<MPQA autoclass="obj" certainty="26.6">Image of the largest antarctic ozone hole ever recorded in September 2000.</MPQA>
<MPQA autoclass="obj" certainty="25.4">Data taken by the Total Ozone Mapping Spectrometer (TOMS) instrument aboard NASA's Earth Probe satellite.</MPQA>
<MPQA autoclass="obj" certainty="20.7">Ozone depletion refers to the approximately 5% reduction in in the total amount of ozone in the stratosphere detected from 1979 to 1990, whereas ozone hole refers to the much larger losses - up to 70% over Antarctica or 30% over the Arctic.</MPQA>
<MPQA autoclass="obj" certainty="19.5">This reduction is held by a wide scientific consensus to be due primarily to an increase in concentrations of stratospheric chlorine from breakdown of human manufactured CFC emissions.</MPQA>
<MPQA autoclass="obj" certainty="4.2">Ozone depletion varies geographically and by season.</MPQA>
<MPQA autoclass="subj" certainty="13.4">Since the ozone layer prevents most <MPQA autoclass="negative">harmful</MPQA> wavelengths of ultraviolet light from passing through the earth's atmosphere, observed and projected decreases in ozone have generated worldwide <MPQA autoclass="negative"><MPQA autoclass="speechDirectSubjective">concern</MPQA></MPQA> and led to speedy adoption of the Montreal Protocol banning CFC use.</MPQA>
<MPQA autoclass="obj" certainty="2.8">In public policy discussions, the term ozone layer depletion is <MPQA autoclass="objectiveSpeech">considered</MPQA> synonymous with the theory that a trend of global ozone depletion, which is caused by CFC emissions, is subsequently allowing more ultraviolet radiation to reach the earth's surface.</MPQA>
<MPQA autoclass="obj" certainty="20.5">It is suspected that a variety of biological consequences, including, for example, increases in melanoma and the destruction of plankton populations in the ocean's photic zone, may result from this increased UV exposure.</MPQA>
<MPQA autoclass="obj" certainty="18.0">1 History of the research 2 General 3 Observations 4 The ozone hole and its causes4.1</MPQA>
<MPQA autoclass="subj" certainty="2.7">Increased UV due to the ozone hole 4.2 Biological effects of increased UV 4.3 Public policy in response to the ozone hole 5 Controversy regarding ozone science and policy 6 Related articles 7 External linksHistory of the researchIn 1970 Prof. Paul Crutzen <MPQA autoclass="speechDirectSubjective">pointed out</MPQA> the possibility that nitrogen oxides from fertilizers and supersonic aircraft might deplete the ozone layer.</MPQA>
<MPQA autoclass="obj" certainty="2.7">In 1974 Frank Sherwood Rowland and Mario J. Molina realised that when CFCs finally break apart in the atmosphere and release chlorine atoms they cause ozone depletion.</MPQA>
<MPQA autoclass="obj" certainty="18.1">These three scientists received the Nobel Prize in Chemistry in 1995 for this work.</MPQA>
<MPQA autoclass="obj" certainty="6.4">They calculated that if CFC production continued to increase at the going rate of 10%/year until 1990, then remain steady, CFCs would cause a global 5 to 7 percent ozone loss by 1995 and 30-50% loss by 2050.</MPQA>
<MPQA autoclass="subj" certainty="9.9">However, the discovery of the Antarctic "ozone hole" by Farman, Gardiner and Shanklin (<MPQA autoclass="speechDirectSubjective">announced</MPQA> in a paper in Nature in May 1985) was a surprise - chemical reactions on PSCs in the <MPQA autoclass="negative">cold</MPQA> Antarctic stratosphere caused faster depletion than expected - and caused worldwide publicity.</MPQA>
<MPQA autoclass="subj" certainty="17.2">Ozone depletion has been observed all over the globe but is greatest at high latitudes (that is, near the poles).</MPQA>
<MPQA autoclass="obj" certainty="2.6">The best known example is the annual thinning of the ozone layer over Antarctica during the polar winter (see ozone hole section below).</MPQA>
<MPQA autoclass="obj" certainty="22.2">Since 1981 the UNEP has sponsored a series of reports on scientific assessment of ozone depletion.</MPQA>
<MPQA autoclass="obj" certainty="27.4">The most recent is from 2002.</MPQA>
<MPQA autoclass="obj" certainty="22.3">GeneralReleasing CFCs into the atmosphere has caused ozone depletion.</MPQA>
<MPQA autoclass="obj" certainty="19.3">Since the ozone layer absorbs UV, this would be expected to increase surface UV levels, which could lead to damage , including increases in skin cancer .</MPQA>
<MPQA autoclass="obj" certainty="20.4">This is the reason for the Montreal Protocol on Substances that Deplete the Ozone Layer.</MPQA>
<MPQA autoclass="obj" certainty="2.2">Although decreases in stratospheric ozone are well-tied to CFC's, and there are good theoretical reasons to <MPQA autoclass="objectiveSpeech">believe</MPQA> that decreases in ozone will lead to increases in surface UV, there is not much direct observational evidence linking ozone depletion to additional skin cancer in human beings.</MPQA>
<MPQA autoclass="obj" certainty="16.9">Ozone in the Earth's atmosphere is generally created by ultraviolet light striking oxygen molecules, which consist of two oxygen atoms (O2), creating two single oxygen atoms, known as atomic oxygen.</MPQA>
<MPQA autoclass="obj" certainty="20.7">The atomic oxygen then combines with a molecule of O2 to create ozone, O3.</MPQA>
<MPQA autoclass="obj" certainty="18.2">The ozone molecule is also unstable and when hit by ultraviolet light it splits into a molecule of O2 and an atom of atomic oxygen, a continuing process <MPQA autoclass="objectiveSpeech">called</MPQA> the ozone-oxygen cycle.</MPQA>
<MPQA autoclass="obj" certainty="15.1">Ozone can be destroyed by atomic chlorine, fluorine or bromine in the atmosphere.</MPQA>
<MPQA autoclass="obj" certainty="10.2">These elements are found in certain stable compounds, especially chlorofluorocarbons (CFCs), which may find their way to the stratosphere and there be liberated by the action of ultraviolet light.</MPQA>
<MPQA autoclass="obj" certainty="10.0">Most importantly, the chlorine atoms so generated destroy ozone molecules in a catalytic cycle.</MPQA>
<MPQA autoclass="obj" certainty="7.5">In this cycle a single chlorine atom would keep on destroying ozone forever, were it not for reactions that remove chlorine atoms from this cycle by forming reservoir species such as hydrochloric acid and chlorine nitrate.</MPQA>
<MPQA autoclass="obj" certainty="8.9">The reactivation of atomic chlorine from these reservoir species is normally slow, but is enhanced by the presence of polar stratospheric clouds which appear during Arctic winters, leading to a strong seasonal cycle in ozone hole formation.</MPQA>
<MPQA autoclass="obj" certainty="15.6">Ozone depletion also explains much of the the observed reduction in stratospheric and upper tropospheric temperatures [1] (http://www.grida.no/climate/ipcc_tar/wg1/223.htm)</MPQA>
<MPQA autoclass="obj" certainty="25.4">[2] (http://www.giss.nasa.gov/edu/gwdebate/).</MPQA>
<MPQA autoclass="obj" certainty="20.6">This is because the reason for the warmth of the stratosphere is absorption of UV radiation by ozone, hence reduced ozone leads to cooling.</MPQA>
<MPQA autoclass="obj" certainty="14.4">Some stratospheric cooling is also predicted from increases in greenhouse gases such as CO2; however the ozone-induced cooling is found to be probably dominant.</MPQA>
<MPQA autoclass="obj" certainty="19.1">ObservationsThe most pronounced decrease in ozone has been in the lower stratosphere.</MPQA>
<MPQA autoclass="obj" certainty="15.4">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 <MPQA autoclass="objectiveSpeech">expressed</MPQA> in Dobson units.</MPQA>
<MPQA autoclass="obj" certainty="17.3">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) [3] (http://www.atm.ch.cam.ac.uk/tour/part2.html).</MPQA>
<MPQA autoclass="obj" certainty="15.8">Substantial reductions of up to 70% in the ozone column observed in the austral (i.e. southern hemispheric) spring over Antarctica and first <MPQA autoclass="objectiveSpeech">reported</MPQA> in 1985 (Farman et al 1985) are continuing [4] (http://www.wmo.ch/web/arep/reports/o3_assess_rep_2002_front_page.html).</MPQA>
<MPQA autoclass="subj" certainty="1.1">Through the 1990's, total column ozone in September and October have continued to be 40-50% lower than pre-ozone-hole values.</MPQA>
<MPQA autoclass="subj" certainty="23.1">In the arctic, <MPQA autoclass="negative">declines</MPQA> are greatest in winter/spring and the amount is more variable year-to-year than in the Antarctic: when the stratosphere is colder the <MPQA autoclass="negative">losses</MPQA> are greater, up to 30%In mid latitudes it is probably preferable to speak of ozone depletion rather than holes; <MPQA autoclass="negative">declines</MPQA> are about 3% below pre-1980 values for 35-60N and about 6% for 35-60S.</MPQA>
<MPQA autoclass="subj" certainty="0.8">In the tropics, there are no significant trends.</MPQA>
<MPQA autoclass="subj" certainty="19.0">The ozone hole and its causesThe cause of the ozone holes is generally agreed to be CFC (Chlorofluorocarbon) compounds which <MPQA autoclass="negative">break</MPQA> down (due to ultraviolet light) and become free radicals containing chlorine high in the Earth's atmosphere.</MPQA>
<MPQA autoclass="subj" certainty="15.7">These radicals then <MPQA autoclass="negative">break</MPQA> down the ozone catalytically.</MPQA>
<MPQA autoclass="subj" certainty="13.7">Ozone <MPQA autoclass="negative">destruction</MPQA> due to chlorine radicals from CFCs can take place in the gas phase, but occurs particularly rapidly on the surface of polar stratospheric clouds (PSC), which form over the poles (particularly the south pole) during winter.</MPQA>
<MPQA autoclass="obj" certainty="6.1">A rise in CFC production has accompanied the ozone depletion and a plausible chemical mechanism for CFC's role in ozone depletion has been proposed.</MPQA>
<MPQA autoclass="obj" certainty="2.3">As a result, a worldwide ban on most uses of CFCs, the Montreal Protocol, was signed and entered into force in 1989.</MPQA>
<MPQA autoclass="subj" certainty="41.4">The photochemical processes involved are <MPQA autoclass="negative">complex</MPQA> but well understood, with UV radiation being involved in both the natural production and <MPQA autoclass="negative">destruction</MPQA> of ozone, as well as the breakdown of CFCs into free radicals and the <MPQA autoclass="negative">destruction</MPQA> of ozone by chlorine radicals.</MPQA>
<MPQA autoclass="subj" certainty="41.4">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.</MPQA>
<MPQA autoclass="obj" certainty="1.9">CFCs are a byproduct of some chemical processes, and were also used in air conditioning/cooling units.</MPQA>
<MPQA autoclass="subj" certainty="3.4">They were also widely used as aerosol propellants prior to the 1980s.</MPQA>
<MPQA autoclass="subj" certainty="41.8">What makes CFCs so effective in <MPQA autoclass="negative">breaking</MPQA> down ozone is that one CFC <MPQA autoclass="negative">radical</MPQA> acts as a catalyst and can <MPQA autoclass="negative">break</MPQA> down many ozone molecules.</MPQA>
<MPQA autoclass="subj" certainty="39.4">Furthermore, these radicals stay in the atmosphere for a very long time.</MPQA>
<MPQA autoclass="subj" certainty="7.6">Scientists have increasingly been able to attribute the observed ozone depletion to the increase of anthropogenic halogen compounds from CFCs, by the use of <MPQA autoclass="negative">complex</MPQA> chemical transport models and their validation against observational data (e.g. SLIMCAT (http://www.lec.leeds.ac.uk/~martyn/slimcat.html)).</MPQA>
<MPQA autoclass="obj" certainty="9.1">These models work by combining satellite measurements of chemical concentrations and meteorological fields with chemical reaction rate constants obtained in lab experiments, and are able to identify not only the key chemical reactions but also the transport processes which bring CFC photolysis products into contact with ozone.</MPQA>
<MPQA autoclass="obj" certainty="14.1">Increased UV due to the ozone holeAlthough ozone, O3, is a minority constituent in the earth's atmosphere, it is responsible for most of the main absorption of ultraviolet (UV) radiation in the atmosphere.</MPQA>
<MPQA autoclass="subj" certainty="13.8">Correspondingly, a significant decrease in atmospheric ozone could be expected to give rise to significantly increased levels of UV near the surface.</MPQA>
<MPQA autoclass="subj" certainty="53.0">Increases in surface UV due to the ozone hole can be partially inferred by radiative transfer model calculations, but cannot be calculated from direct measurements because of the <MPQA autoclass="negative">lack</MPQA> of <MPQA autoclass="negative">reliable</MPQA> historical (pre-ozone-hole) surface UV data, although more recent surface UV observation measurement programmes exist (e.g. at Lauder, New Zealand [5] (http://www.niwa.co.nz/services/uvozone/)).</MPQA>
<MPQA autoclass="subj" certainty="54.0">Because it is this same UV radiation that creates the ozone in the ozone layer from O2 (regular oxygen) in the first place, a reduction in stratospheric ozone would actually tend to increase photochemical production of ozone at lower levels (in the troposphere), although the overall observed trends in total column ozone are still a decrease, largely because ozone produced lower down has a naturally shorter photochemical lifetime, so it is <MPQA autoclass="negative">destroyed</MPQA> before the concentrations could reach a level which would compensate for the ozone reduction higher up.</MPQA>
<MPQA autoclass="subj" certainty="37.3">Biological effects of increased UVThe main public <MPQA autoclass="negative"><MPQA autoclass="speechDirectSubjective">concern</MPQA></MPQA> regarding the ozone hole has been the effects of surface UV on human health.</MPQA>
<MPQA autoclass="subj" certainty="3.9">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 <MPQA autoclass="negative"><MPQA autoclass="speechDirectSubjective">concerned</MPQA></MPQA> that the increase in surface UV could be significant.</MPQA>
<MPQA autoclass="obj" certainty="15.7">UVB (the higher energy UV radiation absorbed by ozone) is generally accepted to be a contributory factor to malignant melanoma (skin cancer ) -- for example one study showed that a 10% increase in the UVB was associated with a 19% increase in melanomas for men and 16% for women ( Fears et al, Cancer Res.</MPQA>
<MPQA autoclass="obj" certainty="23.8">2002, 62(14):3992-6).</MPQA>
<MPQA autoclass="obj" certainty="11.0">So far, ozone depletion in most locations has been typically a few percent.</MPQA>
<MPQA autoclass="subj" certainty="6.2">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.</MPQA>
<MPQA autoclass="obj" certainty="19.0">For example, recent research [6] (http://sci.newsfactor.com/perl/story/15776.html)</MPQA>
<MPQA autoclass="subj" certainty="0.1">has analyzed a widespread extinction of plankton 2 million years ago that coincided with a nearby supernova.</MPQA>
<MPQA autoclass="subj" certainty="22.1">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 oxides that catalyzed the <MPQA autoclass="negative">destruction</MPQA> of ozone (plankton are particularly <MPQA autoclass="negative">susceptible</MPQA> to effects of UV light, and are vitally important to marine food-webs).</MPQA>
<MPQA autoclass="subj" certainty="2.9">Aside from the direct effect of ultraviolet radiation on organisms, increased surface UV leads to increased tropospheric ozone, as <MPQA autoclass="speechDirectSubjective">noted</MPQA> above.</MPQA>
<MPQA autoclass="obj" certainty="6.6">Paradoxically, at ground-level increased ozone is generally recognised to be a health risk, as ozone is toxic due to its strong oxidant properties.</MPQA>
<MPQA autoclass="obj" certainty="9.8">Public policy in response to the ozone holeEnvironmentalists assert that the CFCs have caused so much damage to the ozone layer that the use of CFCs should be banned.</MPQA>
<MPQA autoclass="obj" certainty="12.6">The full extent of this damage CFCs have caused is not known and will not be known for decades; however marked decreases in column ozone have already been observed (see above).</MPQA>
<MPQA autoclass="obj" certainty="4.5">In 1987, the Montreal Protocol was signed, controlling the emissions of CFCs.</MPQA>
<MPQA autoclass="subj" certainty="39.3">To some extent, their role has been replaced by the less damaging hydro-chloro-fluoro-carbons (HCFCs), although <MPQA autoclass="speechDirectSubjective"><MPQA autoclass="negative">concerns</MPQA></MPQA> remain regarding HCFCs also.</MPQA>
<MPQA autoclass="subj" certainty="40.4">Controversy regarding ozone science and policyAny counter-measures which have a <MPQA autoclass="negative">negative</MPQA> economic impact will remain a controversial issue due to the strong economic interests involved, with key questions regarding whether the scientific understanding is strong enough to warrant the proposed countermeasures.</MPQA>
<MPQA autoclass="subj" certainty="35.6">In this context it is worth <MPQA autoclass="speechDirectSubjective">noting</MPQA> that it is commonly <MPQA autoclass="speechDirectSubjective">believed</MPQA> that one reason for the relative ease of introduction of the Montreal protocol was the availability of CFC replacements at little extra cost.</MPQA>
<MPQA autoclass="subj" certainty="42.1">The consensus amongst most atmospheric physicists and chemists is that the scientific understanding has now reached a level where countermeasures to control CFC emissions are justified, although the <MPQA autoclass="speechDirectSubjective">decision</MPQA> is ultimately one for policy-makers and society.</MPQA>
<MPQA autoclass="subj" certainty="39.7">Despite this general consensus, the science behind ozone depletion remains <MPQA autoclass="negative">complex</MPQA>, and some who <MPQA autoclass="negative">oppose</MPQA> the enforcement of countermeasures point to some of the <MPQA autoclass="negative">difficulties</MPQA> experienced in these studies.</MPQA>
<MPQA autoclass="obj" certainty="0.4">For example:Initial studies of the ozone hole were hampered with difficulties .</MPQA>
<MPQA autoclass="subj" certainty="23.7">Most notably, satellite measurements showing massive depletion of ozone around the south pole were initially <MPQA autoclass="speechDirectSubjective"><MPQA autoclass="negative">rejected</MPQA></MPQA> as <MPQA autoclass="negative">unreasonable</MPQA> by data quality control algorithms; the ozone hole was only detected in satellite data when the raw data was reprocessed with modified processing algorithms following evidence of an ozone hole in in situ observations.</MPQA>
<MPQA autoclass="subj" certainty="6.0">This, however, was simply a <MPQA autoclass="negative">problem</MPQA> with the data-processing algorithms for the satellite data and has long been corrected, and so has no bearing on the current situation.</MPQA>
<MPQA autoclass="obj" certainty="19.4">Predictions of ozone remains a difficult science.</MPQA>
<MPQA autoclass="obj" certainty="23.5">The World Meteorological Organization Global Ozone Research and Monitoring Project - <MPQA autoclass="objectiveSpeech">Report</MPQA> No. 44 (http://www.al.noaa.gov/WWWHD/Pubdocs/Assessment98/executive-summary.html#A),</MPQA>
<MPQA autoclass="obj" certainty="4.5">which on balance comes out strongly in favour of the Montreal protocol, nonetheless <MPQA autoclass="objectiveSpeech">notes</MPQA> that projections of ozone loss for the 1994-1997 period made in the UNEP 1994 Assessment had been an overestimate.</MPQA>
<MPQA autoclass="obj" certainty="9.2">Although increased UVB has been shown to constitute a melanoma risk (see above), it has been difficult for statistical studies to establish a direct link between ozone depletion and increased rates of melanoma.</MPQA>
<MPQA autoclass="obj" certainty="7.4">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. time spent outdoors).</MPQA>
<MPQA autoclass="obj" certainty="9.5">Some atmospheric scientists (for instance Fred Singer, founder of SEPP and also a global warming skeptic) and industry-sponsored advocacy groups question or completely <MPQA autoclass="objectiveSpeech">deny</MPQA> a link between CFCs and ozone depletion [7] (http://www.sepp.org/ozone/ozonefranklin.html).</MPQA>
<MPQA autoclass="subj" certainty="13.3">It is common to see completely nonsensical arguments put forward to prove that CFCs cannot cause ozone depletion - for example, that they are heavier than air and so cannot reach the stratosphere [8] (http://www.junkscience.com/may99/freon.htm).</MPQA>
<MPQA autoclass="obj" certainty="5.2">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 [9] (http://www.so.wustl.edu/science_outreach/curriculum/ozone/info/stratosphere/myths/heavier.html).</MPQA>
<MPQA autoclass="obj" certainty="21.6">Related articlesozoneozone layerozone-oxygen cycleMontreal ProtocolScientific assessment of ozone depletionCFCmelanoma, skin cancer greenhouse gasultravioletExternal linksNOAA Stratospheric Ozone Webpage (http://www.ozonelayer.noaa.gov/)</MPQA>
<MPQA autoclass="obj" certainty="27.1">Stratospheric ozone depletion (Antarctic, Arctic, and global) (http://www.al.noaa.gov/WWWHD/Pubdocs/StratO3.html)</MPQA>
<MPQA autoclass="obj" certainty="29.9">at NOAAOzone Depletion And Global Environmental Change (http://www.ciesin.org/TG/OZ/oz-home.html)</MPQA>
<MPQA autoclass="obj" certainty="31.7">at Columbia UniversityScientific Assessment of Ozone Depletion: 1998 (http://www.al.noaa.gov/WWWHD/pubdocs/Assessment98/faq.html)</MPQA>
<MPQA autoclass="obj" certainty="31.7">- by WMO, UNEP, NOAAOzone FAQ (http://www.faqs.org/faqs/ozone-depletion/)</MPQA>
<MPQA autoclass="obj" certainty="32.0">Methyl Bromide Blues (http://www.greens.org/s-r/32/32-15.html)</MPQA>
<MPQA autoclass="obj" certainty="31.9">Five Scientific Questions On The Cfc-Ozone Issue (http://www.sepp.org/ozone/5questions.html)</MPQA>
<MPQA autoclass="obj" certainty="32.0">by S. Fred SingerA Critique of the UN Scientific Assessment of Ozone Depletion: 1994 (http://www.sepp.org/ozone/ozassm.html)</MPQA>
<MPQA autoclass="obj" certainty="31.7">by S. Fred SingerThe Ozone-CFC Debacle: Hasty Action, Shaky Science (http://www.sepp.org/ozone/ozonefranklin.html)</MPQA>
<MPQA autoclass="obj" certainty="31.7">by S. Fred SingerThe Skeptics vs. the Ozone Hole (http://www.wunderground.com/education/ozone_skeptics.asp)</MPQA>
<MPQA autoclass="obj" certainty="31.7">WMO/UNEP Scientific Assessment of Ozone Depletion: 1998 (http://www.al.noaa.gov/WWWHD/Pubdocs/Assessment98.html)</MPQA>
<MPQA autoclass="obj" certainty="31.7">The Ozone Tour (http://www.atm.ch.cam.ac.uk/tour/)</MPQA>
<MPQA autoclass="obj" certainty="28.5">at the Centre for Atmospheric Science, University of CambridgeThe Ozone FAQ (http://www.faqs.org/faqs/ozone-depletion/)</MPQA>
<MPQA autoclass="obj" certainty="11.6">- a superb resource (but last updated in 1997)The Politics of Methyl Bromide (http://www.greens.org/s-r/32/32-15.html)</MPQA>
<MPQA autoclass="obj" certainty="28.5">Jon Shanklin's Ozone pages (http://www.antarctica.ac.uk/met/jds/ozone/),</MPQA>
<MPQA autoclass="obj" certainty="31.7">including a pamphlet (http://www.antarctica.ac.uk/met/jds/ozone/ozpamw7.htm).</MPQA>
<MPQA autoclass="obj" certainty="31.7">Retrieved from "http://en.wikipedia.org/wiki/Ozone_depletion"</MPQA>