Physics Today The Antarctic ozone hole: An updateAnne R. Douglass, Paul A. Newman, and Susan Solomon

Citation: Physics Today 67(7), 42 (2014); doi: 10.1063/PT.3.2449 View online: http://dx.doi.org/10.1063/PT.3.2449 View Table of Contents: http://scitation.aip.org/content/aip/magazine/physicstoday/67/7?ver=pdfcov Published by the AIP Publishing

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42 July 2014 Physics Today www.physicstoday.org

Ozone (O3) is of fundamental impor-tance to our planet. It absorbs solarUV radiation at wavelengths below320 nm that are not absorbed by anyother atmospheric gases. Without

ozone, that high- energy sunlight would reach theground and destroy or damage DNA and other bi-ologically active molecules. In humans, increasedUV exposure leads to increased rates of skin cancerand of eye damage such as cataracts.

Most atmospheric ozone is found in the strato-sphere, about 10–50 km above Earth’s surface. Cre-ated naturally by UV photolysis of oxygen mole-cules (O2), ozone is destroyed by catalytic cycles (seethe box on page 45) involving free radicals pro-duced by decomposition of molecules—both natu-rally occurring and anthropogenic—that contain nitrogen, hydrogen, bromine, and chlorine atoms.1

In the early 1970s, Mario Molina and SherwoodRowland identified chlorofluorocarbons (CFCs),compounds composed of carbon, fluorine, and chlo-rine atoms, as a growing manmade source ofstratospheric chlorine.2 Commonly used at the timeas refrigerants and aerosol propellants, the mole-cules are very long-lived: James Lovelock and col-leagues found in 1971 that the atmospheric concen-trations of the two most abundant compounds,CFCl3 (also called CFC-11) and CF2Cl2 (CFC-12), ac-counted for almost all that had ever been produced.The unfettered growth of CFCs became a major con-cern in the scientific community because additional

stratospheric chlorine would increase ozone de-struction. In 1982 a National Academy of Sciencesevaluation suggested that in 100 years the totalglobal ozone would be depleted by 5–9% if CFC usecontinued to grow at the 1977 rate.

Only three years later, a 1985 paper by JosephFarman, Brian Gardiner, and Jon Shanklin shook thefoundations of stratospheric science: They reporteddrops of about 30% in October ozone levels overAntarctica3 (see figure 1a). Such dramatic depletionwas unexpected in every sense: magnitude, year ofonset, geographic region, and seasonality. Thelosses occurred annually and only during spring,and they took place in the Antarctic lower strato-sphere, where no study had predicted sensitivity tochlorine increase. Satellite-based observationsshowed that the extent of depleted ozone—quicklydubbed the ozone hole—continued to grow duringthe 1980s; by 1992 it was about as large as NorthAmerica (see figure 1b).

Within a few years, substantial research haddemonstrated that CFCs did indeed pose a directthreat to the ozone layer. In response, the interna-tional community of policymakers in 1987 adoptedthe Montreal Protocol on Substances that Depletethe Ozone Layer, which was designed to control theproduction and consumption of CFCs. The cause ofthe ozone hole was still being debated, but dramaticlosses over Antarctica provided a vivid backdrop tothe negotiations.4

Over subsequent decades, the Montreal Proto-col has been strengthened; production of CFCs wasfirst limited and then eliminated, and by the late1990s and early 2000s (the date depends on the spe-cific gas), their concentrations in the lower atmos-phere (the troposphere, from the surface up to about10 km) stopped increasing. Meanwhile, during the1990s the October ozone at the British Antarctic Sur-vey’s Halley Research Station hovered between 120and 150 Dobson units, less than half the value ob-served during the 1960s, when stratospheric chlo-rine levels were close to their natural value. (Named

Anne Douglass (anne.r.douglass@nasa.gov) is project scientist for NASA’s Aura mission and coleader for the Chemistry Climate Model Project at NASA’s Goddard SpaceFlight Center in Greenbelt, Maryland. Paul Newman

(paul.a.newman@nasa.gov) is the chief scientist for atmospheres at NASA Goddard and cochairs the ScientificAssessment Panel for the Montreal Protocol. Susan

Solomon (solos@mit.edu) is the Ellen Swallow Richards Professor of Atmospheric Chemistry and Climate Science atthe Massachusetts Institute of Technology in Cambridge.

Anne R. Douglass, Paul A. Newman,and Susan Solomon

An update

In the 30 years since the ozone hole was discovered, our understanding of the polar atmosphere has become

much more complete. The worldwide response to the discovery was fast, but the recovery is slow.

The Antarcticozone hole

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after George Dobson, who pioneered the study ofstratospheric ozone, Dobson units measure the col-umn density of ozone or other trace atmosphericgas: 100 DU corresponds to the amount of ozone ina vertical column that would form a layer 1 mmthick at standard temperature and pressure.)

The explanation emergesThe hypothesis that CFCs were the root cause of theozone hole3 was puzzling: How could CFCs, mainlyreleased in the northern latitudes, cause large deple-tion over Antarctica? The general features of strato -spheric circulation were known, and ozone depletionwas expected in the upper stratosphere, at altitudesof about 40 km. Nobody anticipated it would be the interaction of stratospheric photochemistry with Antarctica’s unique meteorological and physi-cal processes that eventually explained the ozoneobservations.

Atmospheric transport of CFCs follows theBrewer–Dobson circulation pattern.5 First hypothe-sized by Alan Brewer to explain the extreme drynessof the stratosphere, the flow connects the tropicsand polar regions: Rising air in the tropics is trans-ported poleward and downward in the so-called extratropics at latitudes above 30°.

In the 1970s atmospheric chemistry studies established that CFCs released at the surface arebroken apart by photolysis into free radicals thatcatalytically destroy ozone. The rate of their de-struction increases exponentially with altitude. Inthe lower atmosphere, CFCs are protected from de-struction by the ozone layer until they are liftedabove it; significant losses begin at about 24 km.Their concentrations could thus be tracked to revealthe stratospheric circulation pattern.

The basic pattern soon emerged. Although an-thropogenic CFCs are emitted mostly in the North-ern Hemisphere, they are well mixed throughoutthe troposphere over the course of a couple of years,and air that enters the stratosphere in the tropicscarries with it tropospheric levels of CFCs. Once thecirculation lifts the air above about 24 km, now- vulnerable CFCs are photolyzed by UV radiation atwavelengths of 190–230 nm. The extratropical upper- stratospheric circulation carries CFC- depleted airpoleward and downward. The tropical uplift of theBrewer–Dobson circulation is slow, only about0.4 mm/s, and air density decreases exponentiallywith altitude. Hence most air that enters the strato-sphere is transported poleward below 24 km. Onlya small fraction (about 1%) of the mass of air in theatmosphere rises high enough each year to experi-ence significant CFC breakdown, which accountsfor atmospheric lifetimes of 52 years for CFC-11 and102 years for CFC-12. Observations of tracers showthat it takes five to seven years for air to circulatefrom the tropics through the stratosphere to theAntarctic lower stratosphere.

In the upper stratosphere, the chlorine concen-tration peaked at about 3.5 parts per billion by vol-ume (ppbv) around 1998. Naturally occurringmethyl chloride accounts for only about 0.5 ppbv ofthe stratospheric chlorine; nearly all the rest comesfrom manmade chemicals. Likewise, stratospheric

bromine, another contributor to polar ozone loss,has manmade origins, primarily methyl bromideand halons—molecules composed of bromine,fluor ine, and carbon.

The equivalent effective stratospheric chlorine(EESC) is a parameter that quantifies the effects of chlorine and bromine on Antarctic ozone deple-tion. The EESC is calculated by summing up the Cland Br atoms in ground observations of halogenchemicals, factoring in the time it takes for thosechemicals to reach the Antarctic lower stratosphere,

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Figure 1. The ozone hole. (a) Ground-based measurements (black dots)of the October mean total ozone above the British Antarctic Survey’sHalley Research Station at 76° S, 27° W (black dot in 1970 plot below) reveal ozone decreases from the 1970s through the 1990s. An analysis of the data up to 1985 (vertical line) first discovered the ozone hole.3

Ozone columns are measured in Dobson units: The amount of ozone in a vertical column that would form a layer 1 mm thick at standardtemperature and pressure is equivalent to 100 DU. The decrease in total ozone closely tracks increases in manmade chloro fluorocarbons(CFCs), as quantified by the equivalent effective stratospheric chlorineconcentration (purple, in parts per billion by volume) as described in the text. Also shown are minima of October mean Antarctic ozone fromsatellite instruments: the Backscatter UV spectrometer (BUV, blue) andthe Total Ozone Mapping Spectrometer and the Ozone Monitoring Instrument (TOMS and OMI, both red). (b) Shown are October averagesof total ozone from the BUV (1970, white circle indicates no data), TOMS (Nimbus-7 1979, 1984, 1990; Earth Probe 1996, 2001), and OMI(2007, 2013). The ozone hole deepened rapidly between 1979 and 1990.For scale reference, the US outline is superimposed on the 1990 image.Because CFCs and related bromine compounds are long-lived, recentozone levels remain lower than pre-1970 values.

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and estimating the rate at which the CFCs andhalons are destroyed. The bromine contribution isscaled by a factor of 60 to account for that atom’smore efficient destruction of ozone. With those factors included, the increasing severity of ozoneloss closely parallels the increase in the EESC (seefigure 1a).

In late March the Sun sets at the South Pole andthe long Antarctic winter begins. The dark polarstratosphere cools as IR radiation escapes to space.At southern midlatitudes, ozone continues to ab-sorb solar radiation. Solar heating continues andthermal gradients develop. As polar temperaturesfall below midlatitude temperatures, the pressuregradient forces air toward the pole. The Coriolis ef-fect causes that air to bend to the left, producing avery strong west-to-east jet stream with speedsmore than 50 m/s. That huge whirling eddy—thepolar vortex—encircles the Antarctic region, ex-tends from the troposphere to the mesosphere (at al-titudes from roughly 50 km to 80 km), and preventsair from midlatitudes from mixing with air insidethe vortex.

Air from the upper stratosphere and meso -

sphere, carried poleward by the Brewer–Dobson cir-culation, descends to the lower stratosphere withinthe core of the vortex over the course of the winter.Isolated from the midlatitudes, chlorine and bromineconcentrations within the lower- stratospheric polarvortex are much greater than those at similar alti-tudes outside the vortex.

A paradigm shiftThe chlorine and bromine released from sourcegases typically form inorganic molecules. Through-out the stratosphere, only a small fraction of inor-ganic chlorine resides in atomic Cl and chlorinemonoxide (ClO), the reactive species participatingin catalytic ozone destruction. Prior to discovery ofthe ozone hole, the Cl–ClO catalytic reaction (cycle1 in the box) was considered to be the primarymechanism of ozone depletion, and most inorganicchlorine in the stratosphere was thought to belocked up in reservoirs of compounds that do notreact with ozone—hydrogen chloride (HCl) andchlorine nitrate (ClONO2).

The solution to the puzzle of the rapid declineof Antarctic ozone lay in unexpected chemical reac-tions—heterogeneous ones involving reactants indifferent phases—that liberate chlorine from itsreservoirs in the extremely cold, lower- stratosphericAntarctic vortex. At 20 km, temperatures overAntarctica fall below −80 °C in mid-May; they re-main below −80 °C until mid- October. In July andAugust, large regions of the Antarctic vortex dropbelow −90 °C. Polar stratospheric clouds (PSCs)form at such low temperatures (see figure 2), eventhough the stratosphere is extremely dry—watervapor is present at only 5 parts per million. Obser-vations of PSCs were reported around the begin-ning of the 20th century during expeditions to theAntarctic continent, but the clouds’ importance toatmospheric chemistry was not understood untildiscovery of the ozone hole.

Heterogeneous chemical reactions on andwithin particles in the PSCs convert the HCl andClONO2 reservoir gases into Cl2, a weakly boundmolecule that is easily photolyzed into Cl free radi-cals.6,7 Moreover, PSCs impede chlorine deactiva-tion into the reservoir ClONO2 (see the box). Gas-phase nitric acid (HNO3) condenses to form PSCparticles and reduces available reactive nitrogen.8

Large particles fall out of the stratosphere alto-gether, a process known as denitrification, so HNO3is not returned to the gas phase when PSCs evapo-rate. Still, high concentrations of Cl radicals did notexplain the ozone hole; the Cl–ClO catalytic reactionrequires atomic oxygen, produced when ozone isphotolyzed by UV light, which is nearly absent fromthe lower- stratospheric vortex at the very low Sunangles of the Antarctic winter.

A different catalytic cycleA new mechanism accounts for the catalytic de-struction of ozone in the absence of oxygen atoms.9

Rapid ozone loss begins as the first rays of sunlightilluminate the vortex in spring. In a reaction requir-ing only visible light, not higher-energy UV, the sun-light photolyzes the Cl2 produced by the PSC reac-

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Ozone hole

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Figure 2. Polar stratospheric clouds (PSCs) are key to the large ozoneloss in the Southern Hemisphere.11 Heterogeneous chemical reactionswithin cloud particles release chlorine radicals from longer-lived reservoir gases. PSCs have long been observed at high latitudes in bothhemispheres, but their major role in stratospheric photochemistry wasnot known until after the ozone hole was discovered. Cloud- Aerosol Lidarand Infrared Pathfinder Satellite Observation (CALIPSO) images, such asthis one obtained 1 August 2008, provide details on the composition,extent, and duration of PSCs in both hemispheres.18 As seen in this vertical profile, tropospheric weather clouds extend through the loweratmosphere to altitudes of about 10 km; PSCs are found at higher alti-tudes, in the stratosphere. (Image courtesy of NASA/CALIPSO/C. Trepte.)

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tions in the vortex. As described in the box (cycle 2),the now-free Cl reacts with O3 to produce ClO andO2. Two ClO molecules combine in the presence ofa third body to form Cl2O2, but that dimer is easilyphotolyzed in low light to produce Cl. A relatedcycle involving both ClO and BrO (cycle 3) is alsoimportant; halons and methyl bromide are the dom-inant anthropogenic contributors to BrO.10

The combination of inorganic chlorine andbromine from manmade compounds, extreme coldtemperatures, and sunlit photochemistry accountsfor the development of the Antarctic ozone hole andits seasonal dependence. Cycles 2 and 3 catalyticallydestroy ozone at the rate of about 1–2% per daystarting in late August. By early October nearly100% of the ozone in a layer about 16–20 km in alti-tude is destroyed over Antarctica.11

The ozone hole has been a regular feature of theAntarctic spring since the late 1980s.12 The HalleyStation’s October mean ozone column (figure 1a),the October mean satellite map (figure 1b), andother measurements such as the annual minimumozone column all vary year to year, but they remainwell below the pre-1980 low-chlorine baseline. Dur-ing the first decade of the 21st century, ground-based measurements of CFCs and satellite measure-ments of their breakdown products confirm thatstratospheric chlorine is decreasing as expected.13

Yet because CFCs are long-lived, their concentra-tions decrease very slowly, despite near-zero globalemissions resulting from the Montreal Protocol.Ozone holes are expected to occur for many yearsto come, with their severity dictated primarily byyear-to-year variability in the Southern Hemi-sphere’s meteorological conditions.

Recent contrasting ozone holes illustrate someof the processes that contribute to that variability.Observations with the Ozone Monitoring Instru-ment on NASA’s Aura satellite revealed the largestozone hole on record in October 2006 and one of thesmallest ozone holes in the past 20 years in 2012.14

Figure 3 shows the evolution of the Antarctic ozoneprofiles, spatially averaged over the polar vortex, asmeasured with Aura’s Microwave Limb Sounder inthe 2006, 2010 (a typical recent year), and 2012spring and summer. The similarity of the profiles inAugust and early September suggests similar pho-tochemical processes at the beginning of the Antarc-tic spring. But 2006 had an unusually cold winter,and ozone concentrations stayed low over a broaderrange of altitudes throughout September. In con-trast, both the vertical extent and the duration of the2012 ozone depletion were smaller; there was lessphotochemical ozone destruction in the warmervortex as spring progressed. The ozone increaseabove 24 km in October is from transport into theregion, made possible by the warmer and less stablepolar vortex. The smaller-than-usual 2012 ozonehole is explained by meteorology, not by the smallchlorine decrease (less than 5% since 2006) due tothe Montreal Protocol. (For more on Aura and othersatellites monitoring the state of the atmosphere, seethe article by Tristan L’Ecuyer and Jonathan Jiang,PHYSICS TODAY, July 2010, page 36.)

www.physicstoday.org July 2014 Physics Today 45

Before the discovery of the Antarctic ozone hole, chlorine was alreadyknown to catalyze the conversion of ozone (O3) to oxygen (O2) in thestratosphere through absorption of UV radiation:

But that reaction could not account for the unexpected extreme depletion in the Antarctic. The explanation involves previously unknownheterogeneous and photochemical interactions between stratosphericchlorine compounds and unique clouds that form in the extremely coldAntarctic winter.

The Antarctic cyclesReactions on and in polar stratospheric clouds (PSCs) liberate free radicals from chlorine nitrate (ClONO2) and hydrochloric acid (HCl)reservoirs, which contain most of the atmosphere’s chlorine. The mainreaction, which generates chlorine gas, is

Sunlight creates free radicals:

Cycles 2 and 3 are fast. In the Antarctic, when no ozone is left to scavenge, slower reactions with other atmospheric gases deactivatechlorine:

In the Arctic, PSCs evaporate and return HNO3 to the gas phase. Nitrogendioxide (NO2) produced from HNO3 then deactivates ClO:

ClO + NO2 + M → ClONO2 + M

Cl + CH4 → HCl + CH3

Cl + O3 → ClO + O2

ClO + O → Cl + O2

O3 + hνUV → O2 + O Net: 2O3 + hνUV → 3O2

2 [Cl + O3 → ClO + O2] ClO + ClO + M → Cl2O2 + M Cl2O2 + hνvis → Cl + ClO2

ClO2 + M → Cl + O2 + M Net: 2O3 + hνvis → 3O2

Br + O3 → BrO + O2

Cl + O3 → ClO + O2

BrO + ClO → BrCl + O2

BrCl + hνvis → Cl + Br Net: 2O3 + hνvis → 3O2

Radicals destroy ozone in two dominant classes ofcatalytic reactions. A third body M is needed duringthe reaction to conserve energy and momentum.Unlike in cycle 1, the photolysis in these cycles isdriven by visible light:

Cycle 1

Cycle 2

Cycle 3(and related)

Catalytic ozone loss

Cl2 + hνvis → 2Cl

ClONO2 + HCl → Cl2 + HNO3PSC

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46 July 2014 Physics Today www.physicstoday.org

Ozone hole

What about the Arctic?Although the Arctic and Antarctic are similar insome ways, ozone columns below 180 DU, routinein the Antarctic, have never been observed in theArctic. Chlorine levels are similar in both hemi-spheres, but winter and spring temperatures in theAntarctic are much colder than in the Arctic. At analtitude of 20 km, the area with temperatures lessthan −80 °C spans about 25 × 106 km2 in the Antarcticbut rarely surpasses 10 × 106 km2 in the Arctic. TheNorthern Hemisphere’s large mountain ranges andits broad contrasts between land and sea tempera-tures result in a weaker polar vortex, warmer tem-peratures, and stronger Brewer–Dobson circulation.That stronger circulation leads to greater transportof ozone to the Arctic lower stratosphere and natu-rally higher ozone; year-to-year variability is alsogreater than in the Southern Hemisphere. ArcticPSCs occur less frequently and occupy a muchsmaller fraction of the polar vortex compared withAntarctica. Those temperature and PSC dissimilar-ities lead to much different levels of ozone deple-tion. Occasional cold northern winters, such as the2010–11 winter,15 are accompanied by significant de-pletion, but PSCs in the northern polar vortex havenever persisted into late April. In contrast, theAntarctic vortex PSCs typically persist into late Oc-tober or later, and large ozone depletion occursevery year.

Figure 4 compares annual cycles of the totalozone column from ground station measurements inthe Arctic (left) and Antarctic (right). As the Antarc-tic ozone hole developed, the cycle’s seasonal pat-tern changed. Measurements from the 1960s showspringtime increases in Antarctic total columnozone. But since the mid 1980s, Antarctic total ozonedrops rapidly each spring, from 250–300 DU to only100–140 DU by early October. Values below 150 DUwere never observed before the onset of chemicalozone depletion. Rapid chemical ozone loss greatlyoutpaces the transport- driven increase seen duringthe 1960s. In the Arctic, spring chemical ozone losscompetes with transport by the mean circulation,and Arctic total column ozone to date has displayedlimited decreases within a season. Quantifying thechemical depletion in the Arctic is hindered by uncertainties in the concurrent transport- driven increase,16 but even in such exceptionally cold Arctic

winters as 1997 and 2011 the chemical losses do not overwhelm transport like they do in the Antarctic.

The expected future and world avoidedIn the first years following the discovery of theozone hole, NASA, NSF, and the National Oceanicand Atmospheric Administration obtained ground,balloon, and aircraft measurements to augment the limited observations of ozone and other con-stituents in the stratosphere. That research effort hasadvanced our understanding of the ozone layer andour ability to model the layer and its response tochanges in both ozone-depleting substances andgreenhouse gases. Satellite and ground observa-tions reveal day-to-day and year-to-year details ofozone hole evolution. Modern chemistry climatemodels simulate many facets of stratospheric mete-orology as well as the long-term evolution of ozoneand important trace gases such as ClO and BrO.

In 1985 CFCs had many uses: refrigeration, airconditioning, foam blowing, aerosol propellants,and cleaning solvents. Halons were mainly used asfire- extinguishing agents. In the absence of the Mon-treal Protocol, CFC and halon emissions would havelikely continued to increase. As shown in figure 5,current chemistry climate models can be used tocompare the evolution of the ozone layer for realis-tic and hypothetical CFC scenarios. In the “expectedfuture” simulation, the EESC slowly decreases (topleft panel, blue line) and Antarctic ozone increases(bottom left panel, blue line). The ozone hole is stillpresent in 2014 (middle top image), with averagecolumn ozone around 150 DU. But by 2060 the EESCis approximately 2 ppbv. Although still elevatedabove the background value of about 1 ppbv (seefigure 1a), the ozone hole is closing, with averagelevels above 300 DU (right top image).

The “world avoided” simulation assumes thatCFC and halon emissions increase 3% every year(growth rates in the early 1970s were 9% per year).17

As the EESC continues to increase (top left panel, redline), Antarctic ozone levels continue to decline (bot-tom left panel, red line). By 2014 the EESC is twicethe current real-world level and Antarctic ozone isroutinely below 100 DU (compare the middle bottompanel with figure 1a). By 2060 the EESC is 17 timesits 1980 level, October Antarctic ozone is less than

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2006 2010 2012Figure 3. Ozone profiles inside theAntarctic vortex, in parts per millionby volume as measured by the Microwave Limb Sounder on NASA’sAura satellite. The southern winter in2006 was significantly colder than in2010, and the ozone levels were verylow over a large vertical domainthrough the spring. In the warmer2012 winter, the polar vortex wasweaker, which allowed more ozone tobe transported in at high altitudes andproduced less chemical ozone loss.

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50 DU, and extravortex ozone has fallen dramaticallyas well. Although the year-to-year meteorologicalvariability is too large for us to be able to declare thatthe ozone hole is closing, figure 5 makes clear thatthe Montreal Protocol prevented substantial wors-ening of ozone loss in Antarctica and worldwide.

The present consequences of ozone depletionare not limited to the Antarctic. Antarctic tempera-tures increase during October and November as theSun rises over the continent. The jet stream gradu-ally slows and becomes less stable, the vortex breaksdown, and atmospheric transport resumes betweensouthern polar and middle latitudes. Southern mid-latitude ozone levels decrease due to mixing with ozone- depleted air from over Antarctica, and pop-ulated areas of the Southern Hemisphere experiencehigher UV levels during the southern summer of De-cember–February. In recent summers, the UV index,a measure of sunburning radiation, typically exceeds9—deemed very high by the World Health Organi-zation; in the “world avoided” simulation, within afew decades the UV index would average 25.

The ozone hole also affects the Southern Hemi-sphere’s surface climate. As the Sun returns toAntarctica, ozone should be present, absorbing ra-diation and thereby warming the polar vortex.There is less heating because of ozone depletion,Antarctic lower- stratospheric temperatures arebelow their pre- ozone-hole average during springand summer, and the polar vortex persists one totwo weeks longer. Because the circumpolar flowaround Antarctica extends to the surface, the tro-pospheric jet is strengthened during the southernsummer, which increases the surface wind stressand thereby modifies the ocean circulation. In-creased greenhouse gas levels lead to surface warm-ing in the Arctic and might be expected to have thesame effect in the Antarctic. However, observationsand models show that the ozone depletion hascaused the interior of Antarctica to cool. The windand temperature changes driven by ozone depletionalso change Southern Hemisphere precipitationpatterns.

Important lessons, promising futureThe story of the Antarctic ozone hole is a remarkableone. The ozone depletion was unexpected, rapid,and large: Continental-scale losses of 50% developedin about 15 years. The worldwide response was evenquicker. The scientific community amassed addi-tional meteorological and atmospheric-constituentinformation in the Antarctic and established thecause of the phenomenon within a few years.

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Figure 5. Simulations of the Antarctic ozone hole contrast the “expected future” (blue lines at left and top images at right) resultingfrom curtailing chlorofluorocarbons and a “world avoided” scenario (redlines and bottom images) in which CFC use increases annually by 3%.In the top left, the equivalent effective stratospheric chlorine (EESC) accounts for the influence of chlorine and the more-destructivebromine, both products of anthropogenic gases. The plotted EESC, inparts per billion by volume, is the average over 75°–90° S at 20 km. Thebottom left plots the October ozone column density averaged over thesame region. By 2014 the EESC in the “world avoided” is almost twicethat in the “expected future,” and total ozone is about 80 Dobson unitsless. By 2060 the EESC is 17 times as high, Antarctic total ozone is260 DU less, and large ozone depletions cover the entire globe.

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Policymakers acted to implement the Montreal Pro-tocol. Subsequent amendments to the protocol havefollowed detailed discoveries about the processesthat produce the ozone hole and have largely elimi-nated anthropogenic emissions of CFCs and halons;left unrestrained, the growth of those emissionswould have been disastrous for the ozone layer.

The Antarctic ozone hole teaches important les-sons. Models synthesize knowledge and provideprojections for the future, but if processes such asheterogeneous chemical reactions are missing, thenprojections are hopelessly flawed. Nothing can re-place a foundation of observations that predates environmental change. Long-term Antarctic obser-vations of the stratospheric ozone column werethought to be unimportant. The persistence of Far-man and other scientists led to the long series ofHalley measurements that ultimately revealed theozone depletion. Laboratory studies, focused obser-vational campaigns, and modeling were essential intesting the various hypotheses to explain the ozonehole. Those research efforts broadened understand-ing of the interplay among dynamic, radiative, andphotochemical processes of the stratosphere and established the connection to CFCs.

The future of Antarctic ozone is promising.Ground and satellite observations show that chlo-rine levels in the troposphere and stratosphere aredecreasing. Model projections suggest that ozonewill return to 1980 levels between 2050 and 2070.The ozone hole has been an iconic feature, a testa-ment to the possibility of unexpected and large im-

pacts of anthropogenic actions on the environment.Its disappearance will symbolize the possibility ofprotecting Earth through cooperative actions.

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Phys. 9, 7577 (2009). ■

Ozone hole

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