21.3 Climate Changes

Reading StrategyIdentifying Cause and Effect Copy thetable. Identify the causes and effects ofclimate change presented in this section.

Key ConceptsDescribe natural processesthat can cause changes inclimate.

What is the greenhouseeffect?

What is global warming?

What are some of theconsequences of globalwarming?

L ike most conditions on Earth, climate is always changing. Some ofthese changes are short-term. Others occur over long periods of geo-logic time. Some climate changes are the result of natural processes,such as the volcanic eruption shown in Figure 14. Others are related tohuman activities. In this section, you will learn about some of the waysin which climate changes.

Natural Processes That ChangeClimateMany different natural processes can cause a climate to change. Someof the climate-changing processes that you will learn about includevolcanic eruptions as well as changes in ocean circulation, solar activ-ity, and Earth motions.

Volcanic Eruptions As you can see in Figure 14, vol-canic eruptions can emit large volumes of ash and dust intoEarth’s atmosphere. What you can’t see in the photographis that volcanic eruptions also send minute particles con-taining sulfur, into the air. If the volume of these very fineparticles called aerosols, is great enough, it can cause short-term changes in Earth’s surface temperature. Can youhypothesize why? The presence of aerosols (volcanicash, dust, and sulfur-based aerosols) in the air increasesthe amount of solar radiation that is reflected back intospace. This causes Earth’s lower atmosphere to cool.

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Figure 14 Eruption of MountPinatubo

Vocabulary◆ greenhouse effect◆ global warming

Climate Changes

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Section Objectives21.12 Describe natural processes that

can cause changes in climate.21.13 Explain the greenhouse effect.21.14 Define global warming.21.15 List some of the consequences

of global warming.

Build VocabularyParaphrase Explain the meaning ofthe vocabulary words by using termsthat are familiar to students. Forexample, tell students that global can bethough of as “globe-like.” Point out aclassroom globe as an example.

Reading Strategya. Volcanic Eruptionsb. Possible increase in the amount ofsolar radiation reflected back to spacecan cause lower temperatures.c. Ocean Circulationd. Changes can result in short-termclimate fluctuations.e. Earth Motionsf. Geographic changes in Earth’s landand water due to Earth motion causechanges in climate.

INSTRUCT

Natural Processes ThatChange Climate

Students may think that Earth’s climatewas exactly the same for millions ofyears, and only changed recently as aresult of human activity. Point out thatthere have been many major naturalchanges in Earth’s history. For example,during the Ice Ages that began 2 to 3million years ago Earth’s climate wasmuch colder. At other times Earth’sclimate has been much warmer than it isnow. Emphasize that there are manydifferent causes of climate change, andthat human action is just one of them.Logical

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Ocean Circulation Recall from Chapter19 that El Niño is a change in ocean circulationthat causes parts of the eastern tropical PacificOcean to become warmer than usual. Thesechanges in ocean circulation also can resultin short-term climate fluctuations. For exam-ple, some areas that are normally arid receivelarge amounts of rain during El Niño. Refer toFigure 15. Also, some regions that receive abun-dant precipitation may experience dry periodswhen ocean circulation patterns change.

Solar Activity The most studied hypothe-ses for the causes of climate change are based onchanges in the output of solar energy. When thesun is most active, it contains dark blemishescalled sunspots. The formation of sunspotsappears to correspond with warm periods inEurope and North America. Although variationsin solar output may cause short-term climaticchange, no evidence for long-term variationsdue to solar activity exist.

Earth Motions A number of Earthmotions are thought to cause changes in climate.Most of these changes are long-term changes.Tectonic plate movements, for example, causethe crust and upper mantle to move slowly overEarth’s surface. These movements cause ocean basins to open andclose. Plate movements also cause changes in the positions of land-masses. These geographic changes in Earth’s land and waterbodies cause changes in climate.

Changes in the shape of Earth’s orbit and the tilt of Earth onits axis are other Earth motions that affect global climates. Earth’sorbit, or path around the sun, is always elliptical. But over a 100,000-year period, the path becomes more and then less elliptical. This changein shape brings Earth closer to and then farther from the sun. This affectsglobal climates. Like its orbit, the tilt of Earth on its axis changes about2 degrees over a 41,000-year period. Because the angle of tilt varies, theseverity of the seasons also changes. The smaller the tilt, the smaller thetemperature difference between summer and winter.

Identify four natural processes that can result in climate changes.

Figure 15 Effect of El Niño In1998, bad weather conditions andflooding in Alabama wereattributed to El Niño.

Build Reading LiteracyRefer to p. 334D in Chapter 12, whichprovides the guidelines for outlining.

Outline Have students read the text inthis section about climate change. Then,have students use the headings as majordivisions in an outline. Have studentsrefer to their outlines when answeringthe questions in the Section 21.3Assessment.Verbal, Logical

Earth’s Motionsand ClimatePurpose Students observe howchanges in Earth’s motions affectEarth’s climate.

Materials globe, lamp

Procedure If necessary, review whatcauses the seasons on pp. 481–482 inChapter 17. Use a globe and a lamp todemonstrate how changes in the shapeof Earth’s orbit and tilt of Earth’s axis canaffect global climate. First, set the lampin the middle of a table and turn it on.Move the globe around the lamp in acircular motion. Then move the globearound the lamp in a more elliptical(oval) motion. Ask students how theythink this movement would affectclimates. (It could increase the differencesbetween the seasons.) Next, increase thetilt of the globe’s axis and again movethe globe around the lamp. Ask studentshow they think this would affectclimates. (It would decrease the totalamount of sunlight Earth would receive,and therefore lower global temperatures.)

Expected Outcome Students willunderstand how changes in Earth’smotions affect Earth’s climate.Logical, Visual

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Gifted Many scientists think that globalwarming is not entirely caused by carbondioxide emissions. They predict that globalwarming will have positive consequences forsome parts of the world. There is also debateover the relative benefits of various attempts toslow global warming. Invite gifted students toresearch different views of global warming andreport back to the class.

Some Web sites where they can begin theirresearch include:http://www.cato.org/current/global-warming/index.htmlhttp://www.marshall.org/subcategory.php?id=9http://cfa-www.harvard.edu/press/pr0310.html

Answer to . . .

Volcanic eruptions,changes in global ocean

circulation, solar activity, and certainEarth motions can cause changes inclimate.

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Human Impact on Climate ChangesNatural processes have certainly contributed to many climatic changesthroughout Earth’s 4.6-billion year history. These processes will also beresponsible for some of the future shifts in Earth’s climates. Besidesthese processes of nature, human activities have contributed and willcontribute to global climatic change.

The Greenhouse Effect The greenhouse effect is a natu-ral warming of both Earth’s lower atmosphere and Earth’s surface.The major gases involved in the greenhouse effect are water vapor andcarbon dioxide. These greenhouse gases, as they are often called, aretransparent to incoming solar radiation and therefore much of thisenergy reaches Earth’s surface. Most of this energy is then reradiatedskyward. The greenhouse gases are good absorbers of Earth’s radia-tion, which accounts for the warm temperatures of the loweratmosphere.

The greenhouse effect is very important because it makes life as weknow it possible on Earth. Without this effect, Earth would be muchtoo cold to support any kind of complex life forms. However, anincrease in the greenhouse effect could also prove devastating to Earth’sbillions of organisms.

Studies indicate that human activities for the past 200 or so yearshave had a huge impact on the greenhouse effect. As you can see inFigure 16, carbon dioxide levels in the air have risen at a rapid pacesince about 1850. Much of this greenhouse gas has been added by theburning of fossil fuels such as coal, petroleum, and natural gas.

The clearing of forests also contributes to an increasein carbon dioxide because this gas is released when vegetation is burned or when it decays.

Figure 16 The rapid increase incarbon dioxide concentration since1850 has closely followed theincrease in carbon dioxideemissions from burning fossil fuels. Inferring What do you thinkinitiated this increase in carbondioxide levels?

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Human Impact onClimate Changes

Students may be misled by the termgreenhouse effect. The term may makethem think that the atmosphere isheated in exactly the same way as agreenhouse. The term was coined todescribe the process that heats theatmosphere because it was oncethought that greenhouses were heatedin a similar way. One factor in heatinga greenhouse is the blocking of heatradiation. However, a more importantfactor is that the glass of the greenhouseprevents warm air from escaping byconvection.Logical

Use VisualsFigure 16 Use these graphs to explainhow carbon dioxide emissions affectcarbon dioxide concentrations in theatmosphere. Make sure that studentsunderstand that the emissions amountsand atmospheric concentrations ingraph B are shown in different units.Ask: What unit is used for carbondioxide emissions? (gigatons ofcarbon/yr) What unit is used forcarbon dioxide concentrations in theatmosphere? (ppm, or parts per million)What does graph C show? (carbondioxide concentrations in the atmosphere)Visual, Logical

Students often think that thegreenhouse effect and global warmingare the same phenomenon. Emphasizethat the greenhouse effect occursnaturally and is essential to keeping theatmosphere warm enough for livingthings to survive on Earth. Globalwarming, on the other hand, is anincrease in the temperature of Earth’satmosphere that may be caused orinfluenced by human activities.Logical

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Section 21.3 (continued)

Human influence on regional and globalclimates likely began long before thebeginning of the modern industrial period.There is evidence that humans have beenmodifying the environment over extensiveareas for thousands of years. The use of fire

and the overgrazing of marginal landsby domestic animals have reduced theabundance and distribution of plants. Alteringground cover has affected important climatefactors such as surface albedo, evaporationrates, and surface winds.

Facts and Figures

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Section 21.3 Assessment

Reviewing Concepts1. Describe four natural processes that can

cause climate change.

2. What is the greenhouse effect?

3. What is global warming?

4. What are some of the possible effects ofglobal warming?

Critical Thinking5. Formulating Hypotheses Which would

have a longer effect on climate changes—volcanic ash and dust or the same volume ofsulfur-based aerosols? Why?

6. Formulating Conclusions How do youthink cloud cover might change as the resultof global warming?

7. Synthesizing How might global warmingaffect Earth’s inhabitants, including humans?

Global Warming As a result of increasesin carbon dioxide levels, as well as other green-house gases, global temperatures have increased.This increase is called global warming. Refer toFigure 17. Note that during the twentieth century,Earth’s average surface temperatures increasedabout 0.6°C. Scientists predict that by the year2100, temperatures will increase by 1.4°C to 5.8°C.How will these temperature increases affect Earth?

Warmer surface temperatures increase evaporation rates. This, inturn, increases the amount of water vapor in the atmosphere. Watervapor is an even more powerful absorber of radiation emitted by Earththan is carbon dioxide. Therefore, more water vapor in the air willmagnify the effect of carbon dioxide and other gases.

Temperature increases will also cause sea ice to melt. Ice reflectsmore incoming solar radiation than liquid water does. The melting ofthe ice will cause a substantial increase in the solar energy absorbed atthe surface. This, in turn, will magnify the temperature increase createdby higher levels of greenhouse gases. The melting of sea ice and icesheets will also cause a global rise in sea level. This will lead to shore-line erosion and coastal flooding.

Scientists also expect that weather patterns will change as a resultof the projected global warming. More intense heat waves anddroughts in some regions and fewer such events in other places arealso predicted. What other consequences of global warming do youthink might occur?

Climate 603

Figure 17 Increases in the levelsof greenhouse gases have causedchanges in Earth’s average surfacetemperatures.Interpreting Graphs What yearwas the warmest to date?

Persuasive Paragraphs Write at least twoparagraphs to persuade your friends andfamily to reduce their consumption of fossilfuels. Be sure to explain why the usage ofsuch energy sources should be reduced.

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For: Links on the carboncycle/global warming

Visit: www.SciLinks.org

Web Code: cjn-6213

ASSESSEvaluateUnderstandingAsk students to describe three naturalprocesses that change climate. Havethem describe how each process works.

ReteachHave students write several sentencesthat briefly explain what the greenhouseeffect is.

Answers will vary but should include anexplanation of how burning fossil fuels isthought to be increasing globaltemperatures and why reducing theusage of fossil fuels will slow thisincrease.

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3. the term for an increase in global tempera-tures, due to increased levels of greenhousegases4. short-term and long-term changes in tem-perature and precipitation patterns that canresult in both wetter-than-normal periodsand drier-than-normal periods; a rise in sealevel; an increase in shoreline erosion5. sulfur-based aerosols tend to remainsuspended in the air longer than their solidcounterparts.6. Global warming would probably producean increase in cloud cover due to the higher

Section 21.3 Assessment

1. volcanic eruptions (gases and solids trapand radiate solar energy back to space); oceancirculation patterns cause changes in atmos-pheric circulation that can result in droughtand flooding; changes in solar activity coin-cide with global changes in temperature;Earth motions, including tectonism andrevolution, also cause climates to change.2. The greenhouse effect is a natural warm-ing of both Earth’s lower atmosphere and itssurface that is caused by complex reactionsbetween gases and particles in the air.

Answer to . . .

Figure 16 Industrialization hasgreatly contributed to the rise incarbon dioxide levels in Earth’satmosphere.

Figure 17 1998

moisture content of the air. Clouds wouldalso produce opposite effects: they coulddiminish the amount of solar energy availableto heat the atmosphere, and they couldabsorb and emit radiation that would other-wise be lost from the troposphere.7. It could harm or kill ocean organisms;cause both land and water habitats to shiftor be destroyed; increase the spread ofdiseases; and increase drought, heat waves,and flooding.

Download a worksheet on thecarbon cycle/global warming forstudents to complete, and findadditional teacher support fromNSTA SciLinks.

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Waterlogged soil beneathtrees is acidic and infertile.

Coniferous ForestsThe world’s largest forests extend across the far north,where winters can last for eight months. Thesedense coniferous forests consist of spruces, pines,and other trees that carry their seeds in cones. Theyare particularly suited for coping with cold condi-tions. Animals in northern forests find plentiful foodduring the long days of summer, but the season is briefand cold weather soon returns. To survive the harsh winter, many animals migrate south, while others hibernate.

How the Earth Works

PREDATORSMammals are relatively scarce

in northern forests, so thepredators that feed uponother animals sometimes haveto cover vast distances to find

food. Bobcats may roam manymiles searching for small prey.Wolves hunt in packs for deer

and other large mammals.

Bobcat

FORESTS AND LAKESConiferous forests often grow on land once cov-ered by ice age glaciers. These glaciers scouredthe ground, scraping away soil and creatingrounded hills and hollows. When the glaciersmelted, the hills became covered with treesand the hollows turned into lakes.

CONIFER LEAVESMost conifers havesmall evergreen leaves

that are tough enoughto withstand the coldest

winters. A narrow shapehelps the leaves to copewith strong winds.

White spruce

Distribution of northern coniferous forests

NORTHAMERICA

SOUTHAMERICA

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AFRICA

ASIA

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TROPIC OF CANCER

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ObjectivesIn this feature, students will• describe where the world’s coniferous

forests are located.• recognize the features that enable

conifers to withstand long, coldwinters.

• identify the behaviors and adaptationsthat enable animals to survive innorthern coniferous forests.

Build VocabularyWord Origins Point out to studentsthat coniferous sounds very much likeanother scientific term, carnivorous.Have them contrast the origins of thesewords. (Coniferous comes from Latin andGreek roots that mean “cone-bearing.”Carnivorous comes from the Latin wordscaro, which means “flesh,” and vorare,which means “to swallow up.”)Encourage students to use both wordscorrectly in sentences.

INSTRUCTBellringerHave students list three features thatmake evergreen trees different fromother trees. (Lists might include pointyleaves or needles, pine cones, and stayinggreen all year.) Have students list typesof trees found in their region. Then ask:Are the trees you listed conifers ordeciduous trees? How do you know?Verbal, Logical

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Gifted Mammals are not the only animalsthat hibernate in winter. Many fish,amphibians, and reptiles are also hibernators.Divide students into groups and have each

find out how one type of animal hibernates.Have each group explain how that type ofhibernation differs from mammal hibernation.

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Cold lake water containsfew nutrients but is oftenrich in oxygen.

Caribou

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SEED EATERSSome birds rely on conifer

seeds for food. Crossbillfinches have unique bills

that are crossed at thetips. This helps

them removeseeds from cones.

Clark’s nutcracker, amember of the crowfamily, hides 20,000

or more seeds each fall. It is able to remember the

locations of many ofthese seeds for up

to nine months.

1. A horntaillays eggs deepin a tree trunk.

2. Young larvaebore away fromthe drill-hole.

3. Each larvamatures insidea chamber nearthe bark of thetree.

EATING WOODSeveral insects of northernforests feed on wood. Thehorntail, or giant wood wasp,lays eggs by drilling deepbeneath tree bark with a longegg-laying tube. The larvaehatch and mature inside thetree while feeding on thewood.

Spruce cone

Woodchuck

COPING WITH COLDTo avoid extreme winter temperatures, bears,woodchucks, and other mammals hibernate.During the fall, they build up a store of fat intheir bodies that will last until spring. Theythen go into hibernation, which slows their bodily functions to a minimum.

1. Key Terms Define (a) coniferous forest, (b) predator, (c) hibernation.

2. Climates Describe the climatic conditions that are generally found in northern coniferous forests.

3. Ecosystems How do trees serve as afood source for birds and insects?

4. Ecosystems How are mammals ofnorthern coniferous forests well suitedfor survival in their natural environment?

5. Critical Thinking Developing aHypothesis Deforestation has notreduced northern coniferous forests to the same degree that it has reducedmid-latitude deciduous forests. Whydo you think that northern coniferousforests have fared better than decidu-ous forests to the south?

Red crossbill

Caribou hooves actas snowshoes.

ADAPTED FOR TRAVEL To help them walk

across thick layers ofsnow without sink-ing, caribou and elkhave hooves withbroadly splayedtoes that help todistribute theirweight. Lynx and

snowshoe hares havesimilar adaptations.

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Build Science SkillsHave students read the introductoryparagraph. Ask: What is the featurethat all conifers share? (They carry theirseeds in cones.) Then make a list on theboard of all the conifers they can name.(Possible answers: spruce, pine, juniper,cedar, sequoia, redwood, fir, and cypress)Logical

ASSESSEvaluateUnderstandingHave students create a three-columnchart about the types of animals thatlive in coniferous forests. The columnsshould be labeled: “Type of Animal,”“Feature or Habit,” and “How ItPromotes Survival.” Have studentscomplete the chart using informationfrom the feature. Ask them to write aparagraph describing the plant andanimal life in a coniferous forest.

ReteachHave students collect or draw theneedles of different kinds of conifers.Then have them write a paragraph thatexplains how the leaves are welladapted for cold climates. Displaycollections and paragraphs in theclassroom.

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4. Some mammals, such as elk, have splayedtoes that help them walk on snow. Othermammals build up stores of fat during thefall that will last until spring. In winter theyhibernate.5. Sample answer: Fewer people have wantedto settle in cold, northern areas. The land isnot suited to farming, so humans have notcut these forests to clear land.

Assessment

1. (a) large, dense forests made of trees thatcarry their seeds in cones; (b) animals thatfeed on other animals; (c) a state in which ananimal’s body functions slow to a minimumfor a season2. Winters are long and cold; summers arebrief.3. Some insects eat wood. Some birds eatconifer seeds.

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