larger, but seasonal, decrease in stratospheric ozone over Earth's polar regions during the same period. The latter phenomenon is commonly referred to as the '''ozone hole'''. In addition to this well-known [[Ozone layer|stratospheric ozone]] depletion, there are also [[tropospheric ozone depletion events]], which occur near the surface in polar regions during spring. The detailed mechanism by which the polar ozone holes form is different from that for the mid-latitude thinning, but the most important process in both trends is [[catalytic]] destruction of ozone by atomic chlorine and bromine.<ref>{{cite web | url = http://www.atm.ch.cam.ac.uk/tour/part3.html | title = Part III. The Science of the Ozone Hole | accessdate = 2007-03-05}} </ref> The main source of these [[halogen]] atoms in the stratosphere is [[photodissociation]] of [[chlorofluorocarbon]] (CFC) compounds, commonly called [[freon]]s, and of [[bromofluorocarbon]] compounds known as [[Haloalkanes|halons]]. These compounds are transported into the stratosphere after being emitted at the surface. Both ozone depletion mechanisms strengthened as emissions of CFCs and halons increased. CFCs and other contributory substances are commonly referred to as '''ozone-depleting substances''' ('''ODS'''). Since the ozone layer prevents most harmful UVB wavelengths (270–315 nm) of [[Ultraviolet|ultraviolet light]] (UV light) from passing through the [[Earth's atmosphere]], observed and projected decreases in ozone have generated worldwide concern leading to adoption of the [[Montreal Protocol]] banning the production of CFCs and halons as well as related ozone depleting chemicals such as [[carbon tetrachloride]] and [[1,1,1-Trichloroethane|trichloroethane]]. It is suspected that a variety of biological consequences such as increases in [[skin cancer]], damage to plants, and reduction of [[plankton]] populations in the ocean's [[photic zone]] may result from the increased UV exposure due to ozone depletion. ==Ozone cycle overview== Three forms (or [[allotropes]]) of oxygen are involved in the [[ozone-oxygen cycle]]: [[Oxygen]] atoms (O or atomic oxygen), oxygen gas (O<sub>2</sub> or diatomic oxygen), and ozone gas (O<sub>3</sub> or triatomic oxygen). [[Ozone]] is formed in the stratosphere when oxygen molecules [[Photodissociation|photodissociate]] after absorbing an [[ultraviolet]] photon whose wavelength is shorter than 240 nm. This produces two oxygen atoms. The atomic oxygen then combines with O<sub>2</sub> to create O<sub>3</sub>. Ozone molecules absorb UV light between 310 and 200 nm, following which ozone splits into a molecule of O<sub>2</sub> and an oxygen atom. The oxygen atom then joins up with an oxygen molecule to regenerate ozone. This is a continuing process which terminates when an oxygen atom "recombines" with an ozone molecule to make two O<sub>2</sub> molecules: O + O<sub>3</sub> → 2 O<sub>2</sub> The overall 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 of which are the [[hydroxyl radical]] (OH·), the [[nitric oxide]] radical (NO·) and atomic [[chlorine]] (Cl·) and [[bromine]] (Br·). All of these have both natural and anthropogenic (manmade) sources; at the present time, most of the OH· and NO· in the stratosphere is of natural origin, but human activity has dramatically increased the high in oxygen chlorine and bromine. These elements are found in certain stable organic compounds, especially [[chlorofluorocarbon]]s (CFCs), which may find their way to the [[stratosphere]] without being destroyed in the troposphere due to their low reactivity. Once in the stratosphere, the Cl and Br atoms are liberated from the parent compounds by the action of ultraviolet light, e.g. ('h' is [[Planck's constant]], 'ν' is [[frequency]] of [[electromagnetic radiation]]) CFCl<sub>3</sub> + hν → CFCl<sub>2</sub> + Cl The Cl and Br atoms can then destroy ozone molecules through a variety of [[catalysis|catalytic]] cycles. In the simplest example of such a cycle,<ref>''Stratospheric ozone: an electronic textbook'', Chapter 5, Section 4.2.8, [http://www.ccpo.odu.edu/SEES/ozone/class/Chap_5/index.htm]</ref> a chlorine atom reacts with an ozone molecule, taking an oxygen atom with it (forming ClO) and leaving a normal oxygen molecule. The chlorine monoxide (i.e., the ClO) can react with a second molecule of ozone (i.e., O<sub>3</sub>) to yield another chlrine atom and two molecules of oxygen. The chemical shorthand for these gas-phase reactions is: Cl + O<sub>3</sub> → ClO + O<sub>2</sub> ClO + O<sub>3</sub> → Cl + 2 O<sub>2</sub> The overall effect leads to an overall decrease in the amount of ozone. More complicated mechanisms have been discovered that lead to ozone destruction in the lower stratosphere as well. A single chlorine atom would keep on destroying ozone for up to two years (the time scale for transport back down to the troposphere) were it not for reactions that remove them from this cycle by forming reservoir species such as [[hydrogen chloride]] (HCl) and [[chlorine nitrate]] (ClONO<sub>2</sub>). On a per atom basis, bromine is even more efficient than chlorine at destroying ozone, but there is much less bromine in the atmosphere at present. As a result, both chlorine and bromine contribute significantly to the overall ozone depletion. Laboratory studies have shown that fluorine and iodine atoms participate in analogous catalytic cycles. However, in the Earth's stratosphere, fluorine atoms react rapidly with water and methane to form strongly-bound [[hydrofluoric acid|HF]], while organic molecules which contain iodine react so rapidly in the lower atmosphere that they do not reach the stratosphere in significant quantities. Furthermore, a single chlorine atom is able to react with 100,000 ozone molecules. This fact plus the amount of chlorine released into the atmosphere by chlorofluorocarbons(CFCs) yearly demonstrates how dangerous CFCs are to the environment. <ref>[http://www.eoearth.org/article/Stratospheric_Ozone_Depletion_by_Chlorofluorocarbons_(Nobel_Lecture) Stratospheric Ozone Depletion by Chlorofluorocarbons (Nobel Lecture) - Encyclopedia of Earth<!-- Bot generated title -->]</ref> ===Quantitative understanding of the chemical ozone loss process=== New research on the breakdown of a key molecule in these ozone-depleting chemicals, dichlorine peroxide (Cl2O2), calls into question the completeness of present atmospheric models of polar ozone depletion. Specifically, chemists at NASA's Jet Propulsion Laboratory in Pasadena, California, found in 2007 that the temperatures, and the spectrum and intensity of radiation present in the stratosphere created conditions insufficient to allow the rate of chemical-breakdown required to release chlorine radicals in the volume necessary to explain observed rates of ozone depletion. Instead, laboratory tests, designed to be the most accurate reflection of stratospheric conditions to date, showed the decay of the crucial molecule almost a magnitude lower than previously thought<ref>[http://www.nature.com/news/2007/070924/full/449382a.html Nature]</ref>.<ref>[http://pubs.acs.org/cgi-bin/abstract.cgi/jpcafh/2007/111/i20/abs/jp067660w.html Ultraviolet Absorption Spectrum of Chlorine Peroxide, ClOOCl<!-- Bot generated title -->]</ref><ref>[http://www.wmo.ch/pages/publications/bulletin/ozone_en.html Bulletin - The journal of the World Meteorological Organization<!-- Bot generated title -->]</ref> ==Observations on ozone layer depletion== The most pronounced decrease in ozone has been in the lower [[stratosphere]]. However, the ozone hole is most usually measured not in terms of ozone concentrations at these levels (which are typically of a few parts per million) but by reduction in the total ''column ozone'', above a point on the Earth's surface, which is normally expressed in [[Dobson unit]]s, abbreviated as "DU". 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).<ref>[http://www.atm.ch.cam.ac.uk/tour/part2.html The Ozone Hole Tour: Part II. Recent Ozone Depletion<!-- Bot generated title -->]</ref> [[Image:Min ozone.jpg|thumb|250px|Lowest value of ozone measured by [[Total Ozone Mapping Spectrometer|TOMS]] each year in the ozone hole]] Reductions of up to 70% in the ozone column observed in the austral (southern hemispheric) spring over Antarctica and first reported in 1985 (Farman et al 1985) are continuing.<ref>[http://www.wmo.ch/web/arep/reports/o3_assess_rep_2002_front_page.html World Meteorological Organization (WMO)<!-- Bot generated title -->]</ref> Through the 1990s, total column ozone in September and October have continued to be 40–50% lower than pre-ozone-hole values. In the [[Arctic]] the amount lost is more variable year-to-year than in the Antarctic. The greatest declines, up to 30%, are in the winter and spring, when the stratosphere is colder. Reactions that take place on polar stratospheric clouds (PSCs) play an important role in enhancing ozone depletion.<ref>[http://www.epa.gov/ozone/science/hole/whyant.html#psc U.S. EPA: Ozone Depletion<!-- Bot generated title -->]</ref> PSCs form more readily in the extreme cold of 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.{{Fact|date=March 2008}} In middle latitudes it is preferable to speak of ozone depletion rather than holes. Declines are about 3% below pre-1980 values for 35–60°N and about 6% for 35–60°S. In the tropics, there are no significant trends.{{Fact|date=March 2008}} Ozone depletion also explains much of the observed reduction in stratospheric and upper [[troposphere|tropospheric]] temperatures.<ref>[http://www.grida.no/climate/ipcc_tar/wg1/223.htm Climate Change 2001: The Scientific Basis<!-- Bot generated title -->]</ref><ref>[http://www.giss.nasa.gov/edu/gwdebate/]</ref> 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 gas]]es such as [[carbon dioxide|CO<sub>2</sub>]]; however the ozone-induced cooling appears to be dominant.{{Fact|date=March 2008}} Predictions of ozone levels remain difficult. The [http://www.esrl.noaa.gov/csd/assessments/1998/preface.html World Meteorological Organization Global Ozone Research and Monitoring Project - Report No. 44] comes out strongly in favor for the Montreal Protocol, but notes that a [[UNEP]] 1994 Assessment overestimated ozone loss for the 1994–1997 period. ===Chemicals in the atmosphere=== ====CFCs in the atmosphere==== Chlorofluorocarbons ([[CFCs]]) were invented by [[Thomas Midgley]] in the 1920s. They were used in [[air conditioning]]/cooling units, as [[aerosol spray#Propellant|aerosol spray propellant]]s prior to the 1980s, 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 in the ''ozone cycle overview'' 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 15 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.<ref>[http://www.encyclopedia.com/doc/1E1-chlorofl.html chlorofluorocarbons – FREE chlorofluorocarbons Information | Encyclopedia.com: Facts, Pictures, Information!<!-- Bot generated title -->]</ref> ===Verification of observations=== Scientists have been increasingly able to attribute the observed ozone depletion to the increase of anthropogenic [[halogen]] compounds from CFCs by the use of complex chemistry transport models and their validation against observational data (e.g. [http://www.lec.leeds.ac.uk/~martyn/slimcat.html SLIMCAT], [[CLaMS]]). These models work by combining satellite measurements of chemical concentrations and meteorological fields with chemical reaction rate constants obtained in lab experiments. They are able to identify not only the key chemical reactions but also the transport processes which bring CFC [[photodissociation|photolysis]] products into contact with ozone. ==The ozone hole and its causes== [[Image:160658main2 OZONE large 350.png|thumb|right|200px|Image of the largest Antarctic ozone hole ever recorded (September 2006).]] 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 [[polar vortex]], over 50% of the lower stratospheric ozone is destroyed during the Antarctic spring.<ref>[http://www.nas.nasa.gov/About/Education/Ozone/antarctic.html Antarctic Ozone Hole<!-- Bot generated title -->]</ref> As explained above, the overall 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 dramatically enhanced in the presence of [[polar stratospheric cloud]]s (PSCs).<ref>[http://www.faqs.org/faqs/ozone-depletion/antarctic Antarctic ozone-depletion FAQ, section 7]</ref> 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 [[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.{{Fact|date=March 2008}} The [[photochemistry|photochemical]] 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 hydrogen chloride (HCl) and chlorine nitrate (ClONO<sub>2</sub>). 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 NO<sub>2</sub> from the atmosphere by converting it to nitric acid, which prevents the newly formed ClO from being converted back into ClONO<sub>2</sub>. 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.{{Fact|date=March 2008}} 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.{{Fact|date=March 2008}} 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 heals.{{Fact|date=March 2008}} ===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 sixty-year period.{{Fact|date=March 2008}} * 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.{{Fact|date=March 2008}} * Many{{Fact|date=March 2008}} 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 [[global warming]] theory predicts that the stratosphere will cool.{{Fact|date=March 2008}} * 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 [[UVB]] ultraviolet light from the Sun, ozone layer depletion is expected to increase surface UVB levels, which could lead to damage, including increases in [[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 [[UVA]], 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 [[exponential decay|decreases exponentially]] 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 [[UVB]] due to the ozone hole can be partially inferred by [[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, [[New Zealand]]).<ref>[http://www.niwa.co.nz/services/uvozone]</ref> Because it is this same UV radiation that creates ozone in the ozone layer from O<sub>2</sub> (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 [[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.{{Fact|date=March 2008}} ===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 [[Australia]] and [[New Zealand]], environmentalists have been concerned that the increase in surface UV could be significant.{{Fact|date=March 2008}} ====Effects of ozone layer depletion on Humans==== [[UVB]] (the higher energy UV radiation absorbed by ozone) is generally accepted to be a contributory factor to [[skin cancer]]. In addition, increased surface UV leads to increased tropospheric ozone, which is a health risk to humans.{{Fact|date=March 2008}} The increased surface UV also represents an increase in the [[vitamin D]] synthetic capacity of the sunlight.<ref>[http://www.informaworld.com/smpp/content~content=a723976621~db=all Ozone and solar UV-B radiation: monitoring of the vitamin D synthetic capacity of sunlight in Kiev and Antarctica - International Journal of Remote Sensing<!-- Bot generated title -->]</ref> The cancer preventive effects of vitamin D represent a possible beneficial effect of ozone depletion.[http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=pubmed&cmd=Retrieve&dopt=AbstractPlus&list_uids=17344960&query_hl=4&itool=pubmed_docsum][http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=pubmed&cmd=Retrieve&dopt=AbstractPlus&list_uids=17143048&query_hl=4&itool=pubmed_DocSum] In terms of health costs, the possible benefits of increased UV irradiance may outweigh the burden. [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=pubmed&cmd=Retrieve&dopt=AbstractPlus&list_uids=16159309&query_hl=8&itool=pubmed_DocSum] 1. '''Basal and Squamous Cell Carcinomas''' -- The most common forms of skin cancer in humans, [[Basal cell carcinoma|basal]] and [[Squamous cell carcinoma|squamous]] cell carcinomas, have been strongly linked to UVB exposure. The mechanism by which UVB induces these cancers is well understood &mdash; absorption of UVB radiation causes the pyrimidine bases in the DNA molecule to form [[dimer]]s, 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%.<ref name = "gcrio.org-consequnces">http://www.gcrio.org/CONSEQUENCES/summer95/impacts.html Consequences (vol. 1, No. 2) - Impacts of a Projected Depletion of the Ozone Layer<!-- Bot generated title --></ref> 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 radiation<ref>[http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Retrieve&list_uids=8341684&dopt=Citation Wavelengths effective in induction of malignant me...[Proc Natl Acad Sci U S A. 1993&#93; - PubMed Result<!-- Bot generated title -->]</ref> whereas experiments on opossums suggest a larger role for UVB.<ref name = "gcrio.org-consequnces"/> 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.<ref>Fears et al, Cancer Res. 2002, 62(14):3992–6</ref> A study of people in [[Punta Arenas]], at the southern tip of [[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.<ref>Abarca, Jaime F. & Casiccia, Claudio C. (2002) Skin cancer and ultraviolet-B radiation under the Antarctic ozone hole: southern Chile, 1987-2000. Photodermatology, Photoimmunology & Photomedicine 18 (6), 294–302 [http://www.blackwell-synergy.com/links/doi/10.1034/j.1600-0781.2002.02782.x/full/]</ref> 3. '''Cortical Cataracts''' -- Studies are suggestive of 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 <ref>[http://jama.ama-assn.org/cgi/content/full/280/8/714 JAMA - Sunlight Exposure and Risk of Lens Opacities in a Population-Based Study: The Salisbury Eye Evaluation Project, [[August 26]], [[1998]], West et al. 280 (8): 714<!-- Bot generated title -->]</ref>. 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.<ref>[http://www.ajph.org/cgi/content/abstract/82/12/1658 Ultraviolet light exposure and lens opacities: the Beaver Dam Eye Study. - Cruickshanks et al. 82 (12): 1658 - American Journal of Public Health<!-- Bot generated title -->]</ref> 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.<ref>[http://www.ncbi.nlm.nih.gov/entrez/eutils/escan.fcgi?db=PubMed&uid=9850124&dopt=Citation&field=Title&DateField=MeshDate Racial differences in lens opacities: the Salisbury Eye Evaluation (SEE) project<!-- Bot generated title -->]</ref><ref>[http://www.ncbi.nlm.nih.gov/sites/entrez?cmd=Retrieve&db=PubMed&list_uids=9006434&dopt=AbstractPlus Prevalence of lens opacities in the Barbados Eye S...[Arch Ophthalmol. 1997&#93; - PubMed Result<!-- Bot generated title -->]</ref> 4. '''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 [[redox|oxidant]] properties. At this time, ozone at ground level is produced mainly by the action of UV radiation on [[combustion]] gases from vehicle exhausts.{{Fact|date=March 2008}} ====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 light and they would be affected by its increase.<ref>{{cite journal |author = R. P. Sinha |coauthors = S. C. Singh and D.-P. Häder |title = Photoecophysiology of cyanobacteria |year = 1999 |journal = Journal of Photochemistry and Photobiology |volume = 3 |pages = 91–101}}</ref> ====Effects on Plankton==== Research has shown a widespread extinction of [[plankton]] 2 million years ago that coincided with a nearby [[supernova]]. There is a difference in the orientation and motility of planktons when excess of UV rays reach earth. Researchers speculate that the extinction was caused by a significant weakening of the ozone layer at that time when the radiation from the supernova produced [[nitrogen oxide]]s that [[catalysis|catalyzed]] the destruction of ozone (plankton are particularly susceptible to effects of UV light, and are vitally important to marine [[food web]]s).<ref>[http://sci.newsfactor.com/perl/story/15776.html NewsFactor Network<!-- Bot generated title -->]</ref> ==Public policy in response to the ozone hole== 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 [[U.S. 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 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, [[DuPont]] closed down its research program into halocarbon alternatives. The US 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]]. 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 Vienna Convention 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 on Substances that Deplete the Ozone Layer|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 [[June 30]], [[1990]] edition of The [[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.'"<ref name = "greenpeace-ozone">http://archive.greenpeace.org/ozone/greenfreeze/moral97/6dupont.html</ref> 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 [[asthma inhaler]]s) 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 ([[HCFC]]s), although concerns remain regarding HCFCs also. In some applications, hydro-fluoro-carbons ([[HFC]]s) 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 ([[R-134a]]), which in the United States has largely replaced CFC-12 ([[Dichlorodifluoromethane|R-12]]) in automobile air conditioners. In laboratory analytics (a former "essential" use) the ozone depleting substances can be replaced with various other solvents.<ref>Use of Ozone Depleting Substances in Laboratories. TemaNord 516/2003. [http://www.norden.org/pub/ebook/2003-516.pdf]</ref> ''Ozone Diplomacy'', by Richard Benedick (Harvard University Press, 1991) gives a detailed account of the negotiation process that led to the Montreal Protocol. [http://sciencepolicy.colorado.edu/admin/publication_files/1997.11.pdf Pielke and Betsill] provide an extensive review of early US government responses to the emerging science of ozone depletion by CFCs. ==Current events and future prospects of ozone depletion== [[Image:Ozone cfc trends.png|thumb|250px|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.<ref> {{cite journal | journal = Geophysical Research Letters | volume= 33 | pages = L12814 | year= 2006 | doi = 10.1029/2005GL025232 | title = When will the Antarctic ozone hole recover? | author = Newman, P. A., Nash, E. R., Kawa, S. R., Montzka, S. A. and Schauffler, S. M}}</ref> There is a slight caveat to this, however. [[Global warming]] from CO<sub>2</sub> is expected to cool the stratosphere.{{Fact|date=March 2008}} This, in turn, would lead to a relative increase in ozone depletion and the frequency of ozone holes. The effect may not be linear; ozone holes form because of polar stratospheric clouds; the formation of polar stratospheric clouds has a temperature threshold above which they will not form; cooling of the Arctic stratosphere might lead to Antarctic-ozone-hole-like conditions. But at the moment this is not clear. Even though the stratosphere as a whole is cooling, high-latitude areas may become increasingly predisposed to springtime stratospheric warming events as weather patterns change in response to higher [[greenhouse gas]] loading.{{Fact|date=March 2008}} This would cause PSCs to disappear earlier in the season, and may explain why Antarctic ozone hole seasons have tended to end somewhat earlier since 2000 as compared with the most prolonged ozone holes of the 1990s. 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]] (CH<sub>3</sub>Br).<ref>[http://www.wmo.ch/web/arep/reports/ozone_2002/06_chapter1.pdf World Meteorological Organization (WMO)<!-- Bot generated title -->]</ref> 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.<ref>[http://www.wmo.ch/web/arep/04/bulletin_7_2004.pdf World Meteorological Organization (WMO)<!-- Bot generated title -->]</ref> 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.<ref>[http://www.cpc.ncep.noaa.gov/products/stratosphere/winter_bulletins/nh_04-05/index.html CPC - Stratosphere: Winter Bulletins<!-- Bot generated title -->]</ref> A 2005 [[IPCC]] 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.<ref>[http://www.ipcc.ch/press/SPM.pdf]</ref> 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 &mdash; 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.<ref>[http://code916.gsfc.nasa.gov/Data_services/met/ann_data.html Available Annual NCEP data<!-- Bot generated title -->]</ref> 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.<ref>[http://es-ee.tor.ec.gc.ca/cgi-bin/dailyMaps Select ozone maps, individual sources<!-- Bot generated title -->]</ref> 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 [[March 17]] to 19 when the total ozone cover fell below 300 DU over part of the North Atlantic region from Greenland to Scandinavia.<ref>[http://www.cpc.ncep.noaa.gov/products/stratosphere/sbuv2to/archive/nh Index of /products/stratosphere/sbuv2to/archive/nh<!-- Bot generated title -->]</ref> 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 [[September 24]]. In October 2006, [[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 [[October 8]]. 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.<ref>[http://ozonewatch.gsfc.nasa.gov Ozone Hole Watch<!-- Bot generated title -->]</ref><ref>http://www.theregister.co.uk/2006/10/03/ozone_depletion</ref> 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.<ref>[http://www.newswire.ca/en/releases/archive/October2006/06/c5891.html CNW Group | CANADIAN SPACE AGENCY | Canada's SCISAT satellite explains 2006 ozone-layer depletion<!-- Bot generated title -->]</ref> ==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 [[Sydney Chapman]] in 1930. These are discussed in the article [[Ozone-oxygen cycle]] — briefly, short-wavelength UV radiation splits an oxygen (O<sub>2</sub>) 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 + O<sub>3</sub> → 2O<sub>2</sub>. 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 &ndash; it was estimated that in their absence, the ozone layer would be about twice as thick as it currently is. In 1970 Prof. [[Paul Crutzen]] pointed out that emissions of ''nitrous'' oxide (N<sub>2</sub>O), 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 have an impact on the stratospheric ozone layer. In the following year, Crutzen and (independently) Harold Johnston suggested that NO emissions from [[supersonic]] [[aircraft]], which fly in the lower stratosphere, could also deplete the ozone layer. ===The 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 [[CFC]]s, might behave in a similar fashion as Crutzen had proposed for nitrous oxide. [[James Lovelock]] (most popularly known as the creator of the [[Gaia hypothesis]]) 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 N<sub>2</sub>O, the CFCs would reach the stratosphere where they would be dissociated by UV light, releasing Cl 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 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 [[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".<ref name = "greenpeace-ozone"/> [[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 (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 (CFC's and related compounds) and the chlorine reservoir species (HCl and ClONO<sub>2</sub>) 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 [[halon]]s, 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, Sweden 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.{{Fact|date=January 2008}} Crutzen, Molina, and Rowland were awarded the 1995 Nobel Prize in Chemistry for their work on stratospheric ozone. ===The Ozone Hole=== The discovery of the Antarctic "ozone hole" by [[British Antarctic Survey]] scientists Farman, Gardiner and Shanklin (announced in a paper in ''[[Nature (journal)|Nature]]'' 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.{{Fact|date=March 2008}} Satellite measurements 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.<ref>[http://www.nas.nasa.gov/About/Education/Ozone/history.html Ozone Depletion, History and politics] accessed [[November 18]], [[2007]].</ref> [[Susan Solomon]], an atmospheric chemist at the National Oceanic and Atmospheric Administration (NOAA), proposed that chemical reactions on [[polar stratospheric cloud]]s (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 [[celsius|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 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.{{Fact|date=March 2008}} 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.{{Fact|date=March 2008}} 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.{{Fact|date=March 2008}} 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]] has sponsored a series of reports on [[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[http://abc.net.au/news/stories/2007/11/16/2092527.htm]. ==Controversy regarding ozone science and policy== That ozone depletion takes place is not seriously disputed in the scientific community.<ref>[http://www.answers.com/topic/ozone-depletion ozone hole: Definition and Much More from Answers.com<!-- Bot generated title -->]</ref> There is a consensus among atmospheric physicists and chemists that the scientific understanding has now reached a level where countermeasures to control CFC emissions are justified, although the decision is ultimately one for policy-makers. Despite this consensus, the science behind ozone depletion remains complex, and some who oppose the enforcement of countermeasures point to some of the uncertainties. For example, although increased UVB has been shown to constitute a melanoma risk, it has been difficult for statistical studies to establish a direct link between ozone depletion and increased rates of melanoma. Although melanomas did increase significantly during the period 1970–1990, it is difficult to separate reliably the effect of ozone depletion from the effect of changes in lifestyle factors (e.g., increasing rates of air travel). ==Ozone depletion and global warming== Although they are often interlinked in the [[mass media]], the connection between global warming and ozone depletion is not strong. There are four areas of linkage: * The same CO<sub>2</sub> radiative forcing that produces near-surface global warming is expected to cool the [[stratosphere]].{{Fact|date=March 2008}} This cooling, in turn, is expected to produce a relative ''increase'' in polar [[ozone]] (O<sub>3</sub>) depletion and the frequency of ozone holes. [[Image:Radiative-forcings.svg|thumb|right|[[Radiative forcing]] from various [[greenhouse gas]]es and other sources]] * 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 that "''observed stratospheric [[Ozone|O<sub>3</sub>]] losses over the past two decades have caused a negative forcing of the surface-troposphere system''"<ref>{{cite web |url=http://www.grida.no/climate/ipcc_tar/wg1/223.htm |title=Climate Change 2001: Working Group I: The Scientific Basis |pages=Chapter 6.4 |accessdate=2007-03-04 |date=2001 |work=[[Intergovernmental Panel on Climate Change]] Work Group I}}</ref> of about −0.15&nbsp;± 0.10 [[watt]]s per square meter (W/m²).<ref name="spm_ozone">{{cite paper |title=IPCC/TEAP Special Report on Safeguarding the Ozone Layer and the Global Climate System: Issues Related to Hydrofluorocarbons and Perfluorocarbons (summary for policy makers) |publisher= [[International Panel on Climate Change]] and Technology and Economic Assessment Panel |date=2005 |url=http://www.ipcc.ch/press/SPM.pdf |format=PDF |accessdate=2007-03-04}}</ref> * One of the strongest predictions of the greenhouse effect is that the stratosphere will cool.{{Fact|date=March 2008}} Although this cooling has been observed, it is not trivial to separate the effects of changes in the concentration of [[greenhouse gas]]es 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&nbsp;km (12.4&nbsp;miles), the greenhouse gases dominate the cooling.<ref>{{cite web |title = The Relative Roles of Ozone and Other Greenhouse Gases in Climate Change in the Stratosphere| publisher = Geophysical Fluid Dynamics Laboratory |url = http://www.gfdl.noaa.gov/aboutus/milestones/ozone.html | date = [[2007-02-29]] | accessdate = 2007-03-04}}</ref> * Ozone depleting chemicals are also greenhouse gases. The increases in concentrations of these chemicals have produced 0.34&nbsp;± 0.03&nbsp;W/m² of radiative forcing, corresponding to about 14% of the total radiative forcing from increases in the concentrations of well-mixed greenhouse gases.<ref name="spm_ozone"/> * 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 hyphtosized 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 [[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 staratospheric 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 [http://www.faqs.org/faqs/ozone-depletion 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.<ref>[http://www.phoenixnewtimes.com/1995-05-04/news/freon-easy/full Phoenix - News - FREON EASY<!-- Bot generated title -->]</ref> 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 [[argon]], [[krypton]] and other heavy gases with a long lifetime, they are uniformly distributed throughout the [[turbosphere]] and reach the upper atmosphere.<ref>[http://www.faqs.org/faqs/ozone-depletion/intro/ FAQ, part I], section 1.3.</ref> ===Man-made chlorine is insignificant compared to natural sources=== [[Image:Sources of stratospheric chlorine.png|thumb|280px|right]] 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, ''tropospheric'' chlorine is irrelevant; it is ''stratospheric'' chlorine that matters to ozone depletion. Chlorine from ocean spray 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 [[haloalkane]]s than there is in HCl from salt spray, and in the stratosphere the halocarbons dominate overwhelmingly.<ref>[http://www.faqs.org/faqs/ozone-depletion/stratcl ozone-depletion FAQ, Part II], section 4.3</ref> Only one of these halocarbons, methyl chloride, has a predominantly natural source, 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 measurements<ref>[http://www.faqs.org/faqs/ozone-depletion/stratcl ozone-depletion FAQ, Part II], section 4.4</ref> 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 [[Mount Erebus]] on Ross Island, Antarctica are a major contributor to the Antarctic ozone hole.{{Fact|date=March 2008}} ===An ozone hole was first observed in 1956=== [[G.M.B. Dobson]] (Exploring the Atmosphere, 2nd Edition, Oxford, 1968) mentioned that when springtime ozone levels over [[Halley Research Station|Halley Bay]] 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 completely 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.<ref>[http://www.faqs.org/faqs/ozone-depletion/antarctic ozone-depletion FAQ, Part III], section 6</ref> ===If the theory were correct, the ozone hole should be above the sources of CFCs=== CFCs are well mixed in the [[troposphere]] and the [[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.<ref>[http://www.faqs.org/faqs/ozone-depletion/antarctic ozone-depletion FAQ, Antarctic]</ref> There have been anomalous discoveries of significant, serious, localized "holes" above other parts of the globe.<ref>[http://www.answers.com/topic/ozone-depletion ozone hole: Definition and Much More from Answers.com<!-- Bot generated title -->]</ref> ===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's a "hole" in the sense of "a hole in the ground", a depression, not in the sense of "a hole in the windshield." ==World Ozone Day== In 1994, the [[United Nations General Assembly]] voted to designate [[September 16]] as "World Ozone Day", to commemorate the signing of the [[Montreal Protocol]] on that date in 1987. ==See also== * [[Ozone-oxygen cycle]] * [[Montreal Protocol]] * "[[Scientific Assessment of Ozone Depletion]]", a series of technical reports compiled under the auspices of the World Meteorological Organization and the United Nations Environmental Program. * [[CFCs]] * [[Melanoma]], [[skin cancer]] * [[Greenhouse gas]] * [[Ultraviolet]] * [[CLaMS]] Chemical Lagrangian Model of the Stratosphere * [[Global Warming]], [[ice shelves]] * [[Atmospheric window]] ==References== {{reflist|2}} ===Nontechnical books=== * Dotto, Lydia and Schiff, Harold (1978). The Ozone War. Doubleday. ISBN 0-385-12927-0 * Roan, Sharon (1990). Ozone Crisis, the 15 Year Evolution of a Sudden Global Emergency. Wiley. ISBN 0-471-52823-4 * Cagin, Seth and Dray, Phillip (1993). Between Earth and Sky: How CFCs Changed Our World and Endangered the Ozone Layer. Pantheon. ISBN 0-679-42052-5 ===Books on public policy issues=== * Benedick, Richard E. (1991). Ozone Diplomacy. Harvard University Press. ISBN 0-674-65001-8 (Ambassador Benedick was the Chief U.S. Negotiator at the meetings that resulted in the Montreal Protocol.) * Litfin, Karen T. (1994). Ozone Discourses. Columbia University Press. ISBN 0-231-08137-5 ===Research articles=== *{{cite journal | journal = Geophysical Research Letters | volume= 31 | pages = L12814 | year= 2004 | doi = 10.1029/2004GL020596 | title = On the size of the Antarctic ozone hole? | author = Newman, P. A., Kawa, S. R. and Nash, E. R.}} *{{cite journal | journal = Nature | volume= 441 | pages = 39–45 | year= 2006 | doi = 10.1038/nature04746 | title = The search for signs of recovery of the ozone layer | author = E. C. Weatherhead, S. B. Andersen}} ==External links== {{external links}} * [http://www.theozonehole.com The Ozone Hole] ** [http://www.theozonehole.com/ozonehole2007.htm Ozone Hole 2007] ** [http://www.theozonehole.com/ozoneholehistory.htm History Of The Ozone Hole] * [http://www.eoearth.org/article/Lessons_from_the_Montreal_Protocol Lessons from the Montreal Protocol from the Encyclopedia of Earth] * NASA ** [http://science.nasa.gov/headlines/y2006/26may_ozone.htm New News About Ozone Recovery] ** [http://science.nasa.gov/headlines/y2006/images/ozone/20questions.pdf 20 questions on Ozone] ** [http://ozonewatch.gsfc.nasa.gov/ Ozone Hole Watch (Images, Data, and Information Updated Daily)] ** [http://www.nasa.gov/vision/earth/environment/ozone_resource_page.html Ozone Resource Page] * NOAA ** [http://www.ozonelayer.noaa.gov/ Stratospheric Ozone Webpage] ** [http://www.al.noaa.gov/WWWHD/Pubdocs/StratO3.html Stratospheric ozone depletion (Antarctic, Arctic, and global)] ** [http://www.cmdl.noaa.gov/ozwv/ NOAA's Ozone and Water Vapor Group] ** [http://www.al.noaa.gov/WWWHD/pubdocs/assessment98.html Scientific Assessment of Ozone Depletion: 1998] - by [[WMO]], [[UNEP]] and [[NOAA]] * [http://www.atm.ch.cam.ac.uk/tour/ The Ozone Tour] at the Centre for Atmospheric Science, University of Cambridge * [http://www.nrdc.org/air/pollution/hozone.asp The Ozone Depletion Story] at the [[Natural Resources Defense Council]] * [http://nobelprize.org/chemistry/laureates/1995/index.html The Nobel Prize in Chemistry 1995] - includes links to biographies of [[Paul Crutzen|Crutzen]], [[Mario J. Molina|Molina]], and [[Frank Sherwood Rowland|Rowland]] and to their Nobel Lectures. * [http://www.eoearth.org/article/Stratospheric_Ozone_Depletion_by_Chlorofluorocarbons_(Nobel_Lecture) Stratospheric Ozone Depletion by Chlorofluorocarbons (Nobel Lecture)] - Article by Nobel laureate [[Frank Sherwood Rowland|Rowland]] at the [[Encyclopedia of Earth]] * [http://www.ace.mmu.ac.uk/eae/Ozone_Depletion/ozone_depletion.html Ozone Depletion] - a resource for both adults and schools * [http://www.gcrio.org/CONSEQUENCES/summer95/impacts.html Impacts of a Projected Depletion of the Ozone Layer] * [http://www.wunderground.com/education/ozone_skeptics.asp The Skeptics vs. the Ozone Hole] * [http://www.findarticles.com/p/articles/mi_m1200/is_n21_v140/ai_11651661/print 1991 Science News Article about Emergency Ozone Hole Repair] * [http://www.livescience.com/environment/060630_ozone_hole.html No Quick Fix for the Ozone Hole] - An improved computer model predicts the recovery won't occur until 2068 {{Global warming }} [[Category:Climate forcing agents]] [[Category:Ozone depletion| ]] {{Link FA|fi}} [[ast:Capa d'ozonu]] [[ca:Capa d'ozó]] [[cs:Ozónová díra]] [[da:Ozonlaget]] [[de:Ozonloch]] [[et:Osooniauk]] [[el:Τρύπα του όζοντος]] [[es:Agujero de ozono]] [[eo:Ozona tavolo]] [[fr:Couche d'ozone]] [[gl:Ozonosfera]] [[it:Buco nell'ozono]] [[he:הידלדלות שכבת האוזון]] [[hu:Az ózonréteg pusztulása]] [[ms:Lapisan ozon]] [[nl:Ozongat]] [[ja:オゾンホール]] [[no:Ozonlaget]] [[nn:Ozonlaget]] [[pl:Dziura ozonowa]] [[pt:Ozonosfera]] [[qu:Allpa pachata qatachiq Usunu]] [[ru:Озоновая дыра]] [[sl:Ozonska luknja]] [[fi:Otsonikato]] [[sv:Ozonhål]] [[vi:Sự suy giảm ôzôn]] [[zh:臭氧层]]