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| '''Ozone depletion''' describes two distinct, but related observations: a slow, steady decline of about 4% per decade in the total volume of ] in ]'s ] (the ]) since the late ], 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 stratospheric ozone depletion, there are also ], 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 ] 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 ] atoms in the stratosphere is ] of ] (CFC) compounds, commonly called ]s, and of ] compounds known as ]. These compounds are transported into the stratosphere after being emitted at the surface.<ref>{{cite web | |||
| | url = http://www.sciam.com/article.cfm?id=chlorofluorocarbons-cfcs | |||
| | title = Chlorofluorocarbons (CFCs) are heavier than air, so how do scientists suppose that these chemicals reach the altitude of the ozone layer to adversely affect it? | |||
| | accessdate = 2009-03-08}} | |||
| </ref> 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 ] (UV light) from passing through the ], observed and projected decreases in ozone have generated worldwide concern leading to adoption of the ] that bans the production of CFCs and halons as well as related ozone depleting chemicals such as ] and ]. It is suspected that a variety of biological consequences such as increases in ], ],<ref name="Dobson2005">{{cite doi|10.1136/bmj.331.7528.1292-d}}</ref> damage to plants, and reduction of ] populations in the ocean's ] may result from the increased UV exposure due to ozone depletion. | |||
| ==Ozone cycle overview== | |||
| ] | |||
| Three forms (or ]) of oxygen are involved in the ]: ] atoms (O or atomic oxygen), oxygen gas (O<sub>2</sub> or diatomic oxygen), and ozone gas (O<sub>3</sub> or triatomic oxygen). ] is formed in the stratosphere when oxygen molecules ] after absorbing an ] 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 ] catalysts, the most important of which are the ] (OH·), the ] radical (NO·), atomic ] (Cl·) and ] (Br·). All of these have both natural and 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 levels of chlorine and bromine. These elements are found in certain stable organic compounds, especially ]s (CFCs), which may find their way to the ] 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 ], 'ν' is ] of ]) | |||
| CFCl<sub>3</sub> + hν → CFCl<sub>2</sub> + Cl | |||
| The Cl and Br atoms can then destroy ozone molecules through a variety of ] cycles. In the simplest example of such a cycle,<ref>{{cite book |author=Newman, Paul A. |chapter=Chapter 5: Stratospheric Photochemistry Section 4.2.8 ClX catalytic reactions |chapterurl=http://www.ccpo.odu.edu/SEES/ozone/class/Chap_5/index.htm |editor=Todaro, Richard M. |title=Stratospheric ozone: an electronic textbook |publisher=NASA Goddard Space Flight Center Atmospheric Chemistry and Dynamics Branch |year= |url=http://www.ccpo.odu.edu/SEES/ozone/oz_class.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 chlorine 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 is a 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 (thus a catalyst) 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 ] (HCl) and ] (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 ], 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></ref> | |||
| ==Observations on ozone layer depletion== | |||
| The most pronounced decrease in ozone has been in the lower ]. 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 ]s, abbreviated as "DU". Marked decreases in column ozone in the ] spring and early summer compared to the early 1970s and before have been observed using instruments such as the ] (TOMS).<ref></ref> | |||
| ] 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></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 ] 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></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.{{Citation needed|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.<ref name="epa.gov">http://www.epa.gov/ozone/science/myths/glob_dep.html</ref> | |||
| Ozone depletion also explains much of the observed reduction in stratospheric and upper ] temperatures.<ref name=wg1_223/><ref></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 ]es such as ]; however the ozone-induced cooling appears to be dominant.{{Citation needed|date=March 2008}} | |||
| Predictions of ozone levels remain difficult. The comes out strongly in favor for the Montreal Protocol, but notes that a ] 1994 Assessment overestimated ozone loss for the 1994–1997 period. | |||
| ===Chemicals in the atmosphere=== | |||
| ====CFCs in the atmosphere==== | |||
| Chlorofluorocarbons (]) were invented by ] in the 1920s. They were used in ]/cooling units, as ]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 ], 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></ref> | |||
| ===Verification of observations=== | |||
| Scientists have been increasingly able to attribute the observed ozone depletion to the increase of man-made (anthropogenic) ] compounds from CFCs by the use of complex chemistry transport models and their validation against observational data (e.g. ], ]). 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 ] products into contact with ozone. | |||
| ==The ozone hole and its causes== | |||
| ] | |||
| The Antarctic ozone hole is an area of the Antarctic stratosphere in which the recent ozone levels have dropped to as low as 33% 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 ], over 50% of the lower stratospheric ozone is destroyed during the Antarctic spring.<ref></ref> | |||
| As explained above, the primary cause of ozone depletion is the presence of chlorine-containing source gases (primarily CFCs and related halocarbons). In the presence of UV light, these gases dissociate, releasing chlorine atoms, which then go on to catalyze ozone destruction. The Cl-catalyzed ozone depletion can take place in the gas phase, but it is dramatically enhanced in the presence of ]s (PSCs).<ref></ref> | |||
| These polar stratospheric clouds(PSC) form during winter, in the extreme cold. Polar winters are dark, consisting of 3 months without solar radiation (sunlight). The lack of sunlight contributes to a decrease in temperature and the ] traps and chills air. Temperatures hover around or below -80 °C. These low temperatures form cloud particles. There are three types of PSC clouds; nitric acid trihydrate clouds, slowly cooling water-ice clouds, and rapid cooling water-ice(nacerous) clouds; that provide surfaces for chemical reactions that lead to ozone destruction.<ref>http://www.atmos.washington.edu/~davidc/ATMS211/articles_optional/Toon_Turco91_ozone.pdf</ref> | |||
| The ] 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 (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. 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.<ref></ref> | |||
| 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.<ref>{{cite journal |author=F. Sherwood Rowland |title=Stratospheric ozone depletion |journal= Phil. Trans. R. Soc. B |volume=361 |issue=1469 |pages=769–790 |date=29 May 2006 |pmid=16627294 |pmc=1609402 |doi=10.1098/rstb.2005.1783 |url=http://rstb.royalsocietypublishing.org/content/361/1469/769.full |quote=}}</ref> | |||
| ===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|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.<ref name=Zehr94/> | |||
| * Many{{Citation needed|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 ] theory predicts that the stratosphere will cool.<ref>{{cite web |url=http://www.grida.no/publications/other/ipcc%5Ftar/?src=/climate/ipcc_tar/wg1/351.htm |title=Climate Change 2001: Working Group I: The Scientific Basis |pages=Chapter 9.3.2 Patterns of Future Climate Change |year=2001 |work=] Work Group I}}</ref> | |||
| * 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.<ref>http://people.oregonstate.edu/~muirp/stratozo.htm</ref> | |||
| ==Consequences of ozone layer depletion== | |||
| Since the ozone layer absorbs ] ultraviolet light from the Sun, ozone layer depletion is expected to increase surface UVB levels, which could lead to damage, including increases in ]. 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 because ], 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 ] 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 ] due to the ozone hole can be partially inferred by ] 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, ]).<ref>{{cite web |url=http://www.niwa.co.nz/services/uvozone |title=UV & Ozone |publisher=National Institute of Water & Atmospheric Research, NZ}}</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 ]), 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|date=March 2008}} | |||
| ===Biological effects=== | |||
| The main public concern regarding the ozone hole has been the effects of increased surface UV and microwave radiation on human health. So far, ozone depletion in most locations has been typically a few percent and, as noted above, no direct evidence of health damage is available in most latitudes. 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 ], ], ], ], and ], environmentalists have been concerned that the increase in surface UV could be significant.<ref>http://abcnews.go.com/Technology/story?id=119899&page=1</ref> | |||
| ====Effects on humans==== | |||
| ] (the higher energy UV radiation absorbed by ozone) is generally accepted to be a contributory factor to ]. In addition, increased surface UV leads to increased tropospheric ozone, which is a health risk to humans.<ref>http://www.epa.gov/oar/oaqps/gooduphigh/bad.html#6</ref> | |||
| 1. '''Basal and Squamous Cell Carcinomas''' — The most common forms of skin cancer in humans, ] and ] 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 ]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">{{cite journal |author=Frank R. de Gruijl |title=Impacts of a Projected Depletion of the Ozone Layer |journal=Consequences |volume=1 |issue=2 |date=Summer 1995 |url=http://www.gcrio.org/CONSEQUENCES/summer95/impacts.html}}</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>{{cite journal |author=Setlow RB, Grist E, Thompson K, Woodhead AD |title=Wavelengths effective in induction of malignant melanoma |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=90 |issue=14 |pages=6666–70 |year=1993 |month=July |pmid=8341684 |pmc=46993 |doi=10.1073/pnas.90.14.6666 }}</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>{{cite journal |author=Fears TR, Bird CC, Guerry D, ''et al.'' |title=Average midrange ultraviolet radiation flux and time outdoors predict melanoma risk |journal=Cancer Res. |volume=62 |issue=14 |pages=3992–6 |year=2002 |month=July |pmid=12124332 |url=http://cancerres.aacrjournals.org/cgi/pmidlookup?view=long&pmid=12124332}}</ref> A study of people in ], at the southern tip of ], 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>{{cite journal |author=Abarca JF, Casiccia CC |title=Skin cancer and ultraviolet-B radiation under the Antarctic ozone hole: southern Chile, 1987-2000 |journal=Photodermatol Photoimmunol Photomed |volume=18 |issue=6 |pages=294–302 |year=2002 |month=December |pmid=12535025 |doi=10.1034/j.1600-0781.2002.02782.x |url=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 ] 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>{{cite journal |author=West SK, Duncan DD, Muñoz B, ''et al.'' |title=Sunlight exposure and risk of lens opacities in a population-based study: the Salisbury Eye Evaluation project |journal=JAMA |volume=280 |issue=8 |pages=714–8 |year=1998 |month=August |pmid=9728643 |url=http://jama.ama-assn.org/cgi/content/full/280/8/714 |doi=10.1001/jama.280.8.714}}</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>{{cite journal |author=Cruickshanks KJ, Klein BE, Klein R |title=Ultraviolet light exposure and lens opacities: the Beaver Dam Eye Study |journal=Am J Public Health |volume=82 |issue=12 |pages=1658–62 |year=1992 |month=December |pmid=1456342 |pmc=1694542 |url=http://www.ajph.org/cgi/pmidlookup?view=long&pmid=1456342 |doi=10.2105/AJPH.82.12.1658}}</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>{{cite journal |author=West SK, Muñoz B, Schein OD, Duncan DD, Rubin GS |title=Racial differences in lens opacities: the Salisbury Eye Evaluation (SEE) project |journal=Am. J. Epidemiol. |volume=148 |issue=11 |pages=1033–9 |year=1998 |month=December |pmid=9850124 |url=http://aje.oxfordjournals.org/cgi/pmidlookup?view=long&pmid=9850124}}</ref><ref>{{cite journal |author=Leske MC, Connell AM, Wu SY, Hyman L, Schachat A |title=Prevalence of lens opacities in the Barbados Eye Study |journal=Arch. Ophthalmol. |volume=115 |issue=1 |pages=105–11 |year=1997 |month=January |pmid=9006434 |url=http://archopht.ama-assn.org/cgi/pmidlookup?view=long&pmid=9006434}}</ref> | |||
| 4. '''Increased Tropospheric Ozone''' — Increased surface UV leads to increased ] ozone. Ground-level ozone is generally recognized to be a health risk, as ozone is toxic due to its strong ] properties. At this time, ozone at ground level is produced mainly by the action of UV radiation on ] gases from vehicle exhausts.{{Citation needed|date=March 2008}} | |||
| ====Effects on non-human animals==== | |||
| A November 2010 report by scientists at the Institute of Zoology in London found that whales off the coast of California have shown a sharp rise in sun damage, and these scientists "fear that the thinning ozone layer is to blame"<ref></ref> | |||
| The study photographed and took skin biopsies from over 150 whales in the Gulf of California and found "widespread evidence of epidermal damage commonly associated with acute and severe sunburn," having cells which form when the DNA is damaged by UV radiation. The findings suggest "rising UV levels as a result of ozone depletion are to blame for the observed skin damage, in the same way that human skin cancer rates have been on the increase in recent decades."<ref></ref> | |||
| ====Effects on crops==== | |||
| An increase of UV radiation would be expected to affect crops. A number of economically important species of plants, such as ], depend on ] residing on their roots for the retention of ]. 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> | |||
| ==Public policy== | |||
| ] | |||
| The full extent of the damage that CFCs have caused to the ozone layer is not known and will not be known for decades; however, marked decreases in column ozone have already been observed (as explained before). | |||
| After a 1976 report by the ] concluded that credible scientific evidence supported the ozone depletion hypothesis<ref name="NAS1976">{{cite book | last = National Academy of Sciences | first = | authorlink = United States National Academy of Sciences | coauthors = | title = Halocarbons, effects on stratospheric ozone | publisher = | year = 1976 | location = Washington, DC | pages = | url = http://books.google.com/?id=a2YrAAAAYAAJ&dq=Halocarbons:+Effects+on+Stratospheric+Ozone | doi = | id = | isbn = }}</ref> a few countries, including the United States, Canada, Sweden, Denmark, and Norway, moved to eliminate the use of CFCs in aerosol spray cans.<ref name="Morrisette1989">{{cite journal|title=The Evolution of Policy Responses to Stratospheric Ozone Depletion|journal=Natural Resources Journal|year=1989|first=Peter M.|last=Morrisette|coauthors=|volume=29|issue=|pages=793–820|id= |url=http://www.ciesin.org/docs/003-006/003-006.html|accessdate=2010-04-20 }}</ref> 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.<ref name="Morrisette1989" /> The European Community rejected proposals to ban CFCs in aerosol sprays, and in the U.S., CFCs continued to be used as refrigerants and for cleaning circuit boards. Worldwide CFC production fell sharply after the U.S. aerosol ban, but by 1986 had returned nearly to its 1976 level.<ref name="Morrisette1989" /> In 1993, ] shut down its CFC facility.<ref>http://www.ghgregistries.ca/registry/out/C650-DUPONT-PLN.PDF</ref> | |||
| The U.S. Government's attitude began to change again in 1983, when ] replaced ] as Administrator of the ]. 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 ] 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 ]. | |||
| 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. ], former head of the UN Environment Programme, who was quoted in the 30 June 1990 edition of The ], '...the chemical industry supported the Montreal Protocol in 1987 because it set up a worldwide schedule for phasing out CFCs, which 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.<ref name="Morrisette1989" /> 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 ]s) by 2000 in non-Article 5 countries and by 2010 in Article 5 (less developed) signatories <ref name=hist>http://www.epa.gov/ozone/intpol/history.html</ref> At a 1992 meeting in Copenhagen, the phase out date was moved up to 1996.<ref name=hist/>. At the same meeting, methyl bromide (MeBr), a fumigant used primarily in agricultural production, was added to the list of controlled substances. It should be noted that for all substances controlled under the Protocol, phaseout schedules were delayed for less developed ('Article 5(1)') countries, and phaseout in these countries was supported by transfers of expertise, technology, and money from non-Article 5(1) Parties to the Protocol. Additionally, exemptions from the agreed schedules could be applied for under the Essential Use Exemption (EUE) process for substances other than methyl bromide and under the Critical Use Exemption (CUE) process for methyl bromide. See Gareau<ref>{{cite journal |doi=10.1007/s10784-010-9120-z |author=Gareau BJ |title=A critical review of the successful CFC phase-out versus the delayed methyl bromide phase-out in the Montreal Protocol |journal=International Environmental Agreements-Politics Law and Economics |volume=10 |issue=3 |pages=209–231 |year=2010 }}</ref> and DeCanio and Norman<ref>{{cite journal |author=DeCanio SJ, Norman CS |title=Economics of the 'Critical use' of Methyl Bromide under the Montreal Protocol |journal=Contemporary Economic Policy |volume=23 |issue=3 |pages=376–393 |year=2005 |month=July |doi=10.1093/cep/byi028 }}</ref> for more detail on the exemption processes. | |||
| To some extent, CFCs have been replaced by the less damaging hydro-chloro-fluoro-carbons (]s), although concerns remain regarding HCFCs also. In some applications, hydro-fluoro-carbons (]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 (]), which in the United States has largely replaced CFC-12 (]) in automobile air conditioners. In laboratory analytics (a former "essential" use) the ozone depleting substances can be replaced with various other solvents.<ref></ref> | |||
| ''Ozone Diplomacy'', by Richard Benedick (Harvard University Press, 1991) gives a detailed account of the negotiation process that led to the Montreal Protocol. | |||
| provide an extensive review of early U.S. government responses to the emerging science of ozone depletion by CFCs. | |||
| More recently, policy experts have advocated for efforts to link ozone protection efforts to climate protection efforts.<ref>{{cite journal |author=Mario Molina, Durwood Zaelke, K. Madhava Sarma, Stephen O. Andersen, Veerabhadran Ramanathan, Donald Kaniaru |title=Reducing abrupt climate change risk using the Montreal Protocol and other regulatory actions to complement cuts in CO2 emissions |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=106 |issue=49 |pages=20616–21 |year=2009 |pmid=19822751 |pmc=2791591 |doi=10.1073/pnas.0902568106 }}</ref> <ref>{{cite journal |author=Norman CS, DeCanio SJ, Fan L |title=The Montreal Protocol at 20: Ongoing opportunities for integration with climate protection |journal=Global Environmental Change |volume=18 |issue=2 |pages=330–340 |year=2008 |month=May |doi=10.1016/j.gloenvcha.2008.03.003 }}</ref>Many ODS are also greenhouse gasses, some significantly more powerful agents of radiative forcing than carbon dioxide over the short and medium term. Policy decisions in one arena affect the costs and effectiveness of environmental improvements in the other. | |||
| ==Prospects of ozone depletion== | |||
| ] | |||
| Since the adoption and strengthening of the Montreal Protocol has led to reductions in the emissions of CFCs, atmospheric concentrations of the most significant compounds have been declining. These substances are being gradually removed from the atmosphere—since peaking in 1994, the Effective Equivalent Chlorine (EECl) level in the atmosphere had dropped about 10% by 2008. It is estimated that by 2015, the Antarctic ozone hole will have reduced by 1 million km² out of 25 (Newman ''et al.'', 2004); complete recovery of the Antarctic ozone layer is not expected to 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> | |||
| The decrease in ozone-depleting chemicals has also been significantly affected by a decrease in ]-containing chemicals. The data suggest that substantial natural sources exist for atmospheric ] (CH<sub>3</sub>Br).<ref></ref> The phase-out of CFCs means that ] (N<sub>2</sub>O), which is not covered by the Montreal Protocol, has become the most highly emitted ozone depleting substance and is expected to remain so throughout the 21st century.<ref></ref> | |||
| When 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></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></ref> | |||
| A 2005 ] summary of ozone issues concluded 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>{{cite book |chapter=Summary for Policymakers |chapterurl=http://www.ipcc.ch/pdf/special-reports/sroc/sroc_spm.pdf |title=IPCC/TEAP special report on safeguarding the ozone layer and the global climate system: issues related to hydrofluorocarbons and perfluorocarbons |publisher=Published for the Intergovernmental Panel on Climate Change Cambridge University Press |location=Cambridge |year=2005 |isbn=0-521-86336-8 |url=http://www.ipcc.ch/publications_and_data/publications_and_data_reports_safeguarding_the_ozone_layer.htm}}</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 — 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></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></ref> During March 2006, the Arctic stratosphere poleward of 60° 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.<ref></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 24 September. In October 2006, ] 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.<ref></ref><ref>{{cite web |url=http://www.theregister.co.uk/2006/10/03/ozone_depletion |title=Ozone layer hits new depletion record |work=The Register |date=2006-10-03}}</ref> | |||
| On October 2008 the ] 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 ] ] 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.<ref></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 ] in 1930. These are discussed in the article ] — briefly, short-wavelength ] 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 – it was estimated that in their absence, the ozone layer would be about twice as thick as it currently is. | |||
| In 1970 Prof. ] 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 ] 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 ] ], which fly in the lower stratosphere, could also deplete the ozone layer. | |||
| ===The Rowland-Molina hypothesis=== | |||
| In 1974 ], Chemistry Professor at the University of California at Irvine, and his postdoctoral associate ] suggested that long-lived organic halogen compounds, such as ]s, might behave in a similar fashion as Crutzen had proposed for nitrous oxide. ] (most popularly known as the creator of the ]) 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 ] 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 ] and ] at ]. Neither group, however, had realized that CFCs were a potentially large source of stratospheric chlorine — instead, they had been investigating the possible effects of HCl emissions from the Space Shuttle, which are very much smaller.) | |||
| ] was strongly disputed by representatives of the aerosol and ] industries. The Chair of the Board of ] was quoted as saying that ozone depletion theory is "a science fiction tale...a load of rubbish...utter nonsense".<ref name = "greenpeace-ozone"/> ], the President of Precision Valve Corporation (and inventor of the first practical aerosol spray can valve), wrote to the Chancellor of ] 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 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 ] known as ]s, widely used in fire extinguishers, were a potentially large source of stratospheric bromine. In 1976 the ] 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 ] 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.<ref name = "NAS_report">{{cite paper |title=Causes and Effects of Stratospheric Ozone Reduction: An Update |publisher=National Academy of Sciences |date=(1982 and 1983) |url=http://www.nap.edu/openbook.php?isbn=0309032482}}</ref> | |||
| Crutzen, Molina, and Rowland were awarded the 1995 ] for their work on stratospheric ozone. | |||
| ===The ozone hole=== | |||
| The discovery of the Antarctic "ozone hole" by ] scientists ], ] and ] (announced in a paper in '']'' 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.<ref name=Zehr94>{{cite journal |doi=10.1111/j.1533-8525.1994.tb00419.x |author=Stephen C. Zehr |title=Accounting for the Ozone Hole: Scientific Representations of an Anomaly and Prior Incorrect Claims in Public Settings |journal=The Sociological Quarterly |volume=35 |issue=4 |pages=603–19 |year=1994 |month=November |url=http://www.jstor.org/pss/4121521}}</ref> Satellite measurements showing massive depletion of ozone around the ] 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 ] was rerun without the flags, the ozone hole was seen as far back as 1976.<ref> accessed 18 November 2007.</ref> | |||
| ], an atmospheric chemist at the National Oceanic and Atmospheric Administration (NOAA), proposed that chemical reactions on ]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 ], 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 ] formed over ] 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 ] 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.<ref>{{cite journal |doi=10.1038/328411a0 |author=P. M. Solomon, B. Connor, R. L. de Zafra, A. Parrish, J. Barrett, M. Jaramillo |title=High concentrations of chlorine monoxide at low altitudes in the Antarctic spring stratosphere: secular variation |journal=Nature |volume=328 |issue=6129 |pages=411–3 |year=1987 |month=July |url=http://www.nature.com/nature/journal/v328/n6129/abs/328411a0.html}}</ref> | |||
| 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|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.<ref name="epa.gov"/> These trends were confirmed by satellite measurements. As a consequence, the major ] producing nations agreed to phase out production of CFCs, halons, and related compounds, a process that was completed in 1996. | |||
| Since 1981 the ] has sponsored a series of reports on ], based on satellite measurements. The 2007 report showed that the hole in the ozone layer was recovering and the smallest it had been for about a decade.<ref>{{cite news |title=Ozone hole closing up, research shows |url=http://abc.net.au/news/stories/2007/11/16/2092527.htm |work=ABC News |publisher=Australian Broadcasting Commission |date=2007-11-16 }}</ref> | |||
| The 2010 report found that "Over the past decade, global ozone and ozone in the Arctic and Antarctic regions is no longer decreasing but is not yet increasing... the ozone layer outside the Polar regions is projected to recover to its pre-1980 levels some time before the middle of this century... In contrast, the springtime ozone hole over the Antarctic is expected to recover much later."<ref>{{cite news |title=New report highlights two-way link between ozone layer and climate change |url=http://www.unep.org/Documents.Multilingual/Default.asp?DocumentID=647&ArticleID=6751&l=en&t=long |work=UNEP News Center |date=2010-11-16 }}</ref> | |||
| ==Ozone depletion and global warming== | |||
| There are five areas of linkage between ozone depletion and ]: | |||
| ] from various ]es and other sources.]] | |||
| * The same CO<sub>2</sub> radiative forcing that produces global warming is expected to cool the ].<ref name= ipcc2007>{{cite web | url= http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter9.pdf | format=] | title=Understanding and Attributing Climate Change | work=Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change | accessdate=2008-02-01 | publisher=] | last=Hegerl | first=Gabriele C. | coauthors=''et al.'' | pages=675}}</ref> This cooling, in turn, is expected to produce a relative ''increase'' in ] (O<sub>3</sub>) depletion in polar area and the frequency of ozone holes.<ref>http://earthwatch.unep.net/emergingissues/atmosphere/ozonedepletion.php</ref> | |||
| * 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 ]; the resulting colder stratosphere emits less long-wave radiation downward, thus cooling the troposphere. Overall, the cooling dominates; the IPCC concludes that "''observed stratospheric ] losses over the past two decades have caused a negative forcing of the surface-troposphere system''"<ref name=wg1_223>{{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 Stratospheric Ozone |year=2001 |work=] Work Group I}}</ref> of about −0.15 ± 0.10 ]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= ] and Technology and Economic Assessment Panel |year=2005 |url=http://www.ipcc.ch/press/SPM.pdf |format=PDF |accessdate=2007-03-04 |archiveurl = http://web.archive.org/web/20070221055911/http://www.ipcc.ch/press/SPM.pdf |archivedate = February 21, 2007}}</ref> | |||
| * One of the strongest predictions of the greenhouse effect is that the stratosphere will cool.<ref name= ipcc2007/> Although this cooling has been observed, it is not trivial to separate the effects of changes in the concentration of ]es and ozone depletion since both will lead to cooling. However, this can be done by numerical stratospheric modeling. Results from the ]'s ] show that above 20 km (12.4 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> | |||
| * As noted under 'Public Policy', ozone depleting chemicals are also often greenhouse gases. The increases in concentrations of these chemicals have produced 0.34 ± 0.03 W/m² of radiative forcing, corresponding to about 14% 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 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 ] 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 . | |||
| ;CFCs are "too heavy" to reach the stratosphere | |||
| It is commonly believed that CFC molecules are heavier than air (nitrogen or oxygen), so that the CFC molecules cannot reach the stratosphere in significant amount.<ref></ref> But atmospheric gases are not sorted by weight; the forces of wind can fully mix the gases in the atmosphere. Despite the fact that CFCs are heavier than air and with a long lifetime, they are evenly distributed throughout the ] and reach the upper atmosphere.<ref>, section 1.3.</ref> | |||
| ;Man-made chlorine is insignificant compared to natural sources | |||
| ] | |||
| Another misconception is that "it is generally accepted that natural sources of tropospheric chlorine are four to five times larger than man-made one". While strictly true, ''tropospheric'' chlorine is irrelevant; it is ''stratospheric'' chlorine that affects ozone depletion. Chlorine from ] is soluble and thus is washed by rainfall before it reaches the stratosphere. CFCs, in contrast, are insoluble and long-lived, allowing them to reach the stratosphere. In the lower atmosphere, there is much more chlorine from CFCs and related ]s than there is in HCl from salt spray, and in the stratosphere halocarbons are dominant .<ref>, section 4.3</ref> Only methyl chloride which is one of these halocarbons has a mainly natural source ,<ref>{{cite journal |author1=Y. Yokouchi |author2=Y. Noijiri |author3=L. A. Barrie |author4=D. Toom-Sauntry |author5=T. Machida |author6=Y. Inuzuka |author7=H. Akimoto |date=2000-01-20 |title=A strong source of methyl chloride to the atmosphere from tropical coastal land |journal=Nature |volume=403 |issue=6767 |pages=295–8 |doi=10.1038/35002049 |url=http://www.nature.com/nature/journal/v403/n6767/full/403295a0.html |pmid=10659845 |last8=Li |first8=HJ |last9=Fujinuma |first9=Y}}</ref> and it is responsible for about 20 percent of the chlorine in the stratosphere; the remaining 80% comes from man made sources. | |||
| Very violent volcanic eruptions can inject HCl into the stratosphere, but researchers<ref name=O3F4_4>, section 4.4</ref> have shown that the contribution is not significant compared to that from CFCs. | |||
| A similar erroneous assertion is that soluble halogen compounds from the volcanic plume of ] on Ross Island, Antarctica are a major contributor to the Antarctic ozone hole.<ref name=O3F4_4/> | |||
| ;An ozone hole was first observed in 1956 | |||
| ] (Exploring the Atmosphere, 2nd Edition, Oxford, 1968) mentioned that when springtime ozone levels over ] were first measured in 1956, he was surprised to find that they were ~320 DU, about 150 DU below spring levels, ~450 DU, in the Arctic. These, however, were at this time the known normal climatological values because no other Antarctic ozone data were available. What Dobson describes is essentially the ''baseline'' from which the ozone hole is measured: actual ozone hole values are in the 150–100 DU range. | |||
| The discrepancy between the Arctic and Antarctic noted by Dobson was primarily a matter of timing: during the Arctic spring ozone levels rose smoothly, peaking in April, whereas in the Antarctic they stayed approximately constant during early spring, rising abruptly in November when the polar vortex broke down. | |||
| The behavior seen in the Antarctic ozone hole is 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>, section 6</ref> | |||
| ;The ozone hole should be above the sources of CFCs | |||
| Some people thought that the ozone hole should be above the sources of CFCs. | |||
| However, CFCs are well mixed in the ] and the ]. The reason for occurrence of the ozone hole above Antarctica is not because there are more CFCs concentrated but because the low temperatures help form polar stratospheric clouds.<ref></ref> In fact, there are findings of significant and localized "ozone holes" above other parts of the earth.<ref name="autogenerated1"></ref> | |||
| ;The "ozone hole" is a hole in the ozone layer | |||
| There is a common misconception that “ozone hole” is really a hole in the ozone layer.When the "ozone hole" occurs, the ozone in the lower stratosphere is destroyed. The upper stratosphere is less affected, so that the amount of ozone over the continent decreases by 50 percent or even more. The ozone hole does not disappear through the layer; on the other hand, it is not a uniform 'thinning' of the ozone layer. It is a "hole" which is a depression, not in the sense of "a hole in the windshield." | |||
| ==ODS requirements in the Marine industry== | |||
| IMO has amended MARPOL Annex VI Regulation 12 regarding ozone depleting substances. | |||
| As from 1 July 2010 all vessels where MARPOL Annex VI is applicable should have a list of equipment using ozone depleting substances. The list should include name of ODS, type and location of equipment, quantity in kg and date. All changes since that date should be recorded in a ODS Record book on board recording all intended or unintended releases to the atmosphere. Further more new ODS supply or landing to shore facilities should be recorded as well. | |||
| More info and how to draft a record book and an example of ODS list can be found in web site. | |||
| ==World Ozone Day== | |||
| In 1994, the ] voted to designate the 16th of September as "World Ozone Day", to commemorate the signing of the ] on that date in 1987. | |||
| ==See also== | |||
| * ] | |||
| * ] | |||
| * "]", a series of technical reports compiled under the auspices of the World Meteorological Organization and the United Nations Environmental Program. | |||
| * ] | |||
| * ], ] | |||
| * ] | |||
| * ] | |||
| * ] Chemical Lagrangian Model of the Stratosphere | |||
| * ], ] | |||
| * ] | |||
| ==References== | |||
| {{Reflist|2}} | |||
| ===Nontechnical books=== | |||
| *{{cite book |author=Schiff, Harold; Dotto, Lydia; |title=The Ozone war |publisher=Doubleday |location=Garden City, N.Y |year=1978 |isbn=0-385-12927-0 }} | |||
| *{{cite book |author=Roan, Sharon |title=Ozone crisis: The 15-year evolution of a sudden global emergency |publisher=Wiley |location=New York |year=1989 |isbn=0-471-52823-4 }} | |||
| *{{cite book |author=Dray, Philip; Cagin, Seth |title=Between earth and sky: how CFCs changed our world and endangered the ozone layer |publisher=Pantheon Books |location=New York |year=1993 |isbn=0-679-42052-5 }} | |||
| ===Books on public policy issues=== | |||
| *{{cite book |author=Richard Elliot Benedick |authorlink=Richard E. Benedick |title=Ozone diplomacy: New directions in safeguarding the planet |publisher=Harvard University Press |location=Cambridge |year=1991 |isbn=0-674-65001-8 }} (Ambassador Benedick was the Chief U.S. Negotiator at the meetings that resulted in the Montreal Protocol.) | |||
| *{{cite book |author=Litfin, Karen |title=Ozone discourses: Science and politics in global environmental cooperation |publisher=Columbia University Press |location=New York |year=1994 |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 | |||
| | pmid = 16672963 | |||
| | issue = 7089}} | |||
| ==External links== | |||
| *{{dmoz|Science/Environment/Ozone_Layer|Ozone layer}} | |||
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| {{Global warming }} | |||
| {{Pollution}} | |||
| {{DEFAULTSORT:Ozone Depletion}} | |||
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