Section 2 The Ozone Shield 335

The is an area in the stratosphere where ozone ishighly concentrated. Ozone is a molecule made of three oxygenatoms. The ozone layer absorbs most of the ultraviolet (UV)light from the sun. Ultraviolet light is harmful to organismsbecause it can damage the genetic material in living cells. Byshielding the Earth’s surface from most of the sun’s ultravioletlight, the ozone in the stratosphere acts like a sunscreen for theEarth’s inhabitants.

Chemicals That Cause Ozone DepletionDuring the 1970s, scientists began to worry that a class ofhuman-made chemicals called mightbe damaging the ozone layer. For many years CFCs were thoughtto be miracle chemicals. They are nonpoisonous and nonflam-mable, and they do not corrode metals. CFCs quickly becamepopular as coolants in refrigerators and air conditioners. Theywere also used as a gassy “fizz” in making plastic foams andwere used as a propellant in spray cans of everyday products suchas deodorants, insecticides, and paint.

At the Earth’s surface, CFCs are chemically stable. So, they donot combine with other chemicals or break down into other sub-stances. But CFC molecules break apart high in the stratosphere,where UV radiation, a powerful energy source, is absorbed. OnceCFC molecules break apart, parts of the CFC molecules destroyprotective ozone.

Over a period of 10 to 20 years, CFC molecules released atthe Earth’s surface make their way into the stratosphere. Figure10 shows how the CFCs destroy ozone in the stratosphere. EachCFC molecule contains from one to four chlorine atoms, and sci-entists have estimated that a single chlorine atom in the CFCstructure can destroy 10,000 ozone molecules.

chlorofluorocarbons (CFCs)

ozone layer Objectives� Explain how the ozone layer

shields the Earth from much of thesun’s harmful radiation.

� Explain how chlorofluorocarbonsdamage the ozone layer.

� Explain the process by which theozone hole forms.

� Describe the damaging effects ofultraviolet radiation.

� Explain why the threat to theozone layer is still continuingtoday.

Key Termsozone layerchlorofluorocarbons (CFCs)ozone holepolar stratospheric clouds

S E C T I O N 2

The Ozone Shield

UVlight

Chlorine,Cl

Chlorine,Cl

Chlorinemonoxide,

ClOOzone,

O3

Chlorine,Cl

+

Ozone,O3

Chlorinemonoxide,

ClO

+UV light causes the CFC to breakdown, releasing a chlorine atom.

The chlorine atom reacts withan ozone molecule to createan oxygen molecule anda chlorine monoxide molecule.

The chlorine monoxide molecule then reacts with another ozone molecule, creating two molecules of oxygen and one chlorine atom.

1 2 3

Figure 10 � The CFC molecule inthis illustration contains a single chlorine atom. This chlorine atomcontinues to enter the cycle andrepeatedly destroys ozone molecules.

Copyright© by Holt, Rinehart and Winston. All rights reserved.

The Ozone HoleIn 1985, an article in the scientific journal Nature reported theresults of studies by scientists working in Antarctica. The stud-ies revealed that the ozone layer above the South Pole hadthinned by 50 to 98 percent. This was the first news of the

a thinning of stratospheric ozone that occurs overthe poles during the spring.

After the results from the studies from Halley Bay were pub-lished, NASA scientists reviewed data that had been sent to Earthby the Nimbus 7 weather satellite since the satellite’s launch in1978. They were able to see the first signs of ozone thinning inthe data from 1979. Although the concentration of ozone fluctu-ates during the year, the data showed a growing ozone hole, asshown in Figure 11. Ozone levels over the Arctic have decreasedas well. In fact, March 1997 ozone levels over part of Canadawere 45 percent below normal.

How Does the Ozone Hole Form? During the dark polar winter,strong circulating winds over Antarctica, called the polar vortex,isolate cold air from surrounding warmer air. The air within thevortex grows extremely cold. When temperatures fall belowabout –80°C, high-altitude clouds made of water and nitric acid,called begin to form.

On the surfaces of polar stratospheric clouds, the products ofCFCs are converted to molecular chlorine. When sunlight returnsto the South Pole in spring, molecular chlorine is split into twochlorine atoms by ultraviolet radiation. The chlorine atoms rap-idly destroy ozone. The destruction of ozone causes a thin spot, or ozone hole, which lasts for several months. Some scientists esti-mate that as much as 70 percent of the ozone layer can bedestroyed during this period.

Because ozone is also being produced as air pollution, youmay wonder why this ozone does not repair the ozone hole in thestratosphere. The answer is that ozone is very chemically reactive.Ozone produced by pollution breaks down or combines withother substances in the troposphere long before it can reach thestratosphere to replace ozone that is being destroyed.

polar stratospheric clouds,

ozone hole,

336 Chapter 13 Atmosphere and Climate Change

Figure 11 � These satellite imagesshow the growth of the ozone hole,which appears purple, over the pasttwo decades.

MeteorologyConnection to

Polar Stratospheric CloudsBecause the stratosphere isextremely dry, clouds normallydo not form in this layer of theatmosphere. However, duringpolar winters, temperaturesbecome low enough to causecondensation and cloud forma-tion. These clouds, which occurat altitudes of about 21,000 m,are known as polar stratosphericclouds, or PSCs. Because of theiriridescence, PSCs are calledmother-of-pearl or nacreousclouds. Outside of the poles, thestratosphere is too warm forthese clouds to form. Becausethese clouds are required for thebreakdown of CFCs, ozone holesare confined to the Antarctic andArctic regions.

1980 1990 2000

Copyright© by Holt, Rinehart and Winston. All rights reserved.

Effects of Ozone Thinning on Humans As the amount of ozone inthe stratosphere decreases, more ultraviolet light is able to passthrough the atmosphere and reach Earth’s surface, as shown inFigure 12. UV light is dangerous to living things because it damagesDNA. DNA is the genetic material that contains the informationthat determines inherited characteristics. Exposure to UV lightmakes the body more susceptible to skin cancer, and may cause cer-tain other damaging effects to the human body.

Effects of Ozone Thinning on Animals and Plants High levels ofUV light can kill single-celled organisms called phytoplanktonthat live near the surface of the ocean. The loss of phytoplanktoncould disrupt ocean food chains and reduce fish harvests. In addi-tion, a reduction in the number of phytoplankton would cause anincrease in the amount of carbon dioxide in the atmosphere.

Some scientists believe that increased UV light could be espe-cially damaging for amphibians, such as toads and salamanders.Amphibians lay eggs that lack shells in the shallow water ofponds and streams. UV light at natural levels kills many eggs ofsome species by damaging unprotected DNA. Higher UV levelsmight kill more eggs and put amphibian populations at risk.Ecologists often use the health of amphibian populations as anindicator of environmental change due to the environmental sen-sitivity of these creatures.

UV light can damage plants by interfering with photosynthe-sis. This damage can result in lower crop yields. The damagingeffects of UV light are summarized in Table 1.

Section 2 The Ozone Shield 337

Figure 12 � Depletion of the ozonelayer allows more ultraviolet (UV) radiation to reach the surface ofthe Earth.

Damaging Effects of UV Light

• interference with photosynthesis

• reduced crop yields

Land Plants

• death of phytoplankton in surface water

• disruption of food chain

• reduction in the number of photosynthesizers

Marine Life

• death of eggs

• genetic mutations among survivors

• reduction of populations

Amphibians

• increased incidence of skin cancer

• premature aging of the skin

• increased incidence of cataracts

• weakened immune response

Humans

Table 1 �

Ultraviolet rays penetrateto the Earth’s surfacethrough the ozone hole.

Ozone layer

Ultraviolet rays are absorbed by the ozone layer or reflected back to space.

Copyright© by Holt, Rinehart and Winston. All rights reserved.

Protecting the Ozone LayerIn 1987, a group of nations met in Canada and agreed to takeaction against ozone depletion. Under an agreement called theMontreal Protocol, these nations agreed to sharply limit theirproduction of CFCs. A second conference on the problem washeld in Copenhagen, Denmark, in 1992. Developed countriesagreed to eliminate most CFCs by 1995. The United Statespledged to ban all substances that pose a significant danger to the ozone layer by 2000.

After developed countries banned most uses of CFCs, chemicalcompanies developed CFC replacements. Aerosol cans no longeruse CFCs as propellants, and air conditioners are becoming CFCfree. Because many countries were involved and decided to controlCFCs, many people consider ozone protection an internationalenvironmental success story. Figure 13 illustrates the decline inworld CFC production since the 1987 Montreal Protocol.

The battle to protect the ozone layer is not over. CFC mol-ecules remain active in the stratosphere for 60 to 120 years. CFCsreleased 30 years ago are still destroying ozone today, so it willbe many years before the ozone layer completely recovers.

338 Chapter 13 Atmosphere and Climate Change

www.scilinks.orgTopic: Ozone ShieldSciLinks code: HE4080

1986 1989 1990 1991 1992 1993 1994 1995 19960

600

1,000

200

400

800

1,200

Source: UN Environment Programme.

World CFC Production

CFC

pro

duc

tio

n

(in

mill

ion

s o

f m

etri

c to

ns)

Developed countries

Developing countries

Year

Figure 13 � Chlorofluorocarbonproduction has declined greatly sincedeveloped countries agreed to banCFCs in 1987.

1. Describe the process by which chlorofluorocarbonsdestroy ozone molecules in the stratosphere.

2. Describe the process by which the ozone hole formsover Antarctica in spring.

3. List five harmful effects that UV radiation could haveon plants or animals as a result of ozone thinning.

4. Explain why it will take years for the ozone layer torecover even though the use of CFCs has declinedsignificantly. Write a paragraph that explains youranswer.

CRITICAL THINKING5. Making Decisions If the ozone layer gets signifi-

cantly thinner during your lifetime, what changesmight you need to make in your lifestyle?

6. Analyzing Relationships CFCs were thought tobe miracle chemicals when they were first intro-duced. What kinds of tests could be performed onany future miracle chemical to make sure seriousenvironmental problems do not result from its use?

WRITING SKILLS

S E C T I O N 2 Review

Copyright© by Holt, Rinehart and Winston. All rights reserved.