Environment˜ Environnement˜ Canada˜ - NASA · Evidence of ozone depletion in the stratosphere began to appear in the 1980s.Atmospheric concentrations of most ozone-depleting substances

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Ozone Depletion and Climate Change:Understanding the Linkages

Angus FergussonMeteorological Service of Canada

Ozone Depletion and Climate Change:Understanding the Linkages

Published by authority of the Minister of the EnvironmentCopyright © Minister of Public Works and Government Services Canada, 2001

Catalogue No. EN56-168/2001E ISBN: 0-662-30692-9

Également disponible en français

Author: Angus Fergusson (Environment Canada)

Editing: David Francis ( Lanark House Communications)David Wardle / Jim Kerr (Environment Canada)

Contributing Authors: Bruce McArthur (Environment Canada): Bratt Lake ObservatoryDavid Tarasick (Environment Canada): Canadian Middle Atmosphere ModelTom McElroy (Environment Canada): MANTRA Project

Special thanks for comments to: Vitali Fioletov (Environment Canada)Hans Fast (Environment Canada)

Pictures: Angus Fergusson (Environment Canada)John Bird (Environment Canada)Brent ColpittsRay Jackson

Layout and Design: BTT Communications

Additional copies may be obtained, free of charge, from:

Angus FergussonScience Assessment and Integration BranchMeteorological Service of Canada4905 Dufferin StreetDownsview, OntarioM3H 5T4E-mail: [email protected]

Table of Contents

Summary 2

Introduction 4

The Atmosphere and its Radiative Effects 6

The Dynamics of the Atmosphere 10

The Chemistry of the Atmosphere 12

Biogeochemical Linkages:The Impact of Increased UV Radiation 14

Canadian Research and Monitoring 16

Implications for Policy 20

The Research Agenda 22

Making Connections 26

Bibliography 28

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Figure 1Source: NASA

Figure 1. Compared to the earth itself, the earth’satmosphere as seen from space looks remarkably thin,much like the skin on an apple. In this photograph, thetwo lowest layers of the atmosphere, the troposphere andthe stratosphere, are clearly visible. The stratosphere ishome to the ozone layer that protects life on earth fromintense ultraviolet radiation. The troposphere is the layerwhere most weather activity takes place. The top of thethundercloud has flattened out at the tropopause, theboundary between the two layers. Interactions betweenthe troposphere and stratosphere provide a number ofimportant connections between ozone depletion and climate change.

Ozone depletion and climatechange have usually been

thought of as environmentalissues with little in commonother than their global scope andthe major role played in each byCFCs and other halocarbons.With increased understanding ofthese issues, however, has come agrowing recognition that a num-ber of very important linkagesexist between them. These link-ages will have some bearing onhow each of these problems and the atmosphere as a wholeevolve in the future.

Some of the most important ofthese linkages involve the waythat ozone-depleting substancesand greenhouse gases alter radia-tion processes in the atmosphereso as to enhance both globalwarming and stratospheric ozonedepletion.These changes result in a warming of the troposphere(the bottom 8–16 km of theatmosphere) and a cooling of thestratosphere (the layer above thatextends to an altitude of about 50 km and contains the ozonelayer). Stratospheric cooling creates a more favourable environ-ment for the formation of polarstratospheric clouds (PSCs),

which are a key factor in thedevelopment of polar ozoneholes.

Enhancement of the green-house effect may also be causingchanges in circulation patterns inthe troposphere that are, in turn,altering the circulation in thestratosphere. It is suspected thatthese changes are increasing thecooling forces acting on thestratosphere over the poles andare thus making the formation ofozone holes more likely.There isevidence as well that changes inthe stratospheric circulation maybe altering weather patterns inthe troposphere. Other linkagesbetween climate change andozone depletion are related tothe effect of increased levels ofultraviolet radiation on sun-drivenchemical reactions in the atmo-sphere and to changes in biologi-cal processes that affect the composition of the atmosphere.

The net effect of these link-ages is an intensification of bothclimate change and ozone depletion and possibly a delay inthe recovery of the ozone layeras it responds to diminishinglevels of CFCs and other ozone-

depleting substances covered bythe Montreal Protocol.To under-stand these connections better,researchers are now lookingmore closely at how the tropo-sphere and stratosphere interact(Figure 1). Environment Canadascientists are contributing to thisresearch in a variety of ways,including the monitoring ofozone concentrations and solarradiation levels and collaborationin balloon-based stratosphericresearch and atmospheric modelling.

Both greenhouse warming andthe thinning of the stratosphericozone layer are a result of humanactivities that have changed thecomposition of the atmosphere insubtle but profound ways sincethe beginning of the industrialrevolution more than 200 yearsago. By taking an integratedapproach to ozone depletion andclimate change, governments andscientists will have a betterchance of understanding andmoderating the enormouschanges that human activitieshave had and will continue tohave on the atmosphere.

Summary

Ozone Depletion and Climate Change: Understanding the Linkages • 2

Figure 2a

Figure 2a. Greenhouse gases, such as carbon dioxide, methane, and nitrous oxide, affect climate by retaining heat near the earth’s surface.Concentrations of these gases began to rise in the 19th century and have increased exponentially in the 20th, paralleling the expansion ofindustrial economies and the growth of the human population. Over the past century the average global temperature has increased by about0.6ºC, and significantly greater increases are expected for the 21st century. Warming of the atmosphere is also expected to cause changes inother aspects of climate, including changes in precipitation and evaporation, circulation patterns, and weather extremes.

Figure 2b. Thinning of the stratospheric ozone layer has been caused largely by long-lived chlorine and bromine compounds, such as CFCs and halons, that eventually make their way into the stratosphere. Use of these compounds increased considerably in the 1960s and1970s. Evidence of ozone depletion in the stratosphere began to appear in the 1980s. Atmospheric concentrations of most ozone-depletingsubstances peaked, or their growth rate began to slow, as a result of the phasing out of these chemicals under the Montreal Protocol.Concentrations of ozone-depleting substances are expected to decline over the coming century, bringing a gradual recovery of the ozone layer.

Figure 2bSource: J. Elkins, NOAA

Source: Adapted from IPCC, 2001

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Data from thermometers (red) and from tree rings, corals, icecores and historical records (blue). The shaded area indicatesthe margin of error in the measurements.

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Introduction

Climate change and the deple-tion of the stratospheric

ozone layer have been leadingenvironmental issues for morethan a quarter of a century. Formuch of that time, they have beentreated as separate and distinctproblems, with researchers settingup separate programs to investi-gate the underlying scientificquestions and governments establishing separate internationalarrangements to coordinate control measures.

In many ways, of course, theseissues are quite distinct. Climatechange is concerned with howcarbon dioxide, methane, andother greenhouse gases emittedby human activities are alteringthe climate system (Figure 2a).Consequently, research into cli-mate change has focused largely,though not exclusively, on trendsand processes within the tropo-sphere, the layer of turbulent air,some 8–16 km deep, that is closestto the earth’s surface. Ozonedepletion, on the other hand, isabout how certain industrially produced chemicals containingchlorine or bromine are damagingthe earth’s protective ozone layer,thus increasing the intensity ofultraviolet radiation at the earth’ssurface (Figure 2b). Research intoozone depletion has thereforefocused largely, though notexclusively, on trends andprocesses within the

stratosphere, the layer of stratified,relatively stable air that houses the ozone layer and extends fromthe top of the troposphere to analtitude of about 50 km.Yet, as scientists have come to under-stand more about these issues and the complex physical andchemical processes that drivethem, they have also becomeincreasingly aware of a number of important connections betweenthem.

Recognition of these connec-tions parallels a growing aware-ness among scientists and policymakers that atmospheric issuescannot be dealt with in isolationfrom each other. The human activities that contribute to climatechange and ozone depletion, or toany other air pollution problemfor that matter, affect the sameatmosphere.And because theatmosphere is a very complexentity, changes to one aspect of itcan often initiate changes thataffect other aspects of the atmo-spheric system. Consequently,human activities that alter theatmosphere, even in verysubtle ways,

can have important and surprisingresults.To take account of themany complicated interactionsthat occur in the atmosphere,scientists and policy makers areincreasingly taking a more holisticapproach to atmospheric issues.

The most obvious linkagebetween ozone depletion and climate change is the fact thatozone itself and some of the more important ozone-depleting substances such as chlorofluoro-carbons (CFCs) and hydro-chlorofluorocarbons (HCFCs) arealso powerful greenhouse gases.But ozone depletion and climatechange are linked in many otherways as well, particularly throughtheir effects on physical and chemical processes in theatmosphere and on interactionsbetween the atmosphere andother parts of the global ecosystem.


Figure 3. The contrasting temperature patterns in the troposphere and stratosphere are the result of differences in the way radiative energy is transferred to the atmosphere. The stratosphere is heated from the top down as intense ultravioletradiation is absorbed by oxygen and ozone. The troposphere is heated from the bottom up as the earth’s surface warmed byincoming sunlight, emits longwave infrared radiation, which is then absorbed and re-emitted by greenhouse gases in the airabove. A small amount of heat is also transferred directly to the air by direct contact with the surface and by the evaporationand condensation of moisture. Because the troposphere generally becomes cooler with altitude, the warmer and lighter surface air rises easily. As a result, the air in the troposphere is often turbulent and well mixed. The stratosphere, on the otherhand, tends to be quite stable because the increase of temperature with altitude inhibits vertical mixing of the air.

Figure 4. The basis of the greenhouse effect can be seen if we compare the wavelengths at which different atmosphericgases absorb radiation with the wavelengths at which radiation enters and leaves the atmosphere. The sun, because it is veryhot, emits shortwave radiation, and although the shortest of these wavelengths are absorbed by oxygen and ozone, mostsolar radiation is not absorbed by atmospheric gases. The earth, because it is much cooler, emits longwave, infrared radiation,but most wavelengths in this part of the spectrum are readily absorbed by water vapour, carbon dioxide, methane, nitrousoxide, ozone, and other greenhouse gases. The atmosphere thus provides a very wide window through which incoming sun-light can penetrate but only a very narrow window through which infrared radiation can depart.

Figure 4Source: adapted from Jacob, 1999

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Figure 3

The Atmosphere and its Radiative Effects

Radiative processes have anenormous influence on the

behaviour of the atmospherebecause they govern the amountof energy entering and leaving theearth-atmosphere system andhence the amount of energy avail-able to heat the air, evaporatemoisture, and drive the movementof air masses.This energy initiallyenters the atmosphere as short-wave radiation from the sun, but itis transferred to the troposphereand stratosphere in very differentways, giving these two layers ofthe atmosphere very differentstructures and characteristics(Figure 3).

The stratosphere is heated fromthe top down and is thereforewarmer at the top than it is at thebottom. Consequently, the densestair in the stratosphere is at the bottom, there is little vertical mix-ing of the air, and the stratosphereis very stable. Heat is added to the stratosphere when strong ultraviolet-C (UV-C) radiation fromthe sun is absorbed by oxygen molecules and causes them to split.One of the results of this process isthe production of ozone and theformation of the ozone layer in the stratosphere. More warmingoccurs when the ozone moleculesintercept and are destroyed byintense but slightly less powerfulultraviolet-B (UV-B) radiation. A

beneficial byproduct of theseprocesses is that most of the ultra-violet radiation that is harmful toplant and animal life on earth is filtered out in the stratosphere anddoes not reach the earth’s surface.Some additional heating of the stratosphere also occurs becauseozone absorbs infrared radiationemitted by the earth’s surface.

In the troposphere, in contrast,very little of the incoming solarradiation is absorbed directly bythe atmosphere. Instead, the short-wave radiation warms the earth’ssurface, which then transfers heatenergy to the atmosphere in a vari-ety of ways – partly through directcontact between the surface andthe air, partly through the evapora-tion and condensation of moisture,but mostly through the emission oflongwave infrared radiation, whichis absorbed by water vapour andother greenhouse gases in the airsuch as carbon dioxide, methane,nitrous oxide, and ozone. By re-emitting some of this longwaveradiation back towards the earth’ssurface, these gases retain heat atthe bottom of the atmosphere andhelp to make it warmer. As a result

of this greenhouse effect, theearth’s average temperature is some33ºC warmer than it would other-wise be and the planet is able tosupport life (Figure 4).

Because it is heated in this way,air in the troposphere is generallywarmest at the surface andbecomes cooler with increasingaltitude. Since warm air is lessdense than cool air, the warm airrises and cooler air moves in at the surface to take its place. Thissimple convective flow is modifiedby the earth’s rotation, by surfacefeatures, and by temperature differences between the equatorand the poles.The end result is arather turbulent layer of the atmo-sphere in which air circulates incomplicated and variable patterns,moving energy and moisture fromplace to place.


Figure 5. The stratosphere has cooled significantly since about 1980, mostly as a result of ozone loss but also because of the accumulation of greenhouse gases in the troposphere. Over the middle latitudes of the Northern Hemisphere, the cooling trend hasbeen noticeably greater in the middle and upper stratosphere than in the lower stratosphere.

Figure 6. Polar stratospheric clouds such as these, photographed over Sweden during the winter of 2000, form in the lower stratosphere when temperatures drop below about –80ºC. These clouds support chemical reactions that change stable bromine andchlorine compounds into more active, ozone-destroying substances. Cooling of the stratosphere as a result of climate change and ozone depletion increases the possibility that these clouds will form.

Figure 6

Figure 5

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into much more active chemicals.It is these chemicals that causerapid ozone destruction whensunlight returns to the polarregions in the spring. PSCs are akey factor in the severe ozone-depletion that occurs over polarregions in the spring.

The different heating and cool-ing forces that arise as a result ofozone layer depletion andincreasing concentrations ofgreenhouse gases have beenmeasured globally and studiedusing computer models.Thesestudies indicate that the netresult of all of these contendingforces is a warming at the surface(due mainly to increased concen-trations of greenhouse gases), aslight or negligible temperatureincrease in the middle to uppertroposphere, and a cooling of thestratosphere (due mainly to theloss of ozone).

However, the effects of green-house gases and ozone depletionon the radiative balance are verycomplex and many uncertaintiesstill remain. A number of researchefforts are now under way in an attempt to resolve these uncertainties.

gas concentrations increase, thedownward flow of longwaveradiation increases and theupward flow diminishes, thusexerting a warming force on thetroposphere and a cooling forceon the stratosphere (Figure 5).Although all greenhouse gasescontribute to warming in the troposphere, the effect of individ-ual greenhouse gases on thestratosphere can vary consider-ably, depending on whether theycause a greater reduction inupward emissions at the bottomof the stratosphere or at the top. Carbon dioxide has thegreatest cooling effect on thestratosphere, while CFCs actuallyhave a warming effect on it.Nevertheless, the net result of anincrease in concentrations of allgreenhouse gases is a coolerstratosphere.

Cooling of the stratosphere has important consequences forozone depletion, because it con-tributes to the formation of polarstratospheric clouds (PSCs).Theseclouds (Figure 6), which formonly at extremely low tempera-tures in the lower stratosphereduring the sunless polar winter,provide a medium for chemicalreactions that change stable chlorine and bromine compounds

When ozone-depleting chemicalsare released into the atmosphere,however, these radiative processesare modified in a variety of ways:• Because the most effective

ozone-destroying substances,such as CFCs and HCFCs arealso strong greenhouse gases,the greenhouse effect isenhanced and the earth’s sur-face and the lower tropospherebecome warmer.

• Warming by CFCs and HCFCs is partially offset, however, bythe ozone losses that thesechemicals cause in the lowerstratosphere. Because ozone is a greenhouse gas, a loss ofstratospheric ozone weakensthe natural greenhouse effectand cools the stratosphere.

• Thinning of the ozone layeralso means that less heat isavailable to the stratospherefrom the absorption of UV-Bradiation by ozone molecules.That also has a cooling effecton the stratosphere.

• With fewer ozone moleculesavailable in the stratosphere to absorb UV-B radiation, more of that radiation reaches theground.That contributes to additional warming of the earth’s surface and the lower troposphere.

An increase in the abundance of greenhouse gases producessimilar results.As greenhouse


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The Dynamics of the Atmosphere

As greenhouse gases accumu-late in the atmosphere, they

alter temperature differencesbetween different parts of theglobe and different levels of theatmosphere and in this waychange atmospheric circulationpatterns.There is good reason tobelieve that these circulationchanges may be enhancing ozonedepletion over the poles by weakening some of the warmingforces that act on the polar stratosphere.

One of these forces is known as dynamical heating, and it isassociated with a slow general circulation pattern in the strato-sphere in which air gradually risesin the tropics, moves towards thepoles, and then slowly sinks overthe poles (Figure 7).The compres-sion of the air as it descends overthe poles causes it to become

warmer.The energy that drivesthis circulation comes fromatmospheric waves that penetrateupward from the troposphere.These waves are caused by large-scale atmospheric disturbancesthat are a result of differences inthe way that land and water anddifferent types of land surfacesaffect the heating and movementof the air.They tend to be strongerin the Northern Hemispherebecause of its greater land area,and thus cause the stratosphereto be warmer on average over theArctic than it is over the Antarctic.

Atmospheric waves also affectthe stability of a feature known asthe polar vortex.This is a windsystem that circles around each ofthe poles in winter and isolatesthe polar stratosphere, preventingthe influx of warmer air andozone from the lower latitudes.Because of this isolation, the area

inside the vortex becomes anextremely favourable environmentfor the formation of PSCs andrapid ozone depletion. Before itfinally dissipates in the spring,however, the vortex may occasion-ally be broken down by the actionof strong atmospheric waves, thusallowing a temporary incursion ofwarmer air that makes conditionsless favourable for the rapiddestruction of ozone. Becauseatmospheric wave action isstronger in the Northern Hemi-sphere, the Arctic vortex tends tobe less stable than the Antarcticvortex. Consequently, massiveozone holes, such as those thatform regularly over the Antarcticin spring, have not yet occurred inthe Arctic.

There is evidence from climatemodel studies, however, that thewarming of the atmosphere near the earth’s surface and the

Figure 7. Air in the stratosphereflows poleward from the equator,its movement driven largely by theforce of waves created by storms inthe troposphere. Air enters the strato-sphere at the equator, where the hotsurface generates strong upward currents and thunderstorms. Thetropopause, the boundary between thetwo layers, disappears over the polesin winter, allowing stratospheric air tosink back into the troposphere.Although the tropopause tends toblock the movement of air betweenthe troposphere and stratosphere, airexchanges in both directions alsooccur in the mid-latitudes and thepolar regions as a result of the actionsof planetary waves. The breakup ofthese waves in the lower stratospherecreates a “surf zone” where somemixing of air occurs.

Figure 7


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Smog over the lower Fraser Valley

circulation changes resulting fromit may be weakening these plane-tary wave motions and thus theirwarming effect on the polar strato-sphere. If so, this would have theeffect of enhancing ozone deple-tion over both poles, but especiallyover the Arctic. Such a develop-ment would slow the recovery of the ozone layer that shouldoccur over the coming decades asconcentrations of ozone-depletingcompounds in the stratospheredecline.

Research is also revealing mechanisms through whichchanges in the composition of thestratosphere can affect the move-ment of air in the troposphere.Meteorologists have long knownthat atmospheric pressure patternsat the earth’s surface vary with thesolar sunspot cycle, but they havehad trouble explaining the connec-tion, since the sun’s total energy

output varies by only about 0.1%across the entire cycle. Recentstudies have shown, however, thatthe variation in solar energy ismuch greater in the UV-C portionof the spectrum, where the sun’soutput can vary by as much as10% during the cycle.This isenough to have a noticeable effecton ozone amounts in the strato-sphere, which have been shown tobe about 1.5% greater at the solarmaximum (when the number ofsunspots is greatest) than at thesolar minimum (when the numberof sunspots is least). These changesin ozone amounts cause corre-sponding changes in the tempera-ture of the stratosphere, which in turn result in changes in airpressure and wind flow.

Recent computer simulations atNASA’s Goddard Institute forSpace Studies have shown that

changes in stratospheric airflowalso affect the flow of energydownward into the troposphere,where it can have an impact onpressure and circulation patterns.Such changes may affect the position of the jet stream, whichcontrols the path of weather systems in the troposphere. Asmall change in the path of the jetstream can cause very noticeablechanges in regional climates.Initial studies with models, forexample, suggest that increases instratospheric ozone could havethe effect of directing morestorms into Canada. Further studyis needed, however, to determinehow long-term depletion of theozone layer is affecting surfaceweather patterns.

The Chemistry of the Atmosphere

Ozone depletion is largely a matter of atmospheric

chemistry, but chemical processesplay an important part in climatechange as well, particularly in thegeneration and breakdown ofsome greenhouse gases.The chemistry of ozone depletion andthat associated with climatechange interact significantly in atleast two ways.

One way is through photo-chemistry – chemical processesthat are driven by energy fromthe sun. Many reactions in theatmosphere are of this kind, andthe amount of ozone in the strato-sphere affects the rate at whichthese reactions occur, because itdetermines how much of thesun’s high-energy UV-B radiationreaches the atmosphere near theground.This has important consequences for climate change,because ground-level or tropo-spheric ozone, a significant green-house gas and a major constituentof smog, is photochemically produced (Figure 8). As the ozonelayer thins and more ultraviolet

radiation reaches the earth’s sur-face, the photochemical reactionsthat produce ground-level ozonecan proceed more vigorously.Average ozone amounts overCanada are now about 6%below pre-1980 values. Scientistsestimate that this decrease hasresulted in a correspondingincrease of about 7% in theamount of UV-B radiation reach-ing the earth’s surface. Recentcomputer model research indi-cates that an increase in UV radia-tion will result in an increase inground level ozone in pollutedurban areas where there are high concentrations of nitrogenoxides, carbon monoxide, andhydrocarbons.

The other linkage involves ashort-lived, highly reactive mole-cule known as the hydroxyl radical(OH), which is produced by thephotochemical breakdown ofozone in the presence of watervapour. OH is an atmospheric scavenger that reacts with manypollutants and removes them fromthe atmosphere.These include thegreenhouse gas methane as well as

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Figure 8. Ultraviolet-B radiation providesthe energy for the chemical reactions thatlead to the formation of ground-levelozone, which is both a major component of smog and a greenhouse gas.Measurements taken during a smogepisode in Toronto in July 1999 show aclose relationship between UV levels andground-level ozone concentrations.Computer models suggest that higher levels of UV-B could lead to an increase in ground-level ozone formation in highly polluted areas. Such an increase would not only make smog problems worse but would also add to warming fromgreenhouse gases.

ozone-depleting methyl chloro-form and gases such ashydrochlorofluorocarbons (HCFCs)and hydrofluorocarbons (HFCs) thatare both ozone depleters andgreenhouse gases. In addition, OHreacts with a host of other pollu-tants, including carbon monoxide,volatile organic compounds, andvarious oxides of nitrogen.There issome concern that the demands onthe hydroxyl radical in a heavilypolluted atmosphere may lead to adecline in OH concentrations andthus a reduction in the efficiencywith which methane and variousozone-depleting compounds areremoved from the atmosphere.Slower removal of these com-pounds would intensify theprocess of climate change andslow the recovery of the ozonelayer.Although global hydroxylamounts cannot be estimated witha high degree of certainty, somerecent evidence suggests that asignificant decline in hydroxylconcentrations has taken placeduring the 1990s and that furtherdeclines can be expected in thecoming decade.


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Biogeochemical Linkages:The Impact of Increased UV Radiation

Changes in climate andchanges in the intensity of

ultraviolet radiation can both have substantial impacts on thebiological, geological, chemical,and physical processes that control the exchange of matterand energy between the majorcomponents of the environment–the atmosphere, the biosphere,the hydrosphere, and the litho-sphere. Probably the most familiarof these exchanges is the carboncycle, in which carbon is trans-ferred continuously between theatmosphere, the oceans, livingthings, and the rocks and soils,changing from an almost purelyelemental form, as in charcoal oranthracite, to a simple gas like carbon dioxide, or to one ofnumerous organic compounds asit moves from one environmentalreservoir to another.

The carbon cycle is of centralimportance in the climate changeissue because human disruptionof the natural carbon cycle,

through the burning of fossilfuels and the clearing of forests,is largely responsible for themodern increase in atmosphericconcentrations of carbon dioxide,the most abundant anthropogenicgreenhouse gas. One of the moreimportant links in the carboncycle from an atmospheric per-spective is the so-called “marinebiological pump.” This is aprocess in which plankton, themicroscopic plants and animalsthat live near the surface of theoceans and freshwater lakes,remove carbon from the air andthen transfer it to the ocean bot-toms and lakebeds when theydie. Ozone depletion, particularlysevere episodes such as thespring ozone holes in theAntarctic, present a potentiallyserious threat to this process,because plankton cannot takeshelter from solar radiation. A seri-ous decline in their numbers as aresult of exposure to more intenseultraviolet radiation could there-fore decrease the rate at whichcarbon dioxide is removed fromthe atmosphere.

Similarly, biological changes initiated by changes in climatemight have some effect on ozonedepletion. Methyl chloride andmethyl bromide, for example, aretwo ozone-depleting compoundswhose production and use are nowbeing phased out under theMontreal Protocol. However,because natural sources of thesegases are larger than the humanindustrial sources, anything affect-ing the ecosystems and naturalprocesses that produce these gasescould also have consequences forthe ozone layer. Much has yet to belearned about these natural sources,but coastal marshes appear to beimportant contributors. Fungi,crops such as rapeseed, and soilsrich in organic matter are alsothought to be substantial sources.Warming of the atmosphere andoceans or changes in sea levelcould affect all of these sources by altering the ecosystems and climatic conditions that supportthe natural production of thesegases. Climate change could alsoaffect the rate at which these gasesare removed from the atmosphereby the oceans. Studies of the naturalproduction and removal of thesegases are still in their early stages,however, and it is not yet possibleto predict how climate changemight affect the quantities of thesegases in the atmosphere.


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Figure 9. Ozonesonde data collected over Eureka in 2000 show the annual ozonecycle, with highest levels occurring in the winter and early spring and lowest levels insummer. The plot also shows unusually low levels of ozone in late March and early Aprilbetween about 15 and 20 km. This depleted area is believed to be the result of high levels of chlorine and bromine in a very cold Arctic stratosphere.

Figure 10. At Environment Canada’s observatory at Alert on Ellesmere Island in the High Arctic, background concentrations of greenhouse gases, ozone-depleting substances, and aerosols are monitored daily.

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Canadian Research and Monitoring

Through its ExperimentalStudies Division, Environment

Canada’s Air Quality ResearchBranch is involved in a number ofactivities that are contributing toa better understanding of ozonedepletion and its interactionswith climate change. Some ofthese activities are research pro-jects designed to provide newknowledge about atmosphericconstituents and processes.Others are long-term monitoringprograms whose function is toprovide a long and continuousrecord of atmospheric conditionsthat can be used as a reliable basisfor identifying trends and changesin the atmosphere’s compositionand behaviour.These activities,which also assist in evaluating theeffectiveness of controls underthe Montreal Protocol and help toset strategies for the future, arecarried out through the followingkey facilities and programs.

Ground-based ozone monitoring and ozonesondes

Canadian scientists have beenusing ground-based instrumentsto measure total ozone amountssince the 1950s.These measure-ments are now taken at 12 sitesacross Canada, using theCanadian-developed Brewerozone spectrophotometer.Threeof the instruments are in the HighArctic, at Resolute Bay, Eureka,and Alert.The Brewer network is afundamental source of informa-tion about changes in the state of the stratosphere. At a morepractical level, it also provides

data for use in producing UVIndex reports and forecasts.

These ground-based measure-ments are supplemented by datagathered by small balloon-bornesensors known as ozonesondes.Weighing about 3 kg, these instru-ment packages provide continu-ous measurements of ozone concentrations up to an altitude of about 20 km (Figure 9).

Arctic observatoriesIn 1992 Environment Canada

built the High Arctic stratosphericozone observatory near the Eurekaweather station on EllesmereIsland.The observatory is the maincentre for atmospheric research inthe Arctic and is also a primarycomponent of the internationalNetwork for the Detection ofStratospheric Change, a series ofhigh quality, ground-based stationsthat measure the physical andchemical state of the stratsphere.The Eureka observatory operatesinstruments that measure ozone,nitrogen dioxide, and other sub-stances important to atmosphericchemistry. Canada also maintainsan observatory at Alert onEllesmere Island (Figure 10) thatprovides continuous monitoringof background concentrations ofgreenhouse gases, ozone-depletingsubstances, and aerosols (atmo-spheric particles).Analysis of this data provides informationabout the long-term variability of these substances and furthersunderstanding of the impact of human activities on the atmo-sphere.

The Alert observatory is an offi-cial baseline station for the WorldMeteorological Organization’sGlobal Atmosphere Watch (GAW)program and is one of about 20such stations around the world.Theobjective of the GAW program is to make available ongoing back-ground concentration measure-ments of selected atmospheric constituents and related physicalconditions for every major regionof the globe.The Canadian BaselineProgram began at Alert in 1975with simple measurements of car-bon dioxide. By 1998, the programhad expanded to include measure-ments for other greenhouse gases,such as methane, ozone, and chlorofluorocarbons.

Radiation studies (Bratt’s Lake Observatory)

The Bratt’s Lake Observatory,located about 20 km south ofRegina, is the Canadian link in the World Climate ResearchProgramme’s Baseline SolarRadiation Network.This is a network of approximately 40 stations, spread across all sevencontinents that take very accuratemeasurements of solar andinfrared radiation (Figure 11).Since shortwave radiation fromthe sun and the longwave infraredradiation emitted by the earth’ssurface and atmosphere providethe energy that drive the earth’sclimate system, this informationcan provide evidence of howchanges in radiative energy flowsare affecting regional and globalclimate change. The Bratt’s Lake


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TorontoEdmontonChurchill

Figure 11. Measurements of daily amounts of solar and infrared radiation reaching the earth’s surface should allow the detectionof trends over time. The four-year plot shown here indicates little change in either solar or infrared radiation from one summer toanother. Over the four winters, however, solar radiation has declined and infrared radiation has increased. These changes may be theresult of increased cloud cover during the winters. Many more years of data will be needed, however, before it can be determinedwhether this pattern is part of a longer-term trend or simply a result of the climate’s natural short-term variability.

Figure 12. While increasing concentrations of greenhouse gases lead to warming at the earth’s surface, they cause cooling in thestratosphere. The plot shown here shows the potential effects of a doubling of atmospheric carbon dioxide concentrations on summertemperatures and winds in the stratosphere, as estimated by the Canadian Middle Atmosphere Model. Temperature changes are indi-cated by the dashed lines in degrees Kelvin (1ºK is the same as 1ºC, but the Kelvin scale starts at absolute zero or –273ºC). Windspeed changes are shown by the coloured areas. The model shows the greatest cooling occurring near the top of the stratosphere,with temperatures in the upper stratosphere over the Antarctic cooling by as much as 14ºK.

Figure 13. The graph shows the average daily amount of ultraviolet-B radiation received at Toronto, Edmonton, and Churchill for eachyear since 1965. Values from the late 1980s on are derived from actual measurements. Earlier values are estimates derived from relatedmeteorological information by a statistical model. UV-B amounts begin to increase noticeably at all locations in the early 1980s, shortlyafter the thinning of the ozone layer is believed to have started. The increase at Churchill, however, has been greater than at the other twolocations. This is thought to be due to climatic factors, although it is as yet unclear whether these are related to long-term climate change.

Figure 12Source: de Grandpré 2001

Figure 13Source: V. Fioletov,Environment Canada

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Figure 11Source: B. Arthur, Environment Canada

Rad

ian

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The Canadian Middle Atmosphere Model

Computer modelling is an impor-tant tool that helps researchersimprove their understanding of atmospheric processes, study the impact of natural and human-induced changes in the atmo-sphere, and make long-term predictions about atmosphericchange.The Canadian MiddleAtmosphere Model (CMAM),developed jointly by scientistsfrom Environment Canada and the universities, provides a sophisti-cated representation of importantphysical and chemical processes in the upper troposphere and thestratosphere.The model hasrecently been used to study howthe middle atmosphere wouldchange if the atmosphere con-tained twice as much carbon dioxide as it does now. Results ofthis study suggest that an increasein carbon dioxide will lead indi-rectly to a cooling of the middleatmosphere as more radiation isemitted to space from other areasof the atmosphere.These resultsare consistent with observationstaken over the past few decadesthat show a cooling trend in someparts of the middle atmosphere.Although most of the observedcooling of the stratosphere isthought to be a result of ozoneloss, part could also be the resultof increased concentrations of carbon dioxide (Figure 12). Theresults also show small but signifi-cant changes in the distribution ofozone throughout the middleatmosphere as a result of theincrease in carbon dioxide.

UV Climatology and ResearchMeasurements of the strength of

UV radiation at the earth’s surfacewere not taken on a regular basisin Canada until the introduction ofthe Brewer spectrophotometer inthe late 1980s.When EnvironmentCanada researchers set out adecade later to estimate long-termtrends in UV radiation, the lengthof the data record was still tooshort to yield reliable results. Toovercome this problem, theydeveloped statistical models thatestimated the UV irradiance froma combination of earlier data onsolar radiation, total ozone levels,dew point temperature, and snowcover. Using this method, they estimated that the strength of UV radiation at the earth’s surface has increased by an average ofabout 7% across Canada since thelate 1970s when the thinning ofthe ozone layer appears to havestarted.

Results of the study also show apossible climatic influence on thepattern of UV changes in Canada(Figure 13).When results from different locations in the countrywere compared, Churchill showeda much stronger increase in UVthan either Edmonton or Toronto.The difference is likely due to variations in surface reflectivitycaused by increases in snow coverand decreases in cloud cover.These changes may, in turn, reflecta shift in the average position ofthe polar jet stream.


Observatory, which has been operational since the mid-1990s,provides researchers with a relat-ively non-polluted environment in an area where the climate isexpected to change significantlyas greenhouse gases increase.

Balloon-based ozone researchOver the past 20 years, Environ-

ment Canada has worked with the Canadian Space Agency andpartners in the universities andindustry to collect data about reac-tive nitrogen compounds, chlorineand bromine compounds, ozone,aerosols, and other substances thatplay a critical role in the chem-istry of the upper troposphere andthe stratosphere.This work is co-ordinated under the MiddleAtmosphere Nitrogen TrendAssessment program or MANTRA.To collect the data, researchersuse large polyethylene balloons,about 20 stories high, that carryinstruments to altitudes of, typic-ally, 30–40 km.The instrumentsscan the earth’s horizon at variousaltitudes and record the spectraproduced by scattered sunlight. Atthe end of the flight, the instru-ment payload is separated from theballoon and descends to earth byparachute, where it can be recov-ered.Analysis of the recorded datathen provides information aboutthe chemical composition of theatmosphere at different altitudes.Information from the MANTRAflights not only contributes to abetter understanding of strato-spheric processes but also helpsgovernments determine the effec-tiveness of controls on ozone-depleting substances under theMontreal Protocol.

19

Thanks to the Montreal Pro-tocol of 1987 and the various

amendments made to it in subse-quent years, concentrations ofCFCs, halons, and other majorozone-depleting substances in theatmosphere are now decreasing.As these concentrations declinefurther over the rest of century,the effects of stratospheric ozonedepletion on climate change canbe expected to diminish gradually.Full compliance with the Protocolwould see concentrations ofozone-depleting substances fall bythe end of the century to thepoint where they would nolonger be a significant threat tothe ozone layer. However, it isunlikely that full compliance willbe achieved; therefore, recoveryof the ozone layer will almost certainly take longer.The effectsof ozone depletion on climatechange will also continue longerif CFCs are replaced by substi-tutes such as hydrofluorocarbons(HFCs), which are also potentgreenhouse gases.The greenhouseeffect of these gases, moreover,would be in addition to that ofolder ozone-depleting substancesthat remain in the atmosphere.

While atmospheric concentra-tions of ozone-depleting substances are declining, concen-trations of important greenhousegases, such as carbon dioxide andmethane, that have no direct connection with ozone depletion,continue to increase. Higher concentrations of these gases mayhave the effect of prolonging the

recovery of the ozone layer, mainlythrough their effects on strato-spheric cooling and the formationof polar stratospheric clouds.Recent computer simulations sup-port this notion and suggest thatexpected increases in greenhousegas concentrations could causesevere polar ozone depletion tocontinue for about 10–20 yearslonger than it would if concentra-tions of greenhouse gases hadremained at earlier levels.Thesestudies are very preliminary, how-ever, and further investigation withmore refined models is needed.

From a policy perspective, it isclear that actions to mitigate global warming can have positiveeffects on ozone depletion andvice versa. However, care must betaken to avoid solutions to oneproblem that make the otherworse. Policy makers and scien-tists are now wrestling with thisdifficulty as they look for long-term alternatives to CFCs, halons,and other ozone-depleting sub-stances whose use has beenphased out under the MontrealProtocol. Current restrictions onHCFCs, which have a lowerozone-depleting potential thanCFCs but are strong greenhousegases, illustrate acceptance of theneed to consider the implicationsfor both issues.

But further problems remain.What, in particular, should we doabout HFCs and any other green-house gases now being promotedas alternatives to CFCs andHCFCs? Although concentrationsof these gases in the atmosphereare as yet fairly low, they areexpected to increase significantlyover time if the use of these sub-stances is not controlled. Canadahas adopted the position that theuse of HFCs should be restrictedto the replacement of ozone-depleting substances. Canada also wants emissions of HFCs controlled through mandatoryrecovery and recycling, safe disposal, and the use of otheremission control measures.At thetenth meeting of the Parties to theMontreal Protocol and the FourthConference of the Parties to theClimate Convention, the technicaland scientific authorities of eachbody were asked to provide guid-ance on limiting emissions ofHFCs and other greenhouse gasesthat might be used as CFC replace-ments.Their investigation of theoptions is now under way.


Implications for Policy

21

At the Bratt Lake Observatory, the Brewer spectrophotometer in the foregroundmeasures ultraviolet radiation and the depth of the ozone layer while the solartrackers in the background measure diffuse solar radiation.

The Research Agenda

could, in turn, affect the heightof convection currents withinthe troposphere, such as thoseassociated with thunderstorms,and could possibly affect theintensity of those storms. Itmight also alter the positionsof the planet’s jet streams andthus the movement of weathersystems.

• How will changes in tropo-spheric aerosol concentrationsaffect solar radiation at theearth’s surface? Aerosols aretiny atmospheric particles and droplets. They come from both natural and human-relatedsources. For most of the twen-tieth century, industrial activi-ties substantially increased the atmospheric loading ofaerosols, especially sulphates.Because sulphates contributeto acid rain as well as humanhealth problems, emissions of them have been reducedsubstantially in Europe andNorth America since the 1970s.Although emissions haveincreased with industrializationin other parts of the world, it is expected they too will even-tually fall for the same reasons.Sulphate aerosols reflect solarradiation back to space, thusexerting a cooling force at theearth’s surface and reducingthe amount of ultraviolet radiation that penetrates theatmosphere.As the amount ofsulphate aerosol in the atmo-sphere declines, however,surface warming may increase,as more sunlight reaches the

earth’s surface. In addition, withmore sunlight more ultravioletradiation will also reach theearth’s surface. Since ultravioletradiation contributes to the for-mation of ground-level ozone,which is both a greenhouse gasand a major constituent ofsmog, reduction in sulphateconcentrations could furtherintensify greenhouse warmingas well as lead to more ozonepollution in many areas.

• How will stratospheric coolingaffect the formation of ozoneholes? Since their discovery in 1985,Antarctic ozone holeshave become increasingly larger.The hole that formed inSeptember 2000 was, at thetime of writing, the largest yetrecorded, covering an area of28.3 million square kilometres,a little bit bigger thanNorth America andslightly smaller thanAfrica (Figure14).The holeextended asfar northas the

On the research side, moreneeds to be known about

how the troposphere and thestratosphere interact.The following are some of the moreimportant questions now beingpursued.• What are the major coupling

mechanisms between the troposphere and the strato-sphere and how do they affect climate? We know,for example, that the verticaltemperature structure of thetroposphere is sensitive tochanges in the vertical tempera-ture structure of the strato-sphere.We also know thatwaves propagating upwardsfrom the troposphere reachinto the stratosphere and affecttemperatures and circulationthere. Similarly, waves generatedin the stratosphere can pene-trate downwards and affectweather in the troposphere.Further study of such mecha-nisms will improve our under-standing of how conditions inthe stratosphere affect climatein the troposphere and viceversa.

• How do changes in the vertical temperature structureof the troposphere and stratosphere affect climate?During the past three decades,researchers have measured acooling of the stratosphere anda warming of the lower tropo-sphere.These changes couldaffect the height of thetropopause, the boundarybetween the troposphere andthe stratosphere. Such a change


Figure 14Source: NASA (top); Servicio Meteorológico Nacional, Argentina (bottom)

Figure 14. The Antarctic ozone hole reached its largest extent yet in October 2000, and for the first time extended oversizeable settled areas in southern South America. UV Index values in Ushuaia, Argentina, were close to or greater than 8on five different days and reached 10.1 on October 12. UV Index values for October in Ushuaia are normally around 4.Values greater than 10 are usually found only in tropical areas.

23

UV Index at Ushuaia, Argentina

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vapour in the atmosphere andthat could have important con-sequences for both climate andozone depletion. Scientiststherefore need to determinewhether the amount of watervapour in the atmosphere isactually increasing and, if so,how it is distributed in differ-ent parts of the atmosphere.

• How will decreases in theatmospheric loading of chlorineand bromine affect climatechange and ozone depletion?As the amount of chlorine andbromine in the stratospheredecreases, ozone concentra-tions in the stratosphere shouldgradually increase towards natural levels.At the same time,however, atmospheric tempera-tures will be affected as thecooling effect of ozone loss andthe warming effect of ozone-depleting greenhouse gasesdiminish. Data from monitoringsystems that track ozone concentrations and other impor-tant atmospheric characteristicswill help to verify that expectedchanges are taking place andexpand our understanding ofother changes that may beoccurring.Many of the research initiatives

that are probing these and otherrelated questions are taking placeunder the World ClimateProgramme’s SPARC project,launched in 1992.The acronymstands for Stratospheric Processes

southern tip of South America,subjecting Ushuaia,Argentinaand the city of Punta Arenas inChile, with a population of over100,000 people, to very highultraviolet radiation levels forthat time of year. Since the early1990s, severe ozone depletionhas also been detected in someyears in the Arctic stratosphere.In the spring of 2000, for exam-ple, ozone levels in the Arcticwere depleted by almost 60% atan altitude of 18 km. Ozonedepletion over the poles shoulddecrease as the quantity ofozone-depleting substances inthe atmosphere diminishes, butresearchers need to know moreabout how changes in green-house gas and stratosphericozone concentrations will affecttemperatures in the polar strato-sphere before they can confi-dently predict future trends inozone hole formation.

• How will changes in theamount of water vapour inthe atmosphere affect climatechange and ozone depletion?Water vapour is the most abun-dant greenhouse gas in theatmosphere. In the strato-sphere, water vapour in polarstratospheric clouds also acts asa catalyst that enhances ozonedepletion. Since the atmo-sphere can hold more watervapour when it is warmer,greenhouse warming couldincrease the amount of water

and their Role in Climate, and theproject embraces a number ofstudies designed to improve scien-tific understanding of the physical,chemical, and radiative connec-tions between the troposphereand the stratosphere and theireffects on climate. SPARC’s currentresearch agenda includes suchareas as stratospheric indicators ofclimate change, the physics andchemistry of ozone depletion, theinfluence of ozone changes on climate, energy and gas exchangesbetween the troposphere and thestratosphere, and atmosphericwaves.

Another important area of activity, and one in which SPARC is heavily involved, focuses onimproving the way in which com-puter models of the atmosphererepresent important linkagesbetween ozone depletion and climate change.These improve-ments include better representa-tion of the stratosphere in the general circulation models used tostudy climate change and betterrepresentation of atmosphericwaves. Inclusion of these refine-ments will give us not only greaterinsight into how ozone depletionand climate change interact butwill also provide a better under-standing of how these linkages, incommon with all the other factorsaffecting atmospheric change, willinfluence the future state of boththe ozone layer and the surface climate.


25

An integrated approach to atmospheric issues is alsonecessary because ecosystems and human health areaffected not by just one of these issues but by all ofthem. The puzzling decline of amphibian populations, forexample, may be a result of multiple stresses related toacid rain, persistent organic pollutants, and a number ofother pollution issues. Increased UV levels (caused byozone depletion) and changes in shallow water levels(caused by climate change) may also be contributing to the decline, as they make amphibian eggs more susceptible to water mould, which kills the embryos.

Making Connections

Increasingly both science andpolicy are being driven by the

recognition that the atmospherecannot be seen as a collection ofseparate compartments in whichdifferent things happen in isola-tion. Instead, researchers and policy makers are adopting amore comprehensive view of theatmosphere, seeing it as a singledynamic whole whose state atany given time depends on amaze of interactions not onlybetween its different parts andprocesses but also with those ofother parts of the ecosphere.

To take proper account of theseinteractions, atmospheric scien-tists are increasingly thinking outside the confines of their particular specialties and collabo-rating more closely with expertsin other areas.The evolution of

integrated approaches to ozonedepletion and climate change –and to other atmospheric issuesas well – is now enhancing ourability to understand and moder-ate the enormous changes thathuman activities have imposed onthe earth’s atmosphere over thepast two centuries. Not only willthese approaches help us dealmore effectively with today’s problems, but they will also giveus a much better chance of anticipating and controlling anyfurther human threats to theatmosphere that might occur inthe future.


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Environment˜ Environnement˜ Canada˜ - NASA · Evidence of ozone depletion in the stratosphere began to appear in the 1980s.Atmospheric concentrations of most ozone-depleting substances

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